Back with Jets and some answers

[Gludy]

Re: Gludy

Thanks for the comments.

Jets in smaller boats of the type you have mentioned are really a different aniumal to thge jets I am talking about in cruising boats.

In fact, the slow speed agilty of these boats is outstanding but yes, I imagine that they would use more fuel at such slow speed but this is of no significance compared to the overall use. Do not forget that the Cara Marine claim higher fule consumption at mid range speeds - but if I had a boat that can cruise at 28 knots more efficiently than a prop job, I would not cruise at 15 knots!

You know now matter what energy you deilver to your props, the fact that they are not aligned in the direction of travel loses a lot of energy just pushing water at right angles and not contributing at all to the forward motion of the boat.

What I would like to see explained is how a semi-displacemnt hull can achive so much. I would also like to see contributions about the claim that these larger cruising boats really do not plane - they just lift their bows a little higher but do not properly plane. Therefore we should not be using planing hulls for boats over 30 foot!!!

My first boat did surprise people with its size and everyone raises eyebrows about my next boat - but I am com,pletely bonkers about the whole subject and no matter what I do I try to learn and learn. If that means challanging long held myths- so be it. I cannot lose, I always win because I always learn. I do not give a damn about being right or wrong.

I did my first berthing alongside a wall in a small harbour the other day - about 10 foot away from the shops and with public 3 foot away. It was a tight berth and I handled the boat OK. When the public spoke to me, I explained I was an absolute beginner and someone on my boat questioned me why I had said that wehn no one knew and it was such a good bit of berthing ... I answered because it was the truth and I am not ashamed of it. I also know, that next time, I would just as likely hit something.

I am pleased about the change in the moide of the forum as it keeps threads that deserve to be alive alive and lets the ones that are exhausted die away. This thread, which is already in three parts, would now be pages down the list and postive contributions such as that just made by tcm would be missed



Paul

 

[tcm]

case adjourned!

All agreed.

Actually, i do think it a bit poor that uk boats seem very unadventurous with regard to lots of developments. Seems you do too. Lots of 20-year old items are very much the same on modern boats.

I didn't mean to be nasty in using the word "boring" - as again i think you know! This dateslipping means that the post is on top again, as it was on thursday. With the old method, it wd still be on the first page, but not at the top. It's a "known thread" now, with all others having seen it and just a few contributing. IMHO it should be a bit further down, but still "alive", as before. It should predominate over "today's new threads" as it is doing.

 

[jfm]

Boatbuilders wanted, prev exp making V tablets req

Gludy:
Hi. I think I'll refrain from an engineering/thermodynamics type debate with you - you flipped massively back to semantics above!

And I agree overall mpg is the best yardstick.

But if the Cara boat has massive mpg (all other things being equal) is it becos of the jets, or something else like the hull shape? What other info is there on this Cara boat? Very little on their website, odd they don't publish data if they have launched 2 and are building #3? Do they claim it has a wonderfully efficient hull or issit all down to the jets?

Do they have a demo boat? I guess not, if all built to order. I'll be in Carrigaline for 5 days across the 2nd w/e August so might drop by! If they are trialling one I'll drive alongside and GPS clock their speed!! Carrigaline is an odd place for luxury cruiser to be built, BTW. Not making any specific comments about Cara but before lashing 1/2 million on anything made there I would be very careful to examine it to check they have got the joinery/shipwright skills needed. It isn't Viareggio, and finding cabinet makers in a town whose only other big output (apart from farming) is Viagra, must be quite a challenge for Cara.

 

[Gludy]

Re: case adjourned!

"I didn't mean to be nasty in using the word "boring" - as again i think you know! This dateslipping means that the post is on top again, as it was on thursday. With the old method, it wd still be on the first page, but not at the top. It's a "known thread" now, with all others having seen it and just a few contributing. IMHO it should be a bit further down, but still "alive", as before. It should predominate over "today's new threads" as it is doing.
"
Not true - the third jet thread would be well off the page by now. The first one would be many pages back.

Once a thread passes off the page, it is treated as dead and many who would have contributed do not.

When I log on now, I see which threads I have not seen post on by the number in red brackets - its so easy and enhances the depth of debate on the forum.

Say the jet thread drops to the next page say 3 pages down - then I have some real verified data on the Cara boat - I post it and it would remain buried in the old system - in this system this new real data would be there for everyone who has followed the thread to see.

The old system promoted shallow depth topics and cut deeper threads off early.

I did not mind a damn about the thread being called 'boring" I have enjoyed making you pay for the word all morning (must get some work done here)



Paul

 

[tcm]

Re: mpg and urban cycle

ah , careful

For the sake of an example, lets say that thye prop boat does 2.5 mpg at 18knots, 1mpg at 28. Let's say that the jet boat does 2.0-2.2 mpg at18knots (10% -20% worse than prop) and 1.1 or 1.2mpg at 28knots (10%-20% better than prop).

Now, it remains the case that the pleasure user out for a day may not alway blam along at 90% of top speed. On a nice day, in easy conditions, he'll throtle back to have a comfier ride, and a more extended ride, for half the fuel money. He won't say"But I have to stay at high speed! - because under those conditions I am more efficient than a prop boat!" - cos he's now in a jetboat. Throtlling back will still save money, but not as much. Only inveterate thrashers for whom nothing but 28knots will suffice , will find better mpg. (and they won't be out in F8 waves, I hope)

Would this seem to answer why not so many jets on boats other than mpg-non-critical small speedboats, OR where a ferry operator does indeed have to make the trip at 28knots? Perhaps?

What would be more interesting would be an "urban cycle" mpg figures. You'll only achieve savings if you blast along, but at easy crusing of 20 knots, you'll lose. There might be equal mpg at say 24 knots.


and and, the thread wd still be on the page under the old system, aswell you know! Using the pub analogy, we would rightly be the old gits in the corner rabbiting away, but we'd still be in the pub and not have been thrown out (ie on "today's page") cos the thread has changed today. However, with dateslipping, this thread is front and centre , the main event: Tonite - same pub quiz as the previous five evenings continues!

oh, and obviously a very possible option for rtw hmm? Or at leaast, driving thru fishfarms...

 

[Gludy]

Re: mpg and urban cycle

The urban cycle .... There is no big disaster trhottling back to 15 knots and being 10% less efficient. I accept that tat time folks do that. On the other hand the winds are 4 to 8 much more time than 1 to 4. So, in practice, to be out there there will be waves - those curvy things on top of the water. The result is that any any reasonable sea the engines with a prop job will be varying their load all the time - whilst the nice, whispering jet will always have a nice constant load on the engine at 15 knots, 5 knots or 34 knots.

Then there is the big question of pottering along hitting floating fidges, nets, fish farms etc.

I want a boat that I can take of to the Med for three months - I want to reduce the targets I can hit and have a safe boat in any seas that I might find myself in. Back home I want to crew a 60 footer with just two of us (hence slow speed agility) and a boat I can take out to get to places on nice sunny days that might still say have a force 5 like on the last time out. Hence the type of boat I am looking for has to give me a smooth ride in a force 5.

In addition I want a boat that is very easy to maintain - having just spend over £1k to replace two little 3 inch long stainles steel pipes, I want accesability not just for the engine etc but for all the equipment.

Yes the Cara Marine boat excites me because if the claims are true it meets my requirements. I will be able to visit places and dry out. Need to look at the hull - I can dry out.

I will no longer join the Channel Islands list of boats from Swansea which seems to hit something on every trip. I will be safe, even in a force 8 and have a boat buyilt for it ( I would never want to go in a f8 - but in case I am caught I have a boat that can go in such seas every working day of its life.

I have a fast boat and a boat that is economical at cruising speed and can turn on a sixpence in the marina.

Question?
How may forum members have hit something or touched their props in the last two years?

Question?
How many of our sleek, stylish boats we all like have accesability to every compenent without the need for dismatling half the boat?

I first of all want a boat - secondly, but still important, I want luxury and space.
However, my preference is to make sure above all it is a boat and that the functions of a boat are not compromised for a bit of curved style.





Paul

 

[tcm]

everything from one boat?

Humm.

To answer your questions, I'v not hit anything. Secondly, i've not had an occassion where needed extra manoevrability from a boat. Both these probl to do with decent weather, rather than awful western approaches like what you think is normal, but is in fact amongst most nasty in the world. Thirdly, engine etcaccess in 60 footer is always fairly fine - much better than any 35footer- so this isn't an issue.

For a med boat, you need different things perhaps? Important kit is airconditionaing, and access from rear. Imprtant stuff is not ability to do 28knots in F8. Also, get rid of the greeen carpet in those piccies....

 

[Gludy]

Re: everything from one boat?

" rather than awful western approaches like what you think is normal, but is in fact amongst most nasty in the world."
Do you mean that? Are you telling me that this absolute novice boater last September has been learning in those conditions? Yes, I do think they are normal - I am in for a pleasant surprise evrywhere else? Thats good news!


The access in the Cara is superb and not just to the engines but to every piece of equipemnt form air con units etc. etc.

"For a med boat, you need different things perhaps? Important kit is airconditionaing, and access from rear. Imprtant stuff is not ability to do 28knots in F8. Also, get rid of the greeen carpet in those piccies....
"
All points accepted. There are three access points on the Cara inlcuding the rear. The air con is full environmental control inlcuding built in dehumififier throughout the whole boat - even a cabinet you hang your wet clothes in and the de-humifier dries them!

There is no way the green carpet is staying but then I will be specifiying a lot of the interior - its not a production boat and so each one can be much more tailor made.

The boat has to cover all conditions for me, both at home where it will be based and in the Med. Should be good at flashing over fish farms in the Med.

I have by no means decided on the boat ... who knows what will happen between now and next May when I plan to order my new boat. I am just on a pilgrim's progress so that by the time I get there, I can purchase with full knowledge of the pro's and con's of whatever boat I choose. I will ensure that everyone suffers with me on the way

I will tell you what though - should I purchase the Cara marine boat you will be amongst the first to get on her and be able to report back to the forum .... !!!! Deal?


Paul

 

[tcm]

the med is on the way to Damascus!

Ooh. I have been looking at the site and....it looks fabulous! I am a total convert to the wisdom of a jet boat, especially a nice new one.

And offer of crawling over halfbuilt boat in ireland or alternatively zamming about in med gratefully noted down, photocopied and stuck on my wall! Mind you, i think all boats are fab, really. specially if i don't have to pay.

As regards swansea, yes, i really do think that those seas are amongth the most treacherous that are generally used by powerboats. ok, so cape horn can be rougher but not too many marinas...But seriously, monster tides, prevailing F4, tight marina, means that the med is (relatively) a total doddle.

INcidentally, if you do plan med/uk useage (and even if you don't) then there are some spec things that could be jolly good. One ace thing I chose was a wooden floor, like a sailyboat. Then, nice and cool in summer in med, and no carpet cleaning. And so on. Also, get lightest (colour) wood to maek it all feel big, imho.

 

[Bergman]

Re: More questions than answers

Fascinating thread

Some thoughts occur to me:-

Jet drive propels small mass of water at high speed whereas prop boat propels large mass of water at low speed. Since drag increases as square of speed I would expect jet to have greater losses due to drag for a given amount of power input.

However as power output from a drive equates to mass x velocity squared I would expect a jet drive to give more power output for a given input since the higher velocity is squared as against a higher mass moved at same speed by propellor.

Don't understand the constant load argument. Load on props varies with depth of immersion due to increasing hydrodynamic pressure with depth. Since props are commonly fastened to boat it follows that this only occurs when boat (or blunt end of it) is more deeply immersed. A jet drive is also fixed to boat so I would have expected the same arguement to apply.

Water density doesn't apply - water is incompressable (within practical limits) so density will not vary with depth.

Dont quite follow the planing argument either. If non planing boat is moving at several times hull speed (rootWL x1.4) then it must be moving enormous amount of water. The system(boat +engine+drive) would be so inefficient that I find it hard to believe that the type of drive would make a significant difference either way.

Drag on surface of hull: This I think is most complex part of argument. I thought the idea of a planing boat was that the boundary layer of water close to the hull becomes aerated allowing the hull to pass over the water without actually displacing it. Hence the less wash from a really fast boat. (sorry!)

If this is the case I would worry about a jet drive that did plane since there must be a good chance of air or aerated water being taken into the drive and applied to the impeller. This must be a bad thing causing considerable variation in load possible enough to cause engine damage.

It might be worthwhile looking at engine wear rates and reliability as part of your study.

The inlet to a jet drive will interupt the flow of water along the hull disrupting this boundary layer and causing excessive drag from the area of the inlet and some considerable distance aft of that point. I would expect that this would also create a downward force on the aft part of the hull which would in turn mean that a greater amount of water would have to be moved at any given speed. The sums are too hard for me but I sort of think this would be a square law relationship too so much worse as speed increases.

I think your will have to buy one and try it - gone too far down the road to back out now. I wish you luck

Me

I'll stick to my bits of cloth and string - this is too complex for me.

Thanks again for the thread best reading on here for ages.

 

[Gludy]

Re: the med is on the way to Damascus!

tcm ... come on now, I rpomise an early trip and the boat and you swop side in an instant!!!! are you really that cheap to buy when it comes to boats?????

Thanks for the advice on those Med matters- the better half saw it and swooped onto the flooring point - liked it very much. She is very into the new boat. From here prespective:-

1. Bigger boat = smoother ride rgardless of what things go around to make it work.

2. Boat that can get us smoothly over the f5's to little ports where we can dry out if need be = safer trip more often.

All I have to provide now is the sea keeping of the Cara boat with the interior of the Azimut!!!

Only joking really because she is well into the props/jets stuff and happily puts safety and bigger walkways for the crew (which is her) above polished interiors which she has to polish and keep clean.

You are making me feel a touch proud that I manage in the seas around her - my friend was caught in a f7 last night in his little 18 footer but made it to shelter after a few nerve racking moments - thankfully he had the wind behind him.

I plan the boat as a second home that has to cope here as well as the Med.

As the time gets nearer, I will, without mercy pick your brains on this.

Now ... the only question is - when it comes to boats how easily are you bought?





Paul

 

[tcm]

Re: new thread?

OK.

We're losing any slight advantages of datesliping down here at the bottom of this thread, so, when the time comes for choosing stuff, start a new thread...

 

[Gludy]

Re: More questions than answers

Good points, here are my comments:-
1.
"Jet drive propels small mass of water at high speed whereas prop boat propels large mass of water at low speed. Since drag increases as square of speed I would expect jet to have greater losses due to drag for a given amount of power input."

As regards this one aspect of the system -- yes, the frictional losses through the fast water travelling through the trumpet must be higher. However, when you turn a prop, not only is it no aligned with the direction of travel, and therefore has a vertical and horizontal compenet and wastes energy but the prop itself is turning at high speed in the water and suffers very large frictional losses.

2.
"However as power output from a drive equates to mass x velocity squared I would expect a jet drive to give more power output for a given input since the higher velocity is squared as against a higher mass moved at same speed by propellor."

True but you have to input the energy to produce the high speed jet and it incurs the losses as above in point 1.

3.
"Don't understand the constant load argument. Load on props varies with depth of immersion due to increasing hydrodynamic pressure with depth. Since props are commonly fastened to boat it follows that this only occurs when boat (or blunt end of it) is more deeply immersed. A jet drive is also fixed to boat so I would have expected the same arguement to apply.
Water density doesn't apply - water is incompressable (within practical limits) so density will not vary with depth."

You are right on one sense - it is not the desnity it is the hdrodynamic pressure that changes.
Any prop boat is going to move up and down as well as forwards as it moves through the water lifting and dropping its props. This changes the load on the engines - I was out in a f5 the other day crossing the Bristol channel - my boat has an l.o.a. of 40 foot and I was not slamming but the engines were droning with the varying load on them as we went through the waves.
The jet would always have the same load on the engine no matter what the sea as it is handling water interanlly that always has the same hydrodynamic pressure. The result is that engines and boats last a lot longer.
The jet is making the boat travel forward because of basic newton principle that to every action there must be an equal and opposite reaction - a jet boat would work in space (if it had enough water stored in a balloon beneath it). It does not work by pushing the water but on the basic jet principle. So you set your revs and the load is the same on those engines even in a force 10.


4.
"Dont quite follow the planing argument either. If non planing boat is moving at several times hull speed (rootWL x1.4) then it must be moving enormous amount of water. The system(boat +engine+drive) would be so inefficient that I find it hard to believe that the type of drive would make a significant difference either way."

I am puzzled about this myself. The semi-displacment hull does move with its bow well out of the water but it is not planing. I have been looking at big 50 nd 60 footers in my local waters and I mean looking afresh as the belt along - most of the hull is still in the water and in the proper sense they are not planing. In fact I am beginning to suspect that the large planing hull is a myth as regards cruising boats and the hulls we are buying into are really not the best for us. ... now there is a subject to get things going. In practice the Cara boat is claiming more efficiency at the cruising speeds because its hull is designed for the job not designed to sustain a myth.

5.
"Drag on surface of hull: This I think is most complex part of argument. I thought the idea of a planing boat was that the boundary layer of water close to the hull becomes aerated allowing the hull to pass over the water without actually displacing it. Hence the less wash from a really fast boat. (sorry!)
If this is the case I would worry about a jet drive that did plane since there must be a good chance of air or aerated water being taken into the drive and applied to the impeller. This must be a bad thing causing considerable variation in load possible enough to cause engine damage."

I agree with you on this. The Cara boat does not plane and yet achieves 34 knots with twin 700 hp engines oin a 60 foot boat. I know of a Fairline 52 foot with the same engines, props of course that only achieves 33 knots with a much lighter boat - I think the difference is due to the jets and hull together.

6. "It might be worthwhile looking at engine wear rates and reliability as part of your study."

I have and jets win hands down on this - it is one of the reasons so many ferry boats are going over to jets - less engine wear because of the contstant load. They cannot afford down time or big maintaiance stops.

7. "The inlet to a jet drive will interupt the flow of water along the hull disrupting this boundary layer and causing excessive drag from the area of the inlet and some considerable distance aft of that point. I would expect that this would also create a downward force on the aft part of the hull which would in turn mean that a greater amount of water would have to be moved at any given speed. The sums are too hard for me but I sort of think this would be a square law relationship too so much worse as speed increases."

Nope - not right. The bellmouth intake at the bottom of the boat drwas water in from a fairly thin layer and the water being sucked in is always travelling slower than the water past the hull. As speed increases the depth it draws this water from rediuces to a very thin layer indeed. Frictional losses die to this are very, very small.

8. "I think your will have to buy one and try it - gone too far down the road to back out now. I wish you luck"

I may buy one but there is no guarantee - I believe that my look into this has raised a lot of questions and some answers. I want to find out more. There is already one of these boats in a marina around Soputhampton somewhere - I hope someone spots it anc clocks it for the forum. I am hoping to arrange the gathering of some verified facts soon .... so there should be some interesting posts on this to come. It would be fairly amusing if I end up with another boat after all this but it may happen.

The best person to speak on this would be the owner of that boat in Southampton - if you are lurking around here please speak out.

The thread is really for me to learn from, so it is me that should thank all the contributors - even though I may drive some of them mad .

Thanks for your contribution, it was a good one.


Paul

 

[Gludy]

Re: new thread?

Agreed - as soon as the jet v props issue is over I will start a new thread. There are a number of side issues come up anyway that need a new thread, its just that I am not trying to wear my fingers out typing all at once.

Do not forget , I have not forgotten the tee...hee I got after being put into forced labour at home after your cleaning you boat thread was read by others beside me!

Paul

 

[Bergman]

Re: Just a couple of points

I still don't understand why operating the jet drive deeper in the water does not increase hydro dynamic pressure. You appear confident of this as a fact but can you please explain the mechanism by which this happens.

I don't think you have addressed the question of air/aeriated water being drawn into the intake and its subsequent effects.

Lastly the boundary layer question, related to above I suppose. My thoughts on this were drawn from aircraft design where considerable trouble is taken to maintain and control the boundary layer. Particularly around air intakes to jet engines. If you look at high performance aircraft you will note that the air intakes are usually separated from the fusilage specifically to keep boundary layer air out of the engine. Again the intakes are very carefully designed to control the speed and pressure of the air presented to the compressor. I see none of this on jet boats.

Certainly with ferries and commercial boats the engines will be protected from air bleeding into the water flow by running deeper in the water and at lower speed in relation to the waterline length. I suspect the same may not apply to leisure boats even as large as 60ft.

Have you asked the manufacturers about this - may be worthwhile.

I assumed that some sort of impeller would be used to create the jet flow - is this not the case?

Just as a lateral thought seawater is a relatively conductor, why not create jet by producing a large magnetic field around duct Pass a current through water in duct and whatsits law says water should shoot out at a great speed.

No moving parts
Engines drive generator - add battery and REAL constant load
Steering by shift of magnetic field
wind a bit of copper tube around duct and it will make the tea as well.

 

[Gludy]

Re: Just a couple of points

Thanks for the response.
Here are the answers to the best of ny knowlwdge:_

1. "I still don't understand why operating the jet drive deeper in the water does not increase hydro dynamic pressure. You appear confident of this as a fact but can you please explain the mechanism by which this happens."

The jet is not pushing against the water to get its proplusion. So the hyrdrostatic preassure of the water is immaterial. Put another way, the proplusion is achieved by an equal and opposite reaction force at the point the jet leaves the nozzle under a very great pressure. The pressure difference between the jet and the water is so great that a small change in the hydrostatic pressure would not make any difference anyway. Thats my best estimate of why there is no load change on the engine. In practice there is no load change on the engine whatsoever. Happy?

2.
"Lastly the boundary layer question, related to above I suppose. My thoughts on this were drawn from aircraft design where considerable trouble is taken to maintain and control the boundary layer. Particularly around air intakes to jet engines. If you look at high performance aircraft you will note that the air intakes are usually separated from the fusilage specifically to keep boundary layer air out of the engine. Again the intakes are very carefully designed to control the speed and pressure of the air presented to the compressor. I see none of this on jet boats."

I think that this matters greatly with the higher speed boats. I know even a bolt head in the water on a boat doing 100 knoits can drop many knots off the speed.
If you think of these enourmous drag casued by a brackets, rudders and the props themselves then compared to the very small disturbance in the boundary layer caused the the intake, the appendage drag of the prop system is enourmous.
I am told that at cruising speed the depth of water that water is being extraced from is a little as 1 cm but even this must effect the boundary layer around the hull to ome degree. I think that at the speed we are talking about such losses are are of no significance. The engine load comes from turning the impller inside the jet, the water being expelled out of the back cannot be back pressured back into the system - the engine load is ecaxtly constant. In more technical terms as the jet stream emerges there is free flow of the water and hence the system is not connected to the water outside in any hydraulic system that can back pressure.

3.
"Certainly with ferries and commercial boats the engines will be protected from air bleeding into the water flow by running deeper in the water and at lower speed in relation to the waterline length. I suspect the same may not apply to leisure boats even as large as 60ft."

I am told, this was a problem on early jets. There has been significant development work to overcome cavitation. The water intake is nver out of the water, even with the smaller boat lengths and on the boat I am talking about with a semi-displacment hull, it does not come out even in very high seas.

I had already asked both the boat and jet builders about this matter.

4.
"I assumed that some sort of impeller would be used to create the jet flow - is this not the case?"

Yes one is used. The fact that it is used in a controlled environment means that the impller, in itself, is much more efficient than normal props.

5.
Just as a lateral thought seawater is a relatively conductor, why not create jet by producing a large magnetic field around duct Pass a current through water in duct and whatsits law says water should shoot out at a great speed.

No moving parts
Engines drive generator - add battery and REAL constant load
Steering by shift of magnetic field
wind a bit of copper tube around duct and it will make the tea as well."

Tell you what - you get that to work and I will find the investment to market it!

You are asking some very good questions. Understand that I am no expert on these matters, I am on a pilgrim's progress and gradually learning more and more. What interests me most is a boat that has such a stable hull it can go out to sea in a f8 with a wind against it and do 14/15 knots. (other pilot boats, Nelsons, boats turning back at 10 knots) Then come alongside a big boat on the bad weather side with both the big boat and it doing 16 knots then hold station just 20 cms away from the big boat. After that return to port in a f8 at 30 knots with the wind on the tail. That I find impressive and so did the biggest harbour authority in the UK wh went on to seek its entire fleet of 40 pilot boats to be replaced with this boat. That is a very seaworthy boat/hull with superb directional stability and it is that boat/hull that is being used for a leisure cruising craft. Add to that, the makers has worked with the jets chaps to ensure that the result is the best they can achieve and you have a boat that interests me a lot. Even if, I am being fooled and the performance figures are not true then all that is happned is that I have learnt a lot along the way.

If you fit jets into a planing hull without that directional stability, I think you will have a bit of a skating hull that does not hold the turns etc. It is the combination of jet and hull that is fascinating, not just the jets.

Before I lay out dosh for such a boat, you can beleive that I will have had to have proved to me every point - I am not an easy push over for a salesman. My attitude is very much one of - you make the claims - now prove it.

Keep up the questions, I am learning a lot with the answering!!




Paul

 

[Gludy]

Re: Just a couple of points

A point I forgot to mention was that ofn the water intake into the engine on the jet system I am talking about - it is taken from the jet streamafter the impeller and has therfore passed through a 700 hp liquidiser - should be clear of any significant intake problems!!!

Another way to imagine the jet system that may help is the simplicty of the fact that all this weight of boat is driven forward by two jets of water - imaging a hose pipe with 700 hp behind it. Imagine the force the jet leaving it. Now Imagine immersing that hose pipe end in a few feet of water - would it effect the engine at the other end driving that jet of water - no, there is no chance that anyback pressure could feed back to the impller and hence engine and therefore no change of load.

The engines drive the impller directly without a gearboc between.

Just thought these additiona points and ways of looking at it would help.


Paul

 

[aswade]

Firsthand feedback

I mentioned this topic to my employer and he read some of the posts last night. Rather than register himself and have to remember another password, he asked me to post this reply for him:

tcm is correct about the "urban" usage versus theoretically cruising at 28 knots most of the time. I have been out on various jet boats over the past few years, among them a 44 foot Hinckley Talaria and a 50ish foot Little Harbor. Both of these boats use Hamilton jets and were designed specifically for jets. One of the things that stood out to me in spending an afternoon on these boats was that they lack acceleration versus traditional prop set-up, especially at low speed. When idling along in a no-wake zone at 6 knots, there is a pronounced lag and much slower acceleration when you go to full power. The owners of the boats all said that this was characteristic of jets in these kinds of boats, and that it was a definite negative, especially if you're used to a diesel powered boat of the same size. Still, they loved their boats and felt that there were many definite advantages to jets.

I also know a couple of guys who have yachts large enough to use 42 foot Nelsons as tenders (one brother of an acquaintance even carries a 72 foot sailboat among his tenders- course his vessel is over 300 feet!) One of their captains did say to me a few months back that Cara Marine's boats were the new standard for seaworthiness among pilot boats and they were thinking about replacing their Nelson.

Re: interiors, tcm gives good advice. A nice teak and holly sole in the interior not only looks better but works better than carpet.

A 60-footer with the Cara Marine hull and Italian standard interior? Where do I sign up?

I understand that Cara Marine recently sold their pilot boat designs and molds to Halmatic. I know Halmatic will let you go to a builder of your choice to build a Nelson using their plans. Maybe we can find a builder in Viareggio or on the Adriatic coast who could do a Ferretti standard interior with this ultra-safe Cara Marine hull form.

 

[Gludy]

Re: Firsthand feedback

All I can say is thanks and thanks for relating that first hand knowledge.

I will look at the acceleration angle and come back but it is not a very important factor in my list.

What you say about the new standard of sea worthiness in the Cara pilot boats rings true and it does not surprise me that Nelson bought the pilot boat business out - they now are concentrating on the leisure boat market only and using the same hull as the Cara pilot boat.

I thought that tcm's advice was good to and i will be picking the brains of him and others.

I am being told that the first customer of the boat was very pleased with the very high standard of the interior - lets face it they made a world beating pilot boat, so there is no reason why they cannot surprise with the interior. In any event - all will be able to see it at Southampton boat Show.

"Maybe we can find a builder in Viareggio or on the Adriatic coast who could do a Ferretti standard interior with this ultra-safe Cara Marine hull form." that sounds the sort of boat my better half is asking for.

Once again - thanks for the experience relating to USA boats built for Hamilton jets - I know the picnic boat cuased a big stir over there and long waiting lists.




Paul

 

[mcneilal]

"I will return" was promised, so here I am!

I do not yet have the answers to all the points raised in the previous jet threads but here is what I have been able to gather on some of the main points:-

1. What fuel consumption (mpg) would you expect from say a 60 foot 28 ton boat using props crusing at 28 knots? I guess most would answer 0.4 to 0.5 mpg.

2. What engine power would you need for a typical 60 footer, 28 ton, to gove it a top speed of 28 knots? I guess most answers would be between twin 800hp and 1000hp.

So if I pointed out that a properly designed modern twin jet 60 footer weighing 28 tons just required twin 550hp to give it a top speed of 28 knots and if I increased the engines to twin 700hp you would get 34 knots with a cruising fuel consumption at 28 knots of 0.6 mpg - some 10% to 20% more efficient than props - I guess some would raise their eybrows - but this it seems is the case.

Jet engines require a hull shape suited or designed to them.

For a 28 ton, 60 footer normal boat to really plane you probably need to reach 45 knots - yet few can ever really reach this speed - in other words they never really plane - hence a semi-displacement hull of the same length and weight is achieving 34 knots and better fuel economy at crusing speeds.

Lets drop the speed on such a boat and yes, there is a range from between 10 and 18 knots when props, in the right weather may be 10% more efficent. But if I had a boat with a top speed of 34 knots I would be cruising at a speed of 25 to 28 knots anyway.

Add to that that in a force 8 with the wind abaft the boat, you can motor at 30 knots - and it would appear that the claims for such a boat are either a load of b****cks or we may have to re-adjust our thinking on the type of boats we are buying.

Now it is claimed that this 60 footer 28 tonner can also out manouvre any prop job in sight - like moving sideways in a force 5 onto the beam and closing into a tight spot on the pontoon under total control and very gently - not only that but to do this is easy.

As you have guessed I have been doing more homework but this seems to have come up with what I am informed is am myth. That these large boats with planing hulls really do plane properly. It seems that the experts in this back up that they do not - we just think they do.

What does not work is a 60 foot planing hull (little steerage) using jets when the boat is not designed for it.

These days, modern jets have total control throughout every degree of the 360 degrees - no dead bands, easy to control at a snails pace without touching the throttle.

Comments please - constructive if at all possible!

Paul

Cats and jets are a natural, as are hulls designed for jets. I have been operating jets for the last 7 years and all of my old beliefs have fallen. Jets are not for everyone or for every application, but they definately have their place.