Back with Jets and some answers

[jfm]

Re: Costs v Benefits

Agree with you Deleted User. But I think the cost/benefit doesn't work in that way. They burn more fuel. The debate is, does the customer want to pay more for the vibration-free running and the few other advantages of jets. imho

 

[Deleted User YDKXO]

Re: Costs v Benefits

Er, thats not what Gludy is saying unless I'm being particularly stupid today. He is saying smaller engines for same speed = less fuel. In fact he is quoting 0.6mpg v 0.4-0.5mpg for a 60' shaftdrive boat @ 28knots, a 10-20% efficiency advantage in favour of jet drives. In fact, thats pretty marginal anyway for a lightly used pleasure boat but you can see why commercial operators would be interested

 

[Gludy]

Re: Costs v Benefits

I see your points.

So far as regards my personal requirments, I am into buying a 50 - 60 footer next year and it was in reviewing the market for this, that I came across the Hamilton jet drives on the Cara Marine boat.

During the course of this and the previous two threads I started on the same subject, I have had to investigate a lot - some good points have been raised and I needed to get the answers.

From these investigations, I am confident that the same merits that the Cara Marine vboat has will apply to crusiers in the 30 foot to 40 foot range providing and it is a big providing, the manufacturer accepts that the boat has to be designed for jets and not just a jet proplusion system offered as an alternative.

When the extra costs of jets are mentioned, I think assumptions are being made. There is no need for:-
gearbox
drive shafts
rudders
rudder controls
and all the bits that surrounf and support those items.
Maintainance is actually simpler and less expensive.

So IF someone produced say a 35 footer with jet drives offering:-
1. Great low speed handling/berthing
2. 10 to 20% fuel efficiency at crusing speeds.
3. Very low noise levels.
4. Much less chance of being fowled by nets, floating fridges, lobster pots etc.
5. Ability to dry out.
6. Emergency braking facility like no other boat.
7. Low maintainance costs.
8. A lot less vibration in use with resultant less strain on boat.

And did this for no extra cost ... then would you agree it could be a winner?
I think it would and would force the market to follow.



Paul

 

[jfm]

Re: Costs v Benefits

Gludy:
still an interesting debate (thanks). I remain sceptical about the smaller engine/same speed analysis. Jets ought to be less efficient, so same engine shd give less speed.

Re your comments on 30-40 footers, take a look at the Hinkley boats, made in US. Top kwality stuff. They offer incredible manoeuverability using a joystick, but maybe worth a look through their website to see about efficiency?

BTW, I think they look quite nice too!

hinckleymarine.com or something (there's definitely another word after the hinkley)

 

[Gludy]

Re: Costs v Benefits

Thanks for the tip - I will take a look.

"Jets ought to be less efficient" - you should consider the overall efficiency - as tcm says, the only measure is mpg. I disagree that jets ought to be less efficient"
It all depends on a whole range of facors but it the overall efficiency that matters.

I ask why, if more efficient on a 60 footer they should not be so on a 40 footer?

I have about 35 patents to my name , nothing to do with boats - I tend to look at things afresh - even if my thinking is wrong, that is not important. The fact is that I have entered a leisure activity where I am horrified at how the customer is treated. Rich fools is how we seem to be treated and I do not want to accept that.
I make no bones about it, I am fresh to this game - a novice. However, if others who are more experienced want to shoot me down, please, please do so because I can only gain by learning.

If a engineer from Mars arrived and looked at how we do things - they would probably see things afresh - that is my model ... so yes, I am challenging the established thought - out of that ,I hope I along with everyone else should learn.


Paul

 

[Gludy]

Re: Costs v Benefits

Taken a look at the Hinkley site:-
http://www.thehinckleyco.com/yachts/

They claim for their 30 to 40 foot jet boats:-

"This new yacht will be powered by twin 420 Yanmar diesels and Hamilton waterjets. Weighing in at 26,000 lbs., the Talaria 40 Jet will cruise at 28 knots and post a top speed of 33 knots. With fuel capacity of 340 gallons, her range and cruising speed is expected to be 300 nautical miles. The fine entry line of her bow and 22 degree of deadrise amidships will ensure that her ride will be comfortable, dry, fast and seaworthy. Her draft of 24" will offer her owners cruising options unheard of in boats of her size with propellers. "

They also claim efficiency.

The company seems to make top range high quality boats and they use Hamilton jets. They design the boats for jets.

Does the USA always have to be first!




Paul

 

[Gludy]

Storebro

I have just recived details of the Storebro 62 footer with jets.

I was impressed that the information pack contained a long personal letter and magazines were provided with the pages of interest tabbed for me. Seems like Storebro know how to respond - a little breath of fresh air in the industry.

Anyway, down to the point.
For their boat they list two disadvantages 1. Higher fuel consumption and 2. Sensity to load (top speed drops from 35 knots down to 30 knots when fully loaded.

So it seems that in some cases jets on big boat lose efficiency and on others of about the same size they gain on efficiency.

They also pointed out that they have built large numbers of jet boats 30 to 35 foot for the armed forces and also for the German weekend market.

They use the single joystick on their boats - the boat goes in the direction you point the joystick. Cara use two levers.

My investigations of this matter continue.


Paul

 

[jfm]

Re: Costs v Benefits

Gludy I AM talking about overall efficiency, ie mpg. And I still thing jets ought to be less efficiient, becuase they introduce inherently more drag. The Storebro comments support that. I'm happy to be proved wrong, but it just seems unlikely that you can overcome the massive frictional losses in jets.

I admire the depth of your investigation into this. Perhaps worth your while looking at surface drives? They seem without question the best answer. Tiny frictional losses, and the benefit compounds becuase you have smaller lighter engines. Surely they are the answer you are looking for? And a nice rooster tail too - would make you stand out of the crowd in Cardiff!

 

[Gludy]

Re: Costs v Benefits

"Gludy I AM talking about overall efficiency, ie mpg. And I still thing jets ought to be less efficiient, becuase they introduce inherently more drag. The Storebro comments support that. I'm happy to be proved wrong, but it just seems unlikely that you can overcome the massive frictional losses in jets."

I agree all that matter is the overall efficiency MPG. However in considering this there are many factors. The jets suck in water that is always travelling at less than water speed past the hull. In effect this water is accelerated through the jet until it passes out the nozzles on the stern. There are frictional lossses in doing this - yes.

However, there are also frictional losses in transmittting the power from the engine through the gearbox and shafts plus the large losses in any prop turning in water. They alos have the extra drag of the appendages.

The hull etc is very important - the whole boat needs to be designed for jets.

It was me the supporter of jets, who posted the fact that Storebro claim lower efficiency for jets. In fact they use twin 1050hp with the jets instead of twin 800hp. Now taking about the same weight/length of boat - the Carta marine boat has twin 700 hp engines and returns a top speed of 34 knots (Storbro is 35 knots with the much bigger engines). Cara claimabout 20% greater efficiency with the jets.

Given these seemingly clashing facts, what my inestigations turned up was that the 60 foot boats really do not plane - they lift their bows out of the water and have very little directional stability - especially with jets. The Cara boat is not a planing hull and has high directional stability as it would have being a pilot boat hull. It seems that too much energy is being spent partly lifting the boat with the planing hull and hence jets maybe therefore less efficent with a planing hull.

The Cara boat will be at the Southampton boat show to see.

I am going to take a look into surface drives again. In my initial look I reached the conclusion that they performed best for only high speed boats. However, I really must solve this jet paradox.... I will continue until I have drawn firm conclusions based on factual evidence.

What I can say as a Martian Engineer arriving on earth and taking a look at say my Marina and the boats in it is:-

Facts:-
1. Most of the time throughout the year the sea is more likely to be a 5 or 6, rather than a 2 or a 3.
This my first 'Summer' with the boat most weekends have produced betweena 4 and an 8, rather than a 1 to a 4.

2. Boating is a lottery on what you hit and most boats, at one time or another are going to hit a net, rope, lobster pot ot the botttom. The next boat to me touched his props on the bottom waiting outside the lock yesterday.

3. At any one time, many boats are out of the water losing time of use, whilst the damage from such hits is repaired.

4. Most boats do not venture out if a 5 is forecast despite what may be otherwise nice sunshine weather.

5. Most of the bigger boats need more than two on board to manage themselves through the lock etc.

6. Most boats are very uncomfortable in a force 5 and above.

7. Most boats put design and posing looks above their seaworthiness and hence spend most of their time as floating caravans.

8. Almost all boats are very difficult to maintain with very difficult access to not just engines but many pieces of equipment - they are difficult to maintain and hence the costs involved in coping with things like engine removal are sky high.

I could go on and on with this list - but they way we the public accept so many wrong things coupled with the fact that I will be blowing a serious amount of dosh next year on a boat, has made me take this 'Martian Engineer' look at what is around me.... hence my look at a solution for me.

Paul

 

[jfm]

Re: myth of no app drag in jets

Gludy:
We've covered this before. You say "the large losses in any prop turning in water. They also have the extra drag of the appendages"

Look, jets have a prop turning in water, and a hub for that prop, and a shaft. All those bits of metal are in the water flow, and create appendage drag!! . You dont have to call it "appendage" drag if you dont want, but it's still drag.

Dont confuse the argument by trying to pretend that just cos a jet boat has a flush bottom it has no appendage drag! Of course it has appendage drag, more than a normal shaft boat in fact (because of friction in the duct/nozzle)

 

[Gludy]

Re: myth of no app drag in jets

We are back to semantics.
What matters is the overall mpg – then why is it that the 60 foot Cara marine 28 tom boat does 34 knots with twin 700 hp engines? I know a 52 foot fairline with the same power of engines weighing some 8 ton less does 33knots.

Just to take one part of the system in isolation is a gross over-simplification of the matter. The overall efficiency depends upon many factors and it is mpg that measure that.

You simply cannot state that jets are always less efficient. Its not that simple either way.


Paul

 

[jfm]

Re: myth of no app drag in jets

Gludy, the scatterbrainedness of your mode of debate drives me nuts!! I admire your wanting to get to the bottom of this. But my post was nothing to do with mpg. It just corrected your comment that shaft boats have appendage drag and jet boats do not. That's all. Nothing to do with mpg being the best overall measure (which point I 100% agree with).

The Cara figures are impressive. I'd like to see some verification etc, I wonder if there's a boat test anywhere? I looked on the web but couldn't find anything. The figs look almost too good to be true

 

[Gludy]

Re: myth of no app drag in jets

To be exact:-
The tpe of jet boat I am talking about eg Cara Marine has no appendage drag - i.e. nothing sticking out to cause drag. Shaft drive boats have rudders, brackets and props sticking out as appendages and therefore have appendage drag.

There is no way around that. I know that there are losses internal to the jet drive but those losses are not due to appendage drag because they are not appendages!

By relating to overall efficiency i.e the mpg, a measure we both agree with, we measure the overall efficiency of the system.

My method of debate may drive you mad .... but it has got us to this point ... you find the claimed efficiency of the Cara marine boat to be "almost too good to be true".

So far, you are the only one to have clearly stated that and I agree with you.

I am sure you accept that the claims are worthy of testing and verification?

We are therfore left with the matter of testing the claims. There are no mag reviews as yet - I wish one of the mags would do one soon, The boat will be at Southampton and I am sure reviews and tests will follow.

A very simple test, but not the only one, is if the boat reaches a top speed of 34 knots on those engines without the aid of a planing hull. This is easy to test and so very soon, we will be able to check out the claims and verify or not.

If the claims are determined to be true then the jets in this boat will be proven to be very efficient. In fact, if true, it could or rather should be a lesson taken on board by other boat builders.

I am the one who posted the poorer effciency claims for the Storbro boat, I have no vested interest other than to get to the truth. That I am determined to do. If the claims arew true then there are significant implications.

Paul

 

[Gludy]

Re: myth of no app drag in jets

I thought I should add a further point about the intake to the jets on the bottom of the boat.

This intake is underneath the boat and flush with the hull. When underway, water traveling into the intake travels slower than the speed of water past the hull. The water is drawn from a fairly shallow depth - I understand of the order of a few inches or so and as the boat goes fater, the depth of water it is drwan from reduces and reduces so that at the higher speeds it is being drawn in from a very thin layer.

Any drag due to this intake is very, very small indeed.

My original statement on this matter was that there is no frictional losses due to appendage drag and that holds true.

I am not splitting hairs here. If you start counting in the losses through the jet etc then you also have to count the losses due to the shafts etc in the prop boat - you also have to allow for propellers that are far less efficient that the the 'Propleer' inside the jet system.

Then you have to allow for the fact that props are turning in differnet depths of water all the time causing the varying load on your engine.

The subject becomes very complex, so lets agree to leave it to just checking out the claims of the Cara Marine boat as regards top speed and mpg.





Paul

 

[tcm]

Re: myth of no app drag in jets

why are the props "far less efficient"?

Oh shit, I've put this damn thread on top again for the fifth day running.

 

[Gludy]

Re: myth of no app drag in jets

Apparantly the 'prop' (there is really another name for it) is designed for a particular set of controlled conditions indide the jet and always has the same density of water passing through it. The prop on a prop boat has to cope with a much greater range of conditions and also has the problem of varying depth - hence the changes in load on the engine. as it passes through waves.

This however does not really matter, except as matter of side interest because all that matters is the overall efficiency i,e the mpg and top speed. This was apoint you made earlier and with which I totally agree.

Yes, you put the thread on top again and in so doing you have drawn attention to your question - is this not what you wanted? If not then why ask the question . Furthet you were able to see the points being made by the fact that there were new messages and so be able to ask the question - so the only reason the thread is on top for a short while is because you read it again and you7 were interested enough to ask the question!!! Much better system that only buries the subjects that no one wants to talk about and keeps the subjects being talked about at the top.





Paul

 

[tcm]

selective thermodynamics

The water around a prop, a surface drive or in a water jet will always have the same density - these devices don't "compress" water, so it always hasd the same density. There's slight change in density at a prop than at surface, but then there's varying volumes of water that go past too. The variance of water density isn't a fabulous freedom for the designer, is it?

For a jet to go faster, it will have to suck in and chuck out more water: the water will always be the same density (cos them jets don't compress water) but more of it will thrown out the back or sides. When the boat is accelerated hard, the load will be higher. The same applies to a prop.

The only way that it can be "constant load" is for the load to be constant HIGH load. It woiuld be weird indeed (but not impossible) if the boat engien was started, set at 2,200 rpm (say) and then the jets set to maximum efficiency for higher speed, or set to dump the water (open the nozzle so it does nothing) but permanetly droning away at 2200. Sure, at full speed one could quote decent mpg figures. But off-line (i.e at slower speeds) efficicency would be total crap. I worked for a company making air compressors which did exactly this: everyone looked at "mpg" at top speed, and imagined slow speed mpg would be negligible, which it wasn't.

I can't believe (nearly) that it will work this way - except that you say the engine load is always constant. But the only constant is the density of the water, and it's nearly constant at the props too, or as near as makes sod all difference imho.

It's reasonable to assume that pumping losses will be higher with a jet: instead of a large amount of water being imparted with a litle bit of extra velocity than the hull (like with large-diameter props) the jet gets hold of a relatively small mass amount of water and hurls it rearwards with greater velocity to impart the same momentum. Even at constatnt speed, the vibration might be less but the water jet is screaming away at high revs than the prop. Perceived decibels (to a human) are worse at higher frequencies, so we mind small high-pitched whining, where as louder low rumbling is ok.

Pumping losses (ie all losses associated with getting hold of water with a prop or a jet or whatever) will rise exponentially with fluid speed. Because of the above, fluid speed through a jet is likley higher, so the overall pumping losses higher - but less appendage drag, but question is at what speed is appendage drag low pumping losses on a prop greater or less than zero appendage drag higherpumping losses in a jet? Obviously depends on application, and semi-displacement jet may allow a larger volume jet without the screaming of planing designs.

Agreed, overall and outright mpg is the only worthwhile yardstick, soo we need some figures.

Separately, if (as you're saying) mpg in jetboats of same size as props is the same or better (which I understood that it wasn't) then wonder why they don't usem in endurance/competition with success? Good for driving thru fish farms too?

But! even if it IS better economy and no vibration, then it's still not a finacial winner: all it needs is funny headlights or a funny name (and Caramarine is a fairly funny name) and it will be exactly the same as technically advanced NSU Ro80 or a Mazda RX7, depreciating below the horizon on the first day. We'll let you buy one first. Better still, are there any secondhand? Suppose not.

Finally finally, what's all this "things have changed enourmously in the last ten years?". Seems to me things have come hardly very far at all in ten years. Volumetrically, builders are getting in more useable space than before, they can make a boat look good, but the technology is mostly the same. At smaler sizes the diesels engines aren't so heavy, long-lasting and slow revving, but they blow up sooner.

 

[chippie]

Re: Gludy

Gludy,Like TCM says the jet boats I have observed were notably inefficient at low revs/ starting speeds. On a prop boat the props are hardly turning over when nosing up to a wharf for example, whereas the jet does tend to have much higher revs to get moving and then maintain a low speed. I accept that they can be easily managed in that situation, but there must be a sacrifice in terms of fuel consumption. OK, compared to the fuel used on a day's outing it would be negligible but relevant in terms of overall efficiency.

Where jets excel and are seen most often here in NZ are in shallow stony rivers where the lack of draft is the big bonus.These boats are much smaller than what you are talking about. I suppose not having appendages that can and do get broken has some sort of cost/efficiency value.

Like you I would love to see a comparison/contrast type test in a magazine.

On another matter entirely, I am awed by your steep learning curve in going from being a beginner to contemplating buying a 50 or so footer in about one year. Keep up the postings, I've been enjoying the debate.

Cheers

 

[Gludy]

Re: selective thermodynamics

Thank you for the very good points you have made here.

1. Constant load
With the jet system you set the revs with the throttle and that will establish the constant load on the engine regardless of f8 waves etc. The prop job varys the load in the wave and the engine drones with changing load.

2. The Cara Marine claim 10 to 20% better efficiency at the higher speeds, say 25 knots plus BUT about 10% lower efficiency at say 12 to 18 knots speed range - so the principle of what you are saying is true. However, if stuck in heavy weather, the jet gains in efficiency over the props. If you have a 34 knot boat, the chances are you would be going from a to b at about 25 to 28 knots, not 15 knots and in so doing benefit from the extra efficiency of jets.

3. "I’d imagine that pumping losses will be higher with a jet: "
I agree they must be - but it is the total system efficiency that matters - there are plus and negative points throughout the two systems - as we agree the mpg is the measure.

4. "Pumping losses (ie all losses associated with getting hold of water with a prop or a jet or whatever) will rise exponentially with fluid speed. Because of the above, fluid speed through a jet is likley higher, so the overall pumping losses higher - but less appendage drag, but question is at what speed is appendage drag+low pumping losses on a prop greater or less than zero appendage drag+higherpumping losses in a jet? Obviously depends on application, and semi-displacement jet may allow a larger volume jet without the screaming of planing designs. "

Agree on the thrust of what you are saying here- keep in mind though that the jets align themselves very well with the direction of travel losing no vertical compenent of the energy, whilst props lose a significant amount of energy simply because they are not directly aligned with the direction of travel.

5. "Agreed, overall and outright mpg is the only worthwhile yardstick, soo we need some figures. "
Yes agreed but IF the Cara Marine figures prove to be correct, would this not make the technical use of jets in cruisers a very good alternative to traditional props?

6. "But! even if it IS better economy and no vibration, then it’s still not a finacial winner: all it needs is funny headlights or a funny name (and Caramarine is a fairly funny name) and it will be exactly the same as technically advanced NSU Ro80 or a Mazda RX7, depreciating below the horizon on the first day. We’ll let you buy one first. Better still, are there any secondhand? Suppose not. "

Nope, no second hand boats and you have a very valid point here. It all dpends how the boats are perceived over time. The company has managed to kick Nelson pilot boats big time (now, I understand that, the pilot boat business has been bought by Nelson) and may well do the same in the leisure market - but yest there is a significant risk.

7."Finally finally, what’s all this “things have changed enourmously in the last ten years?”. Seems to me things have come hardly very far at all in ten years. Volumetrically, builders are getting in more useable space than before, they can make a boat look good, but the technology is mostly the same. At smaler sizes the diesels engines aren’t so heavy, long-lasting and slow revving, but they blow up sooner. "
Yes point accepted as regards the marine industry as a whole - this is maybe the whole thrust of this thread - is there not a better way to do it.
I am going to pursue this to the end because I do not want to waste a lot of dosh. Lets hope along the way, we can all learn. For the fact is that it is all theory ob both side because no one who has contributed has experience with jet cruisers of the type under discussion.

Thanks for making such a positive contribution to my boring thread!!!

Paul

 

[hlb]

Re: Gludy\'s Progress!!

I to am astounded at the progress Gludy has made in the field of engineering. I have just been reading back twelve months on the saga about putting a massive rib with 25HP outboard on the davits. The engineering prowess knows no bounds!!

No one can force me to come here-----------
----- I'm a Volunteer!!!

Haydn