Props are old fashioned - jets win?

[hlb]

Re: Calling Haydn

Undeerstud every word John. He means. Some ones talking alot of crap. In a nut shell. It's a wan*ing idea. And he does not aprove Of it. Now have you got the jist of it. Or require more definitions!!

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

Haydn

 

[ParaHandy]

Re: Gludy you are wrong on drag

Not quite in the context you mean but Gludy's not completely bonkers.......if a jet will efficiently get the boat up to 50knots he's at a speed when wing-in-ground effect will lift an appropriately shaped hull out the water - assuming he's got the means of propulsion when the craft tansits from water to air. The trick with wig principles is to create a vertical force/cushion to lift the craft free and water jets might be very suitable for that because the jet is highly directional. Once there, though, drag is virtually nil and fuel consumption will be very low. The craft is still classed as a boat even though speed could exceed 100kph.

 

[MapisM]

1, 2, or 5 knots...

I don't have any personal experience on this subject, but can report the viewpoint of someone who's a pro in this field.
You've probably heard of Seatek, the high performance marine diesel manufacturer. Years ago, I had the pleasure of attending my license course with their chief mechanic, who was getting his licence just because after 20 years or so of trial runs on the lake, the police was fed up seeing him around, driving superfast boats without any license at all...

As a novice to powerboats, I did not miss the opportunity to learn as much as I could from his experience. Besides other interesting things, he told me that:

- based on their tests on different transmissions, the best results were with surface props mounted on sterndrives (surprised?), because even if they absorb more power than the in-line transmission, they're the only way to keep the prop spinning perfectly in line with the hull bottom; at or above 100 knots (yes, kts - not kmh!), it seems that this is of great importance. Assuming a score of 100 for this system, he ranked the in line surface drives just slightly below, at about 95~98. He didn't even consider the traditional in line submerged props, tunnels, semi-tunnels, etc, as they were clearly playing in a different league. But I asked him about jets, which seemed an efficient alternative already at that time. He ranked them at 80 or so, and even worse in terms of consumption.

- he had just repowered a sporty 40 footer with two of their 600 hp engines. During the trials, they noticed some spray coming out of the bracket supporting the surface drives. Looking carefully, they just found two screws whose heads (20 mm. diameter) were coming out from the lower part of the bracket. After removing them and welding those parts, the boat gained 3 knots in top speed.

Of course, these experiences are outside the range of normal pleasure boating, but they give an idea of the efficiency that can be reached by surface drives. I can't imagine any jet whose loss of power is less than the drag of two small screws...
OK, I guess that in the recent years the technology improvements reduced the gap. Anyway, also today, Fabio Buzzi and his FB 80 are eating for breakfast any long distance record around, and they're doing that with 4 surface drives. Don't think he wasn't aware of the jets possibilities...

Last but not least, pleasure boating means also slow displacement or semi-displacement boats. BTW, I'm a fan of them, when serious offshore cruising is the name of the game.
On those hulls, I've never heard of any alternative to props so far.

 

[Bergman]

Re: Heisenberg Uncertainty Princip nearly gets Bergman

Surely

Simply a function of the length of the rope.

Repeat an infinite number of times with infinitely small increments in length of rope.

Wait a min

This is getting silly

I was only joking in the first place

 

[Bergman]

Re: Heisenberg Uncertainty Princip nearly gets Bergman

Surely

Simply a function of the length of the rope.

Repeat an infinite number of times with infinitely small increments in length of rope.

!

Wait a min

This is getting silly

I was only joking in the first place

I'd still like to see it though

 

[jfm]

Re: 1, 2, or 5 knots...

makes good sense mapism. Note, nearly all surface drives (Arnesons etc) are these days tiltable up/down, so the prop can be lined up properly, so they would probly score 105% on your man's scale, because no friction losses in the outdrive unit.

 

[MapisM]

Re: 1, 2, or 5 knots...

Well, actually it was not that long ago...
The trimmable surface drives began to be available, and he took them into account in his viewpoint.
But he argued that in-line drives can never reach the ideal position anyway, because of their structural design - there's just no way to get the axle exactly aligned with the hull bottom: to connect it to the engine, it needs a certain minimum height above the hull waterline, thus creating an angle, which can be avoided only with sterndrives or outboards.
Besides, he considered the trim as a feature more oriented to "normal" applications (where you have different sea conditions, different boat weights, etc.), rather than something which can improve the top speed.
Unfortunately, I lost the contact with him after a while; could have been interesting to get an update from him first hand.

 

[jfm]

Re: 1, 2, or 5 knots...

mapism, thanks, see what you mean about angles and stuff

 

[Gludy]

Re:disadvantages :jets lose. D\'Oh!

As so many points have been raised, I hope I can be excused for not answering every one in this, my first fairly full response - I will however, return to answer any points, I have not answered to date.

Question
"1 Fuel consumption. For a given engine, boat and speed, fuel consumption will be worse. Quite a big issue this, I wd imagine, knocks it stone dead for many, no?
"
My Answer
As far as leisure cruising is concerned, it depends very much on whether you have a 35 knot boat and want to cruise at 25 knots or 15 knots. At the lower speed my research has shown that, jet efficiency is indeed likely to be less. The jet will be far quieter and smoother at those lower speeds (as it will at higher speeds) so the trade off could well be a more comfortable trip.
A 60 footer with say twin 700hp driving a twin jet system should return about 0.65 miles/gallon when cruising at 28 knots with a top speed of around 34 knots.
As you drop below these speeds, you have a point in that the jet loses its efficiency.
So if most of the time you want to cruise at 12/15 knots and do not value the smooth ride of jets, then the props are a better solution.

Question
"Low speed manoevrability. I have a 115 hp jet rib and it is a bit weird at low speed, not at all easy to manoevre. No paddle wheel effect for alongsde berthing, and need to master "gunning" the engine to get it moving a small amoiunt, instead of being able to use lower engine revs on props. "

Answer
Low speed manoeuvring with modern jet designs is a world away from the old clam shell systems and whilst the technique is different, low speed control should show huge improvements over any prop system. - comparing singles with singles and twins with twins of course!! Basically you set the throttles to whatever the conditions require and then simply use the steering and reverse to place the boat where you want. THE ADVICE IS -YOU SHOULD NOT PLAY WITH THE THROTTLES.

Question
" Unfamiliar handling characteristics at higher speeds. Yes! Instead of the boat carving along its route, with the rudders forcing graceful changes in direction, a jet boat can be doing 30 knots in another direction, turn the wheel and it gets shoved sideways up onto its own wake. Then coliholic gets thrown out into the sea. "

Answer
High speed control is again a matter of education. Nobody would dream of throwing the steering wheel of their car hard over at high speed so dont do it with your boat! It is an emergency procedure -as well as a good show-off trick! Just like crash stops in your driving test, you should know what will happen in an emergency and have the confidence that if such an emergency does arise, you can use evry ounce of power to get you out of trouble.

I shall return soon with the answers to more questions.


Paul

 

[Gludy]

Modern jet systems are supplied already anti-foulded. When the hull is being anti-foulded, you can take off the grill and repeat the antifouling to the internal of the jet. So they foul up about the same as your hull.

A lot of points being raised relate to very different jets to those I am looking into for a cruising boat.

Why don't you do a big article in the mags on all this? - I have never seen such an article.


Paul

 

[Gludy]

Re: Gludy you are wrong on drag

The water velocity at the intake of the jet under the boat is usually lower than the water velocity past the hull. The higher the speed of the boat, the shallower the the zone of water below the boat that the water is sucked in from. So the losses due to this are very small.

The tube and nozzle are not unrer the boat but inside and above the water line - these are not appendages causing drag.

If I wanted a boat to go at 100mph - I would look at surface drives. What I am doing is claiming that jets are a realistic alternative to props, and offer many advanatges for most, not all, cruising boats.

Paul

 

[Gludy]

No the jet sucks in water at a lower velocity than the water travelling past the hull - it jets it out at between 1.5 and twice the speed of the boat.

Yes, the part of the intake in the water Inside the boat) and the grill do foul up like a hull does but they are anti-fould just like the hull.

Paul

 

[tcm]

smooth and fast depreciation: NSU Ro80

You're very keen, Gludy.

Firstly, in a 60 foot boat, life isn't exactly "rough" with props. Of course, it depends on sea conditions. But the engines are quite a way away. In a boat of this size, you can play chess in the saloon at 25knots or more. Turn on your car engine, sit inside and it's like that. I mean, sit inside the HOUSE. Nobody sits on our boat and says oh blimminek what a rough rattly old dog - props, right?. Note that there'll still be diesels there.

Second, if the running costs don't get you, the depreciation will. Sad to say that weird albeit wonderful devices often suffer this way when a primitive but good-enough and alrerady-dominant alternative exists. Luxury cars such as NSU and mad yet wonderful Citroens are good examples. Ooer, they're a bit weird says your avergae non-techie punter, and chooses something safer - cos remeber that they have to take an even bigger gamble than you cos they're buying 2nd hand not new- so they'll have a safe option please. Same applies to odd-looking catamrans whioch are technically fab, but look a bit weird, so resale is hopeless.

Soo... consider this shot down in flames cos of
1) the running costs incl gph
2) the poor manoevrability (and I will touch the throttles cos i want to go thru marina at 3knots but touch pontoon at 0.1 knots)
3) the awkward maintenance issues esp as highlighted by KimH
4) the ferocious depreciation
5) the inability to sell it even dirt cheap

A 60 foot boat (any 60 foot boat) will be smooth as silk compared with a 35 footer. You'll be able to choose from lots of reassuringly expensive Princess 50-60 secondhand (or new), and a good chance of selling it later. Note that all the 2n-hand prinnys will be sold (and more) before you get to shift your whacky jet thing. Especially in Swansea...

 

[Gludy]

This does not really apply to twin jet systems for cruising boats. See the RNLI experiments inmentioned in the previous thread - four days boating through interntioanlly place rubbish eventually managed to suck in a rope- opened the hatch, cleaned it out.

Paul

 

[Gludy]

Re: Gludy you are wrong on drag

From what I have researched to date, really high speed boats are better off with surface props.

The jets I am talking about, are for cruising boats in the 20 to 40 knot range.

The efficiency of the jet is very good at cruising speed and drops off to say being about 10% or so less efficient at say 15 knots.

However, at cruising speed in any sort of sea, the load on the engine from jets remains constant, the load on props is changing all the time depending on the depth of the props and so there are efficiency losses with the props due to such factors.

Paul

 

[Gludy]

Re: 1, 2, or 5 knots...

The experience with high speed boats is different - just as you state. The higher the speed, the greater the importance even a small appendage can have - hence the nuts having an effect on speed.

To go to another extreme - lots of Ferries now use jets - one of the main reasons is the lowering of engine maintainace due to the lack of varying load on the engine + a smoother, less noisy ride for passangers + easy to manouvre. However, these boats are as different to the crusing market jets as the high speed boats are.

In practice there are many factors to consider and some good points have been broght up on this thread. I intend to do a summary of it all, once the deabte is over.

Paul

 

[Gludy]

Re: smooth and fast depreciation: NSU Ro80

"You're very keen, Gludy."

I am keen to learn - I am keen not to make a mistake with the purchase of a new boat planned for next year - I am learning from this debate as are others.

Last year I did a really stupid thing - it was not rational - I bought a boat!

Now that was bad enough - but next year I plan to buy a big one and will probably go for a new or very nearly new one. If that does not prove me totoally bonkers, then what does?

Now to answer your points:-
"Second, if the running costs don't get you, the depreciation will."
This is a good point, I was aware of it. You see it does not matter what I believe in, it does not matter what is right or wrong - as regards depreciation all that matters is what the market thinks. However, that does not stop me investigating all the alternatives to hell and back - I enjoy doing and learn all sorts of things in the process.

1) the running costs incl gph - so far, I disagree on this one. Overall with fuel, engine maintainance etc, there is not much in it. I will be responding in detail later.
2) the poor manoevrability (and I will touch the throttles cos i want to go thru marina at 3knots but touch pontoon at 0.1 knots). I really think you are totally wrong here. With jets you set the throttle and the jet control controals your speed of docking. Modern twin jets in a motor cruiser with modern controls out manouvre any twin shaft job - period.
3) the awkward maintenance issues esp as highlighted by KimH. - Anything that is different can be a problem, but so far, I think you are overplaying it a touch - I may change my mind as I check it out.
4) the ferocious depreciation - this could be a big problem. Under investigation.
5) the inability to sell it even dirt cheap - same thing as above.

If I were to look at a prop job, currently it would probably be a 55 fott Azimut - any views?





Paul

 

[jfm]

Re: Gludy Pay attention!

Gludy you are totally and blindly wrong. On the drag, ok if the tube and nozzle and impeller are inside theboat they are not, strictly speaking, appendages so there's no appendage drag. But that is 200% semantics. I'm talking about drag on pumping water through the tube, past the impeller hub, and out the nozzle. That might not be APPENDAGE drag, but it's drag all the same, and it's massive. Sorry, you are totally wrong on this one mate

 

[jfm]

No no no Gludy. OK if the intake velocity is less than the exit, that's fine. But the water doesn't vanish, so you achieve the velocity difference by varying the cross section. You have a large diameter intake pipe, then it narrows at the back (hence me referring to nozzle all along) so the water goes faster at the nozzle end. But that does not change the massive drag. At the inlet, where the velocity is low (which is good, drag-wise) the surface area of the pipe is much bigger (it absolutely has to be) so the drag saving from low velocity is pretty much lost by higher area. Bottom line is that surface drives, not jets, are the efficient answer. Normal cheep n cheerful props are in the middle

 

[tcm]

Re: Azimut! Hurrah! or Sunseeker?

Ooh much much MUCH better idea. Is it the Azimut that has copied a lump of kensington palace and used it as the owners full-width cabin? I think so. Also AZ has the shark's fin windows, quite snazzy bit not too loopy like sunseeker. Tho sunseeker v good name and the 56 is jolly nice I hear. Think this a tons tons better idea.