Props are old fashioned - jets win?

[Gludy]

In the sense you are talking about drag, the drag you are talking about is not expressed as a friction loss for the hull but should be compared to the efficiency of props moving through the water.
Put another way:-
The internal frictional losses inside the jet make no significant difference to the drag of the boat through the water. They should be copmared to the efficiency of the system and so in that sense should be compared to the total efficiency of the props (which have a massive efficiency loss in water.

Not hiding behind semantics - just want to put the right thing in the right pigeon hole.

Paul

 

[Gludy]

Re: Azimut! Hurrah! or Sunseeker?

What is the drag or for that matter the efficiency of a lump of kensington palace in a boat? From what I hear the cost of such chunks in such abodes is about £69 per week so maybe there is some sort of fantastic maintainance saving in having such a chunk?



Paul

 

[tcm]

Re: Azimut! Hurrah! or Sunseeker?

You're probably right. I meant a copy, flippantly referring to large size of owners cabin.

I was interested in the issue you raised regarding drag. Interestingly this was punted around the consumer end of the car industry recently with "Cd factor of .32" being quoted as though it actually meant something useful. The Audi did well, but unfortunately propelled by crap engine: Mpg on a car and boat the only wortwhile measure, of course. Nobody (should) care if the drag is huge or tiny, or if the thing made of lead.

 

[Gludy]

Re: Azimut! Hurrah! or Sunseeker?

I agree very much with that - the only thing that really matters as regards efficiency/drag etc is all taken care of with mpg.

So far, my current thinking is:-
1. Ability for slow manouvering - jets win by a big factor.
2. Overall efficiency - about the same - with some advantages/disadvantages depending on conditions, on both sides.
3. Avoiding lobster pots, nets,etc Jets win hands down.
4. Ability to dry out - jets win hands down
5. Depreciation - jets must lose here and it could be big time.
6. The ride - on both noise & smoothness - jets win hands down.

That is where I am at with my current thinking.

Paul

 

[jfm]

Re One important change

No, overall efficiency much lower with jets than props, in all relevant conditions and across a whole season's boating

 

[jfm]

Gludy is actually Alistair Campbell

Gludy, this is total semantics. I hereby nominate you as forum spin doctor We're saying the same thing, which is that total frictional losses of the jet system are considerably higher than for old fashioned props, at probly all speeds and certainly at all relevant speeds. That's why a jet boat goes slower than a prop job, all other things being equal.........

 

[Gludy]

Re: Gludy is actually Alistair Campbell

I disagree - you are missing the point.

Divide the efficiency into two parts:-
Total frictional hull losses - including appendages - it is lower with a jet.

Efficiency of propulsion system:-
The jet has to pwoer the water through itself and obviously has large frictional losses in doing so.
The prop, gearbox etc also has large losses with the tradional prop set up.

Actually all this overall effect is measured by the mpg - overall - throughout the season. I have quoted a figure for a 60 foot jet boat which is very reasonable.

At crusing speeds jets are effcient - at least equal to props.

At lower speeds they are less efficient than props but even then may be equal in rogh conditions.

I am not talking about single engined small jet boats - I am talking about modern twin jet systems on cruising power boats.

As I currently see it, the there is in effect a prop in both cases, the jet has it internally and probably that is more efficient for a number of reasons than the external prop. However the jet has frictional lossses of the water passing through itself - but then again has a lower hull friction due to lack of appendages etc.
So there are a number of swings and roundabouts there - it all comes out in the overall mpg throughout a season. I am collecting some info on this to place on the forum. Where does the evidence come from that jets, of the modern type I am talking about, on cruising power boats show themselves as greater mpg over a season or any like to like trial?

The swings and roundabouts in this is much more complex than I have painted it here. For example shaft drive props are not in line with the direction of travel of the boat and lose a good amount of energy with their vertical component - however, it does not matter. What should be compared is say a 60 foot cruiser with twin 700 ho engines designed to work with the props it has - against the same boat designed to work with the jet drives it has and the same engines.

As I have stated - I intend to gather some harder facts on this one and post them to the forum.

As regards my new spin doctor title - well, my reputation from those who know me is to be a little too blunt and a little too direct.... its the first time anyone has christened me with spin! However, if the hat fits, I will wear it.

All I am trying to do is investigate the subject and the thread has helped me a great deal with that investigation.

Anyway - I would change sides in this discussion if some witch doctor around here could give ne some nice weather this weekend - its not a good forecast for the Britol Channel and on a Friday night, I would much rather be on board and preparing for my next disaster.



Paul

 

[KevL]

Re: Azimut! Hurrah! or Sunseeker?

<Lurking device off>

Wasn't it Enzo Ferrari who said "Drag coeficents are only of intrest to people who don't know how to build an engine!". But I don't think he had a boat in mind when he said it

<Lurking device on>


--
Tides - Never there when you need one.
Wind - Always there when you don't.

KevL

 

[jfm]

Re: Gludy isn\'t really Alistair Campbell

Gludy, all the spin doctor stuff meant in best possible humour, I agree you have raised an interesting debate

I haven't missed any point. We are saying the same thing which is that friction etc etc can be boiled down to a simple question which is that, all other things being equal, which has the higher mpg over the season, jet or prop?

The answer is most certainly that props have higher mpg, and the reason for that is massive frictional losses inside the jet unit, which are bigger than the appendage drag losses in a prop boat. QED

 

[jfm]

Re: Gludy here\'s some data

Gludy:
This isn't a perfect example but it does illustrate the inefficiency of jets. The big Mercury outboards can be bort as jets. These outbords are exactly the same as the normal ones, except they bolt a jet unit on the bottom instead of a normal gearbox and prop.

The 115hp outboard produces a claimed 80hp at the jet nozzle.

Some of that might be due to lousy pump design, because 35hp is a lot of loss, but Mercury aren't stupid or short of engineers, some of the loss is due to jet inefficiency

Check it out. It's a rather long url !

http://www.mercurymarine.com/cgi-bi...779@@@@&BV_EngineID=cadcegchelegbedcfececjf.0

 

[Gludy]

Re: Gludy isn\'t really Alistair Campbell

"The answer is most certainly that props have higher mpg" - what practical evidence is there for that?

Noone has quoted anything referring to modern jets in a cruiser.. The samples coming in are not the jets systems that I am talking about.

I know that everything is given in good humour - I am never offended.


Paul

 

[Gludy]

Re: Gludy here\'s some data

I would expect a drop in efficiency when the whole jet unit is externally mounted - almost the worst of both worlds.

Modern jets internally mounted have come a long way. Has anyone out there had experience with modern jets in a reasonable sized cruiser?

Paul

 

[jfm]

My data was dodgy, submission withdrawn m\'lud

Why? There's no way external mounting of itself sinks 35hp, no way. The 80hp is a measure of power developed. Even if the external mounting then causes drag, that drag effect is not included in the 80hp figure becos that is, as I say, a thermodynamic measure of the power developed at the jet nozzle.

But your comment has caused a different penny to drop with me. The 35hp difference is in fact misleading and exaggerated. It is net of the "appendage drag" inside the jet unit becos it is a measure of power developed at the nozzle. In contrast, the 115hp rating for the normal outboard is measured at the prop shaft, ie it is torque x rpm, so it is NOT net of appendage drag of pushing the gearbox thru the water.

So my 35hp example actually proves nothing, nothing at all, it was total bolx in fact, for which I apologise, but it was worht a look eh?

But the basic position, that you get higher drag with jets, remains the case. It's just my merc outboard url didn't prove it!

 

[jfm]

Re: Evidence is absent, will anecdote do for now?

I dont have any first hand evidence. I'll look for some. It's just well known that jets lose more power.

And it's pretty much common sense, there's obviously more drag pushing water thru a tapered pipe at higher speed, and the prop shaft, impeller, and hub are all still in the water flow just like on a normal prop boat, so you're guaranteed to lose more power to friction, how could you not?

 

[tcm]

and manoevrability

no offence paul, but your knowledge of moving these things seems entirely 2nd hand. I understand that you are moderately new to boating. In my and in others' exp they do weird things.

I wonder if your unfam with props lends credence to idea that that jets will be easier?

Not so. We've had to do weird things to slow a jetrib down, turn the wheel (where?) and gun it. Coliholic a fairly experienced boater + waterski boater somehow thrown off. Can't see how you say "jet easier - period" when you have little/no exp of them and others with lots more have had strange times.

IMHO handling feedback is proportionate with props, disproportionate and unpredicatable with jets, and I have both. Tell us you know different- not that you've read it in a brochure.

 

[tcm]

Re: enzo

effectively yes- think he said aerodynamics is for those who can't build engines, but same sort of thing. Actually, his cars exact op of what we need in that they are a bit crappo but go like stink in betwen long chats with the AA and 60k depreciation in 18months but er anyway it was a larf, sort of. Anyone intersted in regno V12 GTM?

 

[tcm]

Last-minute evidence from back of courtroom!

http://www.uscg.mil/hq/g-o/cgaux/Publications/Instructions/nonstandard/Ch3.pdf

 

[Gludy]

Re: and manoevrability

tcm - Spot on - as I have already stated I am new to boating and my experience (not theory) is very limited. I do not take offence, for that is the simple truth.
The title of my thread was a question not a statement.... please note the question mark at the end.

My objective is to learn ans this is a very good way of learning.

To balance this, it would appear that there is a very limited experience amongst the general boating population of modern jets with modern control systems in cruising boats. These are a world apart from the jets of 10 years ago and also from small jet boats.

I will soon be gaining some hands on experience of this and will report my findings.

The subject interest me because of some very practical issues:-

1. Given the experience of others all around me in the marina hittting nets, hitting a floating fridge, dmagaing props on rocks, having rope foul their prop and ruin their trip, hitting lobster pots - it would be a major advantage to not have these problems.

2. The ability to be able to dry out - most of the ports I visit dry out.

3. The ease of slow speed manouvering in marina's etc - these modern jet systems are really very special as regards this.

4. The lack of noise and vibration.

5. The simplicity of constant engine load and subsequent lower maintainance costs.


So far, it would appear that the disadvantages for me are:-

1. Probably an overall slight increase in fuel costs in the order of 10% or so at low speeds.

2. Possible, even probable huge deprecation problem because of the very attitude of the market place.

3. Probable slightly more hassle in overcoming fear amonsgt non-familiar service engineers etc

Since entering boating - just last September, I have lived and breathed it a lot. I listen a lot and then out of the mix of advice, I sift and test it so that I can reach my own conclusions. I give weight to experience but do not accept anything as a matter of blind faith.

I go through this process on many things and by so doing I learn fairly rapidly. I also think that others, involved in this process learn along with me.

As I hope to at least semi-retire next year, my next boat is a big decision for me as I may end up going on extended trips to the Med etc. So a boat that can make it past the C.I. without hitting a net, rock or other rubbish offers some attraction to me.

I can say that I have not made up my mind yet. Because I argue a corner, does not mean I belong in that corner. A friend of mine, who is a long experienced skipper but not very technical goes on about one thing as regards jets and its the point you raised - depreciation.

I really think it is not possible to offend me on the forum, I am not in the least offendabl, after all we are only talking about a boat proplusion systems. However, I am not in thge least held back in challanging any beleif and testing it against both theory and practice.

In truth, the apparant lack of experience of forum members with the jets/boats I am talking about means that there is very little experience. This is why a really good in-depth article by the mags would be interesting.


Paul

 

[KevL]

Re: enzo

Think I'd rather have V12 AML but that's downstairs in the carpark at the moment, bet ya can't guess what's wearing it. Mind you A 360 MOD would be nice.


--
Tides - Never there when you need one.
Wind - Always there when you don't.

KevL

 

[jfm]

View from the Pentagon

Thanks for US military URL. Not massively detailed, bit of a noddy manual really. But the Pentagon aren't keen on jets it seems. Gludy, they say jets have reduced low speed manoeuverability, compared with props

Learnt some new jargon though- "Coming Up" - BarryD can use that as he enters the lock, and "Coming Down".