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Old 12 December 2002, 13:20   #11
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Mike G.

Mike G,
JKís comment about selecting a prop which will allow you to reach max revs at wide open throttle is a good guide. But, you must be sure that it is the prop that is limiting you to your maximum and not your engineís rev limiter (whatever that might be). Donít spend money until you are sure. Right, give me some answers.
1) When out on a normal Saturdayís trip, does the engine reach 3800rpm?
2) How much fuel would you be carrying in the above situation?
3) When you load up with 600lts of fuel and a few more people, what is the maximum engine speed?

There are a lot of misconceptions about how propellors work. The problem is, of course, one canít easily tell what is happening under the water. Talk about black art etc, etc. Itís all pish. Normal hydrofoil/aerofoil rules apply. (Sorry JK, this is not a dig at you.Honest.)
It may be worth starting a thread about water flow, turbulence, venturi effect etc. A bit of knowledge about these things will allow one to understand how, for instance, to get a trunk type bailer to empty a boat in half the time, why two boats are forced together when they travel side by side and how the loads on a prop blade cause a boat to move forward.
Anyway, Iím off for now. JW
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Old 12 December 2002, 14:55   #12
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JW

I will be interested in that short of thing. As you might be aware I am doing quite a bit of tech reading on props and I am still in the dark.
Any info on this subject will help (and possibly not only me).
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Old 12 December 2002, 18:58   #13
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If 3800 RPM is the published top RPM, then I would be surprised if the rev limiter kicked in at this speed with no leeway. I would expect an engine to be over revving before the limiter starts doing its thing. I don't know for sure with this engine though -- Alan P, do you know?

Mike, what will the engine rev to in neutral? Although this might be separately limited of course . . .

JW, whilst the behaviour and performance of propellers is (of course) governed by scientific principles they are by no means simple or straight forward. Size and pitch are easy enough, and an estimate of slip isn't too complex. But when you start to include number of blades, blade shape, cupping and running depth things rapidly escalate.

I think that you are unlikely to find many people who can calculate the perfect prop for a given application without a certain amount of trial and error.

John
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Old 13 December 2002, 02:54   #14
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Re: Mike G.

Quote:
Originally posted by jwalker
Mike G... Right, give me some answers.
1) When out on a normal Saturdayís trip, does the engine reach 3800rpm?
2) How much fuel would you be carrying in the above situation?
3) When you load up with 600lts of fuel and a few more people, what is the maximum engine speed?
Thanks for the continuing interest, Jeff and John. Here's a stab at answering Jeff's questions...

1 Normal 3,800 reached when I steadily accelerate from a standing start. The engine has only got 50 hours on the clock so I am taking it easy. But by the time the hand throttle is on the deck we are on 3,800 exactly.

2 Doesn't seem to make a difference whether the tanks are full or mostly empty, I still can hit 3,800. (Incidently, 35kts was achieved on a cool clear day when the air pressure was very high. On normal lower pressure days I lose a knot. Cos of this I think we may have been a bit short of air in the box so I have now installed a funnel on the cabin roof and force air down to the engine via some rather elegant drain pipe. Not yet tested the effectiveness of the mod.)

3 As above. I can still hit 3,800 with full tanks and a few extra people.

I think, with these questions, I can see where you are heading!

John, I have not tested the revs in neutral and won't really be able to do so until I put the boat back in the water. However, one of the Barrus guys did say, I think, that if I dropped a size with a stainless prop the engine would rev over the 3,800 a bit and I would possible gain a bit of speed. I seem to remember he indicated that the overrevving was not a problem.
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Old 13 December 2002, 16:23   #15
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Oops, I've upset the boss.

JK. Sorry, I've obviously rattled your cage. You are right but the difficulty arises because it's so difficult to tell what's giong on under the water. The propellor is just a propellor doing what propellors do. There's no majic involved.

Mike G. Go to http://www.yanmar.com/marine/pdfs2/6LPZP.pdf to get the specs on your motor. Remember, it's diesel and diesels like to work. Look at the torque curve. Past about 3000 it's falling fast. The power stays almost level because the revs are rising but it's hardly gaining anything. Going for the absolute max revs is simlpy not worth it. Look at the fuel consumption aginst power. You'd be using fuel for nothing.

The details you gave me of your boat performance speak for themselves. You put weight into the boat and it still goes just as well. Where do you imagine this extra energy is coming from? Hey, pitch it up, cup the blades or somethin'. Load up the motor and enjoy lower revved cruising. Won't accelerate as quickly though. Aw shit, back to those compromises again.
Cheers for now, JW
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Old 13 December 2002, 17:53   #16
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Hi mike,

I may be wrong here,but I believe you can put pitch on a prop but you cant take it off.

Im thinking about your problem and am wondering weather if the boat is doing the same revs fully loaded as when not then you may be able to squeeze some more top end out of here by adding a little pitch then trying here in a fully loaded condition +400kg,at that point when you get a small drop off in revs maybee 50 or so in a fully loaded + 400kg setup that is the max speed your going to get,by taking the extra load off you may find that she is back to 3800rpm and you now she is as fast as she can go give or take.

If you do it with your alli prop and it doesnt work out then thats your spare,A small amount of pitch added makes a lot of difference so be carefull.

You will loose some exeleration but you may save fuel and could increase speed, I really wonder weather it will be noticible the drop off in exeleration,it is also important to make sure the weight is distributed in the same positions for all the tests.

This is only a suggestion and it comes of the hip so Maybe somebody more informed can put us strait.

Cheers Crazyhorse

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Old 15 December 2002, 17:24   #17
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Re: Oops, I've upset the boss.

Quote:
Originally posted by jwalker
JK. Sorry, I've obviously rattled your cage. You are right but the difficulty arises because it's so difficult to tell what's giong on under the water. The propellor is just a propellor doing what propellors do. There's no majic involved.
Oh don't worry, my cage doesn't rattle that easily!

However, read my initial post again. No one has suggested that magic or black art is involved in propeller calculations. My words were "it sometimes seems to be more of an art than a science", which to many people it does and always will. Saying that normal hydrofoil/aerofoil rules apply isn't particularly helpful to many people.

Like you say, it's difficult to tell what's going on under the water.

What advice would you generally give to someone about choosing the size and pitch of a stainless prop to replace a standard aluminium one?

John
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Old 15 December 2002, 19:04   #18
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JK, I did say, originally, that I wasn't getting at you and that was the case. The text was written off line and I was relating to what is purported to be the case when props are the topic of conversation. While posting, I realised that my comments might be taken as relating to your previous post. That's why I added the rider in brackets.

Now then, what advise. Well, if the SS prop it to be the very same design as the original aluminium one and the aluminium one is not flexing unduly under load, there will be no difference in performance except for that imposed be the extra weight of the stainless steel. This is a bit of a cop-out since this is rarely likely to be the case. However, normally, changing to SS is also a change in prop design and the only thing that will initially get you into the ball court is the pitch of the aluminium prop. Once the SS prop is in use, then an assessment is easier.
But, far better to analise what you need the prop to do, ie, lift the bow, lift the stern, pull up skiers, lowdown lugging power etc., etc.
My own preference, if the engine will allow it, is to fit a Torqueshift. They are very adjustable and, therefore, adaptable.

That's it for now. It's late and my brain is knackered. I'm off to bed. See Ya, JW
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Old 15 December 2002, 20:02   #19
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My advice,

Get your ali prop sorted for the optimum,then buy a stainless as close to the spec of the sorted ali prop as you can get.

All the best

Crazyhorse
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Old 16 December 2002, 02:26   #20
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This really has been an eye opener for me and I thank you all, JW, JK and Crazyhorse, for your input. I will now send off the aluminium prop to get it tweaked to increase top end speed.

I am still of two minds what to do after this.

I am not overly concerned that with more pitch or a bigger prop, the boat will have difficulty getting up onto the plane. It did manage to do that with the Yamaha 165 hp diesel that was in the boat before I replaced it with the Yanmar 300hp job.

Also, before I fitted the Yanmar, I added Bennett Sport trim tabs and with those lowered the 165 popped the boat staright up onto the plane. Once it was up there I retracted them and we motored along perfectly happily at the top speed of 23kts.

Now, with the prop I've got, the 300, even with it's turbo lag, has no need for the tabs and they are mostly redundant. Especially as by adding 600 litres of fuel in two in-line tanks in front of the old one, the CofG has moved forward.

It seems to me that maybe I could go for an 18 3/4 inch prop instead of the 17 3/4 and even increase the pitch from 21" to 23".
I realise the boat could struggle to get up onto the plane with that lot but with the trim tabs down it ought to do it. Possibly?

Supposing that it does work, if I understand JW correctly, I can then look forward to higher speed at lower revs with greater fuel economy.

Or am I talking b**lls?
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