Diesel outdrive props

[Mike Garside]

I have a 17 3/4 x 21 inch aluminium prop at the moment and this delivers exactly the required 3,800 rpm for my Yanmar 300hp engine.

I want to get a stainless prop and keep the aluminium one as a spare but, although I believe that the diameter or the pitch will drop a little to allow the WOT revs to hit 3,800rpm, I imagine 13 1/2 inches is too big a drop in diameter. Even if the pitch goes up from the 21 inch pitch I have now to 23 inch. I guess I will be over revving and lose top end speed.

Am I right in this assumption? Has anyone got any views?

Cheers

[jwalker]

Hi Mike, Yeh, you are correct in your assumption. A wee thought occured to me the other day when you were talking about props and engine revs. Does your engine rev. limit at 3800? ie. to prevent you over revving it.
Judging by your prop size, you must be using a Bravo 2 leg, yes?
JW.

[Mike Garside]

Right on, JW. Bravo 2 it is. I believe it is limited at 3,800 to prevent over-revving.

When talking to one of the Yanmar guys at E P Barrus he said, I think, that exceeding the 3,800 by 50 to 100 revs would do absolutely no harm. But short of having a smaller prop I don't see how I achieve that. The throttle is flat down at 3,800.

What would you do in these circs? Stick with the same size prop as I have now and just swop aluminium for steel?

[jwalker]

Mike Garside, have you managed to keep up with me?

As I was saying. I've come across a couple of diesel boats which have been inadvertently under propped. The test is easy. Get 10 big policemen aboard and see whether it still reaches 3800. (Firemen will do, since they've got some spare time just now.)
This is a possibility because you'd mentioned 35knts as top speed and that seems a bit slow for 300hp. It depends what you carry in your boat though. Like 200mts chain, 3 anchors, 300lts fuel etc, etc. It all adds up.
The big prop will give you a good hold on the water at low speed but, of course, you'll be a bit limited in power until the turbo spins hard. A smaller diameter prop will let the revs rise quicker and the turbo will become useful sooner but you loose out on efficiency at low speeds. It's all a bit of a b***er. Too much compromise. Some guys use Bravo 1's ask them how they get on. (When they're not breaking them that is.) With 300hp, you've gotta keep them seriously cool you know.
Cheers for now, JW

[Brian]

JW
I have a Scorpion cabin rib which must weigh at least 1 ton less than Mikes boat.
Yanmar 300 plus Bravo 1 gives me only 30 knots.
With 26 inch props on a Bravo 3 leg I got 34/35 knots and with 28s on a Bravo 3 I got 38 knots.
Having gone through two Bravo 3s in one year of gentle cruising (about 300 engine hours), I now don't trust the 3 leg and have hence changed to a Bravo 1.
Any comments?
All these figures are with a RPM of 3800.

[jwalker]

It's difficult to equate a single prop drive and a dual prop drive.
Your Bravo3 with 26" blades @3800 must have been under propped. It will take more energy to turn 28" blades at 3800. You don't get anything for nothing.
Bravo 1 & 3 reliabiity is a problem. It is getting better with the later versions of the X drive. I've spoken at length with some chaps from Barrus and they're confident about the Bravo 3 up to 250hp but they insist a shower on the top of the gearcase is necessary for 300hp. Me, I'd put a shower on 250. It's all a bit of brinkmanship. Actually, because of the reliability problem, I've chosen to go Volvo. But that's another story.
I've friends who have an 8.5mtr Tornado with 300Yan. They started off with Bravo 1 and broke it quickly. The replacement also broke. Moved to Bravo 3 and broke that too. Enter Bravo 3X. It failed. There were early problems with accuracy in the gears/bearings.(I've a feeling they broke more than that but I lost count.) They're still with a later 3X plus a shower but they'd like to be without the niggle that they may have to radio for a tow one day. Oh, I didn't mention the cracked engine block in the Pentland Firth. Again, thay's another story.
JW

[Alan]

That wouldn't be the famous Gillespie Brothers would it? (Your friends with the Tornado). Bravo 3 or 3x drives are simply not upto the torque of the Yanmar 300 IMHO. Two drives in 270 odd hours, complete with shower attachments?!! No wonder Brian changed to Bravo 1.

[Mike Garside]

Originally posted by jwalker
Mike Garside, have you managed to keep up with me?

Yup, on the case.

Weight is a problem. The boat is heavy and has all the aerodynamics of, to use Alan Priddy's description, a brick. I also carry 600 litres of fuel and other clobber that inlcudes a rolled up SIB and small outboard (for those mini-break moments).

So I'm realistic as to what the top speed can be, given the weight of the craft and the load. What I do want is to get the best flat-water top speed out of her that I can by choosing the optimum prop.

I have talked to Thomas Bolton at a company called Propellor Revolutions in Poole. He has recommended that I send them my aluminium 17 3/4 x 21 inch prop and they will tweak it and tune it at the cost of about £30 - £40.

After I've then tried this out and if I am happy with the result he suggested that I stay with the same dimensions but get a steel prop from them which they will set up to the same formula.

He did not recommend going bigger or going for a different pitch.

Does this make sense?

[John Kennett]

OK, I've moved all these messages together now!

Mike, if you are getting 3800 RPM flat out, then that would indicate that you have just about got the prop right for the boat and engine.

Changing to a steel prop sometimes seems to be more of an art than a science, but it shouldn't call for a major difference either way.

I would go with the propeller expert on this one. Bear in mind that when he has made your steel prop he can tune or or re-pitch it to a certain degree if the results aren't spot-on first time.

John

[Mike Garside]

Thanks, John. Makes sense, I guess.

[jwalker]

Mike G,
JK’s 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 engine’s rev limiter (whatever that might be). Don’t spend money until you are sure. Right, give me some answers.
1) When out on a normal Saturday’s 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 can’t easily tell what is happening under the water. Talk about black art etc, etc. It’s 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, I’m off for now. JW

[Manos]

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).

[John Kennett]

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

[Mike Garside]

Originally posted by jwalker
Mike G... Right, give me some answers.
1) When out on a normal Saturday’s 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.

[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.

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

[crazyhorse]

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

[John Kennett]

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

[jwalker]

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

[crazyhorse]

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

[Mike Garside]

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?