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Old 07 December 2002, 03:04   #1
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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
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Old 09 December 2002, 19:33   #2
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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.
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Old 10 December 2002, 03:30   #3
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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?
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Old 10 December 2002, 11:49   #4
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Moved here from Buy & Sell-Re.props

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
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Old 10 December 2002, 12:12   #5
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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.
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Old 10 December 2002, 14:32   #6
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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
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Old 10 December 2002, 16:52   #7
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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.
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Old 11 December 2002, 03:56   #8
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Re: Moved here from Buy & Sell-Re.props

Quote:
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?
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Old 11 December 2002, 04:27   #9
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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
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Old 12 December 2002, 10:28   #10
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Thanks, John. Makes sense, I guess.
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