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Old 09 April 2003, 20:14   #21
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JW,

I'm refering to the HydraDrive that Alan P had coupled to his Yamaha for the Spirit-of-Cardif round the world attempt here.

This theory does not apply to the Bravo legs, since:

a) They are not hydrolicly coupled
b) Alan P did not use one to go round the world and therefore i know very little about them

The reason for 3x drives failing is quite different - and i've no idea about this.

As i said previously, and i'm no chemical engineer (electronics actualy) inefficient combustion will cause hotter exhaust gases. It just does!
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Old 09 April 2003, 20:32   #22
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Daniel. Yes, I realised what you were referring to but one cause of 3x drives failing is because they overheat.

Normally, inefficient combustion leaves unburned fuel and this cools the exhaust. You may be relating your thinking to petrol engines where a week mixture does, indeed, cause high temperatures. A diesel engine works by consuming the maximum amount of air it can pass at all times. The more fuel that is injected, the more power it develops, until the point where there is no more air left to burn. I'm not sure what is meant by poor fuel but if it is low grade with a lower calorific value then it will simply produce less power. So, I still don't understand the reasoning.

Cheers for now, JW.
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Old 10 April 2003, 03:57   #23
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Quote:
Originally posted by Pepper
our simple logic
Hey, you said it.
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Old 10 April 2003, 04:06   #24
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I think it's pretty safe to say that there are numerous theories on why Bravo's fail on a Yanmar, my own personal one is that a B3 fails due to the inertia of the twin s/s props, coupled with no shock absorbing hubs.

However, it seems that no one knows the answer, even Mercury or Yanmar, coz I'm sure that they would have fixed the problem by now.

I did hear from a Merc guy in the States, that they regret ever having done the deal with Yanmar for the use of their drives, which is rumoured to have been agreed on a golf course!
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Old 10 April 2003, 04:27   #25
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Whatever the reason actually is the heat generated must surely be a significant cause. The B3x that I have seen/helped remove after failure had got so hot that the paint on it had blistered and discoloured. And this unit was fitted with an aftermarket "shower" as well!

-<=[Tw18]=>- The difference between B1 (or B2) and B3 drives is that the former are single propellors and the latter are twin contrarotating props ala Volvo Duoprop.

Alan
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Old 10 April 2003, 04:47   #26
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Quote:
Originally posted by Alan

-<=[Tw18]=>- The difference between B1 (or B2) and B3 drives is that the former are single propellors and the latter are twin contrarotating props ala Volvo Duoprop.

Alan
simple as that

thank you sir, i should have done a google on the topic but it was late last night and I was off to bed.

thank you
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Old 10 April 2003, 05:51   #27
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Bravo 3

Morning All

Bravo 3 drives fail due to the heat they generate and the fact that the original drives dod not have strong enough shafts and bearings, Mercruiser re built the drives using toughened parts and developed the bravo 3 x leg, We had our drive re built 2 years ago and had the strenghened parts from the bravo 3 x leg put in and so dar no problems.;

Julian
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Old 10 April 2003, 06:13   #28
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My tuppence worth (or two B3x legs worth) on this is.......
I think that if you couple a relatively heavy boat, with an engine putting out more than 250hp and then work this combo in a rib (ie "heavy", "tough" hour-after-hour mode), then it seems too much for the current design.
Move or mitigate any one of these factors and the failure tends to disappear. ie put the combo in say an 8m Open Scorpion, use a 250 Mercruiser engine, install in a family cabin cruiser.
Fitting a showerhead didn't seem to make any diference in my case but common sense says you are better off with one than without.

Actually I am not quite sure where this thread is wandering. Some data might be useful.
For diesel ribs, how many have which engine, and which leg? Separately we need to know the single engined ribs fom the doubles.
All non-ribs should probably be excluded from the stats.
Only then can we say what is popular and what is not.
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Old 10 April 2003, 12:27   #29
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I would assume that Load is not a problem as the use the same engine cofigurations in heavier hard boats......

I could see a problem with shock loads, ie when rib and prop leave the water, and then the sudden free reving to full speed load????

SO is the Bravo 3x unit better then the old B3 units or have people still have them go bang!!!!!

ps Do they fixed for free(warrinty), or are you left crying on to a whole heap of mangled gears????
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Old 10 April 2003, 13:51   #30
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If you speak to Jim at Biboa he will put you strait on a few Misconceptions.
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