Which is the best Stern drive???

[Daniel]

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!

[jwalker]

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.

[Dirk Diggler]

Originally posted by Pepper
our simple logic

Hey, you said it.

[Dirk Diggler]

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!

[Alan]

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

[-<=[Tw18]=>-]

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

[Julian]

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

[Brian]

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.

[TIM]

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????

[crazyhorse]

If you speak to Jim at Biboa he will put you strait on a few Misconceptions.

[TIM]

If you speak to Jim at Biboa he will put you strait on a few Misconceptions


What do you mean by that?????

[crazyhorse]

Tim
If you speak to him he will give his objective oppinions on the best engine and outdrive and also boat Hull.

He has a lot of experiance and knows what works.

I speak to him quite frequently for ideas and advice on what works and doesnt,and what is Hype and isnt.

[Brian]

Tim,
On warranty, I am told that when you first buy your engine, you have bought, in the price, a one year warranty.
When 12 calendar months have passed, your warranty expires no matter how many replacement (ie new) legs you have fitted in the meantime.
This is the official stance.
In practise you may find it diferent, but that sure isn't the way to bet!

PS I agree with you on shock loadings.

I also agree with CH that Jim is an excellent source of valuable opinion and information, and a super top-line guy that I love to death.
However, he is not the only source of valuable information and experience around us, and he does use a twin, non-Yanmar, non-Bravo leg configuration. So shades of Mandy Rice-Davis perhaps?

[Julian]

Tim

The Bravo 3x leg is suppose to be fine now that they have toughened it all up etc, they do fix them for free either, we paid nearly £2k to have ours upgraded, Not good but we had no choice.

Julian

[Alan]

Is not fine if as Brian postulates (good word) you have a heavy RIB - He went through two - one after RB4 and one in Scotland last year. Fried, Boiled and knackered.

I think that the weight thing must be very significant as you report no problems, nor does Mark Wildley and Allen C both who run 300 Yanmar / B3x's in lighter boats.

And Brian was Mandy Rice Davies (showing your age here I think) Scottish with a beard?




Alan

[Julian]

Alan

I agree with you i think the weight of the rib is a contributing factor to the problems with the drives, not sure how much heavier Brians boat is but it must be a quite a bit,

Julian

[Pete7]

The original question was which is the best stern drive, so for my money its a great big Stern Power. Built like a Russian T72. The other good point is the exhaust exits through the transom not the stern drive so no over heating problems. New about £4k second hand £750 to £1k.

Pete

[Alan]

Errm you may be right but the B3x Yanmar 300 combos on Scorpions I have seen have seperate exhaust exiting through pipe and hypalon tube thingy so that cant contribute to the overheating problem. Think its more about power and torque and all that engineering type stuff.

Sternpower drives are used on all the MOD Pacifics arent they? Gotta be a ringing endorsement!

Alan

[Pete7]

Alan, surely a stern drive can't get that hot just from the bearing and gears ? someone said it took the paint finish off ? the exhaust has to be a contributing factor afterall half of a stern drive is in the water ? comments from those who have had one fail then perhaps ? or is it the fact that 300 hp through a UJ and two bevel gears just isn't good engineering for long term use ?

Pete

[Jonny Fuller]

I you drive your aveage car at 30mph, hold your foot gently on the brakes for 1 min, stop, get out, look at your discs, chances are they're near red hot! that's a small percentage of your car's power (maybe 30-40hp) going into the discs! Imagine tearing along at 40Knts, using say 200 of your 300 hp, as your drive begins to fail (for whatever reason) and the friction of the gears and bearings rises, which of course generates more heat, and your boat begins to slow down, every one of those horses that were pushing you at 40, but are now only pushing you at 35, are being turned into heat! 1 hp = 746 watts, so imagine all the electric bar fires you now have switched on in your B3! as the gears fail and the friction goes up, way up,
The heat damage you see at the end of this "chain of events" is started with a small problem within your drive that brings about the increasing friction which spirals into very rapid meltdown!