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Old 24 July 2009, 22:36   #1
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Gearbox.

This is one of a number of inane questions (to the experienced) but the reason I joined this Forum was to gain knowledge, so here goes.

Firstly, am I right in thinking that the gearbox in an outboard is simply to a) allow the selection of forward or reverse direction of travel and b) to provide a reduction gearing?

Secondly, compared with marine turbo diesels, outboards don't seem particularly highly stressed in terms of power output, so would some form of gearbox that offers, like a vehicle 'box, some method of allowing lower revs for the same speed. This might let engines put out higher specific power outputs from smaller engines. Or would the extra weight make it unfeasable?
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Old 24 July 2009, 23:09   #2
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Quote:
Originally Posted by The Beard View Post
This is one of a number of inane questions (to the experienced) but the reason I joined this Forum was to gain knowledge, so here goes.

Firstly, am I right in thinking that the gearbox in an outboard is simply to a) allow the selection of forward or reverse direction of travel and b) to provide a reduction gearing?

Secondly, compared with marine turbo diesels, outboards don't seem particularly highly stressed in terms of power output, so would some form of gearbox that offers, like a vehicle 'box, some method of allowing lower revs for the same speed. This might let engines put out higher specific power outputs from smaller engines. Or would the extra weight make it unfeasable?
Do not understand. What is your objective. Outboards are highly stressed compared with marine deisels.
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Old 24 July 2009, 23:12   #3
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Yes you are right on a and b.

Dunno about not being highly stressed though - a Mercury Verado gets as much as 350hp from a 2.6L engine.

And how many car engines spend all day running around at 6000 revs???

Of course the big advantage with marine engines is that there is a copious supply of cooling water.

There are various systems around for "changing gear" on boats - usually combined with surface piercing props but the prob you have is that the propellers like working at particular rpms. The real answer is a variable pitch prop - common on many displacement boats but hard to do in smaller sizes.
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Old 25 July 2009, 18:12   #4
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2 speed Volvo Conversion

It has been done for Volvo Penta sterndrives.

Was done to get round a characteristic of turbo diesels in heavy boats (marginal on power) where a small propeller was needed to allow the engine to boost and get the boat on the plane. This then limited the top speed as the propeller was then too small for the best top speed.

Outboards tend not to suffer from this problem as they are built for mid range torque.

Having said that it would be technically possible to have a 2 speed gearbox on an outboard but the extra cost would make it pointless - the extra 1000 - 1500 could simply be spent on a more powerful engine.
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Old 25 July 2009, 19:00   #5
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one reason is to give a 90 degree angle from the drive shaft to the prop,unless you do the far eastern way of a long angle shaft and a bus engine and gearbox on the end.
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Old 25 July 2009, 19:17   #6
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And talking of gearboxes I think Yam were the only people to develop a duoprop outboard - it was a 150hp. I think it would make a lot of sense for any ooutboard over say 300hp.
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Old 25 July 2009, 21:25   #7
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Originally Posted by codprawn View Post
And talking of gearboxes I think Yam were the only people to develop a duoprop outboard - it was a 150hp. I think it would make a lot of sense for any ooutboard over say 300hp.
I actually saw one on a rib the other week-first time ever-but it only had one prop fitted.
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Old 26 July 2009, 05:37   #8
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Yes you are right on a and b.
Typical ! You never bothered to read the question . Last time I bothered checking, my out board had a 1.95 :1 gear ratio, so if the engine is doing 5000 rpm, the prop will be doing nearly 10000. Not a reduction box then
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Old 26 July 2009, 05:46   #9
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Typical ! You never bothered to read the question . Last time I bothered checking, my out board had a 1.95 :1 gear ratio, so if the engine is doing 5000 rpm, the prop will be doing nearly 10000. Not a reduction box then
I'm going to stick up for Codders - I think you'll find that your prop is doing roughly 2500 rpm when the engine is doing 5000 rpm.
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Old 26 July 2009, 05:56   #10
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I stand corrected then
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