Gearbox.

The Beard · 25 July 2009, 02:36

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?

Limey Linda · 25 July 2009, 03:09

Quote:

Originally Posted by The Beard

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.

codprawn · 25 July 2009, 03:12

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.

Searider · 25 July 2009, 22:12

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.

m chappelow · 25 July 2009, 23:00

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.

codprawn · 25 July 2009, 23:17

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.

Nos4r2 · 26 July 2009, 01:25

Quote:

Originally Posted by codprawn

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.

Bigmuz7 · 26 July 2009, 09:37

Quote:

Originally Posted by codprawn

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

Poly · 26 July 2009, 09:46

Quote:

Originally Posted by Bigmuz7

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.

Bigmuz7 · 26 July 2009, 09:56

I stand corrected then

codprawn · 26 July 2009, 12:27

Now if I made a stupid comment like that.................

Bigmuz7 · 26 July 2009, 13:47

Quote:

Originally Posted by codprawn

Now if I made a stupid comment like that.................

Thats cos you never admit when your wrong .. and henceforth we now have the Ribnet sport of counterposting a 'codpost'

That said .. It was a fooking 'engineer' that told me that, and goes to show.. you cant believe everything some people say

even the ones that are supposed to know what they are talking about

codprawn · 26 July 2009, 14:25

I ALWAYS admit when I am wrong - if it's a clear cut case of being wrong!!!

The Beard · 27 July 2009, 02:07

Thanks folks. My comment about outboards not being as highly stressed as marine diesels comes from a comparison with road engines. For instance, the Volvo D12 can make 775 hp whereas the road vehicle engine it's based on only makes 440. The Cummins 6BT in a boat can put out 425 whereas in a truck it's only rated at about 260. The Volvo D4 can generate 300hp in a boat yet probably only in the region of 190 in a truck.

Outboards on the other hand are not, I don't think, widely based on vehicle engines. A Mercury 50 is a 1000cc 3 cylinder motor. The Vauxhall/Opel Corsa also has a 1 litre 3 cylinder engine that puts out 55. An Evinrude 90 ETEC is a 1.7 4 cylinder 2 stroke, but a typical 1.6 llitre 4 cylinder 4 stroke car engine easily develops 100 to 120bhp.

I realise this is not comparing like with like in terms of load, drag and wind resistance, but I am thinking in terms of displacement and the origin of the engine.

It would seem from your replies that cost is a major issue, and I presume space is also as an outboard doesn't have the same amount of space as a car. My thoughts about a gearbox with more than one ration though wasn't so much about performance as fuel consumption and longevity, I did think that reducing revs via gearing might increase the life of a motor. Mind you the more I think about it, the more I realise that, with the exception of commercial applications when a diesel might be more appropriate, outboards don't really work the hours that road vehicle engines do, so I think I might have answered my own question there.

With regard to the Mercury Verado, I must admit that output is pretty impressive, but Yamaha and Suzuki both have 1000cc bikes that produce something in the region of 175bhp which if you multiply it by 2.6, equals 455hp without any kind of forced induction.

Thanks very much to everyone who replied. Sometimes I like to know the ins and outs of a f**t, but overall it helps my understanding of the subject. The comment about reduction gearing was also useful as I wasn't absolutely sure whether it was reduction or multiplication.

Limey Linda · 27 July 2009, 02:27

Quote:

Originally Posted by codprawn

I ALWAYS admit when I am wrong - if it's a clear cut case of being wrong!!!

Please post an example

codprawn · 27 July 2009, 02:34

Quote:

Originally Posted by Limey Linda

Please post an example

I readily admitted to being wrong when I thought that rope drum on RNLI ILBs was a winch when it's not!!!

Mollers · 27 July 2009, 06:55

Quote:

Originally Posted by Bigmuz7

I stand corrected then

Said the man in the orthopedic shoes.

Mollers · 27 July 2009, 07:02

Back to the OP, don't forget to add torque to your equation. A lorry engine is set-up to produce massive torque with relatively low BHP. The torque required to spin a prop on a 3tonne boat compared to hauling 17 tonnes up a hill is negligible.

m chappelow · 27 July 2009, 10:10

Quote:

Originally Posted by codprawn

I readily admitted to being wrong when I thought that rope drum on RNLI ILBs was a winch when it's not!!!

yeah but you can put an handle in and wind the rope back like a winch .lol

The Beard · 27 July 2009, 10:27

Think I might just get away with this one, no-one's noticed so far, but a 90 ETEC is of course 1.3 litres not 1.7. Just testing you understand.

The point about torque in truck engines is well made Mollers. On the road any 350bhp truck engine is producing about 1000 nm whereas a 100 bhp car engine puts out about 120 nm
but I find it interesting that as I've been told that a steep torque curve is important to get a boat onto the plane quickly, I've never seen torque figures published, whether in a boat magazine nor a manufacturer's literature.

I run an old Yamaha FJ1200 'bike with an air cooled 4 cylinder engine that puts out 125bhp and weighs about 250kg yet at 70mph is turning over at about 4,000 rpm courtesy of its 5 speed gearbox. It's a fairly compact installation and looking at it is what made me think about some kind of similar set up in an outboard.