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Old 26 July 2009, 08:27   #11
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Now if I made a stupid comment like that.................
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Old 26 July 2009, 09:47   #12
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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
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Old 26 July 2009, 10:25   #13
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I ALWAYS admit when I am wrong - if it's a clear cut case of being wrong!!!
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Old 26 July 2009, 22:07   #14
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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.
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Old 26 July 2009, 22:27   #15
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I ALWAYS admit when I am wrong - if it's a clear cut case of being wrong!!!
Please post an example
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Old 26 July 2009, 22:34   #16
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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!!!
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Old 27 July 2009, 02:55   #17
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I stand corrected then
Said the man in the orthopedic shoes.
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Old 27 July 2009, 03:02   #18
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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.
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Old 27 July 2009, 06:10   #19
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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
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Old 27 July 2009, 06:27   #20
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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.
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