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Old 05 February 2011, 11:36   #31
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Quote:
Originally Posted by jwalker View Post
Power and torque are not inseparable. Torque is simply a turning force...so taking the turning force and RPM power can be calculated. Therefore the converse is true, if you know the power and RPM you can calculate the torque.

What is generally meant by a torquey engine is an engine which produces good power at lowish RPM.
Exactly,

But what some of the other posters have missed is that outboard manufacturers don't publish their maximum torque figures and state at what rpm it occurs.

It's not as simple as doing the maths using max hp and max rpm (HPx5252)/RPM = Torque

I guess the best we get from the manufacturers is to see at what minimum engine speed maximum power is obtained:

An engine that produces 50hp at 4500-5500 rpm produces more torque than one that produces 50hp at 5500-6500rpm.

Hmmm
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Old 05 February 2011, 12:50   #32
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I've said it before & I'll say it again, torque is everything. We all get hung up on HP, but without torque, HP is nowt. E.g, Bugatti Veyron, over 1000HP & delivers 1500Nm, Scania R500 tractor unit = 500hp delivers 2500Nm@1300rpm. Buggati twice as "powerful" as Scania, but lets see it pull 40tonnes of baked beans uphill from a standing start. With a boat, we don't have the luxury of a multi speed gearbox, so we need torque low down in the rev range, otherwise we'd never get on the plane or pull up a skier. It's all very well saying engine X is this HP compared to engine Y, but what we need to know is where in the rev range the TORQUE is delivered, it's no good at 5000RPM because without lowdown torque you'll never get there. As manufacturers seem unable or unwilling to publish torque curves we can only go on experience. And as everybody knows 2st Etecs have LOADS of lowdown torque
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Old 05 February 2011, 18:31   #33
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If you've got the manufacturer's power curve you can plot your own wee graph of torque from that using the formula given in a previous post. Bear in mind that the power graphs will be power at full throttle unless they say otherwise and it's not a common situation to have a boat engine running at full throttle but low revs for a prolonged period. It can occasionally happen eg. climbing the back of a wave or fast acceleration but these are transient. Comparison with a road vehicle is not particularly valid since the sea surface doesn't have prolonged hills for us to climb. Even if there are large waves they would pass under us if the boat was still, unlike a road incline.

PD, in your example at 1300 rpm and high torque that engine will therefore be producing high power too at those revs.
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Old 05 February 2011, 20:29   #34
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PD, in your example at 1300 rpm and high torque that engine will therefore be producing high power too at those revs.
Yup! agreed. But it's still 500hp v 1000hp+, which one is going to get you up the back of that wave when you bury the throttles, 2500nm or 1500nm? As you quite rightly say, Manf's figures are usually quoted at WOT, how often do we run at WOT? It's low down grunt that we need, the ability of the engines to spin the props from low rpm to planing speed under load, is more important (IMHO) than the practically useless HP rating at WOT.
High torque at low rpm
(Milk floats & series wound DC motors spring to mind!)
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Old 06 February 2011, 05:02   #35
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(Milk floats & series wound DC motors spring to mind!)
I think the batteries would make your boat too heavy.....

A good judge of torque is the Squeal Factor when you fill the boat up with a bunch of girlies and give it some breakfast. Am I allowed to say "E-TECs make a girl squeal" or will we get bilged?

I've no experience of a modern 4 stroke on a similar sized boat but the 150 does get up and go
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Old 06 February 2011, 08:53   #36
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Am I allowed to say "E-TECs make a girl squeal" or will we get bilged?
Yep, cos Gotchi's got one, and all he does is squeal like a girl
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Old 06 February 2011, 19:33   #37
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Yep, cos Gotchi's got one, and all he does is squeal like a girl
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