Full displacement trawler power requirement.

[davidej]

In very simple terms power determines the max speed at which the boat can go (assuming the prop is correctly pitched) and torque determines how quickly the boat reaches that speed (ie acceleration)

Where does that bollocks come from?

 

[kashurst]

Unless we want to go slightly faster we almost always cruise at 1600 rpm, regardless of wind, sea etc. But we have stabs, without them one quite a few occasions we simply would not be out at sea, far too uncomfortable.

At a guess our weight is about 26 tons.

what cruise speed is that and what is the fuel burn?

 

[BruceK]

Where does that bollocks come from?

The way I see it is you can talk theory till the cows come home but if I had a 50hp motor taken say from my motorcycle engine, bearing in mind the boat only has one gear and it's a displacement boat, when I pop the gearshift from neutral into forward, without the torque it will stall unless I have a suitable small and rather inefficient prop on that would simply cavitate and slip trying to move a 50 odd foot displacement hull.
I can have Arnie trying to tighten a nut by hand that my 10 year old son will easily best using a spanner, and who is going to argue that Mr Schwarzenegger has less power at his disposal than my 10 year old child?

 

[davidej]

Yes, but all (leisure not necessarily merchant ship) engines are geared down to turn the prop at a suitable speed..

If you were foolish enough to fit a 10,000 RPM m/c engine you would gear it down to rotate the prop to around 1,000 RPM. This would multiply the torque correspondingly

 

[BruceK]

Yes, but all (leisure not necessarily merchant ship) engines are geared down to turn the prop at a suitable speed..

If you were foolish enough to fit a 10,000 RPM m/c engine you would gear it down to rotate the prop to around 1,000 RPM. This would multiply the torque correspondingly

You'd never get it out of neutral without it stalling unless you got a clutch with it too. Surely that is not too hard to grasp?
Or would you like to try putting it into gear at 10k rpm? Give it a go in your car if in any doubt

 

[longjohnsilver]

what cruise speed is that and what is the fuel burn?

We cruise at about 6.5kts, and about 2.5nm per gallon. Top speed is 8.5kts, which we can reach at just over 2000rpm, but fuel use goes up by about 50%.

 

[davidej]

You'd never get it out of neutral without it stalling unless you got a clutch with it too. Surely that is not too hard to grasp?
Or would you like to try putting it into gear at 10k rpm? Give it a go in your car if in any doubt

Do you put your boat engine in gear at full revs.? I'm sure I don't and the morse controls would not let me.

And BTW marine gearboxes have a form of clutch built in.

Anyway this is a bit of a pointless argument as i don't think either of us is thinking of using a motorcycle engine.

I was just making the point that by using a suitable gearbox, a high revving low torque engine could produce as much torque at the propshaft as a slow revving high torque engine,