minimum engine sizing

[nb Innisfree]

Horse Power = Torque x rpm

ft lbs/sec = ft lbs x 1/sec

1 HP is 550 ft lbs/sec (or 746 Watts in metric)

 

So you can see that you can have a small engine with low torque, but revving highly, that develops the same power as a large engine that has high torque but only low revs. The power is the same but the torque is massively different. On the canals it is better to have a large torque value engine swinging a large prop than a small engine revving like fury to turn an egg whisk prop. Not mentioning anyone's boat by name!

 

It is the same principle on the road. Diesel cars are often lower power than petrol cars (in terms of out and out horsepower) and yet they drive as well, if not better, in practical real use on the road. This is because the useful bit is the torque, which a diesel engine has more of than a petrol engine.

Roger

 

Or you could convert high RPM to torque at the prop shaft, where it's needed, by gearing down

[Timleech]

Or you could convert high RPM to torque at the prop shaft, where it's needed, by gearing down

 

Exactly. Best to pick a reduction ratio which gives the biggest prop you can comfortably accommodate. Sometimes worth derating the engine, to give less horsepower but swing a bigger prop. I know it's counter-intuitive but there you go..

 

Tim

[Naughty Cal]

 

 

So you can see that you can have a small engine with low torque, but revving highly, that develops the same power as a large engine that has high torque but only low revs. The power is the same but the torque is massively different. On the canals it is better to have a large torque value engine swinging a large prop than a small engine revving like fury to turn an egg whisk prop. Not mentioning anyone's boat by name!

 

Lucky its got shed loads of torque as well then

[Athy]

[

 

Lucky its got shed loads of torque as well then

 

It should be economical to run then - they do say that torque is cheap.

[Naughty Cal]

It should be economical to run then - they do say that torque is cheap.

 

[Albion]

Or you could convert high RPM to torque at the prop shaft, where it's needed, by gearing down

 

Yes, true, but you can be limited by the available gearbox ratios as to how far you can gear down to swing a large prop.

Roger

[nb Innisfree]

Yes, true, but you can be limited by the available gearbox ratios as to how far you can gear down to swing a large prop.

Roger

 

Agreed

[Timleech]

Yes, true, but you can be limited by the available gearbox ratios as to how far you can gear down to swing a large prop.

Roger

 

Yes, but many builders/owners unthinkingly go for the 'standard'/cheapest 2:1 ratio when others are very likely available which may be more expensive initially but better and more economical in the long run.

 

Tim

[nb Innisfree]

Yes, but many builders/owners unthinkingly go for the 'standard'/cheapest 2:1 ratio when others are very likely available which may be more expensive initially but better and more economical in the long run.

 

Tim

 

I was thinking of a large prop which would be in the region of 500 RPM and a 3000 RPM engine, requiring 6:1 reduction, if one is available?

[MoominPapa]

I was thinking of a large prop which would be in the region of 500 RPM and a 3000 RPM engine, requiring 6:1 reduction, if one is available?

But if you have a hull that can swing a prop that big, why not fit a slow-reving engine too. (Not necessarily a "vintage" lump: a Beta BD3 would fit the bill)

 

MP.

[Timleech]

I was thinking of a large prop which would be in the region of 500 RPM and a 3000 RPM engine, requiring 6:1 reduction, if one is available?

 

They're available for bigger marine engines, fishing boat size for instance, you might struggle to find more than 3:1 in normal 'pleasure boat' sizes.

 

Tim

[Yoda]

7.23x2.5 = 18hp

 

I have 152hp going spare. Anybody want some?

 

You talk sooo dirty!

[Naughty Cal]

You talk sooo dirty!

 

I meant spare horse powers

[RLWP]

I meant spare horse powers

 

Or is it spare horses power?

 

Richard

 

Unless it's telepathy, or the ability to foretell earthquakes or something

[Proper Job]

They're available for bigger marine engines, fishing boat size for instance, you might struggle to find more than 3:1 in normal 'pleasure boat' sizes.

 

Tim

You could always fit a PRM with a trolling valve?

[Albion]

You could always fit a PRM with a trolling valve?

 

Yeah, but the problem with that is that you are just slipping the clutches all the time, that can't be good for the long term life of the box.

Roger