Need me a Gardner....?

[cuthound]

Interesting stuff you have posted.

 

The received wisdom is that it's torque that counts but in my experience this is generally asserted only by posters with a shallow and poor understanding of the effects at play.

 

My personal experience is that blade diameter is far and away the most significant factor. My first but one NB had a 35hp engine and a 16" blade. Performance was pathetic especially in astern. I had to rev the nuts off it to stop in a lock.

 

Then I swapped it (with cash adjustment) for a longer boat also with a 35hp engine but with a 21" blade and slower revving. WHAT a contrast!! AWESOME stopping ability and 7 knots in deep water, easily.

 

I'm inclined to think the blade diameter is the crucial factor. Old working boats with 26" blades and a 18hp JP2 seem to go like the clappers. I'm convinced a modern engine of the same power installed in the same boat with an approriate reduction ratio to drive the same blade would perform in the same way.

 

Just my opinion...

But you need high torque to turn a large prop. Try fitting a 26" blade to something like a BMC 1.5 and see what happens.

 

The old engines used in working boats were large capacity, long stroke engines, usually with only one or two cylinders. A perfect recipe for a slow revving, high torque engine.

 

Modern engines are usually shortvstroke, high revving multiple cylinder engines, which develop lots of power but much less torque.

Edited July 22, 2016 by cuthound

[Tony Dunkley]

But you need high torque to turn a large prop. Try fitting a 26" blade to something like a BMC 1.5 and see what happens.

 

What would happen, if combined with a suitable reduction gear, would be that a lot more of the available horsepower would be turned into propulsive thrust than the pitifully small proportion that usually gets transmitted to the water via small diameter, high revving and very inefficient propellers.

 

You are not taking into account that torque is a constituent part of, and directly related to power. Reduction gearing not only reduces speed of rotation, but it also multiplies torque, . . . by the reduction ratio.

[WotEver]

My understanding is that hp = torque * RPM / 5252

 

So torque = HP * 5252 / RPM, is it not?

 

Tony

 

So the slower the RPM, the higher the torque.

[MtB]

My understanding is that hp = torque * RPM / 5252

 

So torque = HP * 5252 / RPM, is it not?

 

Tony

 

So the slower the RPM, the higher the torque.

 

 

This is correct, for a given power.

 

Although the waters are muddied when turn a prop as the torque load you might measure in the shaft rises in proportion to the cube of the angular velocity.

 

So as engine and blade speed goes up, the torque required to turn the blade rises faster and catches up with the engine torque, at which point the blade won't rotate any faster and equilibrium is reached.

[WotEver]

 

 

This is correct, for a given power.

 

Although the waters are muddied when turn a prop as the torque load you might measure in the shaft rises in proportion to the cube of the angular velocity.

 

So as engine and blade speed goes up, the torque required to turn the blade rises faster and catches up with the engine torque, at which point the blade won't rotate any faster and equilibrium is reached.

So slow it down a bit more and add an even bigger prop . Dunno how you swing a 48" prop on the cut though lol

[IanD]

So slow it down a bit more and add an even bigger prop . Dunno how you swing a 48" prop on the cut though lol

A bit tricky, but this is taking it to extremes...

 

At a constant efficiency, power absorbed by a propeller is proportional to the square of RPM, the blade area (square of diameter), and the pitch -- it doesn't make any difference whether this power comes from a low-revving high-torque engine or a fast-revving low-torque one (or a super-high-rpm electric motor), the only difference is the gearbox ratio needed. Bigger diameter slower-speed props are always more efficient for slow vessels like narrowboats, but the tradeoff is that a boat which is deeper in the water needs more power to propel it at a given speed, and will be heavier so take longer to start and stop.

 

For example, starting from a typical (Beta 43) 18" x 13" prop (24" draught boat) with a 2:1 gearbox (1400rpm prop speed at full power), if you used a 3:1 gearbox instead (930rpm at prop) at the same pitch you'd need a 27" x 13" prop, or more sensibly a 23" x 18" prop (30" draught) -- but since this would be more efficient (better "grip" on the water) you could probably drop the pitch by an inch or so. You'd also get more "bollard pull" (thrust when stationary) which is what matters when starting and stopping (but see below). Going to 36" draught with an even bigger prop and a 4:1 gearbox would be better still, but you'd run into a lot more grounding problems or being very slow on shallow canals.

 

But since deeper draught means you need more power to maintain a given speed and starting and stopping becomes slower because the boat weighs/displaces more, it's not obvious that all this makes sense when you take everything into account...