Ruston Hornsby 2yd

[Tony Brooks]

1 minute ago, MtB said:

 

That's a brave assertion.

 

The relationship of prop characteristics to engine characteristics and transmission reduction ratio is complex than even the prop calc tools suggest, in my personal experience.

 

But I'm sure John.k knows that already!

 

 

 

I agree prop sizing is complex, but the fact remains it is torque that twists the prop round, not power. Power (BHP) has speed in its calculation and at higher revs when torque is dropping it is usual still get an increase of power as the speed rises. The falling torque at higher speeds is why over-propping causes black smoke when the prop speed and hence engine speed can't rise because the torque has started to fall.

 

I find it interesting that people always talk about power for boat engines rather than torque and the torque curve.

[MtB]

17 minutes ago, Tony Brooks said:

 

I agree prop sizing is complex, but the fact remains it is torque that twists the prop round, not power. Power (BHP) has speed in its calculation and at higher revs when torque is dropping it is usual still get an increase of power as the speed rises. The falling torque at higher speeds is why over-propping causes black smoke when the prop speed and hence engine speed can't rise because the torque has started to fall.

 

I find it interesting that people always talk about power for boat engines rather than torque and the torque curve.

 

 

Yes agreed, but it isn't only about engine torque.

 

In the case of a 2YC being replaced with a 2YD, this is easily fixed by having the prop re-pitched shorter to better suit the peak torque happening at higher engine speed.

 

An undesirable (in my experience) side effect of this will be the 2YD needs to run at a higher speed to achieve the same cruising speed through the water that the 2YC needed. So the original prop may well get put back on and the increased engine power sacrificed for the sake of quieter cruising. 

 

 

 

Edited July 15, 2023 by MtB
Tinker with it

[john.k]

I believe there are reduction gearboxes to cope with these factors .........although one guy I rented some space to used to fit engines with step up gearboxes to get more prop revs ..... barefoot waterski ing at 100mph.