How to tell whether I'm over or under or just right propped?

[macd]

Stern to might be taking it a bit too far

It might be an unusual case where you are, but not round here. The effect on max revs is drastic.

 

[lw395]

I(stern to a quay) might be an unusual case where you are, but not round here. The effect on max revs is drastic.

But you presumably don't need the max possible thrust doing this?
Unlike a spot of informal 'ploughing'?

 

[LittleSister]

One good indicator of overpropped is when the engine won't achieve max power rpm with the boat securely tied to the quay.
That can be too much pitch or too much diameter/blade area.
It's underpropped if the boat speed is too low at max power rpm.

Are you sure? I think not. The load on the engine is so much that it won't reach max rpm. I know this from first hand. My boat used to reach maximum rpm of 3500 (with previous gear box and propeller; now I am overprop) but tied to the quay only around 3100.

I said 'max power rpm', not max rpm.
3100 may be about right?

Surely it won't be producing maximum power if it's not achieving maximum revs? I think the power curve for my engine, a Beta 14, is fairly typical. At max revs, 3,600 rpm, it can produce 13.5hp, but at 3,100 rpm it can only produce 12hp.

 

[sailorman]

]Surely it won't be producing maximum power if it's not achieving maximum revs[/COLOR]? I think the power curve for my engine, a Beta 14, is fairly typical. At max revs, 3,600 rpm, it can produce 13.5hp, but at 3,100 rpm it can only produce 12hp.

Quite max revs @ hull speed is correct, i have mine set to about 150 RPM under max revs @ hull spd

 

[Tranona]

Surely it won't be producing maximum power if it's not achieving maximum revs? I think the power curve for my engine, a Beta 14, is fairly typical. At max revs, 3,600 rpm, it can produce 13.5hp, but at 3,100 rpm it can only produce 12hp.

Exactly - the critical word is "can" - it will only produce that power if the prop loads it so that it can. You can get the governed speed of 3600 in neutral, but it will not be producing much power, nor consuming much fuel.

The eternal problem with boat engines (for displacement boats) is that the prop size is essentially fixed so it only fully loads the engine near maximum revs. Below that the engine is never fully loaded as the prop will not demand full power. So your engine at a good cruising speed of, say 2500 is only using probably 7 or 8hp but the maximum it could produce is 10 at those revs. Fortunately displacement yacht hulls need little power to move them in flat water up to around 80% of hull speed, after which the amount of power required rises rapidly. That extra knot and a bit needs a 40% or more increase in power to achieve. Maintaining speed against wind and waves also requires significant increases in power.

This all explains why engine manufacturers specify that the engine must be able to achieve maximum revs within the defined limits before the installation is signed off. However, users will rarely need to venture into the higher revs zone, but the extra power will be there if needed.

 

[lw395]

Surely it won't be producing maximum power if it's not achieving maximum revs? I think the power curve for my engine, a Beta 14, is fairly typical. At max revs, 3,600 rpm, it can produce 13.5hp, but at 3,100 rpm it can only produce 12hp.

Depends on the engine.
Many will produce pretty much max power at 80% revs.
Whatever, IMHO, you want a yacht auxiliary to be able to get well up the power scale while the boat is stopped, not bog the engine down and produce lots of black smoke and not much thrust.
A 'big' prop might give you quieter cruising in flat water of course.

 

[lw395]

...The eternal problem with boat engines (for displacement boats) is that the prop size is essentially fixed so it only fully loads the engine near maximum revs. Below that the engine is never fully loaded as the prop will not demand full power. So your engine at a good cruising speed of, say 2500 is only using probably 7 or 8hp but the maximum it could produce is 10 at those revs.....

The torque to turn the prop is a function of its rpm and also the slip angle the blades are running at.
The torque needed to do 2000 rpm at 5 knots will be way less than to do 2000rpm at 2 knots.

 

[Tranona]

The torque to turn the prop is a function of its rpm and also the slip angle the blades are running at.
The torque needed to do 2000 rpm at 5 knots will be way less than to do 2000rpm at 2 knots.

So? Not sure what point you are making. It is the prop that moves the boat and demands the power from the engine. So you set the revs you want with the control and the amount of fuel (and therefore power) required to achieve those revs is provided by the governor. If the boat is moving, the amount of power required is lower than for example if the boat is stationary or meets resistance such as waves or a headwind. This explains why you may not get full revs if the boat is tied up and conversely if the sails are providing the power to move the boat, the power demand from the engine will be less. The latter scenario is one the causes of bore glazing as the engine might be running at highish revs but with no load.