vic008
Active member
New Vetus, Kiwiprop. How crucial is it to reach the 3600rpm in gear? Presently get to 3000rpm, and can reduce the pitch a bit more, but concerned she will increase revs without a corresponding increase in knots
To rev to 3600rpm(27hp)
- Thread starter vic008
- Start date
Tranona
Well-known member
3000 is a bit low as you are not accessing a potential 5hp+ (20%!). Whether you need that depends on the displacement and LWL of the boat. That determines the hull speed and the HP required to achieve that. The aim is to pitch the prop so you achieve hull speed in the range of 3300-3600 rpm in flat water. 1" of pitch change will give about 250 rpm change. However before doing this it is worth plotting speed through the water against revs in 200rpm intervals from 1800. You can also feed your boat and engine/reduction data into a prop calculator to get the hp required to achieve hull speed as calculated plus ideal prop size.
B27
Well-known member
If your engine won't rev fully in flat water, when you face head winds or choppy water, or run aground, it may be seriously down on revs, power and thrust.
It may be interesting to do a 'bollard pull' kind of test with the current pitch, it gives you an extra data point.
there's often a possibility the engine only hits 3000rpm for another reason, like the governor adjustment, rev counter calibration, fuel supply?
It may be interesting to do a 'bollard pull' kind of test with the current pitch, it gives you an extra data point.
there's often a possibility the engine only hits 3000rpm for another reason, like the governor adjustment, rev counter calibration, fuel supply?
Graham376
Well-known member
When we fitted Featherstream, it was pitched for max BHP, not torque. Punching into wind and chop, 600 revs will make a big difference.
fredrussell
Well-known member
I have a kiwiprop on a Yanmar 27hp engined lightish 31 footer. Mine will only rev to 3.1 or 3.2k rpm, but will be doing just shy of 7kts. I seem to recall that in the YBW/PBO test on folding props the Kiwi prop was not one of the most efficient drivers in fwd gear- don’t know if that will effect max attainable rpm in gear though??
steveeasy
Well-known member
So what speed can you make currently at 3000rpm?. By reducing pitch you may then be able to achieve 3600rpm which is far more important than hull speed.
Id be worried your putting too much strain on the engine if you currently can’t achieve max rpm on flat water.
Steveeasy
LittleSister
Well-known member
I agree with others saying that increase in hull speed is not the important consideration, but availability of maximum power, which requires maximum engine revs. As most boats these days are over-powered, you may already be reaching max hull speed in flat water.)
In short, yes, decrease pitch to enable the engine to reach 3,600rpm or close to it.
You won't 'put too much strain on the engine' at lower revs. At whatever speed you run it, it will only be capable of producing less hp than that potentially available at max revs, which it is well capable of handling. (In practice, it will rarely be producing the maximum potentially available power at any revs - the governor will only provide enough fuel to maintain the particular rpm you have set with the engine control.)
Because you are driving a water propellor through a fixed ratio gearbox, and the way the engine fuel supply is controlled, even if you are over-propped you won't be able to 'labour' the engine at low revs in the way that some (typically older) drivers do with manual gearbox cars by using too high a gear when accelerating or climbing a hill.
In short, yes, decrease pitch to enable the engine to reach 3,600rpm or close to it.
You won't 'put too much strain on the engine' at lower revs. At whatever speed you run it, it will only be capable of producing less hp than that potentially available at max revs, which it is well capable of handling. (In practice, it will rarely be producing the maximum potentially available power at any revs - the governor will only provide enough fuel to maintain the particular rpm you have set with the engine control.)
Because you are driving a water propellor through a fixed ratio gearbox, and the way the engine fuel supply is controlled, even if you are over-propped you won't be able to 'labour' the engine at low revs in the way that some (typically older) drivers do with manual gearbox cars by using too high a gear when accelerating or climbing a hill.
steveeasy
Well-known member
Never a good idea to labour an engine going up a hill in top gear. That’s what it sounds like is happening here. Unless there is another mechanical problem it sounds like the prop is either oversized or the pitch too great.
If it’s underpowered for the prop it will be running warmer than it should be.
Steveeasy
westernman
Well-known member
Don't ever run the engine continuously at more than the maximum achievable revs minus 200 rpm to avoid overloading the engine. I.e. 2800 rpm in your case.
You are losing a lot of power you have paid for! May be you will wish you could use it in adverse conditions.
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