vic008
Well-Known Member
To tell you have the correct prop, then the engine should reach the same rpm ingear and out. Is this statement correct?
RPM in gear &out?
- Thread starter vic008
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DJE
Well-Known Member
Don't think I fancy running the engine up to maximum rpm with no load on it. Doesn't feel right somehow.
But I think the correct statement is that the engine should just reach the speed at which it produces maximum power and not go any higher if the prop is the right size. But some people deliberately over-prop to reduce engine noise at cruising speed.
But I think the correct statement is that the engine should just reach the speed at which it produces maximum power and not go any higher if the prop is the right size. But some people deliberately over-prop to reduce engine noise at cruising speed.
VicS
Well-Known Member
No . It will do that if it is underpropped
awol
Well-Known Member
Diesel engine is governed so throttle position should generate the same engine revs whether loaded or not unless the load exceeds the engines capability, usually indicated with black smoke. Running the engine at full throttle with no load is ok.
Do not try this with a petrol engine!
Do not try this with a petrol engine!
knuterikt
Well-Known Member
No.
You should reach manufacturer max rpm at full throttle with gear engaged and boat running free.
You should reach manufacturer max rpm at full throttle with gear engaged and boat running free.
Graham376
Well-Known Member
+1
When we fitted the new Featherstream, those were Darglow's comments when calculating the pitch stops. Can't be achieved tied to the pontoon.
jwilson
Well-Known Member
In principle yes, but particularly for sailing yachts with fixed props a small degree of "overpropping" is helpful to keep revs a bit lower when motor-sailing, as long as the engine is powerful enough for the hull. My Yanmar has max revs of 3,600 but in calm conditions flat out means 3,400 which with a clean hull and prop gives 7.8 knots.
My gut feeling as the torque curve starts to decline above 2,500 rpm is that a bit more overpropping could be better for most "cruising revs" work, which for me is 2,400 to 2,800 rpm which gives about 5 to 6 knots. If the engine's on I don't want to be going much slower often.
lw395
Well-Known Member
This is common practice these days.
If you overdo it, the downside can be a lot less thrust available in situations like bashing into waves or if the keel is in the mud.
Tranona
Well-Known Member
The engine is governed to a maximum RPM with no load, so you can't over rev it. That maximum should be on the data sheet for the engine. The revs you achieve in gear is primarily a function of the load you put on it - that is the size of the prop. The manufacturer will specify the MINIMUM revs you should achieve in gear in flat water. This is usually around 5% below the rated maximum. So jwilson's 3400 out of 3600 is acceptable. On my Volvo the minimum is 3050 out of 3200. This assumes the engine is in good condition and capable of delivering full power if asked.
So, first establish what the maximum revs should be. Then run it at full throttle in neutral. If it achieves that take the boat out for a series of runs at 200 rpm intervals from 1800 (assuming your maximum is 3000+) and log the speed. establish what the maximum revs you can achieve. If it easily gets up to the rated maximum it may be underpropped, if it fails to get up to the required minimum then it is likely to be overpropped. assuming the engine has enough power to move the displacement of the boat, then maximum revs and correct prop should give you hull speed.
So your statement is essentially correct but you need to understand why, and in particular what you can actually achieve from the engine.