Increasing prop pitch

[DAKA]

I went to a huge amount of trouble to set my boat up with a Prop pitch that allows max RPM thinking this was right for max performance including fuel economy.

For years I have driven my cars round town in 4th gear and not 5th, as I was told this was more efficient than over working an engine.

I have recently been taking notice of my fuel flow meter in my car and I have noticed that 6th gear at 30mph is very economic, poor acceleration.

If I deliberately over pitch my boat prop ( like running in 6th gear ) will the fuel economy increase as the car does ?

If my engines only rev to 75% does it really matter as I will be at cruising speed anyway.

Are these engine computers/flow meters a load of **** ,
6th gear in town cant be right can it /forums/images/graemlins/confused.gif

( question is not about saving a few egg cup fulls of fuel in my car, I have no intention of staying in 6th, just an experiment for the boat)

 

[TonyJones]

AFAIK, you should always be in the highest gear possible that does NOT overload the engine or cause it to lug. The best economy comes from using as few rpm as possible but once you get into that 'lugging' area - especially when going uphill or accelerating - the fuel consumption goes up and the engine can suffer long term damage.

With marine diesels the idea of propping for max rated rpm is that during the cruise the rpm will drop down more into the area of best specific fuel consumption. If you overprop, not only is top speed reduced but cruising fuel consumption is also adversely affected.

Best wishes
TJ

 

[DAKA]

Hi Tony,

Thank you, I agree 100% that is what I would expect.

The flow meter suggests otherwise in that forcing the engine to labour does not increase fuel use, it actually reduces significantly.

Can someone else try it on their way home to lunch ?

Diesel engine with flow meter/fuel computer set to instant MPG,

30mph 4th gear
30 mph 5th/6th gear with engine labouring

 

[[2068]]

You might well get minimum fuel usage with minimum revs, but that's not the point.

With a car, unless you live in the mountains and have overweight children and heavy shopping, it's very hard to "overload" an engine.

With a boat, it's dead easy: just over prop it and run it for a few hours. You will then be forcing it to produce all it can at lower than the designed revs. The resulting bill for the rebuild will outweigh any fuel savings.

dv.

 

[DAKA]

Thanks, I asked and I have listened, I will leave well alone.