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Old 07-27-2014, 06:50 PM   #48
Nomad Willy
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City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,743
Delfin and others,
What I was trying to say in post #39 is that going 6 knots takes the same amount of power over propped, right propped or under propped. It should take more rpm under propped and less rpm over propped. Larry's numbers don't support that. Seems to take 2100 both ways. Shouldn't.

Over propped one should burn more fuel per stroke and under propped less. But under propped the engine takes more strokes and over propped less. Over propped the injectors are injecting more fuel to keep the boat at 6 knots. But the total fuel injected may not be as much as the under propped engine. Or it may be more. It depends on how efficient the engine and prop is making the required power to drive the boat 6 knots. The same amount of work must be done. With the over propped engine there are fewer power strokes and so those power strokes must make more power and thrust per stroke.

Bayview is right though. More information is needed to predict efficiency. I knew I must have been wrong in post #39 as what seemed to be ... didn't make sense. I was thinking that under propped there was less load and reduced fuel consumption. But it takes more strokes so it's not apples and apples.

Delfin when you flew you leveled off when reaching altitude thus greatly reducing power required. So you reduced pitch and rpm at the same time. That wouldn't be possible if you didn't change your angle of incidence.

Yes Scott the GBs were over propped as well as all the Willards. Willy w her original Perkins engine was 250rpm over propped and the boat was only capable of producing 33hp at the resulting rpm of 2750.

So I'm back to square one whereas most boats that are over propped have lower over-all fuel consumption. I think there is a fuel savings but the amount is about 5%. Someone said it was 15% and that may be possible but to get that reduction in fuel burn extreme over propping would probably be necessary. I prefer to be right on or about 50rpm under propped.

And Larry's Willard should get more speed at the same rpm w an inch more pitch.
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North Western Washington State USA
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