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Please be careful with your calcs.

I can't say too much at this stage..

I will say understand your swept area

..And that adding a safety margin to a safety margin ruins your engine,s efficiency.

Think Seriously about where you want your engine to run.

It should be loaded at 3/4 revs...

NOT WITH 20% left over..

Do the proper real world maths..

20% plus 20% manufacturer allowed is nearly half of your top end power !!

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Do you want your engine to be in its sweet spot in flat water, choppy water, even rough water off a lee shore, getting through that reef pass against the ebb, motorsailing, and for added complication on a catamaran, with one running or two? Not an easy decision.

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Your prop should be sized and pitched so the boat, with your normal load, a clean hull and flat water will achieve full manufacturers rated max RPM at full throttle. For most engines, cruising revs are 80-90 percent of that. Diesels like to work.

This is where adjustable props are good. Many boats have the wrong, or at least not the best props, as changing them is expensive.

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Volvo wouldn't warranty my new engine unless I could pull manufacturers max RPM. Had to ditch my old prop and had them do the calcs and supply a 3 blade Volvo folding prop. My old prop would only max out at 2700 RPM so would have strained the engine. 

New prop max out at 3200 RPM as per specs. 

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I've been entertaining myself by reading "The Propeller Handbook" by Dave Gerr.

 

I'm yet to decide if specifying a propeller is a science or an art form. It certainly can't be described as an exact science.

 

I can understand that specifying a fixed pitch prop would be a stressful affair. Certainly gives a big advantage to easily varied pitch props such as Kiwiprop's and other feathering props. Even though the blades aren't as efficient as a fixed blade, It is simple to adjust and match the pitch to achieve max RPM, or optimise cruising speed at peak torque. All that means that, even with the less efficient blade shape, I expect the overall package is more efficient than many fixed pitched packages, because the prop is a better fit to the engine and boat.

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Volvo wouldn't warranty my new engine unless I could pull manufacturers max RPM. Had to ditch my old prop and had them do the calcs and supply a 3 blade Volvo folding prop. My old prop would only max out at 2700 RPM so would have strained the engine. 

New prop max out at 3200 RPM as per specs. 

That's fair. Its bad for an engine to be over-propped.

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That's fair. Its bad for an engine to be over-propped.

Thats the issue with aiming for maximum cruise speed at peak torque, it means you can't get max revs, and generally voids the warranty on new engines. The irony being not a lot of engines are ran at full revs anyway (yacht engines, launch engines are a different proposition).

 

Interestingly Beta have indicated they don't see this as a major issue (targeting higher cruise speed), their logic being that engines need load and its better to load it lower in the rev range than under load it. The caveat is not to over-load it though (i.e. bog down and blow black smoke).

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Down this road last year when cleaning our prop for prop speed got the numbers off it and looked it up on briski's website. Wrong size prop. Took it to Arthur at the prop shop with details of engine and gear box ratio ( a nightmare trying to read that off the back) and got it repitched. What a difference! The engine doesn't blow black smoke and when we had the head off was not coked up so all running sweet. One thing I do though is adjust throttle settings every now and then when underway. Engines don't like spending hours on one setting, it "polishes" the cylinders I've been told.

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Because the kiwi prop is much lighter than any fixed bronze prop, as all the weight involved is in the boss and minimum weight at the blades.

I have had to re-pitch mine well past the optimum recommended in the manual to get close to the speed I had with the fixed original.

A downside I do have is that it has a habit of jamming in ahead mode and the spring not being strong enough  to allow it to either free wheel or feather. I have to dive and hold a blade while rotating the shaft with a 1/2 inch drive tommy bar.

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