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#1 idlerboat

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Posted 09 November 2018 - 12:56 AM

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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#2 Priscilla II

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Posted 09 November 2018 - 06:19 AM

Took me quite a bit of trial and error to find that out too.
Prop manufacturers calculations were miles out and two props later I found that elusive peak torque running sweet spot.
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#3 darkside

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Posted 09 November 2018 - 06:51 AM

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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#4 Island Time

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Posted 09 November 2018 - 07:07 AM

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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#5 chariot

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Posted 09 November 2018 - 01:56 PM

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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#6 Fish

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Posted 09 November 2018 - 03:28 PM

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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#7 Island Time

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Posted 09 November 2018 - 03:36 PM

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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#8 Fish

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Posted 09 November 2018 - 03:45 PM

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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#9 Black Panther

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Posted 09 November 2018 - 05:53 PM

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


.




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#10 Fish

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Posted 09 November 2018 - 07:19 PM

Bet you are a lot of fun at a party

Well BP, I've seen those photos of you in your party outfit, and I think its fair to say you'd be a lot more 'fun' than me at a party.


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