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Old 18-09-2018, 08:05   #1
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Methodology for calculating revised prop pitch

I have reason to believe I'm significantly over-propped, which I can fix at haulout easily enough, but I would like to get my pitch numbers right. With a low-hours Beta 60 and a clean, if heavy-displacement, steel full keeler, I hit cruiser too low (1800 RPM) and can't get above 2,050 RPM in relatively flat seas when I hit hull speed under motor. I do not have evidence of overheat or overpressure, however, which is good.

I've input my numbers into this worthy app (Vicprop - Prop calculator for Displacement and semi-displacement hulls), but the fact is, I have a 19 inch four-blade feathering prop (a Variprop D-107) on a steel full keeler and that isn't going to change. What can change is the pitch in forward (overpitched in reverse means I can stop and back down "with authority" and I don't care if I'm lugging for 30 seconds). But I do care I can't get the engine revving at hull speed a full 600 RPM short of its rated max, meaning unneeded strain and poorer fuel economy at cruise speeds (75-80% WOT).

Any thoughts on how to calculate proper pitch for a given prop width are welcome. It's probably a drop from the present 15 to 11 or 12, but I'd rather do the math before I tinker with the stops in the hub. As a side note, everyone unhappy with their fuel burn or their RPM should understand this topic as it really makes a large difference in terms of efficiency and wear. Here's the prop. The original was an 18 x 13 three-blade fixed prop; this is a 19 x 15 (can be changed) four-blade feathering. It works a charm in every other respect.
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Old 18-09-2018, 10:16   #2
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Methodology for calculating revised prop pitch

Itís probably not linear, but if you have a prop protractor or similar and can measure pitch, Iíd drop it percentage wise equal to what it falls short percentage wise on RPM.

In other words if your 25% short of RPM, Iíd reduce pitch by 25%.

Overproppimg does not increase thrust, it decreases it.
To develop max thrust the prop needs to absorb all the power from the engine at its max power, which is at one specific RPM or course, usually quite close to max RPM, and RPM less than that and the engine is developing less power and therefore less thrust.

Personally, I like to be slightly overpropped as I often motorsail and a little sail assist puts me to being correctly propped.
A little, not a lot and you do have to use your head and not run her at full throttle if you do this.
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Old 22-09-2018, 13:56   #3
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Re: Methodology for calculating revised prop pitch

Typically every inch of pitch equals a change of 200 rpm. So in your case 12 would be a good start. But those are averages, your boat may get a different result.

If you can provide your laden displacement, LWL, beam at the waterline, gearbox ratio and how many bearing surfaces you have I can give you a better estimate for your 19" prop.
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Old 22-09-2018, 14:02   #4
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Re: Methodology for calculating revised prop pitch

I just phoned the manufacturer (max prop) with all my info to hand and the nice people told me over the phone what I would get with each degree of pitch. Awesome.

Find a local diver (the decent local mechanics will be able to recommend one that can do it) and you don't need a haul out.
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Old 23-09-2018, 10:30   #5
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Re: Methodology for calculating revised prop pitch

An issue with using prop calculators is that a max prop and most feathering props are flat, planar blades.
Usually a prop has an aerodynamic profile, an airfoil and blade twist to help equalize thrust across the blade and make it more efficient, Maxprops donít.

So Iím saying I donít know how similar to degree changes a max prop is to an aerodynamic blade with twist, and maybe even cupping.
They are significantly less efficient of course.
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Old 23-09-2018, 17:50   #6
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Re: Methodology for calculating revised prop pitch

If you call a good prop shop or Maxprop, they will get you in the ballpark. Delfin was overpropped a bit and I was off top RPM by around 200, so flattened it about an inch. Wonderful results, by the way. The engine seems happier, and I idle at a slower speed. Being off by 600 is a whole lot of over propping....
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Old 23-09-2018, 21:57   #7
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Re: Methodology for calculating revised prop pitch

Quote:
Originally Posted by S/V Alchemy View Post
I have reason to believe I'm significantly over-propped, which I can fix at haulout easily enough, but I would like to get my pitch numbers right. With a low-hours Beta 60 and a clean, if heavy-displacement, steel full keeler, I hit cruiser too low (1800 RPM) and can't get above 2,050 RPM in relatively flat seas when I hit hull speed under motor. I do not have evidence of overheat or overpressure, however, which is good.

I've input my numbers into this worthy app (Vicprop - Prop calculator for Displacement and semi-displacement hulls), but the fact is, I have a 19 inch four-blade feathering prop (a Variprop D-107) on a steel full keeler and that isn't going to change. What can change is the pitch in forward (overpitched in reverse means I can stop and back down "with authority" and I don't care if I'm lugging for 30 seconds). But I do care I can't get the engine revving at hull speed a full 600 RPM short of its rated max, meaning unneeded strain and poorer fuel economy at cruise speeds (75-80% WOT).

Any thoughts on how to calculate proper pitch for a given prop width are welcome. It's probably a drop from the present 15 to 11 or 12, but I'd rather do the math before I tinker with the stops in the hub. As a side note, everyone unhappy with their fuel burn or their RPM should understand this topic as it really makes a large difference in terms of efficiency and wear. Here's the prop. The original was an 18 x 13 three-blade fixed prop; this is a 19 x 15 (can be changed) four-blade feathering. It works a charm in every other respect.


Call Accutech Marine Prop in New Hampshire. I bought my 24" Variprop 3 blade from them. They know what they are doing and can definitely help you out.

http://www.accutechmarine.com/

I also have a Beta 60 but haven't had sea trials yet so no performance numbers yet.
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