Changing prop diameter.

[Joe Monaco]

I was using a prop calculator from a different web site and I’m wondering about the result. I’ve got a 22000 lb “ avg planning” hull. About 28 feet of water line, 1:483 trans, 420 hp cats @ 2880 WOT and 4 blade 20 x 24 I think cupped wheels. I cruise at 2350 about 20-22knots conditions/weight dependent. The calculator says I should have 21.2 x 22.3 cupped or 23.5 no cup. What does the cupping do for a props performance? If someone can tell if mine are cupped I’d appreciate that feed back too. What would going from 20” diameter to 21 “ do for a boat like mines perf? Anyone have any similar real world experience changing props that could shed some light on my pondering? Is this something worth looking into or would new perf results not warrant the investment.

Topic

[donald roth]

Joe,
If you want to get a handle on what’s going on with your props, it’d be a good idea to read Dave Gerrs book “ The Propeller Handbook”.
It’s not a tech manual, it’s easy to read and understand.
Your best bet is always to work closely with a reputable prop shop in your area.

[Joe Monaco]

Thanks Tony,

When the boat was new to me a few years back I had 1 inch of pitch removed and the blades scanned by the local San Diego prop folks. The boat is heavy and the performance is ok but everyone always would like it to be better.

I contacted ACME via their web site with my boat/motor info. I’ll post what they come up with. I’m not sure the prop technology has changed enough from the boat’s stock props to warrant the investment but I’m curious to see what’s available today.

[Tony Athens]

The question you are asking will depend on the quality of the prop(s) you have now ( they need to be scanned & looked at by a high end prop shop that has learned & seen the difference between a “decent” prop vs. modern prop designed by a company like ACME or Weem……………… Using the HP from the engines is one thing, but making that HP move the vessel fwd ( instead of just “churning water”) is way different…. Basically is comes down to “slippage” vs efficiency.. ..

So, if you really want the most performance from the GPH you burn (MPG) , you have get your props to a shop that really knows the difference………………………. From personal use for close to 6 yrs now, ACME has shown me that they are a cut above on Planning Hulls, plus they are affordable..

Tony

[Joe Monaco]

Thanks for the info. Yes I’ve got probably 3 inches or more room to allow an increase in the diameter. I’m wondering what is the benefit of increasing the dia? Yes the calculator drops my current pitch while increasing diameter. Why would anyone want a decrease speed if not overloaded? If increasing diameter and decreasing pitch only does that why would anyone do that?That’s why I must be missing something in the prop calculators conclusion.

Has anyone increased diameter of their props while decreasing the pitch and been happy they did? What is the improvement you’d be looking for?

[Rob Schepis]

What is your current tip clearance? 3″ or ??. Do keep in mind that a lot of variables effect performance, if you go bigger dia you may need to back off pitch to maintain proper WOT rpm’s thus you may loose some speed… a lot of variables…. You do not want to run no-cup props as the cup adds efficiency & speed and cuts down on cavitation

[Rob Schepis]

Cup could be stamped into the hub, maybe, maybe not and if cup was altered it may not have be re-stamped. You can feel the cup on the trailing edge.

[Tony Athens]

Your props looked quite cupped to me.. A better pic send direct to me would help confirm it.. Look at the trailing edge and feel/see the “tweak” in the shape..

I just looked closer–Can’t really tell.

Tony

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