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  #1  
Old 11-21-2009
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Prop Size for Perkins 4-108

I need a prop size recommendation. I currently have a 2 blade 12x14; and it is terribly undersized. It generates almost no thrust below 1000 RPM and at 2500 I only get about 4.5kts out of it.

My boat is 41' LOA; with about 9 Tons displacement. I'm pretty sure the reduction on the Borg-Warner is 1.5:1. Engine is a 4-108 that generates about 35 HP max with max cruising RPM of 3000; WOT is 3600.

With the current prop if I go up above 2500 RPM I think the prop starts to cavitate (heavy vibration). I think this is due to the small diameter and high pitch; which really is not working correctly. I don't know what my max diameter is; the exit angle is shallow but I might be able to fit a 14-15" diameter prop (wishing I had measured this when hauled out 3 years ago).
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Old 11-21-2009
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I have a 42' 13.5 ton displacement sloop. Perry design; same underbody as a Valiant 40.

Perkins 4-108 with a 2:1 reduction gear & Walter V-Drive. Turn a 3-blade 17" Max Prop. Don't know what the pitch is.

For your boat with the 1.5:1 reduction I'd guess somewhere in the neighborhood of a 16 or 17" 3-blade prop with a 12-14" pitch. But, this is just a guess.

No doubt someone will be able to hit it closer for you.

Bill
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Old 11-21-2009
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12" seems really small diameter for that size boat. Regardless of size you will get much better thrust with a three blade, but more drag. Our boat spins an 18"X 12p with 62hp, 40' 12 tons. IIRC it is most efficient to use the largest diameter prop that will fit.
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Old 11-21-2009
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OK; so sounds like I need to maximize the prop diameter. I'll have to do some checking and see if I can find some pictures when the boat was hauled. The reason for the small prop was probably for racing it when it was originally owned down in Southern California.

An 18"x12 would probably be too large since you have more HP. But that's good info to help getting a ballpark size also. The 4-108 is rated up to 50hp; but that is at full throttle. Closer to 35-40 at cruising RPM.

West by North (Campbell Sailer Prop) spec'd a 3-blade 14"x10; which is lower pitch, larger diameter. That prop is only 1" larger on the radius; I wish I knew for sure if I can fit a 16"-18" prop.
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Old 11-22-2009
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I have 18"x10 3 blade on a 12.5 ton boat w/4-108 perk; it pushes us along @ 4.5- 5kts @ 2200rpm
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Old 11-22-2009
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Do you ever run it higher than 2200? Reason I ask is because there are times when I need to go up close to or above hull speed (3000 RPM) with the current here on SF Bay. It's rare but there are days when the wind shuts off and we have to motor against a wicked ebb. I'd like a correctly pitched prop so I can run the engine up to 3000 and get plenty of thrust. My current prop seems to cavitate between 2500 and 3100; if I go up to 3200 it smooths out but that too high for a normal cruising RPM. I suspect that my problem is being over-pitched and too small on the blade area.

Do you have a 2:1 reduction ratio 'pappy? Do you ever have cavitation or trouble running above 2500 or higher? I realize that many never run their engine up to max RPM.
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Old 11-22-2009
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Quote:
Originally Posted by KeelHaulin View Post
Do you ever run it higher than 2200?
rarely, but I have


Quote:
Do you have a 2:1 reduction ratio 'pappy?
yes, Borg Warner

Quote:
Do you ever have cavitation or trouble running above 2500 or higher? I realize that many never run their engine up to max RPM.
No, never to both questions, but I rarely run over 22-2300rpm's, the few times I have has been late at night, calm seas and I can feel the increase.
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