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Boat Forum / Building / March 2006Estimating best prop pitch with no tach.
I'm trying to come up with an ideal prop pitch. My dealer is of No help. And I have no tach. It's a 20hp Honda with 9.25" dia prop on a 16' planing dory. My original prop was 9" pitch which pushed the boat about 17mph. So I went to an 8" pitch which pushes it 22mph (speed by GPS by the way). Just listening to the revs seems a little low to me and sure enough, I calculate 2904 rpm with my 8" prop without considering slippage. Does anybody know how much I should add for slippage? Recommended RPM is between 5000-6000 so I need to get 'em on up there. ... or is there a better way to do this without a tach? (thinking maybe I should just break down and buy one). Any suggestions appreciated. Thanks in advance Rick Rick, You left out the reduction to the prop shaft of 2.08 in the current version. That leaves you over 6k with the 8". Remember: The Honda vs everyother little bike discussion => Hondas don't sound as fast as any 2-stoke when, in fact, are reving highter. Matt Colie > I'm trying to come up with an ideal prop pitch. My dealer is of No > help. And I have no tach. It's a 20hp Honda with 9.25" dia prop on a [quoted text clipped - 10 lines] > Thanks in advance > Rick Doh!! You're exactly right! Thank you kind sir. Issue resolved! Rick >You left out the reduction to the prop shaft of 2.08 in the current >version. That leaves you over 6k with the 8". [quoted text clipped - 18 lines] >> Thanks in advance >> Rick You mean I just wasted all of six minutes on faulty data? Bah, humbug! :-) Brian W >Rick, >You left out the reduction to the prop shaft of 2.08 in the current [quoted text clipped - 19 lines] >> Thanks in advance >> Rick >I'm trying to come up with an ideal prop pitch. My dealer is of No >help. And I have no tach. It's a 20hp Honda with 9.25" dia prop on a [quoted text clipped - 10 lines] >Thanks in advance >Rick Let's just chew the rag. If you want the motor to spin at 5500 rpm at some speed, what speed would it be? Wild guess: knowing 3200rpm puts you at 22 mph so 5500rpm would provide about 55/32 as much power = 172 % cube root of 1.72 is 1.20 so the target speed is 26 mph. What pitch is called for? 1/5500 (minute/rev) X 26/60 (miles/minute)X 1760 (yards/mile) X 3 (feet/yard) X 12 (inches/foot) = 5 ( inches/rev) no slip plus 10% for slip = 5.5 inch pitch. Ah but now, what is the appropriate diameter? Enough to absorb the power, of course! Wild guess 8 3/4 in to 9 1/2 in. (Can't recall the trades for diameter...) Your mileage may undoubtedly vary. Brian Whatcott Altus OK It sounds as if you have solved your problem, but if a tach is what you are after, I have had good luck with a $24 unit sold by Tower Hobbies for tuning model airplane engines. you would have to put a piece of tape on the flywheel to mimic an airplane prop. I did this on my yanmar diesel and it worked nicely. Discovered that the tach in my instrument panel was off by quite a bit. good luck, Andy >if a tach is what you >are after, I have had good luck with a $24 unit sold by Tower Hobbies >for tuning model airplane engines. >you would have to put a piece of tape on the flywheel to mimic an >airplane prop. Thanks. Yep, this prob solved but the tach idea looks interesting. If I understand correctly, it would work by running the motor with the cover off and pointing the tach at the flywheel? .... and what kind of tape? If it works the way I understand it, you could measure rpms of practically ANYthing with it. Is that right? By the way, they sell three tachs. I read up on only one. Rick it would work by running the motor with the > cover off and pointing the tach at the flywheel? This is correct. .... and what kind > of tape? If it works the way I understand it, you could measure rpms > of practically ANYthing with it. Is that right? I used some shiny foil tape used for ducts (not duct tape) and it worked for me. This was verified by a marine surveyor who also measured my RPM's. > By the way, they sell three tachs. I read up on only one. > > Rick for a gas engine, you can also use a "tune up" type tach. These devices use a closepin type arrangement which clips on the plug wire, and alligator clips for 12volts. These were used by shade-tree mechanics fooling around with cars. I have one made by sears. good luck, Andy |
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