prop quandary

Good Morning all,
I have a Lancer 27 powersailor to which I've mounted a Honda BF 150 outboard. Prop sizing is giving me fits particularly as it's a counter rotation motor. I've tried various prop calculators available online and gotten answers from 16x11px3 to 14.6x9.9px3. The hull is modified by "surface piercing foils" alleged to allow the boat to attain speeds around 18 kts with this motor. The waterline length is 22". The displacement weight is 4600 lbs but adding another 1000 lbs, for fat folks, fuel, gear, water, and stuff seems close to me. The draft (with keel) is 4' 3", the beam is 8' 7", ballast is 1700lbs. Can anyone help me out with this?, the available calculators can't seem to cope with the hybrid design of the boat.

Thanks in advance

Rich

 

Reminds me of a sailboat I saw many years ago that had huge recessed trim tabs to make it into a hull that would plane (with a fairly large engine).

 

Nope, sorry, can't help you.

But I bet you got a really good deal on that motor!

-Tom

 

Are you over or under revving? What is the problem?

 

Using PropClac, "semi displacement hull" shows a 16 x 11, (15.75 x 11.15p ) @ 5000 engine rpm, providing 17.2 kts with 25% slip. This calculation uses a total displacement of 7300 lbs ( 4600 + 1700 + 1000 ).

If the balast weight is included in the 4600lb for a total of 5600 lbs., recommended prop could be a 16 x 12 ( 15.75 x 12.25 ) providing 19.6 kts with 22% slip

Comparing these numbers to the "Supposed" Speed of 18 kts, suggests the above calculated prop sizes shouldn't be too far off?

 

Hey fellas, Thanks for your responses, yes Idid get a really good deal on the motor, half price with 31 total hours on it, and yes the counter rotation is a pain with limited availability in prop sizes/pitches. The problem is underrevving, Tops out at 4100 or 4200, present prop size is 15 X 17P x3. the motor is said to gain 15 HP above 4500 when the VTEC Solenoid opens as well so performance specs change from 135 sub 4500 to 150 above. I wouldn't be bugging you guys about something this simple but it seems to me that the keel is a joker in the deck and can't really be accomodated by the normal prop calculators. I REALLY appreciate the help.

 

4100 - 4200 RPM with 17 pitch is what I would expect, especially with 135 HP. Reduce the pitch by 6 in. ( to 11P.) and you should see a significant increase in RPM,s. Based on your current results and PropCalc, I would suggest that a 16 x 11 (or 12) is probably the correct Prop with 150 HP

 

Just wanted to add: In addition to reducing Pitch to 11 or 12, you need to go to a 16 Dia to limit Slip and get max performance.

I believe the calculations (using a "semi displacement Hull" ) approximate your situation with a keel.

 

Thank you much Jango!

 

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