Alumacraft Porpoising

Hi Randy,

Thanks for taking questions as itís hard to get guidance on boat issues. I have a 2004 Alumacraft Trophy 180 with a Honda 130 HP 4 stroke. I estimate the rigged weight at 2400 lbs. With 3 guys and gear it is more like 3000 lbs. I have a kicker on the back with a 6 gal tank in the splash well. The motor is one hole above all the way down which puts the prop shaft center at 8 inches below the keel. At speed the antiventilation plate/hydrofoil runs about 2 inches below the water. I have installed a planning hydrofoil on the motor. I have 2 props. The original Solas 13 1/4 x 19P 3 blade which runs 5800 RPM 35 MPH GPS and a Honda 13 1/4 x 13P 4 blade which runs 6000 RPM 35 MPH GPS. The 4 blade has a good hole shot. The 3 blade holes hot is poor.  I have tried a Stiletto 17P & 15P 4 blade and the max speed was 35, but the max rpm was 4800 & 5200 respectively. The hole shot was pretty good with the 15 P.  I also tried a Merc Trophy 14 1/4 x 17 P 4 blade which hit 35 at 4750 RPM, but the hole shot was poor. The boat seems to porpoise fairly easily with minimal trim. I could not notice improvement with any prop.  It porpoises at about the middle of the range of the bolt holes for the trim pin. I am looking for a low minimum planing speed to hold in rough water.  A few extra MPH on the top end would be nice, but is less important.  What are your thoughts on raising or lowering the motor to minimize porpoising? Do you have any other set up suggestions or is this boat under powered for the load and for use in rough water?

Thanks,
Ken Giesen

Ken, 

Lower the motor down and run the 4 blade. The porpoise is partly from the high engine and partly from the slow relative speed. The basic boat weight is very high for the horsepower.

Randy

 

 

Please submit whatever questions you have to IHRTechTalk@yahoo.com.  All questions and answers selected will be displayed on this page as they come in.  Note: though all boaters are welcome, priority may be given to I.H.R. members.  Randy can not be held responsible for any advice given.  Though his information and expertise is second to none, he has no control over what you do with your boat.  It is up to you to boat safely and act responsibly, and his advice is only to be used as a guidance for your high performance boat/motor of which you are the one responsible for the risks involved.

Thanks,
Mark C.

 

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