POWER BOATING CANADA
VOLUME 14 NUMBER 3 PAGE 14
ASK THE EXPERT BY STEVE HORSFALL
TRIM TABS TAKE CARE OF PORPOISING

I have a 1991, 29-foot Mach 1 Endeavor (approximately 6,250 pounds). It is powered by completely stock 460 Fords with thru-hull exhaust and King Cobra outdrives.

I am currently running 25-inch hydromotive props. The boat does approximately 70 miles per hour, turning 4600-4700 RPM with the outdrives virtually all the way down. If I raise the outdrives very much I get severe porpoising.

The hole shot is excellent but I am not able to hold it wide open for very long because of the porpoising. This especially happens when on smooth water and a full tank of gas. I was wondering if a prop change would help with the porpoising and be able gain mph. I just do not want to lose top end.

R. Meier
Via e-mail

Your problem is not uncommon for boats which are heavily loaded towards the stern, moving weighty items towards the bow will help somewhat but this is not a very practical solution in some situations.

Having a large set of hydraulically adjustable trim tabs is probably the easiest cure. Set your outdrive trim angle out for the most speed, then adjust the trim tabs down for the least amount of porpoising. Experimenting with the two controls should all but eliminate your problem.

Make sure the bottom is true and straight, especially in the centre, fore and aft direction. You check this by placing a long straight edge from the transom towards the bow, about four to six feet. There should also be no humps or hollows along the edge in the centre of the boat. If there are hollows, they should be filled in and sanded until the bottom is true. If there are "rockers" or high spots, they will need to be ground down flush and sealed. The hollows or "hooks" are more common causes for porpoising than the "rockers".