8 HP vs 15HP

So, I have a 14’ v-hull with a 1993 mercury shortshaft 8 hp that will do about 17mph. I tried a 1987
Evinrude 15 HP long shaft (5 “ extension) and testedit’s mph and got only 19.5 mph (using my hand held
gps). Does that sound correct or maybe I could trychanging the ANGLE of the 15 hp? Just seems like I
should get MORE speed out of the 15 hp then that. Irealize the long shaft is too long but??

Sounds like the long shaft could be your problem. When you hang the engine on the transom, the cavitation plate should be just about even with the bottom of the hull. My guess is the long shaft is hanging much lower which will cause excess drag and effect performance. Another issue could be an incorrect prop on the 15HP. If the pitch is too shallow you'll be spinning a lot of RPM but not getting forward thrust.

As a point of reference I have a 9.9HP short shaft that I've used on a 16' aluminum boat. With 2 adults, trolling motor, battery, and fishing gear I can get 15MPH on a handheld GPS. So, if the 15HP is propped right and the proper shaft length I would think you should see better than 19.5MPH on your boat.

KJ

My '86 15 SS on a 14' boat with just me in it, can get around 28mph on my gps