Trouble getting on plane

[Grew168ss]

90 merc 4 stroke, 16’8 boat, 21p 13 1/4 laser ll, motor runs good, trim works well, hydrofoil, rpm are around 5800-6000 WOT.
went out for a tour last night just me and son, empty boat. Having a hard time getting up to plane, once on plane goes like stink. Take roughly 30-45 seconds to plane. Rather small boat for motor size. Took off a 12 3/4 aluminum 21p replaced with stainless. Won’t get on plane at all with a tube or ski. Any ideas??

 

[Jimwhall]

I was going to say the pitch seems pretty steep for a 90.. my 112 SPL Johnson was running a 15-17 for tubing/towing on a 16' Sylvan and hitting it's redline of about 4800rpm.... but your RPM's at WOT seem good (not sure what the rev limit is for the 4 stroke merc).

Regardless... if you are looking for better hole shot and towing capability I'd reduce pitch to maybe 19, and maybe consider a 4 blade prop. You'll have to watch the RPM's at the upper end but it should help.

YMMV but I had a great deal of luck with Turning Point. They were great in helping me get the right prop, and even had a 'pitch replacement' program for when I went too steep.

 

[roscoe]

Trim all the way down at take off?

Weight of boat?
what boat? age? waterlogged?

hydrofoil my be holding you back if engine is not mounted at proper height.
Foil has to be out of or just skimming across the top of the water when on plane.

 

[Texasmark]

The boat in my Avatar is my stock 90 hp 2 stroke (2.33 gearbox) pushing my 16 1/2' aluminum bass boat kissing 50 MPH with a Ballistic 13x24 3 blade SS. Add a person and I could do 45 easily. I also had a 21P of the same prop and would overrev easily at WOT....not enough pitch.

My best hole shot was with the engine trimmed a little past 90* to the line of the hull. I'd firewall the throttle and it would ventilate but that allowed the RPMs to jump up (like happens with ported props but Ballistic isn't ported) causing the boat speed to immediately fall off, allowing the stern to drop which would submerge the prop more, terminate the ventilation for that instant, prop would grab, but this time at a higher RPM providing a significant increase in thrust and the process would repeat...about 3 oscillations of that and I was gone. I doubt 21P is your problem.