21 Superboat porpoising

**capteliminator** · 07-15-2018, 12:08 PM

I have a 1986 Superboat 21 with a new 300xs. It's on a 10" manual plate. 30 big eared chopper. Boat is porpoising badly. Tried moving motor up and down, which helped the porpoise but it's still there. Also tried a 27 trophy plus and it was worse. Any ideas or advice is greatly appreciated.

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**vnemous** · 07-15-2018, 12:23 PM

First things first get rid of them rollers and put some bunks on it. Put a long straight edge to the bottom and see if its straight any thing beyond that is weight distripution, trim and prop. JMO

**nitro_rat** · 07-15-2018, 01:07 PM

Originally Posted by vnemous

First things first get rid of them rollers and put some bunks on it. Put a long straight edge to the bottom and see if its straight any thing beyond that is weight distripution, trim and prop. JMO

**capteliminator** · 07-15-2018, 01:42 PM

Boat is solid and bottom is straight. I fully rebuilt that roller trailer and there are no dips in the bottom were the rollers rest!! Boat had a V8 Evinrude on it when I bought it. It had a 6" plate it and didn't porpoise. Maybe need less setback?

**gotboostedvr6** · 07-15-2018, 02:21 PM

What mph are you running when it starts porpoising? Does it go away at high speeds?

My 21 had a slightly soft transom which induced porpoise. Replacement cured it completely.

The roller trailer has to go. It will eventually destroy your hull.

**capteliminator** · 07-15-2018, 03:21 PM

Transom dead solid. Porpoise goes away around 45 to 55.under hard throttle it goes away till about 70 then starts again. I checked transom when I changed plates and it's solid. I'm gonna bolt the 6" back on and try again.

**XstreamVking** · 07-15-2018, 04:52 PM

Get a thru hub prop on it and see if the porpoise goes away....Gives a lot more stern lift.

**vnemous** · 07-15-2018, 05:52 PM

whats your prop height in relation to bottom? How high have you been? I run 1 1/2" above pad

**Fullthrottleguy** · 07-15-2018, 06:02 PM

When you're full out how fast will it go and how high is the roost? I think you need more setback or more blade area as it appears to be struggling to hold the bow up. I had the same problem, and went from 6 to 10" of setback, still needed more blade area to help leverage the bow up at slower speeds. Bravo 1 xs with it's 4 blades and 15.25" diameter did the trick.

**capteliminator** · 07-16-2018, 08:57 PM

I ran the boat at all heights. I started at 1/2" up, then went down to 1/2" below. Went down more, then tried it all the way up at 1" above no luck. I ran a 300x before with a 12" and a 14" plate and it was better but would porpoise at 65 up. Changed to the opti motor and shortened setback due to 40 lbs heavier. Boat has never carried the bow well with the merc. Ordered a bravo 1 and drug out the original jackate. It's actually a 5". Do any of you guys run a power jackplate? I'd love to keep the boat simple but a power plate seems lime it would really help.