River rocket porpoising

[Small strokes]

First off I'm new to stvs and tunnel hulls and am slowly learning how to drive them at higher speeds. I recently finished my river rocket and have about 8 hours of seat time in it and am just now starting to open it up. My hull weighs around 750 lbs with 8 inches of set back and motor mounted up the whole way and a 30p prop. The problem I'm having is when I start to reach 90/92 mph the boat starts proposing horribly. I've tried to throttle through it, I've tried bumping the trim up a little and down and it still continues to do it. I usually have my trim indicator between 6 and 7 so the motor is pretty much level with the boat. If anyone can shed any light on what I'm doing wrong here it would be greatly appreciated thank you for your time

[mjw930]

Measure the actual X-Dimension (centerline of prop shaft relative to bottom of the boat). That information will help others diagnose your problem. "All the way up" means nothing.

[Small strokes]

Propshaft centerline is exactly level with the bottom of the hull.

[JPEROG]

Propshaft centerline is exactly level with the bottom of the hull.

It will never handle good there. You need to be 1 1/2 to 1 5/8 above the bottom depending on the prop and water pressure. Go as high as you can while still maintaining pressure. This is assuming that you have a good gear case with low water pickup. With the motor set at its current height it is trying to ride up on the case and there is no stability there so it will rock back and forth causing sponson slap. After you get the height set correctly, then run the boat with positive trim up to 70 mph or so (Where the air is carrying the bow), then bring the trim down as you continue to accelerate up to speed. Fly it level, have fun, and wear a jacket.

Joe

[STV_Keith]

Do you have a ski or Mod VP bottom? My Pro Comp Ski has the Ski bottom. With 6" of setback, I run 1-1/4" above the bottom. With 8", you'd want to come up another 1/4". What prop are you running as well? Props can dramatically affect bow lift or stern lift.

[Small strokes]

Mod vp hull with a 30p lightning et. It's not really chime walking it's more of a porpoise. I understand it's not going to ride like a Cadillac but the way it's slapping can't be right. I'll try raising the motor up a 1/4" as suggested an see how it does. Thank you for the replies

[Small strokes]

I have a sportmaster lower on it atm. The problem with raising it 1 1/2 to 1 5/8 is I can't unless I raise the setback up higher on the transom. Right now I probably have 1/4 to 1/2" that I can move the motor up

[Euroski]

I can be there Monday... afternoon if the weather is good. I'm 30minutes away. Check your PM!

[superboattwin250]

I run mine 1 1/2 to 1 5/8. They usually hop around 65. Then it should flatten out and haul the mail.

[Euroski]

What prop U running? Another thing is U don't want to sneak up on the porpoise zone, but run WOT until above that zone.

[Small strokes]

30 lightning et. I've tried to throttle through but no luck. I got a video of it I can't figure out how to post it. Send me your cell number and I'll send it to you

[Michael Dixon]

Usually this condition is a result of one of three problems. Either motor is too low or not enough trim or there is a hook in the bottom. According to which gear case but 1 3/8 above is where I run a sport master. The lip beside the pad is where you measure from and not from the bottom of the pad. You'll have to add the distance from pad to lip back in the equation. I think it is 3/4? Nine times outa ten the porpoising is just asking for a couple bumps of trim

[STV_Keith]

How much setback is your bracket?

[johnlysh]

If your measurement on x dimension is correct and your prop shaft is even with the bottom that is your problem. Like Joe said you need to be around 1 1/4" to 1 1/2". Bringing the motor up 1/4" won't fix the problem. Remount the Jack plate on the transom and start around 1 1/8" up.

[4Speed]

Any chance you can leave the jack plate where it is and just raise the motor a bold hole or two on the jack? The bolts are much easier to access on the jack than trying to access the transom bolts for the jack under the hatch.