New to me boat porpoising

[wob]

Bought a 1644 gator trax hunt deck (17.5 feet) with a breakaway plate and a yamaha 70 4stroke on it. Previous owner had a mud motor so this setup is new for the boat. At higher speeds, the boat starts to porpoise uncontrollably unless there's a good amount of weight at the bow. I've included some pics that hopefully show how it's setup good enough. Current prop is a 15 pitch. Don't have a tach, but it seems to run pretty strong through the rpm range until it starts bouncing. Should I raise the motor (only one bolt left) or start screwing with props?

Tried to upload pics, but this site is getting unusable.

[murraywader]

Or maybe get a jack plate?

[scdiver]

Where is cav plate in relation to bottom of hull?

Flat bottom rounded corners?

[scdiver]

Where is cav plate in relation to bottom of hull?

Flat bottom rounded corners?

[wob]

If I could upload pics, I'd have all of the answers.

[wob]

I'll try again...20240423_193658.jpg20240423_193658.jpg20240423_193658.jpg


Screw it. That took six minutes

[willyworm]

I'm fairly certain you have the wrong prop. Porpoising indicates that your prop is providing too much bow lift when you really need stern lift. Is it a hull designed for mud motors? If so they're designed to run flat. When you slap an outboard on the back with prop that wants to lift the front end of the boat it just throws everything off.

Without knowing more about the hull design I can't tell ya for sure what prop you need, but it sure sounds like it's not the one you got.

With a F70 the prop will be for a 4.25" gear case. It's likely gonna take some experimentation to find the one that works. Dan's Discount Props used to be an ideal option for procuring one because if it didn't work you send it back and exchange for another, at a small fee. But it's easier than taking a big loss of a new prop that didn't work.

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[wob]

Thanks ww. Was hoping you'd chime in. Unfortunately, I had a bunch of other pics that I can't upload.

[duckmeat]

weld 2 beads on bottom of boat where you would normally have trim tabs or wedges. had the same problem on the same boat. call gatortrax they will tell you exactly how it needs to be done

[Catdaddy]

.....and when all else fails buy a set of transom wedges and be done with it.

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[wob]

20240423_193658.jpg

[SouthernWake]

Call gatortrax but most likely a prop change is in order. They may know which one you want.

[Sheep]

Similar to what Miles said, Id try a ybs prop. You could try a knock off, but a real ybs will be better.

Also know that hull is not designed for that type motor so you kinda have an uphill battle. A hydraulic break away jack plate will make things easier too.

You got another $2500 to spend....

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[triplebeard]

Call gatortrax but most likely a prop change is in order. They may know which one you want.

This right here. Kent sets these up all the time, but I don't recall ever seeing a 1644 with more than a 40hp. When you call them, give them the hull VIN and see if they have record of it having the "outboard package" installed. It's different bracing.

[triplebeard]

Truthfully, and I've NEVER said this before, that F70 is too much motor for that hull.

[Rubberhead*]

The only thing I'll add to the above is that a dent (from going over stumps with a mud motor) in the under side of the boat can cause porpoising problems too. Just make sure there's not a large deformity in the bottom before chasing the other solutions.

[Goin Coastal]

4 blade prop and motor needs to be raised more with that much set-back.

[Fireman]

The farther you set the motor back, the higher the cavitation plate needs to be. It should be up about an inch or so over the bottom of the boat with that setback. start there and work which ever way gets better results. Might want to invest in a manual jack plate to make it easier. Set it where you need it and then lock it down.

[willyworm]

With much of the hull being in contact with the water that's more friction. More friction equals more drag. More drag means more prop slip. So like GC said, don't rule out a 4 blade. PowerTech makes good ones with a plethora of blade shapes, diameters and pitches. Might sacrifice a mph or 2 but you'll get better mpg and it'll likely turn better. Plus you can run the motor up higher while not losing traction.

You've gotten some solid advice to call GatorTrax.

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[wob]

I've got to run a kick up plate. I'd love to have a hydraulic jack plate, but I'm not running both. I'd have to mount a seat on the hunt deck to reach the tiller.

I don't think it's too much motor. I've got friend with an 1850 slowcountry huntdeck with the same motor, and we run about the same top end. I've got more hole shot, though. He's running a 16 pitch prop.

I'm going to borrow his prop, raise the motor, and try out some 2 degree wedges. I'll report back.