porposing at full throttle

Hey guys, thought I would see what the experts here thought before I started throwing money at the problem. I have a 16ft starcraft open boat with a 1993 evinrude 40hp elec tilt/trim motor. My problem is that at pull throttle, the boat porposes, not alot but enough to be annoying. I know I can shift some weight forward, but I would rather not have to lengthen my battery lines and go through all that. Would a hyrdofoil cure the problem? I can't trim the motor down far enough to get it to stop so I thought this might work...my only question though is will it work at full throttle? Thanks.

Pics would help?

Where is your cavitation plate in relation to the bottom of the hull :?:

does it still do it when your trimmed down? After my refurb I had to acutally set my pin (old fashioned, no power trim system) in one hole. I lost a ton of weight doing the build, honestly, close to 1200 pounds of water soaked foam and wood...changed the whole attitude of the boat in the water.

I would try and trim bow down a bit at WOT and see if that doesn't cure your problem.

On the hydrofoil question, I am building one of those too...just becuase I have extra material and a bit of free time...well, and a need for speed! Going to be a simple rig, I have a sheet of heavy gauge aluminum, probably 16X24 that I plan on notching and bolting to my cavitation plate in place of some old two peice rig the PO added.

Hope that helps!

Bob

Yes, it still does it trimmed all the way down. It's not really hard at that point but it's like i run out of trim just before it goes away. Kind of like being in a rocking chair at speed. I'll try and post a pic or two of the boat and motor....I believe the cav plate sits slightly below the bottom edge of the transom.

albanyco said:

I believe the cav plate sits slightly below the bottom edge of the transom.

Click to expand...

Getting this up to even or slightly above the bottom of the hull might cure everything.

It is not trimmed down too far, is it? It may be a shot in the dark but it will cost anything to check.

Nah, I tried trimming up and that just made it worse. Put a stingray on it, so next time out I will see how it does.

if you have a local welding shop that welds alum you can get a set of adjustable trim tabs put on for about 75 bucks
it wil fix that . don't make them to short about 4 inches minumum . stay away from the middle put em as far to the outside as you can ' . those foils have so much drag . if you have weeds ... what a pain in the a$$ just my "two"

The hydrofoil helped me a ton, I have a 50 hp on a 14' aluminum boat. It helps get you on plane quicker and it helped a ton with the porpoising.... My issue was that i had to change the position of motor manually depending on my weight distribution (extra people)-although your power tilt and trim should easily fix that.

You could put a "Hook" in the hull....Below is the best explaination I have found on this from another site. I moved weight forward in my boat to help, but this just may solve your troubles and free too.

"Yep, flip that bad boy upside down, or even tilt it up one side at a time. Take a piece of 2X4 and a rubber hammer. Start tapping on the corrugation closest to the sides of the boat about halfway between the back of the back seat and the transom. Make a nice symmetric hook about 3/16ths of an inch in the middle. That means that if you lay a straight edge on the corrugation, it curves away from the straight edge about the middle between the back seat and the transom. Do this on all except the center and the corrugations next to the center. Do both sides and run your boat, if it still bounces, tap a bit more hook into it. The most I have ever used was about 5/16ths of an inch. DO NOT hook the boat just in front of the engine, DO NOT do anything that would disturb the water flowing to the propeller. I have done a couple hundred of these and have yet to slow a rig down. If you look under the back end of most high performance boats, bass boats, center consoles, almost all glass boats have a built in hook at the back of the hull. This is nothing new. And it sure beats welding trim tabs on the back of your boat."
Danny

I did the tabs on mine, actually build pods that were about 1" above the bottom of the hull. Added the tabs later that are adjustable. check out the build for more details on why and how it worked.

Jamie

Two things to try:
1) raise the outboard up one hole on the transom. When the cav plate and the bottom of the hull are parallel, I like the plate to be about 1/2" to 1" above the bottom of the boat. The more horsepower you have, the higher it can be. With your rig, I'd go for no more than 3/4" to start with.

2) Check the hull for hook as Huntndogs said above, or rocker, which is the opposite of hook. If the hull has rocker, it will be convex just in front of the transom. (Halfway between transom and back seat or about 6" to 10" in front of transom.) If it has hook, it will be concave, as in Huntndogs' pic. Rocker will make it porpoise unless it's trimmed WAY in and it will make it so that you have to adjust trim every time you change speed. I like my hulls flat (zero rocker or hook, or just slightly hooked) but, like Huntndogs said, you have to play with it to see what works the best for your particular setup.

BUT, if you put some hook into it, GO SLOWLY and don't whomp the heck outta your hull to start with or you risk crimping the metal, which is bad.

i like the' puttin a hook in it' heck of alot cheeeper .gonna try that with my 12 ft