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Boat porpoise question


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I have a 2006 Alumacraft trophy 180 with a F150 yamaha, while on smooth water the boat wants to porpoise alot it seems, tilt the motor in to stop the porpoise and theres ALOT of drag and it slow the boat way down, i talked to a alumacraft rep at one of the shows and he said theres a adjustment they can make if i bring it down to the factory, its something to do with adjusting the lower rear edge where the rivets are at i guess, it sounds like they bend that trailing edge down a bit and it helps the way i understand it.... has anyone ever heard of this or know anyone that has had it done? im not sure what hole the motor is mounted in if it matters...thanks!

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Hi Everyone,

I had this issue with my Alumacraft. Essentially, what has happened is when they built the boat, someone got a little "hammer happy" on the trailing edge of the hull. What this does is allows the bow to rise more easily, but harder for the motor to keep it up, hence the porpoise.

What the dealer will do (and you can do this at home...save the trip) is to take a cement chisel and a dead-blow hammer to the seam between the transom and the bottom. It's called "wedging" in which they pull the hull down away from the transom. What this will do is steady the bow, but if it is done too much it will not allow the bow to rise when you trim up.

The procedure is not too hard. Start about 6 inches in from the outside edges of the transom and wedge it open no more than 1/8" Do this evenly on both sides until you are about a foot away from the center on each side. Take the boat out and test. If you cannot trim the bow up, then peen the edge back up a little, test again. Take your time...when you find the right spot, you'll know. you can trim the bow high, but it will not porpoise.

Don't worry...Alumacraft has that little extra aluminum past the rivet line to adjust the hull as needed. You will not split a rivet, nor break the seal at the joint... And...you won't have to worry about the aluminum cracking either...it is soft enough to handle it.

When you find the right amount of wedge, silicone the gap...and you are all set.

A hook in the hull would cause plowing...which if you have that, it would be an area that would be "pushed up" into the hull...and it could be as little as 1/16" BUT it would have to occor before the trailing edge and the transom. A hook is a really bad thing...it would require dealer work... not a good thing to have... With an aluminum boat, that work can get extensive. With Fiberglass, they can basically "fill and level" the gap.

if the trailing edge is open, then "peening" the edge up is what gets the bow to rise... I had to do that on my former alumacraft Lunker 165 mag.

My only other question would be with the set-up before going the route of wedging the hull. If your motor is "over propped" the prop may be able to lift the bow high, but not hold it there...usually occurs when RPMs are on the low side of the motor's designed RPM range. Sometimes, raising the motor off the transom will get you the RPMs needed to keep the bow steady...other times, a prop change is in order.

The only other thing I could think of would be to move weight around in the boat. If you have a bunch of weight in the bow, see what you can move more to the rear. This will allow the motor to trim the boat easier, and it may be enough to get the bow steady... It could be that the bow is just too heavy for the motor to hold it up too..

All part of the fun of setting up the boat for it's intended use... As you can see, lots of variables can come into play...it all depends on how your boat is set up now...

Good Luck with it.


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Hello VMS, thanks for all the info!!! im sure i could probaly get it done myself but im not totally sure and being alumacraft will fix it for free i think i will take a little time and take it down there, i have a couple of other small things for them to look at as well, I would never let the dealer i bought it from mess with it as i have lost my trust in them, i had it in in the fall for this prob and they did nothing, i know this because they told me and they said they never took it out! even though it was there over a month! while it was there i had them add sta-bil and run it the day i picked it up, well come to find out when i picked it up the battery was DEAD!! for that reason i believe they never even ran it that day!! and i didnt notice till i got home! and i did not feel like pulling it back up hwy 10 to the south side of st cloud from elk river, i trusted all was ok but the more i thought about it the more questions i have, i cant wait to see the lower unit lube they "changed" at that same time! im hopeing to see them at the show this weekend.

Forgot one thing, the prop they had on to start with would not even plane the boat with 4 people in it, that was a 21" i changed it to a 19" and it made a AWSOME differance, again thanks so much for the info!!

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