Porpoising when trimmed full down. (pics added)

Bifflefan

Bifflefan

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So I finally got all (almost) the bugs worked out.

Today was the first day I was able to get up to any speed with the boat and I find that it will start porpoising and not stop, in fact it will get progressively worse, until I slow down.
The speedo doesn't work (note the almost) and I forgot the GPS, so I dont know the speed. Im assuming I have to stay below about 30 to keep it stable.
I do have Bennett trim tabs (also dont work yet), but I cant use them.
If I run then down a bit will this help the problem?
I have tried playing with the trim and it only makes matters worse. Also have had the passengers move around with no affect.

Thanks all....:D
 
kenmyfam

kenmyfam

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Re: Porpoising when trimmed full down.

Re: Porpoising when trimmed full down.

How about a photo of the stern and motor ???
 
G

gobbler boat

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Nov 19, 2009
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Re: Porpoising when trimmed full down.

Re: Porpoising when trimmed full down.

I had a 21' Chapparal for 11 years and the boat always handled best w/the trim set between DOWN and 1/4 (so 1/8 on the guage) I bought a used 2009 25' w/19 hrs.
Put it in the water trimmed it down to about 1/8 took off and the boat porposed terribly. Trimmed it down even more and not much better. Buried it on the guage and it planed out fine and I have been running it there ever since. There is more 'trim' left on the transom than what the guage reflects.
To make a long story short, it appears that the guage is not calibrated to reflect the attitude of the boat. Got on this forum to find out how to adjust it, but that was all beyond my capability. Don't know if this will help you, but was just my experience. Good Luck
 
oops!

oops!

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Re: Porpoising when trimmed full down.

Re: Porpoising when trimmed full down.

if it is an i.o chances are that the drive is mounted too high......check the bottom of the keel and the anti cav plate....the should be in line....if the cav plate is higher than the keel....that is the problem....the trim tabs will fix it
 
Bifflefan

Bifflefan

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Re: Porpoising when trimmed full down.

Re: Porpoising when trimmed full down.

gobbler boat said:
There is more 'trim' left on the transom than what the guage reflects.
To make a long story short, it appears that the guage is not calibrated to reflect the attitude of the boat.
Click to expand...

The gauge is correct. And also i know the drive is all the way down. I have ran through the whole travel of the trim and it never gets better.


oops! said:
if it is an i.o chances are that the drive is mounted too high......check the bottom of the keel and the anti cav plate....the should be in line....if the cav plate is higher than the keel....that is the problem....the trim tabs will fix it
Click to expand...

It is an I/O. The cav plate is exactly even with the keel. I do have Bennett tabs but they are currently inop. I figured by triming them down a bit it should take care of this. I was wondering why they were on there, now I know.
Othere than this issue the boat is great to drive.
It has a bit of excessive torque steer under exceleration but a bit of adjusting on the tab behind the prop should take care of it.

Ill see abou t getting a pics or to for Ken.
 
Bondo

Bondo

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Re: Porpoising when trimmed full down.

Re: Porpoising when trimmed full down.

Ayuh,... Put a straight edge on the bottom of the hull...
My guess is, it's either got a Hook, or a Rocker...
The last 4' is the Critical area...
 
Bifflefan

Bifflefan

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Re: Porpoising when trimmed full down.

Re: Porpoising when trimmed full down.

Bond-o said:
Ayuh,... Put a straight edge on the bottom of the hull...
My guess is, it's either got a Hook, or a Rocker...
The last 4' is the Critical area...
Click to expand...

That is another item i checked before i bought it. along with the ventalation plate height.
It is good and straight.
I had a 16' that was a disaster to drive and went through checking a ton of things before i sold it trying to find the problem. These are the hard lessons learned and i dont forget those.
 
Bifflefan

Bifflefan

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Re: Porpoising when trimmed full down.

Re: Porpoising when trimmed full down.

kenmyfam said:
How about a photo of the stern and motor ???
Click to expand...

This is the best shot i have of the stern right now. will post a better one or two when i get home.

picture.php
 
QC

QC

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Re: Porpoising when trimmed full down.

Re: Porpoising when trimmed full down.

I am very surprised shifting weight forward didn't help, but the trim tabs definitely will . . . Where's your fuel? Was it full? Stuff? Even with your comments about people not making a difference I'd still shift as much movable weight forward as possible. Get the tabs working!!
 
B

BoatNoobie

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Re: Porpoising when trimmed full down.

Re: Porpoising when trimmed full down.

what is porpoising?
 
QC

QC

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Re: Porpoising when trimmed full down.

Re: Porpoising when trimmed full down.

A rhythmic bouncing up and down of the bow. Can usually be controlled with weigh shift or trimming in. It's actually very common due to the fact that most boats are fastest with weight aft, bow trimmed up and on the edge of porpoising. Manufacturers actually set them up close to the edge to achieve good high speed numbers as buyers almost always ask and want speed.
 
Bifflefan

Bifflefan

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Re: Porpoising when trimmed full down.

Re: Porpoising when trimmed full down.

QC said:
I am very surprised shifting weight forward didn't help, but the trim tabs definitely will . . . Where's your fuel? Was it full? Stuff? Even with your comments about people not making a difference I'd still shift as much movable weight forward as possible. Get the tabs working!!
Click to expand...

I guess I made need to clairify a bit on the people. That would have been 3 girls ages 3 to 9, total weight about 100lbs.
The tanks is 36 gals, in the center under the floor and it was full.
Im not going to get to worried about this until the tabs are working, then if i still have an issues I will deal with it then.
I belive that I have plenty of speed for tubing and skiing and so on when it is not bouncing so its not a really huge issue.

Thanks all. I will post some new data once the tabs are working.
 
Bifflefan

Bifflefan

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Re: Porpoising when trimmed full down.

Re: Porpoising when trimmed full down.

Here are a couple pics of the leg and transom.

picture.php


picture.php


picture.php
 
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