What did I buy

Yesterday I posted a thread about going from an inboard to an outboard, that was not correct. What I bought is a 26 foot chriscraft express cruser that was redone from top to bottom. The proublem is that it had a 305 inboard, It is converted over to a 305 I/O. It looks like they moved the motor back and added trim taps. I also beleave that they added a hard top to it. What it does is when you bring the speeds up to 20 knotts or so it tips from side to side. It feels like to much of the boat is out of the water and becomes top heavy. It does have trim tabs and I can push the noise of the boat down easy, but this does not help. what I was told it that the hull was designed for a inboard which pulls the boat into the water over a I/O which is designed to push the boat out of the water and on top. Is this true and can this be what is going on. I was thinking of trying to change the hull but not really sure what I should try. The hump is still under the boat where the inboard used to come out at, I was thinking of running that all the way to the back of the boat. good ideal or no? If you have any info please HELP
Dave

 

I'm not a designer, but had been around boats for 50+ years.

Moving the motor aft and attaching a sterndrive dramatically shifts weight, not only in the horizontal plane, but also the vertical. A Mercruiser sterndrive (depending on model) will add between 150 and 200 lbs - right at the transom where there may be the least amount of buoyancy. Then add the weight of the motor and you've put about 600+ lbs at the transom. You've also eliminated the motor's weight from where ever it was. So that bow will certainly rise out of the water (BTW - the transom should be inclined at about 11deg., the same amount as for an outboard). I'm surprised if it will even steer. Perhaps some sort of "box skeg" (ala the Sea Bright Skiffs) might work?

Aside from the trim tabs (which aren't really a very good fix), you need to counter balance that added-weight-aft and removed-engine-weight. But by the time you do this balance, you've increased the vessel's weight by as much as 600 lbs. This is definitely something a designer should be looking at.

Ron Fossum

 

Now we have you! Besides all the rearward shift of weight, you mention a hardtop was added. Yiippees- your right about probably being top heavy. Everything I said back on your other post applies here. Only more so.

 

I had the same problem until I discovered that it was a shift in the fishfinder transducer on the bottom of the boat that caused it.

 

boating dave
You have many things to consider:
Is the boat being pushed faster than it was origanally. Maybe that made a problem that always existed, appearant?
How much has the center of gravity really changed. Sometimes this kind of instabilty can be caused from the CG being moved too far forward.
What appendages, protrusions. hook(concave curvature in the bottom) could be causing ventalation? Old rudder ports,misshapen (altered) strakes, added transducers etc. can cause ventilation. Or maybe ...probably... most likley... the boat is just too heavy. If you load the boat up enough that you are partially riding on the forward curvature of the hull...well I can't guess what would happen.
The other question is...is the trim angle of the I/O correct? If it is pushing your bow down then you have another problem.

Some things to try:
We are just assumming you know what the correct trim of such a boat feels like...you could be driving around on its nose for all we know.
You need to have a friend take some pics of you going by at speed. Post them here...maybe something will become obvious. Keep track of
what is trimmed where and note this on the picture. Also note speed.
Also take couple of pics of the bottom. and post them. Something might be obvious. Without that we are just shooting in the dark.

What year is your boat anyway?

 

Thanks for appearing. I get very concerned when someone gets involved with a major change like this. Rich

 

its not as bad as you think the boat did come as both in it day the hull was the same ,but the i/o had a bump out to get the drive angle and i think most i/o had a aft cabin under the helm it was more like a cave. if they did the repower right they should of removed the shaftlog and rudderport and you said its still there thats part of your first fix ,sounds like your getting air to the drive

 

Could you and DGreenwood go over to Power Boats then Propulsion then open Torque Roll. I really need help on that one. Rich