Boat Porpoising

[buddyearl]

I recently restored a '66 Starcraft Jupiter with and '66 80hp Evinrude. Runs great until I try to go 3/4 to full throttle then the front end starts to go side to side. I attached a picture showing how engine is mounted. Prop says 14P approx 9". Any suggestions on what to do to make this motion stop. Thanks

 

[Maclin]

The Bow moving side to side sounds like bow-steer. That can happen if the engine is trimmed too low and the bow stays down, the front part of the vee catches waves and moves the boat. Porpoising is when the bow oscillates up and down, usually because the engine is trimmed too high for conditions.

Does your setup have power trim? If not, what hole do you have the engine mount's trim adjustment bar in? In your pic, if you do not have power trim or it is not working it looks like the engine is trimmed too far in which would keep the bow down, not letting the hull rise and hit full plane.

 

[old islander]

Agree with Maclin. Try moving the motor one hole up and run it. then two and so forth... you will find your balance point with a little patience and experimenting.

 

[buddyearl]

It's in the 2nd hole from the transom. If I'm understanding correctly I should move it to the middle hole? See picture.

 

[old islander]

Yes, then the next and so on. At some point you will find the "sweet spot" for your rig. Just don't move it too much so you cant see out the windshield due to bow rise. It sounds like your "digging in" right now. As you keep going to the next hole, and the next your bow will ride a little higher each time. Find your and your boats comfort zone. You will know when you find it.

 

[old islander]

One more thing, in that last picture I noticed two small voids on the edge of your wooden motor backing plate. I would recommend you fill and seal those, as well as any others along the edges. Those little voids in the plys of your plywood have a way of sucking up water and causing delamination problems in the future.

 

[buddyearl]

After reading more posts I have "chine walking" My cavitation plate is about 1 1/2" below the bottom of the boat. My transom measures 20" and the motor is a long shaft. I appreciate all your help.

 

[Maclin]

If it is Chine walking then it must be up on plane for the most part. In general , and barring any hull deformities, chine walking is due to a heavier and more powerful engine than the hull is designed for. What HP is the hull rated at? What MPH does the walking start?

Also, what size is the Jupiter? And it seems to be an aluminum hull? Does not take much HP to get a tinny moving, just sayin'...

 

[steelespike]

Chinewalking is usually associated with high speed.I would guess speeds over 40.
Very likely the motor needs to be raised. I usually suggest raising to a point just before ventilation becomes a problem.
When rebuilding the boat did you maintain the transom height? A 20" transom may be slightly under or over that dimension.
It is measured vertically not along the transom.
A 20" motor will very likely measure longer than 20".The mounting bracket is usually setup to fine tune the height.

 

[steelespike]

Upon further inspection it appears the 66 80 hp is 240 lbs,is rated 80hp at 4500 rpm. rpm range is 4000 to 5000.
Brochure says 40 max speed.And they are usually optimistic.Transom is said to be 21".
Just doesn't seem to me that your Jupitor would chine walk at what is very likely speeds under 40.
The Jupitor is rated for 85 hp so your ok with the 80.
While raising the motor a little at a time is a good idea I don't think the 1 1/2 inches is causing handling problems.
Try trimming up.a notch at a time.

 

[Maclin]

I agree with Steelespike's analysis, Trimming up is a good next step. If most of the hull is still in the water then it will act like a displacement hull and when "overpowered" they roll from side to side. Now we know yours is not overpowered but if I leave my 22' Cuddy trimmed all the way down and keep the power on it acts funny, bow-steers and lists from side to side until I (or the trainee driver) trim it up, does not take much to get it all the way up and on plane, similarly does not take much to keep the bow down and cause the plowing issue.

 

[buddyearl]

I replaced the transom and it's 21" Jupiter is a little over 16ft. Original owner's manual for the boat says up to 80hp so I think I'm good there. I'll trim out and see what happens. I sure appreciate all your input. I let you know what I find out. Thanks