Flopping on one side at speed

I have a '74 16' star craft with 85 HP Johnson OB, with a whale tail on the motor and no trim tabs. manual trim with lock in the first (closest to transum) and second hole.
the problem is when running along as i got going faster say around 25ish (have no speedo) the boat just all the sudden flops over one one side.
it does this no matter the side to side loading.
also it is better with the tail than with out.

 

thats called chine walking, and its very hard to stop
In fact you may have to add some wide strakes, or modify the chines with added wooden chines
The problem ss you planing surface, waterplane, it is not supporting the boat

 

i figured as much as i have seen alot of racing boats do this, just alot more extreme than mine. after reading your post whoosh, i have to say...what?
chines are the edges under the boat at the outside edge? and the strakes are the one farther in, right?
im not sure that is worth doing this to as its just a little starter boat that i bought while rebuilding the 30' that i have.
thought it might be good for the kids to play on later, but its a little dangerous.
is there any bandaid fixes that i could do? trim tabs or something?

 

sure, trim tabs will help, bigger the better spose
they are usually fitted to boats with cabins which heel badly, when the wind comes across the boat
sorry about that mate, too bad
I guess next time you will ask the seller, does she chine walk?)
your guess about strakes and chines are right Narrow boats get around this by having chines that are wide and flat , outboard of the deadrise of the bottom, so you may have 15-22 degrees deadrise back there and then a flat chine say anything up to 5-6 inches wide, which support the boat and lifts her to plane too

 

Trim tabs will help you with that, I have the same problem with my 23Winner Flybridge, though the guys at BoaterEd called it prop torque. When it planes it has a starboard list, a bit disconcerting but the trim tabs levels it out nicely. Or you could get a fat lady to sit on the opposite side.

 

Eddy, yours is prop torque, I think. And the other is just dynamic instability - One might call it "chine flopping". "Chine walking" is another thing, as I learned it - an oscillation, almost bouncing, side to side and usually at much higher speeds - kinda like it's "walking". Maybe a fore or aft weight shift could deal with the flopping.

 

Take the tail off. It is lifting the stern of the boat.

 

Take off the Whale Tail, most boats don't need one, and even if it could use one, very few people know how to set it up for it to be of any value.

See how it performs without it and then post the results.

 

Slightly off topic, how do you properly set up one of those Whale Tails?

 

A foil, fin, whale tail, or whatever you want to call it, is just a tool that allows you to do other things that may help performance. Most people think they do the same thing as tabs, or that if you bolt one on, the boat will be faster, get more MPG, ride better, plane faster, be sexier etc, so they bolt it on and head for the water.

What it can do (if needed) is allow you to raise the motor up on the transom and help prevent ventilation. Raising the motor results in less gearcase drag which may increase speed and MPG, plus with the prop higher you may get less bow rise, and with less ventilation the two combine for a better hole shot.
The benefits have nothing to do with it dragging in the water and holding the bow down or preventing porpoising at speed. It should not be in the water when on plane, this is where the negative affects are seen, like poor handling, slower top speed and reduced MPG.

This can work on an O/B, but on an I/O you can’t really do any of the set up (raising it), so you're stuck with the possible bad side affects.

 

Another cause for this kind of thing in older boats is a hollow forming (like sitting on a trailer for years ) in the running surface in the bottom which can cause negative pressure sucking down one side above a certain speed. Put a straight edge along the bottom to check.

 

whats the deadrise on the transom....if you look at a jetski you will see it leans onto only one half of the underside of the hull when it corners at speed ..very tippy and sensitive to the steering ...the deadrise on mine is 30 deg ...is the boat very light for its length /beam so it rides very high in the water when on the plane.... if the back third to half of the boat is in contact with the water over the full width of the beam when its on the plane then it should not tip ....do you need to push the nose down more ?

 

I've seen this happen in much larger boats, a 25 foot Bayliner, A Coast Guard 30 foot motorsurf boat, and several others (senior moment). All were deep vee planing boats. But I haven't seen it happen that drastically in a 16 foot boat. Typically dynamic instability is usually caused by a combo of prop torque and a warped bottom, and a high vertical center of gravity.

Check out the bottom. The planing surface should be straight. Also look at the weight distribution. Is there a lot of weight on one side? Usually they flop to the starboard side. If you have a console on the starboard side and you are the only one in the boat then you have weight concentrated on the starboard side. Move stuff to the port side to compensate for your own weight.

 

Perhaps you try to drive nose too low? Trim the bow slightly up and move some load backwards. The reason for the phenomenom is convex chine line. It is clearly seen when turning the boat. The shine suck down the inner turn side and increase the heeling caused by thrust. If you can drive on the straight part of the bottom and chine, the boat should be OK.

 

Chine walk and prop torque is 2 different things.

It all big engines on little boats.

You might have to move the engine over a bit, Dont ask which way because both ways can work. If its flopping to port its prop torque as the prop turns right hand, moving the engine to the left could cure it.

If it flops right then I would think your have weight probs. Where is the engine now, I mean is it center, and what prop are u using.