Roll Instability on small outboard boat?

[RThompson]

Hi there,

I have a boat here that falls over to port.
LOA 5.8m
B Max 2.3m
Deadrise 18 deg
Outboard 150Hp

My (loose) understanding of roll instability leads me to think that is what is happening.
The hull seems to have a reasonably fine entry with a narrow chine beam forward, and the upward curve of the buttock lines begins aft of where I might expect it to.(no lines plan – just looking at the hull). So the boat has a shorter (straight) run aft than might be desirable.

Another hull (identical) has a slightly different deck (it is lower). The other boat is:
-2% lighter
-CG of deck is 150mm lower (same longitudinal position)
-Has less wind drag.
The other boat has a mild tendency to porpoise, however it has never been noted to heel while planning, although both boats (more or less) heel into the wind.

The issues are not present when using lower Hp rating (or lower revs).
The roll instability (if that’s what it is) is not noticeable on flat water.
The bad habits of the either boat are eradicated or greatly reduced by putting small wedges in the hull at the transom.

So, my questions:
What is happening, and why do wedges help? I would have thought that if the problem was associated with the mid/forward part of the hull, then putting wedges on the back would force the bow down and make the problem worse. Or, the wedges at the back actually move the centre of dynamic lift aft a bit, positioning it on a more favourable part of the hull...

Locating the engine up or down seemed to have very little impact, however I have heard various theories on propeller sizing etc. It has also been suggested that the torque of the prop. in the water will be contributing (the boat heels to port). How much torque, if any, does a prop actually apply to the boat when the boat is running in a straight line?

Any other thoughts?

Thanks,
Rob

[PowerTech]

have you tried adjusting the torqe tab.You know that little zink skeg rite above the prop on the cavitation plate.just a shot in the dark i dont know that mutch about hull science

[RThompson]

Hi Carwun,

Yes, virtually every aspect of the boat that can be easily tested has been-
Engine height, prop spec's, large cav plates or not, Boat CG (and boat displ), Loading and so on. By far the greatest impact is the wedges.

Rob

[gonzo]

If it doesn't do it in flat water, sounds like the hull is ventilating when it hits waves.

[RThompson]

mm, yes, quite possible that its ventilating in the waves.
But why would stern wedges help that? ..I guess if the ventilation was happening by air being drawn in from forward then the wedges would help keep the forward part of the boat in the water.

It goes along OK (in waves) until it gets a wave at just the right angle/size/something and then it falls off the wave onto its side. It stays there for a second or two then rights to an even keel again.
Rob

[gonzo]

The wedges change the trim of the boat. Running flatter may keep it from ventilating.

[nevd]

Rob,

Boat roll due to prop torque can be noted in most boats with large motors. It can be seen in one direction to port during acceleration (and is most marked at about planing speed or a little higher depending on where motor develops max torque) and the boat will roll in the other direction to sbd when decelerating quickly.
Some people even offset the motor or planing pad to minimise the effect.

What type - brand of boat are you having the trouble with and at what speeds is it noticeable?

Your description of roll instability is similar to mine including your wedge analysis.

Regards,

nevd

[RThompson]

Sorry about the delayed response.

The problem reared its head at about 20-25 knots boat speed, in choppy water. 20-25 kn's is also about the max safe speed given the sea/wind conditions associated with the problem. - as said before, no problem in flat water. infact no problem in flat water even with wind.

The boat is an Australian production built boat (FRP). I prefer to keep the brand anonymous.

I imagine the running strakes would have an impact on it as well.
From looking at a dry store boat rack of 50 or so runabouts it would appear that there is no real consensus as to what is "best". Obviously there are different horses for different courses, ...but

ie: Some strakes run all the way aft some are quite short. The widths, depths and angles all vary, some finish with a chamfer some finish abruptly. The number of strakes varies. Some even have dinky little steps, and very technical looking sculpted shapes (marketing?).
Some strakes are parallel to the centreline, some to the chine and others something inbetween. Likewise all the chine flats vary as well.

The two strakes on this boat finish just aft of amidships -the inboard one finishes forward of the outboard one.
Maybe a certain wave condition applies a negative load on the starboard side hull at the very time the aft end of the starboard side strake is leaving the water -allowing it to ventilate from the hole in the water left by the strake...
As Gonzo says the wedge is trimming the bow down thereby keeping the strake in the water well past the leading edge of the planning surface..?
Although given that theory, why did it always roll to port and not to starboard? possibly the added torque of the prop is the "straw that broke the camels back"

Thanks for your input,
Rob

PS. Can anyone point me in the direction of an online paper/article concerning Wedges, trim tabs, Strakes, and/or the general shape of small planning craft?

"different horses for different courses" I like that. With that much power could the boat be riding on the engine (cavitation plate) rather than the hull at the higher speeds. That would create a very narrow pivot point in the stern making the roll more sensitive to waves.

[gonzo]

Professional Boatbuilder had a really good article on that last Fall. Strakes that start forward sometimes channel air all the way to the stern. Every design acts differently, so I'm leery of making categoric statements about your boat.

[Robert Miller]

Originally Posted by RThompson

The issues are not present when using lower Hp rating (or lower revs).

I ask calm tolerance from all those who see things differently....

I believe (looking at the specs of the boat, as you have listed them) that you have well answered your own question.

She seems rather overpowered to me.

Could try a smaller engine.

Robert

[Stephen Ditmore]

Is this boat unstable at rest, or only at top speed?

[nevd]

Rob,

Could you give us an update on what was found/ done to resolve the problem?

[craig mclean]

I have a strong feeling i know what your problem is. I have corrected many boats with similar problems. # 1 reason a boat will porpoise is that you have some rocker in the running surface. How to tell if you have rocker? take a 8' straight edge and get under your boat. Hold the straight edge on the keel and slowly slide it toward the inside strake. check to see if you have a noticable high spot, where the the straight edge "rocks". do this on both sides of the bottem. and make note of the differances. when you have roker its not a good thing and probably was not desighned that way, but maybe the mold became distorted.
what rocker does. when you accelerate the hull pushes a wave, when you acheive plane, the boat goes over that wave and should ride smooth on plane,if the bottem is flat... If you have rocker the boat will go over that wave but make another one to jump over causing porpoise, UP OVER DOWN UP OVER DOWN etc..
the location of the rocker will determine at wich speed it happens. And if you have more rocker on one side than the other, or only on one side, then the boat will roll, or list.
How to fix?
Every one is different. you can try trim tabs but in order for them to cure the problem they would have to be at such an angle that could cause to much drag, losing some speed.
What i do is a little involved but i like the results. I add hook to the bottem with fiberglass then paint it. sometimes the hook will extend 30'' from transom sometimes 15'' depending on the area of the rocker. Adding 3/8" at transom tapering down to nothing can totally stop porpoising, put 1/2 inch on one side can lift it several inches.
Note this is my experience and your boat could have totally different circumstances. But, check for rocker and tell me what you find with the staight edge. Good luck!!!

[craig mclean]

One more thing, If your bottem is completly flat and it porpoises or rolls , its to much engine. And still can be remedied with adding hook, but like i said, to much hook causes drag, slowing your boat. When we design boats under about 20 feet and plan on putting big moters in them we always tool hook into the bottem. it gets the iron out of the hole and keeps the bow down.