round bottom hull, how to stop sliding in turn

hi all, I have converted an 18ft catboat with a round almost flat bottom and no chines into a displacement speed power boat. when it was a sail boat it has a large centerboard. thats now gone, and I think it will not "grab" the water when the rudder is turned, and will probably skid sideways.

I can fiberglass a "keel extension" but dont know the minimum height or length. I'm guessing center or toward the front, and 3" tall and maybe 3ft long. But thats just a guess, and then I rememberd this site!

boat speed about 5 kt

 

I would try it first. My bet that you wont notice any skid on a sharp turn, not with that skeg, and "displacement" usage.

 

by skeg do u mean the keel section ahead of the propeller? it only goes a couple feet...

 

Listen to what @rwatson says. The skeg will be sufficient.

You are right, a skeg is an aft extension of the keel. Compare the skeg with your rudder which might be smaller but still generates enough force to turn the boat. The skeg will be big enough for directional stability. If not, get her out of the water and extend the skeg/keel (probably downwards to increase the projected area).

 

would not a smaller skeg much farther forward be more effective per square inch due to the leverage? that is why bow thrusters are way up front. also, on this boat, the rudder is huge. I may cut that back, but a catboat rudder is really big. since I am building a cabin on this hull there will be windage pushing the boat sideways at times. with nothing on the bottom to "grab" the water, turning into wind could be hard or impossible

 

Probably not an appreciable amount.
This "skid" you refer to is called "transfer" and it happens on all boats to one degree or another.
Practiced skippers intuitively compensate for it without even knowing.
You likely won't notice it.

 

We experienced the difficulty steering in 30' cargo canoes. My solution was to install two runners about 1 1/4 by 4' long close to the turn of the bilge from about the middle to forward.

 

awesome, and that worked???

 

Yes, they started turning in their own length

 

Sounds like you have not tried it yet?
That is definitely a prerequisite to any modification.
From the photo, I don’t think you’ll have a problem.
The rudder is not plainly visible in the picture.

 

Several contributors have suggested that you put the boat in the water and try it before you do anything.
You have asked for advice and this is very good advice.

Installing a larger keel may in fact make the boat harder to turn.
As the prop thrust is always in the centerline direction, ie the prop thrust line never changes wrt the boat, you pretty much need the back of the boat to "slide" and the rudder is the only item that produces a lateral force to the hull.
So the thrust is always centerline at the prop face, the water coming off the one half of the prop, impacts the turned rudder and pushes the boat stern sideways to effect the turn, but the inline thrust is not diminished.
( a bit but not significantly)

Gonzo's example is not the same situation. ie his thrust line changes, I am assuming that it was an outboard, and therefore vectors the thrust laterally. IF his "demi-keels" were at the front, and it sounds like they
were, then the stern of the boat would slip sideways and the front keels could help the turn.

You have a displacement hull, adding a large keel from the front of the boat to the back, may very well help it track in a straight line and resist the lateral rudder thrust.
I doubt that there are many contributors would say that a long keel does not provide directional stability

If you find that the boat does not turn adequately as is, install a larger rudder to produce a larger lateral force

 

You are correct, those canoes are fitted with outboards. However, ski boats have skegs mounted forward of midships to help with turns, and they are inboard powered.

 

Our Mastercraft X14V, ski/wake surf cross over, has such a fin though barely midship. A google search will show that many of the other ski boats have a skeg around midship.
But the reason for the skeg is for directional stability rather than fast turning. Get a capable 190 pound slalom skier doing a course with 15 feet off at 34mph and the big challenge is keeping the boat going straight.

My point above is that any skeg full length or at the back will make it harder to turn. If the back of the boat slips sideways due to the rudder, AND you have short front skegs, it will more than likely help.

The OP is forecasting a steering issue without trying the boat out and trying to come up with solutions to a problem that has not yet shown itself. He is considering making changes that I think will
multiply his assumed steering problem. Ie adding a skeg and reducing the size of the rudder. To me, this is the opposite of what should be done to get more responsive steering.