Unstable steering on displacement boat

[melibe]

Our 27' round bottom displacement power boat has recently become almost impossible to steer in a straight line. This is a beamy heavy displacement boat than runs at about 6.5 knots. It used to run reasonably straight, although you couldn't walk away from the wheel for any length of time, but now it just suddenly veers off into a medium hard turn. There doesn't seem to be any preference for port or starboard.
We made some changes to the deck that changed the fore-and-aft trim a bit but I compensated for that by moving ballast. We also added a couple of thru-hulls about 25% of the water-line forward of the transom.
I've tried adjusting the fore and aft trim to raise the bow by a couple of inches but that didn't help.
The rudder is a flat steel plate. The stock is about 20% or 4 inches back from the leading edge of the rudder. In addition to the post at the top the rudder is supported at the bottom in a sort of stainless cup with a diameter about 1/2" bigger than the post. The gap is filled with a bushing.
Any suggestions or ideas as to where I should look for solutions to this problem?

[Gilbert]

The 20 percent lead on the rudder sounds a bit much, but since it apparently was satisfactory before I am skeptical about that being the problem. In fact for reverse it could just be the appropriate amount of area for balance. I would look at slack in the linkage, couplings, etc., etc. next and see if there isn't an issue there.

[jehardiman]

Originally Posted by melibe

We made some changes to the deck that changed the fore-and-aft trim a bit but I compensated for that by moving ballast.

Was it an old fishing boat? Did you add weight high? Does it happen more at speed? Does she float level, and return to level when heel at the dock? What might be happening is the vessel is lolling due to loss of WP area at speed.

[Easy Rider]

Melibe,
Are you absolutely sure there's no preference port or starboard? Perhaps your ball joint cable anchor is just loose enough to move a bit going one way or the other and a bias applies itself one way or the other depending on which way the helm was turned last. Only thing I can think of that would not favor turning one way.
I have a similar boat (30 Willard) and w cable steering directional stability while cruising was not good. Went hydraulic and now I can frequently not touch the helm for a min or more. Great for me w no AP.

Easy Rider

[tunnels]

Originally Posted by melibe

Our 27' round bottom displacement power boat has recently become almost impossible to steer in a straight line. This is a beamy heavy displacement boat than runs at about 6.5 knots. It used to run reasonably straight, although you couldn't walk away from the wheel for any length of time, but now it just suddenly veers off into a medium hard turn. There doesn't seem to be any preference for port or starboard.
We made some changes to the deck that changed the fore-and-aft trim a bit but I compensated for that by moving ballast. We also added a couple of thru-hulls about 25% of the water-line forward of the transom.
I've tried adjusting the fore and aft trim to raise the bow by a couple of inches but that didn't help.
The rudder is a flat steel plate. The stock is about 20% or 4 inches back from the leading edge of the rudder. In addition to the post at the top the rudder is supported at the bottom in a sort of stainless cup with a diameter about 1/2" bigger than the post. The gap is filled with a bushing.
Any suggestions or ideas as to where I should look for solutions to this problem?

Can you post some pictures of the complete steering set up cables pulleys quadrent and how its clamped to the rudder stock etc etc also the rudder and all bearings attached to the rudder etc etc . Flat plate rudders should be dropped from a great height! , A rudder with a good foil shape could do wonders for what you might have . Can help change the water flow along the bottom of the boat and make the rudder work better !

[Fanie]

Overweight may have it 'lean' to either side... Take everything extra off and go for a spin, see if it does same.

[Gilbert]

I would be very skeptical about a foil shaped rudder doing any good in this instance.
This boat has probably 40 to 50 percent prop slippage. A foil shape would have very little chance of being helpful in this situation.

[tunnels]

Somethng has come loose like the quadrent on the ruddershaft ! If theres a keyway cut in the shaft and a key that would be the first place id be looking or if theres pulleys and cables look for a pulley shaft loose or even the whole pulley carage that holds the pulley where it fixed to the hull , Slack cable will be the give away. Or even the pulley on the main steering wheel shaft keys and grub screws it a complete system look at from one end to the other . A foiled rudder could work more efficently and make the steering work better also a movable trailing edge can be a help as well !!

[Guest62110524]

after you have done as some suggest, you may consider making a foil rudder
abt the worst thing you can have on any boat is a piece of flat plate
if you give me your planform would be happy to look at it
here is an example of how it might look

[Easy Rider]

I don't think there's a downside to flat plate rudders at 6 & 7 knots. I have one on my Willard and am without want.

Easy Rider

[tunnels]

Would still like to see a few pictures of whats there !!! it could help no end otherwise we are all looking into the dark and guessing .

[troy2000]

Since the problem has only recently started, I doubt there's anything wrong with the shape of his rudder. As others have indicated, there's something hinky about his steering apparatus; he needs to check it end to end.