Tartan 40, when motoring, pulls hard to starboard

[jkimmet]

My friend purchased a Tartan 40, 1984 vintage, two years ago. The boat pulls hard to starboard while motoring--higher speed results in greater pull. At 5 or 6 knots if you let go of the wheel, the wheel will turn to it's stop in about 5 seconds and the boat will make a very fast starboard turn, fast enough to knock over anyone that is not prepared.
But if you put the wheel about 1/3 turn to port, making a slow turn to port, there is a spot where the helm is completely neutral.
Also, cranking on 2000 rpm from a dead stop causes the wheel to immediately crank to starboard.
Under sail she is fine.
The prop is a 3 blade Flexofold. The rudder is not original to the boat and we don't know what the behavior was with the original rudder. I attached some photos that show that the rudder is a bit uneven.
Thought I'd look for input before embarking on rudder surgery.

[Cheechako]

The prop rotation has effect. That's normal.
The rudder is balanced, but it's like walking a tight rope, a tiny bit of the "balance" (the rudder portion forward of the shaft) going to one side overwhelms and immediately turns the rudder.
Kinda like when driving at 60 mph, you stick your hand out the window like a wing, once you twist the least little bit... boom... it's gone down or up....

[caffel]

I agree w Cheechako.
But perhaps something has changed to exacerbate this since it was launched.
Bent rudder shaft ? Easy to happen on that style of rudder.
Propshaft extended a bit closer to the rudder ?
Easy to check other Tartan owners on these sites:
https://tone.clubexpress.com/
Owner’s Groups | Tartan Yachts

[jkimmet]

I appreciate any ideas. But "The prop rotation has effect. That's normal." what does that mean? All boats have prop rotation.
I get the hand out the window analogy. I can visualize that if the leading edge of the rudder is a bit off somehow, then while motoring straight ahead the prop wash would push more on one side and cause the rudder to turn. But turning slightly to port results in even pressure on both sides.
What do you think?
When I get to the boat I will inspect that more closely. The boat is up on the hard in northern Wisconsin so I have some time to work this issue before the ice goes out in June.

[dougweibel]

You have probably heard about "prop walk" in reverse. There is a similar effect when motoring ahead. The prop wash coming off the propeller is spinning and due to interaction with the hull and shape of the rudder the effect of the sideways movement of the water is imbalanced. We had a Tartan 4100 that tracked true hands off under sail (when you had the sail trim balanced, of course) but pulled when motoring. I don't think this is unusual, though the degree of pull may indicate a problem.

[Bill O]

This is a known issue w/some older Tartan models.
The shaft was intentionally offset (slightly) on several Tartan models to make it easier to remove the shaft/service the cutlass bearing. This causes the severe pull while motoring and has nothing to do w/the rudder placement or bent rudder shaft.
The general work around this issue is to run the autopilot while motoring.

I believe they have corrected this issue in newer Tartan models.

[malbert73]

I have a 1987 Tartan 40. The shaft is not offset at all. I have a flexofold 3 blade and stock rudder blade. Yes, it will spin if you let go, due to prop being close to front edge of balanced rudder blade. This also is great for close maneuvering as the rudder really deflects prop wash. That is normal. But when steering straight it’s not noticeably pulling.
But, I would say at most my wheel is 1-2 inches off center while motoring, not nearly 1/3 turn.
So, maybe this issue is specific to your friend’s replacement rudder which seems shorter than mine and maybe has more area forward of the rudder post?

[Bill O]

The Tartan 4100 definitely had the motoring/helm issue. It sails well with a light helm, but a different beast when motoring.

To the OP, do deep research on your specific model and contact Ted Jacket to discuss.

If you do decide to do rudder surgery/modification, do more deep research on rudder design, then talk to a naval architect that is an expert in keel and rudder design.

[maine50]

I know a fellow sailor who had the same issue on another make 40' sailboat. He was contemplating a new rudder as a solution. It turns out his steering cables were loose .

[steamgoat]

I believe the problem is the rudder, look at a Tartan 40 drawing,the rudder is more swept back. I think yours has too much ahead of the line of pivot. You say it is a replacement,that’s probably the problem.

[malbert73]

Yes looks less swept than my stock rudder

[jkimmet]

Thanks everybody for the input. Thanks Malbert73 for the rudder photo. That is a very different shape and probably less rudder area forward of the pivot point as well as less distance to the prop. I can see those factors making a huge difference.

[jkimmet]

Looks like the most likely solution is a new rudder. I will attempt to contact Tim Jackett to see if he will offer an opinion.
A new rudder won't happen soon but the autopilot works pretty well.
I appreciate the input from everyone.