why does my yacht turn to port?

My boat has developed a tendency to turn to port; why?

The boat
1986 Van de Stadt 34, built as a racing yacht, subsequently converted to cruising. Hull built to very high standards, strip plank over partial mould. Trapezoidal NACA section lead fin keel, timber/GRP spade rudder, masthead rig (was 3/4 rig originally). Two blade folding prop on p-bracket.

The effect when sailing
Downwind:
Simple - the boat tends to turn to port, requiring helm to be held on centreline to keep a straight course. No discernible influence on boatspeed.

Upwind:
The effect changes with wind speed/heel angle, which is understandable. Overall, I find tacking angles are huge - 100 degrees.

Light weather - lee helm on stbd tack, slight weather helm on port tack. I am referring to rudder angle here, not just feel. The feel is difficult to define because the rudder is slightly overbalanced in most sailing conditions. Boat speed roughly similar on both tacks.

Fresh weather - moderate weather helm on stbd tack (maybe 5 deg); heavy weather helm on port tack ( >10deg). Boatspeed is over 0.5kn greater on stbd tack than port tack - 6.5 compared with 5.8. Not surprising, as the excessive rudder angle on port tack is acting as a brake.

The effect when motoring
Not so simple to isolate because of asymmetry in the prop race, and because the rudder is slightly overbalanced,. But overall the tendency is to turn to port.

Possible sources of asymmetry
I have eliminated the keel, rudder and mast vertical alignment, by measuring each from tip to gunwhale on both sides. All are vertical, within measurement accuracy ( about 10mm). This is confirmed by eyeballing their alignment - nothing visibly out of true.
The prop will clearly have asymmetry in the prop race, but I think I have eliminated that (see later).

The keel section might be a bit asymmetric, but no worse than any other 1980s racing boat. I faired it with templates and glass sheathed it a couple of years ago. That was about the time the port-tendency started to become noticeable, but that might be coincidence. The fairing job was not to racing standards, but better than most production cruising boats.

The rudder is a different matter. It was a nicely faired NACA section foil, but it was overbalanced so I have extended the chord by adding to the trailing edge. I refaired it using ( NACA) templates and it is not bad, but not great - at most +-2mm error in thickness for a 500mm chord. I started doing this about 2 years ago, the same time that I sheathed the keel. I did the rudder in three stages ( one year at a time). I first extended the chord length over the middle third of the span ( because the overbalance was most pronounced under engine, and the prop race covers the middle third of the span). That didn't do much so next year I extended the chord for the upper third of the span, which was still not enough, so this year I extended the lower third, i.e. the chord has now been extended over the entire span. This still gives me overbalance in some conditions (incl under motor), but overbalance in other conditions.

I should add that I have only gradually become aware of the port tendency over the last 8-12 months. Prior to that it may or may not have existed, it is difficult to tell because from 1996 until about 2002 the rudder bearings were too tight so there was no feel to the tiller, and it just stayed at whatever angle it was set to. Then in 2002 I loosened the bearings, which revealed the significant overbalance. This made it difficult to discern any overall turning tendency. It was after I gradually reduced the overbalance that the port tendency made itself known.

hydrodynamic or aerodynamic?
Probably hydro, though it could be a bit of both. I came to this conclusion by taking the boat out under motor, powering downwind at 6kn then bringing the gear lever to neutral. The prop blades usually fold away at that boatspeed, leaving me with symmetrical inflow to the rudder. I did this a few times, and the net effect was that the boat stayed on course if I locked the tiller amidships ( using the autopilot arm), but swung to port 3 times out of 4 if the tiller was left free. When locked by hand, it required effort to keep it in place. The observation is made difficult by the rudder still being slightly overbalanced, so it will go to whichever side a transient force pushes it to.
However, the fact that there was a tendency to port confirms there is a hydro effect, though it does not exclude an aero effect also being present. I changed to a tape drive mainsail 18 months ago, which will have introduced asymmetry ( tapes are only on one side of the laminate), but surely this is not noticeable?

The sub-question as to why this is happening, is what tests can I do to identify the cause? I don't have wind instruments on the boat, just a GPS chartplotter. So I don't know if my tacking angles or leeway angles are symmetric. I could haul the boat out and refair the rudder, but I am amazed that such a slight fairing imperfection can have such a huge effect on performance and directional stability. There is no asymmetry in the rudder sections visible by eyeball inspection, just the +-2mm if you offer a template up - how much more accurate do I have to be with template matching??!

 

From what you say, seems like the problem is in the rudder. Asymmetry is the first thing that comes to mind. Also, the rudder shaft may not be vertical, or may be bent.

 

Dive on the boat and have a look at both the rudder and the prop. I'm betting on the prop.

 

I'm not sure how you used the templates or what the templates look like, but say you have a fore and aft half template that is supposedly symmetric, but has a slight unnoticable bulge in the aft end. The plan is to first use it on one side of the keel or rudder and then the other side. Say the template has Fwd marked on the forward end and Aft marked on the aft end. If it is used both times so the Fwd is forward, it will be alright, in that the slight bulges on both sides are aft and balance each other.

Since it's a supposedly symetrical template and technically shouldn't matter, possibly the template is used on the second side with the Aft end forward. That would double the slight bulge error, in that the bulge is forward on one side and aft on the other.

It's very hard to see but it can have a noticeable effect. Don't ask me how I know.

 

I agree with Gonzo sounds like the rudder to me. Possibly a bend shaft or warped blade. Also the slight bulge on the aft side of the keel could be producing "lift" in much the same way as a airplane wing. Just a thought....

 

Thanks for all the suggestions. I am inclined to agree with the general opinion that the source is probably the rudder. Specific replies below.

PAR: I have dived on the prop and it is all symmetric as far as I can tell - difficult for a simple p-bracket installation to change its symmetry over the years.

SamSam: Note sure I understand how you could fit a NACA template back-to-front without noticing, but it is not relevant in this instance because I used full templates (i.e. both sides), and slid them up from the rudder tip to the appropriate height for that section. Could still be warped (twisted) though.

Gonzo: Yes, it could be explained by a bent rudder shaft, though it was not detected when measuring from the rudder tip to the sheerline on each side, and the rudder aligns closely with the keel when viewed from aft. The bearing at the root is on the centreline within 2mm.

It has been suggested elsewhere that I do a Dieudonne spiral manouvre test; this would tell me how much of a problem I have, but I don't see how it helps identify the cause.

Before I take the boat out of the water and re-fair the rudder, I am going to try fiddling with the mast rake. Not because I have any great belief that it is the cause, but it is easy to do and might just give me some insightful result. (desperation is setting in!)

 

I was thinking of this, where there is no mention of NACA templates or whether they were full templates, etc.

Is this your boat, more or less? It looks like there might be 3 different keel arrangements.

I suppose you haven't noticed any hollows or bumps in the hull itself?

 

turn to port

I am not a designer or any thing like that so this is from my experience but I did have exactly the same problem on a friends yacht I used to race.
we checked aliagnment,etc etc and could find nothing. UNTIL one day I went for a quick tack round a mark and the rudder came loose on the shaft. All the welds internally had brocken down one side and had been for some time judging by the pitting and corrosion on the face and so it was not apparent at rest, just moved when loaded when the boat heeled. eventually all failled. rebuilt rudder, straight as an arrow after that.
I dont want to be a doom and gloom merchant but might be worth checking for any play.
how to check it ---I would have to hope that some one might come up with a solution. I dont know how much twist a normal rudder experiences so I just jam the tiller , grasp the trailing edge and give a damn good yank. not tehnical I know, but I have not had any more rudders give up on me --------YET:!:

Kim s

 

Easy to check. Have someone hold the blade tight and move the tiller

 

Yes, that's the design of my boat. In fact, if you have lifted this pic from the Van de Stadt catalogue, it is my very boat that is photographed in the catalogue being built using the cedar strip plank method. There have been several hundred built to this design. Mine has the deep keel and rudder shown by the solid line, though there are some different rudder designs from the design office I believe.
The hull is very fair, being built as a racing yacht and competed very successfully ( before I bought her!).

 

Now that's a very interesting idea I hadn't come across before; I'll check it at the w/e. There has been not an inkling of corrosion or structural weakness, but you never know. Thanks.