panache
11-09-2009, 01:49 AM
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??!
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??!