Originally Posted by Panope
The most traditional of all cruising boats, Josh Slocum's Spray, had such awesome directional stability that she would hold course on any point of sail with nothing more than a lashed helm. ......
Certainly hull/rig type must be a large factor with successfull sheet to tiller steering. I wish I was savvy enough to explain why.
Letcher alludes to Spray in particular and long keels in general: he points out that, while a fin keel
has a LOT less yaw resistance than a long keel
, when a fin is combined with a decent sized rudder
, positioned well aft, the combo has an overall length dimension not significantly less than most 'long' keels from the cruising fleet. (Where the rudder
is often some distance in from the stern, and the forefoot often cutaway - although not in Spray)
The fin/rudder combo is obviously more efficient (in the same way that a plane develops more lift
from a separate wing and tailplane than if they were joined)
and being able to develop lots of lift
in the desired direction with little drag is a good thing from the point of view of active yaw control, (as with a decent wind vane
or autopilot) but does not tell the whole story for passive yaw control (sailing with helm lashed)
The fact that not all the intervening space is filled in, means that there is not the same degree of passive damping against external yaw-inducing forces.
Letcher says of modern underbodies "such boats are quicker to turn off-course when the helm is simply let go, and have less ability to damp out oscillatory yawing that might be induced by self-steering gear
nevertheless, all his experiments were conducted in fin keelers, and he says
"it has been amply demonstrated that boats with short keels and separate rudders can be made to steer themselves quite adequately by windvane
or sheet-to-tiller gear
, so length of keel for self-steering ability need not be a consideration in the choice of the design of a voyaging yacht".
(Which I'm not alone in loudly endorsing)
Elsewhere he points out that the points I numbered 1) and 2) in my early post are less applicable to beamy boats. He doesn't add the qualification which in recent times I think becomes important : PROVIDED they do not take advantage of their beam to fit a tall rig.
I say this because his points 1) and 2) presume that the primary cause of weather helm from heel is what I think of as the "spanner" effect, due to the drive on the sails being applied far out to leeward because of heel, 'screwing' the bow up into the wind.
I think he's right to point out that this predominates (and he had done formal research
into that exact question); most sailors have been misled to believe that weather helm from heeling is entirely due to the immersed shape of the hull
being so lopsided.
While I believe this is not true for the general case of cruising boats, it may be a significant contributor for very beamy boats which nevertheless heel enough to make the underwater shape very asymmetrical when on the wind (eg most modern racing
Returning to Spray, I would imagine that Joshua sailed her pretty upright, and because of this, point 2) would not be much of a factor, and the hull form would be more like a cruising catamaran
in terms of being course-stable downwind.
Secondly, Joshua had a very long bowsprit
on Spray, AND fitted her with a mizzen (purely to assist steering) shipped as far aft as it could go, on the taffrail IIRC - and this complies as well as any boat could with Guzzwell's point 7) about exaggerating the spread from foresail to aftmost sail.
Thirdly, the rudder was probably very heavily built. Letcher doesn't talk about this in relation to Spray, but Wanderer III was an enigma because she sailed thousands of miles in varying windstrengths (admittedly, with the main reefed more than would normally be the case) with the tiller free, not even lashed!
Letcher reckons this could have been an accidental happenstance: when the boat heeled, gravity acting on the rudder swung the helm to weather in proportion to the amount of heel.
I've considered trying a pendulum on a more modern boat to simulate this effect, but it occurred to me it would have to be well damped.
The rudder on Wanderer III would have been well damped by the water
immersion and flow, and (althought Letcher does not mention this) the rudder rake, while not as considerable as on Wanderer V, would have added a tendency to self-centre with gravity, which I reckon would have been valuable in light winds, which generally defeat 'free rudder' attempts (unless the boat is capable of a decent turn of speed in drifting conditions).
So a subset of these characterisitics is probably sufficient to plausibly explain Panope's coursekeeping with sheet to tiller (the short, divided rig must have helped in her original guise, and the long keel would still)
and to me the whole nine yards certainly explains why Spray was such a smug pussycat in that department, requiring no assistance.