Rudders, course stability...

OK, Paul,

maybe not a milestone, but that rudder surely should be an eye-opener.

It has grown into a dogma that long keels with integral rudders are best for cruising, while short keels are not. Try to suggest modifying the rudders of a Rustler 36 or an OE 32, and I bet one will meet a storm of protests. Surely, long keels mostly make sailboats directionally stable, and also adds yaw damping, which is very good for cruising. However, the downside is that their rudders are both unbalanced and they sit in the dead water right at the end of the keel. The result is that the tiller response is nothing like that of a freestanding rudder with an efficient foil section, fitted way aft.

I see now that many fin-keeled yachts in the cruiser-racer league are not directionally stable. They may still be easy enough to handle, as long as the rudder is OK, but this instability puts big loads on the rudder in a seaway. In case it quits, the boat becomes almost impossible to get back under control.

My Johanna was close to perfect when it came to course-stability and easy helming. There was a ‘propeller shaft skeg’ plus a half-size skeg at the rudder. The rudder was thus supported by it, but also had a generous balanced section below it. The result was that the hull was (just) directionally stable, while tiller forces were very light, and the rudder response was good. However, her finkeel was on the small side and with a rather in-efficient section, so upwind performance was not quite as good as one should expect.

For serious cruising, I think the best underwater profile is a longish keel of some area (or with a cb. added), and with an independent spade-rudder way aft. To protect the rudder from destruction, it should either be supported at the lower end, as on LC, or it should be designed to kick up in case it hits something hard.

Arne

Anonymous wrote:

Anonymous wrote:It would not surprise me if Paul could control LC even with the fore-sail dropped.

Arne

LC could sail under main alone on all points of sail, without any issues once I made my rudder modifications.

NB. LCB is going to get similar mods to her rudder.

Rudders, rudders, rudders...

Paul,

I am glad to hear that the new rudder of the mighty La Chica passed that ultimate test by controlling the boat with only the mainsail up.

I think LC’s rudder should be regarded as a milestone in yacht design.

Not only does it handle any sail forces, but I bet it improves handling in a nasty quartering sea as well.

Another boat, which was much improved with a new, balanced rudder, was Karlis Kalnins’ Otterbelly (Magazine 74, p.26).

My own 6.5m centre-boarder, Frøken Sørensen, came with something that could only be described as an oversize rudder. With its streamlined blade set in its lowest, forward position, it was well balanced and easy to turn. Even though FS’s hull had very unbalanced lines, with a thin, sharp bow and a wide stern, the boat behaved perfectly well, and never broached on me  -  mostly thanks to that rudder.

Last, but not least, we should not forget Bob Groves’ (Badger copy) Easy Go. Bob fitted her with a big, well-balanced rudder (no skeg), and his home-made Belcher OGT Mk I windvane was able to steer that vessel directly, with no trimtab or servo (See Mag. 51, p.16).

Soooo,  I guess I’ll keep on nagging about rudders.

Arne

Anonymous wrote:It would not surprise me if Paul could control LC even with the fore-sail dropped.

Arne

NB. LCB is going to get similar mods to her rudder.

Arne wrote:

My armchair guess is that the big Tystie, with her shallow, broad-chord, unbalanced (?) rudder, would be a challenge to any steering system, manual or automatic, in particular with the sloop rig.

Not so, Arne! I have to indulge in some Norwegian-style plain speaking, and say that your armchair doesn't sail as well as Tystie does. How do you imagine I could surf at 10 knots on a regular basis, if I didn't have enough control? 

The rudder  has 100mm balance, which is the maximum possible with the prop just ahead of it - it overbalances if the helm is put over hard, due to the prop wash. With the low AR sloop rig, I moved across the mast on a run, so that the helm loads were in no way excessive. 

I designed the servos with 30 degree inclination of the power axis, so that they trail, and resist fast yawing. And I also designed them strong, with plenty of servo blade area. This helps a lot downwind, because the biggest problem when travelling fast is that the apparent wind is varying hugely. This is the major reason that raceboats not constrained by the rule for this race, to use only technology available in the '60s, use electric autopilots. Even the Minitransat 6.5. This is the major reason that I kept the speed under 7 knots, to keep the apparent wind stronger and more constant. The problem with many vane gears is not a lack of power, it's a lack of sophisticated control of that power.

Or, Arne, you could install a Split-Junk or Aero-Rig for a balanced single sail plan reducing your rudder loads downwind  :)