Alan B Martienssen wrote:

Many thanks for the plan!  I may, initially, simply thicken the existing rudder, doing an aft extension on a plywood boat is not the same as welding something onto steel. I'm sure it can be done, but it's a much bigger job. The effect of my ssimpler job  will be measurable by sailing with La Chica.

This could be done fairly easily by laminating a number of layers of timber onto the back of the existing stern structure, much as rudder would be laminated up. Ideally there would also be several layers of double bias fiberglass cloth brought back over the existing hull structure to further tie everything together. The project would not be technically difficult, just involve some work with epoxy and fiberglass. If I was doing the conversion I would not do the extension aft from the keel to support the rudder, because really this will not do a lot to strengthen things, cantilevered so far out and aft of the keel. I would go for a stern hung rudder with balance area forward below the waterline, or build in a shallow skeg with the stern modification so as to get the lower rudder pintle further down so as to provide more support for the rudder. 

Annie Hill wrote:Looks like Zebedee is determined to beat La Chica at this year's Tall Ships!

Zebedee almost certainly will beat LC unless it's a predominantly windward course as it was last year. Then I'm under no illusions, LC will loose as we cannot win against Zebedee in a drag race, she has more sail area and a longer waterline.

And neither of us can hope to beat Tystie, no matter what the wind direction. So what would LC like? Lots of windward work and light winds (6-10kts) :-)

Paul,
I am not with you here. Flat plate keels, sails or whatever, surely produce lift. They just produce much less lift for a given area. La Chica’s new rudder, with her good hydrofoil section, surely must be very powerful in its own right, but the added distance from LC’s turning point (somewhere around the middle of the wl?) also means that the rudder has an easier job in producing the required turning moment. The end result is that you get away with lower rudder angle, which means reduced drag.

You say that you not only go faster to windward, but also point higher. Remember then, if speed goes up with 10%, the “square rule” means that the lift of the keel, rudder etc. goes up with 21%, or in other words, you get the needed lift (of the keel + rudder) with less leeway.

Remember, leeway is the keel’s angle of attack. No lift without leeway.

Arne

PS: Dirty keels or rudder: Yes, definitely! My Johanna simply died to windward with only a tiny bit growth on her fin keel. No surprise then that glider pilots clean and polish the wings before competitions.

 

Arne, yes I do believe that cleaning up the flow has something to do with the increased performance of LC's rudder that's also why I kept a reasonable gap between the keel and the leading edge on the rudder. Also it is critically important to maintain the required foil section, even small deviations from the correct shape can have relatively large effects on the lift generated.

For example, if the rudder becomes dirty with just a light amount of fouling, I notice it immediately and you can see the flow is visibly disturbed. You also notice that she needs slightly more helm.

I suspect that as the only efficient foil on the hull (I doubt that the keel develops a great deal of lift)  the new rudder may not only be more efficient at steering the boat but is also helping to lift it to the windward.

What is notable when Zedebee and LC beat to the windward together is that LC is not only sailing faster, she is also pointing higher. This was not the case before I made the rudder changes, then Zedebee had the edge.

Presumably LC is sailing faster because with the rudder mostly nearly centred there is a lot less drag but the fact that she is also pointing higher seems to imply that it's helping to lift the boat to the windward as well.

Many thanks for the plan!  I may, initially, simply thicken the existing rudder, doing an aft extension on a plywood boat is not the same as welding something onto steel. I'm sure it can be done, but it's a much bigger job. The effect of my ssimpler job  will be measurable by sailing with La Chica.