Traditional rudder design query

Absolutely, its why I'm trying to work out how the tried and tested and developed over generations ideas worked, how they compare and how to use in current designs. My sailing started in dinghys with all the usual dagger/centre boards and then fin and long keel yachts, but I'm interested in building a tradional boat and seeing how it works out for me. For instance, the deep bow and rudder acting as the sole rrquired lateral resistance as pointed out by Myszek rather than using a centre board is very attractive, and something I want to try

 

mckee , working boats of britain ,read it maybe

 

robert simper, john leather

 

Everything is a trade off, design wise. Your deeper bow will have more grip, but will it be less reliable in a tack? Im sure you have seen these if you are already interested.



Dayboats

His other Cobles all retain the more usual centreboard and retractable blade rudders, and from one owners report, it sailed well, but did mention how the boat would slide off to lee with the board raised. If you can live without the need for pointing ability, then stuff under the boat is un-neccessary.

 

My crude idea about Coble looks like this:

Hull length 4m, width 1.6m, hull draught 0.32m, 0.9m with rudder. Displacement about 400kg, 10sq.m sail area (probably could be more).
Could be origami-folded of ply:

In 20deg heel, the immersed part looks interesting:

With some help of XFLR5, I tried to evaluate the induced drag and lateral balance. When upright, the boat seems to be well balanced with the rudder turned 3-4deg leewards and 4-5deg of leeway. When heeled by 20deg, she is balanced with the rudder 6deg windwards, without any leeway. In both cases, the effective draught is about 0.7m (i.e. the induced drag is as small as for the ideal foil of this depth). Not bad.

regards

krzys

 

There is always leeway.

 

I meant the leeway with respect to the centerline. When heeled, the immersed hull is rotated by a few degrees, plus some helm to balance the boat. This produces a sufficient angle of attack to go straight along the centerline. Not very important, just interesting.

krzys

 

Your coble design looks to have 3/4 of the wind effort well forward of the only lateral resistance, so it will bear away continuously in any breeze, probably won't want to tack, except at speed, and steering could be vague and slow. Heeled, there is not much chine angle to assist the leeway. Improved by a deeper more pronounced V at the bow, and a long under slung rudder like the originals. Could be ok if you have an outboard to get out of trouble, but I think you will be finding lee shores quite often under sail. A long shallow keel may help a little with the lateral resistance, but hinder quick steering. Your transom, rudder pintles, and gudgeons will need to be very strong, with that long deep rudder blade.

 

I'm not a sailboat builder yet, but maybe a sharp bow with lateral drag effect is an idea.

 

Interesting
I see it's got the widest beam in the centre rather than fish form as the old coble designs I've looked at had. Must be a beach boat thing mainly I suppose

 

would the old traditional coble designs have the same issues with tacking etc

 

I personally have never sailed one, so can't say definitely either way. They seem to usually have had oars ready to use, so maybe that was the answer to tacking through the wind when the hull didn't want to. At speed they would be maneuverable, but in light winds or heavily laden they probable weren't. Do a word and photo search for cobles, there seems to be a wide variety.

 

You may be right... or not.
This layout is so uncommon, that little is known about its hydrodynamics in practice. Traditiona Coble works, as well as traditional Bragoc etc. How much variation around these designs is possible? Nobody knows. I tried to estimate the lateral resistance and balance using XFLR5 software, but this is a rough estimation. The closest modern boats are beach cats, like Hobie 16 - also a relatively shallow hulls and deep rudders. They are, however, much faster, so they need less lateral area.
It would be a great task to test this layout, its, potential and limitations.

regards

krzys

 

Yes I could be wrong. Try it out with a model, and see if it's OK or not. Get wacky and extreme with it. It's all a balance, so you could make it work with some model experimentation, without spending hundreds or more. A single lee board on a tether would make a big difference, and not be needed all the time; no need for one going down wind. When not in use, a lee board can be a cutting board, table, wind break, and many other uses.
Beach cat Hobies have asymetric hulls giving the lateral resistance, instead of a centreboard, which you can't do with a mono hull, maybe on a prau or outrigger though, also.
Don't believe it can't be done, but believe it may consume lots of time and effort making something work. Just do it if you want to.

 

Small 1 Ton SailBoat
Yaw/Leeway: 6°
Munk Moment: 300 Newtons

"For instance, the deep bow and rudder acting as the sole rrquired lateral resistance as pointed out by Myszek rather than using a centre board is very attractive, and something I want to try"

The Hull with Yaw/Leeway and Roll/Heel ....create lateral Force at the Bow/Stem and at the Stern

If lateral Force at the Stern = lateral Force at the Bow/Stem ... Then

1) Yaw Moment can be huuuge
2) Lateral Force = 0