Keel and rudder size

[davef]

Hi there... is anyone familiar with any design guidelines for setting the size (area) of the keel and rudder. I'm finishing up the design of a 20' daysailer. Pretty light displacement (1400lbs w/o crew). I've got plenty of guidance about setting up the lead and such, but can't find anything giving me general rules on the proper area. The keel will be a retractable daggerboard style with a lead bulb (around 450lbs) if that matters or helps.

Thanks in advance.

Dave

[WayGray]

I have seen guidelines for the area of keels and rudders in design texts but can't remember where now. What I did for a boat I built while in Panama was to build a trial daggerboard (not so easy for centerboards) and rudder which I would use, then cut down some, until finally I started to notice the lack of area. At that point I knew what too small was. Then I built the final board and rudder of slightly larger dimensions and more refined construction.

[mmd]

Check Skene's "Elements of Yacht Design" and Chapelle's "Yacht Designing & Planning" for their take on rudder sizing. Daggerboard size is a compromise between balance, resistance, hydrodynamic lift, and strength. A daggerboard with a lead weight on the end should be carefully engineered to ensure that the board and the housing are adequately strong to resist the big moment exerted by the dynamic motion of the board in a seaway. No "rule-of-thumb" on a bulbous retracting keels - just number-crunching engineering.

[John E Hardiman]

davef;

A couple of things:

1) 1400# for a 20' daysailer is heavy, even for 1980's boats. This leads to questioning the hull form and the profile of the plane of laterial resistance. Is it narrow and deep or wide and shallow? The choice of area ratios keel:rudder will hinge on this fact.

2) The ratio between the keel:rudder is linked to the laterial stability and desired maneuverability of the hull. Deep narrow hulls have good laterial stability and need only small rudders adaquate to pivot the hull, the fin keel is balance for the sail plan and to prevent excessive leeway (let's not talk about tacking or lifting daggerboards here just yet :D ). In shallow wide hulls, the rudder provides most of the laterial stability of the hull and the balance to the sailplan while the fin keel just prevents excessive leeway.

3) The depth of the forefoot compared to the rocker (maximum depth) of the hull will greatly effect the size of rudder needed.

4) Bulbs are not the be all - end all of what to do with the extra weight. A poorly considered bulb can make a good boat into a dog and a well designed bulb can make a tender boat more unstable. Before adding a bulb, get a copy of Hoerner's two books: Fluid Dynamic Lift and Fluid Dynamic Drag and pay attention to what what he says about wingtip fittings. Then go back and plot a couple of dynamic righting curves (with wave orbital effects!! :eek: ) and see if making the board a foot longer or wider with the lead in a extended tip is a better option.

5) Try to find the underwater profiles of boats of like displacement and size which are considered "good" boats. Plot out their sailarea:displacement, sailarea:laterial plane, sailarea:ballast, laterial plane:moving rudder, etc. This is the best way to put yourself into the ballpark at a first go.