Keel and mast positioning, for dummies.

Hello, I'm the dummy.

I'm designing a 20' sporty daysailer. If you don't want to know anything about it, just skip to the question at the bottom. I've built a cedar strip canoe, but that was too easy, and I love to sail, so a sailboat is my next project. The hull design is basically an Utimate 20 with a 7' beam instead of 8'6", lighter hull, lighter centerboard ballast (250 lb bulb instead of 450), shorter freeboards, and a flatter bottom with 6" radius chines. It is going to have a fractional rig with 250 feet of SA. Trapeze wires will be probably necessary for anything over 10 knot winds. Construction is going to be fiberglass over 1/2" spruce strips and balsa frame. I'm going to use NACA 0012 foils. The whole thing should be about 850 lbs (+ or - 500lbs). It probably won't be worth a darn, but I'm going to give it a try anyway.

Anyhow, I'm missing a very basic element of the design, and I'm not sure how to figure it out. I know that the location of the keel and the mast are essential for neutral handling, stability, and so forth, but I don't know how to figure out exactly where they should go.

In my sketch, I've got the front of the centerboard in the dead center of the boat - the boat is 20 feet, so the centerboard starts 10 feet back. I figure the centerboard foil should have a chord line of about 14-16 inches, but again, I don't know how to figure that out. I'm planning on putting the mast 18" ahead of the front of the keel, because it seems to look right on the sketch, but I'm not super confident about those placements.

TO SUM UP MY QUESTION:
Is there a mathematical way to determine the correct placements of the keel and the mast? What happens if the keel is placed too far forward or aft? What happens if the mast is placed too far forward or aft?

Thanks folks. Have a great day.

 

Welcome to the forum.

Well, you're right in that, the centerboard and mast placement in relation to each other and the boat is critical, if you expect it to handle properly.



Making the changes to the hull and ballast ratio as you've suggested will make one tender *** boat, that's for sure. The relationship of the mast and board location are in reference to the LWL, not the length of the boat. The bow and stern overhangs are essentially ignored.

I hope you haven't mounted anything yet, as your centerboard is too far aft at 50%, though this is difficult to say, if the LOD dimension is used. The Ultimate 20 places the chord of the board, about 42% LWL and the mast is about 35%, judging by the drawing shown. These are just eyeball estimates, so don't cut anything.

Post some drawings of your proposed changes and strongly consider more ballast, not less. Also with the beam reduction, you're probably going to need to move displaced volume around a bit, to get some bearing and satisfactory lines, for a sport boat.

The usual concurrences for these types of changes, is a stronger grasp of sail and hydrodynamics. In your case, an adjustable mast step is a real good idea, so you can move the stick fore and aft as you dial in the best placement. Of course this screws with any staying arrangements you might have, so maybe some tracks for the stays too.

 

Without having a look on design or read youre story, please read the principles of yacht design. I think you will find youre answer there

 

This question has been discussed in previous threads; for instance http://www.boatdesign.net/forums/sailboats/center-effort-vs-center-lateral-plane-36687.html

The most common mathematical method used to determine the location of the mast uses the centroid of the sail area with the sails on the centerline of the boat, and the centroid of the side view area of the hull and keel/centerboard. The common terminology for the centroid of the sail area is Center of Effort or CoE, and for the centroid of the hull and keel/centerboard area is Center of Lateral Resistance or CLR. The method calls for the mast to be located so that the CoE is ahead of the CLR by a certain percent of the waterline length typically called the "lead". The amount of lead used depends on the sail rig and hull shape, and is generally best determined by comparison with other boats that have similar sail rigs and hull shapes which are known to be satisfactory. There are numerous variations on this method. Most books on sailboat design describe this method and have recommendations on the amount of lead. There are numerous variations on this method.

If the mast is too far aft the boat will tend to have weather helm, that is it will tend to turn into from the wind when close hauled or on a reach. This results in a pull needed on the tiller (when sitting to windward). Too far forward and the boat will tend to have lee helm, that is it will tend to turn away from the wind when close hauled or on a reach. This generally results in a push being needed on the tiller (when sitting to windward). A moderate amount of weather helm is generally considered desirable.

The CoE-LCR-Lead method is frequently claimed to be based on a balance of aerodynamic forces which it is not. What is commonly refered to as "CoE" is NOT where the aerodynamic forces act while sailing. Likewise for the "CLR" is not the where the hydrodynamic forces act. So trying to figure where CoE should be relative to CLR on a first principles physics basis doesn't work. Lots of boats have the CoE ahead of the CLR, and have a weather helm. However putting the CoE in a similar relation to the CLR as other similar boats known to be successful is a good pragmatic, emperical way to locate the sails. Fabio Fossati in "Aero-Hydrodynamics and the Performance of Sailing Yachts" in discussing the use of such methods has said "they [lead method] are only valid for producing a boat design that we can consider balanced with respect to a series of previously analysed projects; otherwise they are excessively disconnected from the fluid dynamics governing the problem, and from this point of view there is room for in-depth research, ..."

Since you are building the boat yourself you may want to consider designing it so that the mast can be easily relocated fore/aft to tune the balance.

 

The reason for 'lead' as discussed above is to account for the driving force of the sailplan hanging out over the water to leeward due to boat heel. It's not that the lead cannot be calculated, it just can't be calculated in 2D. The fact that you are making a tender boat makes the lead calculation difficult but likely manageable.

If you use the standard practice you will get decent results while the boat is upright. If you can't keep the boat upright, weather helm is going to be a problem and you have a choice to make -reef earlier and more often, add some ballast, or increase the rudder size and tilt the rig forward. Having a boat that gets a bigger percentage of it's lift from the rudder is not that bad as long as it is operating at an efficient angle. In fact, you are building a race dingy so the rudder proportion SHOULD be more like a dingy than a keel boat, reflecting the additional control you will need to tame the beast you have created.

 

why not just copy the mast and keel location of the Ultimate 20? presuming you will be using a similar rig, it should balance in a similar manner.

There are varied and complex reason for different rudder and keel sizes, but for small boats the rule of thumb is the keel/dagger board area should be about 5 percent of the sail area, and the rudder about half of that. Make the rudder a lower aspect ratio than the keel so it resists stalls.

 

Common adjustable keel root and adjustable foil to bulb detail that allows fine tuning of keel and bulb placement

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(OK, that didn't work. Anybody know how to attach a photo on this site?)

That's the design - not quite the final draft, but a good starting point.

Thanks for the information. I'm not a pro and I really appreciate the advice.

I know that the lead weight is minimal, but I want to keep the boat light, and I don't mind being out on a wire (I rather enjoy it, actually). Also, the shape of the hull is meant to provide stability in lieu of extra centerboard weight; the flattish bottom and chines should move the center of buoyancy abruptly to the leeward side of the boat when heeled. If it proves to be too much to handle, I'll cast a new centerboard weight for it. My inspiration for this boat came from the Laser SB3/20, but I wanted to be able to hike out. I figured that if you design the boat for hiking, you don't need as much CB weight. It wouldn't surprise me if I'm wrong, but that was my assumption.

PETROS - I thought about that, but the U20's CB is canted at an angle, which really complicates the design. Thanks for the 5% rule of thumb; I might even make it a little larger than that because I really want the boat to be able to point upwind. Since you mentioned stalls, I've got a basic question for you. Forgive my ignorance but all of my experience is in aviation. Does a foil in water stall just like a foil in the air? I've always assumed that it doesn't because air is compressible an water is not.

SKYAK - Thanks for that info. The boat is going to be more dinghy-like than keelboat-like, so I might want to think about using a larger rudder.

DCOCKEY - Wow, sooooo much great information. I'm not even close to finished with reading it all yet, however, I think you're right - I should plan on adjusting the position of the mast once or twice before the boat is done. I'll just keep the mast step on a movable block until I've got the mast positioned right.

J - I will. Thanks.

PAR - That's really interesting. I never noticed that the CB was so far forward on a U20. If you look at the SB3/SB20 the CB is aft of the Center of the LWL, maybe 55% at the center of the CB. How can 2 boats with similar design and purpose have such different keel arrangements. I can see that the rocker of the U20 hull is moved fore, while the SB has more rocker aft. I'm really just baffled by the whole topic.

 

stall has nothing to do with compressibility, it has do whether the flow is attached at the foil leading edge or not. High aspect ratio surfaces are more efficient in terms of lift to drag ratio, but area also much more stall sensitive. It is easy to stall a rudder by over controlling it and not realize it, so with a lower aspect ratio it makes it more stall resistant. An AR of about 1 will not stall for up to 45 deg angle of attack, but a rudder with an 8 or 10 AR will stall at only 15 deg. not as much an issue with the keel, an efficient high AR keel will allow you point higher (though the stall will be more sudden, you can usually feel it when you push it too far), but if the rudder stalls in rough conditions or with an inexperienced helms man, you can loose control of the boat in a panic situation, you might end up going for a swim at best.

I do not think a beam to length ratio of about 3 will make too tender of a boat as PAR thinks, in fact the original design has a rather large beam for typical sailboat. the wider water line beam will come up on plane sooner (less wind) but it will have more drag in light air when in displacement mode. Your change will just make it a better light air performer, especially making it lighter. You also might come up on plane sooner because of the weight savings.

What DCockey writes is the way most small sailboats keel/sail plans are located, but they are not that precision. the fluid mechanics of both sail, hull and keel are much more complex, but over the centuries it has been found to be a simple way to approximate it close enough to make it work. Expect to have to fine tune the sail location, mast rake, etc. this too is normal, it is easier to just build it and fine tune it later rather than try and make accurate calculations (too many variables!).

You can post pictures by braketing the picture location with "[" and than "img" and than another "]", than paste in the picture location, and follow with "[" and than a "/" and than "img" and than a "]". I can not write it out for you or you will see nothing.

 

So, I've decided to make a few changes to this since I drew it up. First of all, the rig needs to change. I was considering using a Hobie Miracle rig that I could scavenge off of an old boat, but I really want to have a backstay to help strengthen the rig and move the shrouds forward and out of the way. I'd like to use an SB3 rig, but the price of the equipment would cause my spouse to blow the whistle. I haven't priced U20 parts yet, but it carries a bit too much sail and again, it has no backstay. I'm not sure what I'm going to do about the rig at this point, but I'm taking suggestions. I'm working under the assumption that I am unqualified to design my own rig and sew my own sails.

Whatever rig I used is going to be mounted further forward.

 

My assumption for a tender boat is based on a few things, 20% narrower beam, 45% reduction in ballast, the same draft (also assumed) and the same sail area. Hardened bilges or not, she'll likely be tender, certainly in comparison.