Modified CB advice needed

I am building an 18' daysailer. I am interested in building a finn keel in place of the CB. Are there any texts out there that might help me close in on the critical specifications? I am refererring to the placement of the finn and the associated CLR, depth, weight, center of mass. I have plenty of sources for how to build a robust assembly and attach, and the oak keel is as big or bigger than some other finn-keel designs so I believe that it can be done structurally, but I have no clue how to make sure that I still have a good, safe sailer and have not ruined the whole thing from a performance standpoint. I am capable of working through any calculation for the hull, sail rig... that may be required. Thanks for the help folks.

 

I think a fin keel would be better that a cardboard box.

Well I don't know what a CB is? Does every one else? A very nicely written question and one that deserves a better response than that of mine but what is a CB.

 

Im guessing,centerboard?
Steve.

 

A fin keel is usually ballasted and it adds little stability until the boat is well heeled over. Since most small centerboarders are human-ballasted, they are designed to sail upright generally.
The hull form, then, is likely flattish in shape and doesn't do as well if heeled beyond a certain point.
I'm assuming you are looking for more final stability and are willing to give up the convenience of beaching, trailering, etc..
That said, adding ballast sinks the hull all things considered, meaning a lower payload. If not ballasted, the one advantage would be the open cockpit.
You are no doubt aware of these issues. Added ballast would work best in a round-bottomed hull and would allow slightly more sail area to be carried. However, if the hull underboady was similar to, say, a Lightning (shallow vee), I'd guess you'd have no performance advantage at all. Some boats such as the 14 ft Mercury have ballasted keels, but I'm guessing the Mercury can sail at greater angles of heel to take advantage of the fixed keel.
If unballasted, as said. the trade-off is between cockpit openess and beaching/trailering inconvenience. However, it's doubtful the rig would change or the CLR would change. Additional wetted surface would reduce low speed efficiency and possibly higher speed efficiency as well. You definitely won'y do as well off the wind if you can't raise the board.

 

If ballast is added the bouyancy tanks should be increased in size for safety.

 

Why do this in the first place? The added problems with draft, reduced landing options and problems with trailering etc. are not worthwhile for a little gained stability.

Changing to a steel or aluminum centerboard may give the same stability result, with similar impact on performance and much better utility for beaching. You would also be able to switch the boat back to it's original board when selling.

If it is a daggerboard design, addling a bulb to the bottom, and inserting from the bootm wil get you closer to the U-20 type lifting keel configuration. performance AND stability in the same boat. Ther have been Laser 5000 twim wire skiffs modified in this way.

--
Bill

 

If a space problem in the cockpit, think about leeboards. If you think performance will increase, any centerboard can be shaped like a fin keel and still lift. If for ballast, the hull should be redesigned completely to accomodate the ballast.
A canoe-bodied boat like a dory or a norse type design can carry more weight and simply become a larger boat in doing so by automatically lengthening and widening. A boat with a transom is another kettle of fish. Once the stern sinks under water, dynamics change drastically. The boat becomes sluggish.

 

Much depends on the design you're building. There are several issues to consider, some of which will be structural, including the rig amazingly enough.

What design is it? What is your major contention with the centerboard? Have you considered a sub keel to house the centerboard, rather then a fin keel, which could offer you shoal draft and a case free interior?

In other words, more information is required.

As to placing the appendage, plan form, sectional shapes to employ, balancing it under the CE, locating the CLP, ballast considerations and it's CG, etc., there are no easy answers, as each has to be determined per a specific boat basis.

 

Sorry guys for the sketchy details (and PAR before you ask, you are correct in thinking that I am the guy who asked a question about a hard chined, sawn framed boat elsewhere. That is the OTHER design; not that which I am asking about here. Yes I have TWO eighteens lofted which I did to be able to see full size the general shape and make a more informed decision which to build bases on the aesthetics of the lines, degree of bend in frames, ...). Anyway, the boat is Edson Schock's design 282 from Mystic - very similar to an O-boat, about 166 sf of sail area, traditional construction, 1 3/4" x 5" keel. So yes Alan, it is very much a traditional round hull form. The interest is to gain cockpit room and permanent mooring vs. trailer sailing is not a concern. It seems however that I am in over my head in considering this, which causes enough concern that I don't want to ruin a fine design, to make me leave well enough alone. I am interested if anyone has any final thoughts and do appreciate those above.

 

I like an open cockpit--- my fifteen foot keelboat is completely open and I love it. It's about as comfortable as can be even with a crowd aboard.
Can you do it? How clever are you? The stresses involved may require you to build up a portion of the boat around the keel. If it's round-bottomed, then all the better, as the floors will be deep there.
Enough on structure. Your boat may easily handle another 200# of weight in the form of a lead casting added to your fin, especially a round-bottom that adds beam more quickly than harder chines when weight is added. Some weight can be subtracted when the centerboard case is eliminated.

Others can inform you on a NACA section foil and so forth. Balance is the main thing. The area of the keel is likely moving, losing depth but gaining fore and aft. How much will this affect balance if the CLR remains the same?
To my thinking, not much, if at all. If it was me, i'd not change the CLR. It's difficult to comment without having sailed the boat first. It may be that any shift forward or aft would be a refinement of the original design, which may have been less than perfectly balanced. Talk to other owners of the same design about those characteristics.
More opten than having a lee helm, boats are often designed with enough weather helm to forgive inattention to wind shifts by tired or inexperienced sailors. With a centerboard, slightly "raising" the board is actually moving the CLR aft, and better balancing the boat, since the board isn't raised hardly at all, but simply swung back a foot or more.
Any case, as long as you get informed input from various sources, which would entail putting up drawings or naming the boat you're building, you will do alright.
What design is it? Is there a link to a study drawing?

 

I too like wide open boats. I built a 22' sailing cruiser nearly 30 years ago. It sort of looked like a Catalina 22, for no apparent reason, but the styling trends of the day. I found I didn't use the cabin at all and was tied of climbing over the damn thing so I hacked it off. It's now a 23' 6" open boat (I put a counter on it), with a yawl rig and small, flush foredeck to give the waves a place to land.

I'd start from scratch and work out the centers from what you have. The plans should show the CE, CB and CLP. Arrange your appendage to align the CLP where it was with the board 3/4 to 7/8 of the way down, with an acceptable lead for the rig and appendage combination (which will be similar to the centerboard lead). I'd add a percent or two over the original lead if using a fairly narrow fin.

As far as structure, it's hard to comment without plans in front of me, but generally you want to tie as much longitudinal support into the floors that hover above the new appendage. Since you'll be eliminating the case, you'll want substantial floors in that area. These serve two purposes, to transmit compression loads from the rig to the hull and to resist torsion loads from the appendage.

Personally, I'd use a stub keel and house the centerboard below the sole. You get the best of both worlds. No case dividing the cabin/cockpit, reduced drag with the board up and shoal draft if for nothing else, to make trailering easy.