Centerboard Design Help Needed

[BrianR]

I'm working with Marc Barto's plans for the Melonseed. They call for a Daggerboard. I'm thinking of converting it to a centerboard design.

I've read up on some articals about lining up the location of the Center of Effort on your sale in relation to the center of lateral resistance on the boat, etc.,

Anyway, I've made a small cutout of the daggerboard as it extends into the water, and I've essentially placed it on a pivot point on the plans. to see how it would work, or look. It looks to me that if I didn't change the shape of the board, it would have to pivot so far down, that the line attached to the board to pull it up into the case would probably vibrate in the water.

I'm wondering if I dould design the board in such a way as there remains the same aount board below the hull, although not as deep as a daggerboard. So long as my center of lateral resistence remains relatively close (as measured from bow to stern), would it make a difference if it were not as deep as the daggerboard design?

If I strive to keep the board as deep as the daggerboard, in order to achieve the depth, I can't seem to figure out how to modify the shape to avoid having the rope vibrating in the water, and still have it fit into the case.

Forgive me for not being up to snuff on my terms and descriptions. I'm a relative newbie to this. I've built a kit kayak, and this is my first "real" sail boat. To many of you, this will seem a foolish question, I know. But I've asked and received your help before - for which I offer my continued thanks.

I can't seem to raise Marc Barto - last I heard he was in Washington or something. Then I heard O'Connell's boat works, etc., but I've had no luck - although I'll take any tips if anyone knows how to reach him.

[Stiletto]

I've seen a daggerboard shaped centreboard somewhere (cant remember where) that was set up like a regular centreboard with the back curve cut out of it to get the daggerboard profile and foil shape. The proportions of the housing had been changed to allow the slightly longer depth to be retracted (longer slot), and somewhat stronger bracing for the larger side loadings.

I would suggest you think carefully before changing what appears to be a successful design.

[Ian McColgin]

Regular pendants and hold-down pendants will vibrate if left hanging outside the trunk.

Departing from John Gardner's design for the 18' gunning dory, I did something like that to make Leeward's CB rtunk low and comfortable. Instead of a ballasted board and lines, I experimented with two systems.

Plan A was to have a hemi-circle top to the board around the pivot pin, which put the pin a bit higher than normal and I liked that anyway. I aimed to have a pair of lines going to turning blocks at the upper forward corner of the CB trunk.

Plan B, actually executed when I ran into a good machnist amused to solve my problem, was a sewtpped and keyed pin that went in one side of the trunk and stuck an inch out the other, the keyway being let into the board itself. Given the high location of the pin, I could actually put the board in or out afloat, not that I ever wanted to.

A lever on the outside gave the board control and I was fairly clever about how it all sealed.

I was going to make a moderatly elaborate mechanical way of controling the board depth but found in practice that a doubled bit of firm bungee with siezings such that I could put the handle through at any given depth did the j ob just fine.

I found that the lever was not powerful enough to push the board down if a beaching had ground sand into the trunk, so I put in a capped FKhole near the after end of the trunk and put a bit of bronze reinforcement on the approptiate part of the board's trailing edge so'e I could open it up, insert the FKstick and bash the board down.

If you can find an wrecked old Dyer sailing dink, they had a terrific lever system that would do the trick for you.

So, you can do what you want, but why? The CB trunk for a pivoting board will take up about three times as much inside space, is a more complex and heavier system, and generally clutters a good idea.

G'luck

[N. Scheuer]

Is there any way for you to check out one of the current production fiberglass Melonseeds built by Crawford Boatbuilding? Perhaps a visit to Crawford's shop?

The original Crawford design, copied from an old wooden hull, had the banana-profile daggerboard, as shown on Chapelle's drawing for The Smithsonian. Later boats were produced with a choice of either daggerboard or centerboard. Currently, only the centerboard model is being produced, because it iseasier to control in very shallow waters.

Either Crawford model sails exceptionally well in all conditions.

My wooden SIMPLE GIFT has a heavy White Oak daggerboard, which I prefer simply because it is faithful to the original design.

Moby Nick

[Philip Maynard]

I think daggerbaords are deep for 2 reasons, 1 because that shape lends itself to a smaller case for the same amount of board area, and 2, a deep skinny board (high aspect ratio) is more efficient - look at modern rudders, they are deep and narrow. I have not seen Barto's plans but I would expect that the shape of a centerboard is going to be dictated by the area available in the boat that you are trying to put it in and that constraint is going to override all others. If you can get the same area and center of lateral resistance and get it to fit - I would consider that a success and the fact that the board is a different shape (wider and not as deep) a minor tradeoff to the benefits of going with a centerboard.

Gardner's "Classic small craft you can build" chapter 4 is devoted to triming a small boat - the relationship between the sail and keel area. I found that interesting but sometimes seemed contradictory and maybe dated but still helpful.

I have a very rough sketch of what I did for my melon seed, for what it's worth. If you email me, I can send you a copy. phil@pmaynard.lunarpages.com

[Wild Wassa]

Originally posted by MIke:
"I do not like the daggerboard arrangement in small boats in more open bodies of water, either."

How come? ... Do you not cleat the boad or secure it with shock cord? ... or what else comes into play, Mike?

Warren.

[ 10-10-2004, 09:49 PM: Message edited by: Wild Wassa ]

[Ian McColgin]

A boat meant for open water should be 'self-rescuing.' A daggerboard inevitably has a trunk with an open top, so for safety the top of the trunk should be at gunnel level. Depending on other aspects of the boat, this can be more of an intrusion than a lower pivoting board that comes up into a closed trunk.

An advantage of the lever system is that then the trunk can be absolutely enclosed. If you use pull/pull lines you have holes that will admid some water when the boat is swamped.

I think this model of boad is a bit too flared in the topsides to make good use of a leeboard but worth a consideration. You can go with a single board as done with a pivot pin, the strain is no more whether the board is actually to lee or on the weather side. The only difference would be if you allow the boat to heel so much that when the board is to weather it's too much out of the water and less effective. Such a board has the advantages of a pivot in that you can trim the underwater profile, and none of the fussiness of trunks. Only problem is that you need a verticle landing that really needs to be at least 4" from gunnel down, giving 2" around the pivot pin for a landing to hold the board and you have to make it strong enough that all the lateral strain of the sailing rig can be held by that 2" - not hard at all in a small boat - but the verticle may be hard to arrange.

G'luck

[DougWilde]

A pivoting daggerboard-shaped centerboard without lines in the water was designed by Uffa Fox for his Lively, Fifty Wooden Boats page 44. May not be the solution you want, but it is interesting. Also a classic solution is a rod to raise and lower a centerboard which itself need not be submerged when the board is lowered.

Doug

[paladin]

I don't know if Marc Barto is still at O'Connells boat works or not, but I had coffee this morning with his former "associates"/Roommates at MAS Epoxy and his cell phone number is ...I don't know Nuuuuthiiing!

[ 10-14-2004, 12:48 PM: Message edited by: paladin ]

[Wild Wassa]

Thankyou Ian and Mike. Mike's comment didn't make sence to me, at the time, being a wet dinghy sailor. I only sail in wet self rescuing dinghies. Self rescuing? ... (well I think that's is up to me on every occassion unless I really blow-it badly or wreck a dinghy ... which is not out of the question).

The last wreck that I fully participated in, ripped the transom right off the dinghy and bent the dagger, the buoyency tanks filled quickly, water didn't need to come in through the dagger slot this time. A bullet smashed/knocked down the dinghy, the rudder stayed attached to the transom in it's normal position, while the boat was ripped off the transom. The tow back into shore was far more harrowing than the damage to the dinghy, it was at speed. We shouldn't have tried to use a bullet to catapault the boat out of a gibe ... I still do though.

Warren.

[ 10-12-2004, 11:51 PM: Message edited by: Wild Wassa ]

[adampet]

Warren...A bullet? I'm not sure if I understand :confused: Can you edify me?

[BrianR]

Again - you've all come through for me.

Thanks so much for that cell phone number. You know what? It worked! First time! He didn't even ask where I got his number from!

After speaking to him, I'm convinced that I'm going to stick to the plans. He swears by them, points out the daggerboard case never leaks, saves room in an already small cockpit, and changing it would require changing the rig to accomodate the new CLR.

Anyway, thanks again. I really appreciate it.