change from daggerboard to centerboard

[Alan Peck]

This is a question that came up in my eariler post about small lapstrake boats. Normally a daggerboard is no problem on small boats because it doesn't go all that deep, perhaps 18 to 24 inches.

However, my sailing waters, the north florida gulf coast, are extremely shallow in many places. My past experience with daggerboards has not been pleasant in these shallow waters so I would like to have a centerboard instead.

Several of the designs I have looked at to date (12 to 15 ft.) have daggerboards. Is it reasonable to think that I could redesign the trunk for a centerboard. I've got the feeling that there is a lot more to it than meets the eye. For example changing from a daggerboard to a centerboard might effect the helm and result in excess helm in one direction or the other.

Therefore, I think I should probably not consider changing this basic design item. On the other hand, maybe its not that big a deal and I would hate to reject a design that otherwise fits my needs very well.

Anyone with any advice on this?

Thanks

[Buddy Sharpton]

Yep. I have both kinds. The boat is not going to care which it is as long as it is the same shape and location ubder water. There is a little more drag from the openslot of a centerboard, but so what-any centerboard design has that "detriment". The daggerboard puts an awful strain you must build for if it runs hard aground, but so what- all daggerboards have this detriment. Copy the centerboard construction details from a similar boat or design for the how to make. Principally both dagerboard and centerboard trunks need to have their foward edge strengthened athwartships by a center seat or deck beam or a strut added solely for that purpose. But that would already be provided for in the daggerboard trunk design you're building from. You need only to lengthen the trunk to be about one inch longer than the board measuring from the pivot bolt as your referencr point for measuring.

[johnw]

The type of centerboard that bumps up most gently in shallow water is the type traditional to catboats and sharpies, which drop down to show the water a triangle. Daggerboards and modern centerboards tend to be much more vertical. If you decide not to drop the board down to vertical, make sure the area with the board down is centered in the same place as it was with the daggerboard.

[DougWilde]

Grab your copy of "Fifty Wooden Boats". Turn to page 44 and look at Uffa Fox's Lively. This basically is a dagger board that retracts. If you study the plans closely you will see that there is no center board pin, rather that the board is on rollers to roll across the top of the case.
This, by the way, is a lapstrake design that I think would lend itself to glued-lap construction. I purchased the plans and they are lovely.

Just another option to consider.

Doug Wilde

[skuthorp]

I have a 12' yacht similar to a fireball (basicly a shaped box with a sail and quite fast) that comes with a 3'6" dagger board. I made an alternative curved centreboard that revolves up on impact for the same reasons (shallow water). Theres been a stream on the subject lately, including collapsible ones , and mine is very similar. The only real problem is if you hit a sandbar when sailing hard on a reach, you go over very quickly but you can walk away.

[Bruce Hooke]

There was a good article in WoodenBoat just a few issues back about a daggerboard that would swing up to clear obstructions. This might be a good way for you to go unless you sail so much in thin water that the daggerboard would have to be 'up' all the time. The design was aimed at dealing with what happens when a daggerboard hits a single obstruction like a rock...

[Chadd Hamilton]

I'm glad this topic came up again. I've got an Acorn Tender with a daggerboard and I am worried that I don't have it secured enough to survive a hard grounding ect. Basically, it is epoxied to the inner keel and has the center thwart as a horizonal support. Should I add 4 through-bolts to the case to further secure it?

Doug, the centerboard design you are speaking of sounds alot like the centerboard design on the Thistle Class; the centerboard is lowered into the case and has rollers on each side that act as stoppers. A pulley system at midship allows for lowering and lifting. I have a Thistle and can get some detailed pics if anyone is interested.

[DougWilde]

Chadd,

I'm not familiar with the arrangement aboard a Thistle. I've uploaded a scan of Lively to my site and give the address below.

Inboard profile of Uffa Fox's Lively (http://www.cox-internet.com/wilde/Images/Lively.jpg)

Its 238K and I didn't want to inflict that on anyone without sufficient bandwidth, without warning.

The arrangement really is clever because the 'board isn't attached to the boat but by the cable.

Doug

[johnw]

Looks like when it's down, the cable is out, so when it kicks up the board will stay back where the cable holds it. It will kick up until it hits the back of the case, leaving the boat with about a foot more draft than it would have if it had pivoted on a pin and retracted into the case. This may be why the design hasn't swept the world. I've sailed a sharpie to windward right into a beach, smooth and peaceful as you please, while someone following in a much smaller boat (an El Toro) first stubbed the daggerboard, then when he pulled that part way up, stubbed the rudder. Once he got those up, he had to paddle in with the daggerboard and with very little dignity. The design for Lively combines the long, intrusive case of the centerboard with a board that won't kick all the way up (especially if the cable isn't secured -- it might just move back in the case.) For racing I can see the point of the design. You can have the board down all the way, so that it is an efficient foil, and still move the center of lateral resistance aft if you need to. For cruising, a workboat design would be better.

[Dave Hadfield]

You might want to look at Jim Michalak's ideas re:leeboards. He builds them as centerboards with open framing located on the outside of the boat -- in use on both tacks; just one for fairly large "small boats". Very practical.

One negative aspect regarding changing designs to this though, is that the leeboard has to be hung on the part of the side that's facing fore-and-aft, ie the point of max beam. This means the sail plan has to place the center of effort there too, or very close.

Still, if you haven't decided on boat type yet, I would give his site and plans a good once-over.
http://homepages.apci.net/~michalak/1sep01.htm#LEEBOARD%20ISSUES

[bud]

Blue Earth - I was once sailing across a bay a mile from shore and my daggerboard struck what was probable a submerged piling and I can tell you what happens. The board jams against the keel which pivots the top forward, chopping the cap and forward strut down the middle like an ax into cordwood. Water enters the boat. But still, I'm not sure that in a boat with so little room I would give up any space to a centerboard. I'm just careful now to set the board halfway up unless I need it all. Everything's a compromise.

[Chadd Hamilton]

Bud- I thought that's what would happen. I am afraid that the whole case might break off at the base and leave me with a slot and water pouring in. I'll have to play the daggerboard I guess.

Doug- That centerboard design is very similar, if not identical, to the Thistle. Works really well IMO.

[htom]

Leeboards do NOT have to be at the point of max beam.

You can put them forward or aft of that point, providing spacers on the leeboard or the hull to align them correctly.

It is easier, and doubtless cheaper, to put them at the point of maximum beam, but there's no intrinsic requirement that they be there. Indeed, if your attachment allows them to move fore&aft along the hull, you can use this to adjust the steering balance.

[johnw]

BlueEarth -- I thought the Thistle had a pivot bolt. Not having one is the only thing unique about the setup shown for Lively. Fox used many centerboards that looked like this, but this is the only one I recall without a pivot bolt. Fox designed many centerboards that looked like this, but the others I've seen included a pivot bolt. Generally he used a slot in the board rather than a hole, so that the board could easily be removed. If your Thistle didn't have a pivot bolt, I'd be interested in how it acted when you ran aground. Most race courses are set up so that this is not a problem, and most racing boats are sailed in such a way that they don't run aground much. I don't think Lively's setup is a problem in ordinary use, I just don't think I'd care to run aground at speed with it.

[Dave Hadfield]

Quite right, leeboards don't have to be at the point of max beam. It's just easier to locate them there. Not as much structure required to keep them pointing fore and aft.