Heavy Question

[Ted Chism]

I am starting on my centerboard for a 25' sharpie - 7' long, 30" high, and 2" thick out of white oak all bolted and drifted and epoxied together. Probably gonna weigh upwards of 150 lbs - fairly heavy piece of equipment. However, Chapelle and the others all show a lead insert but don't say how much weight we're talking for how big a board.

So, for a board of this size - a boat of this size, how much weight is about right? I'm thinking maybe 40 - 50 lbs, but that's just a guess.

Any ideas on how much weight to add would be appreciated.

Also, am I correct that I am going to need some kind of winch to get it up (careful here) or will I be able to get it into the top of the slot by hand?

[Lewisboater]

84"x30"x2" = 5040 cubic inches/1728 cubic inches per cubic foot is 2.9166666667 ish. times 45lbs/cubic foot density of White Oak and you are looking at 131.25 and change lbs for the board blank not including the metal bits. Seeing as sea water is 64 lbs/cuft and the amount of wood in the water will be about 2 cuft you have residual buoyancy of 38 lbs to obtain neutral buoyancy plus you'll have the weight of the rest of the board pushing downwards...so I would say you would need at least 60 lbs of lead to keep it down against the push of water at speed... unless you plan on pinning it in the downward position...then you could go with a bit less. Just a rough guess though. To paraphrase..."I think you're gonna need a bigger winch" because the more of the board that emerges out of the water the heavier it will get until you are pulling up the whole 200 or so lbs.

[Gib Etheridge]

Just figured it out and posted it only to find that Steve had done it sooner and more accurately.

I don't know that I've ever seen it done, and it may even be common practice, but I have an idea regarding the pivot bolt that might interest you.

Sometimes pivot bolts down near or below the waterline leak. That can be dealt with, but the possibility can be eliminated.

If your pivot bolt is down near or below the waterline and your lanyard breaks the board will hang almost straight down. It may even swing out of the slot entirely, except in way of the bolt, and twist or get jammed sideways. One thing for certain is that if it falls that far it will be a real pain, or worse.

I suggest that if the pivot were placed at the upper forward corner of the board and the leading edge of the board were cut on a rake sufficient to allow the board to lower as far as it should and then stop because the leading edge of the board hits the forward end of the slot both leakage and the possibility of the board getting "overextended" would be eliminated.

On pages 63 and 64 of Chapelle's Boat Building he shows plans for a 24' sharpie and a 24' Hampden boat with drawings of lead weighted centerboards for each. You may be able to scale surface area ratios from the drawing.

[Ian McColgin]

Modifying Gib's notion, on his bigger centerboarders like the she-pungy schooner, Capt Pete Culler gave the front edge of the board a radius around the pivot point which is centered from leading edge to back (up and down when board up) and just that far back. That positions the pivot above the waterline. Capt Pete did this not to avoid leaks - easily done if you think about it - but to make dropping the board convenient when the boat is in the water. It also puts more of the board in the trunk which reduces twisting stresses on the trunk and keel.

Drifted oak is great. Forget the epoxy. It won't help in the slightest. Goblin, a 12T Alden 43' schooner, was over 60 years old when I had her. That fabulous drifted oak CB could take the ground on a hard beat. I could feel the board, which had deflected sideways a bit as we grounded, straighten out, giving the stern a sassy wiggle as if to say "Pffft, you silly sandbar."

[michigangeorge]

I am starting on my centerboard for a 25' sharpie - 7' long, 30" high, and 2" thick out of white oak all bolted and drifted and epoxied together. Probably gonna weigh upwards of 150 lbs - fairly heavy piece of equipment. However, Chapelle and the others all show a lead insert but don't say how much weight we're talking for how big a board.

So, for a board of this size - a boat of this size, how much weight is about right? I'm thinking maybe 40 - 50 lbs, but that's just a guess.

Any ideas on how much weight to add would be appreciated.

Also, am I correct that I am going to need some kind of winch to get it up (careful here) or will I be able to get it into the top of the slot by hand?

Ted, I only addressing your last question (winch?) and would strongly advise you plan on using a block and tackle setup in lieu of a hand crank winch. The B&T setup is so much faster, easier and quieter than a winch and you won't bang your knees (or knuckles) on a piece of rope :-)

[James McMullen]

Drifted oak is great. Forget the epoxy. It won't help in the slightest.

Ian is absolutely right about this. Epoxy ain't gonna do a damn thing to help a 2" thick waterlogged white oak blank stay stuck together. If it just horrifies you not to have some glue in addition to your drifts then you can use 5200. But the epoxy will be a complete waste of time and effort in this application.

[BriMac]

I strongly second the block and tackle suggestion

[Ian McColgin]

Goblin's board was handled by a 3:1 block and tackle that came up through the cabin house on a turning block and then the B&T was horizontal back to the aft end of the cabin. It rode in a shallow copper sheathed U carved in a slat planted on the house to keep the moving block from scaring things. It was fine for raising and lowering the board when the boat was in the water. Had enough weight to keep it down when underweigh but enough bouyancy to be an easy lift for any crew member. When the boat was out for the annual shave-and-a-haircut and I wanted to drop the board for painting I had to be very careful to snub the line around its cleat lest it get away from me. It was easier to jack it from below to get it back up rather than dead lift with the B&T.

Marmalade has a very powerful worm gear winch, which she really needs given the weight of that plate steel board. I very much preferr the wood. Were I building a new Marmalade (as Jim is) I'd modify the keel to accept the wider trunk a wooden board requires even though this plays havoc with the interior. Maybe. Maybe not.

Side trick - If you're like me and run aground now and then (daily) and especially if you have a wire pendent, put some sort of spring at right angles to the centerboard pendent so that it will take up the slack when the board is pushed up by the bottom. This will keep the pendent from catching between trunk and board. If you have a rope pendent, you could make it thick enough to fill the trunk and thus never catch unless you drop the board so far down that there's some pendent exposed. I believe that any designer who plans for an exposed to flowing water pendent is one step below the designer who designs a board that when down has some open slot exposed. Doing both is really inexcusable.

Marmalade's


The white line leads up over a shackle to a bungee cord that's stretched along the overhead to the fore end of the skylight. Picks the pendent right up.

G'luck

[Ted Chism]

Thanks, guys, all good information and much appreciated. I have planned out so the pin will be at the center of a radius and just above the waterline and the board should not need to drop all the way out of the slot so I think I am good on those points. Good info on the epoxy - I will use 5200 between the planks but will cover the whole board with epoxy to reduce abrasion against the trunk (which is also coated). I am still debating between a block & tackle and a winch - I have a 2 & 3/8 slot so I suppose there's room for a B & T in there without getting hung up. With a B & T I could also control the board from the cockpit without having to dash up into the cabin if I am short handed.

Steve, thanks for working out the amount of lead I will need. It's usually difficult for me to approach problems that way.

[Ian McColgin]

I'd not bother with 5200 between the planks. Either the drifts do the job or nothing. Think about the twisting strain. If the drifts are down the center and if the planks are touching, then you have almost an inch on each side of the drift to take the compression of a bend in that direction. If you have 5200 in there, it just allows wiggle room which will gradually get worse as the drift holes enlarge and the planks pull away.

Somewhere I posted a pic of a very dried out rudder of this construction that I'll try to find and put up below. You'll see that a long dryout is not that big a deal. That rudder always closes up in a week. Just don't stress the boat or put the sticks up till she's taken up rightly.

As to epoxy on the surface. Pointless. Epoxy is not that abrasion resistant anyway and once it gets scraped enough to let water under to the planks, it will start cracking and peeling off. Especially with oak. Just red lead for priming and bottom paint. Even if you use Ecopaint for the bottom, get a little copper poison for the board since it's in the dark and the Ecopaint does not work as well without sunshine. And you need the red lead to keep the copper in the bottom paint from interacting with the drifts. Unless you're really cool and go with bronze drifts. In any event, it's not worth the worry. THe board will get scratched and it really will not matter.



G'luck

[Ted Chism]

Ian,

I bow to your experience and will do as recommended.

Thanks, I appreciate it.

Ted