Centerboard of DOOM!

[John C. Harris]

The earliest drawings of PocketShip indicated a steel centerboard. The challenge of having to torch a plate of steel to shape and get any sort of shape on the leading and trailing edges was a major drawback. But the biggest issue would be the weight of the steel board---I think it was about a hundred pounds. This required a tackle arrangement for hoisting, and Murphy's Law demands that the tackle would get tangled or fail somehow right when you need it. In college I worked as a rigger and and did penance in the bilges of big cruising yachts with terminally jammed centerboard hoisting apparatuses. NO FUN.

A lighter aluminum board would be nice. You can work aluminum with woodworking tools. You'd have to mind corrosion---etching and coating would be required. But that would be even more true for a steel centerboard!

What usually jams centerboards is beach sand, gravel, or mud. I wonder if a looser-fitting board is actually MORE subject to jamming, as more sand and gravel can get into the slot.

Ultimately, a centerboard is a moving part that lives underwater. Sometimes they jam. <Shrugs> My insistence on inspection ports in the CB trunk is to allow you to unjam the board if needed. In two hundred launches of PocketShip #1, the board has hung up a couple of times, and was easy to dislodge through the inspection ports.

What might be the BEST solution would be a neat 3/8" hole with a grommet in the bridge deck. You can insert a metal push-rod in the hole and push the board down if it ever gets jammed. I know of centerboard boats that have the push-rod in a neat mount on the centerboard trunk.

Once in awhile I daydream about a fixed-keel PocketShip. That could be very cool, and still trailerable.

[truenorth]

That's interesting -- and not hard to install. The bridge deck worries me because it could actually encourage things to fall down there. I'm envisioning a hole somewhat below the pendant hole angled to insert a rod to engage the centerboard. This would provide less force than through the bridge deck but more than through the decking plates. Think that could work?

[Bflat]

Come to think of it, I've seen such holes and rods on centerboard cases. Great idea. I think I'll go ahead and do that when I come to it.

Rather than a grommet I'll just make a simple hole lined with epoxy on the bridge deck. Then I think I'll make a flush sacrificial plug made of balsa so that it can be easily broken though if/when the time comes.

Bob

[Bflat]

Hi Truenorth,
One possible problem I could see with putting the hole below the pendant exit hole instead of on the bridge deck is that the rod would have to get purchase on the thin trailing edge of the centerboard. It would seem to slip off and/or cause damage to that edge and/or the trunk lining.

For what it's worth,
Bob

P.S. I did take your advice and installed the bulkheads and floors without the cleats. It made glassing and sanding there much easier. I'll add the cleats later, after I epoxy and sand them at the bench.

[DanaDCole]

Please check John Harris's post on page one of this thread if you have not already. He makes several suggestions, the last of which is a hole in the bridge deck allowing you to use a metal rod to push the CB down. I have a couple thoughts about that very good idea: First, I might want to put a plug in the hole instead of just a grommet. Second, wouldn't a wood dowel be less likely to do any damage?

I'm pretty sure I'll put in that hole. It looks to me like it should be maybe an inch or two aft of the pendant sheave. It should be relatively easy to avoid the pendant, although if you release the pendant from its cleat, it shouldn't hurt anything to push it down along with the centerboard.

Any thoughts? I've got a long time to think about this--haven't even finished the centerboard yet!

[Bflat]

Dana,
My post was in reply to Truenorth's post which was in response to John's post about a bridge deck hole.
You may notice, I suggested an aluminum centerboard, to which John seemed to approve of although I'll be sticking to the plywood centerboard as designed since I find its thickness and the clearances to be good.
He then suggested perhaps the best idea, that of a hole in the bridge deck, to which I said I'd probably do when I get that far in the build.

Yes, a dowel would be a good first try, but at a 1/4 inch or so, might prove too bendy in which case a metal rod would be the best second choice. I'd like to keep the hole rather small.

For what it's worth,
Bob

[DanaDCole]

My apologies. I didn't realize there was a third page of postings and was replying to the last post on page 2! When I found my post at the bottom of page 3 I then found out there were several postings above it that said about the same thing I was saying--oops! as the gov of Texas would say.

Re the wood dowel. Perhaps I was thinking of too large a hole in the bridge deck. I was planning on a 1/2" hole, which should allow me to insert a 3/8" rod, even with the grommet. Does that seem too large a hole?

[DanaDCole]

On reflection, I realized the trunk opening is a full 1" wide. I don't see any reason why the hole could not be 5/8" or even 3/4" in diameter as long as you are very careful to get it perfectly centered. Either of these would allow insertion of a fairly large diameter dowel. The larger diameter wood dowel means less possibility of breaking or bending, and less possibility of damage to the CB, both because it is wood and because it is bigger around.

I'd appreciate any thoughts on this. It does not seem to me that a hole, even as large as 3/4" would present any structural problems in that particular spot. A removable plug would keep objects from falling in the hole--either a purchased plug, or Jon's idea of a balsa plug should work fine, although I suppose the manufactured plug could cause a sitting problem or tripping hazard.

Also, are there any ideas about the exact location of the hole?

[Bflat]

Hi Dana,

Actually, the balsa plug was my idea

Anyway, I don't think a bigger hole would cause any structural concerns and, of course would allow for a bigger dowel. Personally, I want as small a hole as possible so am thinking of one that would allow for a 1/4 inch metal rod. If I broke a dowel in the trunk my day might suddenly become a lot worse.

Either way, I think a plug would be needed. Without it, I imagine a narrow beam of sun light "painting" its way across a part of the inside of the trunk and board every day.

In looking at the plans, it looks like a good place for the hole would be several inches behind the sheave right where the highest point of the raised centerboard is. The pendant might get in the way, but may be easy enough to work that aside with the rod. Better still, might be a little ahead of the sheave to leave the pendant completely out of the way.

I'm trying to not over think things, but am starting to consider ways to protect the pertinent edge of the board from possible "rod damage." I have some Kevlar on hand, but the stuff is tough to cut and fuzzes up rather than sands. But, they do make bullet proof vests with it. I have some graphite cloth that might do. The simplest might be to just epoxy a thin strip of oak as a sacrificial covering. That would be installed on the completed, glassed board. The edge in question stays in the trunk so looks and hydrodynamics aren't really an issue. Such a strip could even be notched or given a hole to accept the rod for greater purchase.

Another course of action would be to not even drill a hole until if and when it's needed, but then, when it's needed could be at the worst possible time (Murphy's law).

My launch is quite a ways away, but this stuff is sure fun to think about.

Bob

[DanaDCole]

Sorry about the mistaken credit--that's what I get for relying on my memory. You've got some pretty cool ideas. I think the hole would need to be aft of the sheave, though, for maximum leverage on the board. You're right, these things are fun to think about. The sacrificial oak strip seems like a good idea, as long as you attach it such that you know it can't fall off and muck things up in there itself.