Centerboard to Trunk Clearance?

[UCanoe_2]

How much clearance should there be between the centerboard and the inside of the trunk? In the attempts to achieve a good foil shape, I have added thickness to my CB. It's now about 1-1/8" maximum thickness. A couple layers of glass cloth and a little more filler should bring that dimension to about 1-3/16". Sticking a pair of inside calipers inside the trunk at several locations, the minimum inside dimension appears to be about 1-7/16".

So when everything is complete the clearance will turn out to be about 1/8" on either side. Is there a preferred measurement? I don't want to have the clearance too tight and risk jamming the board with debris.

Also, how fair is fair? What are the usual tolerances for the board profile? Now it is within +/- 1/16" of the planned cross-section according to my pattern. I'm getting tired of mixing toxic goop, and I am really really sick of sanding. Perfection seems to be pretty elusive, and anything, even a hunk of scrap plywood, will be an improvement over the original damaged state. As GreenLake pointed out in another thread, this boat is less than perfectly symmetrical.

As of a couple days ago, the CB weighed 29 lbs. I think I started with more than the class maximum weight of 25 lbs., and figure it will come out to 30 lbs. when all is said and done. I'm not racing, so who cares as long as there is not excessive strain on the fittings, etc. For crusing purposes, excess junk in the boat or excess beer consumption by skipper and crew can easily add more than 5 lbs.

[K.C. Walker]

Yes, but extra beer weight on the skipper and crew is movable ballast… so is better!

Your centerboard is going to be a serious improvement even if you stop fairing right now.

[GreenLake]

I agree with K.C. on all points here. Let me emphasize just one:

From my own experience I would say that the stock CB (esp. if heavily used or aged) is so marginal, that even half-baked improvements make a noticeable difference. From everything else I've learned in sailing I would estimate that doubling the effort will give you 10% extra in performance (at best), a ratio that seems to be steady no matter how often you double your efforts

[kokko]

I had a lot of slop in my cb. First I squared off the hole that the cb handle fit through, and then I bonded two lengths of PVC to the inside of the cb trunk from below. The pieces were each about 3/8" thick, 2" wide and 16" long. I used 3M 4200 to hold them in place. It has significantly reduced the slop, and the thud when I tack

[UCanoe_2]

Kokko, that's a good idea. Shimming inside the case would reduce the stress on that square hole, which on my boat wore out, causing the CB to get stuck and preventing it from lowering completely.

A couple of questions:

How much clearance do you have now between the CB and the shims?

Where did you get the PVC material? Maybe home improvement store plastic trim material?

I have a small tube of 4200, but not enough to do this job and no local source for more. Is this a polyurethane adhesive? What kind of substitute should I look for out here in the mountains?

[Alan]

TAP Plastics has PVC sheets on their website, here:

http://www.tapplastics.com/product/plas ... sheets/531

[K.C. Walker]

I shimmied my centerboard by screwing strips of high density polyethylene to the centerboard (at the top of the board and at the trunk opening). I was able to get a very snug fit this way but because the high density polyethylene is slippery it works very smoothly. This is also available from TAP Plastics. My boat came with Formica shims at the centerboard trunk opening, I understand this is not an uncommon practice and works pretty well. Use contact cement and you can stack strips to get close fit.

[hales2]

To: ucanoe
Re: Centerboard weight and clearance

Are you working with a DS 1? What was the weight of your centerboard when you finished? I have a DS 1, weight of my CB is only 21#. I drilled a hole in it (it is hollow) and just bought some lead shot from the gun store. (Got some weird looks from the guys with my boat shoes and all). So I am thinking of pouring 5# or more of these big BB's in to my centerboard, then patching the hole. BTW, I like your idea of adding weight by applying fiberglass to the exterior of the board itself.

My big concern is: will the board still retract ok, given the pivot point location? Can I go to 30# you think? If I pour in any more lead shot, will I have to join the NRA?

Thanks
Mike

[kokko]

Just considering the physics, adding 5# or 30# of weight to the centerboard will not affect the stability of the boat as compared to crew and gear weight. It may also structurally weaken the board, and put undo stress on the cb catch or the pin

[hotwheels]

The centerboard/trunk design on the DS 1 could use some improvement. Shimming the board is good, use plastic sheet 1/8" thick. Make it big, cover the whole head of the board. Shim for minimum clearance, you do not want any side movement as the boat rocks. I thru bolted the CB pivot. I very carefully drilled the handle and shaft, the cover plate and the opposite side of the trunk to clearance a 1/4-20 bolt. I tie the shims to the board with string and install the board and shaft into the trunk. I then insert the thru bolt and tighten for minimum movement of the board side to side yet easy up and down. I double nut the thru bolt to lock in the clearance. Without the thru bolt the loading of the centerboard can spread the trunk and cause the centerboard to be loose. I install a rubber washer under the port side to prevent leakage, just like the starboard under the handle. This arrangement does not depend on the ridgedity (or lack of) of the trunk to maintain c/b clearance. If someone can instruct me I will shoot some pictures and post them. I will also email pictures. I currently have the c/b out of the boat (unrelated problem) and can take some good pictures.

[UCanoe_2]

hales2, I did not weigh my board after I completed it. 30 lbs. is my best estimate. I rigged a 2:1 tackle to make it a little easier to raise.

hotwheels, you have a good idea. Even after rebuilding my CB and making it about 1/4" thicker there was still more slop than I like. How did you align your through bolt hole in the proper place on the port side of the trunk?

[GreenLake]

For picture instructions see the section that says "website info" - where there are several threads that discuss posting pictures. If any instructions conflict, always take the later ones, as the sit was updated some time ago and some details changed slightly.

[hotwheels]

I used a drillpress to drill the thru hole in the handle/shaft. I clamped the square of the shaft in a table vice and brought the shaft thru the table. I wanted the hole to be square with the flat sides of the shaft, not the flat (?) end/handle face of the shaft. I flattened the tapered end of the shaft with a file and center punched it, "eyeballing" the center. I then carefully drilled a smaller "pilot" hole in the shaft. I then enlarged it to clearance for 1/4-20. Using the shaft as a guide in the trunk, I drilled the port side of the centerboard trunk. I then installed the retainer plate on the handle and from the port side of the trunk, drilled a hole in the retainer plate. I now have holes in all three pieces, the port side of the trunk, the shaft/handle and the retainer plate. The holes are more or less on the same center line, at least close enough for everything to pivot just fine. I then added a piece of 1/8" plastic to each side of the c/b, make these plates big, cover the entire top of the c/b. I then fine tune the friction with a nut on the starboard of the trunk, just enough to keep the board down at speed. I lock in the adjustment with a "jam" nut. It's a complicated process, but it get's the job done right. If anyone needs, they can mail me the shaft/handle and I will drill the first and most important hole in the process. With this as a guide the other holes will be easy. I can also take pictures, although I don't know how to post. If you send a email to kj4amh@gmail.com, I will reply.

[GreenLake]

I've pointed you to instructions on posting pictures.

Here's a picture of my own, showing the port side of a CB that was prepared by a PO to your specifications.



Note the reinforcement of the CB trunk wall and what looks like a bushing in the hole (the rubber gasket shown is the old one, prior to being replaced).

I don't shim my board and the bolt is tensioned just enough to keep the gasket tight, so there are no leaks on the port side. (Leaks on the starboard side are controlled by tightening the bolts on the triangular holding plate.)