Thoughts on centerboard trunk

[Bill Heil]

When visiting Rob's site, I noticed he was building the centerboard trunk right in the boat. I never thought of that, I always built the trunks outside of the boat and then installed them. One other thing is that I fiberglass the inside of the trunk, usually with two layers. You just never want something to go wrong with the inside of a centerboard trunk after it is in the boat. i also build the board at the same time as I am building the trunk, to make sure the trunk fits the board with the proper clearance, etc. Also, when I install the trunk in the boat, it put temporary spacers in the trunk so it can't close up on me during the installation process. I also taper the plywood along the inside (where the board fits) of both sides of the trunk so I can glass the trunk to the outside of the boat bottom. The taper is enough to make up for the thickness of the glass i am putting on. This ensures a strong, leak free trunk installation. Maybe, I am a little intense about this, but I sure don't want to fool around with a centerboard trunk after the boat is built and gets some age on it. Bill

[Joe Nelson CS#35]

I think the advantage of doing this way is to ensure an accurate cut of the slot. I am doing it the same way now...but after the slot is cut I am taking the case back out to glue it together and drill the pin for the centerboard pivot.

I ran my glass long on the bottom of the case and didnt epoxy the bottom edge. Then I can pull the glass through the slot and wrap it around the hull bottom. You can get access from under the boat to coat the bottom insides of the case with epoxy. I did this to avoid a high spot in the case at the lower edge. Then you dont have to taper the bottom edge to allow for glass thickness. Should be a seamless/consistent surface inside the case.

I also felt my board was a little tight after test fitting it in the case. So I added a few layers of epoxy to one side of the king posts to enlarge the interior size of the case.

Joe

[Brent]

I built the CB case and CB at the same time, glassing both the board and the case. I too had to widen the king posts a little to get a good board fit. I installed the port side of the case first, then the starboard so that I could get each individually tapered to fit. Next time I wont do it this way, I think it would be easier to epoxy the case sides together, then fit the whole assembly into the boat.

My router with a round off bit did a great job on the hole in the hull. I used 3" fibreglass tape to further bind the CB case to the boat hull. The tape got covered with the 6 oz cloth I covered the entire hull with, leaving a smooth transition around the hull.

Spacers were essential during the case installation to keep the sides lined up properly.

[Docpal]

I know this is an old post but it was the most applicable topic when I ran "centerboard trunk"through the search engine.

My (latest) question is this; I've just finished coating both the insides of the centerboard trunk, and the centerboard itself with multiple coats of graphite/epoxy glop. The board is a little wider than "spec"and so I've widened the trunk spacers accordingly to get the 3/16"gap which Graham suggests in the plans. However when doing a dry fit it seems to me that there is much more "play/slop" possible for the board than I feel comfortable with. SO I am asking anyone who has used the graphite coating whether or not they thing I can narrow the gap down to 1/8" ( or even less)  without a binding issue...Thanks!

[Norman Colter]

Graham's specs for the Princess centerboard trunk says 3/32. (actually says "If clearance is less than 1/16" the board will bind, if clearance is more than 1/8" it will rattle") so anywhere in between should be fine. My advice FWIW would be more towards 1/8". 

Norm

[PAR]

Make up the difference with a shim on each side of the board at the pivot pin. Mine were Teflon disks, which made a very slightly snug fit at the pivot. The advantage is the Teflon will permit the board to move easily and the sung fit will eliminated any rattling. HDPE, UHDPE, PVC and ABS will work too. I just happen to have a little Teflon lounging around at the time.

[Docpal]

Norm/Paul,

Thanks for the info and hint. I'm probably going to try a combination of the two thoughts and take a little off the king posts as there is waaaaaay too much play for my liking, but I will also use something slick as a washer in order to fine tune the final gap.

[tom151]

Docpal,

I haven't read the details yet - so forgive if I'm restating directions or old (good) advice.

Not to be omitted... consider that at the place where the board exits the c/b slot is the high load area and so be sure that the contact patch is NOT a point/edge load. Making this area of the case intoa 'flat' surface bearing against the c/b does a good job in reducing the side loads that can break a board. This belt & suspenders thinking might include added layers of fiberglass on the board in this local area.

So, if this needs doing (adding to either the case or the board) you may not need to take anything off of the king posts.

My 2-cents,

TomH

[Docpal]

Tom,

Good point, thanks! I think what I will now try is a "dummy"assembly where I'll use some scrap wood as temporary king posts and then try to somehow suspend the entire case in a way I can measure the gap at the lower edge. Once I get a real feel for my clearances I can do whichever fix(es) will give me minimal slop, but solid support. I have a little advantage in the fact that I have embedded 5 pounds of lead near the bottom of the board so between the graphite case and board, (plus the weight) I'm pretty sure I'll be able to dial this in to the point the where the board glides up and down cleanly, and evenly.

[Joel B]

Whats the purpose of the 5 lbs of lead?

[Docpal]

A number of earlier posts mentioned that there was an issue with the boards lifting while underway, and making speed. A few people mentioned the possibility of using added weight to keep the board down, and since I was at that point of the building process I thought I'd give it a try....