Converting to centreboard

Well, here goes! I hope that you will see below either a photo or a link to a photo of my centreboard, most of the way through the project, but not quite finished.
My first post in this topic gives the reason for doing the job in the first place. I'll try to post some photos in chronological order, warts and all!
John Booth
Red Kite 53814

Picture copied and re-saved and linked here by Admin , 24 Oct. 2020

This is what started it all off. I was just leaving the lake shore when I was hit by a gust. I pushed the board down too soon. The boat took off like a rocket, and the board hit a rock. Wham! Crack! The case split open.

I had had similar issues before, although not so serious. As I was going to have to replace the case anyway, I asked David Sumner of the Dinghy Cruising Association if he had any plans for his centreboard conversion, and he did. So the job started! More to follow!

NOTE by Admin: I have edited this to show the picture. @Jollyboy : it's all in your court now ;- )

The question has to be asked. Why change from daggerboard to centreboard at all? It's quite a lot of work, and I think I probably wouldn't have done it if the daggerboard case hadn't cracked open. That being said, I saw David Sumner's conversion and thought it would make a lot of sense if, like me, you're sailing in very shallow water much of the time, risking hitting underwater obstructions. It also makes it a lot easier for the crew.
Given that I was going to have to scrape a lot of paint off the bottom of the boat, around the case opening, I decided to scrape the whole hull and repaint it. When I did that, I was shocked to see that the glass tape on the outside of the joins was barely adhering to the hull at all! I doubt if it was giving any structural strength at all. See this photo: It just peeled off easily with my hand.

The first thing was to consider the design drawing that David Sumner sent me. I haven't asked him if I can put a photo of his plan on this forum, so I don't have a photo of that at the moment.
I have to add at this point that I have never done a job like this since I built Mirror 26465 Lucy Lastic with my dad in 1971! I was therefore a little apprehensive. Perhaps I had bitten off more than I could chew? In any event, I couldn't use Red Kite with the crack in the daggerboard case as it was, so there was nothing to lose.
David's original design appeared to have case sides made out of 3.5mm ply. This seemed a bit thin to me, so I upped it to 6mm. Here is a photo of my design:

There are many things to sort out when trying to design a board and case like this. To cut a long story short, I made the board out of 12mm ply. The big question was how to make the effective area of the centreboard (i.e. the bit which protrudes below the bottom of the hull) as close as possible to that of the daggerboard? The lateral area of the standard Mirror daggerboard below the bottom of the hull is 57cm x 37cm, i.e. just over 2100cm². What would the effective area of the centreboard be? There are alternatives. You can have a long, slim, higher aspect ratio board, but if you do that, you need to cut a longer slot in the bottom of the hull, to allow the board to pivot up into the case, but that weakens the hull. Or, you can build a not-so-long board which is wider from front to back. If you do that, the centreboard case needs to be taller, requiring the thwart to be raised. That gives even less headroom under the boom than there already is. So eventually, I went for a good old British compromise! I made the board slightly longer but no wider, ending up with a centreboard with an effective area of 1743cm², or approximately 83% of the size of the original. In practice it has not affected the boat ability to go to windward in any noticeable way.
I'll do another post with a photo of the centreboard, and details of the centreboard case.
John
Red Kite 53814

The next step was to remove the thwart and the daggerboard case. I think I have been more fortunate than some, in that the 1976 brass screws holding the thwart and the centreboard together all came off without any great disasters. I know some people have had the screwheads snap off. I removed the screws holding both ends of the thwart to the supports attached to the side buoyancy tanks, and then took out the screws holding it to the daggerboard case. I removed the thwart, complete with the wedge-shaped support pieces which attach the thwart to each side of the daggerboard case. I was able to reuse these support pieces when I put the new thwart back in.
I then turned the boat upside down, and removed the brass keel bands on either side of the daggerboard slot, so that I could scrape away the paint and glue, and access the screws which hold the case onto the hull. Once I had done that, I turned the boat right way up again and set to the task of removing the case from the hull. At this stage, having removed the screws mentioned above, the case is only attached to the hull by resin and tape. I used an electric heat gun to soften the tape and resin. This is a fairly sticky job. You have to be careful not to heat the plywood so much that the glue holding the ply together boils! I worked very gingerly, gently applying heat, withdrawing the heat gun and gently pushing a broad bladed chisel between the case and the hull.

This is a warts-and-all description of the project, so I am telling you the things that went wrong as well as what went right. I managed to get the port side of the case loose without problems, but in my enthusiasm to get the case out completely, I bent the half-removed case to starboard (having warmed the resin first), and in so doing managed to peel off the top layer of ply from the starboard side of the slot!

Having done that, my immediate worry was that I had weakened the hull. In practice, it was not so great a disaster. Later on when installing the new centreboard case, I was able to strengthen that area with thickened epoxy, and it appears to have worked.

I then started building a mock-up of the case. I did that as I had the necessary wood in my garage, and I wanted to see what problems might await me before I made the final version. It was good that I did so, as there are all sorts of hidden problems awaiting the unwary amateur boatbuilder! The first was that the vertical transverse bulkhead which has the two openings into the storage boxes at the front of the cockpit is in my boat not quite vertical, so I made the mock-up of one side of the case and stuck pieces of cardboard onto it to fill in the gap. If I had made the front end of the centreboard case absolutely vertical, it would have fitted fine where it touched the bottom of the cockpit floor, but there would have been a gap of about 3mm at the top. Although I guess I could have just filled that with epoxy, I thought it was better to cut the case side pieces slightly forward of vertical at the top end.

More to follow......

Next, I had to decide on the size of the centreboard. The effective area (EA) of the standard daggerboard is 2109cm², i.e. 57cm deep below the hull x 37cm wide The EA is the area of the part of the board which protrudes below the bottom of the hull, so not the full area of the board (which is approximately 33cm longer). I don't think it's possible to make a pivoting centreboard which retains the same area as the daggerboard. The centreboard case would have to be about 10cm higher than the existing case, which would make it virtually impossible for the crew to change sides below the boom. Unless of course you like to make things tortuously difficult for your crew.... ;-).

The board I eventually made has an EA of 1750cm² - 83% of the original board's size. The leading edge is 67cm long. The trailing edge is 58cm long, and the width is 28cm. I retained the same thickness of board - 12mm. That creates a board which is 10cm longer but narrower, giving a higher aspect ratio, and a longer leading edge, which I hoped would partially make up for the smaller area of the new board. In practice I haven't noticed any difference in windward performance. Here's a picture of the board.

The case was made simply to match the size of the board. I had to scratch my head a bit to work out how to match the curve of the bottom of the case to the shape of the hull. I eventually used a batten marked off in 5cm intervals, and slid a ruler down each 5cm.

That worked ok, but I made another error. I didn't take into account that the boat was sitting on its launching trolley, which was slightly distorting the bottom of the hull, pushing it slightly upwards. That was exacerbated by the fact that there was no stiffening effect from the daggerboard case, as it wasn't there any more! In practice, it didn't matter greatly, and the hull and case came together later without any visible distortion.

This is what the mock-up of the centreboard looked like in place:

I made the final case out of 6mm Okoume marine ply which I bought, along with most of my other supplies, from Fine Boat Kits near Kendal in the English Lake District. I would highly recommend them to UK Mirrorists. They are highly experienced and knowledgeable, and very easy to deal with. David Sumner's plan suggest that he made his case out of 3.5mm ply, which seemed a bit thin to me. Here is a photo of the assembled board:

I used some 6mm offcuts to make strengthening supports for the pivot bolt which can be seen here. In effect, the pivot bolt is support by a total of 12mm of ply on each side of the case. The gap in the case is 17mm wide. I bought the pivot bolt from, I think, Trident UK. It is a 10mm A4 316 Marine grade stainless steel pivot bolt and is intended for an Enterprise.

The next step was, I felt the point of no return. That was cutting the centreboard slot in the hull. Up to this point, it would have been possible to get cold feet and re-attach the daggerboard case, but once the slot is cut, there's no turning back!

Before gluing the sides of the case together, I just remembered in time to paint the inside of the case! I then had to chamfer the underside of the bottom of the case to match the curve of the hull. That took ages! Finally I was ready to screw and epoxy the case in. That went much more easily than I expected. Here is the final case screwed and glued in place. I haven't screwed the top cap onto the case yet here.

You may notice that I attached vertical battens to each side of the front of the case, so that I could bolt it onto the transverse bulkhead. That worked ok, but, as Sod's Law would have it, just behind the bulkhead, where I couldn't see it, is the edge of a vertical batten supporting the bulkhead, precisely where the hole for the bolt was. I therefore had to glue a second batten alongside it, to give a wider flat surface for the nut and washer to tighten against.

This is a photo of the centreboard and case after finishing the project.

I also made an inspection hatch at the back end of the case, in case I get any stones or debris jammed in the case.

I replaced the keel strip with an aluminium strip, and installed a proper slot strip to prevent water slopping up into the case. I don't have a photo of that at the moment, but will put it up here on the forum asap.

The whole project took me about 9 months, but that was because I wasn't focussing on it all the time. If I had been, I reckon I could have done it in a month. Red Kite sails very well. As mentioned, I have found no obvious reduction in her windward ability, given the slightly smaller board. It's also much easier to vary the depth of the board, and my crew are eternally grateful!

If you have any questions, I am happy to answer, if I can!

Fair winds!

John Booth
Red Kite 53814