Mold making from a plug

So I am looking to repair the trailing edge on a couple of daggerboards that I bought used. The trailing edge is in really poor shape. I was thinking of making a mold of the trailing edge from a new daggerboard to aid in the repair process. If you were going to do it this way would you make a fiberglass mold or would you make one by laying the new board into some type of plaster. On one board I will probably have to lay some glass for stregth. On the other I think I can just rebuild the edge with thickened epoxy like I have done on some Hobie 18 boards many times. Previously I had clamped a straigted edge on the trailing edge, packed it with thickened epoxy and then sanded to shape. While don't think the mold will save any time, in fact it will probably negate any extra sanding with the time it takes to make the mold, I thought it my be fun to try something different and pick up a skill I can use later. Seeing as new boards are now 850.00 a peice, I probably should make a mold of the entire board and start a production line.

I have used and like Gypsum, Ultracal 30, as a molding material. It is good for a limited number of parts, which I have never exceeded.

Dan

I can't offer much on mold-making but..
To repair a damaged trailing edge, make flat strips of fibreglass or carbon lay-up about 1-2mm thick and about 25mm wide and as long as the repair that you need, cut a slot down the trailing edge with a Dremel disc and insert the strip with some epoxy as glue. This will give a nice hard edge that can be filled to and faired

Thanks Paul. Thats an idea that I had not previously thought of. I just may give that a go.

The easist way to make a mold from a plug is to use modeling clay, gelcoat and fiberglass. There are several posts of the internet on this subject, so I will just give a summary.

You are going to make two half molds. Lay your plug on some plywood. Place modeling clay around the entire perimeter of the plug from the plywood to the HALF line. The modeling clay supports the plug and delimits the half mold everything above the modeling clay will be your half mold.

Wax the top of your plug and spray it with gelcoat. You will need about 2 coats more than you think you do. You will not be waiting until the gelcoat hardens. Recoat as soon as it is firm enough to take a new coat. The gelcoat is just to give a slick smooth finish to the inside of the mold.

As soon as the gelcoat can support it start laying fiberglass/polyesther resien. You are not building a lightweight hull. You are building a heavy, sturdy, cheap mold. Solid glass needs to be 1/8 to ¼ inch thick depending on the size and shape of the mold. Personally I cut a lot of 1x6 inch strips and lay 2 layer of glass then a core material soaked in resien. 2 layers of glass, more core etc … until I think it is stiff enough. For cheap core I use the blue shop paper towels or the non woven material used for cheap car covers.

When it is cured pop the plug out and cut the excess material off so you have a half mold. Leave a little extra so you can match the two halves. Repeat the process for the other side.

For a rudder repair you can get away with only 1 mold half and eyeball the other side. I am repairing a pair of EPO rudders that were broken into pieces so I will build a new skin and use the old rudders as a core.

Both ideas are good. One thought your finished product is only as good as your plug / mold so it looks like you will have to repair at least one board. Paul has a very good idea on how to fix the trailing edge; it is good to have some type of fiber material in the repair and a good anchor to your part being repaired.