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t_train 09-11-2012 04:34 PM

Building a daggerboard trunk
I have a 7.5' rowing dinghy that I got from a salty sailor friend for cheap, and I want to modify it for sailing. I have never done any fiberglass work, but I have a basic understanding of it, and what is involved in glassing in a daggerboard trunk.

If any of you have done this and run into any problems that seem to be typical of this sort of project, I would love to hear about it before I start.

Also, for some reason, I'm bent on gaff rigging this thing. I've whittled a 12 foot red cedar mast, which I oiled last night, and it is GORGEOUS. I need to make the boom and gaff, but before I do, I want to figure out what sort of sail area I'll have. I have a friend with a sewing machine that is designed for making sails, biminis, dodgers, etc, and was hoping to find an old sail that was maybe ripped that I could cut and make a smaller dinghy sail from.

Anyways, my question here is, how large of a sail can I go with before it becomes too large? The daggerboard I have will give me almost almost a 3 foot draft (it is 4 feet overall), so I'm thinking I can get a fairly large sail here for the size of the boat. I'd say the boat weighs about 100 pounds. It will be a cat rig, but I think at some point I may decide to put a bowsprit on the little thing and put a very small jib on there. I want the saltiest little dinghy in the neighborhood!

Thoughts and comments?

t_train 09-11-2012 04:59 PM

Re: Building a daggerboard trunk
Oh, also, I'll be building this thing strong and I'd like it to be capable in strong winds. The standing rigging will be a little oversized, and I've been toying with sail area/displacement numbers. The popular 14' sunfish, which I know is a much different design, weighs about the same as mine will after I add a mast, boom, gaff, rigging, etc, and it carries a 75 sq/ft sail and draws a little over 3 feet with the board all the way down. I thought around 75 sq/ft might be a good starting point for the mainsail. I can always reef.

The headsail, if I decide to go that route later, will probably be no more than 12 or 13 sq/ft.

fordo 09-11-2012 06:27 PM

Re: Building a daggerboard trunk
To start, what does the dighy look like? Was it designed for rowing with a fairly narrow transom or does have a broad stern to carry a small outboard? If it was designed for an outboard it probably won't sail that well.

You daggerboard trunk should go at the center of lateral resistance, usually where the middle seat is. On dinghies designed for sail the seat and trunk are usually built together. This is good as it provides strength to the daggerboard trunk.

What is this boat made of? If it's fiberglass you'll have to cut out a slot in the bottom and grind off the paint on the inside of the hull to allow the new epoxy and fiberglass to bond. You make the trunk out of plywood covered with light fiberglass cloth, both inside and out.

You are talking about a lot of sail area for that size boat; more than twice as much as on an Opti. Remember capsizing a Sunfish is no big deal; capsizing your dinghy means you have to be rescued. Start Smaller. Good Luck

t_train 09-11-2012 10:36 PM

Re: Building a daggerboard trunk
The dinghy is fiberglass, with a wide transom, so I guess more built for an outboard than for rowing. I know it won't sail like a dream, but I think with the daggerboard down and good sail area, it could sail fairly well.

I know generally how to build the trunk. I was more looking for potential problems I may run into, or what not to do and why.

It would kinda suck to capsize the boat because I have too much sail up, but the boat has built in sealed floatation fore and aft. If completely full of water, it will float. A bucket or hand pump will get me going again, so I shouldn't need rescuing. Maybe starting with a smaller sail is a good idea, but I don't want a fat, slow dinghy. I want it to be fun to sail... something to not only get me to shore, but I'd like to want to take it out for fun.

nickmerc 09-12-2012 10:46 AM

Re: Building a daggerboard trunk
It sounds like to hull design is your limiti factor for fun sailing. You may want to consider a lug or balanced lug rig. The gaff rig will add weigh aloft which is not your friend. The lug rigs will still look very salty and are extreamly easy to sail and make. You may want to consider no standing rigging. This will drive the design of your mast step.

For ideas on small sailing dinghys look at This will give you some ideas of how they are designed and rigged. I am not affilated other than I am considering building one of their sailboat designs.

bratzcpa 09-12-2012 11:04 AM

Re: Building a daggerboard trunk
I built a sailing dinghy (pram style) last year. We've been sailing it now for 2 summers. It works GREAT. Tons of fun, and it actually heads into the wind pretty good.

here is a link to the blog I put together at the site (Bateau) where I got the plans and lots of forum support.

Anyway, you'll see that there is a daggerboard trunk built into the middle seat. It works quite well. The only weird thing is that when I'm rowing (e.g., no daggerboard in), the water splashes inside the trunk and gets my butt wet. I need to make a little tight-fitting plug to close that off when rowing (or towing).

Also you'll see that sail setup that I am using. I ordered the sail "kit" on ebay. The mast and boom comes appart and it stores in a mesh bag. Easy to store and erect. It feels like the sail size is a little big for the boat (8' LOL), but maybe that's just me.

Good luck, mark

Bateau2 - Builder Forums • View topic - Montana built D5

Hudsonian 09-12-2012 11:29 AM

Re: Building a daggerboard trunk
Consider leeboards in lieu of a daggerboard. The fore and aft location of the board(s) (daggerboard or leeboards) significantly effects the center of lateral resistance of the hull and you will seek to have the center of lateral resistance directly beneath the center of lateral effort of the sailplan. You can estimate both of these before you locate the daggerboard trunk or leeboards but to a significant degree you will be designing by trial and error. You will find this much easier with leeboards rather than a daggerboard. Further leeboards don't require you to pierce the hull. You may find this article interesting: Leeboards

baboon 09-12-2012 11:59 AM

Re: Building a daggerboard trunk
I have not built a dagger board trunk, but have built a skeg box for a kayak which is similar and have sailed lots of dingys. Better plywood saturated with epoxy will work well, you can add a layer of glass cloth on the outside for extra strength. Make sure you saturate all the parts that will face inward with 2 coats of epoxy prior to assembly since you can not reach it once the box is together. The top of the box will have to meet the center thwart unless your dingy has a deck to attach it to.

Where the trunk meets the hull and seat it is best to use a fillet of thinkened epoxy for extra strength. Wood flour (very fine sawdust) works fine.

As to design, I would just look at the 100's of sailing dingys out there that are about the same size as yours, take a few measurments and go for it. You got it cheep, and you can build a trunk and simple rudder with under $50 worth of materials. There are several sites on the net describing making sails out of Tyvek house wrap, a great way to experiment on size and design. If you are a little off on for/aft placement of the mast or centerboard you can adjust the rig a bit, add a jib, or just consider it a learning experience.

t_train 09-12-2012 12:51 PM

Re: Building a daggerboard trunk
Cool, good advice so far guys. Thank you. I'm checking out these links now. Bratz, I like your pram. It looks good. I had actually wanted to build a pram and looked at plans online, but I came across this dinghy shortly afterwards and decided to get it. How much does that pram weigh? I wish this dinghy was a little lighter. If I kept it on deck, I would have a hard time getting it over the side myself without marking up my topside.

bratzcpa 09-12-2012 02:21 PM

Re: Building a daggerboard trunk
train: the plans state that the finished weight is 65 lbs. I think mine is "close" to that, probably a bit heavier. This was my first project with the Stitch n Glue method, so I think my seams and resin work are thicker than ideal. I also have some additional hardware (e.g., cleats, flotation foam, bow eyes, oarlocks, etc.) that weren't specified, so that adds some weight too.

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