a question about building a dinghy

[dragonwim]

I'm fairly new to boat building, just getting started really. I'm about to build a dinghy, ruffly 12 feet long and about 4 feet wide. I'll ask that you forgive me if I come accross as an idiot, but I don't know alot of terms when it comes to boat building. When I'm building the dinghy do I need to put a fin on the bottom of the hull for stability or can I leave it off? I am building from plywood, and boards. The closest body of water to me is the tennesse river, and thats the only place i plan to take it.

[alan white]

You don't need a fin. A dinghy is short though, and a skeg would help it to track straight. It would be about one third of the length of the bottom aft, ending at the stern. Research some dinghy designs and see what skegs look like.

[dragonwim]

Thanks for the tip, it probably will help with handling. The part i was asking about was the centre board, or at least that's what it's called on the diagram i found. The reason I'm asking is that the boat I'm making has a flat bottom, and i'm wanting to know if i need it to help keep balance from the mast and sail.

[alan white]

Not all dinghies sail. Most probably don't. Therefore I assumed you referred to a skeg. A sailing dinghy usually has a centerboard, yes. However, I doubt you'll find anyone here who is going to walk you through designing your own boat.
Your best bet is to find plans for a dinghy that appeals to you and build it. If you do insist on building your own design, read a lot of books and in a few years you might have an idea about how to design a small boat.
Anyone can, of course, "design" a boat that floats in the same way anyone can design a pair of shoes if duct-taping boards to your feet counts.
Search the web for some easy-build plans. Then ask advice here about the merits of both the design and the build method.

[tinhorn]

While we're on the topic, please allow me to chime in about the different bottoms of a few dinghies I have. One in particular is a 12' Ensign that I bought to restore simply because I liked the looks of it. The skeg on this boat runs the entire length! I was pondering whether this full-run skeg actually helped keep it in a straight line, or if it was just for stylin'.

My AC dinghy has the shorter skeg, but it looks like someone stuck short sections of PVC pipe to the plug, creating a ridge on each side of the skeg. Do these serve any real value? Would they help if they were actually parallel to one another? Or was it most likely a sneaky way to make their boat a bit different than the boat they were splashing the mold from?

[tinhorn]

I hope that answers to my questions on dragonwin's thread are useful to him--I'd hate to completely hijack his thread.

The picture below is of a daggerboard trunk I removed from a sailing dinghy. (The seller said it "seeped" a bit. Har!) I'm going to build a new trunk from fiberglass, but I question the excessive slot in the old one. Do I really need 2" of room, as well as 1/4" of side clearance? I've never sailed a dinghy, but this seems kinda sloppy.

[alan white]

I would suggest you have 1/4" extra overall slot width (FG or steel/iron board) UNLESS the boat will be kept in salt water continuously. Marine life can grow on the board, especially if it's not removed annually and coated with bottom paint. Simply painting bottom paint onto the portion that sticks out is asking for trouble.
More width for a solid wood board, less for fiberglass, use your judgement.

[Petros]

Originally Posted by dragonwim

I'm about to build a dinghy, ruffly 12 feet long and about 4 feet wide... When I'm building the dinghy do I need to put a fin on the bottom of the hull for stability or can I leave it off?

If your intent is to row or motor it only, than no, you can leave it off. A skag as noted will help you keep it in a straight line.

If you intend to put a sail on it than you need some kind of centerboard or "fin" to keep it from drifting sideways when the wind blows the sail. You can simply this however by putting a "side board" on it instead of the centerboard, it saves a lot of building time. The side board needs to be strong enough to handle the side forces from the sail, but it is a lot easier to build than having a center board box to make.

[alan white]

Originally Posted by Petros

If your intent is to row or motor it only, than no, you can leave it off. A skag as noted will help you keep it in a straight line.

If you intend to put a sail on it than you need some kind of centerboard or "fin" to keep it from drifting sideways when the wind blows the sail. You can simply this however by putting a "side board" on it instead of the centerboard, it saves a lot of building time. The side board needs to be strong enough to handle the side forces from the sail, but it is a lot easier to build than having a center board box to make.

Very true. Called a "leeboard", it is probably one of the most underrated board types ever conceived. See Phil Bolger designs for an idea of how they function.

[Ike]

I am building from plywood, and boards.

If you go to some sites that offer plans for stitch and glue boats, bolger, Sam Devlin and the like such as bateau.com, jem Watercraft, you will find you don't need the boards, only the plywood. You may need some small board stock for things suchs as rub rails, trim, a skeg, but nothing for structure. Stich and glue building eliminates all of that. Most small boats built this way just use plywood. If this is just for rowing or a small electric motor, or small gas engine, then you don't need a centerboard or leeboards and stitch and glue will pretty much eliminate any chance of leaks. You will need plywood, epoxy resin and fiberglass tape. Go to Sam Devlin's site and study the stitch and glue method http://www.devlinboat.com/stitchandglue.php. Don't let the size of the boats there scare you. It works equally well for dinghies. see mine at http://newboatbuilders.com/pages/fl12.html

[dragonwim]

Originally Posted by Ike

If you go to some sites that offer plans for stitch and glue boats, bolger, Sam Devlin and the like such as bateau.com, jem Watercraft, you will find you don't need the boards, only the plywood.

I'm actually trying to build a sailboat that's out of a video game, the only reference pic i have don't show anything below the water, which is why i was asking about a center board. and since i am building a sailboat i will need to use one, which is good to know since I'm still in the planning stages, but I'll probably be building it in the stitch and glue style. thanks for the links, I'm sure they will be very helpful when i draw up my plans. i'll post pics on this thread when i build the boat so yall can see the outcome.

[lewisboats]

Not trying to be mean here but...just because some game developer drew a boat shape don't be fooled into thinking it is actually a boat. There are a lot of things that go into designing a safe and properly performing boat than just drawing the picture and making it work in a video game where anything is possible. In the real world you have little things like Physics and natural laws to deal with...especially with a sailboat..which interacts with two entirely different media at the same time. Let me throw a few things at you...when you know what they are and understand their relationship to each other...then you can start working on your boat plan. Hydrostatics, Fluid dynamics, CE, CLR, Metacentric height, initial stability, ultimate stability, dynamic stability, scantlings, CG, reefing points, Jib, Main, Spinnaker, Genoa, Mizzen, Leeboard, Daggerboard, Foil, Galvanic action, Osmosis, Anchor rode, boomkin, topping lift...Oh how I could go on...but you get the general idea. Go with a plan from someone who knows a bit about what it is about. You should be able to find something out there that is somewhat similar to what is in the game...otherwise it probably isn't feasible.

[dragonwim]

actually, i happen to know a little something about design, i have a degree in design. and befor you ask, no not graphic design. the boat in question is similar in shape to the ones used in Venice in the canals. the main difference being that it's shorter and has a sail. i also know that what some game designer thinks looks cool might not be practicle in the real world, which is why I'm spending time researching different boat plans to gain a better understanding of their design, so that i can make a boat that will look similar to the game, but will actually sail. i also plan to build a scale model to test and see if there will be any changes i need to make to the design so that it won't just roll over in the water.

[ancient kayaker]

I built a small (10' x 4') boat last year for rowing and sailing which is fairly close to what you have in mind. I omitted the skeg, shown as optional on the plans, because I planned to use her in very cramped waters and needed maximum agility. The flat-bottomed hull is difficult to row in a straight line but I found that rowing her with the rudder about 1/4 immersed solved the problem; I modified the tiller so it could be lowered to the floor so I could steer her with a foot. This helped out my limited rowing skills.

This is an option you may care to try. Some hulls track better than others and do not need a skeg. A skeg can always be added after the build, so you can try the boat and see if it really needs one.

Although I have designed and build several canoes I did not feel it wise to design this boat, not least because I did not know what the stresses of sailing would be and needed data on construction and scantling sizes. Compared to the cost of materials and the time involved in building and finishing even a small boat, the price of a set of plans seems very little in return for the peace of mind of knowing the boat will work as intended, and much less hassle than making and testing a model, but this is your own decision.

[dragonwim]

thank you for the suggestions, i'll keep them in mind. the main reason i'm gonna build a model is to make sure that the design will work. the front of the boat has an odd design were the prow curves away from the direction your heading, and the neck of the figurehead juts forward. i want to make sure i can get this right before i try to build full size.