Beam in dinghy design

[Clinton B Chase]

Well, I am heading confidently through the prelim. design process on a dinghy that I intend to build in a couple/three years. My question to those who have designed and built dinghies or have a lot of experience sailing them -- I have some but not enough -- is what are the things to think about in determining the beam of the boat. What I know is that more beam will help the boat stand up to a larger sail area. More beam may make the boat more initial stability something I'd like since by that point I may have two kids to think about out on the water. I also know that more beam can kill upwind performance, which my chosen rig -- a standing lug -- is already in need of. So far my dimensions are 14' 10" LOA and a beam of 5' 10". I like the idea of space for kids and gear inside the boat since I might want to use this as a way to get out to some islands to camp and sail around. Thoughts about beam?

Cheers,
Clint

[Lewisboats]

Less than 3:1 ratio would seem to be a bit beamy to me for a skiff/dinghy. For a length of 14' 10" I wouldn't go with a beam of more than 54" (+/-1"). Of course...Catboats tend to run 2:1 or a bit more and I have done a few hull designs in this range. You will want to balance the rig with the hull to obtain the maximum form of performance. A Beamy boat will carry a lot with reduced performance in other areas, whereas a narrow boat will outperform a wider boat while being less able in stability and carrying capacity. Of course...hull form in other areas will also affect the perfomance.

Steve

[mmd]

Don't let all the "approved" performance and geometric ratios drive you batty. Unless you are designing a dinghy for competition, the primary function of the boat should be the comfort and safety of the persons in it. After that is taken care of, then see if you can tweak the form and rig for performance gains.

When you consider the shapes of classic small boats, remember that the designers/builders of these didn't use fancy formulae; they shaped the boat for its intended purpose, be it carrying passengers swiftly to their desinations (Whitehall skiffs) or cargo in rough waters (dories). In almost all cases, the boats are designed from the inside out; that is, the cargo space was defined and the boat was built around that requirement. Length-to-breadth and sail area-to-displacement ratios were alien concepts to the design process in most cases.

My advice would be to define the boat's beam by working out how much foot room you need in the sole, how much seat room you need for your bum, how far from the edge of the opposite seat to the backrest of the seat you are on is comfortable when the boat is heeled and you are bracing your feet on the opposite seat, and how wide the washrails/side decks (if any) are. Make the accomodations comfortable for the occupants while underway and the maximum beam will follow. Once that is established, then you can tweak the waterline beam by adjusting the flare of the hull. That done, you should then do a quick check of the form stability of the hull and the approximate loaded weight. With this in hand, you can then look at how much sail you need to move the boat, and how tall the rig can be without causing undue heeling moment in the expected operating breezes.

G'luck and have at 'er!

[Thorne]

Outboard motor or not? -- that makes a difference in the width aft as well as transom rake.

If not and you'll be rowing her sometimes, then the aft section should be narrower. Personally for smaller sailboats I've always liked the option of using a small outboard on an adjustable bracket on the transom, particularly when boating with kids.

I really like MMD's "designed from the inside out" description!

;0 )

[Bob Ketterling]

I am in the process of building Oughtred’s Gannet that is about the same dimensions. You may want to look that design and compare your ideas with it. Two nice things about a boat of that length; it is about the longest boat your can build in one stall of a standard garage (at least in my garage) and if you are building it out of plywood you can get the planks with only one scarf joint in each plank. Keep up posted on your progress.

[Canoeyawl]

There was a very interesting article in WB a few years back by Basil Greenhill about an 18th century ships boat replica built from a lines plan in Fredrik Chapman’s Architectura Navalis Mercatoria (I think!). If I remember correctly it was a ten-foot vessel with a five-foot beam and was a great success. A lovely little craft she carried his entire family and performed all manner of tasks well. The boat was designed around utilitarian tasks and convention was very different in the past. The article was informative and shifted my thinking a bit.

This article would be good reading if you wish to design a burdensome dinghy.

WoodenBoat - Greenhill, Basil, author:/"TELKKA: A New Boat from the 18th Century," 60:68

[johnw]

I also know that more beam can kill upwind performance, which my chosen rig -- a standing lug -- is already in need of. So far my dimensions are 14' 10" LOA and a beam of 5' 10".

When Commodore Munroe wanted to improve windward performance of his Presto-type sharpies, he increased the beam. Going to windward, you need stability. I think your dimensions sound fine, as long as she carries enough sail and has an efficient centerboard. I'd only make it narrower if you intend to row long distances.

[Clinton B Chase]

TX for thoughts all. I will have a pretty large standing lug and I am tall so I like the beam when seated...the CB trunk will be a great foot rest too. The Gannet has been a very useful boat for comparison as has Paul Gartside's dinghies.

The other thing I am considering is the keel. I have seem outer keels that are very low profile like Iain Oughtreds and then keels that come way aft and then stop and fair into the bottom of the hull like Gartside does...like a cutaway keel. i know Paul does this b/c it helps the dinghy tack, but what does the rest of the keel do? I don't think it is for rowing is it? Is it add a little lateral resistance when the board is up? IO does not have this type of keel with the cutaway aft so I wonder what the difference is? Maybe Paul does it for rowing afterall...he hates outboards..it would help the dinghy row straighter afterall. IO doesn't mind the idea of an outboard -- nor do I -- so no keel? TX in advance.

Cheers,
Clint

[sv Lorelei]

Clinton: Don't overthink this! Go back and read mmd's post. Make a list of what your performance expectations are for this boat and then design around them. Choose elements that will help the design achieve it's objectives, and when you're not quite sure, go with what looks right.

IMO if you're looking at a standing lug rig, there may not be too much difference between a keel or a board.

[johnw]

A good board will be more effective than a shallow keel, and you can pull it up and reduce wetted surface and resistance while rowing. On the whole, I think it's worth the trouble.

[Clinton B Chase]

Clinton: Don't overthink this! Go back and read mmd's post.

It annoys me when people say "Don't over think this". How the hell can I do that when i have no idea what I am doing! i am on the very steep part of the learning curve, making observations about other designers' work and asking questions. I've read mmd's post once and got a lot out of it...as I draw and make calculations...or comparison's with other proven boats...i am constantly going over my criteria list in my head. Paul's boats have a very interesting shape back aft and I wonder how he does it and what effect it might have based on the sailing people on this forum have done in similar hull types...if you can't answer my question please do not reply. Thanks for everyone else taking time to help me out...worthwhile reading your posts.

Cheers,
Clint