sailing dinghy questions

Low blow!...but Danged if it doesn't come close. Still...I think if you are going to go that route you need to go with a Goose rather than a Duck. Check out Storer's plans for the longer, bigger Goose.
http://www.storerboatplans.com/Pdr/pdgoose.html

 

The Duck is lighter and less unwieldy toting around.
http://www.storerboatplans.com/Pdr/pdr.html

 

Although the PDR and it's evolutionary sister the Goose are technically boats, to me they just seem ... butt ugly. So do Pugs, Shar Peis and Italian Mastiffs - all dogs I like very much. If you are going to put in the effort to build a boat, it may as well be aesthetically nice - nobody ever lost points for being pretty.

Continuing on the Michael Storer theme, I'd build a Goat Island Skiff. To be honest, building a small boat of it's size takes almost the same amount of effort as a PDR, but the result is something to be proud of, instead of something you want to wear a mask when sailing.

Just my preference. I'd rather walk the beach with Jessica Alba than Roseanne Barr.

--
Bill

 

Bill, if Roseanne took a liking to 'ya, you'd tell Jessica to pack sand. Of course this assumes a better prenuptial with Rosie, which would have to be the case, because she can have any of the Tom Arnold's in the litter, to pick from . . .

 

A boat that leaned a bit more to the Jessica Alba side of the equation would be nice... The PD Goose actually fills the bill, mostly, but we are tending towards a pointy nose design aesthetically. In that vein the Goat Island Skiff is a beautiful boat that should be a joy to sail, however it's just too big for our purposes. For transportation and storage, 12 feet is about the max.

So, the basic idea that's forming in my head would be a 12 foot boat with a widish beam, say 5 feet, carried well forward from the stern, with a somewhat abrupt taper to a pointy bow. An exaggerated freeboard could help with load capacity a bit. She'd probably be a catboat, maybe a gaff rig to keep the mast shorter. Sound doable? (well, remotely possible? )

 

Hey everyone,

I am the second partner in this little nautical endeavor. As far as my sailing experience goes, I have sailed a little-known 13' design (the Satellite) that has a molded fiberglass hull and sloop rig. I have sailed it many times with my Grandpa along the coast of Southern California in up to 4 foot swells. I have sailed it solo in the harbor just to say I could do it, but we sail to enjoy each other's company.

I have also sailed an 8' Sabot that my Uncle built in the late '60s or early '70s. My Uncle lives in Houston area and we took that out to a lake nearby his house. Of course, that was a solo experience. (Cool part was that there was a giant fountain in the middle of the lake, and so it was fun to sail under the fringes of mist! )

My Uncle also has a 14' fiberglass boat, sloop rigged, with a covered deck on the front. We sailed out of Galveston, TX and it has a center-board with kick-up rudder. (Came in handy a few times, too!)

As for the current boat project, here are the reasons behind the requirements:

We can build the boat outside on a corner of the school's RC plane field, but it will probably be stored in a 12' x 12' room or two.

The three person requirement comes from a third partner who does not want to help build, but will donate money to the project if he can just ride along in the boat. With a passenger on board, we were thinking that it would be nice to have someone in the bakc operating the main and tiller, and someone in the front operating the jib. That way, we will not be clambering around on the passenger on a tack.

Being in college, we want to keep the cost down to $100 - $150 per person. For a two-man boat, that leaves us with a budget of $300, and for a three-man boat, $450 - $300.

Also, the larges vehicle any of us own is a pickup with a 6' bed. Fortunately, the lake is literally right across the street from the school.

And last of all, we are engineering students, so we like to design/build stuff!

So there you have it! Our motivations and goals in a nutshell!

 

Hey good to see you here! (I just happened to log on now, I'm not here 24/7, honest...)

At the risk of becoming too technical, how are the sail(s) and daggerboard placed wrt the hull when designing? I know the sail(s) act as wings and the component of the resultant force parallel to the hull is the 'motivating' force, but can the sails be assumed to act as wings to the degree that the x-component of the center of effort is approximately 1/4 of the mean aerodynamic chord back from the luff? Also the daggerboard should be placed such that the x-component of its aerodynamic center is slightly forward of the center of effort of the sail area for a slight weatherhelm, correct? Does the center of gravity of the boat/hull play a significant role to the degree that immitating the proportions of similar designs is not sufficient?

Thanks as always,
Ryan

 

Both can be true (well stringers anyway)... In 99% of cases that you'll see its design inertia, but the most sophisticated and lightest wooden boats tended to have stringers unless they were very "tortured" three dimensional shapes. Its a long while since I've seen a boat built like that though simply because cored construction (even plywood or cedar core) with fibre/epoxy skins is so much better for a lightweight boat.

On daggerboard placement - the fashion in the high performance boats at the moment is to have the rudder contribute to leeway resistance quite significantly, so the board tends to be smaller and maybe further forward. This makes the position rather less critical. Evaluating "weather helm" is more tricky than might be thought - it doesn't matter (indeed it may be good) if the tiller is pulling on your hand and wants to zoom off to leeward provided that it doesn't need to be offset from the centreline to keep the boat sailing in a straight line. The "weather helm" of tiller pulled 15 degrees towards you to keep the boat straight may well be a lot more about hull shape and just not sailing the boat upright than it is about 6 inches fore and aft placement of the daggerboard.

But this is mostly talking about far lighter and higher performance boat than you folks probably require... For instance 3mm ply decks and topsides are great for light weight, but not so great if the way your boat will be used means they get holes in them once a month...

 

The usual position of a daggerboard on a small craft such as you are contemplating tends to be right under the Center of Effort of the sail/sail set. Your weight distribution will affect balance as much as anything else and you can probably weight steer such a boat almost as easily as using the tiller...so board placement, while important isn't absolutely critical. Another way of doing it would be to make the case longer than the board is wide, then you can vary the position of the board and use plugs to fill in. I am using this on the scow I am building...12" board with a 16" slot and various plugs to allow me to position my board how I need. This, along with wedging the mast in at various rake angles will let me fine tune the rig or even change it without having to re-do anything major.

But why not go with a 2 piece boat? Build a GIS or something similar then make 2 bulkheads and lop the front off at the correct length. Transport and store nested or at least stacked then bolt them together when you get to the water...THAT should tickle your engineering bone.

 

I strongly urge you to investigate the work of Phil Bolger, a fine naval architect who produced a lot of simple-to-build yet good-sailing boats in your price range, all of them very simply rigged, which keeps the cost down. If the third person is only occasional, a small (12') boat may suffice. If he will be a regular crew, you'll want a larger boat. It is no coincidence that every boat I can think of, 16' or less...Comet, Snipe, Lido 14, Rhodes Bantam, Wayfarer, etc., is a two-man dinghy. It's not until the 17-19' range that you see three-man crews as in the Thistle and Lightning. The reason is that while you can physically cram three people into, say, a Penguin, doing so destroys her sailing qualities, the very thing that gives pleasure and satisfaction underway. While none of Bolger's boats are one design classes like those I have alluded to as examples, they sail just as well in most cases and are a lot simpler and cheaper to build using the previously-mentioned tack-and-tape method of construction. He wrote several books that showcase his work; I'm sure they are available through your library.

Paul

Edited to add: And then there is Herb "Dynamite" Payson, a Maine boatbuilder who specializes in Bolger designs. He also sells plans under authorization from the late naval architect and has published two books about the boats and how to build them, Instant Boats and The New Instant Boats. I'd certainly want to read one of the books before tackling any plywood boatbuilding project. His website: http://www.instantboats.com/ While she is somewhat larger than the parameters you originally listed, the Light Schooner is the right size for a crew of three young men. I've seen a larger copy of her plans; she is extraordinarily appealing. You might consider building a small boat as practice. If all goes well and you are still game, you could sell the small boat and use the proceeds to build the schooner. Two men should be able to build one of the small boats in a weekend.

Good luck, Paul

 

Thanks for the good info guys- lots to think about.

Those are great ideas for the oversized board slot and the raking mast lewisboats; that should definitely help us out in the trim department. We have actually discussed the possibility of a modular boat, as it would really help for transport and storage, however that may require more wood, and is more complex to build (spare time is a bit short).

I've started laying out a basic hull shape, and I've hit a design decision point- structurally, I'd like to integrate the lower end of the mast into some other structure, say the daggerboard trunk, but this would locate the mast further aft than ideal for a single sail. Because of this, a sloop rig looks to be more appropriate, at least for the hull structure. Is a sloop rig on a boat of this size advisable from a rigging/handling standpoint?

Also, where approximately is the sail/sail set's center of effort? Is it the centroid, or further forward, i.e. aerodynamic center?

Thanks,
Ryan

Now I'm off to the school library to see if I can locate one of Phil Bolger's books- thanks for the suggestion Paul!

 

I think that you're at risk of making a big compromise on sailing ability, usability and cost in order to avoid making a small compromise on hull structure...

Whatever rig is appropriate for the use your boat will be put you should put the mast and foil in the right place for the boat to function correctly and sort out the structure to match rather than vice versa. Unless you are planning something really extreme in terms of large sail area, high rig loads and ultra light weight there really isn't going to be a downside to an appreciable separation between mast and board.

Start thinking about your internal layout - how your three crew operating the boat, what people will sit on, whether you will be carring stores (eskie and sandwiches?), buoyancy tanks, that sort of thing. When you've got all that pencilled in you will proably find that decisions on internal structure are much clearer because the benches, decks or whatever to sit on are also going to be key parts of your load bearing structure.

 

This'll help:
http://www.boatbuilding-links.de/Jim-Michalak/sail-math-and-thoughts-of.html

 

It's one thing to make adjustments to an existing design, maybe even wholesale changes with fundamental elements of the original being moved, but to design the whole thing from a blank sheet, appears to be well outside your present scope of understanding. I'd strongly recommend you consider one of the low cost and even free plans that are available, if you'd like to have any reasonably level of success to come of this, particularly if you're on a budget.

Yes, with the help of some quite primitive and basic diagrams found on line, possibly a discussion forum or two, maybe some free or low cost software, some of the questions you need answers for can be provided, but there's considerably more to the skill set necessary then you realize. It takes a far understanding of several engineering disciplines to get a reasonable small boat. Anyone can design a pig that doesn't sail well, maneuvers like a truck and cost three times what it needed to, though can hold all the stuff they thought they'd need.

The trick is to design a boat that actual fits your needs, uses enough material to tolerate the loads you expect with a reserve margin, but little more so, you can actually afford it and have it sail well, possibly fairing pretty good against other boats of similar dimensions, because there's nothing worse then getting beat by a 12 year old in an over loaded small yacht tender.

I say this because of the type of questions you're asking and most importantly, because of the questions you're not asking. If you're going to venture farther from shore then you can swim back to, it would be nice if you had some level of assurance that she'll bring you back. And lastly, it's just as easy to build a real piece of junk as it is, a boat you can be proud of and does all the things you hoped.

 

gggGuest- I will try to design the structure around the proper positioning of the components rather than vice versa, however, in the interest of simplicity of the structure I would like to integrate the daggerboard, mast, and midships seat into one unit. By using a sloop rig, the mast can be placed near the daggerboard while still maintaining a balanced rig. What I am asking is that, is a sloop rig on this size of boat too troublesome to consider? If a catboat configuration is a significantly better option, then I agree that the mast must be placed to balance the sailplan first and foremost.

lewisboats- Thanks for that link! That clarifies many questions that have been bouncing about in my head. As an aeronautical engineering student I still find it hard to accept that the center of effort is at the centroid of the area and not further forward as on an airplane wing, however a good deal of the sail area immediately aft of the mast is rendered ineffective by the separation bubble aft of the mast, so the centroid may be a reasonable rule of thumb simply by happy coincidence.

PAR- I recognize and appreciate your concerns regarding our inexperience, and I too have seen many an amateur become overwhelmed by a project they thought they could handle in other disciplines with which I am more experienced, however at the risk of sounding stubborn, one of the primary ideas behind this project is to build from our own design work

Thanks for the input as always,
Ryan