Skin on frame dory - why not?

This idea has been rattling around in my head for a couple of weeks and so I wanted to throw it out there and see if the rest of you think it's possible. The idea would be to build a rowing dory something similar to a lapstrake method, but instead of planking it run longitudinal stringers and skin it like a kayak. I'm thinking about weight savings since I would be building this primarily for my mom to row for exercise and she would have difficulty getting a large boat on and off the dock, trailer, or beach.

Thoughts?

 

I don't think that a dory, a flat bottom boat, would be lighter with that construction. Also, soft skins make boats less efficient.

 

there were some small skin on frame rowboats shown on the forum last week, i don't remember the thread name though.

 

I'm presently building an alloy tube frame, Dacron covered, lightweight rowing boat. My inspiration came from Dave Gentry's Ruth: http://gentrycustomboats.com/RUTH page.html

I'm pretty familiar with shrunk fabric covering, from aircraft work, and can say with confidence that a flat bottomed hull like a dory would not be well suited to the technique. Add a shallow V to the bottom, with a keel strip, and the fabric will retain its shape under pressure loading better. Add an extra couple of panels (like Ruth) and the fabric will hold its shape even better and stay tauter.

I personally doubt whether the very slight flexibility of the fabric makes one jot of difference to efficiency for such a light displacement hull, provided that the skin is properly tensioned and decently finished. My experience with aircraft showed that the slight flexibility of fabric seems to have little or no impact on viscous drag when compared to a solid skin at the same sort of speed. If there were a noticeable difference then it'd be more of a problem for an aircraft, where the velocity and dynamic pressure is greater (not to mention the viscosity difference).

There are some threads over on the Wooden Boat forum covering Ruth that might be of interest: http://forum.woodenboat.com/showthr...sliding-seat-rowing-wherry-project.&highlight
http://forum.woodenboat.com/showthr...-plan-comes-together-RUTH-launched!&highlight
http://forum.woodenboat.com/showthread.php?119700-Building-Mr.-Gentry-s-SOF-Ruth-Wherry

Jeremy

 

Holy Moo-cows, that's beautiful! Hows your coming along? Pics?

 

The build has been a bit slow, as I'm relatively newly retired. Despite me thinking I'd have lots more time for projects, the reality has been many more household tasks that have materialised from nowhere.........

The build of my alloy frame version of a wherry that's a bit like Ruth is here: http://uk-hbbr-forum.967333.n3.nabb...g-boat-was-going-to-be-a-canoe-td2424625.html

The build has progressed a little more, in that I managed to get the strong back finished yesterday, so with luck I may have something boat-shaped to photograph later today. One big advantage of skin on frame the way that Dave Gentry does it is speed of build - he built Ruth in a week. With my design I've had to spend a lot of time on detail design, working out how to fix alloy parts together, make a stem and transom from foam composite etc, all because I made a stupid bet with myself to not use any wood in the construction. Had I opted for a wood stem and transom the whole thing would be a lot quicker and easier to build.

I doubt I'm going to make my design goal of under 10kg, but I'm confident that the finished boat won't be more than around 14kg (about 30lbs) so should be easily car-toppable and a hopefully nice to row.

Dave Gentry's plans for Ruth are free, just email him and he'll send you a set. I'll put my plans for the alloy tube version up on the HBBR site for anyone to use as I finish them.


Jeremy

 

Hi Jeremy - thanks for the kind words!

Cthippo - yes, your idea has great merit. One can often build a boat in SOF, and make it lighter, but just as capable, as the same boat in wood. And, it will usually be much faster and more inexpensive to construct, as well.

First, you'd have to decide which style of dory you are interested in. The flat panels of a banks dory are not so conducive to SOF, but most of the other styles definitely are. John Gardner's books are the "go-to" reference for dories.

Shorter boats will be lighter for your mom to carry around, but might not be so satisfying to row, speed wise. Something like my "Ruth" wherry, already mentioned, is fast indeed, but it is narrow and not nearly as stable as a big fat (round hulled) dory, peapod, etc. Picking the right boat is half the fun!

I did just complete my "Shenandoah Whitehall," a new 14' SOF rowboat design. It is pretty heavily built, and came out at about 55lbs or so. Stripped down some, that's likely not too much for a mom to wrestle with, and it will still make good speed. No doubt there's a dory or two in the 13'-16' range that will suit her.

Anyway, yes, what you want is very do-able!

DG

Shenandoah Whitehall

 

I know that feeling. I decided I was going to use only recycled wood and no metal fasteners on my current kayak build. Those two decisions have resulted in many many hours of frustration because I either couldn't find the piece of wood I needed at the recycled building materials store or because I was trying to attach something under strain with pegs and clamps when one little screw would have solved the whole problem.

Ah, the challenges we create for ourselves. Your project looks awesome, can't wait to see the finished pictures.

Do you have plans for that design available?

 

Seems like some of you are into self flaggelation. Wood forbidden, simple metal screw forbidden, and such. I salute you for sticking with your premise but I also feel some compassion (and puzzlement) for the pain that you've forced yourself to endure.

 

I have built two skin-on-frame sailboats in addition to 8 kayaks and one canoe. there is not reason it could not be used on a small row boat. As pointed out however you do not want large unsupported panels, the fabric tends to bow inward. That is not so much as problem as it will contact the frames and create a draggy shape.

Here is a picture of a sailboat made almost all of salvaged materials. It is 14 ft x 4'8" beam, weighs about 60 lbs I would guess. That is me and my daughter in it near Port Townsend on its first launch:



Picture of the frame using mostly kayak type construction, steam bent ribs, lashed stringers, with some screws to save time. Friend Floyd helping in picture:

 

Here's an umiak from the Port Townsend Wooden Boat show. Pretty much a dory in shape but ancient in heritage and seaworthy in arctic conditions, which means ice and rough beaches.

 

Another take on the idea.

 

So it sounds like it can be done successfully, but it needs to be the right design to begin with. Thanks for the input everybody!

 

As mentioned by others the dory style that is made from flat planks will not translate well to SOF, but many glued lapstrake designs should be OK. They typically have many planks and rely on the plank-to-plank chine joint for hull integrity so the angles bewteen the planks are moderate: not too acute since that would result in a narrow, weak joint.

That type of shape can be rendered using SOF with enough stringers to define the shape, one at each chine.

If you can get a copy of Building SOF Boats by Robert Morris there are a couple of SOF rowing designs, a Pram Dinghy and a Providence River Boat. Even if they do not suit you, they will give you workable scantling sizes.

 

The weather was kind today, so I went out this morning and started fitting bits of my SOF wherry together. I bonded the alloy gunwales and keel to the transom and stem, then had to wait for the epoxy to cure (they'll be riveted to the composite as well). After lunch I decided to fit the frames and stringers, so by around 15:30 I had something that looked like this:



Many thanks to Dave Gentry for giving me the inspiration to try SOF. I am pretty satisfied with my four hours work today - I've gone from a pile of bits to a pretty near finished alloy and composite frame, nearly ready for covering.

SOF is a mighty fast way to build a boat!

Jeremy