Skin on frame dory - why not?

You weren't kidding when you said it would go quick. Congratulations and nice work. I wish I had the skills and equipment to work in metal.

 

Congrats! It looks great, and yes, it's always satisfying when a pile of sticks suddenly looks like a boat!

 

Interesting comment from Jeremy about using 100% metal really just for the sake of doing it. What are the advantages of metal in SOF construction and would a wood/metal mix be easier to construct?

On the plus side - assuming aluminum - there is dimensional stability and zero rot. Aluminum is not much harder than wood so is almost as easy to work. I suspect there is a cost downside and of course there is the question of reliability of fastening unless one can weld aluminum ...

 

Thanks for the kind words, guys. I did a fair bit of work on getting the scantling sizes right for alloy, as it's governed by stiffness not strength. The alloy tubes are ultimately much stronger than larger section wood, but I designed it so that the deflection between support points when under load should be the same as typical wood components.

The total cost of all the alloy tube would have been around £85 (~$138) delivered to my door, or about £75 (~$122) if I picked it up from the metal factor (hard to do because of the 5m lengths). I suspect this would be close to the cost of decent boat building timber over here.

There's very little in the way of special tools needed, really just a drill and hacksaw with a fairly fine tooth blade. The frames are bent around a simple peg jig (being fairly thick wall the tube doesn't distort much). The frames are attached to the keel, gunwales and stringers with 4mm diameter stainless steel pop rivets - strong, quick and easy to use. Each rivet has a shear strength of well over 800lbsf, so is probably much stronger than a wood epoxy joint of the size typically used on a boat like this. To aid longevity in salt water the rivets could be dipped in sealant before being fitted, as they used to do when riveting alloy seaplanes together.

Using wood for the stem and transom would be a lot quicker and simpler. I used foam covered with epoxy glass (simply because of my choice not to use wood anywhere) which caused a couple of hours extra work. If doing it again I think I'd use over-size pieces of ply with holes bored in for the inner stem and transom, bond the tubes in place, then add an outer transom and stem fitted using the expanding-plug-and-rivet technique I used to attach the gunwales to the ends of the frame tubes. Once all had hardened I'd trim back the excess ply to the tubes, ready for covering.

I'm planning on doing some filling and fairing today, ready for covering later on, but the temperature dropped below freezing again last night, so using epoxy filler is out until it warms up a bit.

Edited to add:

Here's the obligatory lightweight boat photo. There can't be many 16ft boats you can lift with one hand.



and another shot of the frame off the jig, almost ready for covering:





Jeremy

 

Great image and info, Jeremy. Can you give us the diameters and wall thickness(es) used for the various bits? I have a small pile of Ally tubing that I might put to creative use if it’s suitable. I think wood pads between round tubing would help structurally - I used oak for a project of mine and it worked and looked good.

 

Thanks for the kind words.

There is more detail on the trials and tribulations that have led to this boat over on the UK Home Built Boat Builders Rally forum, here: http://uk-hbbr-forum.967333.n3.nabb...g-boat-was-going-to-be-a-canoe-td2424625.html

The tube sizes I used were:

Gunwales and keel - 1" x 18g
Frames and stringers - 1/2" x 16g

All preferably 6082-T6/6061-T6 alloy, but 6063-T6 should be good enough and maybe easier to get hold of.

The frame to stringer and keel joints are really simple and just use a single 4mm diameter stainless steel pop rivet, 20mm long. The 1/2" tubes are really stiff and robust and don't crush when through riveted like this, which makes life simple. I'm not sure that anything more complex is needed, as these joints seem pretty strong.

The gunwale to frame joints are slightly more complex, they use a 1/2" hole drilled into the underside of the gunwale (which is the neutral axis, so not really stressed) and a matching 4mm hole on the top of the gunwale, with both holes aligned to the angle of the frame. The frame ends are fitted with 3/8" diameter alloy plugs, with 4mm holes drilled through them, internally tapered on the inner end and split with two saw cuts, to form an expanding plug. When a 4mm stainless rivet is pulled from the top of the gunwale tube it expands the alloy plug and pulls the frame tight into the gunwale tube.

The whole fastening system is pretty quick and easy, quicker than I'd thought it'd be.

Jeremy

 

Thanks for the data. I bet your hand got a workout with all those rivets, unless you have an air tool ...

 

I do happen to have an air pop rivet gun..........

Having said that, there are only 35 4mm rivets in the whole frame, so it wouldn't be too hard to do with a hand rivet gun (although stainless rivets are, admittedly, pretty tough to pull).

BTW, I weighed the frame last night. As it stands, without the covering, floorboards or seat, the hull weighs 8.2kg (~18lbs). I reckon it will be over my 10kg target finished weight, but not by much, maybe around 11kg or so (about 24lbs). I reckon this is probably pretty light for a 16ft rowing boat.

Jeremy

 

Sounds good: weight targets are never met, not in boats or Aerospace!

 

I was down at Georgy Dyson's shop yesterday looking at skin for my kayak and he's building metal framed bidarkas that are sewn together with twine. Very impressive.

 

hey gonso, I live in Olympia. you bring a 20' double kayak in wood or plactic down here and I 'll beat your pants off in my 20' double SOF. you don't know anything about SOF lol
Outlaw

 

curious as too the covering:?:
(on the first pic).

 

I have thought a lot about doing a quick and easy SOF build. Like others in this thread I have given myself a material limitation to work with. Everything must be available at any common hardware store/ lumber yard. No special order or expensive parts.

I thought that EMT (electrical conduit) would be a great material for the frame. It can easily be bent with a simple conduit bender. Tie the joints together with wire. (or maybe pop rivets)

I am not sure what to use for the skin, what is cheap and readily available?

Does not have to be pretty, just work ok. As long as it is easy to replace it doesn't even have to be long lasting.

C.O.
http://whatsintheshop.blogspot.com/

 

The short answer is "not much".

One possibility is cotton duck fabric which Walmart carries in their sewing department at most stores. They also sell a waterproof outdoor fabric which I have used for spray skirts and the like. If you use the cotton you will have to coat it to make it waterproof, which is not a huge deal. I used a 2 part epoxy based paint I was able to source locally, but any heavy duty paint should be OK, so long as it's waterproof.

Another option I considered was to get a painter's drop cloth which is made from a material similar to cotton duck, but comes in larger pieces.

For reference, the "right" answer is heavy nylon fabric coated in something. I think 6 and 8 ounce are common weights, but I was able to get a big piece of 12 oz from George Dyson for cheap because it had a small defect in it. I know you're limiting yourself to hardware store items, but file this away in case you run across a local supply.

 

Drop cloth, when coated with paint, becomes a fabric that will rip so easily you wouldn't believe it.

Use clear vinyl. The stuff they use for Jeep windows. It's cheap, readily available, and doesn't even require a coating. But since you probably don't want a clear covering to show off your conduits, just use spray paint made to stick to plastic. 12 oz will be plenty tough-maybe even 10 oz would be okay.