Introduction: 240cm Skin on Frame Sailing Pram With Mirage Drive
I tried to build the lightest and cheapest (~150€ total material bill) sail-boat I could come up with. Space was an issue. I keep this boat upright on my parking lot in summer and on the back wall of my grandmothers garage in winter.
The boat would have to be light enough to get it up on my car on my own.
Safety was not an issue. My local sailing grounds are "Alte Donau", an old sidearm of the Danube river in Vienna.
As a bonus I wanted to use the Mirage Drive.
If you do not have that space you might want to wait for my next boat.
It will be a collapsible 4m sail boat (to be built...maybe 2015 ...unless "Winds Of Winter" is published which would delay things).
Step 1: The Design
You can find lines on the internet or in books.
While you can adapt round hulls the most suitable designs for SOF (skin on frame) boats are derived from stitch and glue designs.
You obviously are limited to convex shapes.
I use three dimensional degree 2 Bézier nurb curves. Meaning there is a beginning point, end point and one weighted point that you move perpendicular to the curve to create it.
This makes sure they are all fair.
When creating your own design or modifying existing hulls you also want to watch volume distribution.
I just copied some designs from books and from the net, drew them and decided to go with this little pram.
Step 2: What You Need
- A jigsaw, bandsaw or scrollsaw (slow but very accurate):
- Epoxy (though you could also lash everything together with artificial sinew or use screws.) 500g+hardener will be enough.
- A circular saw or pre-sawn wood.
- Japanese saws are handy for cutting stringers to length.
- cable ties and ratchet straps are handy.
- A rasp, some files if you want to go slow or get some proper finish.
- Clamps...you can never have enough clamps.
- An electric drill. There are also rasp bits available which are a great.
- I also used some sanding tools, hand-planes, a multitool for cutting and a router for rounding off the frames. A router is not needed though it makes things easier.
- Plywood. Either marina grade ply or the 12mm phenol coated ply I use (cheap at 14.70€/sqm and light at 7-9km/sqm, depending if it is european birch or lighter Chinese wood of lower quality). You want to make sure it does not de-laminate when wet. I placed some scrap pieces in water for several weeks with no ill effect.
- Wood for stringers (longitudinal wood). I use cheap construction grade FiTa (stands for Fichte/Tanne...fir/spruce are sold as one). 48*25*3000mm, rough sawn surface. I have also used pine and larch...whatever is available and has little knots.
- Polyester cloth, the heavier the better. Also some weaves are preferable but anything will work. Length and width are determined by your design.
- Bamboo mast.
- Tyvek cloth for the sail
- Some ropes
- Ply or XPS foam for the cockpit.
- Paint for the hull. Cheap water based acrylic paint works well. Latex paint can also be used. Everything that sticks to the polyester cloth. You will want to try a batch first.
Step 3: Let's Start Cutting
I start by putting out PDFs from CAD. I have access to a large printer and can print most things on one sheet. You can also print on A4 or A3 using fitting crosses and assemble everything with tape. I like to put measurements on my prints so that I can control the scale on the print.
Glue or tape them to your ply. If you cut them out on the line and tape around it you can cut through the tape. If you only print half frames make sure you have a way to position them properly on the other half.
I like to round over the edges with a router. If you have not one handed edge router the thing gets heavy if you use it from above. You can also mount it under a table.
I came up with a slotted design built along a double stringer backbone which forms a very rigid box and is necessary for the Mirage Drive.
I also removed some flesh from the transom and bow. A lot of work with a 20mm router, not really worth the effort. All those cutouts do add up though...maybe 1-1.5kg saved.
Now everything should slot together nicely.
you can also do away with the plywood stringers but then you would have to mount all the stations on a strongback to proceed.
Step 4: Fitting the Stringers
Prepare your frames with a rasp to take the stringers. You want to go over all hard edges, those will break your stringers if you don't. Never try to bend stringers over hard edges.
The stringers are 19*19mm. Cut them on the table saw and either use a thickener or your handplane to plane them. I also run the plane over all edges till they are really soft.
If they won't bend you need to soak them in hot water. A steam box is the proper way but it works fine to wrap them into towels and pour hot water on them. I pre-bend them and dry them on the jig with a hot air blower.
You can then bend them into place and secure them with cable ties.
Other woods bend easier. Poor grade pine just needs some more attention.
On this boat I added a ply bottom. I can now drag it around in the grass without worrying too much and the cloth would have been to short in the first place also.
An alternative skin could be made of truck plane PVC material (I will use that on my collapsible).
Step 5: Skin, Paint and Rubrail
Use staples (preferably V2A,
stainless) or little brass tacks to mount the skin on your frame.
The staples on the gunwales (gunnels) will be hidden under the rubrail. I like to use brass tacks where you can see them. You could also hide staples under a rope on the transome and the bow if you prefer that.
Never mind small wrinkles just tighten it the best you can. After mounting the skin you can apply head to further shrink the cloth. Never get too close or you will melt a hole in the fabric!
I used expensive PE paint on the pram. It's smelly and unnecessary. The little canoe was painted with inexpensive water based acrylic...this is what I use now on all my SOF boats.
Use a roller to apply the paint.
2-3 layers will be enough.
Screw a strip of wood over the fabric through the gunnels. It can be any size. Mine is only 6mm. Screw it on with stainless screws. They are cheap if you buy them lose and rust does not look good on the boat. I previously used screws that where only brass covered....the head where the bit scratched them started rusting after some time outside.
The rubrail can easily ne exchanged if it gets damaged.
You can also screw rails on the bottom if you want some more protection.
Step 6: The Sail
The sail I am using is called a 'balanced lug'.
It is most convenient to rig and self balancing. You just pull it up and it is set.
I use Tyvek. It sews well, is cheap and available without print.
There is a 3mm rope sewn in around the sails perimeter.
Ring-rivets on the corners.
There is not much wind most of the time here anyways so I use a longer mast to get some good head clearance. The sail can be lowered or reefed in better conditions.
I installed a Mirage Drive...I'll do 5 km/h upwind or in no wind at all ;).
The Mast is a simple bamboo pole.
It is put into the mastpartner (that's the hole on the bow) and stepped into the maststep (the maststep is a block of wood right under the mastpartner).
You can build your maststep or mastpartner so that your setup allows for different angles of the mast. It is not that important with a balance lug though as it will balance itself anyways. Just make sure the mast is not leaning to one side.
The sail is tied to the yard (top) and the boom (which you should avoid with your head). The yard is lashed to the sail and you can add a parrel which is a loop around the mast so that the yard does not come away from the mast.
The boom is only connected to the sail on the corners (it is easy to see in one of the videos).
Then there is the tackline which keeps the lower front tip down and is attached to the mast and lower front corner of the sail.
The rope which is used to pull the sail up is called a halyard. It is attached to the yard some centimetres behind the mast.
Step 7: Some More Notes.
You can add inner gunwales strengthen the structure.
I like to sit on removable XPS foam. It's light an inexpensive.
Try cheap materials. I tried the cheap water based acrylic on my sons little canoe (which was actually built in 2 days) and it turned out perfect + it is easy to draw on it with a permanent marker. The acrylic has now lastet for 2 summers and is still doing good.
Use pigmented paint! Transparent varnish (even with UV protection) will not protect the fabric.
Our first boats skin broke down after a summer due to UV damage.
Another option, which I have not tried yet, is transparent Vinyl or PVC. (Search for "Dave Gentry canoe UGO"
to see what I mean)
Please ask any questions you like!
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