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May 26, 2019, 11:36 PM
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Build Log

Plywood IOM

I dithered a lot over whether to start this log, as it commits me to keep going. But it will turn out better if I can get some feedback along the way.

I have designed and built 8 IOM's now, all using shadows and strip balsa to make a plug. Finally I have one that makes me happy but it drove me nuts finishing it off. What I used to find enjoyable turned into one long chore and that started me thinking about the IOM class.

As I understand it this was intended to be a constructors class, simple and restricted enough that home builders could be competitive. According to this potted history, an initial principle would be that
boats permitted by the rule would be capable of being built by non-expert builders, either from a kit or from scratch, or inexpensively by a commercial builder, without being at a disadvantage in terms of performance when compared to yachts built using an unlimited amount of time and other resources.
It does not seem to have worked out this way. Building an IOM from wood strips or fibreglass requires a lot of work plus a reasonable amount of equipment like jigshaws and dremels, if not an entire woodworking shop. Or you can buy one, paying several thousand dollars for a new one. Either route is not that enticing for a newcomer. So, I wondered if it is actually possible to make a competitive IOM easily and cheaply and preferably without using power tools? This my attempt.
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May 27, 2019, 12:12 AM
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As far as I know the easiest way to make a boat is the 'stitch and glue' method used to make dinghies. So altered my software to cope with multiple chines and give me the developed shapes of the panels. Then I made a 0.6 scale one out of cardboard to see if it worked and it did. So followed up with a full-scale version made out of 3mm balsa (see the picture).

This took about one day to make and looks good, but it had two faults. Firstly it it a little bit twisted, not surprising as I used no shadows or frames to make it. Second I don't really want to use balsa because it dents too easily and swells horribly if it gets wet (anyone with a contrary view please pitch in with your experience).

My deliberations about what to use instead of balsa got resolved when I bought 2 sheets (2400 x 1200) of 2mm gaboon plywood for $26 each. These will keep me going for years. Then I drew a straight line on the ply and pinned out the sheets of panel lines along it (my printer only goes up to A4 sheets) and pricked the outlines through at abut 5cm spacing. (Next time I will add more in areas of high curvature). I joined the points with a black pen and used a craft knife to cut the panels freehand.

Then I coated the inside of each panel with epoxy to waterproof it and add strength. It seems quite strong and stiff enough not to need fibreglass on the inside. I also put a plywood strip into a glass of water and left it there for a week to see if the plywood glue held (it is rated as 'water-resistant'). It did hold, and this test gave me a great idea which I used later. Finally I made small holes at the 5cm spacing along each edge (again inserted by my software as they need to be measured along the curve))

To stop twisting I decided to use two external frames, and cut those out with a craft knife as well. The support frame shown is designed to allow access all round the hull for the wiring. I found some old house wiring and stripped out the copper wires and then just started wiring all the bits together and into the frame.

This took two a whole days. So far so good.
Last edited by Swiftsure; May 27, 2019 at 12:14 AM. Reason: spelling
May 27, 2019, 03:46 PM
Will fly for food
davidjensen's Avatar
I remember this IOM was also was a multi section plywood boat. The construction method is lighter and can look great. Keep posting your progress.
May 27, 2019, 06:33 PM
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Oops! Major hold-up. I had big problems trying to wire up the bow and stern because the angle of the middle panel changes too much over the last 20 cms at each end. While the panel is developable the rulings at the ends lie at a steep angle to vertical which means the panel has to be bent as much across its width as along its length, and this is really hard to do. At the stern I broke the panel and had to glue in a splint, and then it tore the wires right out of the stern when I tightened them. So I started all over again with a redesign which has a gentler transition for this panel at the ends.

These photos show the whole sequence done again with the new design. I took more care cutting out and used a 1mm drill bit this time to make the holes for the wires (last time I used an awl which made the holes too big). I forgot to say before that the lines plans also shows how much chamfer is needed to make the joints tight so I added these by eye using a sanding block.

This went together much easier at the stern this time but the bow had to be rewired three times before it went together. The lowest four panels are all quite narrow in this area so the internal space is very cramped for the threading the wires through the holes. In the end I used a needle and rigging line to sew everything together loosely and then tightened it up. Next time I might use another external frame close to the bow to help with this.

Next I used CA glue to spot-glue the panels together, then I cut and removed the wires and did a bit more spot gluing. Next I will use exopy to make a glue fillet on all the joins.
May 27, 2019, 10:12 PM
Classic wooden RC sailboater
SeattleRCSailor's Avatar
Cool project! This looks like an IOM version of the Tippecanoe T37. It's a panel boat, and was the first RC sailboat I built. You buy the kit and get everything with it including epoxy, fiberglass, spars, servos, even the radio. If you followed through with you project to that extent, you have an IOM kit in a box.

If you have access to Rhinoceros software, it does developable panels.

If I recall correctly, the T37 doesn't use stitch and glue but instead uses tape. I can also tell you that after making five plywood kayaks (from Pygmy Boats) that were all ostensibly "stitch and glue", that you don't need to stitch anything. Tape works perfectly and frankly much, much better than wired stitches. If you can tape together a 17.5' kayak from 4mm plywood, then you can certainly tape an IOM.

To help with the twist, try using at least one, but probably three, interior transverse frames. They can be temporary until the epoxy cures.

This will be fun to follow and I wish you the best!
May 27, 2019, 11:17 PM
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In the late 80s Bantocks first IOM Rhythm was hard chine ply.I still have one,and we built many sisterships.
John Spencer (the ply guru) was active at this time also and published quite a few ply IOM plans.
May 28, 2019, 06:27 PM
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Thanks very much for your feedback.

I searched for other plywood designs and found two here, one by Bantok and one by Spencer so thanks for the tip.

Using temporary internal frames is a good idea - I had not thought of that. Half the problem is the lack of space in the bow and we'd still have that to deal with. I've been using a pair of long-nosed pliers to thread the wire but this is a challenge for my eyesight and dexterity.

Can you explain more about 'tape and glue' please? I knew about using f-glass tape along the seams but all the references I can find still require stitching the panels together first somehow.

Meantime I noticed that my sample soaking in a glass of water had got a lot more pliable, not surprisingly. So I gave the first hull a soak and then had another go at stitching up the bow and stern (after switching to 3mm spruce plywood at the stern which spoils the look a bit). Success this time, but it needed a forest of wires.

Now I've got the unexpected issue of having to decide which hull to continue with.
May 28, 2019, 08:42 PM
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On the Bantock boats we simply taped the panels together with masking tape.We made many with 1.5mm hoop pine ply,my own one was 1 .2 mm Gaboon.
Bevel the ply and then tape together.Apply thickened epoxy glue along the inside of the joint.
May 28, 2019, 09:25 PM
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That does sound easier, thanks. The thinner ply would help too.

And did you glass the outside. How many layers and what weight?
Last edited by Swiftsure; May 28, 2019 at 09:30 PM.
May 28, 2019, 09:48 PM
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No glass needed,you could put a small strip of glass over the joints on the inside,we didn't and the boats are still in one piece.Yes the lighter ply makes it all easier
May 29, 2019, 12:36 AM
The wind is free, go sailing!
Scratchy101's Avatar
There is also a Frank Russell design plywood chine IOM called the Paper Clip Mk2.
Look at the bottom of this page for the link to the free download:

May 29, 2019, 01:37 PM
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Janusz Jawien's Avatar
Hi Art,
Thank you so much for this Paper Clip link! This thing looks so cool !
I am off to Staples to print the plans !
May 29, 2019, 05:03 PM
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Thanks for the new links. The Nekst certainly sets a high standard for finish!

Good to hear that I won't need a f-glass skin - that will save weight and time. And it seems that thinner plywood (1.5mm or so) would have been fine so I could have saved weight there as well.

Have now built the fin/mast box. I usually start with a commercial fibreglass box and add the extra bits so this was a bit more complex this time.. The various components and end product are as in the photos. The 'antlers' will help align it all and transfer loads from the top of the fin. Still just using a craft knife to cut everything out but my little jigsaw would have been much quicker for this. Still needs a coat of epoxy on the outside.
May 30, 2019, 09:26 AM
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Have you looked at the scharming design? I did one of the older ones as a rg65 and found it quite good. Really skinny.

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