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by John Welsford, Marine Designer – Auckland, New Zealand

How to fit pieces up to an awkward shape.

Even a really nice curvy boat like this is nowhere near as difficult to build as it looks. Alan Churchwarden Photo. 

When we’re building a boat like the Pathfinder above, the outside of it is actually pretty easy.  But getting good fits inside, where parts are really odd shapes is one of those jobs that the professionals make look simple, but which unless you know how they do it, can be very frustrating.

Below is another Pathfinder, this one built by Paul Groom.  Note that this was his first significant boat build, well done that man.  Note that the seat tops have to be fitted to the inside of the hull, the outboard motor well and the transom.  The have to fit to the centre case brace, and still have a nice curved shape on the inside edge.

Not only that, but forward of the center thwart, alongside the centre case and forward up under the foredeck there is a raised area which gives enough space to sleep two people comfortably, or to sail with a couple of those little fold up deck chairs. Real luxury!

So there are a lot of parts to fit, complicated shapes, shapes that need to fit fairly well or you’ll have issues.

The inside of Varuna, Paul Grooms Pathfinder.  Note the seating and the raised floor, all of which has to fit the sides of the boat, the motor well, transom, centre case and forward bulkhead.  Good fits are not only good workmanship, they save bucks on epoxy filler too.

Fitting a seat against the side of the boat, or a bulkhead into the hull, or any other of the myriad parts of a boat that can't be measured with a set square and ruler is for most a matter of trial and error.

But error, when working with material that costs as much as top grade marine plywood, is not something that we can afford.  So, how do the professionals get those hairline close fits?

Piece of cake! They use a “jogglestick”.

Yes, it sounds odd, and a verbal explanation does not seem to get the message across. To say “uneven joggles” and “multiple outlines” just confuses the issue.

So here is a photo essay, I’m fitting the stern sheets, or seat into SEI my new 15 ft double ender.

This is the stern area of SEI, the prototype of a new design that I will have plans available for very soon. I’m fitting the stern seating while there is as little other structure in the way as possible, and want to cut the seat top which acts as the top of the aft buoyancy tanks in two pieces joined down the middle on the spine piece you can see there.

I’ve chopped the corners of a leftover piece of hardboard so it will fit into the space where I want to make a fitted 1/4in panel seat top.  That’s the “template sheet”.

Any thin panel will do, something that won’t pain you too much if you throw it away later.  It does not have to fit perfectly, but it does need to sit flat, or in the same plane as the seat.  No sag or humps, you cant see it but the inboard forward end is actually sitting on my little cordless saw, its just the right height so as you can see you don’t need to frame it all up or go to any trouble, whatever is handy will do.

Here I’ve laid a piece of hardboard in where I’ll trace the jogglestick shape so I can reproduce the sticks position on the plywood permanent material. Note that the shape is not anything much like the finished item will be.  As long as the hardboard template sheet is big enough, and full length of the finished item the shape is not important.

You cant see it but I’ve put a couple of very fine nails in there to ensure that it cant move while I’m doing the jogglestick thing.

The jogglestick, it could be any piece of scrap with an odd shape cut in the side, but don’t make the notches even, they need to be sufficiently “odd” to make it obvious where it will sit when transferring the position of the points.

Make your “Jogglestick”.  Just a strip of scrap plywood, with an uneven zigzag cut into one side.  The idea is that when you trace the outline of the jogglestick, you can then put the jogglestick back in exactly the same place later on. Because the zigzag is uneven, it will only fit in one place. When you are marking out the finished item the point, which you’ve placed up against the side of the boat, the edge to be fitted, will be in the same place relative to your template sheet.

With the jogglestick laid on the template sheet, and the point of the stick up against the side to be fitted to, run the pencil around it making sure it cant move while you do so.  Do this at regular intervals around the component.

The next thing to do is to mark in, using the jogglestick, the centreline. Or the “other side” of the piece so you can reference the edge that will fit the boats side.  If it’s a straight edge, then just the end points is enough, or even two points well apart along the center.

Once that’s done then mark in, by laying the jogglestick point against the shape to be fitted to, then running the pencil around the “joggle”.
I number the “points” which makes it easier to identify the markers.

Pick up and mark the corners, any slots, angles or odd shapes.  Remember that you are going to use this to produce the finished shape on the permanent material, plywood in this case.

I put markers about a handspan apart plus the markers for the details.  Until you are well practiced with this, more markers are better than fewer.

When this is all done, take the template sheet out and lay it on the material from which you will cut the finished piece, in this case I used the centreline markers to position the template sheet relative to one edge of the plywood.

Secure the template sheet so it cant move, I popped a couple of finishing brads through it with my cordless nailer, but that’s over the top for most people, a pair of fine nails and a hammer will do it.

With the template sheet tacked down to the plywood seat top, I’m about to transfer the points onto the plywood.

Then, put the jogglestick on item one, line up the shape and mark the position of the point on the plywood.  Tick the number to show you’ve been there then move on.

You’ll see that the jogglestick can only fit the outline one way, so the point can only be in one place.

Put a fine nail in where the point mark is.

Continue, using the jogglestick in the same way to outline the odd shapes, in this case the slots where the seat top fits around the frames.

With all of the marks transferred, and the nails in place, bend a nice even batten around the nails and mark the edge of the piece to be cut. You may need to adjust a few nails a little, go for a fair curve and slightly bigger, you can plane this down if needed but its not so easy to add material.

Marking the position of the jogglestick point by placing the stick over the lines you’ve traced.

Note that where there is a break in the curve, such as would be where the line of the seat top crosses a chine or plank lap join, run your batten from that angle to the next, you cant put a batten around that angle so do the curve on each side separately.

With a batten wrapped around the nails that have been driven into the marked points, I’ve run a marker pen around it to show where to cut.

Do the details, and after pulling the nails and removing the template sheet, cut the piece to shape and trial fit.

Its usual to have a little work to do on the edges, especially where what the piece will fit to is angled such as the side of the boat is.  In this case I’m not worried about a perfect tight fit as I’m going to fillet it, the small gap making it easy to get a really strong epoxy joint.

Note that I wanted to produce a curved edge for the after edge of the seat, and although I’d drawn it on the template sheet, I did not cut it until after the plywood had been marked out so as to leave a larger working area to mark the edge with the jogglestick.

And that’s it!

Nice fit, first time!  Well, there were a few tiny shavings taken off inside one of the frame slots.  But close!  The whole thing took around 20 minutes.


So what does SEI look like? Here’s the draft drawing. Plans will be available in the new years.

SEI Pic.

4.45m (14 ft 7in ) long;
1.58m (5ft 2in) beam;
draws 0.150m /0.900 (6in board up, 3 ft board down);
sail of 7.1 sq m (76 sq ft) and will weigh in at around 70 kg (155 lbs).

John Welsford
Marine Designer.

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