Forums: Whisstock Designs: Another question New Topic Post Reply
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Posted: Sep 6th, 2013 at 15:36   |   Subject: Another question
Hi George thanks for your quick response. My grid will be painted by tonight and drawn tomorrow. I fully understand this. What I'm unclear on is when practicing drawing the frame I get one line for the futtocks. I'm clear that I'm drawing the larger offsets to bevel down to smaller offsets.

How do I get the corresponding lines to cut out to complete the one form? In other words if one line is drawn all I'm making is a cut. This one answer will really break the code for me on cutting all forms out.

Thank you
Posted: Sep 6th, 2013 at 17:16   |   Subject: Another question
Hi Mike

I'm answering both posts here in one go!

Frame -450 is quite complex because it has a two-layer 9mm ply bottom section (2 x 9mm = 18mm), a solid 18mm timber middle section (the 'futtocks') and a laminated beam that is 25mm sided (thickness).

However all these components are flush (they finish level with each other) on the aft face. So the beam protrudes 7mm further fwd (25 - 18 = 7).

This is reflected in the Table of Offsets which give the aft and fwd faces of the frame (-450 and -432) and the fwd and aft faces of the beam (-450 and -425).

Ok - you need to draw out BOTH sides of the frame, around the hull all as set out in the instructions, Section 4.1. You use the larger dimensions all the time. The instructions give you the actual offsets (widths) and heights to use as follows:

Mark in the sheer point by measuring 976mm up from the dwl (the larger of the two sheer heights - neglecting for the moment the fwd face of the beam) and 405mm square out from the centreline (the larger of the two sheer offsets - again neglecting the fwd face of the beam).

Mark in the hull offsets on their appropriate waterlines, using the larger offsets which are all those for the aft face. So, on wl 800 measure out 387mm, on wl 600 measure out 359mm, on wl 400 measure out 319mm, on wl 200 measure out 263mm, on the dwl measure out 178mm, and on wl -100 measure out 102mm.

The rebate is a constant 50mm offset (like a 50mm buttock) so we shall be measuring heights for this. So, measure out 50mm and measure down 147mm below the waterline.

Note that you can, when drawing out the hull, neglect the sheer height on the fwd face of the beam. This is because the beam is thicker than the hull part and the boat gets narrower as it goes fwd. So the sheer offset on the fwd face of the beam is 4mm less than on the fwd face of the hull part of the frame (396 - 392 = 4). The sheer height on the fwd face of the beam is 1mm higher than on the fwd face of the 18mm thick hull part of the frame (977 - 976 = 1). BUT - mostly it is higher because it is narrower, so the height has 'climbed' up the deck camber.

When you set the beam section of the frame out, you follow instructions:

Now measure out the deck heights . On the centreline measure 1039 (the larger of the heights) up from the dwl. On b 100 measure up 1035, on b 200 measure up 1022 and on b 300 measure up 1002; the sheer height for the beam is 977 (still at 405 Offset). Measure the heights on the other side of the boat as well Join the points with a batten as before, right across the boat in one go. Across the deck (and the coachroof top) don’t mark one side and then the other as you will get a “hard” join on the centreline, whereas the curve should continue smoothly across the centreline and down the other side.

So here we have used the largest height of the beam at the sheer (977) at the largest hull offset (405). This is not entirely "correct" but it will be near enough in practice.

If you want to be entirely correct, you could set of the beam height 977 at the correct offset (392) and then continue the curve until it hit the hull offset of 405.

So now you have got the entire frame, both sides, drawn out on the floor. Now you make up the all the frame components as set out in the instructions and assemble them and bond them together so that they match their outline on the floor.

Then you transfer the waterlines and buttock lines from the floor to the frame so as to be able to mark the bevels on the appropriate faces of the frame. The instructions tell you how to do this. But basically you can either mark in the full measurements – or you can subtract the smaller from the larger and mark that amount in fro the edge, as the instructions, Section 4.5:

To set out the bevels we need to refer to the Table of Offsets again. On this frame, the fwd face is the smaller around the hull and the aft face is smaller on the sheer height and across the deck heights.

The bevels can either be marked by setting out the outline of the frame again on the smaller face, or by simply subtracting the smaller dimension from the larger and marking this distance in from the edge on the smaller face - but this will only be accurate if the edges are planed up accurately and square. Offset bevels are measured horizontally along their waterline and Height bevels are measured vertically along their buttock.

So on the sheer, on the fwd face (of the beam) measure and mark an offset bevel of 13mm (405 - 392) back in from the edge. Or simply measure and mark the offset of 392. At the 18mm thickness of the hull part of the frame the sheer offset bevel is 9mm (405 - 396). On wl 800 measure a bevel of 9mm (387 - 378); or simply measure the offset 378. On wl 600 measure a bevel of 9mm (359 - 350); or simply measure the offset 350. On wl 400 measure a bevel of 9mm (319 - 310); or simply measure the offset 310. On wl 200 measure a bevel of 7mm (263 - 256); or simply measure the offset 256. On the dwl measure a bevel of 7mm (178 - 171); or simply measure the offset 171. And on wl -100 measure a bevel of 6mm (102 - 96); or simply measure the offset 96.On the rebate line (at Offset 50) measure a height bevel of 4mm (147 - 143) on the fwd face; or simply measure the height -143. Join the points with a batten as usual. Repeat on the other side of the boat.

On the deck we need to mark the height bevels. On the centreline and b 100 there is a 1mm bevel off the aft face. On b 200 and b 300 there is no bevel. At the sheer there is a height bevel of 3mm off the aft face. This sudden change to 3mm bevel may seem odd but it comes about because the offset bevel at the sheer is smaller on the fwd face so the sheer height on the fwd face is nearer the centreline and has thus ‘climbed’ up the camber of the foredeck. Join the bevel points with a batten as usual.

Now plane and spokeshave the edges of the frame so that they match the bevels. Try not to take any material off the existing larger face. To help prevent this, run a pencil line around the edge of the frame about 2mm away from the larger face. As you are cleaning the edge off, make sure this line remains visible until the last few strokes of the plane or spokeshave.

So, to sum up, you draw the entire frame out, both sides, to the larger dimensions. Then you make the frame and assemble it so that it matches its outline on the floor. So the frame is made to the large dimensions. The you bevel the edges back to the smaller dimensions.

I hope this is clear. It sounds horribly complicated when you write it all down, but in fact is is not when you get to do it.