Do you provide any "how to" instructions to guide us through the building process?

Yes, Dudley developed this method by designing and then building the first Didi 38, "Black Cat". He wrote a guide which is supplied with each set of plans sold for this method. It is also updated when necessary, drawing on the experience of other builders. You should also read the article on radius chine plywood construction on this site.

How can I accurately plot the tangent lines on the bulkheads? Just by the center of the radius and the intersection point of the hull (side) line and the arc? It seems that, depending on the thickness of the pencil line, this intersection point may very well move up or down a couple of centimeters.

The correct way to find the centre point for the radius is to measure inward from the edge of the bulkhead by the radius measurement. Do this at two points on the side and two points on the bottom. Join the points with straight lines, which will be exactly parallel to the bulkhead side and bottom edges if correctly done. The intersection of these two lines is the centre of the radius.

Now draw a line from the centre point to the side, meeting the side at exactly 90 degrees. The intersection of this line and the side is the upper tangent. Do the same from the centrepoint to the bottom, at 90 degrees to the bottom. This intersection is the lower tangent. Your arc should meet the side and bottom at these points but there will be no guesswork as to the exact position of the intersection.

How do I set out the splay on the corner of the bulkhead for the sheer clamp? (Dec 2002)

The detail below is from the Didi 26 but the same principle applies to the other designs. First draw the basic outline shape of the bulkhead (to the inside of the hull and deck skin). Next, bisect the angle formed between the hull and deck. This is done by measuring inward the same amount (say 100mm or 4") and drawing lines parallel to both edges as shown by the green lines on the diagram. Draw a line from the sheer point through the intersection of the two green lines, shown by the middle red line. This line is bisecting the angle.

Next measure on both sides away from the red line by a distance equal to half the width of the sheer clamp. In this case it is 32mm because the sheer clamp is 64mm wide. Draw new lines parallel to the first red line. The distance between these two lines is the same as the width of the sheer clamp.

Draw a new line connecting the points where the two outer red lines intersect the bulkhead edges. This is the cut line and will give an edge equal to the width of the sheer clamp.

Do I need to fit blocks on both sides of each bulkhead for fastening the sheer clamps?

No, the timbers to receive the sheer clamps are one side of the bulkhead only. I have shown them on the front face forward of the point of maximum beam and on the aft face aft of maximum beam. This allows you to plane the outer face to match the angle which the sheer clamp crosses.

Am I right in thinking that it will be a lot easier fitting the stringer doublers to the bulkheads after the stringers are fitted, rather than before set up?

Yes, it is much easier to fit them afterwards. I don't know that they are really necessary because the epoxy fillets spread the bond well into the structure anyway. There is no sign of strain in the structure of my hull after 4 crossings of the South Atlantic and 20 000 sea miles, so the epoxy fillets are very effective. The doublers are fiddley to make, probably a bit more time consuming than doing fillets around the stringers.

Note that these doublers are only shown on the earlier designs, ie the Didi 34 and 38.

Do I need to do epoxy fillets all round the bulkheads against hull and deck?

Yes, make fillets on both sides around the full perimeter of each bulkhead.

Do I need to do epoxy fillets along stringers against hull and deck?

No, there is more than enough gluing area to the stringer faces. A small fillet is useful for neatness and to make painting easier but it need be only a few millimetres (1/8") radius.

I have planed the sides of the stem to line up with the stringers and the backbone to match the angles of the bulkheads. How do I shape the transition between these two at the radius?

The shaping of the backbone in the forefoot area must be done carefully. Aft of where the lower tangent intersects centreline the backbone is planed in a simple V-shape to match the bottom angle. From there forward it has to fair into the curve of the radius chine so you will be planing a curved surface onto it rather than a flat.

Do this work slowly and use battens tacked onto the edges of the forward few bulkheads as a guide to establish the angles. Remember that in cross-section it is a curve but fore and aft it is almost straight, so that is what you should use for guidance. The end result will be a gradual transition from the backbone shape at the lower tangent to the stem shap at the upper tangent.

In the guide to construction you call for the sheer clamps to be be fitted before the stringers. Would it not be better to fit the stringers first to make the structure more rigid before fitting the sheer clamps?

It is probably not important which is done first, as long as you ensure that the bulkheads stay properly in line throughout the process.

The sheer clamps are large and require a fair amount of force to fit. Do you have any further points to assist with this operation?

I fitted those by myself so it took inenuity and a bundle of clamps.

I dry fitted both sides, starting with the sheer clamp held close to its midpoint with a G-clamp to the appropriate bulkhead and the ends supported on saw horses. I then worked towards the ends, clamping it to each successive bulkhead. When it was all positioned, I eased each G-clamp in rotation to allow the structure to realign if needed. Then I glued in position, again by easing each G-clamp in rotation and pushing the glue into the joint with a spatula then fitting the screws.

If I bend a piece of plywood round the radius so that it is sitting squarely on the tangent stringers and the 2 stringers in between, it makes a curve, which is fair to the eye, but is not a true circle. It does not match exactly the curves at the bulkheads, which are geometrically accurate. What can I do to correct this?

There are two possible reasons for this. The one is that you have not planed the tangent doublers to match the radius and they are forcing the plywood out of line.

The other is too much or too little "tension" in the plywood when fitted. I found that small differences can be made by adjusting the tension on the plywood, ie pulling it tight between tangents pulls it in hard against the stringers and straightens it out between stringers (tending towards a multi-chine shape with the stringers as the chines) whereas pushing some slack into it gives a nice round curve.

On the subject of fairing, would you expect the radius to be completely true to the eye when viewed from any angle, with no detectable flat or high spots? Will slight variations, say less than 1mm, which are just visible from some angles if you look very hard, be lost when the anti fouling and boot stripe are on. On Black Cat, would it be possible to find high or low spots if one looked hard enough?

My hull is very fair on the flat surfaces but the radius is not totally fair, which would have involved filling and long-boarding, a long and slow process.

I set a long jack plane to a very fine setting and made it really sharp. I ran this longitudinally over the radius surfaces to take off the high spots and leave the lows. I think I was at it about 2 days, not trying to achieve a perfect surface but rather what my eye would accept. I still see the imperfections but others who look at her don't see it and compare her favourably with a GRP production boat.

I think sometimes that some of us are too critical of our own work. Most of the radius is under the water so for aesthetics sake, concentrate on the aft end, where it sweeps up out of the water. The rest needs to be good enough that you still feel proud of your work.

The patterns you sent me show the dimensions for a 12mm skin. As I intend to have a skin of 18mm as suggested by you for cruising, how do I go about it? Should I just add the further 6mm on the outside or should the bulkhead size be decreased, which can be very complicated?

You must increase the skin thickness to the outside, so the bulkhead remains unchanged. That way the hull becomes a little bigger, to give the volume needed to support the extra material weight. The result is that your boat will not float deeper even though you have made it heavier.

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