Didi Mini Builders' Notes - Updated October 2007

Please visit the Didi Mini Kit Construction Project for photos and explanations of the Didi Mini under construction.

Also visit our radius chine plywood FAQ section for answers to questions raised by other builders.

The lower part of the bow, in the region of the radius, has hollowness when seen on plan. This is contrary to the rest of the hull, which is convex. This can be seen on the lines and construction drawings and in the diagram below. The broken black lines indicate stringers installed to their designed curves. The solid green lines span the hollow and have been added to this diagram to highlight the reverse curve that the stringers must take when correctly fitted. You can see the same thing by laying a straight edge on the stringer lines on your construction drawing.

This hollowness needs special care to be taken when fitting the stringers. The stringer ends must be cut at the correct angle to butt against the sides of the backbone and they must be fitted so that the line of each stringer, projected forward, meets the front edge of the backbone on the centreline. This can be checked by making a saw cut through the outer corner of the backbone from the backbone centreline to the outer face of the stringer. When the stringers are correctly fitted, the saw cut will line up exactly with the outer face of the stringer. This saw cut can also be used as a guide for planing the bevel onto the side of the forefoot portion of the backbone to receive the forward edges of the hull skin.

If you fit the stringers incorrectly, they will come into the side of the backbone a long way in from the edge and they will be a straight line or convex from the front through to bulkhead 2, whereas this full section should be concave. If you have already glued these stringers into the structure in this manner then you will need to build up the outer face of each stringer forward of bulkhead 1 and plane it down between bulkheads 1 and 2, to produce the hollow curve. It will also be necessary to build up the inner face of the stringers where they are planed down on the outer face, to get them back up to the specified size.

Mini-transat radius chine plywood kits

The skin thickness is specified on the drawings as 6mm for the sheet plywood areas and two layers of 3mm plywood for the radius. If 3mm plywood is not available in your area, you can sheet the radius with two layers of 4mm.

Before fitting the first layer, use a belt sander to chamfer the inner surface of the ends of each strip where it overlaps the tangent doublers, so that it lies flush with the rebate in the 6mm sheet plywood alongside it. If you prefer, you can glue the strips first, then use a belt sander to sand the edges flush with the rebates but be careful not to damage the rebates. After fitting the second layer, use the belt sander to remove the excess thickness and fair it smoothly into the surfaces of the 6mm plywood. Use a straight edge on the 6mm plywood to ensure that the radius plywood is not higher than the flat plywood, which will show as a hollow along the radius.

You should now have the radius plywood fairing in flush with the surface of the sheet plywood of the sides and bottom. There will be a slight increase in weight, of course.

The junction between the forward end of the side hull skin and bulkhead 0 must be reinforced to resist cracking the joint from side skin flexing in large waves. Do this by making an oversize high density fillet (75mm radius) on the inside in the corner between the hull side and bulkhead 0. On the outside of the hull apply a single layer of 75mm [3"] wide glass tape over the joint between the capping and plywood skin.

The rudder is specified as cedar core with a layer of 450g/sq.m biaxial glass covering it. Add another 3 layers from the top of the rudder stock down to the step at the start of the foil. Below that taper the glass off at 100mm width per layer. This will distribute the pintle loads better into the rudder structure.

The Mini 650 Class is tightly controlled, so you must ensure that your boat fits the rules if you want to race in the class. The Didi Mini hull is designed to the limits of the measurement box if accurately built. In any boatbuilding project there are many ways that inaccuracies may come in that can affect the final size. Check the overall size of your boat during construction so that you can make adjustments where needed to ensure that your boat will measure within the rules. In the design we have the allowance for a bow block that can have material planed off to reduce overall length.

The beam is not as easy but also has to be checked. This is particularly necessary if you have increased the skin thickness by using thicker plywood or fabric covering over the outside. Please read in the rules how the measurement is taken. You will have to plane down the skin to achieve the 6mm designed skin thickness locally near to the sheerline at bulkhead 7, thereby compensating for the extra thickness that you added.Even if you have not increased the skin thickness you should check that your beam complies. It is easier to adjust when it is bare wood than when you have it all painted and looking pretty.

The Mk1 version has separate linkages from the tiller to each rudder, which results in awkward rudder geometry when the tiller is turned. On the Mk2 version we have changed this to a single link bar between the rudders, which is attached at its midpoint by a flexible linkage to the bottom of the tiller. We recommend that Mk1 builders make the change to this system also. Any Mk1 owner who wants to make the change, please email us to request the revised drawing.

Mike O'Neill has done some experimenting with rudder toe-in angles on his Didi Mini "SCM" (ex "Warlock"). He has found that 7 degrees of toe-in between the two rudders gives good feel to the helm and reduces drag on the windward rudder. To achieve 7 degree toe-in, set the link length so that the rudderstocks (narrower portion above the waterline) are 30mm further apart at the aft edge than at the front edge. These measurements must be taken at the same height above DWL for front and back.

Piers Dudin is sailing his Didi Mini Mk2 "Thunder" from South Africa to UK and has broken two keelbolts. This may have resulted from initial incorrect fitting of the keel but has highlighted the need to increase the keel bolt size. If you use the fixed keel option and have not yet built your keel, please change all of the keelbolts to 12mm or 1/2" diameter 316 S/S bolts. If you want a drawing showing the larger keelbolts, please email Dudley Dix to request it. If you have already built and/or fitted the fixed keel, there are two possible solutions to strengthening the keelbolts:-

1) Cut openings through the splay plates at the top of the keel to give access to the bolt heads. Remove the 10mm bolts and replace with 12mm bolts then repair the splay plates. You may prefer to cut off the splay plates, replace the bolts then fit new splay plates.

2) Leave all existing 10mm bolts and fit two extra 10mm bolts on each side. These must be 100mm forward of the 78mm OD tube and 75mm aft of the tube for maximum benefit. These positions are both measured from centreline of the tube. This will need access openings through the splay plates at the new bolt positions, in similar manner to option 1.




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This page was updated 22 October 2007

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