We’re still enjoying a bit of late summer sunshine but it’s never too early to plan your end-of-season projects. To that end, our very own epoxy guru, David Johnson talks us through how he built a rudder for his Sandhopper boat last winter.
This is the second development of the thin section MK 1 version rudder I made and have sailed on my Sandhopper boat for the past eight seasons.
The MK 2 version has a much thicker section and is a typical four-digit NACA 0008 or 10 profile, with the maximum fullness at 30% chord. This gives good lift over drag for the slow speeds and leeway of a small keel boat with tolerant stall characteristics at high angles of attack. Incidentally, it’s a similar aerofoil section to that of WW1 aircraft design.
I owe a lot of my preliminary design work to a really comprehensive article from Gougeon Brothers, Inc. entitled ‘How to build Rudder Blades and Centreboards by J.R Watson’. A good friend and Naval architect Neal Pawson tidied up the finer detail. I might add that a misspent youth as an aero modeller and cadet sailor gave me a basic understanding of aero/hydrodynamics.
Light and strong
I built the rudder using laminated strip planks of cedar for lightness, spruce for stiffness and a twin Sapele ‘Spar’ for aesthetics. Everything was bonded together with WEST SYSTEM 105 Epoxy Resin® and WEST SYSTEM 205 Fast Hardener®, along with WEST SYSTEM® 403 Microfibres.
I did the profiling with the aid of accurate templates and a combination of electric and hand planes and judicious sanding.
I rebated a 3mm square channel all around the foil leading and trailing edges, and also carried this on through the tip. This I backfilled with WEST SYSTEM 105/205 epoxy and WEST SYSTEM® 406 Colloidal Silica. The result was a high density centreline to shape to and a very hard protective edge for the rudder during use and transport.
I finished the rudder in WEST SYSTEM 105 Epoxy Resin and WEST SYSTEM 207 Special Clear Hardener™ with 280g/m2 glass fabric sheathing. I achieved the final surface finish by burnishing with increasingly finer wet and dry paper, then polishing with cutting compound.
The tiller is infused epoxy/carbon, using PRO-SET® INF-114 and PRO-SET INF-210 Infusion epoxy. It is removable via hardware bonded socket screws and is much lighter than alloy tube, with immediate feedback.
Fitting requires considerable effort and involves pushing the rudder to the class maximum, away from the transom. My thinking here was that this pushes the rudder into clearer flow, away from the turbulence created by the Sandhopper’s bilge keels and increases the effectiveness of the rudder turning moment from the CLR. It also makes it much easier to remove the rudder from the transom whilst in the mooring.
All the rudder fittings are bolted through, with the fixing holes backfilled with WEST SYSTEM 105/207 epoxy to create hardware-bonded bushes.
I’ve called the rudder Sweet Spot MK 2 in homage to my retired work colleague, prolific foil builder and world champion sailor Peter White, who is now a highly successful golfer!
The finished weight is 4.5kg. I’m very pleased with the finished product, there is something symbiotic about clear finished carbon fibre and natural wood.
Many thanks to David Johnson for his contribution.
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