August, 2014

The Rubbing Strakes

The Rubbing Strakes for Pícara are made of solid Mahogany wood stock. With a thickness of 9mm, the rubbing strakes run from the bow to the transom reaching a maximum width of 50mm between stations 5 through 7. I opted for a multi-piece rubbing strake for my boat.

The method I chose to fit the rubbing strakes is to avoid any plastic nails or screw holes on the face of the rubbing strake. Not the easiest method, but I want to learn how to do it. And it has taken me the longest to learn it.

The biggest challenge is to secure the rubbing strake tightly against the gunwale while gluing them together. Most importantly, ensuring that the top edge of the rubbing strake tightly fits against the top edge of the deck, without any gaps.

The problem is, how to clamp the rubbing strake against the gunwhale? Where to clamp it from? The gunwale's angle and curves resemble the movement of a Chinese Dragon.

Many thanks to Keith Callaghan for providing a drawing on the rubbing strake clamp detail with a timber batten. This clamping method proved very useful.

I'd also like to thank fellow Merlin Rocket builder/sailor Jeremy Deacon, Miles James and Al Jackson for their build photographs, drawings and expert advise.

All recommendations for this particular method included the temporary addition of wood blocks screwed under the gunwale every 150mm or a 8mmx12mm strip stapled the length of the gunwale using 15mm staples, stapled every 75mm.

I deviated from using the wood blocks and instead I used metal eye screws because I can easily drill a pilot hole at the desired gunwhale angle per location and the eye screw provides a strong pad for the clamp foot in parallel to the gunwale. 

Here are the eye screw fitted on Pícara in preparation to fit the aft starboard section. 

View of the eye screw fitted on Pícara in preparation to fit the forward starboard section. 

First dry run clamping the bottom of the aft piece first. I leave 1 mm or so of extra material on the top edge so I can shave and sand flush with the deck. It also leaves most of the stock at the bottom where it will be shaped to the right width per station according to the rubbing strake drawing.

As you can see, the lower clamping torques the top of the rubbing strake away from the deck.

View of the eye screws underneath the rubbing strake piece where the clamp food can get a good grip.

Another view of the eye screws underneath the rubbing strake piece.

Next, I clamp the upper side of the rubbing strake tightly against the edge of the deck. I also added a piece of scrap wood over the inner edge of the deck for protection. Now I have a perfect joint.  Of course, once epoxy with colloidal silica is applied, things can get very slippery. All of a sudden the static energy can be released and the piece can move out of place. Given the warm weather we're experiencing in Austin, Texas, I continue to use West System's 209 Extra Slow Hardeners. It fully cures in 24 hours.

A blue tape protects the top outer edge of the deck to avoid epoxy contamination over the area that will be varnished.

The same process was followed for the rear port side piece.

For the forward starboard piece I have to solve for a different problem. The forward deck provides no opportunity for clamping anything from so I had to get creative to implement a counter-torque technique recommended by Keith Callaghan.

The counter-torque technique consists of adding a rectangular stick of wood about 12" long placed between the rubbing strake and the outer foot of the lower clamp.

The creative piece was the addition of a temporary 4" x 4" cedar timber about 6' long over the deck to secure the bar clamps from. I used scrap pieces of 4 mm plywood between the timber and the deck for protection. I secured the timber with larger clamps at the breasthook and at aft deck frame.  

But the bar clamps would slip off the 4" x 4" timber so I clamped a scrap piece of 15 mm x 30 mm x 6' pine stock along the back of the cedar timber so the bar clamps can get a great grip. Clamps, clamps and more clamps. Had to get more. Said it before, say it again. You can't have enough of them. And the slipping and moving stopped!

View of my enhanced clamp collection from the bow during the dry run.

The same process was followed on the port side. View of the port side while setting up the clamps in place before gluing.

Now with protective blue tape on the outer edge of the deck while gluing the forward port side rubbing strake piece to the gunwale.

View of the clamps from underneath looking from the rear.

View of the clamps from underneath looking from the bow on the port side.

After removing the clamps, I scarfed the forward and aft rubbing strake pieces in preparations for the mid-section piece. I projected a tangent line off the gunwale to the rubbing strake and proceeded to shave and sand until a smooth flat scarf joint surface was achieved. 

View of the aft piece after shaving and sanding.

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Fair winds and happy sailing to you!