Fixing A Broken Finn Boom -- Part One

As with all things getting older, a Finn Boom is likely to break, especially if fitted with boom hangers where the mounting holes are vertical. The bottom hole is in expansion mode as the boom is loaded and unloaded and eventually, snap, a race DNF.

Those who have Finn boom hangers with vertical holes or whose boom is cracked will find the second part of the fix beneficial. Especially if the boom is cracked and weakened where the mainsheet block shackle has worn away and damaged the boom under the hanger.

Scroll down to the bottom of the second part of the boom fix project to see how to fix a flogged out boom pin hole or corroded outhaul exit block mount.

broken boom repair

First step is to cut away the material around the break. Use a dropsaw to get square cuts, removing only around 2mm from either side of the breakpoint.

Remove all dross, round sharp edges and clean the inside of the boom, to a depth of 200mm, using scotchbrite pads to remove all aluminium oxidization. Use a file and disc grinder to create a roughened surface, for the adhesive to bond to, on both the inside of the boom and the outside of the inserts.

broken boom repair

From 2mm wall thickness, 100x50mm, aluminium box section cut two 320mm long, 70mm high and 12mm wide L shaped inserts. Clamp insert, all square and snug, halfway inside the boom.

Measure down 30mm from the top of one section of the boom and drill three 5mm holes at 30 mm centres, starting 30 mm from the cut edge.

Holes are for temporary rivets to hold the inserts when gluing the insert into the boom section. Rivets will be replaced by permanent countersunk ones later.

broken boom repair

As welding would alter the temper of the aluminium and make the boom softer, the inserts and boom sections are glued together using a two pack, Methacrylate type adhesive.

Available in handy 50ml dispensers and mixing nozzles, they are ideal for all manner of fixes, but beware they have a limited working time before they set, so line up, prepare and be ready before commencing gluing operations.

Simply apply a decent size bead on the insert and slide it around to spread the adhesive. Pop rivet to hold securely whilst the adhesive sets.

broken boom repair

Measure down 30mm from the top on the other half of the boom and drill three 5mm holes at 30 mm centres, starting 30 mm from the cut edge.

On a substantial, straight and square support (at least 2 metres long) set up the other half of the broken boom square, level and on a straight line. Clamp securely. Create a separator between support and the end of the boom (use insulation tape) to prevent the boom being glued to the support.

broken boom repair

Slide the two boom sections together and line up and clamp the two section square and on the straight line. Drill the pop rivet holes through the insert.

Separate the sections and apply a decent size bead of adhesive on both inserts. Slide it around to spread the adhesive evenly.

Slide sections together, lining up both boom sections square on the support and on the straight line. Clamp down and pop rivet to secure whilst the adhesive sets.

Finn in the background was in the shed for major Port/Starboard damage repair.

broken boom repair

Two halves have become one whole boom again.

broken boom repair

Drill out the temporary rivets, countersink the holes and insert monel countersunk pop rivets. This will allow the outer plates to fit flush on the face of the boom.

Part One, to repair the broken Finn boom, is complete.

Procedure to fit the outer plates, in Part Two.

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