Building A New Finn Boom

When we repaired my broken Finn boom, the break was joined using both internal and external sleeves. Whilst this made for a very strong boom, it suffered from being stiffer and heavier then desired. (view the broken Finn boom repair here).

Combined with my stiffer mast, the combination always felt over-powering in the heavier breezes, it just never seemed to allow the leech to open.

Building the new boom started way back in 2009 but it never got finished, hanging in the shed and being used as a straight edge or support structure for other building or repair projects.

new finn boomConstructed from three alloy extrusions. The top out of a single 32x32mm box section and the bottom from two 43x15mm sections, each with a wall thickness of 1mm. Glued together using a two pack, methacrylate type adhesive. This adhesive requires a 2mm spacing between the surfaces to be joined and this was uniformly achieved by placing small 2mm alloy strips at intervals between the sections.

With the 2mm gaps, the boom cross-section was right on the minimum dimensions for a Finn boom (77x32mm).

Bolt is a safety measure to prevent leech tension opening up the glue line and zippering the top extrusion from the bottom two.

new finn boomOutboard end showing the machined capping glued on to alleviate the leech tension loading away from the sail track slot. Capping doubles as a fairlead and exit block mounting for the outhaul control line.

First outhaul 2:1 purchase is on the outside of the boom.

The bottom edges of the boom are quite sharp and to protect ones head from the scythe it can become, a bit of foam padding was glued on.

new finn boomMain sheet hangers are simple dyneema cored loops through the machined fairleads.  Loops can rotate through the fairleads to bring the mainsheet purchase loading in line with the boom centreline when reaching or running, removing any twisting motion from the mast gooseneck when the boom is loaded.

Bottom edge of the new boom is reasonable sharp so we will need to keep an eye out for chafing on the loops.

Saddles on each side are to hang the outhaul, kicker and boom bungies from. Soft mainsheet loops leave no anchor points for these bungies.

Main sheet block is our home made 40mm soft attach block. Designed to prevent pulley clash when at maximum traveller and the is boom sheeted hard on deck.

new finn boomKicker lever is a nice bright red anodised copy of our standard lever available for our Finn Shop. More details here.

new finn boomKicker lever rotatates around a stainless steel barrel. Acetal plastic insert serves as load distributing bearing for the kicker lever barrel plus as sail feeder and outhaul fairlead.

Second 2:1 outhaul purchase inside the inboard end of the boom.

new finn boomInboard End showing the two kicker pulleys and the outhaul exit pulley.

Acetal plastice backing pads inside the boom inboard end to spread the compression load of the 16mm boom/mast bolt.

Handy fittings to bolt the kicker pulley onto.

2mm plates glued to the sides to padded out the 32mm wide boom to 36mm in order to fit snuggly into the mast gooseneck yoke.

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