Hollow wood paddle boards require internal blocking to support the fin box and optional thrusters. You can use either wood or foam for the internal blocking. Both materials result in a stronger fin box than commercial foam boards because of the internal fishbone frame is connected to the outer skins. Wood blocking should be at least 1/2″ oversized in all directions. Foam blocking should be at least 1″ oversized unless you have a really high density variety.
I am currently using 2″ wide foam cut offs that a local home builder gave me that would have otherwise ended up in the landfill. Discarded foam is very easy to find as there is currently no practical solution for recycling it.
Bottom attached to fishbone and rails started.
Pl Premium or 3M 5200
Locate where you want the centre fin box with the bottom skin facing up. Drill a 1/16″ pilot hole to locate the centre fin box and and the optional FCS thrusters. Flip the board over and find your pilot holes. Scrape away the adhesive fillets from areas that will receive blocking. This will allow the blocking to fit tightly against the fishbone as you do not want epoxy to be able to leak into cavities when you install the fin box later.
An angle gauge is being used to set the right angle for the blocking. If you do not have an angle gauge I would cut a short piece of scrap and find you angle by trial and error. Once you have you angle you can transfer it to the blocking and be confident that it will fit.
With the blocking’s length set scribe the correct height directly on the blocking. SUP’s use a standard 10.5″ longboard fin box so you will have to repeat this procedure on more than one rib section. On higher volume board I make sure the blocking is 1/2″ taller than the fin box being installed. Wood blocking does not have to extend all the way to the opposite skin for strength. As long as the blocking is 1/4″ deeper than the fin box before it is installed you will end up with the desired 1/2″ oversizing. (1/4″ plus the 1/4″ for the bottom skin) With foam blocking it is safest to extend it to the apposing skin for additional support.
Here is and example of foam blocking being installed with PL Premium. Wood blocking can be installed with Titebond 3, Pl Premium or epoxy. FCS inserts should have a 2″ cubed block of wood glued or epoxied over the pilot holes to support the forces of the fins. I do not usually install thrusters on boards that are not being built for surfing.
Steps for installing FCS style inserts:
- Flip board so the bottom skin is point up.
- locate where you want the fins to go and drill a 1/16th pilot hole through the bottom skin.
- Flip the board back over.
- Epoxy wood cubes at least 1.125″ over your pilot holes.
Seal any gaps wear the blocking overlaps lightening holes. Voids will cause problems when you attempt to flood the fin box air even worse allow wood chips to spew inside the hollow board while routing the fin box.
As much as I despise styrofoam’s effect on our planet, it has no chance of splintering when routing holes and the size pieces required can be upcycled from someones trash. Every new flatscreen TV has enough foam for many boards. This picture shows blocking for a leash cup and the vent on either side of the fin blocking.