Trunk Reinforcement for High-aspect Centerboards

Written by Ali Meller, 01/15/2003

Gybing centerboards create a point load on the side of the centreboard trunk when the airfoil section starts to come into the centreboard trunk as the centreboard is raised. You can often see the point on the board that bears on the side of the centerboard trunk. It is the worn spot at the thickest part of the airfoil section, at the top of the airfoil section. Some gybing centreboards are built with a distinct wide point on the airfoil section that is just below the bottom of the centreboard trunk when the board is fully down, but when the board is raised, stops it gybing. Other gybing centreboards do not have this addition and rely on the airfoil section itself coming into the centreboard trunk. In either case a point load is applied to the side of the centreboard case as the board is raised. This point load is applied to the centreboard trunk wall on an arc described by the point on the centreboard rotating around the CB pin as the board is raised. However, the point load rapidly reduces in force as the board is brought far up into the trunk.

Lindsay, Parker-hulled Lindsay, Hamlin and Waterat 505s were built with a special reinforcement running vertically between the reinforcements at the floor and the centerboard cap, to handle this point load.

High aspect (HA) ratio gybing centerboards with shorter root chords and shorter head chords have this point load moved forward, typically forward of where the vertical reinforcement in existing boats was placed. Some of the HA centreboard designs have rather smaller heads, and they have lower centers of area, possibly increasing the loads they place on the centreboard trunk. The possible increased load is not the problem, the fact that the point load may be applied forward of the vertical reinforcement is the problem.

According to Larry Tuttle at Waterat, it is desirable to extend the existing reinforcement forward to handle the new location of the point load. Waterat recently repaired a 505 where this load has cracked the plywood centerboard case sides. This reinforcement extension can be done using materials similar to those used in the original reinforcement.

Some HA centreboard designs require the centreboard bolt/pin to be moved further forward in the centreboard trunk. The new bolt hole should be reinforced.

This article assumes the existing reinforcements are intact and not broken away from the underlying centreboard trunk sides. If the existing reinforcements are damaged, they need to be repaired; if they have broken away, perhaps they can be removed and a new wider reinforcement installed to both replace the existing reinforcement and extend forward the reinforced area.

These are photos of three boats that had these reinforcements added.

  1. Waterat 8263 -- A forward tack bag boat with a lifting centreboard pin, built in late 1989 and delivered with a Waterat standard CB -- had the CB trunk reinforcement extensions added by Waterat in 2002. The forward CB pin hole reinforcements were added at the same time.
  2. Rondar 8776 -- built in early 2001 -- did not have any special reinforcements. I added reinforcements similar to those on a new Waterat. The boat was delivered with the forward CB pin location for High Aspect ratio centreboards.
  3. Lindsay-hulled wood-look Waterat 7200 -- A forward tack bag boat with a lifting centreboard pin -- had the original reinforcement. I added extensions to the reinforcements similar to the way they were added to 8263, and also added the forward CB pin hole reinforcements.

For the 8776 and 7200 projects, parts were obtained from Waterat, as I do not have easy access to a table saw, table planer and other power tools, that make basic shaping of the spruce side reinforcements and mahogany centreboard pin pieces much faster and more accurate. I did cut the Waterat-provided parts to fit. Parts were bonded into the boat, glassed over, sanded and painted.

On 8776, the vertically grained spruce reinforcement was bonded in by flipping the boat on its side, inserting a full thickness centerboard trunk shim, and then borrowing a large cinder block from the dinghy park to use as a weight, or "vaccum brick" as Steve Clark calls it. The shim ensured that the centreboard trunk was not deformed while the reinforcements were being bonded in. The bonding area was sanded, and the part bonded in with an epoxy and cab-o-sil bonding paste. The boat was flipped onto the other side in order to do the other side's reinforcement. The bevelled edges were filleted and glass taped in, and the whole thing was sanded, glassed over, sanded and painted. I was able to obtain some SP Systems Hi Build (primer), the paint used on the Rondar cockpit floor and CB trunk sides, from Rondar via Guck, Inc., so the area is painted the same colour as the rest of the centrboard trunk sides.

A minor problem was that the forward bailer impinged slightly on the area of the reinforcement. I cut away a very small portion of the reinforcement and did not tape or fillet it in the area of the bailer. 8776 also had bulkheads added under the forward face of the forward thwart as part of this project. The bulkheads were made from six mm okume marine plywood. These bulkheads are full width at the height of the thwart, full height next to the seat tank, and about two inches high where they tie into the back of the CB trunk reinforcement. They are filleted and glass taped to everything they touch, glassed on the forward face, and painted. Though subjective, my crew told me the boat's rail felt stiffer in waves after this modification.

On 7200, the vertically grained spruce centreboard trunk side reinforcing parts were carefully fit (measure, cut, sand, check, sand) into the recess between the existing reinforcement and a forward reinforcement for the lifting pin slot. Once both sides fit very well, the area was carefully sanded for bonding, the parts coated on their bonding surfaces with straight epoxy and then a bonding putty of epoxy and cab-o-sil (WEST calls it colloidal silica), and wood was jammed between the inside face of the seat tank and the part, to keep appropriate pressure on it. Excess epoxy squeezing out was spread around with the square end of a mixing stick, to fill any remaining gaps. Once cured, the area was sanded and glass cloth bonded over the new part and into the surrounding structure. Once that had cured, the area was sanded and painted with the same colour Awlgrip (desert sand or desert tan) two part linear polyurethane as had been there before. I did not use a special CB trunk shim -- I should have to be completely safe -- but 7200 had a fair bit of structure surrounding the area where the new reinforcement went in, and I am confident that I did not deform the CB trunk by bonding in the reinforcements.

The installation of the the forward centreboard pin reinforcing pieces -- horizontally grained mahogany -- was done at the same time. These were cut from the Waterat-provided stock, the back edge was bevelled to fit against the existing vertical lifting pin reinforcement, the part was epoxy coated and then sanded smooth on its external faces, and then it was bonded in, again using a piece of wood jammed against the inner face of the seat tank, to keep the part up against the surface it needed to bond to. It will be clear Awlgripped to match the adjacent finish, shortly. Again, I should have used a full thickness CB trunk shim to be completely safe. The actual centreboard bolt hole has yet to be drilled and sealed; that is best done after fitting the new centreboard into the trunk to verify the appropriate location of the centreboard bolt hole.

Photos of USA 8629 CB Trunk Reinforcements from Jesse Falsone: