Mast Bracing
Page updated 23 May 2006
A common desire of owners of the two-berth SeaHawk is to remove the two bulkheads and mast bracing supports forward of the two quarter berths to create the more open cabin found in the "four-berth" boat. (See the Brochure for comparative pictures of both types.) However, this is NOT a project to be undertaken lightly!
The bracing is designed to take the lateral stresses on the mast step. (Fore and Aft stress is handled by the substantial plank of wood moulded into the upper surface of the cabin roof, on which the mast sits). Clearly, it would be foolish to compromise the structural integrity of the cabin roof by removing the bracing, without installing an alternative.
Original Four Berth Bracing
The method originally used can be seen in the technical drawing (532kb), sent by a Dutch owner, for the Mk2 Four berth boat. This shows that the existing ¼" bulkheads were replaced by 5/8th inch plywood brackets supporting the end of the back-rest shelf. These are attached to a pair of solid mahogany bracket of the same thickness. The top end of these are then attached to a 1½" thick by 2" wide laminated wooden beam. What the drawing does not indicate is that this beam is moulded into the roof of the cabin, as can be seen from the photograph.
Alternative Bracing (#1)
When Bill Tinsley, bought Bantam's Drift, the boat shown undergoing conversion to a four berth by the previous owner, he realised it would need the mast bracing. He reported:
"I have decided to make a 50mm 'Finn Ply' bulkhead across the hull just below the front bunk level, bonded to the hull and bolted to the existing bunk support moulding. This will carry a plate to support a 50mm stainless steel mast support tube with a square flange at each end and through bolted to the tabernacle at the top. This will in effect give a rigid mast stepped at the keel in the conventional manner with all the down thrust from rigging and lateral loads carried down to the keel."
Later he sent the photographs here, writing:
"I have now completed restoring the mast support arrangements that the previous owner had removed in the interests of an open plan. I made a new bulkhead forward of the bunk mouldings, bonded to the hull and bolted to the front of the bunk moulding. A stainless steel tube with flanges both ends connects this to the new bulkhead and through bolted to the tabernacle at the top, in effect extending the mast to the keel. The whole structure is now very rigid. The attached photo's should help to make this clear. ... The single pillar gives the illusion of space and still leaves the V-berth for two children."
Whilst this is clearly a very workable arrangement on this four berth conversion, the owner admits it would not suit a boat still fitted with the galley moulding seen on most two berth boats.
Alternative Bracing (#2)
This elderly two-berth boat appears to have much thicker bulkheads than is normal. Although they are not appreciably cut away, without the bracing struts the cabin seems much more open and usable. They provide the necessary strength by extending to the cabin roof, where they are fixed to a laminated roof beam, similar to that which would be encased in GRP in the standard four-berth boat.
This approach certainly works for a two-berth boat, although in this case the galley is home-built and doesn't use the conventional moulding.