Tuesday, September 11, 2007

Raised Deck Modification

I've been working on the design of a modification I've been contemplating since before I decided for sure to build the Tiki 26. I think the basic design of the boat as drawn is great, but like many owner/builders, I want to fit my boat to my needs and requirements and I think minor modifications within reason can be done without detriment to the design's proven capabilities.

As I've stated before, I wanted to build the smallest boat that would work for my intended sailing, and for me the Tiki 26 is it. A Tiki 30 would have more interior room in the cabins, but at the expense of much more time and cost to build, as well as more difficulty hauling out and demounting for trailering. Sitting headroom in the cabins is all you can expect on any of the Tiki designs short of 38 feet, and the Tiki 26 has this. What I wanted was a bit more elbow room inside as I am 6'2", and one way I saw to accomplish this was to build a "raised deck" version that would allow me to also raise the bunk levels a corresponding amount to gain a bit more width there due to the flare in the V-hulls. One famous Wharram catamaran that was modified to an even greater extent to gain more interior room is Rory McDougall's Tiki 21 Cooking Fat, which he sailed around the world. The extra volume created by the raised decks contributed a lot to the ability of such a small catamaran to handle the conditions of the open ocean, adding reserve buoyancy where it was needed.

Although I did some sketches and even built a rough model before I began building the boat, I knew I wouldn't be able to determine for sure how the raised deck modification would work out until I had a hull built. I left my options open by cutting the cabin bulkheads separately and leaving extra material above the sheer on the installed bulkheads. I went ahead and raised the bunk levels by 3 inches, knowing that I could make this up in the raised deck or if I changed my mind, by simply raising the cabin tops 3 inches, which many Tiki 26 builders have done. Since I've been away from the boat for awhile in Florida, thoughts of these modifications were weighing on my mind quite a bit and I decided yesterday to go ahead and start working out the details to see if it's going to work.

The first step was to cut the extra bulkhead material above the sheer level to a slight inboard angle, so that the new side deck panels will be cambered in by a few degrees. I then made up some Doug fir stringers 1 3/4" by 3/4", the same dimensions as the other stringers in the hull, and sprung these in place against the bulkheads to get a fair curve. Maximum height above the sheer is 3 and 3/4", decreasing to 2" forward at bulkhead 6 and aft at bulkhead 1. The raised deck section will begin with the this 2" bump in height just behind the front crossbeam, so that the transition will not be noticeable. It will end 4 feet aft of the cabin at bulkhead 1, where it will blend into the rear trampoline beam I will be installing there. The new stringers for this raised deck panel will be on the inside top edge of the panel, let in flush with the cambered edges of the bulkheads. The decks themselves will have the same camber as in the plans, and the cabin bulkheads will follow the design lines, beginning at the top edge of the raised deck sheer. The mast and aft beams will fit in their chocks atop this raised section forward and aft of the cabins, with all loads transferred to the reinforced hull sections below at the true sheer. Beam lashing chocks will be located below the hull sheer as designed.

Although I'm not ready to begin permanently building these raised decks just yet, I've mocked it up with stringers clamped into place and the cabin bulkheads clamped above them. It's hard to visualize the appearance of the decks from these photos, since the tops of the forward bulkheads are still straight and do not show the graceful deck camber that will be there, but you can see the lines of the raised deck section at the outside.

This is a view at the stern showing how the section will terminate at bulkhead one behind the cabin. A rear trampoline beam will be mounted on the lower section just aft of the transition.

Here is a view of the raised section with pattern material in place. You can see the slight inboard camber here. This seam will of course be faired and glassed into the outboard sheer stringer below it.

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