The First Sea Trial

by Dermot Tynan in Microtransat

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Posted on Sunday, October 27, 2013 at 08:00

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On September 1st, we launched the hull in Aughinish bay, without sails, rudder or electronics. The purpose was to see how the hull performed in open water, with the keel attached. As the keel wasn't properly attached or sealed in place, the main compartment flooded with water, but as the compartments are individually water-tight, this wasn't an issue. It did lower the boat in the water somewhat, but not to any significant degree.

As the compartment wiring wasn't completed, and the deck panels weren't glassed in place, they were attached to the hull using duct tape. Not pretty, but it works.

The immediate good news was that the hull didn't sink! That's worth celebrating. Also, I was extremely impressed with how she moved through the water. The hull maintained a very straight line, with no noticeable turn to port or starboard. The bow and stern waves looked impressive, even when forcing the boat through the water at a faster-than-hullspeed rate. Overall, I was impressed with the seaworthiness of the hull and her ability to hold her course.

The bad news was that the keel is way too heavy, and too far forward. The boat is too far down on the water line, which makes it relatively sluggish, and the extra weight isn't necessary. The long keel does a lot to keep the boat going straight, and resisting the lateral forces which will be imposed by the sails. The original design specification was for a keel of around 12kg and a hull of around 4 or at most 5kg. In fact, the hull is weighing in at 7kg and the keel is a whopping 18kg. This puts the total displacement at 25kg which is just too heavy. The keel is formed from two stainless steel threaded rods, covered by two sheets of marine ply, shaped into a NACA airfoil. At the base of the keel is a lead weight.

Since that initial test, I have removed approximately 6kg from the lead keel. I have also angled the keel back a bit by about 10 degrees, to soften the leading edge and move the weight back. Recent sea trials have shown this to have been quite effective, but there is more to do. I'm still not happy with the natural pitch angle of the hull, and would like to see more freeboard at the bow and less at the stern. The keel is bolted to a thick aluminium plate which distributes the load. I will re-drill the plate to move the keel back further along the longitudinal axis. I will also plane or sand back the plywood fin, to make it thinner and lighter. Structurally, the strength comes from the two threaded rods, so it is OK to reduce the width of the keel.

Overall, the hull still performs admirably, and it will be great to see her progress through the water under sail.

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