Hull Design

Where do you start when designing a new hull? I chose to look at the best current designs, the prowler mk1 and the Axeman 7 (this is in 2005).  I had already built a Axeman 7 hull a couple of years before, so the mould was sill lying around. Comparing the Prowler to the Axeman it was apparent that the shapes of the two were close, the prowler had rounded chines and a flat bottom compared to the sharper chines and rounded bottom of the Axeman.

The design approach adopted by many of the current builders is less is more. Foiling moths tend to spend most of the time in the air, so reducing windage and weight is perhaps more important than how the hull handles in the water. 

The Geko design follows this approach too. The following sections describe each aspect of the hull design:


To simplify construction the Axeman mould was modified so its bottom was flat cross ways, I also rounded the chines with a 50 mm radius. In retrospect adding the radius to the chines was unnecessary. The current thinking is hard chines are favourable because when the hull skims the water the hard chines shed water reducing drag. Another plus for hard chines is the build process is much easier!

Rocker was slightly reduced compared to the axeman 7 with no rocker in the aft section of the hull. This was inspired from windsurf board design, where the fastest boards have no rocker in the rear of the board to promote fast planing. 

Free board

Since the hull doesn’t have to contend with waves the freeboard was reduced from 500mm at the bow to 330 mm to save weight. The decision to do this was made after the hull had been moulded. I had been toying with the idea while designing the hull, but no other moths had done this. At this time the first mistress appeared from full force boats with a much lower freeboard. I figured that if is was good enough for Mr May I too would adopt the smaller hull concept. Since i had to cut the hull shell down i was able to measure the material cut away - 1.2 Kg saved!


The following table lists the offsets. Section 0 is the bow and section 6 the stern.


    Depths (from water line) Widths (from center line)
Section Distance from Bow chine & Center line gunwale Beam at flare Beam at Chine
0 0 -0.194 0.15 0.01 0.004
1 0.559 -0.191 0.15 0.072 0.067
2 1.118 -0.183 0.15 0.123 0.121
3 1.677 -0.167 0.15 0.149 0.147
4 2.237 -0.14 0.15 0.156 0.147
5 2.796 -0.104 0.15 0.138 0.108
6 3.355 -0.048 0.15 0.081 0.025

An excel spreadsheet showing the lines can be downloaded here


Three bulkheads are installed. The front bulkhead is 0.9m from the bow. It provides support for the fordeck, crewdeck and racks.  It extends 0.2m above the gunwale.

The centre bulkhead is located at the rear of the daggerboard case. This allows it to transfer some of the foiling loads the the hull skin and stops the hull bending.

Transom to stop the water getting in!

Crew Deck

The crew deck has to carry out 4 functions. Provide a place to stand, provide attachment for the tramps, mainsheet attachment and rack support.

The attachment for the tramps and mainsheet is realised in the same manor. The crew deck extends 25mm beyond the hull skin. Through this carbon tube is glued allowing the tramp lacing and mainsheet to be attached. To ensure the carbon tube is bonded to the carbon skin correctly 5mm of foam core is removed around the hole using a bent nail and power drill.