Fathead on the 505

Proposal and an answer to a respons from the 505 Association:


Some years ago (2004) I presented the International 505 Class Association a suggestion for changing the main – providing it with a fathead. I had used that shape since 1992 on my boats (inspiration: Andy Smith, Tornado, 1990? and the multihull, F 27).

For a boat I designed in 2001, I also wanted to add a flexing upper part of the mast – the wind is quite gusty on the Mälaren inland lake where I sail. Having no time to build it I bought a used 49 – er mast from Anders Lewander, North Sails. He did not like my idea of putting a fathead on the mast, and recommended using the “classical” design. But the fathead was an important part of my basic concept, and so a 49-er mast received its first fathead. Something I gave notice about to that class.  And a few years later the fathead appeared on the redesigned 49-er rig – but with a less radical shape than I had suggested.

Regarding the main on my boat (SK 2000), we had to make some redesigns, because of the composite soft upper part of the mast. Finally I had to reinforce that part and add a completely new design for the main, with a wider top. It was cut by Next Sails, with some later adjustments by sail-maker Ingemar Jerling,


With close to twenty years positive experience of fathead mains, and also having experienced the 505 in the neighborhood, I now make a new proposal to the 505 ICA – rough outlines attached, together with my earlier proposal. The mast has the same dimensions as before. But to add cloth higher up, the top of the fathead is 700 mm wide, with a 123 degrees angle between mast and the top of the headsail. The boom is shorted to 2400 mm. The leach is changed to 6800 mm. – with the advantage of lifting the boom and so making more room for the crew. Allowing a 70 mm curvature to the mast adds more alternatives for tuning the sail.


With hope of receiving some serious thoughts about the proposal – as the main has to change sooner or later, as happened with the 49-er.  A new touch of efficiency also to the 505, a great classic boat updated into a new era, because of the possibilities new sail material offer. The body already has its carbon, so why not allow the driving force it´s part of the future?


Comments, answering a response on the matter by Jurgen Waldheim:

Thanks for information on the 505 main – the process has already started. Good! Anyone contemplating the picture with the three boats will understand that a change is necessary. The sooner, the better.

I have experienced the whole process from recutting an existing regular, top-angled main, through gradually changes, ending in a kind of final design on the SK 2000. With this background I would recommend the 505 class to consider a main that is close to the SK 2000 – the latest one, with the NEXT sail. See pictures, with the drawing showing my final solution.

Something that must appeal to the 505 is the angled top, for several reasons. A practical advantage is that you can use the present masts, and getting more cloth higher up.

It has also importance for the flow over the sail: The pointed top leads the tip vortex smoothly away, reducing induced drag and so lessens loss to the driving force.

Using this design would not only put the 505 In par with modern fathead rigs – it would pass them, with the 505 suddenly in the forefront. The wonderfully shaped hull would get a matching main.

More arguments: The forward end of the sail, that initiates the driving force, becomes longer, and makes the sail area more efficiently used.

The trailing edge, on the contrary, should be straight, to minimize length, and so reduce drag.

Allowing more curvature to the mast is favorable because of possibilities to flatten the forward part of the sail, so lessen heeling force and allowing sailing more closehauled.

Finally – there is no point in making small changes to the existing sail. I must be better to make a radical change that should last for several decades … until new materials may influence again.


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