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 Post subject: The daggerboard decision
PostPosted: Tue Jan 10, 2017 5:38 pm 
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This year I am going to experiment with substituting a high aspect ratio (long and skinny) daggerboard for the Mirage drive which is essentially a low aspect ratio (short and squat) daggerboard. A high aspect ratio daggerboard is more efficient than a low aspect ratio one in that it has less associated drag for the same amount of lift. Reducing drag increases the efficiency at which wind is converted into forward motion rather than heeling.

I will, of course, stand on the shoulders of those who have gone before me in this forum, mainly Stringy and Skua who did great work way back in 2008. Stringy showed how the cassette that fits the Mirage drivewell could be modified to take the daggerboard (DB) from the Hobie Adventure -- a very elegant solution. He reported: ‘A bonus with the DB, apart from stopping sideways drift when going upwind, is that it adds stability when sailing. With the DB in the down position, heeling is slowed noticeably, and you have more time to adjust the sheet in strong gusts.’

I will start with the Stringy approach. I have a daggerboard from a windsurfer that I think will do the job. It is much longer and narrower that the combined fins of the Hobie Mirage drive but about the same total surface area. It is also a bit thicker than the Hobie fins. However, this approach it is not perfect for my purposes because it means taking the Mirage drive out of its slot and a Sport doesn’t have much storage space to spare, particularly if it is also carrying a trolley.

I think the Skua approach of a sleeve or ‘sock’ over the mirage fins has promise because it leaves the Mirage drive in place. But Skua’s sock (made of aluminium) covered both fins and its low aspect ratio meant there was a lot of drag. While pointing was good, said Skua, ‘The performance was gone. Hardly any picking up of speed in a gust. It just heeeeled’. What I will do differently from Skua is fit a high aspect ratio fibreglass sleeve over the front fin only that will clip to the mast of the rear fin for a bit more rigidity. Maybe the drag problem will still exist because of the trailing Mirage fin but it is worth a try.

A discussion of potential daggerboards is not complete without mentioning hydrofoils. Most of us familiar with the foils on high performance yacht classes such as AC72 catamarans and International Class Moths that lift the hulls of these boats clear of the water. Now I don’t reckon this sort of foiling is practical for recreational or expedition kayaks. But I think foiling leeboards (Bruce foils?) of the sort seen on both sides of state-of-the-art monohulls like the Edmond De Rothschild in the Vendée Globe Yacht Race may work with kayaks. When a boat is moving and heeled, a foiling leeboard that is in contact with the water generates both lateral (resistance to sideways movement) and vertical (righting moment) forces.

Implementable in a kayak? I don’t see why not (though I could be mistaken). There could be appropriately shaped foils on either end of a (detachable) bar that is mounted on gear tracks on the rails of the kayak at the point that balances the forces generated by the sail and the below water foils. There is already something like this with the inflatable ama system that Hobie sells as an option. Essentially the amas would be replaced with more sophisticated and efficient foils. Some reinforcement of the mast receiver may also be required to cope with the increased power.

Maybe I’m dreaming but what I like about the idea of kayaks with foiling leeboards is it could blur the lines between monohulls and multihulls by giving monohulls some of the righting moment advantages that multihulls have. Monohull kayaks with foiling leeboards should be much lighter than catamarans and trimarans and would probably also tack faster. In my opinion, foiling leeboards could enable development of highly seaworthy sailing kayaks suited to events like the wonderful Everglades Challenge.

I don’t think I have the time, knowledge or construction skills to build efficient foiling leeboards but it is fun to think about the possibilities…


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PostPosted: Wed Jan 11, 2017 8:37 am 
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Too reduce heeling, you'd pull the daggerboard up a bit (or use a smaller one). This will result in more slip but will reduce heeling. It's a trick many sailors use in very heavy wind situations to help prevent capsize.


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PostPosted: Wed Jan 11, 2017 7:02 pm 
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Tom Kirkman wrote:
Too reduce heeling, you'd pull the daggerboard up a bit (or use a smaller one). This will result in more slip but will reduce heeling. It's a trick many sailors use in very heavy wind situations to help prevent capsize.


Interesting, which suggests that part of the response to an overpowering wind gust is to push a pedal forward to lay the Mirage fins flat against the hull.

There is an exhilarating video here that shows the Edmonde de Rothschild foiling. Of course, I only have to narrow my eyes at little when looking at the video to imagine my Sport equipped with foiling leeboards storming down the same seaway.


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