Surfboard-like CLR on downwind racer

[Sketch]

The design spiral can sometimes spiral out of control. In designing a 46' LOA cruiser for reasonably quick single or shorthanded ocean passages, my thoughts - for a different kind of boat altogether - brought me to places that surpass my experience and education. I hope to gain some insight and critique from you, the forum members.

Brief background: I am a believer in fore and aft water tight bulkheads, keel stepped masts, and speed (for both fun and safety). I am pleased with my initial hull dimensions, drawings, and calculations, however in order to advance the design I needed to define a rig. While a large roach B&R fractional rig with removeable cutter stay can certainly do the job I admit a keen interest in freestanding rotating wingmasts.

In order to get a suitable lead between CE and CLR without using a headsail (and consequently a forestay tensioniong system to account for mast rotation) the rig that lends itself most easily to freestanding rotating wingmast(s) is the cat rig - in particular the cat ketch. While my eyes have grown more accustomed to two spars carrying identical sails, aesthetically speaking, two different sized sails looks better to my eye - with the cat schooner edging out the ketch.

After struggling to establish an appropriate lead between CE and CLR (for the freestanding cat schooner rig) I began to wonder about moving CLR aft. While this would be nothing like the orignal boat, what if you moved the keel all the way back like on a surfboard? Certainly, there are many complications with this idea (not the least of which is steering - rekindling the idea of forward rudders). But the idea that most intrigued me was that with CG moving aft, downwind surfing (even with fine bow entry angles) would reduce the tendency to bury the bow. Going upwind would require waterballast forward.

If this new boat where designed so that it could go upwind with a certain displacement (albeit with the weight at the ends of the boat) and shed the forward weight when going downwind, could you not get a faster downwind surfer? Is this craziness? Has anyone tried this before?

I have found precious little information regarding weight in the ends of the boat other than that it is bad. Most discussions of weight distribution deal with transverse distribution and how it affects rolling moments. Lars and Eliason discuss gyradius, which incorporates longitudnal weight distribution, but little is said relating specifically to weight in the ends of the boat.

The basic idea is to trim the bow down when going upwind (because CG is so far aft) and shed weight (trim bow up) when going downwind. Any thoughts?

Respectfully,

Kevin Barry

[Andy P]

This is what dinghy sailors do!

ie sit fwd in light winds to get the bow down ( or the bottom of the transom at least near the surface)

and in strong winds hanging aft off the transom to keep the bow up.

[sorenfdk]

Your idea sounds interesting - I'm not sure if it'll work, though.
But why would you place the keel so far aft? Did you remember to use only half of the mizzin area when finding the combined center of effort?

[mattotoole]

Originally Posted by Andy P

This is what dinghy sailors do!

ie sit fwd in light winds to get the bow down ( or the bottom of the transom at least near the surface)

and in strong winds hanging aft off the transom to keep the bow up.

The reason dinghy sailors move forward in light air is to put more weight into the rounded bow sections (less wetted area per displacement), and lift the flat rear sections (more wetted area per displacement) out of the water. This reduces wetted area and drag.

[Andy P]

Originally Posted by mattotoole

The reason dinghy sailors move forward in light air is to put more weight into the rounded bow sections (less wetted area per displacement), and lift the flat rear sections (more wetted area per displacement) out of the water. This reduces wetted area and drag.

But.... the bow sections can be just as flat as aft!

and it's the drag of the immersed transom that has much more effect than the change in wetted surface.


and lines
http://boatdesign.net/forums/showthr...rson#post14381


The old 18' skiffs used to reduce displacement by some of the crew jumping overboard on the downwind leg to the finish.

[Sketch]

Soren,

I did not think it appropriate to use half the mizzen because on a cat schooner the sails are almost in the same place as for on typically rigged marconi. That is, the smaller cat sail forward replaces the jib, and the mizzen - I figured that was what it was called - is actually the largest sail, roughly in the same place as the main on a marconi (it must be moved aft, more in the place of a cutter's main in order to have a reasonable aspect ratio on the forward sail). I understand that radically changing an underwater profile simply for rig aesthetics not the best way to design boats.

With regards to moving the keel all the way aft (and its influence on CG and steering) I was taking an extreme view to see what might possibly work. If it sounds a little fishy, well it probably is. Incidentally with twin forward rudders and a tab on the aft placed keel, the whole starts to look like a barracuda...

Regards,
Kevin