Gardner,

  The other antidote to your sail area and mast height conundrum could be the swing wing rig proposed by Kim and Rudolph.

    http://wharrambuilders.ning.com/profiles/blogs/sailing-with-pha-tiki-30-n119-1

  In theory, this will create noticeably more lift than a single-skin sail, and do so with noticeably less drag.  Bertrand Fercot's experience bears this out -- he noted that he could sail his Tiki 30 faster and tack easier than another Tiki 30 with a traditional Tiki rig.

  You'd also have more lightly-loaded sheets, a simple reefing system, lower loads on the sail, no need for a boom, and a round mast (no "sailing" while moored) without rotating bearings.  The sails would also keep their near-perfect shape even in extremely light winds.  And given the greater efficiency of the sail, you'd either be faster at a given mast height, or the same speed with a shorter mast.  That's something to consider.

  It might not be extreme on a dead run, but given that a powerful multi can sail faster by tacking downwind than by running, and that these sails will hold a nice airfoil shape at a better angle of attack, this would probably make a proa a great downwind boat in most directions.

  I can't decide if I'd be willing to make the leap.  All the theory appears to be correct, but it would mean either lots of trial-and-error on your own, or alternately, working with a sailmaker who could analyze the sail's performance in an airflow, specify the right wishbone shape, and then cut the sails to match.

  But Rudolph, Kim, and Fercot make good arguments.

        - Mike

 

On 5/25/2010 10:33 PM, Gardner Pomper wrote:

 

Hi,

I have been occupying myself with what seems to be my favorite hobby; designing harryproas that I never get around to building. One of the things that occurred to me, is that I don't seem to have a good way to predict performance. I have been using the typical SA/D and Bruce number calculations, but I can't get these to provide me with number I can believe, based upon my personal experience and the speeds shown by Rare Bird on the Youtube videos.

Here is a table I compiled:

Boat Type Length Disp Sail Area SA/D Bruce #

ME Cat 30 catamaran 30' 6000 500 24 1.2

ME cat w/screacher catamaran 30' 6000 720 35 1.5

Elan 7.7 trimaran 25' 2000 310 31 1.4

Rare Bird harryproa 50' 10000 774 26 1.3

Harry harryproa 40' 3500 452 31 1.4

Harrigami harryproa 35' 2500 333 29 1.3

Camper (mine) harryproa 30' 2000 392 39 1.6

Contrarry (mine) harryproa 38' 3500 728 50 1.8

It is a little hard to read, but basically it says that Rare Bird should be slower than my Maine Cat with the screacher up, or my Elan trimaran at any time. The Maine Cat never got over 12 kts, and that was surfing down a wave. With the screacher in 15 kts of wind, it would top out at about 10 kts. The Elan may have hit 15kts at one point, but that is when it was blowing 20+ kts and I was terrified of a capsize.

So, given that the longer, sleeker, harryproa hulls are the deciding factor, what metric should I use to compare a harry design with "normal" multihulls?

The reason I ask is that I may be overreaching for sail area on my designs, based on these numbers.

Thanks,

- Gardner

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