I have two points to discuss:  What's the advantage of having such a slender lee hull?  But it is interesting to ask, if the lee hull was 10:1, how much slower would it be?

  The longer the lee hull is, the more fore/aft righting moment the boat has, the more resistant to pitchpoling it will be, and the smoother/safer it will be in big seas.

  In this case, the hull length is partially shaped by the boat geometry.  What's the fastest boat, with two double bunks, a galley, and a head, that can fit into a 40' length (either because of marina restrictions or because this allows the lw hull to be shipped in a container).

  Give the weight-to-windward harryproa geometry, this gives us the 27' windward hull with the previously-mentioned accommodations.

  If the 20:1 beam gives enough flotation and fore/aft righting moment, then great.  Narrower means less wetted surface area, smoother wave-piercing, and less material (less weight and cost).

  If the 40' length is kept, a 10:1 beam increases the weight.  If the same weight is kept, 10:1 reduces the length and the sea-kindliness.

  The narrower hull and higher speed might mean for more race wins based on absolute times, but that might not matter to most people.

  There are two other areas where these things help, though, that are more universal:

    - Low-wind sailing.  Making progress in 3-5 knots of wind requires forward motion so that the foils act as foils.  And that forward motion also means a breeze in one's face, which can have a big impact in the summertime -- that's the difference between sitting around and sweating and happily sliding along.  Particularly if you're heading downwind, where tacking downwind in a high-performance boat is fun and cool, while ghosting straight downwind with almost zero apparent wind is unfun and sticky.  Minimizing wetted surface area matters a lot in very light wind.

    - Seas over 2'.  Sailing a lightweight multihull into 3' seas or higher is not necessarily a fun experience.  One of the reasons a proa will be our next multi is that our current 27' catamaran just pounds and bounces in 3' seas, and is even worse in 4' to 7'.  We go from looking down into the water off the bows, to seeing only sky, then slamming back down. We even have to lean out just to see where we're going at times.  Having that long, slender lw hull to smooth out the motion would make a huge difference.  Forget trying to beat the next guy on the water -- this is about making reasonable progress while enjoying life.

  Since we're not retired, we can't restrict our sailing days to perfect weather, so we want to be able to enjoy the days where the wind is at 4 knots, and also the days where the seas are 3' and higher.

  It's also possible that the combined width the the hulls matters for things like fitting into a container for shipping, fitting into a marina slip, and/or fitting on a trailer, in which case the narrower hulls are useful.  Though it's tough to say.

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  Of course, not everyone shares these goals.

  It's possible that being able to carry more cargo is the priority over speed and seakindliness for a particular build.  In which case I imagine Rob and Steinar would be willing to double the lw hull width in the plans, and upgrade any materials specs, once the plans are purchased and it's time to build.

        - Mike

 

I assume he was trying to mock me for a stupid question. But my questions was serious. I have read that the ideal is in the area of 14:1. That may have been the ideal given different constraints than what was used for the design of this boat though.

For me the answer (speed) is not as given as for you guys. Because I have been playing with simulations of hulls. And from what I could learn, these light hulls have a pretty negligible wave resistance. So the width is not as important as in heavier boats. The wetted surface is the important parameter. But on a second look, it looks like the hull is already stretching the beam a bit away from the ideal. So I think this explains it. If it is made wider, it will have higher wetted surface. When the dimension are close to the ideal beam, large changes in beam results in only small changes in surface area. But if the beam is already stretched, an increase might results in a noticeable increase in area / decrease in speed. So it's a matter of how far from ideal it is now, if it can be stretched without a penalty.

On Fri, Oct 25, 2019 at 3:52 PM '.' eruttan@yahoo.com [harryproa] <harryproa@yahoogroups.com.au> wrote:

 

| 20:1 sounds cool.

I did a spit take Doug. Hilarious.

| I love it.Nice layout of the cockpit (and cabin).
| I have two points to discuss:What's the advantage of having such a slender lee hull?

Fast to sail.
But it is interesting to ask, if the lee hull was 10:1, how much slower would it be?

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Posted by: Mike Crawford <mcrawf@nuomo.com>

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