Subject: Re: [harryproa] Re: Schooner v. Unarig
From: Mike Crawford
Date: 12/9/2006, 8:19 AM


  You're entirely right.  Those loads will definitely be transmitted through the boat, and the structure will have to be designed to handle them.  Additionally, with a 9m beam, there will obviously be torsion when one hull hits a wave before the other does. 

  There is a proa design out there with a single large beam that allows the hulls to pitch independently, but I personally have doubts about trusting such a joint long-term.  Besides, a single aka with independent hulls leaves no room for a large deck with massive trampolines.  Why get a large multihull if you can't stretch out?

  The difference in the unstayed mast would be in the way the structure is loaded.  Given a catamaran or trimaran with a stayed mast, there's a lot of stress the structure has to handle.  There a large compressive load in the center where the mast is stepped, large point loads on the hulls/amas at the chainplates, loads due to righting moment while heeling, and also wave loads. 

  Compounding the matter is the problem of keeping the structure torsionally stiff enough to keep the leeward stays from getting too loose as the boat twists.  This not only puts more force on the forestay, it also allows a jolting point load on a chainplate when the mast snaps back in the opposite direction due to wind/wave action and/or twisting in the structure.

  That's a lot of loading to handle while attempting to keep the boat from twisting lengthwise.  If there were four symmetric stays, either shrouds parallel with the mast step and fore- and mainstays, or two shrouds on a side, there's less torsion to deal with.  But when compared to swept shrouds and a forestay, there's a lot of stress induced in the structure while flying a hull.

  While the harryproas will still have to keep themselves together and relatively stiff as each hull hits different waves, they won't have to deal with the additional huge wrenching forces induced by stays in tension with a mast in compression.

       - Mike

Chris Ostlind wrote:

Hi Mike,
I understand the part below about the compressive stress on the boat due to lack of shrouds, but the other two (torsional and point load stress) kind of escape me with regards to a freestanding mast.
Is not the mast point loaded at the mast partner location (the deck in most cases) once dropped into place and placed under wind load by the sail(s)?
Is not the moment generated through the mast focused into the hull torsionally through the partner location as fulcrum and then, subsequently, to the mast step?
----- Original Message -----


The nice thing about a freestanding mast is that there won't be all sorts of torsional stresses on the boat, nor point loads and compressive stresses on the mast. 


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