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19884Re: Easy build catamaran thoughts invited.

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  • graeme19121984
    Oct 5, 2009
      Doug,

      arguments go back and forth as to whether trimaran centre hulls ever plane or not. There's some big guns on either side of the debate. I go with the view that for most intents and purposes they don't really plane. It's unlikely the centre hull ever lifts the others. With heeling under sail the windward hull and centre hull are jacked up as their weight is loaded onto the leeward hull. The centre hull waterline beam is reduced which increases its fineness. That's why tris can get away with a central hull fineness from down to about 6, whereas for catamarans it's about 10. That's partly because the loaded leeward hull decreases in fineness. The wind-loaded tri sheds wetted area, but the cat gains it. The tri centre hull is really still slicing the water in displacement mode.

      I concede the possibility of a bit of planing going on. Is it really "planing" though if there's more than hydrodynamic forces lifting the hull (hydrostatic in the diplacement mode lee hull resisting aerodynamic in the rig)? OTOH could it be the case that if there is any hydrodynamic lift at all then that is "planing"?

      Tris that have been designed to truly plane on their hulls (and that will mostly be on the lee one) have been fast. BUT they haven't been race winners, nor even good boats. They're not handy. They need the big wind - without it they're dogs - with wind comes waves - with waves comes punching - so how do they get up, then maintain the plane when there is wind? They have hulls that have comparitively very low drag at planing speed, which to a certain extent continues to reduce with more speed, but below planing speeds such hulls are comparitively very draggy. A planing power boat hull doesn't have to be so versatile.

      Big fast [read $$$$$$$!] tris now go literally "all out" and plane on foils, eg http://www.ziltmagazine.com/video/nummer42-2009/hydroptere/hydroptere.html 55.5kts! If this is accepted, it has the world sailboat speed record. 61knots capsize. This French boat is a prototype for a larger one. Wow! (11ft hydrofoiling Moth dinghies have beeen clocked at 40kts!)


      Graeme

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      --- In Michalak@yahoogroups.com, Douglas Pollard <dougpol1@...> wrote:

      > I guess what I was thinking when I wrote the previous post is. That the
      > modern 30ft'ish trimarans I have seen have very narrow center hulls that
      > widen out above the water line. They basically have a long narrow foot
      > well in side the boat with bunks on either side. These extremely
      > narrow center hulls also have very high hull speeds. But a small
      > trimaran would seem to need a wide hull amidships to have any usable
      > space. A wide hull would have to plane. I'm wondering if the bow would
      > be able to climb on top the water and drop down on a plane at some
      > speed. With the outer hulls not planing the main hull would have to lift
      > them as well. AS they lift at the bow the hull in the water would get
      > shorter so hull speed would go down. They would not likely plane with
      > such a narrow none plaining hull shape. Would they then drag the
      > center hull speed down. I don't pretend to know much about trimarans
      > but these are kind of my what if's. I would like to hear some
      > thinking on these musings. Doug
      >
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