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Catamaran musings


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  #1 ScottWidmier

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Posted 16 August 2007 - 05:37 PM

From time to time when dreaming of my "next boat" my mind flutters to a catamaran.  The speed, double hulls for storage/berths, and a big net trampoline for lounging really piques my interest.  Basically, a cruiser which expands upon touching the water to give you lots more lounging area.  Now, I like the idea of shallow draft hulls so have been toying with a catamaran design using hulls similar in shape to a sharpie or dory giving the advantages of building ease, shallow draft and traditional looking shearline.  I also was thinking of lower-tech rigging than you see with most catamaran designs. 

Then I found wharram designs (www.wharram.com) which have a lot but not all of the features I am thinking of.  On the plus they have the nice shearlines, simple rig (sprit..yea!) and simple dory hulls.  However, the hulls are deeper draft with a much narrower bottom than I was thinking of.  Here is a picture of the cat they have in the size range I am thinking of (23').
Posted Image

So, I am thinking of a catamaran design that flies in the face of tradition with shallow draft hulls and either daggerboards or leeboards giving the needed lateral resistance.  The only designer I have seen advocating shallow draft wider hulls on a catamaran was Bolger when he designed a little cat for club racing.  I am thinking of two dory shaped hulls 24' long, 4' at the widest point, and with a bottom 2' at its widest.  Then I run into a lot of questions including:

1)  How tall should the hulls be...I would prefer shorter both for reducing windage and looks.
2)  Shape of the transom?  I am leaning towards pointed.
3)  Rocker amount and shape?  Should I give it rocker in the aft for good speed under displacement or reduce the rocker in hopes of getting on a plane.  If the latter, should I make the stern wider?
4)  Form of lateral resistance?  For simplicity and ease in shallow-water sailing, I am leaning towards leeboards which mount on the inside of each of the hulls.  No through-hull to worry about and simple kick-up for beaching or shallow water.  The windward board could be raised for speed.
5)  Engineering of crossmembers?  Needs to be easy to expand to sailing width on the water.  Big bonus if I could reduce the width for using a boatslip.  Add to this that the crossmembers need to support the mast...

Just had a thought for #4 and #5!  I could build a centeral member or "hull" that is never ment to touch the water but supports both the mast and a centerboard.  The crossbeams would attach to this centeral member.

Anyway, I would love to hear the input from folks on my musings.

  #2 Greg Luckett

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Posted 16 August 2007 - 07:45 PM

Sort of like what was used in Water World?

  #3 JeffM

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Posted 16 August 2007 - 09:10 PM

Deeper hulls can be designed to provide all your lateral plane.  That means you could actually SAIL in thin water instead of just floating in it.  The few extra inches of draft could be well worth it.

  #4 ScottWidmier

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Posted 17 August 2007 - 11:16 AM

I think the boat in waterworld was a Tri and much more modern that I was thinking of.  However, that windmill drive was kinda cool! 

Jeff, I guess that is why most cats have deeper hulls.  However, shallower hulls allow pulling up onto the beach and you can sail in shallow water with a centerboard...it just bumps along the bottom.  Besides, the downside to a catamaran is the difficulty in tacking where you have to have a lot of momentum or get caught in irons.  I was kinda thinking leeboards or centerboard would be better for turning than lateral resistance in a full-length hull.

  #5 ScottWidmier

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Posted 27 August 2007 - 05:29 PM

I just got "The Wharram Design Book" with all of his tri designs in it along with a lot of his philosophy.  Being a publication from Europe, it also has some topless and naked women in it which took some fast talking with SWMBO  :o

Anyway, he answered my question regarding hull shape.  In addition to giving you lateral resistance which allows you to sail off the beach, the V hulls also prevent pounding in chop or waves.  A flat bottom is ok for a sharpie which heels as it sails but a catamaran sails flat.  His first catamaran that he designed and built to prove cats could be ocean cruising boats had a flat bottom with lots of flair forward and she pounded and pulled a huge pile of wake behind.  He goes for V hulls with lots of flare which is supposed to help the cat ride up and over waves that hit from the side.  He also advocates stiff crossbeams that are lashed on making for a flexible connection which takes grounding better and gives the boat more initial stability in rough water. 

I love the idea of a cat that is sailed flat with both hulls in the water but still has the fun of being suspended over the water between two hulls.  Given the simplicity of these designs and the fun they could be as daysailers or even dingy cruisers, I have put three of his cats on my possible next build list.  He has a 14' and a 17' with storage but no cabin space and a 21' with cabin space.  The 17' has lots of built in storage in each hull and can accomodate a dome tent on the platform.

  #6 dstgean

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Posted 23 January 2009 - 11:15 AM

You could also check with Richard Woods or Bernd Kohler--both of whom have dory shaped hulls.  They tend to have a higher performance potential than the Wharrams...however, remember the wharram is designed for safety not speed.  You could also check out Ray Alridge's little cat--dory shapes there too.  Most of the dory bottoms are substantially narrower than you might suspect.  If the plans weren't so spendy, I'd be building a Woods Wizard as an open cat rather than the cabin/nacelle boat.  Cats make for great little cruisers; I'm sure you'll do well.

Dan 

  #7 paul_stewart

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Posted 23 January 2009 - 03:06 PM

:o =

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