Multihull Structure Thoughts

I was going to buy one of those some years back, but sailing it home was too scary. I would have thought kept light it should be fast. The one I was looking at in Auckland, was open deck.

 

There was an item by Jamez about a cat that was built from a quarter mould. In effect a mould of one quarter of a hull EG bow section to half length and to mid gunnel height or full gunnel hiegth. If the hull shape is done correctly, you can mould 4 sections and glue them together to form a full hull. Good in theory harder to do in reality. Current cat hulls generally have fuller stern sections and finer bows. Please assume I am talking about foam glass or solid glass builds in this item.

The most extreme of simple shapes I know is done by John Lindhal in his A class cat he built for his son. The shape is the same on all 4 sides of the hull. So, one half mould is required to do all 4 hull sides (first 2 jpegs). The hull sides are joined at the keel and centre deck line. This concept with a higher bow section could be done for a larger cat.

A quarter mould as in the Jamez jpeg Mitchell cat, can be done if it is not quite a quarter mould. The mould would need to be a “quarter mould” EG 60% of the hull length. You would mould the 60% length then mould a 40% section from the widest part of the mould to a point short of the bow. This will provide a wider stern section to add to the bow section. Again, you have 4 components that “glued together” will form a hull.

All good in theory but the hull is only 15% of the total boat build and you are compromising the hull shape for a perceived simplicity of build. The reality is most modern designs can be mainly built of flat panels with only the bottom required to be shaped if you wish to have the ultimate performance. If you have a genuine cruiser, you can get away with a flat panel chine hull shape. The weight, sail area and foil shape have more effect on performance than well done chines do.

The surface area of a rounded bottom mould will be less than a hull quarter mould, so it will require no more work and will be able to attach to a flat panel build for the rest of the cat. This is basically the approach of Grainger, many of the Schoinning and Spirit designs. Also using the mainly flat panel approach often allows the purchase of pre manufactured panels such as Duflex which can speed up a build. Please also learn vacuum bagging and resin infusion. Both techniques will produce better, often stronger and lighter components.

Building a cruising cat is a big project, do not get lead down theoretical paths that “simplify” a build unless you really gain something. Rob Denny’s Intelligent Infusion in cheap female moulds is a well thought out solution to a fast hull builds that suits proa hull shapes. Parts of that approach can be applied to cat hull shapes and components. Some details of Intelligent Infusion is at: Intelligent Infusion – Harryproa http://harryproa.com/?p=1845

The easiest way to build a cat I know is to build big component units and do a minimum of joints as taping together and fairing component joins is long and painful. Also, if the flat panel work is done well, there will be a minimum of fairing required all around.

The jpegs give various ideas.

 

Jim Antrim has used half moulds for several of his floats with good results. Anything that saves a little work is a good thing!

 

A couple more pics of another Mitchell. This one built from the same 1/4 mold as the previous. The quarter shot shows the hull shape quite well which is mirrored below the waterline. It can be seen that the hull is basically symmetrical double ended, which is not the best for pitch dampening - ask any Wharram owner. It looks at though there should be standing headroom in the hulls. The owner (of a different version) I talked to (who is a serial multi owner with wide experience) said he felt like it was an improvement on the wharram minimalist style and with dagger boards and a decent rig sailed acceptably well - although obviously not a race boat. Guzzi, did you come over and see the boat you were interested in, in person?

 

Alexandre Fortabat Design has designed a high volume charter cat that has a few interesting features. The Chartercat cat is 42 x 23.5 with a displacement of 27,800 lbs. The fixed aluminium mast carries an upwind sail area is 1,050 square foot. The draft over the low aspect ratio keels is 4.6 foot. The hull length to beam is approximately 8 to 1. The numbers indicate the performance should be good but as a charter cat it will have its limitations. Twin inboard engines are standard.

The 42 foot cat has been designed with an emphasis on volume and comfort for charter. It features 4 large en-suite cabins in the hulls and an original layout on the bridge deck with a large galley and seating in the cabin connecting to an aft cockpit with a lounge area aft with BBQ and sunbeds. There is a forward cockpit connected to the bridge deck cabin for all manoeuvres. An additional crew cabin can accommodate the captain in the forepeak.

The build is foam glass, mainly flat panels with moulded hull bottoms.

This would be a good design for a spacious cruising cat if wanted to travel. The displacement and simple rig will provide a good base for long distance cruising. The jpegs give some idea.

I will be upgrading the Index on the first page (with the Moderators help) in the next day to a length based index which should be more useful then I will be taking a short break prior to Christmas.

 

No. I knew they are symetrical front to back but the waterline is full length, so a 36' waterline or whatever it is goes some way to deal with the pitching problem. I saw videos of it sailing and it seemed to move ok. The hulls are really slim. I can't remember how skinny but very. I was interested because it was a lot of boat for not much money, and being glass and aluminium beams less of a headache than ply.

 

This is a test of the first page of the new 9 page index.

 

The new index for the front page if it can be read. The new index has 3 major columns per page. Each major column has 5 sub columns.
First column is length in feet (1 is for general issues before the start of boat length starting on column 1 of index page 2).
Second column is page number on this thread
Third column is G general issues, C cat, T trimaran, P proa, M monohull
Fourth column is C composite, P plywood, A aluminum, S steel O other (steel).
Fifth column is boat name and or some description

Can someone please comment if the index pages are understandable and readable before I request the Moderator to replace the indexes on the first page of the thread with the new length base index. Thankyou.

 

We have an index problem. I will try and fix in the next day. If you want to read the content click on a page attachment and press the the angled arrow in the top right corner and it will show you a page in readable text. More to come.

 

The new index for the front page if it can be read. The new index has 3 major columns per page. Each major column has 5 sub columns.
First column is length in feet (1 is for general issues before the start of boat length starting on column 1 of index page 2).
Second column is page number on this thread
Third column is G general issues, C cat, T trimaran, P proa, M monohull
Fourth column is C composite, P plywood, A aluminum, S steel O other (steel).
Fifth column is boat name and or some description

Can someone please comment if the index pages are understandable and readable before I request the Moderator to replace the indexes on the first page of the thread with the new length base index. Thankyou.

 

The images in post 2289 have an issue when viewed in the lightbox viewer because of the transparent background against the gray.

Let me swap those images in a minute by removing the transparency and also converting from RGB to paletted for more efficient file size.

 

Thanks Moderator. I have changed the PNG to Jpegs which appears to have solved the problem. I will cancel post 2289 and 2288.

 

The jpegs work fine for me. Or I swapped out the original pngs for comparison too. (I set them to 16 color paletted for file size and no background transparency.)

 

Moderator. Thank you. I will wait a day for any comments and if people are OK with the index, then the index can then replace the first page index.

 

So"r "stands for rigging in the 3rd column.