Multihull Structure Thoughts

The final on Chris White multi’s for a while. The original Discovery tri was designed in the early 1980’s as a fast day sailor of 20 x 15.25 foot and can be folded to 8.5 foot for trailering. Its claimed to weigh 525 lbs and displace about 1400 lbs. The mast was either an aluminium section or a wood wing of 23.5 foot long carrying 175 square foot main and 60 square foot jib. There were many versions of this model built ranging from fast day sailor to mini cruisers with removable dodgers. This version “easily does 12 to 16 knots. I've been told these boats have touched 20 knots” according to one owner. The folding mechanism was simple and the floats could be folded up or down depending on your trailer design.

The Discovery 20 was built from 3 layers of western red cedar to about 6 mm thickness in a constant camber approach. The hull halves are joined at the keel line with an epoxy slurry and glass taped over inside and out. The cross arms are timber top and bottom flanges with plywood faces with a ply forward wing shape. Home built boats were often built from 2 layers 3 mm ply with 6 mm ply decks and 9 mm ply bulkheads. There are no stringers with floors etc acting as stiffeners. There are gunnel strips on all hulls. Light glass and epoxy cover all hulls.

The Discovery 20 was a good design but in about 2016 a company wanted an updated version of the Discovery from Chris White and the Discovery 21 was born. The Discovery 21 is 21.25 x 15.75 foot that can be folded to 8.5 foot. The 21 weighs 620 lbs ready to sail and displaces 1400 lbs. It carries a 30 foot carbon fibre wing mast with 180 square foot main and a 99 square foot jib. The boom is aluminium. This is a much more powerful tri which required much larger floats which are longer and have more buoyancy. The 21 was also available as a kit or as a complete boat.

The structure was also changed. The main hull and floats are moulded in halves from carbon fibre, epoxy either side of a Nomex core. The hull halves are taped together and have 6 mm or 12 mm ply bulkheads inserted to hold shape and locate the cross beams. The cross beams have 12 mm and 6 mm ply with carbon fibre fairings and Douglas fir internals. The rudder and board are carbon fibre epoxy and foam.

The Discovery 21 will be faster than the Discovery 20 across the wind range although it may not top 20 knots. The 21 will need to be treated carefully in strong winds with the power the rig can generate. Interesting boats. The first set of jpegs is the Discovery 20 the later Discovery 21.

 

Carbon Fiber Mast,....wing mast developments...WOW

This particular issue of ProBoat discussion the new wing mast developments of Randy Smyth were even more INTERESTING ! This is really exciting new technology.

Basically the leading edge of this rig is a carbon fiber tube,...but one with a 'D' shape. Naturally the smooth curved side faces forward into wind. The aft flat side of the D forms a very stiff straight connection athwarships,...it greatly stiffens the mast from bending sideways, but without traditional spreaders.
Professional BoatBuilder - 170 - Dec-Jan 2018 https://pbbackissues.advanced-pub.com/?issueID=170&pageID=59

We are all mostly familiar with the std elliptical shape of mast to gain fore-to-aft stiffness without spreaders, but then we would all use spreaders to gain sideways stiffness. We even build our first carbon mast in that same manner.

Now there is a BIG change. This is exciting technology, and it is patented.

 

POLLEN was constructed 1991 as a cruiser racer trimaran for fast long distance passages. The tri is 51 x 35.75 foot weighs 12,125 lbs and displaces 17,000 lbs. The sail area is a mainsail of 710 square foot with a jib 303 square foot on a 7/8 rig carbon fibre aero rig. The Aero rig was fitted for easy handling, safety and improved visibility for single handed sailing. POLLEN now is controlled with a single sheet. Average cruising speed of 13 knots and peaks of 16 knots were measured on the first voyage to Spain. Boats with Aero rigs are given a 5% penalty in racing according to the RORC CHS system as they have an “acknowledged superior efficiency to Bermuda rigs”.

A small deviation. The materials used in the mast etc are not specified but Carbon fibre (CF) is not one magic material, there are more than 30 grades of CF. There are some CF’s that have similar characteristics to e glass. Then you move up the scale to eg T300 CF which is at the lower end of the strength and modulus range. T1000 CF has double the tensile strength of T300 CF but also costs more. If you need very high modulus CF you may need a different product like M60J which has over twice the modulus of T300 CF. Also, you have to match the epoxy resin you use with the strength and modulus characteristics of the CF you are using. Masts etc require an understanding of the CF and % of material required in each axis to obtain the best mast structure. We have discussed the fabric layouts in an earlier post for masts but please leave it to the experts for the design unless you are prepared to do some heavy maths or learn from your mistakes. Just copying someone else’s design without understanding the grades of CF used may not work. Back to the tri.

The tri is claimed to be fiberglass but the internal shots clearly indicate ply and timber construction which probably has an e-glass fiberglass covering. The ground tackle is 24 kg fortress anchor with a 5 kg fortress anchor. This tri is an interesting solution to one persons cruising needs.

 

This is for easter reading. These books are modern classics based on real experience by a man who thinks a cruising boat for his wife and 2 daughters is 78 foot long capable of doing 300 mile days without any additional crew. He has been there and has done several circumnavigations. The books are from "Dashew Offshore, Beowulf Publishing, and the FPB team have been blessed with the support of the cruising community for many years, and we’d like to return the favor in a small way. We are making Mariner’s Weather Handbook, Surviving the Storm, Practical Seamanship and Offshore Cruising Encyclopedia available for free as PDF files. We hope Mariner’s Weather Handbook helps you avoid the need for Surviving the Storm, and that Offshore Cruising Encyclopedia and Practical Seamanship enhance your tools for successful cruising. If these books help a few of our fellow cruisers have a more enjoyable experience, and perhaps stay out of difficulty, we will have been amply repaid." The address is to access the PDF's (they are large)
SetSail FPB » Free Books https://setsail.com/free-books/

 

I like Dudley's work and those cat designs of his. Combine these flat panel methods promoted by Kelsall with the 'shoe' of the Solitary cat I mentioned above.
Another construction method I had referenced, borrowing ideas from Kelsall's extensive ventures in this flat panel concept. I posted this over HERE
Weekender/Picnic PowerCat https://www.boatdesign.net/threads/weekender-picnic-powercat.33751/page-3#post-872912

 

I recall that distinctly, and posted something about it at the time. Forgot where I posted it, so long ago.

 

Tara Vana was the boat that inspired me to draw up my sportfishing/sailing cat,...
Gamefishing Design - a 65' mast-aft sailing catamaran. http://www.runningtideyachts.com/gamefishing/
"I'm well aware that once a large gamefish is hooked, the vessel must get rid of all sail quickly, with minimum effort, and get under power." All 3 of my sails roller furled up, and there was no traditional mainsail to haul down.

Lock Crowther (whom I had met at an Annapolis Boat Show event), and I had a few discussions about my aft-mast rig design I had created for my version, particularly since he had also designed a aft-mast boat, 'Shotover', I believe it was called.

 

I have a good friend up in Ocean City MD who has one of these. He loves it

 

Hi Phil,
What if you were to use a new wingsail rig like that developed by Randy Smyth for his Sizzors cat?...very little rigging holding the mast up,and very little compression loading to the hull,...etc

 

Resin Infusing Carbon Composites
very interesting, just had to bring it back up

Another posting that deserves repeating,..
Carbon Beams

 

Underwing Clearance

I've long contented that we need a center nacelle design that 'attacks' those peaky little waves up front that grow into bigger ones that slap at those flat surfaces of most wingdecks.

https://www.boatdesign.net/threads/wave-splitter-on-catamarans.5741/#post-458374

https://www.boatdesign.net/threads/flat-bottom-bridgedeck-vs-other-shapes-in-heavier-sea-states.56646/#post-789136

https://www.boatdesign.net/threads/bridgedeck-centreboard-why-dont-they-work.57051/page-4

I took it to extremes here,..

 

X Beamed Multihull

When I saw this X beam mentioned it took me back to an old subject thread I had posted in 2006
X-Beam Cat and the Giant
X-Beam and the Giant https://www.boatdesign.net/threads/x-beam-and-the-giant.14679/

 

Dragonfly 25 Trimaran ?
I looked thru this whole subject thread and never saw one mention of the Dragonfly trimaran? She had won the Micro Multihull Worlds for 2 years, and the Round Britian Race in 1985

The Original Dragonfly Trimaran | Small Trimarans http://smalltrimarans.com/blog/the-original-dragonfly-small-trimaran/


I became the original importer to the US, but could never get enough boats (only 2 if I remember correctly). I subsequently redesigned the vessel and we built molds and 6 boats for our 26 foot Firefly trimaran
Trimaran Design http://www.runningtideyachts.com/trimaran/

 

Brian. Thank you for all your input. Your experience and thoughts are welcome. We all need to learn about the thinking behind the concepts and structures of multihulls to improve the breed. Many home builders think they can "modify' a design by removing things, adding a thicker panel, putting in a 30 HP engine instead of a 10 HP outboard etc not realizing the effect its having on the overall design. Your sort of experience helps get the message across there are better ways to do things. Building a resin infusion carbon fibre structure is a little harder than an e-glass polyester structure but the resulting boat can be stronger, faster and not that much more expensive. Information helps people understand and make choices.

 

A little history, CSK (Rudi Choy et al) designed 36 foot cat Toru. Toru is 36 x 15.5 foot weighing about 5000 lbs. The mast is a 46.5 foot aluminium section of 205 x 120 mm with 2.4 mm walls. It carries a 380 square foot main, 225 square foot jib, 558 square foot number 1 genoa and a 1450 square foot spinnaker. The boom is 180 x 130 x 3 mm section. All the rigging is 5.5 mm 1 x 19 stainless steel. The hull length to beam at the 28 foot waterline is 14.45:1. When the hull is overloaded or heeled the L:B is closer to 16:1.

Now the elephant in the room. The asymmetric hull shape. This is a 1966 build when CSK design model was very asymmetric hull shapes. Over time CSK has significantly reduced the asymmetry and some of their design have symmetrical hulls. Again, a learning experience. The initial hull shapes worked well in rough water and could maintain good speed reaching and running but the shape required dagger boards to get the best upwind and a lot of CSK early designs pitched a fair bit.

Construction is 3 layers of aircraft grade Skita spruce (similar CSK cats had 9 mm ply on the outside of the hull and 6 mm on the inboard side of the hulls) over sawn 12 mm plywood frames and closely set stringers. Deck ply over stringers. To quote an owner: “This, not only unique but classic C/S/K has been hidden away in dry storage for over 30 years. The hulls were built from the same American Airforce Sitka Spruce that was left over from Howard Hughes' Spruce Goose. Toru was dubbed the “Little Spruce Goose" or "Son of Spruce Goose" and when last surveyed the Toru was classified as above average construction.”

The jpegs show the boat. The PDF’s are the actual study plans from CSK. Interesting history.