Open Source Design 20M Cat

even if you gotta do your own thing somehow! the cost of a pro looking it over, or sketching something up is flat out free insurance at your budget range. lets say an idea goes awry for 2 days of work and materials, possible epoxy material right there bill there could be 2k or much much more.


Barry

 

Boat design is not the dark art many of it's practitioners make it out to be, which is why there are so many different designs out there. There are many boats and people to get ideas from and, as long as you stay within well defined parameters, you will not go far wrong. Anything you try that is different to the norm should be proven, either by someone else or by yourself at a smaller/safer scale. Or, you should be willing to accept that it may not work. This applies to building, sailing and designing.
I have a number of clients who are designing or co-designing their boats. The cross fertilisation of ideas that results is very satisfying for both of us.

A 20m boat makes sense in terms of speed, seaworthiness, load carrying, living space for 5 couples and party space for 20. It does not have to be any more expensive to build and maintain or harder to sail than a 15m of the same weight. IF you have the length without the weight, big rig, beam and unused space that is inevitable with a conventional catamaran design.

One solution is a conventional cat with extra length added to the bow and sterns, an approach taken by Bob Oram which works well as long as the extra space is not used. Another solution is a harryproa. One long hull (which is always to leeward) containing the rig and rudders, and a short hull (always to windward) containing most of the accommodation. 60-70% of the total weight is in the short hull, so the beam can be reduced without affecting the righting moment. Overall weight is lower than a same length cat.

The key to fast building (and low cost) is minimizing labour. Infusion using flat panel moulds and tables reduces the labour by about 50% over conventional methods by eliminating the messy, dusty and soul destroying sanding, wet laminating and fairing. Done intelligently, it also makes the internal fit out quicker.

Not at all silly. It gives you the opportunity to see what works where and what doesn't. Worst case is you pull the boat out of the water after a season's sailing and finish it off. It works best with a simple basic layout, with plenty of common runs for power and plumbing.

Comfort is about many things: staying dry/shaded, while being able to see the sails/horizon and communicate with the crew while sailing; keeping off pitching foredecks; ease of setting, reducing and storing sails, including extras; shallow draft (easy scrubbing off, ability to sail in shallow water and anchor close to the beach); couches you can lie on; ease of motion (loads and living near the pitch centre of the boat); open living areas; enough electricity to run all your goodies; big fridge; seating for all your crew for dinner; easy/low maintenance (unstayed rigs, shallow draft, minimum deck gear, rot and corrosion free materials); ease of getting in and out of the tender; anchoring and sail tending without yelling; party space (big deck) to name a few.

Tame sailing means ease of use, safety and control. An unstayed rig (schooner or biplane with a wishbone boom or ballestron) is the single biggest contributor to these. The ability to easily and completely depower the rig on any point of sail, in any breeze means less work, less worry and higher average speeds (no need to reef at night or in dodgy conditions). Tame also means easy handling. Tacking or gybing conventional rigs in big winds and seas is dangerous and hard work. Shunting harryproa.com/?page_id=468 is neither. Shunting is also a much more reliable way of sailing in crowded areas and of correcting accidental groundings and entanglements with crab pots, etc.
Most sailor's biggest fear is being caught out in a storm. With searoom, the tamest solution is to drop the sails, lift the rudders/daggerboards and go below with a good book. A 20m x 10m raft, drawing 2-300mm will be almost impossible to capsize by wave motion. If it gets too hairy, or the wind is from where you want to go, a drogue or sea anchor that can be easily launched without going on the foredeck will hold you in place. Without searoom, a boat that will sail/tack with a scrap of sail set and that does not get caught in irons by poor helming or a bad tack (ie a shunter) is a lot tamer than one that does.

High speed is different to performance. Performance allows you to enjoy sailing your boat, and enables you to sail in light air, to make ground upwind and to beat off a lee shore in a gale. One of the keys to performance is light weight. Alloy makes sense for 20m conventional cat hulls (internals, decks etc will be beeter in composite), less so for long skinny ones. The amateur built 20m harryproa at http://harryproa.com/?p=726 is on track for 3.5 tons sailing weight. It does not meet your accommodation needs but shows what is possible with infused foam/fiberglass construction. The professionally built 18m at http://harryproa.com/?portfolio=harryproa-cruiser-60 will weigh ~4 tonnes and could be altered to meet your requirements. Light weight means fewer materials to buy and process, lower loads, smaller rig and motor.

If you want clarification of any of the above, please ask.

 

This seems can match Your budget of newbuilding alu cat!
Will PM a link to look at the pics of 49-ft lady.

Hv a nIce evg,