Multihull Structure Thoughts

When a male mould is used is the outer skin wetted out first and vacuumed pulled against the foam and then turned over for the inside? Or are both sides of the core laminated at the same size with perforated core? How does one pull a vacuumed on a male mould.

 

Slingshot. Vacuum bagging is the first step to resin infusion. Vacuum bagging is common over male molds. The attached is a guide to vacuum bagging. Also look at Professional Boatbuilder magazine Issue 103 starting on page 142 (look at an earlier post to get the web address for PBB back issues). This is an excellent article on resin infusion. Take the advice of Rob Denney. Get some simple equipment and do experiments on flat panels from just glass to foam and glass one side (= male mold) to glass foam glass full test. A lot can be learnt. Dont spend money on good equipment for experiments simple reinforced garden hose, plumbing pipes and caps for resin traps, garden hose fittings etc. The vacuum pump can even be an old vacuum cleaner for small experiments. Real panels for boats requires some serious equipment but sea container refrigerator motors, reasonable hoses and fittings, pressure gauges etc are obtainable relatively cheap. Finally work out how your going to clean all the equipment prior to doing your first run as resin is difficult to clean up when cured. Female molds are the best but real world says a male mold for boats with complex shapes is cheaper for most home builders.

 

A slight deviation from building in glass. Ian Collombin a Canadian did a 28 x 8.5 foot 5500 lbs power catamaran for his personal boat in 2013. The interesting part was the construction method. He built the hulls from 3 mm aluminium that was initially shaped then glued (yes glued) along the keel line with methacrylate adhesives. The hulls were “tortured” into shape (not like ply just pushed into a deck mold). The aluminium to be glued was sanded with 36 grit sandpaper cleaned (no sodium chromate wash was used) and glued. The bow and the stern had some aluminium welding. The hulls then had 5 aluminium honeycomb bulkheads installed. The bridgedeck had some aluminium beams and a floor of aluminium honeycomb. During the WW2 the british glued together a few 52 foot aluminium patrol boats and during the 1970s some aluminium sorc racing boats were glued aluminium.

 

Slingshot. I forgot to include a very good web site about building a 39 foot farrier trimaran with male molded vacuum infusion. The construction of the first trimaran float http://www.fram.nl/float1.html The guy who built this boat did a lot of good work and was capable of infusing complete hull halves in one shot when he really got into it.

 

You will be surprised at how much of the skin is covered by fiberglass bonding bulkheads and furniture to the hull.

 

Bigcat 1950. Just want to know if you are the same person who had plans for the following? If so did you go ahead with the project? We could all learn a lot from your experiences. The following is a write up I found.

65' Catamaran, radius chine, wing sail

Radius chine, wing sail, unstayed masts, vinylester, balsa, hard chine, full floatation, buoyancy, amateur construction, home built, vacuum bagged, vacuum infusion, resin infusion, junk rig, Polynesian, , laminating table, tandem rig, bi-plane rig, catamaran, multi-hull, polyurethane, shoal draft, balanced rudder, keel, skeg, Tim Dunn, Seattle. I have deleted the old web address for privacy but if you have a current web address to share, good.

 

Fujin is an interesting cat for several reasons. But its main reason is about where the edges of a racer cruiser design are. Fujin is 53 foot long and displaces 6120 kgs (13700 lbs). Its top speed was reported at 31 knots just before it capsized in a Caribbean race. The boat can partially foil. The boat has been rebuilt by Gold Coast yachts and is racing again. The designer and owner wanted a high performance cruiser. This they designed and got one and more. The interesting features are the raised bow pieces so forebeam can clear the water. The central pod to allow full headroom in the central cabin but keep the cabin profile low. The chine bow to add buoyancy but reduce width on deck to assist in recovery from a bow dive. The foils on the rudder and curved dagger foils to provide lift.

 

BigCat 1950. I only received a partial reply to your last post. Could you please resend it.

 

Same as on anything complex. With a lot of coating to make it airtight and messing around to get the bag right.
Because fitting foam on wet resin is difficult, convention is to vacuum the first skin, then clean it up and vacuum the perforated core onto bog (often in sections to minimise gap size), clean that up and vacuum the outer skin on. Then clean, fill and fair it. The time and waste are huge. if it is not wet out by machine, about 50% of the resin ends up in the breather cloth, plus peel ply and non reusable bag.
With infusion, all the layers are laid up and wet out simultaneously. However, it is still tricky getting the foam into compound curves, so a lot of builders use segmented foam which conforms to curves, but the opened cuts need a lot of resin to fill them. It is better is to either jam them between overhangs on the mould (see the hull in post 704) or spend a lot of time heating and bending them. Infusing on a flat surface is an order of magnitude easier. Compare the work Hennie (one of the skippers on the Dutch harryproa, Blind Date, great guy) did on Fram with Intelligent Infusion to get an idea of the difference.
Big Cat
Another benefit of Intelligent Infusion is that the bulkhead landings are included in the infusion. The bulkheads are glued in, there is no secondary laminating or filletting required. Saves days of messy, sticky, weight adding building.

 

For simpler multihull shapes that uses mainly flat panel materials there is a build method that is easier. I will use Wayne Barret M series of trimarans to demonstrate but many other designers use the same technique. The M series can be built in ply or foam glass. The M60 gives an idea of the shape of the boat and its main hull. First you cut out frames in MDF, set up the frames then cut out the panels for the boat then put the panels into the frames and tape the interior joints. Insert bulkheads next and tape them in. By now you have strong structure that can be taken off the frame rolled over and the outside finished. The designer needs to provide accurate patterns for each panel that will inserted into the frames.

 

For those that like ultra high performance multi's the Seacart 26. This boat is less high tech than its larger sister Seacart 30. The 26 is more of a day sailor. A fun design.

 

Thanks for the links Oldmulti. Lots of info with the trimaran. I like the look of the M80 and how 5e flat panels go together.

 

Following from the post about the M series of trimarans build approach I will show another designers approach. Kurt Hughes has more rounded hull shapes for his boats and as a result uses slightly differing techniques. He resin infuses panels that are flat or nearly flat, places those panels in a MDF frame structure, then adds strip plank foam glass strips at chine joints giving a nearly round hull shape. The final glass layers on the chine is then done. Dudley Dix does the same with his cats but they are mainly plywood. The attached gives the idea. The first 2 pictures are of Dix 55 foot ply cat the last is o Dix 43 ply cat. The boat can also be done in foam glass.

 

The Hughes build technique is one that I could get into. I had seen a similar page regarding his cabin house build but not for the hulls. I have sailed on a Hughes 65 charter cat in SanFrancisco and it was quick even with a whole herd on board. Also, sailed down the Baha with a Hughes 60 it had 14 people and all their gear for a winter season and it was still reasonable quick. I think both these boats sailed well with a load because the hulls had a medium beam width, but 60’ plus also helps.