I carried on doodling my proposed 10m LOA catamaran design at odd moments over the last few months. Progress on this will have to stop for a while since we plan to go cruising in our present (monohull) boat for much of the rest of this summer. Below are a few views of the design as it stands at the moment. It probably does not look a whole lot different to the pics I posted here before, but there is now more internal detail - the previous drawings were just a shell with the main structural bulkheads inside. If I decide to continue with the catamaran design when we get back from sailing this summer there will be plenty still to do. Although there is now quite a lot of internal detail for the hulls, there is little for the bridgedeck structure between the hulls and almost no detail at all for the rig, sail handling arrangements, mooring and anchoring arrangements, electrical system etc. Also at some stage I would want to re-visit the finite element analysis of the structure. The initial simplified analysis indicated high stresses around the access openings into the hulls and also to some extent around the big windows. The wood work in these areas is now strengthened but another round of analysis would be needed to see if it is strengthened enough. Initially, the only load case considered was the 'close to diagonal capsize' situation. I might now look at a few more load cases, perhaps including situations like a heavy crew person jumping down onto the deck from a quayside - that kind of event can cause higher local stresses than sailing situations! Towards the end of the design process I would need to work out how to cut some hundreds of odd shaped pieces of plywood from standard sized sheets. The method I had in mind was to print out the silhouettes of all the pieces to scale on stiff paper, cut them out with scissors then jiggle them around on cardboard rectangles representing the blank sheets of ply, then finally re-draw them with 2D CAD to obtain those vital DXF files - it sounds like quite a job, does anyone know a better way?
A few points regarding the design for anyone who might be interested:
As before, the structure is basically wood and epoxy. I like the idea of foam core and resin infusion with glass or carbon fibre, but I am pretty sure that would be more work to build despite the claims made. My choice of wood and epoxy is primarily for easy building, although it is also true that the materials are cheaper, even allowing for getting all the plywood CNC cut, which would be nice. Nearly all the shell is plywood, in a range of thicknesses from 9mm down, but there would be small sections of strip planking at the turn of the bilge. The curved front of the bridgedeck structure might also be strip planked. Each hull has a knuckle a few inches above the waterline. The idea is that the lower hull below the knuckle would be built first, using female formers so that internal sheathing and structure could be fitted while the skin is still supported in the formers. I would hope that this lower hull would then be a sufficiently rigid unit to act as a base on which to assemble the rest of the hull structure. Glass epoxy sheathing above the knuckle would be thin, external only, and mainly for surface abrasion resistance. Sheathing below the knuckle would be inside and out and would be a bit thicker, with the glass fibers accross the grain of the strip planks.
I am keeping the structure demountable, so that it can be transported on a large lorry, preferably one of those lorries with a hydraulic crane for loading. The two main cross beams fit into 'trunkings' in the hulls and are fixed with an arrangement of wedges. Certainly I could use bolts, but I quite like the wedge scheme since it avoids some of the problems that can occur when loads are transfered between a low modulus material such as wood and a small load bearing section of a higher strength and higher modulus metalic material.
I have retained the moveable wheelhouse shelter, I think this is a nice feature allowing one to sit in shelter near the wheel, idly watching the autopilot at work, or alternatively to slide the shelter forward to stand at the wheel for better visibility, or to fold the shelter right out of the way in nice weather. I could make it a ridgid shelter but then sooner or later someone would try to walk on it, so it would have to be quite strong and heavy. Most people will not try to walk on a canvas shelter, hence that can be much lighter. Also a folding canvas shelter allows the mast to be lowered in a tabernacle arrangement. I agree with a previous post here that the mast will not often need to be lowered for navigation, but I am not keen on climbing up masts so I like the idea of a mast that can be fairly easily folded down to allow all parts to be inspected/maintaned.
My present boat, which I built in 1978, has a pivoting lead ballasted centreboard and over the years I have got into a bad habit of using the centreboard as a depth finder. I suspect that I may not be able to instantly drop that habit, so I have designed this catamaran with pivoting centreboards and dinghy style pivoting rudder blades, although of course no ballast is needed in the centreboards. There is a pivoted part in each centreboard case to closes the centreboard slot flush with the hull skin when the centreboard is fully lowered, giving little more water resistance than with a typical dagger board arrangement. I am not sure if it is worth also considering some kind of dingy type centerboard flaps for when the boards are only partly down. The centreboard cases are set inboard of the hull centrelines and there are also beaching 'skegs' that I have drawn on the hull centrelines. I wonder if these should be a bit outboard of the centrelines to keep them further from the centreboards?
The transom hung rudders are controlled from a steering wheel. The general arangement of this boat makes it difficult to include tiller steering and the proposed wheel steering system is really quite simple and should have little friction. I realise that the steering will be heavy if one ever needs to steer with the rudder blades part up, perhaps steering with the throttles of the two engines would be a better option for manouvering in really shallow water. Both rudders can be fully raised and turned 90 degs inwards to lie flat against the transoms for harbour stowage. Transom hung pivoting rudders are said to be prone to ventilation of the rudder blades, although I dont think that has been much of a problem with my present boat. To help avoid such ventilation, the lower rudder mounts are designed so that they form a 'fence' just below the nominal water plane and extending round the foreward part of the fully lowered rudder blades. The wedge under the transom might even recover a minute amount of energy from the sternwave - well who knows!
The design has sprouted a curved beam, or mainsheet horse, above the aft main cross beam. This would house an integral mainsheet track, I am thinking of a home made traveller with rollers that run under rails made from lengths of aluminium rod. This structure also provides bracing for a pair of dinghy davits and it acts as a robust fence that would make it hard to fall off the back of the boat, the downside is that it would also make it more awkward to climb down from the cockpit into the tender.
I have sketched a small gas powered fridge in the galley, but if necessary this could easily be changed to electric before the design is completed. I would think that refrigeration could be the main electrical load on this vessel, so gas refrigeration might significantly reduce the requirement for battery charging and allow lighter batteries to be fitted. I just dont know how practical a gas fridge would be on a boat, although I think that there is one type of American catamaran that has them fitted. I have the installation manual for a Dometic gas fridge and this indicates that it is OK to run it with the normal motion of a vehicle on the road, but what about a catamaran, does anyone have any thoughts, or ideally experience?. Since this is a catmaran it is possible to install the fridge in an enclosure which ventilates straight downwards to the open air, I imagine that should mitigate some of the safety issues, sorry if am sounding like an engineer at work!
I have sketched in two outboard engines, 8 to 10 HP each. I wonder if the luxury of power tilt on the engines is worth the extra weight? I have shown the engines fitted aft of the aft cross beam which allows a good spacing between the props for manouvering using the engine throttles. I have also considered locating the engines further forward within the bridge deck structure, which has some advantages, but it would mean that the distance between the props would need to be reduced to something like 2.5m. Would that be enough to turn the boat through the wind in a stiff breeze I wonder?
If I actually build this design I would be thinking of normally sailing it with just two persons on board, so the accomodation has one double berth in the starboard hull and this could be supplemented by using the 'L' shape settee in the port hull as an occasional single berth. The spaces forward of the main bulkheads would probably be left empty, or used for storage of a few light items, but if more berths become wanted there is a possibility to use one or both of these spaces as a single berth cabin, the tops of the forward bouyancy tanks are more than long enough for berths. These 'cabins' would offer excellent privacy, but not wonderful elbow room. Also, as drawn, there is no second escape route from these spaces, which might be considered unacceptable. It might just be possible to provide such an escape route, but for the likely use of the boat, this doesent seem worth the trouble at this stage. There is full standing headroom at the entrance to each hull and in the galley and shower/toilet areas, but only sitting head room elsewhere. Being used to cruising in a much smaller boat we are happy with these compromises. Generally, I am trying to keep down weight and windage whilst avoiding a purely racing inspired design and accomodation that feels like living in a pair of pipes.
