Anonymous wrote:

I thought I would put any updates about my little catamaran build here, seen as how my posts are so few and far apart there is no point in a separate thread, and it is after all a minimalist cruising multihull.

Progress has been a bit slower than I would like, but that is to be expected with all the other things we have going on in life. There has also been a hiatus of several weeks whist I have been organising the sale of Footprints which has included making a new sail cover for her. Anyway this afternoon I finally managed to get in several hours of work and did the initial fairing of the plywood on the hulls, cleaning up all the rough edges and tidying up the chines. So, I now have two very floatable objects. If ever we have a flood up here on the top of our hill I have the makings of a double canoe so as to ensure our survival!

The next step now is filling of all the screw holes, and other areas of filler needed, a good sanding, and then into fiberglass. Pictures below of what are looking like two slender and potentially very fast hulls. 

Very pretty, David.  I can see why you might not want to put a mast in each bow, though.  It will be most interesting to see what rig you eventually use.

Anonymous wrote:

Hi Jeremy,

I think explaining the dynamics of the pacific proa will help the monohull family to make better sense of it all....

Only true Pacific Proas of all multihulls have almost Zero "aka" (cross beam) loads, Cats and Tris put huge loads on their cross beams, forcing them to be seriously engineered resulting in ether heavy or expensive beams.

With the mast placed on top of the "vaka" (big hull on a proa) and a shroud connected to the "ama" (small windward hull). Mast compression is over the hull, vs over the akas on cats. Aka compression as a result of heeling is totally eliminated, since akas are to windward and shroud is lifting the windward ama, vs trimarans that are loading leeward akas more and more as wind builds, since the leeward ama on a trimaran is, counteracting wind and heeling forces. These heavy loads are some of the biggest problems in multihull design, resulting in expensive engineering. 

Pacific proas have none of this due to a genius design of a vessel that excepts the wind from one side only. "A shunting boat", meaning it can sail in both directions, as long as the ama is to windward. The mast is on the vaka and the shroud lifts the ama, for that is a pacific proa..... And that friends is the simplest, cheapest, lightest and strongest multi possible. Just remember a stayed mast is the foundation of a low stress vessel, now, low stress is what adds strength, by reducing weight..................... 

Very interesting rig you're working on, Good Luck Jeremy.

all d best.

rael.

Rael, thanks for the 'good luck' wishes.

As for the theory on  pac proas.... I have been meaning to make comment on your 'la shunk' thread, so will take this as a cue to go ahead (refer that thread for post to come). Current build is a Pacific/Oceanic double canoe that is to be rigged a shunter, because the companionway hatch needs to be kept on the lee side of a dodger facing the weather, and at the same time allows the bigger of the two hulls to be the primary displacement hull, to lee.

Al weight to w/ward of this lee hull's CB, can be carried by a shroud( if rig is stayed), or elelse there could be a cantilever mast stepped in an 'aka' structure between the hulls.

Going with a stayed split rig ( as per the older type Kanak and Tuamauto double canoes) will no doubt be the lighter and less costly way to go, so is what I can plan on for the immediate future.

Beam of this Pahi will be 5M, and as designed and built so far, will be joined by wooden kiato( traditional 'cross spars' as they are known by pacific people), but as already mentioned, a structure carrying rig load from a cantilevered mast ( something akin to Richard Newick's akas on proa Cheers, but fabricated rather than laminated using wood) is a possibility, if testing on a small prototype  shows promise.

Using traditional kiato keep's a lid on costs, and allows me to fit the individual hulls into tight spaces and onto small trailers while in the building stage. Also, the beam dimension can be varied, If I really need a bit more, or possibly a bit less, rather.

Weight is relatively low, since no heavy ballast is required for stability, given that all crew, stores, gear and rig, serve to keep the craft right side up - like a catamaran, except that I have the option of packing stuff in such a way as to lighten the smaller hull (the ama) if it is beneficial to do so. 

More important to me, is that I can build with and live with solid wooden boards, in the way of cabin soles, furnishings and slatted decks, rather than be so obsessed by weight reduction, that I begin looking at a tramp stretched between 2 hulls, or even worse; a tramp stretched between one hull and an Ama that has no internal volume for living space.

As a teenager, living life again, a double canoe, minimalist in the extreme, would be the way to go, now the closest I'l get to that, might as well be an experiment in what is achievable, using our/my currently available resources.

Agreed, Oceanic people using stone or shell tools to shape, then fasten and rig their canoes with vegetable fibre, developed amazing craft..... Ive spent a major part of my life in quest of knowledge covering this subject. As well, the development of a bamboo raft into a sailing platform with a battened reef-able Junk sail, is equally interesting.

Dynamics of form stability apply equally to both the bamboo raft  and  the canoe/raft outrigger hybrid.

Still, the double canoe slots right in to this scheme of floating craft, and from what we know, is the better load carrier of the lot, if sailing performance is required well.......as you like to say"keep on shunting". 

 

An explanation might do in covering comments made(in my last posting on this thread), about stayed rigs etc....

When multihull talk comes around to weight reduction being the pivotal issue, I get nervous and ready to think of monohulls or any way around it....even an unproven proa concept, which might explain why 40years on from starting with the shunting paradigm, am still at it.

Trimarans and foiling multi's lead the march taking boating in the aerospace direction, which all or mostly use a widened platform for rig support spread.

So, wider beam dimension plays along with  stayed rig, until it comes to a knockdown or self rescue ability, then we should want less of it, and the Gugeon's G32 shows how less beam can can provide something of benefit, even with a stayed rig.

For a long time I have wanted to use mast buoyancy to prevent full capsize, and still think a free standing mast could do the trick. But it  probably means reliance on crew body weight for stability as well as recovery.

This is the precept behind my fixed mast rotating rig design, and stretching the bounds of possibility to work on a heavyish plywood bridge deck catamaran( such as the (Eco6) looks unrealistic....even a stayed rig of the G32 config would likely fail to provide both performance and self rescue function on the Eco 6 , and my experience with catamarans shows that a min beam of 4metres makes good sense on the NZ east coast.

Jeremy wrote:

What I had in mind, was building on the structure to gain more mast bury beneath the bridge deck and using this girder as a pivot/ mounting point for a kick-up stressed ply foil, that doubles as an OB motor bracket.

There is already a lot of bury in the bridge deck, 900mm at the designed mast position. I have also thought about options such as you suggest, but a maxim to keep in mind in any small multihull is REDUCE WEIGHT. I am already going to be adding weight in a longer cabin top with the aft cabin bulkhead more vertical, and higher bulwarks on either side of the cockpit, so I have to be careful about whatever else I put into the boat.

Jeremy Walker wrote:

Going with the idea of using a wishbone boom and luff spar rig on a free standing mast (as above) on a Kohler Eco 6,  does strike me as a possibility.

Structural changes to accomodate the mast, might include changing out the daggerboard for a kick-up board on the centre line, and still save most of the accomodation space and layout.

Only a cursory look at at the Eco 6 material (available on the net) has led to this thinking, but as an alternative to stepping twin masts, I think it would be worth pursuing in depth, because  both the daggerboard's vulnerability to strike damage ( unless a foam packed collision box is included) and the space robbing twin masts are asking for improvement.

I have thought a lot about daggerboards, and keels, and lift up leeboards on the inside of the hulls, and a central kick up board, and I am coming from 5 decades of sailing and cruising experience. I have decided to go with the forward sloping dagger board as designed. Yes, it could be vulnerable to potential damage, so I will need to be careful when in shallow water, but I think the risk of damage is very minimal. I like simplicity and the dagger board as designed is the most simple solution. Having already built the rudder blades I now know that the stressed plywood daggerboard will be very quick to construct, and the daggerboard case will also be quick and easy to build. There is only one daggerboard which is in the port hull.

But rig ideas are still very much 'on the drawing board'.