Anonymous wrote:
What about a stayed mast?
Howard,
from time to time, I have also been pondering this subject. One thing is how to support a freestanding mast. That may in many cases be solved, somehow. The other challenge is to find some sort of rule-of thumb as for the strength of the mast.
A third question is how to make an automatic sheet-release or sheet-brake device to avoid capsizing or overstressing the mast.
One can quite easily find the max righting moment of a cat by multiplying its displacement with half the beam. However, the resulting righting moment is so far beyond that of a monohull, that it would require an overheavy mast (aluminium), or an over-expensive one (carbon).
Then there is the question of simply staying the mast. There are two arguments against that on a monohull:
· Since the shrouds has to be from the mast top, the shroud angles will be very fine and the unsupported pole (mast) under compression will be very long. In other words, the shrouds will add stress to, rather than relieve it from the mast.
· Shrouds are in the way when we want to square out the sail for downwind sailing.
However, on a wide beam catamaran the shroud base will be so wide that the compression stress may be reduced enough to make it work. A bit serious engineering has to be done, but a cat is a result of serious engineering, anyway. A pair of shrouds, set well forward, and a pair of aft shrouds, set as far aft as is practical, is all that is needed. One may even support the mast at the partners and step to reduce the problem with compression loads (reducing the ‘compression length’).
As for sailing downwind, a cat may just as well be broad-reached instead of running straight before (faster). It may therefore not be such a big disadvantage with those aft shrouds (Btw. forget about running backstays)
As for the matter of making a sheet-release devise. I think the simplest and most reliable would be to make a sort of sheet brake or elastic devise, set up to ease the sheet as the load exceeds a preset limit. This should be easy enough to make and adjust.
From the safe position of my armchair -
cheers,
Arne
Arne:
I see lots of reasons not to use a stayed mast, particularly the fact that not having a hundred components to fail, and to inspect and maintain and replace is a huge load off the mind, and pocket book. And of course you are not working around stays and shrouds, or in this case just aft shrouds and a forestay.
In the original post, I was pondereing the comparison of monohulls and cats in terms of mast strength based ONLY on righting moment. On a monohull righting moment is the only factor, as for practical purposes they will not move sideways or accelerate under the force of a gust at anything like enough of a rate to absorb the forces.... a multihull is a different kettle of fish. At least a light weight one is. "whiplash acceleration" is the norm, so where a monohull rolls, a multihull accelerates. This tells me that something must be able to absorb the initial force... flex in the mast, perhaps a step and partner setup that utilizes polyurethane rubber for some give, perhaps both, but like a spring taking up that force, it will be released as energy driving the boat forward more than lifting a hull. Take Sagitta.... which is as you know a boat that appeals to me greatly..... 2.7T loaded displacement... 50cm draft, it doesn't take much to get it going. Then look at for example an Alberg 30 Same LOA, but 4.5T displacement and 130cm draft. The dynamics are radically different.
An aluminum mast on a cat should probably be made from 6061 T6, 7075 T6 would be better in a spring temper, both of which have some spring and resilience to them. Neither is available in tapered columns like a flagpole or light pole such as Roger Taylor used on MingMing II..... Probably 6063 or one of the 50 series. Wood, particularly Douglas Fir or Sitka Spruce would be good in this respect, as would fiberglass or carbon fiber. I've looked at ways to construct a carbon fiber mast, and do not see a really simple method. Probably the most interesting method would be to construct from pultruded unidirectional carbon fiber rod, which runs from one source about $25 per meter in 12.5 MM dia, but they don't list long lengths. This sort of material is often used in wing spar construction for the spar caps..I'm not quite sure how they would be incorporated into a mast, but have several ideas.
Considering the destruction of these annual hurricanes in the Caribbean and Gulf of Mexico, one would think that somewhere there are some wrecked Freedoms with salvageable masts. But how does one go about finding them? It would be interesting to know a bit about Pete's masts on Oryx. The wood he used, the dimensions, etc. He was clearly pretty conservative. All I know is 53.16 sq.m sail area, 8.5M mast above deck, hollow douglas fir..... Almost the same exact sail area of the stock shorter KD 860, a ton more displacement, 1.4M more LOA. I'd be interested in how he engineered the masts. The original was listed as having an RM of 5600 KGM. The beam being the same, the displacement 33% greater, Oryx would it seem have an RM of 7465 KGM.
I'm not sure I really understand righting moment and how it plays into mast engineering. Apparently at some point the boat above would take 5600 kg to lift one hull. The fact that the total weight of the boat is 3000 kg, suggests that this point would be quite low. It definitely is not a vertical force on one hull, which would seem to be less than 1500 kg. As iit is listed as KGM, I'm assuming that it is torque, probably about the rotational axis of the lleeward hull, or 5600 kg at a point 1M out from this rotational axis. That suggests that one would divide this figure by the meters to the linear distance to the vertical CofE to find the force that would need to be applied to the sails to lift the hull. Thus the higher the CofE, the lower the force required to overcome the righting moment. That is how I read this, but it is a static value only, and wind is not static. Say the straight line distance to the CofE from the rotational axis is 10M, only 506 kg would be required to lift one hull, but that force would have to be at right angles to the line between these two points. In a gust, the forces could it would seem reach or exceed 10x or the actual righting moment of 5600 kg at that point. Since the vectors do not align, the need mast strength goes up. The vertical CofE on a Bermuda rig is liable to be fairly low, well below half the total luff height of the sail. On a junk rig it will be considerably higher as a percentage of the luff, so a shorter mast is called for. The question is how much force per sqm of sail area does one need to design for? The calculations are all doable though it all looks pretty complicated to me ;-(
Fused sheets or slip clutches are clearly called for, also mast failure is preferable to capsize I would think when far out to sea. The mast itself should be the ultimate "fuse" to protect the life of the crew.
H.W.