Sliding CRoss ArM type multihull!
I’ve been spending some time today with the microMOTH model. It was conceived of as a monohull with a sliding rack"Trapeze Power Ballast System" Pictures can be seen on the microsail website under monohulls; to understand better what follows you might want to take a gander at those pix especially the one showing the PBS at full extension to one side. The PBS on the microMOTH is two .25" carbon tubes fixed at each end with a cart that slides on the carbon tubes. By sliding the entire rack as well as the cart an effective beam of almost 6’ is created for the
microMOTH.Mechanically, the PBS,which has been used on several different type boats, works extremely quickly and well. The microMOTH has two “sissy floats” attached to each end of the PBS rack to prevent the boat from turning over when it is first put in the water–the boat is not designed to sail on the floats -just on the two hydrofoils.
Now comes the good part: it occured to me today why not beef up the tubes in the rack so that they acted like "normal"cross arms and attach hulls at each end instead of “sissy floats”. And lose the main hydrofoil-- until I figure out how to retract it anyway. You now have a trimaran with a beam when the crossarm is centered of 3 to 4 feet BUT since the two outside hulls+ crossarms now slide side to side you have an “effective” beam of between 6 and 8 feet!! Effective beam means that since the cross arm and hulls slide out to one side the beam is effectively almost double the ACTUAL beam! This system could be used in the multiONE class or the F48 class. It has several advantages over a normal trimaran: 1)high righting moment can be achieved without flying the main hull,
2) the lee hull ceases to be a load carrying hull since as the cross arm slides to weather the lee hull is clear of the water allowing the main hull to be the effective “lee” hull–and with a t-foil and anti dive planes it can be extremely resistant to pitchpole(see5 below) the system is much lighter than ADDING a PBS to a boat: the crossarms and outside hulls ARE the Power Ballast System!
4)VERY IMPORTANT!: if there is a capsize the PBS is simply centered and the buoyancy of the lee hull rights the boat!
5) at no additional weight the sail winch can slide the cross arm assembly aft to resist pitchpole even more.
6) the cross arms are supported by the mast with trapeze wires; that allows the relationship between the cross arms and the hull to be adjustable such that the mast can be canted to weather a small amount(10 degrees more or less) or to leeward a small amount.
7) Since the boat will not sail with any appreciable weight on the outside hulls they can be smaller than normally required consistent with their duties in righting the boat from a capsize.
This is the first non foiling multihull that I’ve thought of that MIGHT be able to compete with a retractable foiler…I think it would decimate any normally configured cat or tri in any condition… It has tremendous
speed potential especially in rougher conditions where foilers are weak but since it is so lite it will have standout performance in any condition more than likely.
PS-if this idea turns out to be any good I may try to get some patent protection but regardless any individual is welcome to try this concept if they want to.If you have questions just ask…
So basically what you have is a swichable proa? Where does the lee hull end up? won’t it end up dragging in the water? Also what happens in light air, does it sail as a proa with the float to windward or leward, or does it sail as a tri? Sounds like a nice idear anyway.
That is a neat idea, but I think you might have it backwards… The main hull with the winches, mast, etc is heavier than the floats, so you would want the main hull to be to windward so that the weight can be used to resist heeling moment.
If you slide the floats so that the leeward float is right next to the hull and the windward float is extended way out to windward, then you only have the weight of the windward float (very light, I presume) to react heeling moment. This will not provide much heeling resistance unless you are going to also add some sliding ballast that shifts out onto the windward float.
Since you aleady have quite a bit of weight in the main hull, I would try using that weight first and see how far it gets you when you put that out to windward. Obvoiusly, you would need to have a float that could resist the heeling moment, but since the moment arm would be twice as long, the load transfered to the leeward float would be half as much.
I’m not sure I see the geometry of how the system can be used to right a capsized boat. You may want to diagram that out a little better and make sure that the center of mass ends up on the right side of the center of flotation to cause the boat to come upright. I think it could be done, but you should diagram it out to make sure…
On a full size trimaran ultimate stability is gained by flying the main hull. But that is extremely risky and is more likely to end in capsize or pitchpole. In the SCRAM concept the weight of the two crossarms and the two outside hulls plus the battery provide the initial righting moment.As well, the boat could be sailed as Will suggests up to a point and that is that you don’t want to fly the main hull because it is more stable and therefore faster not to–at least theoretically.The other reason not to sail with the hull to leeward is wetted surface: by using the sliding cross arm for stability the outside hulls are only minimally in the water keeping drag low. The capability on the multione version would be there to add up to two pounds more ballast.
The lee hull remains clear of the water most of the time.
The righting moment of the boat is gained by the seperation of the weight of the windward hull from the boyancy of the leeward hull. If the leeward hull is the main hull, then you would need to have quite a bit of weight in the windward hull to give you adequate righting moment. If you use the main hull as the windward hull, then you already have a lot of weight in that hull from all the servos, the rig, etc. But you would need to make sure that you had adequate boyancy in the leeward hull (the float).
So Doug is suggesting something like a gybable proa with some sort of mass added to the windward hull. That hull would be minimal in terms of floatation. My question is how much mass would you have to add to that hull and would that eqtra mass result in a very heavy (and ultimately slower) boat.
My suggestion is to use the weight that you already have in terms of radio gear and rig and move that to windward. What doug is asking is whether the extra wetted surface area of having two big hulls in the water would create more drag and ultimately a slower boat.
Doug’s boat would perhaps look something like and international Canoe with a sliding hiking board (with some small floats on it for light wind). My boat would look like a catamaran with the rig mounted on the windward hull instead of in the middle of the cross beams…
Will
BTW Dick, when doug said “Ultimate Stability” I think he meant final stability or the limit of stability. In other words the limit of the stability of a Trimaran is when the main hull flies. You cannot gain more righting moment than that…
Ultimate stability= Maximum righting moment. Maximum righting momenton a cat is when the weather hull just clears the water and the crew or movable ballast are in there maximum windward position. Max RM in a tri is the same except referencing the MAIN hull.
Visualize the SCRAM concept as a “normal” tri whose cross arma slide across the main hull.
Tacking and gybing would happen as it normally would.
Flying the weather hull on an rc cat or flying the main hull on an rc tri WITHOUT a power ballast system is EXTREMELY tricky. A PBS offers more control and the SCRAM system offers the maximum control and power while virtually eliminating pitchpole. It seems like it might be able to stay with a foiler in rough conditions and would certainly seem to be superior to any “normal” configuration that doesn’t use movanle ballast. It would also seem to be superior to a movable ballast system similar to any I have used in the past on a multihull which are add ons: the SCRAM system incorporates the very structure of the boat as part of the movable ballast system.
The SCRAM system MAY be able to incorporate hydrofoils in some configuration–I haven’t gotten that far yet…
One thing seems clear to me:it offers the possibility of a very lite weight, highly powered ,extremely stable platform that would only require one extra Guyatt winch- keeping costs low–maybe even lower than a retractable foil boat.
Dick, in determining maximum righting moment you have to find the exact point that that occurs. On a cat the exact point it occurrs is when the windward hull just clears the water and is no longer displacing any water. This assumes the “crew” is max out and that the windward board is max up.
On a tri the “max point” is when the main hull just clears the water and is no longer displacing any water.
In other words if the cat was sitting level it has less righting moment than it does at the point the windward hull just clears the water.
In design of either type multi it is important to know what this angle is because it is your maximum power point meaning that you must(should) reduce power if the boat heels even a little further because the boat is now losing power to carry sail with each degree more heel.
On non foilers this angle is sometimes used to establigh the ama angle on a tri relative to vertical --the same for cat hulls sometimes with one theory being that at the maximum power point the hulls will have minimum wetted surface if they are angled correctly…(as well as having a symetrical waterline)