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Post by Doug Lord on Jan 31, 2004 16:37:36 GMT -5
What do people think is the proper bem to length ratio for the main hull of a tri and its amas? How about for a cat?
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Post by isammis on Jan 31, 2004 23:41:04 GMT -5
Although I have never actually sailed a Trimaran, I have designed and built two different catamarans. One was 52" long and 36" wide, and the other was 48" long and 32" wide. To tell you the truth I never did find the right combination. I really think a catamaran could use a moveable ballast. I have a F48 in the works and it is 48" x 48". Since two hulls should only be in the water at a time and the distance between hulls is only 24" I don't believe tacking will be a problem. Also I don't think a moveable ballast will be needed until the very upper wind speeds since I will have a wider boat to counter the forces exerted. Infact these are the reasons why i started getting interested in RC tri's. It just made more sense to me that a RC tri should perform better than a cat based on these basic principals. How wide was that first red tri(sorry can't remember the name) that is shown in your video on your website?
Ian
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Post by Doug Lord on Jan 31, 2004 23:54:54 GMT -5
Ian, the F3 is 71" wide foil tip to foil tip and 56"LOA. The PBS equipped D4Z is 48" wide but the overall width of the two hulls together is 33". I think there is huge room to experiment on "normal" cats and tris with twin foils ala CBTF that would eliminate the daggerboards entirely and dramatically improve tacking and upwind performance. Though the idea eliminates (probably) the retractability you could have with a daggerboard it would permit the maximum beam (with no tacking problems) that is ,unfortunately ,limited to 48"...
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peter
New Member
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Post by peter on Feb 3, 2004 5:55:38 GMT -5
I found that with the mini40 cat that I had, the beam was as wide as you would want to go to get the boat to still tack reasonable quickly. It was 33" wide, and had underslung rudders. The steering was set up so that the windward hull had a tighter turn than the leeward hull.
I don't think it is worth while building cat's at 48" wide as they are nearly impossible to tack at any decent speed.
The main thing to work on when designing your boat is to get the (rocker) correct.
The tri's need to be as wide as possible (48") to help with stability. What we are working on over here is to have boats (tri's) that sail on one hull for controlled periods in 5 knots and over.
We are achieving this on a regular basis, the only thing stopping it is the inconsistant winds.
We have all gone away from the cat's as it is (without using) movable ballast or similar to hard to carry full rigs and match the tri's over a series of racing.
Hope this information helps someone.
Peter
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Post by Doug Lord on Feb 3, 2004 10:14:25 GMT -5
I think if a cat is designed to carry a max of 3lbs. movable ballast so that it floats with overhangs when the ballast is removed in light air that it could compete with a tri in those conditions and blow it away in stronger wind.The critical thing is that the wetted surface of the cat is reduced with the ballast removed and that with the ballast added and flying a hull-only then does the cat float right at it's designed waterline -not below it as so many cats do when flying. If the boat used twin movable foils instead of conventonal rudder and daggerboard tacking would be equal to or faster than a tri with a "normal" foil configuration. The cats foil areas have to be designed so that in light air the two forward and two aft don't total much if any more than the tri's two foils. Tris flying the main hull will probably need a rudder on the amas so that part could balance out. None of these options can compete with a properly designed retractable foiler that starts foiling in 5 mph and up.But the cat with movable ballast and the right hull design has great potential.
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Post by MultiFast Is MultiFun on Feb 3, 2004 11:29:30 GMT -5
A classic example of "why" all big boat ideas don't fit small r/c boats!
Most beach cats sail at their designated waterlines in light air, but below the waterline when heeled and on one hull. The designer simply must come up with an "average" displacement. Unlike Doug's suggestion, it is not possible to change the weight on a full sized beach cat by nearly 30% or more. Thus the cat is designed for light air performance and in heavy air added weight (crew) to keep flat, or heeled to one hull also changes the design waterline.
If you take an r/c cat, I (personally) feel it needs to be designed for the normal expected racing conditions in the local venue where it will be sailed most often. With the r/c cat, you have the option of removing the ballast completely for light air - something that isn't possible on a real cat. 3 lbs. of weight on a cat represents darn near 50% of the weight of the total boat without ballast. So do you design the boat to float at the added weight WITH ballast, or do you design it to float at the unballasted weight? And the second part - is regardless of which choice, do you design for one-hull (heeled) sailing or both hulls in the water?
It is recognized by those in Europe having sailed the Mini40's that the top rig is almost too much. Yet, the top rig on the NIGHTMARE when Mark owned it seemed fine for Arizona winds. Again, as a personal opionion, I would design the boat to float flat with top rig, and begin stepping down sail area yet retaining a light platform. If you design/build for maximum weight and maximum winds, light air performance will suffer, as you are only removing 3 lbs. of weight. The only concern would be having enough hull volume to support the boat on one hull. Then again, based on the current design thinking, it looks like the big boats are now being designed to almost get the entire leeward hull UNDER the water - or darn close to it, allowing it to penetrate waves rather than riding up and over them.
The issue of tacking a cat, could be addressed by using a jib that can be held via second servo to the new windward side of the boat, allowing the jib to push the bows over. Once across, the jib is allowed to move to leeward and re-trimmed. Does require an additional channel.
I also think, the mass of a cat is much less than a trimaran, and that too inhibits the cat from carrying itself through a tack due to it's light weight and little if any momentum. Again, this is proven by monohulls where some weighing a bit more will perform better in light air because they can carry through the tack with their added weight (mass). I also haven't seen any r/c multihulls rigged with a uni-rig (main only) which is inherently a design that creates weather helm. If the boat is "pulled" toward the wind, falling off and sheeting out on the main at the appropriate time could also help in tacking. Finally, tacking problems are also related to jaming the rudders over hard during a tack like a monohull, which simply will stall out the boat - especially a double rudder cat. A slow arc is required for a tack - not a fast spin-on-the-board like one would do after sialing monohulls.
This means a designer must also make the call - acceleration in puffs in light air and downwind, or upwind stability in heavy ar and mass to carry through a tack? I think in most cases, they try to find the happy medium and shoot for performance in those conditions.
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peter
New Member
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Post by peter on Feb 4, 2004 5:36:54 GMT -5
The cat that I sailed had double rudders and after setting up the "throw" properly I had no problem tacking. The thing that I think is wrong to do with a cat is to build them at maximum beam. One of the guys that sailed with us tried this. The boat was a bullet in a straight line but lost out big time when it had to tack. Hull volume makes a big difference in both classes,(cats and tri's) but we have found that hulls that sit "in" the water and not "on" it are the way to go.
We are of the opinion that we are very close to the ideal hull shape and configuration, the thing for us to work on now is sail design.
But the big thing to remember when sailing r/c boats whatever the class, monohull or multihull, you can have the best design in boat hull(s) and sails but "luck" stills plays the biggest part. I say this because all you need is to miss a wind shift and you get out of sync. Your "luck" just isn't in.
Peter
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Post by MultiFast Is MultiFun on Feb 4, 2004 13:37:32 GMT -5
Peter -
how about amount of rocker in hulls?
Hard to tell from web photos, but if you take a side view of the hulls (floats or cat hulls) and draw a line from boat to stern at the point of waterline (or at point where transom turns into the bottom of the hull), about how much rocker are your guys using? ie: measure from string line down to maximum depth of the hull rocker - and at what point (from the bow, perhaps) is the maximum rocker located?
Kind of interested to see how mine match up with yours. Will try to measure mine tonite to share with you.
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peter
New Member
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Post by peter on Feb 4, 2004 20:02:14 GMT -5
The rocker we are using on our boats is around 15mm [5/8"] on all hulls whether cat's or tri's.
Will be interested to compare what you are using.
It seems to me that by the replies coming in on the other forum that the sail material that is being used around the world on multi's is quite a bit heavier than what we are using here. We have found that 50 microns is about right for light airs and 75 microns is good for the heavier winds, at least over here that is.
What do you use?
The material that we are using is becoming very scarce and we need to find something else.
Peter
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