current cataraman beams


OK, you have my interest. What is the “current” length/beam ratio for fast cataramans?
If I think of a mini40 trimaran the length/beam is basically the same, but looking (briefly) at the two hull version the beam seems narrower. Am I right?


Yes Jon -

Like it or not, I try to use a scale of full size boats for relative sizes when possible. It may not meet the “mathematical” reasoning, but it works for my “eye”. Angus has another thread going about scaling and looks.

I will try to find the technical article that I have somewhere that discusses the beam of beach cats. Yes, they have moveable ballast, but the writer (designer of the Supercat) shows a relationship between narrow and wide beams. Somewhere there is a point when the narrow beam of a cat (2 hulls) that has sideways tipping tendency changes as beam gets wider until the boat has great sideways tipping resistance, but then suffers from forward tripping (pitchpole). Trying to get that balance between tripping and tipping correct is what is hard.

Most modern beach cats are hampered by pre-defined dimensional restrictions imposed by class rules. Thus we have a development class like the “A” Class that is 18 feet x 7.5 feet (5.5 x 2.3 mtr). We have the Tornado with dimensions 20 foot x 10 foot, (6.0 x 3.0 mtr) the Supercat (Now called the AC20 I think) that was 20 feet x 12 feet (6.0 x 3.7 mtr). Many class rules - development or not - were simply selected due to the legal size of road and highway load limits ! How’s that for a design reason? Of the above “wide” platforms - the Tornado is broken down - or trailered “tilted”. The Supercat had telescoping beams. Both so they can be trailered. My 18 Square Meter class (developmental) only had a length restriction. This allowed designer/builders to test theories with the majority of boats ending in the 11-12 foot (3.1 - 3.7 mtr.) wide range. Of course the “C” Class wound up (by rules) at 23 feet x 14 feet (7.0 x 4.27 mtr). An aberition, but it does sail, is the MicroSAIL “foiler” which is nearly 72 inches (1.82 mtr) wide and less than 50 inches (1.27 mtr) in length. The builder/designer (Doug Lord) indicates the wide stance is a benefit for anything riding foils for stability, with the length being somewhat secondary and having an adjustable “T” Foil on the rudder for 3-point foiling. I have sailed one of his boats, and while the speed when foiling is impressive, two factors contribute to it’s overall success… 1) Weeds and 2) low winds. If it can’t foil, it has lost almost total advantage of the foils. It might be a hoot in strong breezes, but water ponds small in size and choked with weeds quickly show the problems associated with “anything” dragging below the water. Also noticable is the drag of the foils in light winds where foiling isn’t possible. Just a sidenote since his beams are significantly wider than his boat’s length.

I think most current/recent builders of multihulls take it to the extreme and if 48 inches (1.2 mtr) is the limit, then they go that wide. For a Mini40 cat, I think I would stay somewhere around 38 inches (.96 mtr) as maximum beam - or slightly less and deal with tipping in gusts. For a trimaran, that width (total) might be adjusted upwards but not over 45-46 inches (1.14 - 1.16 mtr). One has to remember that when tacking, a single hull pushed sideways through a tack is less resistance than multiple hulls - and being able to cross the eye of the wind quickly and easily is an answer to keep boat from getting stalled in “irons” while tacking. A “slow thumb” is also beneficial - carving turns while tacking instead of “slamming” a tack common in monohulls.

By staying at the narrow dimensions for total platform beam, tacking may be improved, tripping is reduced (not eliminated) and only sideways tipping is encouraged.

So where would I go for a 48" (1.2 mtr) long multihull for a beam? For a cat I would stay at/under 35" (.889 mtr) and for a tri probably would stay at/under 42" (1.066 mtr.) as a starting point. A true ratio from big to little using the 18 Square “real” sizes" would calculate to a beam for a cat of 29 inches (.736 mtr). It is because of lack of moveable ballast - skipper and/or crew - that I would increase this a bit for the cat. Since the tri is primarily a monohull without lead - I would build to suit “most” local wind conditions. Wide with floats well off the water for light winds and narrower and floats closer to water for stronger winds. Again - a lot of guess work since there are so few of these racing to get accurate information on performance.

Dick -

can you provide a link to this topic?

OK - it’s on “another” forum for r/c boats. Called “Elastic Tape Measure” - should be able to get to it from here:

Sorry - thought it was on this forum.