RC Boat and HM Help

[TimClark]

Hi, some friends and I have all been building our on RC catamarans and I was doing some math to figure out what sail size would be needed. I completed the RM and other items by the way that you used to find the necessary rack extensions for a Contender if I was to add an asym onto the boat.

This is where the problems come up: I was calculating the possible SA in a general wind speed of 10 knots. These are the following stats for the boat/hulls.

Max Beam: 2 feet
Hull Weight: 3 lbs
Length: 4 Feet

The following are the stats I got for the seperate sails:

4 Sq. Ft. Sail
RM: 3 ft-lbs
CE: .75 feet
CG: 1 Feet from CL
Sail Load(10 knots)-1.72 lbs

I multiplied 1.72 by .75 to get 1.29 ft-lbs. I then divided by the CG(which I assumed was 1 ft from CL because it is a catamaran)

9 Sq. Ft. Sail
RM: 3 ft-lbs
CE: 1/3 ft
CG: 1 Feet from CL
Sail Load(10 knots)-3.88 lbs

I multiplied 3.88 lbs by 1/3 ft to get 1.29, then divided this number again by 1.

How does the number that I get above compare to the RM of 3 ft-lbs. If the above number is less than 3 ft-lbs than I'm assuming the boat won't capsize, or is it in relation to the rig and sails? Any help or spotting in miscalculations would be greatly appreciated. Thanks.

Tim Clark

[messabout]

Looks to me like you are using the fore/aft location of the CE. You must use the vertical height of the CE to estimate the overturning moment. Nine square feet is a lot of sail for such a small RC boat, especially in 10KN of breeze. If you can make it work, the boat will surely go like a bat out of hell. I'D try some smaller sails and sneak up on the maximum that you can carry little by little.

Doug Lord can probably give you some good estimates of what the boat can carry without a lot of unneccesary calculations. Empirical data beats paper data most any time.

[TimClark]

I know it is a huge amount of sail for an RC boat, but I was just trying to figure out what the hell was going on. I was getting the same final number for the 9 ft sail as the 4 ft sail.

Also, for a catamaran would the CG always be on the windward hull? For example, the boat has 2 feet of beam, would the CG be at 1 ft from CL?

Tim, I don't get your "dividing by the CG"? For your purposes the distance between the Center of Lateral Pressure-about 1/3 down the daggerboard under the hull- and the CE is the Heeling Moment arm. Multiplying that by sail pressure will give you a close approximation of HM(heeling moment).
Measure off your plans.
As I said earlier you should plan on at least two rigs if not more-race boats have at least three rigs and some(in classes that allow it) will have 5 rigs-or you can have a reefable rig.
Th Formula 48 class LOA 48" Beam 48"(MAX) allows a maximum of 1396 sq.in. of sail but on a cat w/o movable ballast you'll probably be limited to around 7-8 knots with the full rig.
A good beam on a 48" cat is around 33" or even a little more. RC tri's almost always outperform
RC Cats especially in lite air. Cats smaller than around 48" can be very difficult to control in any wind because they've lost a whole lot of pitch stability in the act of being scaled down and will pitchpole in the blink of an eye.
Cats are ideal candidates for movable ballast systems but very few people have even tried this option-probably because it can add $400 to the cost.

[TimClark]

Doug, if I did a trimaran, how would you go about building a 48 inch trimaran with unlimited beam and sail area?

I would think a 48 inch main hull with a beam of around 2 inches and the 2 "floats" being about 40-44 inches and a little skinnier? Would the rudders come off of the main hull or the secondary hulls? I'm totally new to the trimaran side of things so this would be wierd for the design aspect to me, but if I could get more speed out of it, then I might as well do a tri.

[TimClark]

I redid the math for the HM after I saw what you posted above. The distance between the CE and the CLR is about 1.45 feet. For the HM in 10 knots I got 2.494 ft-lbs for the 4 sq. ft. sail. Thanks Doug for sorting everything out. I think I am just going to do a catamaran but probably add another 10-12 inches of beam on to the boat. Thanks for all the help.

TC