"Super" Cobra

[pmisuinas]

So when surfing the web looking for outrigger plans, I came across this boat--the Cobra. Link is

http://www.john-tom.com/RCPowerBoats/RCPowerBoat.html (scroll down to see it).

Given my model airplane background, I couldn't resist, so away I went. I must say boats do seem to build faster than aircraft.

Anyway, it was designed as a catamaran but without a step in the hulls. Overall size is 35" long plus a couple of inches for the rudder and 15" wide at the widest point. It was designed for engines, so rpms would be limited to the 10-12K range, depending on the engine. Seemed like a good candidate for an electric to me, as I can use a high Kv outrunner on 5 or 6S with a small diameter high pitch prop--which should really make it move.

Given the beam, I felt it looked more like a hydro than a cat, so proceeded to make it more like one. I added a step, 1.5 inches in front of the CG, and also put a mini-sponson in the back, a la a rigger. I felt this should make the boat get up and go (pun intended). Given I am adding in what I expect will be more power via an outrunner, I decided to rename it to "Super Cobra" Besides, if it doesn't run well, I can always convert the hull back to the original no-step cat design--but I don't think I will have to.

However, as I am not really a boat guy, I do have a couple of questions for input.

As shown in the picture, the boat has an air rudder that extends into the water. Should I maintain this, or maybe just go with a water rudder only (King of Shaves style rudder, maybe? or something else?) I think I at least want to keep the original if only for the looks, but if there is info that says otherwise, I do want to hear it.

The other picture shows a center mounted turn fin. Again, should I keep this? And since I expect performance to be jazzed up a bit over the original, should it be deeper? Plans show a fin that extends 1 3/8" below the hull. Again, my lack of FE experience shows, as I don't have a clue about turn fin size and depth.

Appreciate any input and opinions--and when I get it on the water, will be sure to mount a GPS and get a speed. Thanks…Pete M

[Ub Hauled]

seems like an interesting project, keep us posted.

[pmisuinas]

So ordered the motor for this, I am getting a Scorpion HK 3026-1400 (max 80 amps) which should allow me to use a prop that will be well over 100mph pitch speed. The helicopter motors tend to be a bit more robust and have better built in cooling vents. Will be interesting to see what speed on the water the pitch speed translates into...Pete M

[Ub Hauled]

Pete, I don't want to burst your bubble, but don't count your chicken... it will probably will not reach 100mph with that motor... it actually may be to weak of a motor to push 35" of hull...

[pmisuinas]

Oh, I agree completely with that--it's only pitch speed that will be over 100mph. Efficiency losses, hull drag, etc. all mean the boat will never go that fast--but it also should do a litte better than the 40-45 mph other airboats manage...Pete M

[pmisuinas]

So I am gathering parts and proceeding with building. Decided to go with a water and air rudder, they will not be connected but will move together. Have the cowl pretty much done. Engine mount is done except for waterproofing. I am waiting on my radio system to arrive so I can have it to set up servo and movement. I will be using a CC HV-85 ESC (because I have one) or may end up using a Turnigy 120--will wait and see what current draw will be. I will use a small fan to blow over the ESC since it will be enclosed. Tubing simulating engine exhaust stacks will be the exit for the air flow. Power will be 3S or 4S, again depending on current draw, but don't expect to go beyond that. I have room for a 9" prop but don't think I will end up using one that big--again, at the risk of sounding like a broken record, will depend on current draw.

Should not be too long before the fiberglass goes on. Two layers for the bottom, one layer elsewhere...

[pmisuinas]

Wow, doesn't take long for a thread here to get buried, does it Anyway, have the rudder servo installation done and rails/velcro for the battery packs. I assembled it all to find out where I will need to put the packs to meet the CG, and also cut the top to allow access to same area. The front section that has the motor will be glued down, the last two thirds will be the hatch. Only assembly I have left to do is the centerline turn fin, and I have to add a couple of air intake scoops to the cowl (the exhaust tubes are functional and will be the air exit points). As noted, I am not going to install an air rudder, want to see how the standard water rudder will work, if it won't turn well I can always put it in later--it won't be hard to do.

I'm hoping to find time to fiberglass it over the long weekend, but a lot of family stuff going on so it will likely not be until Monday. Getting there...

[pmisuinas]

it actually may be to weak of a motor to push 35" of hull...

I guess I don't understand your comment since the motor won't be pushing the hull--it will be pulling it. And as previously noted, I realize the boat will reach 100mph--but the pitch speed of the prop should be over that (1400kv * 14v * .8 efficiency * 10 inch pitch / 12 inch per foot / 5280 ft per mile * 60 min per hour * .8 efficiency = 118 mph pitch speed). This of course depends on the current draw not exceeding the motor/ESC capability, otherwise will have to cut down the diameter/pitch of the prop.

Again, my expectation/hope is it will do better than the commercial air boats that are on the market...