RC Boat Advice

Scheer · March 8, 2018

I have two RC boat projects that have been on my to-do list for years now and I really need to get them done. I used to tinker with RC cars years ago so I've got the RC basic's down, but I'm clueless when it comes to the boat specific stuff.

The first project is to convert a Miss Budweiser boat from a nitro engine over to brushless. My grandpa bought this for me pry 20 years ago to run on the lake at his cabin, and now my 8 year old son goes up to the cabin with them every now and then so I'd like to make it easier to use as I find nitro a PITA to deal with and want to make it as simple as possible. The plan is to epoxy a mount like this one into the hull to match up to the existing propshaft, throw in a brushless motor and esc that have water cooling (the water pick up and lines are already in the boat) and upgrade it to a 2.4Ghz TX/RX. I've done some reading on motor KV and prop sizes, but its a little over my head so the plan is to go with a ~2300kv motor and just buy a ton of props until I find one that works best. Really all I'm looking for here is a sanity check to make sure I'm not missing something obvious as I think this one will be easy.

The boat is boxed up in the attic right now, but I did find a 10ish year old photo of it:

The second project is also going up to our cabin, but this one is for me. I want to build a large somewhat replica of a pontoon boat to use for a platform to light fireworks off of. We've started a small tradition with several of the groups who go down to the cabins of hot gluing fireworks together on styrofoam cooler lids, and then someone takes them out in a small jon boat, and lights them out on the lake.

Currently my plan for this one is to use 3-4" PVC pipe with endcaps as the pontoons, then build a sealed frame between the pontoons for the RC electronics, and the system to remotely light the fireworks. The top will just be a flat platform with a few sets of connectors for hooking up the electric matches. I've found 1/4", 1/2", and 3/4" sheets of PVC locally that I can use for building the frame to keep everything waterproof and hopefully long lasting, and plan to make the top 2' wide and 3' long so it is plenty stable. Now for the questions:

Will flat ended PVC end caps cause too much drag for this thing to move in the water? It doesn't need to be fast, so long as we can drive it 50' off the shore and back in a minute or two.

Do I need to worry about cooling the motor and ESC if its only going to be used for short periods of time?

With it being a large block essentially, am I better off with a single prop and rudder, or is it better to do dual prop? If I go dual prop, do I even need rudders or can I essentially "skid steer" it?

Thanks in advance!

straight_stewie · March 8, 2018

2 hours ago, Scheer said:

The second project is also going up to our cabin, but this one is for me. I want to build a large somewhat replica of a pontoon boat to use for a platform to light fireworks off of. We've started a small tradition with several of the groups who go down to the cabins of hot gluing fireworks together on styrofoam cooler lids, and then someone takes them out in a small jon boat, and lights them out on the lake.

Currently my plan for this one is to use 3-4" PVC pipe with endcaps as the pontoons, then build a sealed frame between the pontoons for the RC electronics, and the system to remotely light the fireworks. The top will just be a flat platform with a few sets of connectors for hooking up the electric matches. I've found 1/4", 1/2", and 3/4" sheets of PVC locally that I can use for building the frame to keep everything waterproof and hopefully long lasting, and plan to make the top 2' wide and 3' long so it is plenty stable. Now for the questions:

Will flat ended PVC end caps cause too much drag for this thing to move in the water? It doesn't need to be fast, so long as we can drive it 50' off the shore and back in a minute or two.

Do I need to worry about cooling the motor and ESC if its only going to be used for short periods of time?

With it being a large block essentially, am I better off with a single prop and rudder, or is it better to do dual prop? If I go dual prop, do I even need rudders or can I essentially "skid steer" it?

I don't know much about electric RC. I'm a nitro guy myself, so I'll skip over that portion of the post.

However, building catamarans (real catamarans) is something that I do have experience with. Of course, mine are always wind powered, and hold people, but I think the lessons apply.

The first thing you want to do is write down this equation: Buoyant Force (Pounds) = Pounds of water displaced. So, if you know the outer diameter and length of your pontoon, and it's dry weight, buoyant force becomes B_lbs= V_{cubic feet}* 62.42796 - W_lbs. Where V is the outer volume of the sphere in cubic feet, 62.42796 is how many pounds a cubic foot of water weighs, and W is the dry weight of your pontoon in pounds. This equation tells you how much weight, in pounds, a pontoon can support.

Knowing the above, there are two approaches you can take:

You can design your pontoons, and then limit the design of your boat to however much your pontoons can support. Keep in mind the weight of any payload you will need to carry.
You can design your boat, and then design your pontoons to support that weight. This is my preferred method of doing it, although it may require multiple design iterations to zero in on the perfect motor/pontoon combination. If you don't care about perfection, then forget about that part.

I would recommend against a flat front face that is perpendicular to the oncoming water. This is because a flat front end will cause the pontoons to dive under forward movement. It's a common problem in river barges, and it's why you almost never see a pushboat without a barge in front of it: Many a real pushboat has been sunk because it had a flat front end.