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Old Jun 29, 2014, 06:32 PM
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Need help choosing hardware. 25" WildThing - first boat

I started on my first RC boat scratch build a few days ago and I am to the point where I need to mount my hardware. It is a 24.5" WildThing constructed from balsa that will eventually be fiberglassed.
I am building this to try out RC boating, so I want it to be somewhat fast but it doesn't need to be a champion racer. I am thinking about 30-40mph. I also don't want to dump a ton of money into the electronics.
I plan to get this HobbyKing 50A watercooled ESC. When it comes to the motor, I don't know where to start! This Turnigy AquaStar 3520 looks like it'd be good, but I don't know what I should be looking for as far as KV goes. NOTE: I have lots of experience with electric planes so I know the basics but I don't know how to choose a boat motor and boat prop.

Any recommendations for a 50A motor and prop setup for a 25" V hull?
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Old Jul 02, 2014, 12:00 PM
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Very nice hull, do you have any plans for it that you could send me?
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Old Jul 02, 2014, 01:43 PM
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I have the plans saved on my computer and I'm away now but I can access them later for ya. You can buy the official plans online if you search for Wild Thing plans. The plans I use were made from a CAD model but they worked well. It's a pretty easy and fun build! I cut out all the pieces from about $15 of balsa sheets.
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Old Jul 02, 2014, 01:44 PM
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Do you have experience with RC boats and could help with my motor situation? Does any one have any tips picking a motor and prop?
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Old Jul 02, 2014, 01:56 PM
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I have experience but its the kind of experience that would only allow me to tell you what not to buy haha. A set up i tried was a 5900Kv motor on 3S with a 160A ESC. It was way too high RPM and just broke everything and in the end everything would just overheat after 10 seconds. You should generally keep your RPM at 30000. The motor you were looking at is suggested 4S which would put it at about 25000RPM which is a decent speed. For a propeller for that motor i'd say a diameter of 30mm and 2 or 3 blades. I'd get a double check on my information though. Hope it helped a little bit at least =)

EDIT:
To make sense of the KV you take the KV rating which in the case of that motor is 1700. You then multiply that by the voltage of the battery which with a 4S is 14.8V. This puts the RPM at 1700 x 14.8 = 25160. This is the RPM without load though and will be lower when propped.
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Old Jul 02, 2014, 02:32 PM
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Hi,

I would suggest a 2-3s setup

2s = 3300-3700kv motor
3s = 2500-2800kv motor

I would go for a setup like this:
Turnigy 60a ESC http://www.hobbyking.com/hobbyking/s..._Boat_ESC.html
Motor: http://www.hobbyking.com/hobbyking/s..._Inrunner.html
Or
http://www.hobbyking.com/hobbyking/s...ner_Motor.html

One thing im unsure about, is if the hull can handle 30-40mph or not
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Old Jul 02, 2014, 03:00 PM
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For a boat that length you want the prop 2.5" from the transom,
On mine which I ran on a .15 nitro engine the centre of the stuffing tube was 1/4" from the bottom, no offset. If I put that boat back on the water I think I'd raise the stuffing tube 1/8".
Used a cheap Chinese 3-3/4" long rudder worked good enough.
The turnfin I used was 3/4" wide x 1" exposed beneath the mount, 90 degrees to the bottom and as far out as you can mount it. I only used the one on the right side as I really only turn right!
The prop I was running on a .15 converted rc car engine was a x440 with the tongue removed and a little bit of a back cut.
I have a Lehner basic XL which is a 3650 motor 4200 kv I've been toying with installing, probably run a pair of 2s 5000mah packs in parallel. As you know as voltage goes down amp draw goes up. I'll use a 180 or 200 amp esc if I try it, I know exactly what amperage my SV 27 pulls on 4s, If I double those numbers it puts me too close to the limits of a 120 amp esc for sustained running. For the price difference it's not worth a stinky boat. (My 29" P-spec mono race boat reeks from a blown up esc!)
I think I'll be pulling 90-100 amps solid, spikes will be 150-160 amps. That's pretty much double the SV's numbers. It's a little smaller at 25" but the design of the Wildthing can handle much more speed than the SV hull, I ran it a number of times against the electric SV's and it was faster and handled much better. I can probably run a bigger prop than I did on nitro but the boat will weigh more.
HMMM all I need is a 1/8-.150 collet, motor mount and an esc.......
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Old Jul 02, 2014, 03:00 PM
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Thanks guys. Bostong, I was actually just looking at that second motor there. I have lots of 3s batteries already and it is already hooked up for cooling (not sure how necessary the air circulation is though). I'll probably go with that motor and HK's 50A boat esc... I'm sure it won't live up to the quality of other ESCs but it's half the price.
What factors would prevent the hull from handling 30-40mph? Strength or shape/design?
I've never had an RC boat so I don't know exactly what 40mph feels like so I guess I am aiming for it to just be relatively "fast" and impress me :P
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Old Jul 02, 2014, 03:06 PM
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Sibe, thanks for the info about prop placement! I hadn't put much thought into that. Just curious about why 2.5" from the transom. Does that give the most efficiency? And then is that subsurface drive or surface drive?
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Old Jul 02, 2014, 03:08 PM
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Quote:
Originally Posted by Ham Sammiches View Post
Thanks guys. Bostong, I was actually just looking at that second motor there. I have lots of 3s batteries already and it is already hooked up for cooling (not sure how necessary the air circulation is though). I'll probably go with that motor and HK's 50A boat esc... I'm sure it won't live up to the quality of other ESCs but it's half the price.
What factors would prevent the hull from handling 30-40mph? Strength or shape/design?
I've never had an RC boat so I don't know exactly what 40mph feels like so I guess I am aiming for it to just be relatively "fast" and impress me :P
I have never seen that hull in action (video) and not sure if its strong enough - design looks pretty solid though

Maybe others with more scratch build experience can chip in

Saving 20 bucs on ESC might not be worth it
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Old Jul 02, 2014, 03:12 PM
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Quote:
Originally Posted by Ham Sammiches View Post
Sibe, thanks for the info about prop placement! I hadn't put much thought into that. Just curious about why 2.5" from the transom. Does that give the most efficiency? And then is that subsurface drive or surface drive?
Rule of thumb, if I remember correctly, that it should be about 10% of hull length sticking out the back (use a strut or stinger drive)
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Old Jul 02, 2014, 03:17 PM
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Originally Posted by bostong View Post
I have never seen that hull in action (video) and not sure if its strong enough - design looks pretty solid though

Maybe others with more scratch build experience can chip in

Saving 20 bucs on ESC might not be worth it
I will also fiberglass and epoxy it so I think it should hold up. I guess we'll see, I might learn a lesson in boat building

That motor says 40 amps and the ESC is 50 so I feel safe there but better quality is always better... If the motor is water cooled, can I run more amps through it than they suggest or is that figured in?
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Old Jul 02, 2014, 03:17 PM
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Hi Ham,

For a fast electric, you aim at 30.000 rpm under load and choose your Voltage and motor accordingly.

Bostong made some good suggestions, getting you in the low rpm segment of the fast boats (25000 rpm under load).
This in turn needs a rather large prop to reach your target speed, where 40 mph is doable, but will need some experience in setting up and dialing in a boat, not to mention light chop at the most to avoid nasty flips.

A big prop has a lot of propwalk, which needs to be counteracted by installing the drivetrain offcenter to the right by 4-5 mm.

As it's your first fast boat, I'd settle for somewhat less topspeed, which greatly improves the handling.

While you're at it, consider building your Wildthing with a floodchamber.
You have no idea how relaxing it is to be able to run your boat full throttle and not worry about what to do when you flip it.
With a foodchamber it'll selfright.

If you go for the second motor, the watercooled outrunner, keep in mind a 50A ESC is too small, the spikes getting out of the hole and accelerating out of a corner will be often more than twice the stated max Amp.

Regards, Jan.
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Old Jul 02, 2014, 03:39 PM
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Originally Posted by pompebled View Post
Hi Ham,

For a fast electric, you aim at 30.000 rpm under load and choose your Voltage and motor accordingly.

Bostong made some good suggestions, getting you in the low rpm segment of the fast boats (25000 rpm under load).
This in turn needs a rather large prop to reach your target speed, where 40 mph is doable, but will need some experience in setting up and dialing in a boat, not to mention light chop at the most to avoid nasty flips.

A big prop has a lot of propwalk, which needs to be counteracted by installing the drivetrain offcenter to the right by 4-5 mm.

As it's your first fast boat, I'd settle for somewhat less topspeed, which greatly improves the handling.

While you're at it, consider building your Wildthing with a floodchamber.
You have no idea how relaxing it is to be able to run your boat full throttle and not worry about what to do when you flip it.
With a foodchamber it'll selfright.

If you go for the second motor, the watercooled outrunner, keep in mind a 50A ESC is too small, the spikes getting out of the hole and accelerating out of a corner will be often more than twice the stated max Amp.

Regards, Jan.
Lots of helpful info here, so the watercooled 2842, is that a good motor for a beginner boat? I'm fine with less than 40mph. I can tune it up as I become more experienced. I didn't know the currents jumps that much when accelerating. Looks like I'll go with your 60A suggestion bostong It has such a high burst current rating (380A. Whoa.) I should be fine with that ESC right?

I thought I would skip the floodchamber at first but I am doing this boat as a fun challenge so I think I better give it a shot. You're right, it'll also make it more relaxing to run.

Do you have any prop suggestions Pompebled?
Also wondering what size rudder I need
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Old Jul 02, 2014, 04:09 PM
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If it was me, I would do a setup kinda like this, only leaving out the trim tabs. (Only the outside hardware)

http://kintecracing.com/Prince_Upgrades.html

You can find the rudder under the "hardware" section - its the 3 inch rudder.
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