hemiblas

Ok I'm building an electric Aquacraft hammer boat. I wripped out all of the stock electronics and I have added:

12C 3600 mah 2S lipo battery 7.4Volts with deans connector
Feigao 8L motor with 3072 rpm/v brushless motor with cheezy stock watercooling that came with boat
120A brushless hextronik esc with the aquacraft cooling jacket
Aquacraft supervee standard 3 blade 40mm prop

Does anyone see anything wrong with this setup? Should it be a safe first run? Am I going to overheat the speed control or motor?

I'm calculating somewhere around 22k rpm for the submerged prop. Any input is welcome. Thanks.

kyrus

Can't comment too much on the setup, but I'm really considering the same. Anixous to see how it works for you...

CadillacDTS2001

i think your prop will be the most problematic part of this.
the prop is to big. You should try a small prop like a octura series X427-X432 for starters, and work your way up. you don't want to over amp it and burn it up.
I personally would start with those props mentioned

also with your esc, the motor will cog on start up probably quite a bit.

hemiblas

I have been working on it this past week and its all put together and ready to go. I put one plate under the motor and the other plate over the motor to get less of a drive angle between the motor and shaft. Other than that I am keeping the stock radio. I have tested it out of the water and the prop spins just fine. I am anxious to try it out. I have been playing around with brushless motors for a while and it seems like it should work just fine. My only concern is that the prop seems a bit much for this boat. I was considering using the 37mm prop in the nitro supervee instead. It should fit better but I have to wait till I put in another order as my LHS doesnt sell these.

Wheelnut

My Feedback: (81)

What about prop angle? Are you going to keep it a sub surface drive? It will hop in and out of the water like a porpose.

hemiblas

Yeah you are right on the cogging. I bought the esc cause it was cheap and it coggs quite a bit at startup. On an 8s motor its really bad. Seems like the larger the motor the less the cogging. Not sure why though.
Ya I am concerned about the prop size. It does seem a bit large for a sub-surface drive. The other one I own is very small though and I know it would work with that one. I may try a couple of short runs and check temps as I go. I will look into the prop you mentioned and see if I can pick one up before my first run. Thanks for the advice.
So do subsurface drives just not work for speed? I agree it should jump out of the water with the prop angle, but will it go fast or am I just wasting my time. I left it the same cause I didnt want to change the shaft.

Wheelnut

My Feedback: (81)

If you decide to go surface drive, you have some work ahead of you. There is a guy on RCU that has done a surface drive conversion on a EP Hammer successfully.
Here is what happened to me with a kyosho sunstorm 600

http://www.youtube.com/watch?v=vI9Ics-gajs

hemiblas

Man its a small world. I saw your video a couple months ago when I was searching for brushless boats on youtube. I think I have a good idea of what will happen now. Looks like I am prob going to try it anyway and see what happens. At worst I think I will toast the speed control.

speed3-inactive

That setup should not pull more than 65amps so the speed control should be fine but the batteries I think you should be concern about.

hemiblas

Are there any calculators ou there where I could try and figure out how many amps I would be pulling, top speed etc. I was thinking it would be close to 30A so I figured the battery would be ok, but thats a good point. I would hate to blow up my nice new lipo.

speed3-inactive

I have done static test with a grauper k42 fully submerge with a grauper 700bb 8.4 on 6cells and 7cells. Basically the results indicate that the increase in prop speed is directly proportional to increase torque. I tested the kv, rm and no load current on the 700bb before I used it for the test. On 6 cell on pulled 11.4amps on 7cells it pulled 13.2amps. Subtracting the no load current from what it pulled give me a mulitplier to multiply the torque constant by. Using the motor resistance times amp draw I got an estimated voltage loss. In short for the k42 prop to turn at 8000rpm it needed a torque of 8.11 in.oz/A. Therefore to turn the same prop at 16000rpm it would need double of what it produce at 8000rpm. The k42 has 2.31 inches of pitch so I figured that would be close to the aquacraft prop in loading charteristics. You said your motor has a kv of 3072rpm/v that means the torque constant is .441 in.oz/A (1355/KV). At 7.4volts your motor should rev anywhere in the 22500rpm range without load. Let say your motor needed to turn the aquacraft prop at 18000rpm to run effiently then it likely your motor also needs to produce somewhere around 20 in.oz/A to give that rev. At 20 in.oz/A your motor will draw close to 50amps depending on the noload current (load useful current + no load current).

hemiblas

Cool. Thanks. 50A is a bit much for my battery to handle. I think I am going to try this thing with my 4 cell ib4200 stick pack I have laying around. I will probably take temps of the batts, motor and esc at the end of the run to see where I am at. I am probably going to run this thing next weekend. I will post my results. Even at 4.8V the prop seems to turn pretty fast in free air.

headhunter23

You can also try flipping the prop to the reef racer, I did that because I have a similar setup, the motor seems to run just fine at 6 cells nimh. Plus its cheap to try, its slightly smaller than the original hammer prop.

Ivan.

hemiblas

I did not know the reef racer prop fit also. thanks for the info. Right now i have the stock prop which is 30mm and the miss bud prop which is 32mm and also fits. I know the setup would be ok with either of these two props. I have run a 4436rpm 8s motor on the miss bud prop with a 2S lipo and it was fine. Everything got a little warm, but prob in the 110-120 deg range which is acceptable considering I was doing a lot of starting and stopping and the water cooling was not effectively working. This is the L motor with more torque and a lower rpm/volt by quite a bit. So I was looking at something bigger than a 32mm prop, but maybe not as big as the 40mm prop.

Wheelnut

My Feedback: (81)

Where's the vid?

hemiblas

Ok, I just ordered the 37mm aquacraft prop instead. The 40mm barely fits anyway and it scraped the hull a little from trying it out. I am going to run it with my 6 cell ib4200 stick pack instead on the 2S lipo. I will try and get a vid this weekend when I run it.

neil.cocker

Hi,

My hammer is in the process of an upgrade to a Feigao 540L 12 2048KV. I have a water cooled 60AMP esc. I intent to run on just 6 cells to start with. Do you think this motor is too much for EP Hammer hull? Will running only 6 cells keep the amps down?

speed3-inactive

For that kv try 8 or 10 cells.

ryan_t888

I am also concerned with two things here.

1 - Battery! 12C, 3600mAh @ 80% is only 35A

2 - This is a submerged drive, not surface. I believe a 40mm or even a 37mm is way too big. Start small and work your way up. Don't start with disaster.

Neil - You can keep the amps down by using more cells and dropping/decreasing the size of prop used.

Ryan

hemiblas

Yeah I'm thinking of trying on 4 cells with the 37mm prop or trying to get a smaller one like you mentioned. I got the 37mm props in the mail today and they both came broken. So I'm going to have to send them back. Looks like my testing may take a while longer than I thought. In the mean time I am going to look at my LHS to see what kind of props they carry.

Hey Neil, what size prop are you running?

neil.cocker

Hi guys,

I'm running a stock 37mm prob, but am trying to find a smaller one to fit the stock 3mm shaft. Our props in the UK are all for 4mm shafts. You say more cells equals less amps. In let case I might go for two 3600 mah six cell packs.

Neil.

hemiblas

For the same motor more cells equals more amps. I agree 37mm is kinda big for a sub-surface drive and you might want to start with a lower cell count than 12. Its better to run more cells with a lower kv motor. That will give you less amps and be easier on your speed control. An example is a 2000kv motor on 12 cells would be the same power output as a 4000kv motor on 6 cells. The 4000kv motor would be pulling twice the amps at half the voltage. Its the amps running through the esc that heat it up.

headhunter23

I still think your trying to run a prop that's too big, what you could do is gear down the motor... I'm currently buying one just because the rpm is too high. The boat runs great on the reef racer prop, if you trim the prop down a little it actually runs faster top speed but sucks ass accelerating and turning. The hammer prop pulls a lot more heat. With the reef racer it's a lot cooler and can almost be run without any cooling. I'm currently finding the cooling system kinda sucks and is getting blocked up somewhere. Thinking of cutting it off and strapping a bigger tube on the back of the boat.

ryan_t888

For the same motor you can decrease current by increasing voltage and still have the same power output. For example run the 8L at 50A and 7.2v or 8.4v and 43A with a smaller prop. Either way results in exact same power input.

There is no direct relationship between kv and power. A 2000kv motor will not necessarily have the same power as a 4000kv motor.

Also the hammer has a submerged drive.

Ryan

hemiblas

Yeah you are right. My example is only correct if both scenarios have the same size prop. A smaller prop will decrease current draw. Heck no prop and it really decreases current draw.