building my first fe, need advice

[william zamastil]

ok so i got an old deep vee that i ran for years with a k&b 3.5 outboard. its a dumas mach 3.5, been out of production for about 25 yrs. i ran this boat long enough that i have it trimmed so its very stable in the straights and corners really nice, the motors tuned so that itll run around 45 mph consistantly.

what id like to do is convert it into an fe, ideally id like to stay in the same speed range(really dont think itll handle much more speed). im thinking if i get the weight and cg the same it should handle the same, other than the different torque curves.

this is where i need the advice, which of these motors do you think would be best all around

Ten Shock TS-VZ2240/1020Kv

Leopard 4074/1050Kv

Leopard 4082/1250Kv

im thinking of running two 4s 45c in parallel thru a swordfish 300 pro

i run for fun so this boat doesnt have to be the fastest on the water, i do want it at the same speed it is on nitro, but id like to see 15-20 minute run times without excessive temps.

thanks for your help

 

[william zamastil]

oops, i meant to say im thinking of running two 4s 45c 5000 mah in series

 

[jayt]

Kvs are too high for 10 minutes of run time on 8S. Closer to 900 to 950 will still give ~45 mph and lower amp draws, meaning longer run times. A typical 4S boat that size needs around 1700 watts to run 45 mph. On 8S that should mean around 55-60 amp draw on average, so with 5000 mAh packs figure around 4-5 minutes of run time at mostly WOT. You will get about the same run time with the 4S parallel setup but 8S will give much lower temperatures. You can extend the run time by not going WOT the whole time, but on 4S the ESC temps at part throttle will get too high. On 8S that is much less of a problem.

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[william zamastil]

thanks for the info jayt

so what Kvs should i look for to get into longer run times and still be able to hit 45?

would a more efficient motor get me better results?

i was trying to match the rpms of the nitro engine to the electric motor, is this not the right way to look at it?

 

[jayt]

Spinning an m445 (if it fits under the cav plate) at ~25,000 rpm will get you there, so a Kv of 900-950 would be good for 8S. But you won't get 15 minutes of run time with 5000 mAh batteries. To do that on 8S you'd need to draw an average of just 15 amps - no way to hit 45 mph very often. If you keep the 55 amp draw you'd need 17,000 mAh of battery capacity which would be too heavy to let you achieve 45 mph. Small 45 mph FE boats just don't run for 15 minutes at near full throttle. Heck, I can't remember my old Dunlap 3.5 tunnel runing for more than about 5 minutes on a tank at WOT.

A more efficient motor would help a little, but probably not more than 10%.

.

 

[HTV Boats]

JayT has the math correct but your into a serious overkill to replace 3.5 K&B power. Your big ESC weighs over a pound and carrying 5000ma at 8S is a weight penalty. Our 4S spec tunnels run faster than any K&B I ever had using 2500-3000ma in a heat. propping down you could get 3-4 minutes run time with a lighter and cheaper setup. 1800-2000kv around 1 to 1.5 watt motor. (1000-1500mw) Buy extra batteries and just come in allow a little cool down, change packs and your back on the water while your charging another pack. Another option would be say a 3674 / 1400kv Leopard on 6S with say a 200a Swordfish or T-180 ESC. Two 3S in series balance a mono well. I haven't run offshore class but maybe someone can chime in on what they use to race 5 minutes. Matching your old 3.5 power might allow you to use props you already have. An M440 or X438 would give you a fair run time. Mic

 

[jayt]

Mic is right, I was trying to lead you to the conclusion that you won't get the run time you want with one set of packs. Too many other new FE boaters don't believe expert FE boaters when they are just told "you can't do that". They have to be shown.

While 45mph could be a stretch on a Limited setup, 40 would be relatively easy. And a larger motor would do it no problem. Just realize that the faster you go the shorter the run time will be.

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[william zamastil]

well i can understand what you guys are saying, and i have to admit i wasnt paying attention to ohms law. i dont have any reason to not believe you thats why i asked.

the 4s packs are an effort to future proof, in the aspect that if it goes well with the deep vee i can see a vs-1 in my future, i have a couple of 3.5 lower units laying around.

i guess i should clearify that if i convert the vee itll be an inboard, so the cavitation plate isnt an issue. with the 3.5 this boat really likes the 438 prop, a 215 on glass water.

after talking with a sparky at work today he confirmed that looking to match rpm isnt the way to look at it, but instead compare hp. so on jayts advice, a 1700 watt motor would be a really close hp match to the k&b, both about 2.25 horse. 1700 watts has probably got an advantage, so with that being said could i go lower on the watts and pick up some run time? or is it gonna get to close to the weight to horsepower ratio?

i thought about a smaller sc, but i was going for the insurance, so WHEN i do something stupid hopefully ill be covered.

one other thought i had, since this is a run for fun boat and test bed. what about an overdrive gearbox i know it adds weight but could i cut back on batt cells and or a smaller motor without getting up against the weight to hp ratio?

 

[HTV Boats]

You do not need to re-invent the wheel here. If you want to use 4S cells for a later project then using what is established already for a "P" mono would be easy since that's right where your hull fits. Forget gears and also pay attention to KV's as that's where RPM ranges are obtained. That's the beauty of FE. A P mono runs anywhere from 1800kv to 2500kv. 1900-2220 probably the heat racing range. 1800 for less horsepower or longer run times and 2500 and up is a SAW setup.

http://www.keithbradleyboats.com/?wpsc_product_category=motors

I run a 1515 1Y 2200kv for heats and it is popular in P mono. M445 prop gets high 40' to low 50's. Smaller prop will get longer runs and less speed. 1250 watts continuous and 2500 peak. Look at the CC1512 1800 kv with 1000 watts continuous and 2000 peak. Would be more efficient and fit what your doing with a 180-200amp ESC plenty of headroom.

Mic

 

[william zamastil]

thanks mic and jay

with all of this info i guess ill be sticking with nitro for a while longer cause i dont think batts are at a point that ill get what im lookin for

thanks again