For P-Limited - Why Would Anyone Run A 1600KV Motor vs. 1800 or 2040?

[msnrochny]

I’m starting to get into FE racing (I’m a long-term gas boat racer). So, here’s my question as a new electric racer. Why would someone planning to do closed course heat racing, go with a 1600 KV motor over a higher KV motor, which would turn higher RPM’s (e.g., say an 1800 or 2040 motor), and meet the legal can size dimensions?

I’m running P-Limited with 4s on a 32” carbon fiber TFL Pursuit Mono. The boat comes with an SSS 4082 1600KV motor, and a Seaking 180 ESC.

For racing, would I be better served with a higher KV motor, and if so, what? Thanks.

[Speed3]

If the 4082 1600kv is legal you could run a bigger more efficient prop at a rpm the smaller motors will not match even though they have a higher kV.

The bigger prop increases acceleration, cornering speeds and because of the prop efficiency the it can beat a smaller prop even though the smaller prop may rev a little higher.

[TRUCKPULL]

I am running P-Limited with 4s on a 32” TFL Pursuit Mono.

Dynamite #3835 (36mm X 56mm) - 1500KV - 2Y - 6 pole Motor

I can turn a larger Dia. prop. with more pitch then any 1800kv or 2000kv for 4 MIN,s and run cooler.


Larry

[msnrochny]

I am running P-Limited with 4s on a 32” TFL Pursuit Mono.

Dynamite #3835 (36mm X 56mm) - 1500KV - 2Y - 6 pole Motor

I can turn a larger Dia. prop. with more pitch then any 1800kv or 2000kv for 4 MIN,s and run cooler.


Larry

Larry -

This is very helpful - thank you! Do you mind if I ask what prop(s) you run with your motor set-up?

Mike

[fweasel]

kV is not horsepower. Higher kV does not necessarily mean more speed.

[Mad Hatter]

I've been curious myself, but it makes sense. More kv draws more amps. Therefore more heat, less run time, less torque. Necessitating a smaller prop. Hence less forward motion per revolution. Volts x amps = Watts. Watts is power. RPM is not. Thanks for re-enlightening me. -Ken-

[msnrochny]

Yep, I think I’ve been clear on KV not being HP, but unclear on the “sweet spot” between higher RPM vs the ability to pull a bigger prop.

I’m curious if there’s a majority opinion out there as to whether higher KV with smaller props is a better racing set-up or whether lower KV with bigger props is the way to go?

[JestDanny]

If I did want to get into P-limited racing, in a mono, cat, hydro, opc tunnel what speeds am I looking to achieve if I was wishing to be compete, say in a race in NAMBA/IMPBA at Anywherevillie USA. Are some hulls better suited for low kv big wheel and others better off with high kv small wheel?

[TRUCKPULL]

Ever HULL has a MAX DIA. of Prop that it likes.

The trick is to find that MAX. Dia. along with the Setup to go with it.

By setup I mean -
-Motor
-KV
-ESC
-Strut - Angle - position and length
-Rudder - position and length
- Trim tabs - position and length
-Servo - Torque - Speed - Mechanical advantage of you rod location to the rudder.
- Radio - your setup within it.

Every little bit of the things above can make a difference in your boat running ability.

For racing - last but most import- your driving.


Larry

[HTVboats]

Ever HULL has a MAX DIA. of Prop that it likes.

The trick is to find that MAX. Dia. along with the Setup to go with it.

By setup I mean -
-Motor
-KV
-ESC
-Strut - Angle - position and length
-Rudder - position and length
- Trim tabs - position and length
-Servo - Torque - Speed - Mechanical advantage of you rod location to the rudder.
- Radio - your setup within it.

Every little bit of the things above can make a difference in your boat running ability.

For racing - last but most import- your driving.


Larry

Bingo: This is the correct info.

Another way of thinking about motor KV in the same watt or size can is like gears. If I had a prop that ran fast but wouldn't make 6 laps either on battery or overheating, lowering KV from say 2000 down to 1800 would be like gearing down a rear gear in a car and lessening the load. Heat should go down and run time increases. You might be surprised the actual rpm or speed may not change. The real trick is finding the motor and prop that work with any hull.
Mic

[Speed3]

In my with an aquacraft 3656 1800kv, the stock revolt motor. I found on 4s that the m545 was one of the best props. It was almost the fastest based on GPS and at the same time pulled less amps than the other props that was comparable speed wise.

The other props I tested was the m447, x646, x642, m645, grimracer 4255, ABC 1814 17 45 2 blade.

The ABC was slightly faster but pulls more amps.

I am familiar with the aquacraft revolt. When I first decided to upgrade. I bought an m545. When I used the m545 on 4s with the stock motor the boat was blowing off the water every time. I was able to set the trim tabs to a point that it was able to run at way higher speeds without blowing off the water. I ran a leopard 4092 1730kv on 5s with a m545 sucessfully. I ran the leopard 4092 2080kv on 4s with the m545, I ran the tp 4060 1950kv in it as well. If you drop the tabs too much it runs wet and sometime cause the boat to learn on one side. I you don't drop the tab enough I will be very loose and blow over easier. If you find the sweet spot you can achieve much higher speeds with stability in a straight line.

I guess the same principal applies to all mono hull as well.

The revolt hull is a very fastest hull, I have the proboat mystic. The blackjack 29 I think is the same basic hull. I have gotten the revolt to run faster and more stable than I could get the mystic to do.

[Speed3]

I think 1800 to maybe 2100kv is the sweet spot for 4s. 1800 not as fast as the 2100 but the ability to manage amp draw and get cooler electronics and your longer runtime goes to the 1800kv.

If anyone is familiar with a tp 4060 1950kv and the leopard 4092 2080kv. These motors have a lot to offer on 4s if you are run a boat 29 to 32inches. I would like to suggest a 45mm prop example m545 for those motors.

[Mad Hatter]

I know this from my street racing days as a late teenager. My 66 GTO with a 427 Chevy welded in would blow the doors off of every 327 Chevy powered early (67-69) Camaro, in spite of the fact that some of these small blocks were built to run at 8000 + RPM. My valves would start floating at 6500 RPM, but by that time they'd be little spots in my rear view mirror! So let this be a lesson. I beat them with torque, which is shear twisting power, not high RPM Horsepower. The secret seems to be to use a bigger can (motor) with less KV, And more battery power. This will permit you to run a larger prop (tires), and get the traction we all want so much on the water, it's fluid. Yes there are limitations to my theories, let's not get ridiculous, a prop that's obviously too large for the hull, (you don't want the boat to spin around the prop) or batteries that are heavy enough to sink it. I guess what I'm trying to say is hit it with a bigger hammer. But not too big! And cubic inches = cubic power. RPM is not power! Thanks & have a great New Year! -Ken-

[MarkF]

The fastest spec motor for a mono is the 1500 and a 2015 abc prop. For hydro I like the 2000kv and 545 but never played with an 1800 or 1500. For cat a 1500 and 1817 3 abc prop. These are some of the winning setups racing in district 19. Of course you can make any kv motor work well with the right prop and setup combo.

Mark