Motor/ESC combo for 28 inch mono

[Old School]

I suppose that this question has been asked a multitude of times. I have a 28" mono of an older design that I am converting to 3/16" stinger drive. The purpose of the upgrades is purely for "sport" running and if 60 - 70 km/hour is achievable then I will be happy.

I am thinking along the lines of a Leopard Hobby 3674, around 1700 - 1900kv, pulling around 80 amps coupled to a Hobbywing Seaking 120 amp ESC and running on 4S cells (3300 mamph/4S/40C or 60C). With 25C discharge I could go to a 3700 mamph battery. Propellor to start testing I was thinking a two-bladed Octura 40 mm diameter with 1.4 pitch.

Any suggestions please. I am limited in that a LP 40XX series motor is too large to fit and I feel that larger than suggested may simply make the boat uncontrollable. Also, I wish to limit myself to 4s voltage.

[Insanity Plea]

Sounds like you have a good plan to start with. The 1900kv on 4s will be 28,120 rpm so you're definitely in a safe zone from that aspect, seems like 2000-2200kv is the sweet spot on 4s. Get the highest C rating you can for the battery, with 40c being the minimum. I would also consider a x440/3, I've seen about 4mph difference on my applications between the x440/2 and x440/3. Good luck and enjoy!

[Old School]

Many thanks.

[rol243]

the 3 bladed prop will suck up more battery power and only give more speed in the turns. i say x440 x 2 blade .or a 220 x 2.

[Insanity Plea]

The 3 blade will suck more power but it will also increase speed on the straits, in the turns it will maintain a higher speed than a 2 blade.

[Old School]

Many thanks, I did intend to experiment with several prop configurations.

[R2315]

I run a 2200kv on 4s with a seaking 180a esc, 2 2s-5000mah 70c packs in my 28" mono and I'm pretty happy with it I have a couple of vids in the video section if your interested x440 and m440 props.

[Old School]

Many thanks

[CraigP]

Another important consideration goes to handling issues with a 3-bladed prop. It increases prop torque considerably, thus can promote more difficulties from prop walking. I would recommend taking any negative angle out of the strut if you run some and start there. Sometimes a touch positive can help offset the torquing issues up on the front of the boat.

[Old School]

A thought maybe in initially using a two-blade to sort out the boat's behaviour and then progress to a three-blade to gauge any performance gain. Any change in behaviour would then be as a result of the three-bladed prop.

[Fluid]

There are lots of misconceptions based on limited experience in the hobby. High Kv is not always the best way to go. Neither is the use of a 3-bladed prop. Like most things, it depends.

One example - in my club the P Limited Cat class is dominated by 1800 Kv motors. Those running 2000 Kv motors can't swing enough prop to keep up. Several Nats winners run 1500 Kv motors in this class, more rpm is not always the 'sweet spot'. It depends.

Another - few SAW records are held with 3-bladed props. The advantage of these props is reduced prop torque, an advantage in "some" hulls. Top speed is usually best with 2-bladed props, but what matters is what works. Any absolute statements here are usually wrong.

Smaller hulls like the OP's usually benefit from smaller diameter props. They cause less prop torque and less prop walk, and reduce the tendency for chine walk. With the smaller props, higher rpm is needed to keep the speed up, meaning higher Kv.

Multiplying the Kv by the voltage does not represent the actual rpm while running. Usually at best this value is 15% or higher that the actual loaded rpm. The higher the Kv the more sensitive the setup is to prop changes - a small change can send the amp draw through the roof and damage the ESC or motor. Lower Kv is safer, especially for less experienced boaters.

A 3-bladed prop will not increase amp draw unless the same diameter is used. The major advantage of the 3-blade is the ability to use a reduced diameter for less torque reaction. Reduce the 3-blade diameter by 10% to 20% from the 2-blade for similar amp draw and thrust.

This is all boils down to the OP's original question about his setup. I believe that his motor/prop ideas make good sense. He should be close to his goals, particularly if he upgrades to an x642 prop. I would suggest a higher mAh and minimum 45C pack. I assume a surface drive with a stinger or adjustable strut.


.

[CraigP]

This is good stuff... I have some questions and was hoping Fluid would answer. I've been thinking that the lower kV motors have less loaded bog, because they develop more torque. Another thing I have seen is that people use 3.7V as the nominal voltage to apply to kV. But 3.7V is close to discharged levels. I would think that 3.9V, the average between 4.2V (full charge) and 3.6V (low charge) would bring the bog factor more into play...

On my data logger, using a 1050kV motor, I'm only seeing about a 1200rpm bog factor. Are we not compensating for loaded motor bog by using a lower voltage in the calculation?

[Old School]

Many thanks for the detailed information.

[Insanity Plea]

All hail king fluid

[CraigP]

I can't get any answers out of Fluid... I must be the court's jester!