3s or 4s in Mini Hydro keeping the same max rpm the same.

[martin]

As title says using say a 3400kv on 3s or 2550kv on 4s the max unloaded rpm is the same. Will they both produce the same speeds or will the 4s set up take a larger prop due to the higher voltage & lower amp draw. Their for producing more speed than the 3s set up, Both motors are the same just diff kv. Thanks Martin.

[sampit]

Well put Martin, I wish I knew the answer to this aswell.
Its had me wondering.

[Rumdog]

What size boat? What motor size (dimensions)?

[martin]

Boat is a JAE Mini Sprint that ive run on 3800kv outrunner on 3s that runs very well, The motors im looking at using in future are either the Leopard 2860 or 2850 around the KV that i mention in the first post. So 3400kv on 3s & 2550kv on 4s.

[Turbo Dan-O]

I would think the 4S setup would be better with it's lower amp draw. This is my thinking with my small bolt. I am running it on 4S with the stock 2604kv motor. Of course starting with small props and hoping to work up to an X632.

[martin]

Given the 2 examples i mention ie same unloaded rpm but with 2 diff kv motors & diff voltages is the 4s setup going to be faster simply because you can run larger props because of the lower amp motor on 4s & lower kv. If you were to run these 2 setups on the same prop would the speeds be the same as the unloaded rpm would be the same.

[NativePaul]

I would go 3s so you can prop down and race if you ever change your mind about not doing so.
With the motors having the same rotor and weight of copper both will be capable of the same power output and swinging the same prop.
If you are limited by space to say a 50A ESC, it's less than both motors are capable of and 50A at 4s is more power so you would be able to spin a bigger prop and go faster, if you have fitted a 120A ESC things will even out. Are you not happy with your HET1w30?

[martin]

Hi Paul, Im just modding the tub yet again to make more room inside. This time im making it quite a bit bigger inside so i can use diff set ups re motors & esc sizes & also able to get 4s in if i want to as well. Im going to a 120amp esc or even larger if needed, Ive got the HET 1W-30 also some Leopards 2845 4420kv, 2850 4430kv & 2860 3400kv all of which i will be running on 3s 60c. Im also looking at the Leopard 2860 2730kv & 2850 2950kv to poss use on 4s. I havnt use the HET motor yet. Martin.

[larryrose11]

Martin,
is your HET 1W-30 motor this one?
http://www.highendrc.com/index_eprod...roducts_id=167
From the site:
-KV 4400
-No. of poles: 6
-Max Current: 120 A
-Power Nominal: 1400 Watts on 4 cells

running this motor with a 60-100A ESC on 4s with a small prop would really, really scoot, but IM a noob.
Is there a problem with spinning small balanced and sharpened props that fast?

[martin]

Yes that is the HET motor, 4s on that motor in a boat is to high even on 3s its a hot set up that requires short runs as its doing close to 49000rpm unloaded. The sort of amps these small motors can pull a 120amp esc is the best choice ecspecially if you use larger props for more speed. Martin.

[larryrose11]

I was thinking about my previous post, and your right about 3s is too much for a boat with that 4400 kv motor.
I understand that with the KV that high, the amps would be huge, and amps = heat.
Is the RPM a problem tho? Suppose you use their Typhoon EDF 2W-23
http://www.highendrc.com/index_eprod...products_id=86
the motoe has a KV = 2950, -Max Current: 75 A
Running it on 4S nominal has an loaded RPM of 38000 (est from OSE calculator) or so, but the current would be much lower, hence lower heat.
Assuming you could keep it cool and had a good prop, and the hull could handle it, would that RPM be a problem?

[NativePaul]

I like High RPMs, and its not the automatic disaster that some will make out I have raced with 5 minute runtime on setups near 50k unloaded rpm effectively and know folk that have gone higher still, but there are issues and as a self confessed noob I suggest you gain some more experience before trying it. The problems with High RPMs are finding a prop small enough, props small enough not being big enough to push the hull efficiently increasing the slip angles, and bursting motors. Also at very high RPM going up just a couple off MM in prop can make the difference between an easy running cool setup and smelly black sludge in the bottom of your boat.

Don't forget a boat isn't constantly loaded like a plane, it may well be doing just 38k when things are smooth but when you hit a wave the prop comes out of the water and it unloads to 50k, when it hits the water again it wont resume normal running hight instantly it will splash deep and fully submerge your prop which is doing 50k and pull LOTS of amps, if your gear can take it, if not you'll be cleaning smelly black sludge from the bottom of your boat and buying new gear.

[eric_laermans]

A lot of ferry here. 3s or 4s with same RPM... well. What is the difference between the motor you use on 3s and the motor on 4s apart from kV? Allright, you found out about Ri (measured in mOhm). The more copper you add, the more Ri you get. And, is that what you want? Nope. Therefore 3s would have my personal preference.

[martin]

Wouldnt the lower kv motor have a lower amp draw so larger props could be used but still have the same max rpm on 4s producing more speed than the higher kv motor on 3s. Martin.

[eric_laermans]

Dear Martin, the answer is yes and no! Okay, the lower amp motor on 4s could use a bigger prop, but on the detriment of runtimes.

Let me give an example. In Europe we run Hydro I both on 2s and 3s. Given a certain RPM for both setups the props of 2s and 3s cannot differ in size theoretically. The weight of the LiPo is limited to 280gr and the racetime is 5:00. There might be an advantage of the 3s setup needing smaller and lighter ESC, cables and connectors, but that might also be leveled out by the higher Ri (internal resistance, Ohm) the motor on 3s has.

In practice there are sometimes differences in what LiPos manufacturers can offer. Sometimes a 2s package may contain more energy weighing 280gr, sometimes this is true for a 3s package.