Matching Prop and Motor...

[pmisuinas]

Hi, would like to know how I go about deciding on what prop should be used with a given motor. Right now I am thinking of using in a model a brushless motor, either a

28-35-2200kv inrunner or a

35-36-1500 outrunner.

Choices are based on weight of the motor. So how do I pick a prop with the idea I won't exceed the current rating of the motor? I know outrunners can run larger props more efficiently but again, how do I at least get a starting point on what prop to use?

This would be for a sport boat, looking for spirited performance, power would be 3S lipo. Thanks for any information....

[Flying Scotsman]

The only way to answer that question is what hull. Choose the hull first, not the motor
Douggie

[ED66677]

agree with Douggie!
given that you want to use 3S (witch one? capacity? max amp?) a 25" mono will be ok but the 28-35-2200 might be too small and the 1500KV outrunner won't push it fast, 2500Kv is what you want under 3S!

[Ub Hauled]

that's the million dollar question...
the answer is: Experience.
Listen to the Pros and one day things will start clicking...
With the motors suggested, you will be able to run micro hulls (up to 22")
and at 22" if you it's a mono, it may be a challenge to get keep the temps down
depending on the size of the prop.

[pmisuinas]

that's the million dollar question...
the answer is: Experience.
Listen to the Pros and one day things will start clicking...
With the motors suggested, you will be able to run micro hulls (up to 22")
and at 22" if you it's a mono, it may be a challenge to get keep the temps down
depending on the size of the prop.

Well, that's the crux of what I am asking about--how do I know what size prop I should be using to keep the temps down? As for listening to the pros, that's why I am here--to listen to what they have to say...

So if the hull needs to be defined first, let me tell you what it is. It is a 33" mono hull (it's actually the Dumas PT-109 kit) and I will be doing a twin drive--either 2 inrunners or 2 outrunners, each with its own shaft. The recommended Dumas system is a single 6v motor and running hardware--the specs I have on the recommended Dumas 6v motor is 10,000rpm, the kit drawings indicate it is a 540 size motor so I am guessing about 6-7 ozs in weight--so the twin system I want to use should not be too much different in weight. As far as the battery packs, they are 25C 4000mah packs so I don't think I am going to have a current problem. What I am trying to avoid is a motor problem--I don't want to overprop the motor and have heating problems. And if I am using the wrong motor, then that's easy enough to change--so if a 2500kv motor is the recommendation, it helps to know that as well. I picked the ones listed based on the Dumas 10000rpm/6v numbers, meaning about 1650kv, and allowing for what I thought might be a reasonable current--the inrunner will do about 15amps constant, the outrunner 45amps. And of course I will be at about twice the voltage. So what prop should I be starting with?

[Ub Hauled]

Ok, I'd use the 1500Kv motor, with the same size prop that came in the boat originally to start with, for two reasons... the outrunner is more powerful and it's a more appropriate RPM.
If you feel that it's the scale speed you are happy with and the system is at an adequate temperature after 1 minute run, let it all cool off and try a 2 minutes run, if it is stone cool, try 3 minutes, and so on... the idea is run it for a short time in the beginning to make sure the setup is proper... if you think you need more speed and the system has low temps, well... prop up!

[Steven Vaccaro]

I'm will UB, the 28mm motor is to small, go with the outrunner and stock props.

[pmisuinas]

LOL--I would be happy to use the "stock props" if there were any. The kit is just the boat, no running hardware at all. The recommended drive system Dumas sells (with stuffing box, 1/8" solid shaft, etc) comes with a 2 blade clear plastic prop that I think is about 3/4" in diameter (don't have it with me right now, so it could be 1"--but certainly no more than that). Dumas designed the kit for a single shaft setup, says their 6v 5000prm motor will give "scale speed" while the 6v 10000rpm motor is supposed to be "fast". I'm looking to improve on that a bit, so going to a twin drive setup while trying to keep the overall weight about the same.

So this is why I am asking how do I match a prop to a motor--I don't have a "stock prop" to start with, otherwise I could do just what was suggested--run the motor with it, check the temp/performance, then change accordingly. That's what I'm trying to get to, that starting point that I can then change--but still don't have a clue as to where that is...

I guess I can go ahead and start with the clear plastic props--but they sure look a bit on the small side...

[Tony]

I think you can find the exact "stock prop" for that boat on evilBay...

[Flying Scotsman]

Check out http://www.mackproductsrc.com/index.htm
They offer the goodies for scale boats and are of a high quality

Douggie

[Ub Hauled]

try to find some info on the manufactures site about how many mm the clear props are in dia., from there you can go and get Octura props, or any better quality ones.

[pmisuinas]

OK, I got an input that a starting point is a prop diameter the diameter of the motor, or a bit larger for an outrunner. So it seems I may start with a 35-40mm prop and go from there--is this way OTIS (out there in space)?

[Ub Hauled]

I see a little problem, since I have not owned direct shafts, I am not sure how it works on the prop stub department, if you go below 37mm shaft, it'll be a 1/8 shaft and above it'll be 3/16... just something to think about. Me personally, I think that a 40mm prop is too much for that motor,
that is just my opinion.

[pmisuinas]

Well, most of the props I have looked at seem to be for a 3/16" shaft, so I will likely go with that. And if the props are too much for the motor, can always go up a size in motor (not necessarily diameter but in length), that would provide additional torque--just means I bust my weight goal, but an extra ounce or two per motor should not be that big a deal...

[Steven Vaccaro]

I have a rtr pt-109 thats 40" long. It comes with (2) 550 size motors and runs well. The motors size you gave should have more torque than the motors in my rtr boat. The proboat pt-109 uses a 3 blade prop. I would suggest maybe trying those.

http://www.horizonhobby.com/Products/Default.aspx?ProdID=PRB3621

http://www.horizonhobby.com/Products/Default.aspx?ProdID=PRB3605

http://www.horizonhobby.com/Products/Default.aspx?ProdID=PRB3604