Propeller size

[William Martin 1Participant @williammartin1]

Hi everyone I am new to brushless motors and I’m struggling to decide how to match the correct size propeller to the motor and the boat .
I am scratch building a 32” Perkasa. It is all wood with a 1400kv brushless motor running on 12v lead acid battery. How do I work out the correct propellor ( two blade , three blade and what size)

[William Martin 1Participant @williammartin1]

Matching it with the correct motor

[Colin BishopModerator @colinbishop34627]

Hello William,

We have some members who should be able to help you but the combination of a brushless motor and a lead acid battery my be inadvisable as a brushless motor suitable for a 32 inch perkasa is quite likely to draw more current than the battery is capable of giving. plus the weight of the battery will affect performance quite badly.

Lead acid batteries are only really suitable for slowish scale boats with a large displacement such as tugs and even then they are not the best choice, NiMH batteries are preferred.

SLEC market a Perkasa kit at pretty much the size you are building your scratch model and their recommendations for the power train can be seen on this link:

**LINK**

The kit was designed by Dave Milbourn who had a vast amount of exprience with this type of model so you can be confident that his setup will work. The kit is slightly smaller than your boat and has a 1000kv motor so I guess your motor should be OK, others can confirm this.

Colin.

[Charles OatesParticipant @charlesoates31738]

What many of us do is but a few cheap nylon propellers, say 30, 35, 40 mms 2 blade standard props plus a couple of 3 blade ones. Then we try them out on the model preferably with a wattmeter/ ammeter. When you're happy with your choice you can either stick with it or invest in a cast metal good quality prop. Doing it this way helps you get experience of the effect on speed against amps drawn of the different sizes. I'm pretty sure you'll end up using a standard 35 mms prop with that motor.

Now that's said, you've got a major problem with the lead acid battery, it's completely unsuitable for both the Percasa, because of its weight, and wrong for that motor because it can't cope with the current needed.

There are 2 types of battery you can use, nimh. ( nickel metal hydride ) or lipo ( lithium polymer) if you're new to all this I suggest the nimh battery because lipos need informed and carefull maintenance that comes with experience.

The highly regarded component shop is a popular source of supply, you'll find them on the webb. You might re consider the voltage, perhaps starting with a 7.2 volt pack which will give over 10000 revs on the prop. I run a model with that motor, so I think you'll be surprised what it can do.

Charles.

[Chris EParticipant @chrise]

Which 1400kv motor?

[William Martin 1Participant @williammartin1]

Thank you both for your prompt reply to my questions. I have been searching the web for some advice on this problem and up until now have had no luck. I will certainly be changing my battery choice as you suggest and will be on the lookout for a reasonably priced watt mete.

[William Martin 1Participant @williammartin1]

Sorry Chris I didn’t answer your question. The motor is a Robitronic 3652 3250kv. The website said that it was 1400kv but looking at the writing on the motor it seems that I’m wrong.

[Chris EParticipant @chrise]

That is a very high kv! 24000 rpm on 7.4v and 36000 on 11.1v. Even if we assume a loaded speed of 75% of those numbers it is a prop speed that is well outside my experience level and well beyond the safe operating speed of many props.

Hopefully somebody who has used that sort or rpm can help.

Edited By Chris E on 15/07/2023 13:56:54

[Ray Wood 3Participant @raywood3]

Hi William,

It really doesn't matter what size motor you use you will have throttle control , I would go with a X35mm plastic prop to start with as others are saying lead acid batteries are good for tug boats

You don't want a scale boat going Mach 2 it would look very un scale like !!

Regards Ray

[Charles OatesParticipant @charlesoates31738]

I totally agree with Chris, that's an incredibly high kv for a fast scale model, 1400 kv would have been fine. To give you some idea, I have an rttl, about 1 meter long, with a 1400 kv motor, 2836. It has way more power than I need, it looks daft when going flat out, so I use the throttle a lot.

,

[William Martin 1Participant @williammartin1]

The motor and speed controller were bought as a present. They were bought as a hydra 50 set which was the speed control and a 1400kv motor. The boat wasn’t ready for the motor until recently so last weekend was its first test on the water with a two bladed plastic 45mm prop. Its performance was pretty poor. I couldn’t test it for long as I hadn’t checked that the prop was tight enough and lost it when putting the boat in reverse. .

i built the same model from a kit for my son, over twenty years ago and that runs on a 540 brushed motor. It goes really well on 12v lead acid but you have explained why after a short while it slows down and after a short time it will pick up speed again. I take it that the battery is catching up.

[Chris EParticipant @chrise]

I am guessing that a 45mm prop is far too big for 3250kv 35mm motor. As it sounds like you were trying to run it hard last weekend you may well have overheated the motor/esc combination.

Battery voltage drops with load but I haven't ever noticed a lead acid battery recovering as you suggested. Assuming it is a 7-10A lead acid battery you are well over its sensible max load which is about the same as the battery capacity.

It would be really useful to know how many amps you are pulling.

Edited By Chris E on 15/07/2023 16:41:24

[Colin BishopModerator @colinbishop34627]

Yes, the problem with the readily available SLA batteries is that they are not suitable for high drain applications, Pulling too much current drops the voltage and probably generates heat. Hence they recover a bit after a few minutes but you are almost certainly damaging them internally and shortening their life.

Digging around on line I assume you have bought this:

**LINK**

(or similar from another supplier)

If you should have got a 1400kv motor and they have included a a Robotronics 3250kv then it looks like they have made a mistake and should be asked to exchange it. The 1400kv makes far more sense in the description in the advert. (Otherwise you are putting an F1 engine into a hot hatchback!). No way would you put a 3250kv motor into a scale boat as described in the advert, it wouldn't just scare the ducks, it would overtake them!

Unless you want to try out LiPo batteries with their particular characteristics and charging requirements then a safe bet would probably be to use a 9.6v 4600 NiNH pack.

**LINK**

SLA batteries shouldn't be discharged below 50% of their rated capacity to avoid damage but NiMH cells can deliver virtually all their capacity so they effectively have almost twice the running time for the same weight. Charging is simple with no special precautions needed.

Going back to my earlier post, the SLEC Perkasa advert specification provides a practical benchmark to work from. A 35mm prop should be fine.

Colin

 

 

 

 

Edited By Colin Bishop on 15/07/2023 17:50:23

[Colin BishopModerator @colinbishop34627]

Chris E,

Lead acid batteries do recover to a degree when abused. If your car won't start you might keep trying until the battery won't turn the motor over anymore . (modern cars often prevent this by measuring the voltage). But if you leave it for ten minutes or so you can often get another couple of attempts out of the battery.

Colin

[Chris EParticipant @chrise]

Colin

I know what you mean with an essentially flat battery but I have never seen it happen repeatedly with a partly charged SLA battery. There again I have never abused one so badly.

To clarify I read it that you are suggesting a 35mm prop with a 1400kv motor and not the 3250kv motor which we all seem to agree is unsuitable.

[Colin BishopModerator @colinbishop34627]

To clarify I read it that you are suggesting a 35mm prop with a 1400kv motor and not the 3250kv motor which we all seem to agree is unsuitable.

Yes, this accords very well for Dave Milbourn's recommendations for the Perkasa kit he designed which is only marginally smaller than William's model.

A 9.6v supply should give a pretty decent speed I would think but it does depend on the weight of the model. As always, it is finding a balance between motor, battery and prop. There are always trade offs.

Colin

[William Martin 1Participant @williammartin1]

Thank you all for your very helpful comments. The motor wasn’t miss sold ,my son showed me that I asked for that particular size (3250kv) I have no idea where I got the size from. I am going to take your collective advice and when Im able to l will buy a 1400kv motor and a watt meter and change batteries.
I will post my results when I get things sorted.

[ashley needhamParticipant @ashleyneedham69188]

we use the very common unbranded 2212 motors here at the pond. despite the markings, they are 28mm outrunners, and the 1400Kv one will easily turn a 35mm prop, and only maxes out at about 12A on an x50 prop. Its something like 100 plus watts, cost from ebay about £8.

Ashley

[Dave Cooper 6Participant @davecooper6]

Hi William,

You have a lot of tech' data to digest here, but I'm a firm believer in the 'good-old bath test'…..

Put the boat in the bath /kiddies paddling pool or, whatever. If the lead acid battery is too heavy it will show up on the boat's waterline. It will also show up if the model is bow or stern heavy or just right, or it may be listing to port / starboard.

Hang on to the stern with one hand and with the other give it a good burst of power….does the 'pull' feel strong ? There should be quite a wake of water on full power and you should have to hang on quite firmly to stop it hitting the taps !

Batteries : NiMhi's are quite easy to manage – no more difficult than charging a car battery really. LiPo's are more of a mental hurdle perhaps. However, modern chargers are now pretty fool-proof (well, within reason !). Stick to the recommended charging instructions. Use a fireproof safety envelope and watch, over time, for any bulges. Some of my LiPo's are coming up to 8 years old now and still going strong….perhaps I should 'time expire' them…

Personally, I find lead-acids a bit of a pain these days, although I do keep one on the bench just for testing purposes. If you do switch to LiPo's (my recommendation for a fast electric). Take a fully charged spare with you and change it over part way through a sailing session.

Happy modelling,

Dave

 

 

 

Edited By Dave Cooper 6 on 19/07/2023 23:35:49

[ashley needhamParticipant @ashleyneedham69188]

As a for-instance….the Component shop.

see. **LINK**

and “wattmeter”. **LINK**

component do much the same thing

Wattmeter easy to use and will show current draw. Use this figure and compare to motor spec to see how loaded the

motor is. Ashley

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