Powering a Billings Slo-mo-shun

[JeremyParticipant @jeremy15845]

First thanks to all for their advice on batteries.  In the end, it proved simple to split the Tamiya type 7.2V NiMh battery into two sticks.  The far end of each stick is connected to the other by a welded tag.  I have cut this in two which has given me good surfaces on which to solder the connecting wire.  The length of the Tamiya connector cable was enough for the required distance between the two sticks.

 

However, am I wasting my time and will I have sufficient power to make the boat plane over the water?  I am using a Graupner 600 Race 7.2V motor with a 25 amp Mtronics Viper ESC and the 7.2V battery pack.  The prop is a 2 blade 35mm S type.  All as recommended by Cornwall Model Boats.  In Dave Abbott’s kit review of this model, he used a 600 motor but it is not clear from the pictures what batteries he used and what the total voltage was.  Whatever he used, his photos show the boat skimming over the water.

 

Advice from another forum suggests that I will be underpowered:

Jeremy.
That’s a big boat for a 600 motor on only 7.2 v & a 25A ESC.
The quick maths tells us that you are limiting yourself to 175Watts. I
don’t think that is enough power to get a boat that size up on the
plane. These wooden boats usually end up pretty heavy too. The small
size prop you will need to keep the amps under 25A probably wouldn’t get
the boat going fast enough to get up.
I don’t want to be a downer, it’s just the way I see it.
We used to run 700 size motors on 12 cells for this sort of thing, &
a 50A ESC. Even with cooling they used to get a tad warm.
I hope someone can prove me wrong & has managed to use a 600 on 7.2 V for one of these.

Paul.

 

Is Paul right or will I be OK.  I am still at a stage of the build where hardware can be changed so any advice would be appreciated.  I would prefer to steer clear of equipment that needs water cooling and have so far discounted the use of a brushless motor and Lipo batteries.  I am not after extreme speed but would like Slo-mo-shun to plane as designed.

 

Jeremy

 

 

 

 

[JeremyParticipant @jeremy15845]

[ashley needhamParticipant @ashleyneedham69188]

Jeremy. This is a bit of an unanswerable question I fear. Unless someone comes forward to tell you what they used when they built an identical craft, and if it planed or not you will be in a bit of a suck-it-and-see situation.

 

So saying, there are various models of 600, I had a quick look and Graupner do a much more revvy version to the one you have, and there are various “hot 600s” to try.

 

The point being they all use the same mounting, so, barring batteries, you have scope to swap motors a bit if performance is not…sufficient…and what IS sufficient performance?? how much plane do you envisage..for the money and run time.

 

Going to a 700 would mean a different mount, and unless you can get something to mount either a 600 or 700 then a swap to  a larger motor later on might be more tricky.

 

The other thing is, the setup as suggested by Paul  sounds very juicy and would give the batteries a hard time, or you would need some larger capacity batteries.

 

Ashley.  

[JeremyParticipant @jeremy15845]

Ashley.

 

Thanks for your advice. I will stick with what I have and see what happens.  Just one further question – is the 25 amp ESC big enough for the Graupner 600 and 7.2V battery combination?  I don’t envisage long runs at speed but I don’t want to risk the ESC melting!

 

I see that Dave Abbott has written a feature currently on this web site so he clearly is still around.  Can I send him a message asking which 600 motor and batteries he used on his review Slo-mo-shun?  I am not sure this works on this site.

 

By the way, contributions from a U.S. site suggests that 7 cell and 8 cell NiMh cells are commonplace over there and you see them on many supplier web sites.  Over here, they don’t seem to exist.  Is there any reason for that?

 

Jeremy

 

[ashley needhamParticipant @ashleyneedham69188]

I think a 25 amp ESC should be enough for this combination. You can always check, mind you, with an ammeter to be safe, and put a suitable fuse in in any case.

 

I would imagine you could send dave a message or failing that I am sure Colin could manage this.

 

Americans always do things differently! any of the various battery suppliers will make up packs to order without extra charge i am sure so I wouldnt worry about getting 6, 7, 8 or 9 cell packs, whatever you want.

 

Ashley

[Mark Beard 1Participant @markbeard1]

This is precisely why I am so keen to find a way to correctly size motors, props, gearboxes and batteries during the build, BEFORE testing on the water.  Working on it…

 

As yet I can’t help you with picking the motor for the hull, you still need to find someone who’s built and run a similar setup.  But if you go with the Speed 600 Race motor then I can give you my twopenneth on the ESC.

 

An analysis of the characteristics of the Speed 600 Race motor shows that at 7.2V, the maximum shaft power will be 226W and it will draw 66A.  Of course without some very effective cooling, running like this will fry the motor.

 

At peak efficiency, (which is where we want a displacement hull to operate, not a fast planing hull), this motor will draw 18A and deliver 95W of shaft power.  Given that a fast boat will run harder than this, I conclude that a 25A ESC will be too small for comfort.

 

Personally I would use an ESC rated at no less than 40A, and preferably 60A.

 

When you do get it on the water, and can report on how it performs, I’d be very interested to hear and add to my growing data on motor-hull matching.

 

Mark.

[TelstarParticipant @telstar]

Hi Jeremy   have you ;ooked at this on the forum

http://www.modelboats.co.uk/forums/postings.asp?th=38379&p=7

scrole to the bottem for a photo of a billings slo mo shun running

cheers Tom

[JeremyParticipant @jeremy15845]

Hi Tom

Thanks for the link – somehow I had missed that forum.   Phil Wink’s build log will be very helpful.  I think I will start with the brushed Speed 600 which Dave Abbott used in his kit review as I already have it and I have no experience of really high speed boats.  I have completed the hull without any provision for water cooling although I imagine this could be retro-fitted if required.

Cheers

Jeremy

[JeremyParticipant @jeremy15845]

Slo-mo-shun upgraded with a water-cooled 2760Kv brushless motor with the same 7.2V NiMh battery as before. As the U-tube link below shows, It's now a lot quicker.

Jeremy

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