Model Boat Mayhem
Technical, Techniques, Hints, and Tips => DC Motors (Brushed) and Speed Controllers => Topic started by: flag-d on April 27, 2006, 06:21:02 pm
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???I have an MTB, 1/24th, 53" LOA, about 15lb displacement. Currently running 4x 500E's on a pair of 6V gel cells (=12V). Props are 35mm 3-blade brass. Would like a little more speed from her but don't want to 'kill' motors or ESC's (mTronik Marine 15's) or batteries.
1. Can I use bigger scale type props?
2. How about plastic/nylon racing type 2 blade props but same diameter or smaller?
3. What about bigger motors? Say 600's?
4. What about NiCads instead of gel cells?
Lots of (simple!) help required here. I'm not after blistering performance and 5 minutes of run time, just a 'little bit more' get up and go.
Thanks in advance :)
Mike
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The Johnson 550 fan cooled type motors run about 17,000 rpm on 12 volts, would give improved performance, but am unsure what current these would pull under load.
Might be worth trying 2, as they are quite cheap, on your present set-up.
And if it is what you want go for the other 2.
Bob
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Thanks Bob, I'll have a look the next time I get down to my model shop. I've decided to do some tank tests with a decent multimeter to see what sort of current the motors are drawing. This'll help with motor/prop combination testing and also tell me if I'm over-loading the gel cells.
Mike
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I have always found that the answer to this sort of problem was resolved by a call to Alistair at Model Motors Direct........................ very knowlegeable man .
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Hi,
Do model motors direct have a website?
Thanks
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no >:(
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I have always found that Alistair does lots of fine motors, but is a little bit light on motors for large warships.
his larger motors do tend to be slower running, great for Merchant Ships with their low drain specifications, but have only found the 777 motor for warships and this is a little small for a large Patrol Boat.
Bob
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I have always bought my motors from motors direct for the last 5 years, I build large warships, his Buhla motors power my 7ft 7inch NORTH CAROLINA, and IRON DUKE, and his 777 will power my next project, all sail at good scale speeds. ron h
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Hi ya there, Flag D
Can you tell us what type of M.T.B. this is? Because of the size you say and the number of propshafts .
The only vessels that spring to mind are the Camper and Nicholsons M.T.B. (I am sure these were only 3 shafts)
The other one is the Fairmile D which was a 4 engine - 4 shaft M.T.B. However, the size you give 1/24th, 53" LOA, about 15lb displacement, this is a little bit small for the Fairmile D,
The reason I ask is - do you want the model to come onto the plain? because if its a round bilged hull it doesnt matter how much power or what size motors you are going to put in, you are going to reach a point where the hull willgo no faster and you are just literally wasting power.
Aye
John E
BLUEBIRD
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Hi Bluebird
Yes, it is a Fairmile D and I can't type: LOA should have been 58", which is actually very slighter longer than 1/24th. I'm not sure that 'D's ever came up onto the plane. I have seen a couple of photos where the forefoot is showing but this is only when crossing a wake as far as I can tell. I think (which is rare in itself!) that she's just a little bit sluggish. Any idea how to scale speed? At full size the max speed was around 32 knot designed but down to around 28 or 29 in reality and often less. What would these equate to at 1/24 scale? Do you just divide by 24? Maths was never my strong point!!!
Mike
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Hi ya Flag D - right now we know where we go
she is a Fairmile D - right - on the old model Boat's forum - there was a long thread about scale speeds - but no longer available to us obviously.
However, the jist of the thread at the end of the day was - IF IT LOOKED RIGHT AS A MODEL - IT WAS RIGHT - so....what you are basically looking for is - you are not looking for the model to come out of the water as on the plane, but, to create a decent wake at the bow.
This can be achieved in two or three ways...not only by playing with the speed but playing with the weight in the boat. Looking at photographs, as you say, the forefoot of the boat doesnt come out the water unless she is crossing a wake...
You can imitate the above by shifting weight slightly to the stern of the boat with the set up you have and see if this gives the effect which you desire.
Try this and let's know how you get on.
Fitting really big motors and props to this type of hull WILL NOT really achieve spectacular performance.
Aye
John E
BLUEBIRD
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You're supposed to get a realistic scale speed by dividing the speed of the full-size vessel by the square root of the scale. In other words, divide 30 knots or so by the square root of 24.
Jonty
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One problem with using this formuls is that the wind and waves are not to scale.
If you look at pictures of the wave formation along the hull, when your model looks similar the speed should be correct.
No need to go into Frouds formula etc, bacically if it looks about right it generally is.
Bob