Hello Paul

Lets get down to the nitty gritty!

I`ve just bought six 7.2 volt stick batteries from a well known model supplier and two MFA 700 motors and two recommended ESC`s

The batteries and ESC`s come fitted with Tamiya plugs

The plugs and wires attached are supposed to be "fit for purpose", yet the experts on this forum are telling me to change the plugs, to Deans plugs

The supplier wouldn`t dare sell hazardous equipment, so why should I change the plugs?

Bob

I`ve already said, the props are too small

Bob

You will all be pleased to know that I am totallyt confused by all of this. !!!!!!!!!!!!!!!

When working with electricity it is worth mentioning to use safe test equipment. Whilst working in Building Services, the HSE inspector was keen that all multimeters were to their latest guidance. It is a good document too.

If anyone has an older type multimeter with older type probes (see the HSE sketch), consign it to the WEEE skip. Don't buy an older type from a car boot sale, bring and buy sale or elswhere. If new leads can't be bought for your old multimeter, buy a new one.

You could argue, I've had this one for many years and it is only used on low voltage equipment, but if you had a fault on a household appliance or circuit (mains voltage) you would be tempted to test it with the old multimeter and this is where the danger lies.

Here is the HSE guidance, as previous it is an excellent document which is a free download.

**LINK**

Tony

At the risk of confusing everyone even more I will throw in my threepennoth to the debate. I spent a lot of time (and money) developing the propulsion system for my 1:24 scale Elco PT boat and while that is a planing hull and Brutus is not, there are some similarities. I would guess they are around the same size and weight. PT602 is 40 inches long and weights around 13 lb.

According to MFA's data sheet the RX15+ motor has a free running speed of 22500 rpm, and at 7.2 volts will be drawing 2.4 amps. The maximum efficiency point is at 15500 rpm when the motor draws 10.3 amps and delivers 55.6 watts.

PT602 has 'conventional' 30 mm diameter 3 bladed brass propellers, chosen because they are near enough scale size. It is fitted with a pair of Graupner Speed 700 Turbo C motors (part number 7307). These have a free running speed of 14600 rpm on 9.6 volts. Under load they drive the props at around 9200 rpm, as measured by a tacho and take 21.6 amps as measured by a wattmeter. The input power to each motor is 180 watts.

Based on the above data I get the impression that the RX15+ is not really a very good match to the propellers fitted to Brutus, ie it runs at too high rpm and delivers insufficient torque. Even if it was geared down, I suspect that you really need more power too, particularly if you are thinking of going to bigger propellers. In the Brutus installation I suspect the overall system is operating way below maximum efficiency. My experience with PT602 suggests the best way to find out is to instrument the installation rather than spend a lot of time and money trying to find an optimum installation by trial and error.

I agree with Diede's comments about the Tamiya connectors as I found they were unreliable on PT602 which takes >20 amps. The XT60 type he recommends are excellent and I now use them in PT602 and have a long term plan to standardise on them in everything eventually

Bob, before you embarque on even more steps into the unknown with multi motor drives I suggest you should try and measure the propulsion system performance. In view of the interest on the forum I am quite prepared to meet up somewhere and bring my tacho, wattmeter and thrust meter and we could measure both Brutus and Ellie so that you know where you stand now. We are on holiday from next Friday, off to see Kim in the Isle of Man, ferries permitting, but if you have any suggestions when and where, let me know. Lyme Park tomorrow is a slim possibility, is that when you are planning to be there?  Would there be anywhere suitable to make the measurements?  You really need to be able to tether the boat to the pondside in a location where you can read the instruments fitted to it.

Gareth

Edited By Gareth Jones on 14/06/2014 08:25:35

I have read this and agree it is a great article that addresses the wiring in an easily understood manner.

What I have trouble with, and it seems that nobody on here is different, is selection of an apprpriatte sized prop for any given application. So far, all the advice I have read or been given boils down to " Try this size and if it doesn't work, try something else !"

Surely there must be a better way? Is the answer simply to use the biggest motor that will fit so that it doesn't have to work too hard ? We don't all have deep pockets and a bottemless drawer of spares floating about.

Mike

Mike, this is very true! The problem is that there are simply too many variables involved – it is a moving feast!

MOTORS have their own characteristics depending on size, windings, torque and type.

PROPELLER characteristics are governed by diameter, pitch and blade area (and your boat may need a particular type/size of scale propeller which in turn will dictate your motor options.

BATTERY selection will be governed by voltage, capacity and of course weight!

Then there is the BOAT itself – size, hull shape and required performance level.

All these factors influence each other and there are not many hard and fast rules, just principles such as that an electric motor will work best the nearer to its free running RPM you can get when working in the boat. There is also a rule of thumb that for brushed motors don't use a prop of greater diameter than the motor (unless it is geared down). Of course this doesn't apply to brushless motors! If something gets very hot then there is massive inefficiency and power loss somewhere in the system (it could just be the prop shaft binding in its tube!)

The usual practice is therefore to find another model which approximates to the same size and type as yours and if that runs successfully then either adopt that drive train or use it as a starting point for your own boat. The kit manufacturers will frequently recommend power train setups for their models and you can use those as a guide, the other alternative of course being to ask questions on Forums such as this one to draw on other people's experience which is freely given and which may give you a more precise solution.

It is always best to have power in hand as you don't need to use all of it but it can be useful if you need a bit more oomph to get you out of trouble such as avoiding collisions etc.

Colin

To avoid confusion between threads the electrickery measurements on Brutus are presented on the Brutus thread.

Just for completeness though, while I use a digital multimeter for troubleshooting, I always use a wattmeter to measure motor performance as it will measure much higher currents and displays volts, amps, watts and milliamp hours all at the same time. The one I use is shown here but many others are now available, some much more cheaply than the one I bought about 8 years ago.

Gareth

I agree too, always use the analogue meter in preference to the digital one.

It's the same with watches – analogue ones are far better than digital ones as you can 'see' the time rather than have to read it and also see at a glance how long you have got if you are doing something like going out etc. A picture is worth a thousand words! Technology does not always mean progress.

Colin

Len, a good point well made. But at the end of the day most of us just want to play with our model boats and have them running well enough.