Sean. Various MFA boats, slightly larger than yours use a Torpedo 850 motor, this appears normally to be mated to a 50mm X type 2 blade prop. Your 600 motor is possibly (depending on type) not quite as powerful at the 850, but I recon will be spinning much faster. So,,,a 40 ish mm prop should do you, it is always best to start smaller, as putting too large a prop on an engine will start to overload it and pull far too many amps, ruining your motor,and endurance. 2 blade "racing" type that is. Must be some fast-electric bods out there to advise???

As to propeller shaft length, you first have to decide where you are going to mount the motor, what length of coupling you will use, and where the prop is going to be sited at the under-end of the boat (nautical term!). Once these factors have been decided, then get a bit of dowel approximately the same diameter of your intended propeller tube, tape a 50mm disc to simulate a prop the end of it and poke the other end  through ther hole you have left for the shaft . positioning the motor in its intended position, line the stick up with the motor shaft, allow for the coupling length, position the dummy prop, made oversize to allow for other sizes to be trialled and mark the stick at a point near the end of where the coupling stops an hey presto ! the length you need is …not known… as prop shafts come in "standard" sizes, so you will have to then consult a list of sizes and pick one approximately suitable for your boat, and permanently site the motor to suit. Personally I would allow space for a 50mm prop to be fitted, just in case.

Personally again., for a fast boat I would make the rudder reach to just below the prop. You can always cut a large rudder down a bit if the turning performance is a fierce.I assume here that you are not worried about a scale prop/rudder appearance>?  Ashley

 i am going to finish a  perkassa 1/32 scale as the plan is not too scale what size rails do you

lads  suggest  and it had a12v marlin with it what battery and prop setup do you suggest

and what e s c or the mfa mechanical speed controller does 12v i believe the cost is a 

bit  of a issue as i am on a fixed income so please help lads

well thanks for your time mick

Mick. I have not built a perkasa before, but a 1/32 model soldier stands approx 54mm high, so his waist will be at about 30-35mm high, and if i was building this , then that is the sort of hight I would go for. Conversely you could find a supplier who sells stanchions for 1/32 and see what hight they are. I am not familiar with the Marlin motor, but at the risk of sounding obvious as you say its a 12v motor than a 12v battery is what you want!!   Cheapest option would be a 12v SLA (gel lead-acid), either a 12v single battery or two 6v ones. Look on the Maplin website for battery details, cost/size/weight. If the hull is watertight how about putting it in the bath with weights to simulate the control gear and motor and see how much weight you have to play with in regards battery capacity. Fast electrics normally use NiMh batteries as they generally have a higher current delivery and are much lighter, but as I say I dont know about your motor (will have a surf after this) and they will be more expensive. It depends on how fast you want to go. The MFA speed controller is cheaper, but will run your battery flatter much quicker than an electronic one. You need to know the likely current draw of your motor, and could just put any old prop on, say, a 40mm one, and measure the current whilst floating the vessel in the bath and connecting the battery to the motor via an ammeter. Those nice little viper esc`s are about £25 for a 25A version, if you could keep the current draw to 20A by using the right size prop, then this is the way to go.Ashley

Mick, I do believe MFA sells a 50mm two blade prop for use with both the MFA 800 and 850 motors, and the gel batteries are also recomended. The boats they sell to use trhe 850 motor in are I think a bit bigger than yours so you might usefully try a slightly smaller prop to start with, as this will give a saving in current consumption. Ashley.