Hi John,

It’s a difficult one! The recommendations on the Mobile Marine Models website seem to be geared towards making a towing model. If you are only expecting to do free running then it looks overpowered to me. A 40mm prop is very big for this size of model.

The 555 motors would be too heavy I think, for such a small model.

It also depends on how ‘accurate’ you want the underwater appearance to be – only the fish will see it when afloat!

Personally I think your 385/35mm combination should give pretty good performance.

I am currently building a motor yacht based on Vic Smeed’s Silver Mist (see separate topic). It is only 21 inches long and  is fitted with two MFA 360 3-6 volt motors driving 25mm 3 blade props using 2:1 reduction pulley drive. Initial domestic tests have shown it trying to climb out of the bath on a 7.2volt battery! Photo below.

If you want to go down the gold plated option route the Prop Shop will sell you  prop and nozzle combinations. However their combos are measured in inches. You can see the details here:

https://www.prop-shop.co.uk/products.php?cat=kort-nozzle-propellers

There are independent suppliers of fixed Kort nozzles but from what I can see they all start at 40mm.

Hope this helps,

Colin

Interestingly only last weekend I was at an event where a small pusher type tug was on the water frequently.  It spent most of the day flying around at a ridiculously over scale speed looking extremely unrealistic and very toylike, which was a great shame as it was actually quite a nicely put together model.

You are quite right to consider not overpowering a model as it can completely destroy the credibility on the water.  I have always strived to achieve as close to a scale speed as possible, with just a little in reserve for those crash stops.  I think Colin is spot on with his suggestion as the 40mm may look a little bit out of proportion and two 555 motors will weigh quite a bit.  It may well be worth considering putting an ammeter in line with the motors and do some tests in the bath before you put the deck on to give you an idea of the current being drawn and therefore what duration you might expect with the battery you choose.

Have you done any ballast tests to see what weight you have available for propulsion/battery/electronics etc?

I use 2x 385 motors in an sms Emden, 1.1 metres long. Its overpowered a little just using 7.2 volts, the motors can take up to 15 volts.. It is worth considering using the same as Colin, i can heartily recommend using the simple pulley drive, it’s efficient and cheap , and the 360s are good motors.

Well said Colin.  Pulleys, gears and toothed belts all work fine.   I prefer the latter as they seem easier to adjust for less side load on the motor and prop shaft.

Len.

i