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[David 48]

Please can some one tell me how to work out  what size motors to use  I have just started to build the  Fairmount Alpine. I do not want to underpower or overpower. It has 50mm props in nozzles Thank you.

[Shipmate60]

The easiest way is to find a model of the same size and weight and if it performs as you want yours to ask!!
A general "rule of thumb" is the motor casing should be the same size or larger than the prop on direct drive.
If it is the Billings Kit here is a Utube video of her using Graupner 700bb Torque Motors.

CLICK HERE

[ACTion]

Make that Speed 720BB Torque motors - there's a world of difference between these and 700BBs, which are fast-revving and not suitable for tugs ungeared. Less expensive alternatives would be ACTion 755 or Mobile Marine Models' T4A. All are suitable if running on 12 volts.
One suggested installation is here http://www.action-electronics.co.uk/pdfs/Billings%20Fairmount%20Alpine%20Dwg%201%20-%20Mick%20Burke.pdf; although you don't have to use all of the functions of a Robbe-Futaba F14 radio like Mick Burke's model does! This first page is basically the same way that Dirk Schricker's model is set up.
DM