servos are generally quite cheap 3-5 pound (as far as rc is concerned) but a servo wont work without the radio gear which is when the bill starts piling up. A basic radio kit on ebay can cost between 20 and 30 pound on ebay (not including postage). Now i have tried the budget route of buying in ebay auction a 99p futaba transmitter 2 channel with sticks. I managed by luck to match it with a hobao receiver. here is a picture of a servo connected to the receiver then it all powered by a battery pack:
doing it the way i have done has been long and a pain in the stern. in the end ive had so many problems doing it budget that ive spent 30 quid trying to rectify the problems. I should have just bought new in the first place.
im trying one of these next. ive heard good reports about these radio kits. there cheap for what they are and do the job:
its called an fs gt2 they can be as little as 18 pound on ebay brand new.
i seriously doubt the cars you have judging by there size and price in argos that theyl have anything you can use. the motor will be lacking power, youl have a major engineering task to modify there steering mechanisms to work for your boat. it can be done but you have to build a gearbox for them with a self centring spring on it.
theres more to come
