LiPo batteries are lighter and more powerful, but they are expensive, need special chargers and should not be fully run down. These constraints suggest that LiPos should only be used if you are familiar with their care and requirements - otherwise choose Ni-Mh AAAs, which are much more robust.
Note that FLAT battery packs are much better for EeZeBilts. Because the batteries generally sit on top of a high sub-deck, you need to keep the weight as low as possible, otherwise the boat will be unstable.
You can see the general principles. I have made up an approximation to the hull of a Terrier or Curlew, then epoxied 4 sections of brass tubing as shown. The rudder post is cut to just below the deck level, and a length of Dacron fishing trace is tied onto the servo arm, threaded around the brass tubes through conveniently cut holes, round the tiller arm then back to the servo.
Small brushed motors MUST be suppressed. They generate a lot of interference, and all the electronics are very close together on an EezeBilt. 2.4Ghz rejects airborne interference quite well, but does nothing to stop interference passing down the motor leads and into the ESC, etc. Buy three 0. 1 microfarad ceramic disk capacitors for a few pence each and attach as shown. Here is a pack of 50 that I bought from Bitsbox on Ebay. If that does not cure things, install ferrite rings on each motor lead.
Small brushed motors only need a thin plastic tube connector to the propeller shaft. Thin fuel tube, or insulation from house wiring leads will do nicely.
Adding Radio Control
The original EezeBilt boats were cheap models designed to carry a small motor and a 4.5v flat battery. A modern model boat expects to carry a motor, battery pack, receiver, electronic speed controller, servo and some bulky connectors to attach these together. This will create space and weight carrying issues as well as increasing costs!
These problems are not so great for the Large Range of EezeBilts but start to become acute for the Small Range, which are 10.75 inches long. Luckily, modern miniaturization comes to our aid, but even so we are working at the limits of readily-available low cost technology. The hints below may help direct your purchasing if you are looking to add radio to an EezeBilt. I will assume that we are talking about the Small Range - some of the constraints can be relaxed for the larger boats.
In the UK you can use 27Mhz, 40Mhz or 2.4Ghz to control a boat. If you have the option, 2.4Ghz is more attractive.
2.4Ghz does not require checking for frequency clashes at the poolside, is somewhat better at interference rejection and uses a much smaller receiver aerial which will actually fit in a small EeZeBilt. Cheap Chinese-built 2.4Ghz radios are also available at around £30 or less.
For performance, use either a 280 or 300 brushed motor for the larger EeZeBilts - seen on the left here. These are cheap, and you should expect to pay only a pound or two - sometimes as little as 30p if you buy in bulk. For the smaller EeZeBilts, a 180/18 or 120/12 - seen on the right, will produce a sparkling performance with 4.8v of NiMh.
The slower small EeZeBilts such as the Otter will be badly overpowered with such motors, and need a slower revving low-drain motor. Three are shown in the background - a 100 with its distinctive flat sides, and two even smaller specialist motors from Action Electronics - their small servo motor and cassette motor..
These sizes of motor are relatively inefficient, and often generate a lot of spark interference. Small brushless motors exist, but they are not cheap, and more importantly, are not capable of flexible low-speed use without a lot more money being spent.
Electronic Speed Controllers (ESCs)
Some very cheap brushed ESCs available from China for a few pounds a unit. Make sure you do not get brushless ones - these are intended for a completely different kind of motor!.
At these prices you often don't get a reverse. If you really want this, and a better quality set, try the P52, P68 or P78 from Action Electronics - but it will cost more!
The bulky connectors should also be unsoldered from the ESC, and in many cases the ESC can be connected directly to the motor and battery case with quite thin wire, saving a lot of space and weight.
A sub-micro servo can be used to work the rudder, costing around £5. Here are three cheap ones that I picked up from Hobby King - the left-hand one was under £2!
Servos are usually connected to the tiller by a metal link. For a Small EezeBilt that may result in a lot of wasted space if the servo is mounted centrally, or the need to build the servo into the stern where it will be inaccessible later. I have found that a closed-link cable linkage is a workable way to get a cheap, flexible and unobtrusive mounting system. Here is a test rig showing the concept...
Once the line is tied, just enough tension to take up the slack can be applied by wrapping turns of the line around the tiller arm. Move the tiller arm to one side, use a small screwdriver to loop the line round, then centre the tiller and the line will easily slide round it.
Once the line is tight, a little superglue at the tiller end and the servo will render it permanent. Note the rudder post soldered to a small flat sheet (of copper, in this case). The rudder connection to the bottom of the hull is always a weak point on EeZeBilts....