Hi there Andi
With reference to running your model on 3 motors, I am running an M.T.B. on 3 motors - with three 3 blader 30mm brass propellers. Also, the power supply for this is 2 x 6 volt 4 amp gel cell batteries the motors are MTroniks 500's. The batteries are divided as follows; 1 fits midships, 1 fits behind the 3 motors at astern. You will possibly find, if you put a lot of weight at the stern of your particular model, that she may not come up to the plain properly - she may just bury her stern into the water - due to all of the weight factor being at the stern
This model comes up on the plain very easily, but, the duration of the model is, at the max 10 minutes. I do have the facility though of shutting the 2 outboard motors off and running solely on the centre prop - and - doing it this way - her duration on the water is increased dramatically. However, her performance is way down.
The model I am currently building, The Whaleback, is also a 3 motor setup. I also have the facility on this one of shutting the outboard motors off. I also hope to be running this on NiCad packs just to see how it will perform.
The way I control switching on and off the outboard motors - on the M.T.B. I have 2 speed controllers from Electronize and the speed controller which controls the 2 outboard motors - the output transistor and the circuit board has been 'beefed up' to compensate for the extra load drawn by the motors. The signal wire from my RX goes to a microswitch, switching the signal off and on to the speed controller. This gives me the facility of switching the outboard motors off and on without having to use heavy duty relays on the power feed side, to the motors. This aforesaid method would be the ideal method, but, it entails either a very heavy duty micro switch in the region which can handle in the region of 30 amps, or, a relay that can tolerate 30 amps switching.
I have noticed to date that when using a Futaba RX or a HiTec RX switching the motors in and out I get no 'chatter' from the speed controller relays. However, when I use the miniature one from Cirrus or the Sanwa one - when I switch the speed controllers off using these RX's - I do get a lot of 'chatter'.
Now, on the new build (i.e. the Whaleback) I have incorporated a mixer along with 3 speed controllers from Electronize and on this one I am using a twin micro switch setup to switch the 2 outboard speed controllers off via the signal input - this does not affect the control of the mixer or the centre motor.
I am currently trying to negotiate for a mixer that is capable of reducing the speed of the centre motor; when operating all 3 motors together you will find (the same as I do) you have an affect when turning to either port or starboard - the model will have a preference to turn tighter in one direction than the other - this is due to the fact that your centre prop will be the same hand as one of the outboard ones. Therefore giving one and a half times more thrust to that side of the model, in a turning scenario. In real life to compensate this, when doing manoevering they reduce the revolutions of the centre prop.
You will also find you will always get a certain amount of cavitation; with smaller diameter propellers.
Hope this is of some help.
aye
john e
bluebird