Is my Ferry drunk?????

scottie2212 · 15-12-2004

Hi Guys, hope someone can help with this problem -

I am building St Columba a Sealink car ferry - pictures and info can be found on my web site (see sig)

The problem I have is that she won't sail in a straight line. She 'wanders' to both port and starboard indiscriminately.

She has twin motors (identical), twin rudders and a bow rudder (which has little effect when sailing). She continues to turn after the rudders have been centered. She sails straight and true in reverse. I have added bilge keels but, apart from preventing roll, this has not corrected the problem.

Can anyone suggest what may be causing this and how to correct it.

Thanks for any assistance you can give me.

Scottie

noobee captain · 16-12-2004

Scottie,

are you running the two motors from a single channel and speed controller or are they separate?

Noobee

scottie2212 · 16-12-2004

Quote:

Originally Posted by noobee captain

Scottie,

are you running the two motors from a single channel and speed controller or are they separate?

Noobee

Hi Noobee,

I use two channels through two controllers but I have made a mod on the transmitter which allows one stick to control both motors or, at the flick of a switch, have individual control. I'm fairly confident that its not a motor problem as it wanders both ways.

Scottie

adzam · 16-12-2004

faults like this can be difficult to diagnose scottie.

it`s going to have to be a process of elimination, does it still happen if one motor disconected,are the prop shafts flexing under load is anything binding on the shafts os is there a bent shaft ?.

i`m presuming you`ve already been down this road, but it`s maybe worth starting from scratch again and checking from a fresh point of view.

i have heard of people having problems with the direction of rotation of the screws.a model i have , had a similiar problem ( but it`s single screw) that it constantly pulled one way, when teh builder replaced the prop ( i cant remember if it was from a left hand turn to a right hand turn or vice versa) this apparently alleviated the problem. maybe a slight twist was built into the hull ( it`s plank on frame) but no obvious reason could be found. just some thoughts on where to start scottie .

scottie2212 · 16-12-2004

Thanks for the reply Adz,

If I run just one motor it goes into a permanent turn which the rudder is only just able to correct, opposite turn happens with the other motor.
There's no flexing or binding on the bench, even when I simulate a load.
I'll try swapping the props over, I have them set so when looking from the stern, the right hand prop turns anti-clockwise for forward - this I am told is the 'normal' way for twin screws to be set-up.

adzam · 16-12-2004

no probs scottie,

i take it the motors are the same type /load rating . and not one new one old .

on your radio set there isn`t any mixer functions or settings that have been inadvertantly switched on ?
another thing to check m8 would be to measure the current draw of each motor under load ( in the bath ) with an ammeter.

couple more thoughts

Uncivil Serpent · 17-12-2004

Hi Scottie,

One other thing you could check is the fact that when building her you did in fact purchase a pair of props, ie one left hand and one right hand. You can tell this by looking at the pitch of the prop.

Again, I know its an obvious trick but you never know what came in the packet.

Regards

Chris

scottie2212 · 17-12-2004

Thanks for the thought Chris. Yes, I do have left and right hand props.

I'm beginning to wonder if the shape of the hull at the stern, under the water line, has something to do with what's happening. Is it possible that turbulence in the water could be having an effect?

Scottie

Uncivil Serpent · 17-12-2004

Hi Scottie,
One other thought for this at the moment.

Is it possible for you to get to the motors easily? The reason I ask is that you could try reversing polarity on the motors so they spin the other way, having the right hand prop spinning clockwise, left hand. looking the way you were, spinning anti-clock, that way any turbulence from the props is thrown outside the line of the hull and not into it. You may also be able to do it on transmitter switches.

As I said, just a thought but it may work.

Chris

scottie2212 · 26-01-2005

Thanks to everyone who made suggestions regarding this problem. I've had her on strong black coffee for a while and she's now as sober as a judge.

The solution - BIGGER RUDDERS - over twice the surface area of the ones on the plan.

I'm guessing there is a formula for calculating the required rudder area for a hull, does anyone know if this is true and, if so, how it might be applied to a scale model?

Scottie