Is my Ferry drunk?????

[scottie2212]

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. :sobbing: 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]

Scottie,

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

Noobee

 

[scottie2212]

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]

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]

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]

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 mate would be to measure the current draw of each motor under load ( in the bath ) with an ammeter.

couple more thoughts

 

[Uncivil Serpent]

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]

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]

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]

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. :thumbs-up

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

 

[Bluewavestudios]

Your Ferry may not be drunk but I have been on that one a few times sailing from Holyhead....was it the boat not sailing straight or was it me after a few beers...??? Hard to tell as it gets quite rough on that Irish Sea sometimes.

Those were the days of the booze cruise on the Ferry from Holyhead, Hit Dun Laoghiare for a few real pints of Guinness and the sail back home getting even more drunk and duty free's too !!!

The HSS is quicker now so not so much drinking time on board but at least you can make it into Dublin when you get there.....Loads of Bars around Temple Bar area.......Many Happy days spent there....and very drunken ones too

Regards.........Mark :beer2:

 

[magpie]

You're right there Scottie. I've found the need for much over scale rudders on all my builds. The dynamics of models are obviously quite different to that of full sized boats. (Don't ask me y. Too tekicle for mi) I recently designed a motor sailor and it sailed well except for terrible steering down wind a bit like your ferry. It went it's own way either side and was hard to get back. The rudder was based (like the boat) on a popular full sizer and I doubled the area and added a bit more ballance to help the servo. It sails beautifully now. I realise this is a bit after the horse bolted, but I thought I'd add it anyway.

Jason

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[new to trains]

i had a similar problem on an 1 to 12th scale aerokits lifeboat, after lots of messing and trials, i actually ended up with both motors running in the same direction, i replaced the ' handed' props with two props that were exactly the same ( these were also 2 bladed sports type props not scale items) i got better thrust with these than the scale 3 bladed handed versions, the boat was far more stable and better under control, the on load motor current draw was halved instantly and the turning circle was much improved- the boat would also stop far quicker in an emergency and i never had to replace rudders but had to set slight rudder trim to counteract the torque effect of both motors running in the same direction, but my ' drunken boat' problem was gone as a result !