Model Boat Mayhem

The Shipyard ( Dry Dock ): Builds & Questions => Working Vessels => Topic started by: ianb on August 26, 2009, 01:02:53 pm

Title: Puffer Wiggle
Post by: ianb on August 26, 2009, 01:02:53 pm
Well, my Mountfleet Puffer had her first test session at the pond recently. She took a lot of ballast, 2 SLA batteries midships and about 2 lb of large bolts in front of the mast.

The Puffer is very stable and the waterline is about 1 inch below the deck level. The prop is well immersed and she is not trimmed down at the bow.

I tried to set up the rudder servo for a straight hands off run at full throttle and couldn't do it. The stern twitched randomly left and right. Brought her in and checked for play in the rudder linkage, none found: I use the helicopter ball links. Changed the servo, no change in the behavior. No wind was blowing on the test day.

Rudder control is very positive at any speed and the turning circle is good. I sail alone and the receiver is SPCM so that it shuts everything down if the signal is corrupted or lost. The same equipment works just fine on my other models.

So, I am stumped. Any ideas as to what may be causing this?

Ian
Title: Re: Puffer Wiggle
Post by: Martin [Admin] on August 26, 2009, 02:09:23 pm
Have you done the usual range checks?
Title: Re: Puffer Wiggle
Post by: ianb on August 26, 2009, 02:49:07 pm
Yes. Tx and Rx both working as they should. The Puffer antenna runs up to the top of the mast, so reception is excellent. Antenna is well clear of any other wires and the motor is suppressed. The problem occurs at both close range and at a distance, so I think that the radio link is good.

With the SPCM radio the throttle is set to center stick if there is any loss of signal ie failsafe position.

Ian
Title: Re: Puffer Wiggle
Post by: FullLeatherJacket on August 26, 2009, 02:53:55 pm
Ian
It sounds like RF interference is being generated somewhere inside the model. Check for metal to metal joints rubbing against each other. Also twist the two motor wires together along their length between the ESC and the motor.
FLJ
Title: Re: Puffer Wiggle
Post by: ianb on August 26, 2009, 03:24:40 pm
FLJ,

Good point. I'll check that aspect of possible internal RF out tomorrow. Its 11:15PM here.

The motor wiring is all paired inside that spiral wrap, and there are no loops or excess wire. The Rx is mounted in the forward part of the hatch opening with the antenna going up the mast. No other wiring up there.

The twitching had a period of 5 to 10 seconds - go straight for 5 to 10 seconds, twitch left or right,go straight for 5 to 10 seconds but on a slightly different heading and so on. Didn't notice any change in the prop wash or in the steam engine sound.

Thank you
Ian
Title: Re: Puffer Wiggle
Post by: Shipmate60 on August 26, 2009, 03:46:41 pm
What motor is in her and is it suppressed?

Bob
Title: Re: Puffer Wiggle
Post by: DickyD on August 26, 2009, 06:13:36 pm
Yes. Tx and Rx both working as they should. The Puffer antenna runs up to the top of the mast, so reception is excellent. Antenna is well clear of any other wires and the motor is suppressed. The problem occurs at both close range and at a distance, so I think that the radio link is good.

With the SPCM radio the throttle is set to center stick if there is any loss of signal ie failsafe position.

Ian
Wakey wakey Bob. Been a hard day ?  {:-{
Title: Re: Puffer Wiggle
Post by: OMK on August 26, 2009, 06:47:40 pm
I had the exact same symptoms on one of mine. All wiring/range testing/linkages were fine. Not sure if this could be relevant, but the problem went away when I stuck a smaller rudder in there. Fuelled with that knowledge I re-installed the larger rudder and swapped out the servo for another (beefier) one. That too cured the problem.
Title: Re: Puffer Wiggle
Post by: ianb on August 28, 2009, 11:07:42 am
Well, an update seems called for.

I tried the puffer out again today. First I disconnected the rudder servo receiver connection and clamped the tiller arm. Then I disconnected everything else except for the ESC and set her to cross the pond. 3 to 5 seconds a little twitch left, then a twitch left again, then a twitch right, and so on. All this was at full throttle, I didn't try part throttle.

I think that PMK may be on to something as I have found that the rudder is very sensitive around center, so have had to put in about 50% exponential on the D/R in order to make it less sensitive, yet give me full throw when needed. Maybe my rudder is too big (don't fancy changing it at this stage) but I can consider putting in a beefier servo.
Full throw is about 45 degrees L and R, is that about right?

Or maybe I'll just have to live with it!

If I get it solved Mayhem will be the first to know.

Thanks to everybody.

Ian
Title: Re: Puffer Wiggle
Post by: FullLeatherJacket on August 28, 2009, 12:12:52 pm
Ian
Are you saying that the rudder linkage and servo were totally disabled for your test? If so then the problem has to be some sort of motor-speed glitch which is causing a prop-steering effect, as there don't appear to be any other forces which could be in play.
I have to say that the rudder movement you have seems excessive, especially for an unbalanced rudder which is so large anyway.
FLJ
Title: Re: Puffer Wiggle
Post by: Colin Bishop on August 28, 2009, 12:45:20 pm
It may be possible that the hull is hydrostatically slightly unstable and that the waterflow over the short hull at certain speeds can cause it to veer from one side to the other. As the rudder, even if not operating, is effectively part of the hull then changing its size may well have an effect.

The same effect occurs sometimes on full size yachts which are short and beamy as well.

Colin
Title: Re: Puffer Wiggle
Post by: derekwarner on August 28, 2009, 01:28:21 pm
My need the three person test  %%  

1) sit the puffer on a sheet of glass
2) person 1 .....holds the rudder firmly
3) person 2 .....gently holds the hull
4) person 3 .....gently actuates the RC...in fwd & astern + port & stdb
5) person 2 .....whilst gently holding the hull.......accepts the rudder servo movement to allow the rotation mass of the hull
6) persons 1+2 ...... stay ship bound whilst person 3 takes the R/C tx further >>>>>>>>>>>>>>>>>>away
7) simply testing & visually watching the R/C rudder function out of the water ....without load or pressure will confirm ZILCH

However the 'restricted' tests at close range should visually define the conditions/actions & hence the source of the R/C glitch  O0 ...................however this may only partially resolve points offered by others.........  :o Derek
Title: Re: Puffer Wiggle
Post by: dreadnought72 on August 28, 2009, 01:50:19 pm
I'm leaning towards Colin's suggestion. A hull speed which results in twitches.

Andy
Title: Re: Puffer Wiggle
Post by: snowwolflair on August 28, 2009, 03:03:11 pm
Is it an old servo, if so is its centre point on the feedback pot wearing out?  Old problem you dont often see these days, but your symptoms match.
Title: Re: Puffer Wiggle
Post by: Colin Bishop on August 28, 2009, 05:20:27 pm
I think Ian said he disconnected the rudder servo and clamped the tiller arm and it still happens in which case it isn't anything to do with the rudder servo.

Colin
Title: Re: Puffer Wiggle
Post by: Martin [Admin] on August 28, 2009, 05:28:03 pm

Any chance of a picture of you rudder set-up Ian?
Title: Re: Puffer Wiggle
Post by: ianb on August 29, 2009, 03:37:51 am
Here's some pictures of the rudder set-up. Hope they are easy to see.

Thank you for your comments FLJ, I too wondered about the possibility that the radio and/or the ESC was glitching. Because the twitch was to both the left and right at random, it seemed to me that an intermittent cut or loss of power would always cause the twitch to be in the same direction. Maybe I was incorrect in this assumption.

I have cut back the rudder servo travel to about 70% of full which gives me a swing of about 30 degrees. Haven't tested it in the water yet.

All up weight is 13 kg, maybe too heavy?

Really appreciate all the interest in my problem.

Ian
(http://img268.imageshack.us/img268/4028/47753398.th.jpg) (http://www.postimage.org/image.php?v=aVPR_pi)
Title: Re: Puffer Wiggle
Post by: Martin [Admin] on August 29, 2009, 03:43:10 am
Lovely neat installation!
Title: Re: Puffer Wiggle
Post by: ianb on August 29, 2009, 03:47:59 am
Sorry, screwed up the pictures. I'll try again.

Ian


(http://img198.imageshack.us/img198/4536/73693261.th.jpg) (http://www.postimage.org/image.php?v=Pq1wrP6i)

(http://img198.imageshack.us/img198/3546/76244288.th.jpg) (http://www.postimage.org/image.php?v=Pq1wrRBr)
Title: Re: Puffer Wiggle
Post by: ianb on August 29, 2009, 03:53:53 am
Thank you for the kind words, Martin.

What are you doing up at this time in the morning????

Ian
Title: Re: Puffer Wiggle
Post by: Martin [Admin] on August 29, 2009, 05:22:59 am
Always loking after the interests of all my Mayhem friends... and I'm also slightly mad.....  %%

Title: Re: Puffer Wiggle
Post by: OMK on August 29, 2009, 06:55:10 am
Da Boss: Be careful though because you can get some real weird comments from some real weird folk if you post in the small hours of the morning.

Janb: Pardon me if this sounds blatantly obvious, but have you tried any of what Derek Warner has suggested yet? Namely, how does it behave when it's not in the water?
Furthermore, take a bow for the neat electrical installation, but how is the wiring beneath the deck? Do you have sufficient distance between the clean wires and the dirty ones? By that I mean are your servo wires and receiver wires well separated from those on the ESC and the motor?
Silly question, I know, but we're all shooting in the dark here.
For what it's worth, it seems that your hull/ship design may not be the cause of the problem after all, which may point to one of electrical interference.

<edit>

By the way... Yep - 45-deg's either side of neutral sounds about right.
Title: Re: Puffer Wiggle
Post by: OMK on August 29, 2009, 07:08:16 am
One other thing.......

This too may sound a bit cheesy, but I just noticed that the linkage on the servo horn seems a tad close to the actual body of the servo. Do you suppose it may be causing it to bind or snag?

Hmm... on seconds thoughts, perhaps not, else you would have noticed by now, right?
Title: Re: Puffer Wiggle
Post by: ianb on August 29, 2009, 08:28:18 am
PMK

Thank you for your comments.

I have all the power cables on the port side and all the signal cables on the starboard.

No, the linkage does not bind on the servo, A good point though, which I have now checked again.

Next weekend, weather permitting, I hope to have a chance to try out what Derek Warner has suggested.

Will keep you posted.

Still stumped, as usual.

Ian
Title: Re: Puffer Wiggle
Post by: OMK on August 29, 2009, 09:49:59 am
Well, that's about as far as my fault-finding knowledge with this particular teaser goes. Sorry I couldn't be of any help.
Keep posting though. Hopefully you will get there in the end. I for one would love to know what the cause of the problem could be.

Hope your weather stays better than ours.
Title: Re: Puffer Wiggle
Post by: Bunkerbarge on August 29, 2009, 09:58:47 am
I'm not sure if we are now going up the garden path with this and getting a bit bogged down with irrelevencies.  If the model was sailed with the rudder disconnected and locked and only the ESC powered up then the problem is surely not with the electrical side of things.  Colin has rightly identified that the issue is with the hull and rudder design or combination and how they behave hydrostatically.

One thing I haven't seen mentioned, although I may have missed it, is what motor and therefore how fast is this model going?  Don't forget that a true speed for one of these vessels was no more than about nine knots however most models seem to progress across the ppond considerably faster than that!  Going too fast with the model down to it's marks will quite often cause instability, especially with a hull having the hydrodynamic efficiency of a brick!  In conjunction with this is rudder size and integrity of the linkages.  Any slack in the rudder, no matter how insignificant it may seem will lead to instability with such a hull as this down to it's marks so that should be checked out as well.

If this effect does not come to like at slower speeds I would do no more than run her slower by either controlling it or adjusting the range on the ESC channel of your radio.  You don't really want to play around with changing the rudder or removing ballast or even the motor, but if it is more stable at a slower speed, then run it at a slower, more scale speed.

Title: Re: Puffer Wiggle
Post by: FullLeatherJacket on August 30, 2009, 08:41:15 pm
Just back from a model show and seen the pictures. The tiller arm looks very very short, even if it's true scale. Any play in the linkage will be more significant the shorter the tiller arm and take-off point on the servo disc are/is/am. I'd suggest you double the length of the tiller and move the pushrod further out on the servo disc.
It may well be another "irrelevency" to the problem in hand but it won't hurt.
Wasn't there a thread a while back about a canal boat which did similar loopy things? Did the bloke ever solve that prob?
FLJ
Title: Re: Puffer Wiggle
Post by: ianb on September 05, 2009, 06:16:59 am
Update on the problem.

I went to the pond this morning with a friend to follow derekwarner decoy's suggested tests. Results; radio connection is reliable and positive with the Tx antenna down to a distance of about 200 feet. Over that distance and the motor shut down, with the rudder holding its last position. This is the way it should work according to JR.

Next, I tried the puffer at a slower speed, about 2/3, to see if the twitch re-occurred. It did, but not as much as before and not as often.

As to the questions from Bunkerbarge; the motor is a Model Motors Direct 555 and the "full" speed is less than walking speed, perhaps 2 mph. Definitely not a hotrod! There is less than 1/32 rudder free play at the aft end furthest away from the hinge.

An idea came to mind, maybe completely wrong, but then I am usually so.

The pond is man made and about 20 inches deep. I know that there are many small rocks, about 4-5 inches round, here and there on the bottom. I wonder if the prop wash goes down, hits a rock and is deflected back up which affects the steered course just a little. Or the fact that the puffer is quite heavy with a very unstreamlined hull shape may make her very sensitive to water flow changes reflecting from the pond bottom. What think you?

If you tell me I'm waaaaay off base, I won't be offended.

Ian
Title: Re: Puffer Wiggle
Post by: derekwarner on September 05, 2009, 08:40:41 am
Ian......considering all of the comments from experienced members on Mayhem...  ;D :-)) & your comments & tests....

Forget the size of the rocks in the pool, forget the 1/32" radial arc at the rudder extremity, forget the hydrodynamic "built like a brick" of the hull....& my understanding of JR R/C equipment is that it is second to none  ..........

so  >>:-( I suggest that irrespective of the motor suppression installed ............the motor is the culprit  <*< >>:-(...in that it is providing a spurious intermittent EMF signal that is not being filtered out...& hence the Rx jitter  O0 ....Derek
Title: Re: Puffer Wiggle
Post by: Martin [Admin] on September 05, 2009, 08:47:36 am
Any chance of a video clip Ian?
Title: Re: Puffer Wiggle
Post by: ianb on September 05, 2009, 12:00:23 pm
Sorry Martin, no chance at all.

I don't possess a video camera, couldn't operate it if I had one, couldn't upload a video to save my life, and can't even post pictures without you having to correct my efforts.

No, video is way beyond my limited technical ability.

Ian
Title: Re: Puffer Wiggle
Post by: GG on September 05, 2009, 04:34:51 pm
Time for my two pennyworth's?
All the supplied information suggests,
1) the rudder servo and linkage are not to blame
2) no evidence of motor interference
3) the rudder is completely, or almost so, covered by the propwash
4) the model has a very bluff bow shape

The third point would mean that the the rudder is unaffected by small changes in heading.  The water flows past the rudder just the same and so it exerts no corrective action.
The last point suggests that the models "centre of resistance" lies ahead of the models "center of gravity", possibly moving further forward as the speed and associated bow wave increases.  This makes for an unstable model which will not hold any heading when disturbed.
I suspect that the as the model builds up speed, it becomes directionally unstable and starts to weave from side to side.  This slows the model and reduces the instability allowing the model to run straight again.  The the whole process repeats again.  This ties in with a description of the model's behaviour.
One way to test this theory is to enlarge the rudder.  Extending the rudder rearwards will not take it out of the propwash and might be of limited value.  Extending the rudder downwards, and thus out of the propwash, is like to be more effective.  It will now be in the free flowing water stream and thus notice as soon as the models heading changes.

I cannot help but think that a lot of confusion has arisen through the use of the word "twitch" which to me usually means a violent motion caused by sudden rudder movement.  Hence, my use of the word "weave"
GlynnG
Title: Re: Puffer Wiggle
Post by: ianb on September 06, 2009, 06:23:27 am
GlynnG,

Thank you for your very succinct post. You are quite correct, the word "weave" is a far better choice than "twitch". Mea culpa.

The cause of the weave may be exactly as you have so carefully reasoned. If so, then there is nothing I can do in a practical sense to alleviate or cure it ie "its in the nature of the beast".

The best thing to do may be to just accept it, and make small corrections to the steering as required in order to maintain an approximately straight course.

Thank you to everybody who contributed many valuable ideas and much good advice. Much appreciated.

The Mountfleet Puffer has been a challenging but very satisfying build. She really looks great on the water. I'm happy, and isn't that why we build model boats?

Ian
Title: Re: Puffer Wiggle
Post by: GG on September 06, 2009, 02:28:56 pm
Glad to help Ian,
              If nothing else, this thread illustrates the difficulties of trying to sort out problems on an Internet Forum.  If the problem is not described with 100% clarity (not impossible but never easy!) it can encourage people to launch into their own pet theories.  A good example is external interference, often claimed when the true problem can lie closer to home.
I'd be tempted to try adding a temporary rudder extension to your model, perhaps some thin material held in place with waterproof tape.  This might confirm the problem and possible solution, perhaps too late for this model but handy in future?
Just returned from sail my latest model which has this problem.  I quickly learnt to sail at less than full throttle and keep applying small rudder corrections.
GlynnG
Title: Re: Puffer Wiggle
Post by: ianb on September 20, 2009, 09:56:18 am
Final update on my problem if any body is interested.

I installed a good piezo electric gyro, left over from the days when I tried to fly model helicopters, at the approximate CG of the puffer. 50% gain at the rudder neutral was put in.

What a difference! Almost a straight course without any correction from me. No weaving that I could see.

I am coming to the conclusion that the hull in it's present form, ballast, speed, rudder size etc., or the combination, is just not stable in direction when on the water. Why? I don't know.

So, my problem seems to have been solved with an item from my parts box.

Thank you to everybody who contributed ideas and suggestions.

Ian
Title: Re: Puffer Wiggle
Post by: Colin Bishop on September 20, 2009, 12:27:54 pm
Thanks for the feedback ian. Some hulls are just like that - the Battleships Rodney and Nelson were full size examples with poor low speed steering characteristics.

Colin
Title: Re: Puffer Wiggle
Post by: farrow on September 23, 2009, 11:09:43 pm
Many years ago, I skippered a motor barge with a similar shape to your model. An yes she was a COW to steer, you never knew which way she would sheer, but when manoeuvring she would turn on a sixpence. I ended up with large biceps and frayed nerves, as when you was going up a river you never knew if she would veer off course and hit something.