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stability problems

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sammyk13/12/2020 13:14:16
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104 forum posts
33 photos

Hi all fellow builders, I am looking for some advice on one of my models.it is a girl class tug built to a plan from model boats with a modified superstructure as per full size boat ,picture should be in the photo gallery. the model sails well but is very tender to the point the rudder throw was reduced to the minimum. during a normal sail I turned it across the wind and it started to heal, I put in opposite rudder to try and correct the problem but a small gust of wind ruined the day and she turned turtle and sank. all this happened in slow motion which was weird. the question is I could build another hull slightly wider and deeper but how much would need to be added to make the model sail well without looking out of scale. regards sammyk

neil howard-pritchard13/12/2020 15:05:07
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1625 forum posts
1059 photos

there is no way it should have done that if it were sailing down to its water line, or you had made the superstructure out of heavy material.

did you have ballast in the boat...........the only model i have ever known to be that tender is the Billing Calypso which did have a habit of turning upside down in a wind.............but your tug has low free board and a reasonably deep draught for good sailing properties...............something must have been inherently wrong for it to pop over.

an old now deceased friend once built a "DOG" class tug similar......................and it sailed in even the roughest of waters.

Colin Bishop13/12/2020 15:18:10
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The model does look a bit top heavy to me. As Neil says, if the superstructure is built of thick material it could be the problem.

Does the boat float at its designed waterlne?

What battery are you using?

Is there any ballast on board?

Colin

Charles Oates13/12/2020 15:29:52
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597 forum posts
50 photos

Agreed, that should be a stable model, obviously the c of g is too high, the question is why? As Niel said, is the superstructure too heavy or are the internals, especially the battery not low enough. I once saw a model with a lead acid battery stood vertically, that was a nightmare. If nothing seems obvious, take some pictures of the internals, and some showing the outside and inside of the superstructure so we can all give some informed help.

Charles.

Malcolm Frary13/12/2020 15:39:02
943 forum posts

Scale boats are an oddity in modelling.

Considering model rail, if your loco is too heavy, you say "hooray" because it will pull more without slipping. If you build a plane too light, it flies more easily, if a bit heavy, more power usually sorts things.

Scale boats have to be the correct scale weight. If too heavy, they don't float. If too light, they tend to sit too high in the water and become unstable, fall over and sink.

The weight also needs to be in the right place, not only fore-and-aft and side-to-side, but the height is also critical. Too high and you get instability. I have yet to notice any model boat with the CoG too low. The main item governing canter of gravity is usually the battery. In a tug, probably an SLA. These can be mounted laid on one side, which lowers the CoG as opposed to one mounted stood up. If a lighter type of battery is used, ballast will be needed, preferably as low in the hull as possible.

A few years back a newcomer to the hobby tried a Puffer, which was vastly under ballasted. Despite being repeatedly advised not to use it in the wind conditions that day, he persisted. Predictably, when the wind hit it, it headed away, lost all control and eventually foundered. We never managed to recover it.

sammyk13/12/2020 19:17:44
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104 forum posts
33 photos

Hi, fellow models and thanks for your answers, the model had a nimh 9.6 volts in the hull as low as possible with very little ballast, and the superstructure was made from styrene sheet which I weighed and it came out at just over one pound which seams a bit heavy .It was on is intended waterline .I do not want to build another hull to the same spec and have the same problems hence the question how much could be added to the depth of the hull and the width without looking odd .the hull was made with a plywood keel and formers with balsa wood planks finished in fibreglass cloth and resin. unfortunately I do not have any pictures of the inside of the hull as after a few weeks of wondering what to do with it I salvaged all the good stuff with the intension of building a new hull and after the removal of the shaft there was not much to keep so the hull was binned . with hind sight I should have thought a bit longer and rebuilt the super structure, the getting of wisdom is never easy. thanks again for taking the time to read and answer my posting, added some build pictures to give an idea of how she was built. regards sammyk

Edited By sammyk on 13/12/2020 19:32:27

Colin Bishop13/12/2020 20:16:01
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4756 forum posts
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From your photos and comparing with the full size ship it does look like the superstructure is a bit out of scale with the hull..

Where did you get the plans from as they don't seem to be listed in the Model Boats range?

Colin

sammyk13/12/2020 20:46:00
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104 forum posts
33 photos

Hi Colin ,my apologies I was wrong I have just checked the plan its from marine modelling 1998. regards sammyk

neil howard-pritchard13/12/2020 22:00:37
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1625 forum posts
1059 photos

yes, looking at the top hamper, and especially the brass used for the stanchions, rails mast and other odds n sods.........she was definitely top heavy......

if you are going to keep the top as us, i would suggest an extra inch to 1.5 inches in the depth of the hull, and put heavy ballast in to t.......i mean heavy as in lead shot, or flashing cut into small pieces so that it sits low in the hull, thus lowering the c of g.......if you use "lighter" ballast, such as sand or even resin you need more of it to take down to waterline and because of this it raises the point where batteries and such have to sit higher on it and thus defeating the object by raising the c of g again.

Malcolm Frary14/12/2020 10:59:43
943 forum posts

The superstructure does look like it was built to a different scale to the hull. I've got some drawings tucked away somewhere where the drafter decided it was a GOOD IDEA to draw the superstructure to a larger scale tan the hull to help "clarity".

An over-sized superstructure will act as a sail when hit by a side-wind. The force of the wind will cause a lean, in a case like that, a high CG will not help.

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