Stability and freeboard

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richardabeattie

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To come clean at once this is about a large antique pond yacht. 30" on the deck plus a long bowsprit and rigged as a cutter headed gaffer with a main topsail. A beauty. But she sits very low in the water and even a moderate wind pushes the lee rail under. I've installed a rudder servo so want to keep the water out - and there's nobody on board to play the mainsheet!

I can see two options. I could extend the keel downwards so that a reduced amount of lead on its bottom edge would keep her sufficiently stiff. Twice the depth and half the weight? But that means butchering the existing keel and would look awful when she's on her stand. Or I could fit a removable flotation collar around top of the existing keel to lift her a bit. But how would jacking her up like that affect her stiffness in a wind?
 
Or could you pay close attention to waterproofing the electronics so that she can dip her rail under the water without killing the control system? That'd preserve your ability to control the boat without wrecking her appearance. Pictures please.

Another thought. Could you make a shoe to fit on the bottom of the keel, made of lead, to increase the stiffness, but removable so as not to destroy the looks when on the stand? Hmmm, that might be difficult as all you'd be doing would be increasing weight, therefore further decreasing freeboard, which might solve one problem but create another.
 
The pushrod to the rudder goes to and fro and as it does so the angle changes so making a waterproof gland would be difficult. In any case she just looks wrong sitting so low. More weigh on the same keel would turn he into a sub.
 
Your first idea is best.

Laminate some strips of wood to make a shape the same as the existing ballast part of the keel, which you now don't need. Leave a slot in the middle by leaving out a part of one lamination. But a piece of SMC-GA-5518750 carbon strip (from http://sussex-model-centre.co.uk/shopexd.asp?id=22721) and make a double thickness extension which fits the slot and fit a bulb to the bottom of that.

You'll need a pin to hold the extension in, but it's removable and you can also use the slot for an even better display stand.

The only downside of a deep keel (other than grounding in the middle of the pond) is that pitching can increase.

I've just cast a couple of lead bulbs (model sized) and can resize a drawing if you tell me the weight. I can also show pics of how I did it.
 
The pushrod could have a bellows on it, sort of thing found maybe on waterproof toggle switches. I would reduce the weight and increase the draft as suggested.
 
Can you post some pictures of the servo to rudder linkage & servo mounting. I am sure ( As a 1OM sailor) that there are loads of ways of sorting the linkage to prevent water entry. This must be preferred to altering the design of the hull which will affect sail balance etc
 
Sorry if I'm asking a daft question here but (as an R/C car racer) have you already fitted a waterproof servo? I've no particular experience with them but have seen videos of the RC truck fraternity driving underwater with modern waterproof electronics, so for occasional or even semi-continuous immersion I would think one might be the simple answer and for a model boat seems a sensible fit in any case.

See a test of a Traxxas unit here: http://rctruckstop.com/2011/12/15/is-it-really-waterproof-plus-how-to-waterproof-a-servo/

sold via https://www.amazon.co.uk/Traxxas-TR...8&qid=1462650972&sr=8-1&keywords=traxxas+2075 other waterproof servos also available eg by HPI and Savox...
 
I used to have a self steering non-audio model yacht when I was a kid. It had a rubber band system connected to the rudder and mainsheet so that it eased the main in a gust. It worked brilliantly. Even if you don't add a mainsheet servo, there may be something you could do.
 
Thanks for all these constructive replies. Waterproofing the electrics would only work if I can seal the pushrod and I like the idea of using a silicone tube based bowden cable system. And I also like the idea of making up a dummy weight for display and a smaller weight on a longer keel for sailing. But I now think that my own idea of increasing the freeboard by a removable buoyancy collar is wrong. If the boat floats higher that must mean that the distance from the centre of buoyancy to the weight at the bottom of the existing keel will be reduced and the boat will be less stiff. What I now need is an expert on metacentric shelf height!
 
When related to full sized sail boats and therefore presumably RC models initial stiffness comes not so much from the pendulum effect of the weight in the keel as from the form factor of the hull. The self righting effect of the ballasted keel does not start to really come into effect until something like 30 degrees. However it is the shape of the hull that gives initial stiffness. So a wide flat bottomed boat will be far more stiff than a deep vee or semicircular shape. (cross section). Think here of a catamaran as the extreme example.
So I would suggest adding balsa hull shape at the chines. This will give buoyancy but more importantly will move the centre of buoyancy further out when it heels. Or put another way the boat will try to float on the extensions at the chine so that the whole mass will work to stand it up again. This righting force works from the beginnings of heel.
From there I suggest wider side deck so that water will not enter even with the gunwhale under. You may be able to reduce the mass of the keel if you increase the stiffness.
The additions to the chine area would make the midships cross section closer to a flat bottomed U shape tapering to less pronounced at the bow and stern so maintaining streamlined shape. good luck olewill
 
I do not think I have ever seen chines n a gaffer.
the problem in changing hull shape from the design is that as the boat heels the rudder has to compensate more & from a distance the helm has to be able to control this. Plus the drag of the rudder becomes a problem. Even if it is big enough to counter the force needed.
As for electrics, it is not just the servo that needs attention but the receiver & aerial plus battery & mainsheet servos that have to be considered as well. Presumably the OP has found a way of ensuring the mainsheet exit through the deck is watertight. If he is not operating on 2.4 then he needs an aerial up the mast or backstay so another exit for that as well
 
Thanks again. This is an antique model of quite high value so I can't make radical changes to the hull form or deck. All the electrics are below deck so, if I can seal the hatch and use a bowden cable arrangement so that the pushrod hole can be closed, that takes care of the risk of flooding. There's no mainsheet winch and no possibility of one. I think it has to be an extended keel with a reduced weight on the bottom of it. That might not help the heeling much but it would give her more freeboard.
 
richardabeattie said:
Thanks again. This is an antique model of quite high value so I can't make radical changes to the hull form or deck. All the electrics are below deck so, if I can seal the hatch and use a bowden cable arrangement so that the pushrod hole can be closed, that takes care of the risk of flooding. There's no mainsheet winch and no possibility of one. I think it has to be an extended keel with a reduced weight on the bottom of it. That might not help the heeling much but it would give her more freeboard.
Click to expand...

So with no adjustment of mainsail how do you expect to sail it anyway?
The mainsheet winch could be quite a simple arrangement such as a servo with a single arm
If you cannot make changes to the hull how do you intending to extend the depth of keel etc?
Are you not just wasting your time.
 
I am not trying to win races! The mainsheet will have to be let out suitably on the day. There is no room for a second servo. The foresails are self tacking. Extending the keel simply means removing the existing weights and attaching a smaller weight to a temporary bolt on flat plate keel extension. I'm sorry you should think I am wasting my time but this is a model that's crying out to be seen in use.
 
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