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Author Topic: False keels again  (Read 1498 times)

Will

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False keels again
« on: April 20, 2018, 10:50:53 AM »

I'd like to pick the assembled brains of the forum and ask if my idea for constructing a false keel would work.

I hope to build a hull for a working schooner rig, just over 4 foot long.  I will need a false keel, both to act as ballast and to provide righting balance.  I reckon on around 20 kg of ballast.   I plan to use a stainless steel bar, 1m long, x 60mm wide x 3mm think embedded into the wooden keel of the model.  Basically I will construct a sandwich of wood - steel - wood.  I'll then build the rest of the hull in wood.  When that's finished I'll have around 35mm of steel sticking out the bottom, for the central 1m length of the hull.  I can then use this to attach ballast and a fin keel, and gives me an easy way to experiment and adjust trim.

But the questions that I have are:
Would the turning force  cause the steel to break the wooden structure of the hull?
Would the different thermal expansion of the steel rule out the idea?

Many thanks
Will
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tigertiger

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Re: False keels again
« Reply #1 on: April 20, 2018, 11:24:24 AM »

If you know the lines of your boat and can work out the volume of the hull below the waterline, you can work out how much ballast you will need. 1 liter of water = 1kg, minus the all up weight of your model (you can guesstimate this if you don't know).


As models get longer, they need less righting force. As your weight will be in the keel, that is already low down. If you use a keel fin it only needs be short. A schooner rig is much shorter (height) than a bermuda rig on a racing yacht, and the tilting forces will be much smaller.
The sandwich should be no problem, I have seen molten lead in some plans sandwiched in the keel as ballast for sail boats. With only 1m of length and 3mm thick, I don't think expansion will be a problem.
Turning forces also no real problem.
If you pre-drill your steel bar with several equidistant holes before fixing in the wooden keel, the trimming the ballast will be easy.
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tarmstro

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Re: False keels again
« Reply #2 on: April 20, 2018, 07:23:49 PM »

It would be good to know the size of the boat you will be building. Ideally you should provide plans of both hull and rig, so that people on the forum can help you do some calculations....
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roycv

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Re: False keels again
« Reply #3 on: April 20, 2018, 09:02:39 PM »

Hello Tarmstro, I see you are on post number 2 so welcome.
The text says that it is 4 feet or 1.2 metres, and is a schooner and is heavy at 20+kgrms.
Are you building model yachts!
kind regards Roy
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tarmstro

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Re: False keels again
« Reply #4 on: April 20, 2018, 09:51:53 PM »

The text says that it is 4 feet or 1.2 metres, and is a schooner and is heavy at 20+kgrms.


If the displacement is 20kg, then the ballast must be less than that. Think about these weights:
  • Hull
  • Rig & Sails
  • Rudder
  • Fin/Keel
  • Ballast/Bulb
  • Electronics & Controls (if any)
The weight of ballast should be simply the total designed displacement minus the weight of all the rest of the parts. Sounds obvious, but that gives you the maximum weight of the ballast. Meaning that if you make all other items lighter, you can have a heavier ballast...  And only then you calculate the lenght of the fin/keel, and for that you take into account the sail area and the overall height of the rig.


Example. My RG65 has a designed displacement of 1000gr. Rig is 70gr, rudder 10gr, fin 60gr, hull 120gr, electronics 150gr --> total is 410gr, which then allows for a 690gr bulb as ballast.   Experience in RG65's has shown that for the bigger rigs used a 35-40cm long fin is good for a ballast of around 700gr... so my fin is 40cm long.


We really need to know what the design looks like, how it will be built (carbon, fiberglass, balsa wood, hardwood, 3D-printed?), what the rig looks like, etc.... so some kind of specific advice may be given....


In any case, 20kg for 120cm seems way to heavy. Modern Marblehead sailboats are about that size, and only weight 5kg max!


More info please!
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tigertiger

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Re: False keels again
« Reply #5 on: April 21, 2018, 02:01:23 AM »

When I first got started, I got hung up about sails design, ballast, etc. On working models, there is no need. There is a fundamental difference between working boats, and racing boats (real or models).

Racing boats are designed for speed. Weight and drag are minimized, while forward thrust is maximized.If we just think about those for the fin keel for a moment.There are hideous calculations for maximizing the righting moment, while minimizing the drag of the fin keel. If you lengthen the fin keel you can reduce the weight of the fin keel required, but you will increase the drag. If you have a fin keel bulb this also helps. There are also the effects of scale and water density, the larger the model (everything else being equal/to scale) the shorter (relatively) the fin keel needs to be to achieve the same righting moment.
Efficiency = speed over the water.


Working boats (with the exception of clippers) were designed to carry the largest load, at least cost, at an acceptable speed for that trade. The surface area of the hull is far larger (comparatively) and so is the buoyancy. I have a working model hull and 13KG of ballast took it down to the waterline. I could easily add/remove 2kg and she would still sail well.

Schooners are usually working, but sometimes pleasure craft.

On sail plans. For racing boats, these are usually fixed by class rules, but change with the weather conditions. The same is true for my model schooner. I don't have different sail sets, I just remove sails. I have also significantly altered the sail pattern on my Mary J Ward model, mostly for aesthetic reasons (skippers fancy). In the pictures you can see different sail set ups. I could even remove the flying jib.

The final factor is wind. There are some boats that are known to be good in light winds, others better in moderate winds. One reason the J Class races (real world) of the early 20th century fell out of favour is that the boats were so finely tuned, that they could only race in light weather. The boats had reached the pinnacle of speed, but so many races were cancelled due to the boats not being suitable for heavy weather that people lost interest in the events.

On working models the aim is to get the boat to function satisfactorily, not to squeeze every last knot out of the design. To some degree, I am more interested in how the boat looks on the water, and with that in mind, the lower in the water, the more impressive the wake of the boat in calmer waters, and the more stable in rougher weather; but she will be slower.
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roycv

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Re: False keels again
« Reply #6 on: April 21, 2018, 04:17:28 AM »

Hi TT I do envy you your Mary J Ward schooner, they really look nice, I think your post echos most of my thoughts on scale sailing yachts. 
If you are beginning from scratch it can be a bit of  compromise where the sails are etc.  I made the J class yacht Endeavour, from the Amati kit and converted her to RC.
I decided to set up the sails so that in a good wind she wouls sail without the big quadrilateral jib sail.  in a light wind she had that sail re-attached, the difference was surprising without the quad' she looked like any other yacht, with the quad she was a J class.

The quad is a very big overlapping sail that I kept cutting back until it would fly on the either side when tacking.  I must have halved the sail area but it hardly affected the way the yacht sailed.
It is best to start with what looks right and then tinker.

All the above would lose you a race with a class racing yacht, but who is racing?

Going back to the original post.

I think 20 kgrms seems exteremely heavy for a 4 foot long boat, have you got your sums right?

Secondly do not worry about whether steel is going to cause a problem, it won't.

Thirdly as far as the strength of the structure goes and I have never lifted a model yacht of this weight, If you can hold the boat at or near the bottom of the keel out sideways and it stays intact then it is strong enough.

If you worry about the wind then a 20 mph wind has a force of 1 pound on one square foot of sail.

I shall put those schooner pictures in with my own photos, really nice, thanks TT.

Just one thing Tarmstro, many model boaters come on here to ask a question, they do not always have or want to share all the calculations etc.

kind regards to all,
Roy
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tigertiger

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Re: False keels again
« Reply #7 on: April 21, 2018, 05:17:53 AM »

For a 4 ft working boat 20 kg seems ok. The 13 kg I mentioned before was in a 3 ft fishing boat hull. Although it would be a lot for a J class or any other race boat, of that length.
Thanks for the compliment about the pictures Roy :D
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Will

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Re: False keels again
« Reply #8 on: April 21, 2018, 09:09:17 PM »

Thanks for all the replies.  I was a little careless with my question, and should have said that the length was the waterline length, and the 20kg was an estimate of the displacement not the weight of ballast.  So obviously I need to deduct the weight of the model and control gear from that. 

I'll think some more about the steel keel, but it's reassuring to know that no-one has suggested any obvious flaws with the idea.

Roy's figure of wind speed and force (20mph = 1lb per sq. ft.) is useful  :-))  Though I might have to convert to metric to avoid brain seizure!

Now to go off and give this project some long thought.
Will
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tigertiger

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Re: False keels again
« Reply #9 on: April 22, 2018, 03:08:14 AM »

There are large scale sail models, of less than 4 ft, that do not have a fin keel. The weight is sandwiched in the keel. From what I have read, light winds present no problem.
One other thing to consider, for ease of handling and transport, is to have it so that the steel ballast can be slipped in and out of the keel sandwich, using a pins or bolts (one at each end) to secure it. The steel could then be inserted at the waterside. Any protruding bolts would cause some drag, but probably no more than a layer of barnacles. If you think of the drag caused by an auxiliary motor and prop (when not in use) being similar, and auxiliary motors on sail boats are not uncommon in the real world, or in models.
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MikeK

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Re: False keels again
« Reply #10 on: April 22, 2018, 08:48:50 AM »

Peter Wiles does a fitting for the 'A' Class yachts which have heavy ballast fins, that consist of a carbon tube that goes through the hull to somewhere suitable above the water line and braced and secured. Then a CF rod with a threaded brass insert and SS screw at one end that is a snug fit in the tube. This is secured into the fin (read false keel !) and inserted up the flush cut tube in the hull bottom, then secured with the SS screw inside hull. A small guide pin is best secured in fin top to make sure the fin is in line F&A

His website is at http://www.pjsails.co.uk/ where there is a catalogue of parts. It is not the easiest part to find , but a quick email to Peter will put you in the picture

Mike
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