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    The Games Chest
    Making an Emma Jr.
    50 Posts ยท 11 Followers ยท 83 Photos ยท 220 Likes
    Began 24 days ago by
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    Latest Post 7 days ago by
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    RNinMunich
    Fleet Admiral
    ๐Ÿ‡ฉ๐Ÿ‡ช Germany
    ๐Ÿ“ Making an Emma Jr.
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    Hi Roy,
    Re: "does 'wandering discussion' mean 'Off Piste'?"
    In a manner of speaking yes.
    Coz it often causes an avalanche of digressions๐Ÿค”๐Ÿ˜‰
    Doug๐Ÿ˜Ž
    Young at heart ๐Ÿ˜‰ Slightly older in other places.๐Ÿ˜Š Cheers Doug
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    roycv
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    ๐Ÿ‡ฌ๐Ÿ‡ง United Kingdom
    ๐Ÿ“ Making an Emma Jr.
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    Hi all I usually make my keels from laminated wood or plywood. The outside surface being vertical. As I have said I also make the cross section like an aerofoil and here the various layers of ply can work for you.

    As you sand the rounded fore edge of the keel tapered up to about 1/3rd. the width of the keel and then tapered down to a near point. You will see the contour lines created by removal of the wood layers as sanding progresses. This can help to make the 2 sides symetrical in shape.

    Picture shows a trial rudder I made but did not use. You can see the nice contours which you can then match to the other side.

    This was 1/8th. ply cut to include a bend in the rudder shaft and then 2 layers each side of 1/16th. ply glued together with any PVA glue and clamped together for 24 hours in a vice.
    Roy
    roycv
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    ๐Ÿ“ Making an Emma Jr.
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    re post 41.
    Length of keel and weight.
    A 20 mph wind has an effect of 1 pound per square foot of sail. Work out the area say 3 square feet.
    Find the centre of area of the sails and measure how high that is above the water line. Let us say it is 15 inches. The 3 square feet now changes to 3 pounds and multiplied by the 15 inches gives 45 inch-pounds.

    So if the keel weight of 4.5 pounds is chosen it should be 10 inches below the water line and the keel length can then be measured.

    It could also be 9 pounds at just 5 inches but the displacement of the yacht would not allow that much weight. So take the all up weight of the yacht and aim to have a keel weight of 40% of this. Then divide this into the inch-pound figure for your yacht.

    The area is a bit draw it and see if it looks right. It is a good idea to make an aerofoils section of the keel as well.
    Bulb keels are very affective but slabs of lead flashing, low down, making the keel thicker are also effective.
    Regards
    Roy
    roycv
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    Hi Doug does 'wandering discussion' mean 'Off Piste'?
    Roy
    RNinMunich
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    ๐Ÿ‡ฉ๐Ÿ‡ช Germany
    ๐Ÿ“ Making an Emma Jr.
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    Hear hear Ron.๐Ÿ‘
    But I'm puzzled as to why you have put 'Emma Jr' on the Open Forum and not in a Build Blog?
    Folks tend to respect a Blog as being more personal than the Open Forum which invites wandering discussion.
    Cheers, Doug๐Ÿ˜Ž
    Young at heart ๐Ÿ˜‰ Slightly older in other places.๐Ÿ˜Š Cheers Doug
    RossM
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    ๐Ÿ‡จ๐Ÿ‡ฆ Canada
    ๐Ÿ“ Making an Emma Jr.
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    I am sorry Ron. You are correct
    Force nothing, waste nothing, leave nothing undone
    RossM
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    Alessandro,
    You used the term
    CENTRE OF THE HULL
    Possibly lost in translation, does this refer to
    CENTRE OF BUOYANCY OF THE HULL?

    It is best ABOVE the centre of gravity.
    CENTRE OF BUOYANCY will shift as the hull heels over.

    Does centre of hull and centre of buoyancy refer to the same thing?

    You are correct the righting moment will have to be discovered EMPERICALLY. I am not looking forward to that series of measurements and calculations
    Force nothing, waste nothing, leave nothing undone
    AlessandroSPQR
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    ๐Ÿ‡ฎ๐Ÿ‡น Italy
    ๐Ÿ“ Making an Emma Jr.
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    Yes, Ross is right, we invaded your topic a bit, sorry Ron.
    But by doing so your magnificent work will get more visibility.

    My intention was only to reassure Jumpugly. His model will have no problems because, as already said, it has space to spare and does not have sails.
    The sails are the big problem because they raise the center of gravity and (especially when the wind is abeam) tilt the ship to one side.

    However, in this regard I want to clarify something that many people are confused about. Let's say they don't have clear ideas.
    In scale naval models we can more or less easily obtain a "weight stability" (in Italian manuals this term is used when the center of gravity is below the center of the hull).
    In reality (with a few exceptions) almost all ships and boats (passenger ships, ferries, motorboats, oil tankers, ocean liners, cruisers, aircraft carriers, destroyers, frigates, corvettes, auxiliary ships, icebreakers, container ships, but also ancient galleons and vessels, as well as naos, caravels, carracks, triremes, liburmes, etc. etc.) only had "stability of form" (in Italian manuals this term is used when the center of gravity is above the center of the hull).
    It follows that the righting thrust exists when the metacentric height is positive. In other words, the ship is able to right itself only within a certain angle of heel, not beyond.
    Ships with "stability of form" beyond a certain angle will no longer be able to right themselves and will capsize.
    Most ships, even in adverse weather conditions, do not suffer dangerous inclinations (except for a few cases in where tragedies occur).

    Ross, regarding your fin, I am not able to establish how long it should be, calculations should be made with all the data available (almost impossible to find). It is much easier to do empirical tests.
    As you know, the longer the fin is under the keel, the less weight you need to put at the end of it.
    If you want less draft you need to increase the weight at the end of the fin or the thick fin (in case the weight is distributed and does not have the final bulb), so as to be able to shorten it.
    Logically the weight can be increased within certain limits. Until reaching the waterline.

    I completely agree with luckyduck: it is important to already place the necessary weights (not considered ballast) as low as possible (at least under the waterline).
    Since I made the project from scratch I was able to make the choices I wanted and I managed to position the relevant weights as low as possible. I am referring to the two batteries, the motor and the propeller shaft, the large electrical cables, etc. etc.
    In this way I managed to obtain a good righting thrust (I cannot say without calculations whether I obtained a "stability of form" or a "stability of weight") even without ballast.
    When I then added the 400 gram ballast (in place of the keel) I obtained a truly satisfactory righting thrust.
    All empirically.
    Doing as luckyduck says is better. Create a stable ship already without ballast (if that is possible).
    luckyduck
    Sub-Lieutenant
    ๐Ÿ‡ฌ๐Ÿ‡ง United Kingdom
    ๐Ÿ“ Making an Emma Jr.
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    The other option you have, which I have used in the past, if you have sufficient internal space at the keel level, is to use an oversize battery - the sealed lead acid batteries are a very effective extra weight set on the internal keel, when you also need battery power for winches. I sail my Bristol Pilot Cutter with no external fin/keel weight, as the water needs a smooth hull surface to avoid catching weed.
    RossM
    Commander
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    ๐Ÿ“ Making an Emma Jr.
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    Yes Ron, Your first drawing is my next calculation, HOW DEEP A FIN KEEL DO I NEED FOR A RIGHTING MOMENT?

    I have to find the weight of the final vessel and the ballast required to keep it at waterline and the length of the keel for the righting moment.

    I do like your idea of an interchangable keel for different conditions.

    I don't think I can get that budget for multiple fin keels past the Auditor-General๐Ÿ˜‰
    Force nothing, waste nothing, leave nothing undone
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