Where’s the “waterline”

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This topic contains 7 replies, has 4 voices, and was last updated by  Tony Thresher 5 years, 8 months ago.

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  • #51599

    Andy Weir
    Participant

    Hi there,

    I’m planning things out for when I attach my new transom to the back of my boat, and I’m trying to work out where to put rudder fittings…

    … in the class rules, it shows the depth of the rudder from waterline, but I can’t find a definition of the waterline…

    … is it the outside of the hull skin? or the end of the keel? (or somewhere else?) as there’s a 25mm difference between a “short” rudder at the hull skin, and a “long” rudder at the keel (10mm tolerance on rudder plus 15mm max depth of keel).

    Thanks,

    #53515

    Tony Thresher
    Participant

    Andy,

    I think you are correct to point out that there is no definition of the waterline in the rules and something that needs clearing up. Rather than measuring from an undefined waterline which is difficult to determine easily we should go for either the underside of the keel or the underside of the hull at the centreline. I have always taken the measurement from the underside of the hull but for no particular reason.

    An important thing to do will be to clear up the rule so that all current rudders that comply with the depth measurement, measured either from the underside of the keel or the underside of the hull still comply. This would mean a slight increase in the depth tolerance I suspect.

    Before we go down this route hopefully someone will be able to give us an easy waterline definition, that is one that mere mortals can easily measure to!

    Tony

    #53516

    Andy Weir
    Participant

    Thanks for that Tony (I thought I was missing something),

    forgot to take into account the keelband – that makes a total difference of 32mm.

    Another one that’s a bit woolly is the hole for the tiller – the bottom of the hole on my original transom was 75mm down from the apex of the curve (and not a straight line between the two corners) – it was also a bit wider than 300mm…

    … so how “curvy” could you make the sheerline, and still be legal (from the camber detail – MKIV 6mm rise in 90mm – others 6mm rise in 50mm)?

    I think a good datum (for both these measurements) would be a straight line between the two corners.

    #53517

    Michael Brigg
    Participant

    The drawing right at the start (On page 1) of the class rules appears to show the waterline/datum plane passing exactly through the bottom of the stem at the bow, and through the bottom of the transom (keel band included) at the stern.

    This same “Datum plane” is also marked in the drawings of the Plate case on page 25 of the class rules. (These are linked to the Technical Information section of the “About the firefly” tab, linking to the “Documents” section of the RYA site.)

    http://www.rya.org.uk/SiteCollectionDocuments/technical/Web%20Documents/Class%20Rules/Firefly%202013160113.pdf

    The full drawing and extension of this Datum plane will be evident in the original drwings, and I seem to recall Alastair Curry had a copy of these that he bought along to the Hamble Nationals. Perhaps all it needs is a quick perusal of the original drawing to settle the argument.

    The “original” rudder fittings also give a clue, with the pintle being mounted pretty much directly on what is simply an extension of the keel band.

    A final guide would be to look at an “original” fairey built “Fixed” rudder in position on an “Original” Bronze Pintle/keelband assembly, and take measurements from there.

    As you point out, with uncertainty about measurement from the outer skin or keel (or even keel band) there is a 25mm (or more) opportunity to exploit the uncertainty in the rules, but with little point as it seems likely that the technical committee will sort this out. It sems likely to me that the lowest point on the transom (ie the Keel band) will be the easiest point of measurement, the most consistent with the original build (ie Keel band/Pintle assemply) and also the least likely area to allow exploitation.

    The 40mm +/-10 refers to how far away from the transom you may put your pintle. (We don’t want to turn the boat into an i14 or skiff with rudder gantries 1 foot or more away from the boat!) A good reason to allow this tolerance is that the commonly available metric fittings for the Rudder Gudgeon have a diameter of @4cm, with an eccentric hole for the Pintle. If fitted to the smaller original fittings, at extremes of steering this causes the pintle to shear off.

    #53518

    Tony Thresher
    Participant

    Thanks Michael. I was hoping you might come up with the definitive answer. Of course the Rondar hulls dont have any keelband so perhaps underside of keel might be better just for consistency?

    Tony

    #53520

    Guy
    Participant

    Tony, I have sent you a reply by e amil on this. See what you think? Then if we agree on the current rule interpretation, I’ll post it here very shortly. I think we have an answer and it is very close to what has been said here already I believe by Michael .

    #53521

    Tony Thresher
    Participant

    Our technical secretary has come up with the answer which can be found on the old rudder diagram in the class rules. In the notes it states

    ‘The water line is defined as the line perpendicular to the transom passing through the intersection of the aft side of the transom and the outside skin at the centreline.’

    This has got to be where to measure from as there is no variation in the point from which one measures. The keelband (3mm to 7mm) and keel (11mm to 15mm) tolerances adding up to a possible 8mm difference dont come into the equation.

    I have not been long in the class but have heard some stories of how Guy used to extole the virtues of the old spoon rudder. Thank goodness he did and used one for so long otherwise we’d have completely forgotten the diagram still existed!! Time for this definition to go on the straight rudder diagram page I feel.

    Tony

    #53522

    Tony Thresher
    Participant

    Andy,

    Yes we were missing something as you can see from above!

    On your tiller port question remember the sheerline is defined as where the extended line of the outside of the skin meets the top of the deck. The bottom of the tiller port should not be more than 75mm below the sheerline at the the transom. So your old tiller port complied with this if it only measured 73mm from the top of the transom at the centreline.

    As for the curve on the top of the transom I’m unclear where you got your measurements from? The figure I’ve used is 30mm to 35mm above the sheeline at the centre but this is what I’ve arrived at by measuring some transoms in old boats and allowing for the fitting of a substantial enough piece of wood to fit a top rudder fitting to take a rondar stock. No need if you are going to fit pintle and gudgeon below the tiller port but still good practice as it gives you something to screw your mainsheet track onto.

    Tony

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