Research Note
Leeboards
John Harland
The leeboard does indeed function in a similar way to a center-board or a
dagger-board, but in some instances does so in much more sophisticated way.
De Hoogaars by J van Beylen mentions three features which contibute
to the superior effectiveness of the device:
- The tumblehome of the hoogaars' side result in the leeboard on the lee side
being vertical, when the vessel is heeled to the wind. A centre- or
dagger-board would be tilted, and hence a little less effective, in this
circumstance.
- The 'drag' on the lee side would tend to force the boat's head off the wind.
To counteract this, the edges on the inboard side of the board are faired off
front and back, so that the leeboard presents to the water a sort of
airplane-wing section, which generates 'lift', and tends to push the vessel to
weather.
- In some cases, the leeboard is angled a few degrees towards the centreline,
so that the water exerts pressure pushing the ship to windward. van Beylen says
that although this may be effective, that it makes raising the board more
difficult.
The Dutch word for 'leeboard' literally means 'sword' which fits well enough
for the long straight narrow type of leeboard found in some craft. It is less
apt for those leeboards which are egg-shaped. I am sure there is some reason
for constructing them in this way, and perhaps someone can explain the
underlying principle. Dagger-boards differ slightly from centre-boards and
lee-boards in that the centre of their lateral resistance stays in about the
same position relative to the centre of effort of the sail-plan, as the device
is lowered. With centre- and lee-boards, the CE continues to move forward, as
the board is immersed.
Some of these features are found in the side-rudder of the
Viking-ship....another ancient artefact which is much more sophisticated than
it at first sight appears. Once again, there would be the problem of drag on
the starboard side, and this is counteracted by using an airfoil section to
produce 'lift'. Also the Viking rudder is 'balanced' unlike the later midships
rudder which works on the barn-door. See Seamanship in the Age of Sail
pp. 69-71 and:
'Two Danish Side Rudders' by Ole Crumlin-Petersen: Mariner's Mirror
Volume 52 (1966) pp. 251-261. This has a nice clear drawing of the Varså rudder, and
by drawing a line down the axis of the shaft, one can see the surprising degree of
asymmetry of surface ahead and abaft the axis.
{John Harland}
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