I have been thinking very carefully about Dick Lemke’s observations about what happens when you let a forestay/headsail luff go seriously slack off the wind.
It has just dawned on me that this phonomenon is already well known. It is an axiom in high performance dinghies (particularly those odd few with spinnaker) that off the wind you let the rig go forward ‘over the bow’ and that this helps keep the bow up.
Taken simplistically this is obviously pure witchcraft: moving the centre of effort forward has no effect on trimming moment - the boat is a rigid beam, not a piece of elastic.
However, if we view what is happening as an effect on the shape of the headsail, we have pretty much exactly the effect described by Dick and are back in the realms of academic geometrical argument rather than folk tales.
off the wind you let the rig go forward ‘over the bow’ and that this helps keep the bow up.
My understanding is that letting the rig forward affects steering. It reduces the steering effort, reducing resistance. Less rudder input reduces speed loss due to steering input.
Letting the rig forward places more weight forward, and moves CofE forward. Both are more likle to press the bow deeper, not lift it.
Lifting the bow is a different issue. When the bow depresses, it increases forward displacement. Pushing more water out of the way increases resistance. The resistance tries to slow the bow. The stern tries to overtake the bow, resulting in a broach.
The question is “how to lift the bow”. In full-size, moving the crew aft helps. Another way is to plane fast enough that the bow is riding its own bow wave or by ‘surfing a wave’. Another way is from hull shape, eg the scow config with a spoon shaped bow.
All of these are hard to do in a Footy (at least no one has yet shown us how)
The forward transfer of weight is minimal. I estimate 0.5 mm headdown trim on a conventional Footy hull.
Downwind the CE is in 3D is not much affected by mast rake. The big differences are transverse. Further, the hull is a rigid beam: it does not matter whether the same horizontal force is applied at the bow or the stern (think about it, it’s right).
Your explanation of a broach is quant. The added resistance tries to slow the entire boat.
Moving the rig forward increases the ‘lead’ ie moves the CoE (centre of Effort of the sail plan) forward of the CLR (centre of lateral resistance).
This seems to apply even when running. Increasing the lead, increases its leverage in the direction you want to travel, so less rudder input is needed.
I suspect that the improvement seen in the ‘loose jib’ incident has more to do with depowering the jib, rather than it acting as a spinaker. It was a windy day, and the boat was already overpowered.
On lead, the biggest effect with a planing hull is when the forward part of the hull lifts out of the water moving the CLR way aft, much further than anything that might be achieved by letting the rig flop forward.
As to overpowering, you may well be right: the geometric explanations of spinnakers ‘lifting’ the bow are unsatisfactory and a reduction in trimming moment will obviously have dramatic effects.
Barrett, 0nly just a millenium. Viking longships, which were in use until about 1250, were astonishingly fast and seaworthy (see the film of Robin Knox-Johnson and a Viking cargo ship reproduction).They were built of clinker planks swen together and must have flexed hugely.
if i remember they were stitched around a centeral beam, [this is just what i think i read in national goegraphic] so while you are prolly right, that they flexed quite a bit, it would seem that they were attempting to stop that flexing…?
Barrett - for God’s stake stop apologising. Your comments are (almost ) always intersting and just as valid as anyone :zbeer: else’s!
Construction methods varied (not that we have many examples). Looks look cargo ships had a relatively solid centreline (logical if they were to carry loas) but drakkar (longships, warships) did not have a lot. Prhaps just to save weight. Remember that Viking military technology included the portage - like carry your ship across land. Eirik the Red and his Trailer Sailor!
The ability of a spinnaker - asymmetric or otherwise - to lift the bow of a high performance dinghy is indeed well known. In skiff classes with overpowered rigs (such as the the Australian and New Zealand 12 footers, and the International 14) the sailors know that it is actually more difficult to keep the boats from nose diving in overpowered situations downwind without an extra up, than it is under main and jib alone. Rather than explain it - I recommend you just do a Youtube search for “12 foot skiff”. The results are pure entertainment but demonstrate the principle.
What dinghy sailors also know, is that keeping the centerboard down during highspeed downwind legs results in “tripping up”. Our deep-keeled RC sailboats suffer downwind in part because of the fact that their keels are so deep. Hence the trade off between the deepest fin the class rules allow (and the uphill and reaching power arising), and downwind control issues. The extremes available in the Marblehead and 10 Rater classes are good examples.
) always intersting and just as valid as anyone :zbeer: else’s!