Hull Shape Considerations
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Hull Shape Considerations

Hull Shape Considerations

By Julian Bethwaite

A very interesting conversation developed on Sailing Anarchy week or so back and it has had me think about what is planing upwind, why can some boats do it and others can’t, even though they have similar righting moment (RM), and to a lesser extent, as we have just gone through a re-tooling process, what are acceptable variations to a one design concept.

It’s a biggish topic, but all inter-tangled because I’m not quite sure people get it.

I come back to the fundamental issue which is – what is the biggest difference between air and water?    Both are fluids and most naval architects would have you believe they’re similar.  Why?  Because most research has been done on wings and fuselages of aircraft.  A few % points difference in Coefficient of Lift or Coefficient of Drag makes a huge difference to fuel burn, so it’s worth investing $b in research because the savings are worth $b’s.   Whereas, the sum with respect to a sea-freighter, it’s still important, but a few % difference gets negated by how much cargo can be carried.

Air is compressible whereas water is not.

This even affects the car industry where water in the cylinders can bend crankshafts.

So when it comes to boats that are going to plane upwind, as compared to a boat that does not, mm’s count.

Going back to Sailing Anarchy, the argument centred around what was planing upwind?  Crack any boat off say 30° and spring the sheets, and within reason, most boats will plane.  Technically they are going upwind so they are planing upwind.

But the more meaningful definition to me and I believe to the assembled multitude on Sailing Anarchy was a boat that can plane upwind achieves a meaningful higher VMG than they would have had they remained displacement sailing and pointing.

So a couple of definitions:

RM = Righting Moment.  How far you move the crew weight and how much weight you move sideways from the CoB (Centre of Buoyancy)

hull-shape-diag2Righting Moment and Sail Carrying Power – click on diagram to enlarge

VMG = Velocity Made Good.   Very few sail driven boats can sail straight into the wind (a few with blades can, but most would consider those boats “out there”, and they are pretty slow) so you need to tack.  If you assume 45° tacking angle, that means for every 100m sailing through the water you are actually making ≈ 70m to windward (sine 45°). If you did that in 30 secs, you are doing 2.3 m/s VMG.  1knt ≈ ½ m/s (or 1.7ft/s) so your VMG is approx. 4.6knts.  Speed through the water is 100/30/0.5=6.666 knots.

This would be a good example of a Flying Dutchman sailing in displacement mode upwind.

So the next definition is displacement, often referred to as Hull Speed (HS).

Start from the simple formula, HS = (√ water line length) x constant.

Because my father was a pilot and speaks in imperial (feet and knots) I will stay there but there will be metric equivalents.   So length in this case is in feet, the constant for salt water is ≈1.4 and the result is in Knots (fresh water is very slightly different).

So take a 29er LWL = 14ft, so √14 = 3.741 x 1.4 ≈ 5.238knts.

So what does that mean?

Because of the nature, the viscosity, density and environment of the water that a sailing boat operates in, as it moves across the water it pushes the water down and out (from the surface of the hull at the bow, the bow wave) and what flows out, must flow back! It does and the time it takes for it to go out and come back to ostensibly the same point, in the case of a 14ft LWL boat, travelling at ≈ 5.2 Knts, that water flowing back (the stern wave) happens at the transom.     So if you travel slower than that, the stern wave will form in-front of the transom, if you travel faster than that it will form behind the transom.

If the stern wave happens behind the transom, i.e you are moving faster then HS, the boat will go nose up, as the stern falls down into the trough in front of the stern wave and you end up sailing “up the bow wave”!

Just like pushing anything uphill, it’s hard work and the heavier it is, the harder it is to push.

Same goes with a boat.   The result is you get a pretty significant spike in the drag curve.

So the attached graph is based on hard empirical data, peer reviewed, yada, yada, yada.

It is the 470, done May 2016 and it is the 49er, pre re-tooling 2005.

hull-shape-diag1Drag curves for 470, 49er and 29er hulls – click on diagram to enlarge

Picked these 2 boats because they are very similar in terms of weight, particularly hull/rig/foils weight (near identical) and they are also pretty similar in waterline length (LWL) but mostly because we have the data to hand.

The 29er curve is my best calculated guess.

With the 470 you can clearly see the hull speed (HS) bump, so for a 470 to exceed HS it needs a considerable increase in grunt.

The 49er has a much flatter drag curve at this point, what my father often referred to as a dynamically humpless hull!   49er has almost double the “grunt” (more correct word is ‘righting moment’ (RM), less correct is ‘power’) than a 470, but if you now look at the 29er curve, because it has similar RM to a 470 and would have a similar if not slightly lower drag curve than a 49er (because it’s lighter) then the reason why a 29er can’t help but plane upwind, most of the time, whereas a 470 has to hold off until circumstances are favourable, becomes obvious.

Switching direction a little, what is an acceptable tolerance when we retool?

We start with what is realistic.  Moulds move, does not matter how well they are built, +/-1mm in the length of a 49er/29er has to be accepted.

So when we re-tool, I insist on an absolute minimum tolerance of +/-1mm under the chine, it matters there, because water is uncompressible and that 1mm moves the water alongside it, and so on, and so on!

How far, the best example I have was Helen, who worked for me for 15 years, besides being an accomplished glider pilot and really amazing with FRP she also represented Australia at the 1976 Montreal Olympics in the one man (woman) scull, think we now call them K1’s.  In the St Lawrence seaway, at 10m deep, they achieve much faster times than in the rowing channel at 4m deep.    So if a K1, at its weight, affects water 4m down, a 29er has to be moving water more than that.    Add to that, Helen still paddles but these days up and down the Danube, often in a 2 woman K2.   Though it’s not meant to be competitive, can’t change her so they exploit the pressure wave, so when the water is restricted from moving down and out, it heaps up beside the boat, forming a pressure wave, and apparently the “grunt” required to paddle up this wave, by a following K2, is near insurmountable.

Coming up from the chine with a 9er, because they are sailed flat, it’s less critical, so +/-1mm at the chine, ¼ of the way up 2mm, ½ way up 3mm, ¾ of the way up 4mm and at the gunwale +/-5mm.

With respect to the deck, provided you don’t increase the beam, industrial tolerance here is +/-10mm and it’s now more about ergonomics than hydrodynamics so that’s just fine.

Those are my parameters, not ISAF’s or ISO’s.    I have imposed them every time we re-tool.

Both the 29er and 49er are re now represented accurately electronically and we can photo scan anytime relatively cheaply to verify a hull if we need to.

But it’s also horses  for courses.  Some people believe it’s a cube law, because it’s 3 dimensional, but I will hold with convention and accept it’s a square law, even still a 29er that regularly exceeds 20knts (occasionally 30) is bending double or triple the amount of water that the 470 does, so it’s only fair that tolerance is +/-1mm (where it counts) whereas the 470 is +/- 17mm over its entire surface.

I won’t get into “humpless hulls” now because it will consume another 3-4 pages, maybe next week’s edition.



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