A few notes.

1 - The article did not take observed boat speeds and work backwards to make a correction fit. An approach was taken by which a different number was used in place of the standard 1.34, one method being to use hull length to beam ratio if I remember right. They then applied the approach to various different cats of the time to see how close it worked and reported the findings. In most cases it was fairly close.

2 - Don't know, but I don't believe the article was penned by a journalist. Of course, just because a journalist were to write something does not make it wrong. People with wonderful credentials can be just as wrong.

3 - The modification of the constant is in recognition that not all displacement boats get trapped by the wave system. Planing hulls climb over the bow wave, lighter displacement hulls with a narrow form cut through them. In this case the attempt is not to imagine that the wave system magically moves faster, by goblins or other means, but simply to characterize a hull type that doesn't get trapped. Is it an appropriate extension of the original formula then? Maybe not, but does that make it wrong? It probably tries to move towards a simplistic estimation of how the hull plows through the water regardless of the wave system using beam to length and displacement.

Anyway, out the cats I've sailed, all have greatly exceeded the traditional hull speed (using the traditional without the goblins method of calculation), and the only one that I would characterize as planing to any degree has been the I-20.

Of course, then there's the Farrier tris where the main hull has planing sections and the floats do not...

So how was this correction factor derived ?


To me it still sounds like adjusting the gravitational constant to have the formula for falling speed reflect the true speed of a bag of feathers better.

Wouter

Once again, by looking at the hull shape and characterizing beam to length ratios, displacement, perhaps prizmatic coefficient... I don't have the exact formula used in this case, but I do remember that they did not work backwords. But I have seen other approaches that use the displacement of the hull, and again, this is because it is known (as you have also stated) that narrow hull forms and those of lighter displacement can break out of the wave system without planing. ULDB monos are another case in point (although they may also plane). So again, they're not trying to make the wave system faster. They're trying to characterize the speeds of the boats.

And checking your numbers back against real data can help refine your approach as long you don't get into the "for a Hobie-18 use this factor, for a Taipan 4.9 use this other factor" trap. If you find from emperical testing that the calculations are more correct for light displacement boats of a certain LWL range if a certain factor is used, then that's ok.

And yes, because it takes something that worked for a specific case based on a certain principle and tries to make it reflect something else you can argue the merits of it. I think when you're out of the wave trap and still displacement that displacement, prizmatic coefficient, and water line length will act to determine a ballpark theoretical speed for the hull. So this actually may be useful even though it no longer holds to the principle it started with. Do I have all the numbers? Nope. But I also know that just because a Hobie 16 can do more than the speed indicated by the traditional hull speed formula does not mean it is on plane. And this may just be a good way to get a ballpark estimate, although it doesn't work in the traditional wave theory sense.

Note that I say ballpark - because any of these basic formulas take no real life things into account. It's strictly the hull in the water. Nothing about how efficient the rig is, upwind or downwind, how ungodly huge the wind resistance of the cabin structure is, how far the boat is heeled, or how many cheeseburgers the crew has snarfed in the last 12 hours.

Go back to the days of old when racing design formulas would restrict waterline length at rest. Large over hangs front and rear dramatically increased the waterline length when the boat was heeled.

If it's ballpark does it matter? To some people. At least they can have a better explanation as why cat hulls are not constrained by the usual thinking on displacement hulls. If I had a dime for every time somebody told me my cat was fast because it planed... Another explanation - multihulls are fast because you multiply the waterline length by the numbers of hulls, and thus the equation almost works again. I have to remind them that the boat should then slow dramatically when you fly a hull...

Anyway, good discussion, and yes we'll be debating this again soon. With respect to the original question of whether the V40 can do 40 knots. I suspect somebody came up with a theoretical max for the hull form of around 40 mph, and somebody turned that into knots because it sounded good that a V40 could do 40, and nautical types like to talk knots, donchano. The fact that the boats will never actually achieve that in real life doesn't matter unless they put the boats into speed trials...

While we're talking about Volvo 40's <img src="http://www.catsailor.com/forums/images/graemlins/confused.gif" alt="" />....

note the BOLD text below!

Okay lets put this discussion to bed.

One last note. Your last post did remind me alot of what the Texel and ISAF measurement handicap systems are actually doing. It is surprising accurate, even though you won''t find a physics basis to the coefficients and formulae.

Is this actually called heuristics ?

Wouter