I need to ask this question in simple terms as there are probably and extreme number of variables like fin size, tail width, length, rocker, rider technique, rider weight, volume, board shape etc.
All other variables being equal and using the same rig of say 7.5m, what difference in wind speed will there be for planing threshold vs board width from say:
70cm to 85cm?
85cm to 100cm?
I realize there will be a difference, but how significant is it?
Has someone got a general formula for it?
The reason I ask is I currently have a Kona with a 7.5m and I'm 84Kg (with gear). The board weighs about 16Kg which is about 8Kg more than a short board, so I add 8Kg to my weight (92Kg). If I put all these variables into a sail size calculator it comes up 16 knots which I have tested to be true. According to the calculator, if I change to a 9.5m sail, the threshold would drop to 13 knots. In similar terms I am trying to figure out what a measurable difference board width would make in addition or instead of a larger sail.
That's a very difficult question to answer for most of the reasons you have listed, but there are two things that stand out as making it impossible;
One is that fin size is enormously significant, and the second is that there is a big difference between the amount of breeze required to get planing, as compared to the amount of breeze required to stay planing. So if you have good pumping technique it is quite possible that you could pump yourself up onto the plane in, say, 12 knots and then sail straight past someone who is not only lighter than you, but also has a bigger sail, board and fin!
Manufacturer makes good board = I buy board = Good day sailing = After sail beers = beer gut = Manufacturer responsible for my beer gut 
The laws of physics and hydrodynamics mean that a wider hull will need less speed to plane than a thinner hull, whereas in lighter/subplaning winds a thinner longer hull will glide better/faster than a wider hull.
But there's not a lot of diff b/n a 100cm hull vs an 85cm hull, depends what you want it to do. a formula board with large fin might start planing at 10 knots speed but go straight up wind, whereas a 85cm Slalom board might plane at 10.5/11 knots speed can be sailed across the wind and more fun??
en.wikipedia.org/wiki/Unsprung_weight
The board is unsprung. Your body (well most of it) is suspended by your legs and harness.
Unsprung mass is usually factored in at about twice sprung mass in terms of how bad it is for performance.
So board weight does make a big difference, more than going on a diet would.
Regarding width and early planing, I think it's more to do with planing area rather than width. I tried an experiment with a friend's F2 Guerilla vs my Mistral Flow. Both had similar sized fins. The F2 was a good couple of cm wider than the flow.
Which planed earlier? The flow, by a significant margin... we tried swapping sails and riders to confirm it, it was definitely the board that was the issue.
Aus1111 is spot on. I was sailing my (old) Formula board and 9m sail the other evening in 9-10 knots of wind. It was a really struggle to get on the plane but once going it was as if the wind had suddenly strengthened. I was able to fully lean back on the sail and power along at 17-18 knots speed. As soon as I tacked or gybed it was again a struggle to initiate panning! Apparent wind is a marvelous thing!
To add my 2c worth to the original question: I think given equally efficient hull shape the 85 to 100cm width difference is quite significant for early planning and especially upwind performance. Starboards 2001 Formula board was about 80cm wide. Within a year they has suddenly grown to 100cm because it was so obvious that there was a performance gain. This gain would have been mainly in upwind performance but this is similar to early planing performance.
I remember I first tried a 100cm Formula with a 6.5m sail in about 11 knots. I could not get anywhere near getting my best light wind slalom board planing but immediately I plugged the sail into the Formula, two pumps and I was planing. My conclusion: The big, efficient board makes more difference than piling on sail area.
I recon if you put your 7.5 rig on a formula you would get at least another knot or three threshold of light wind planing compared with your Kona.
Another significant factor is rocker, a board with straight rocker should always plane noticebly earlier then the same board with more rocker and a smaller straight section.
dear Nobody,
I've got a Kona too, and weight about the same as you. I use a 7.0 sail on my Kona (Gaastra GTX 7.0), and I reckon I can plane in about 12-13 knots with a few solid pumps. I also use an adjustable outhaul.
Needing 16 knots to plane sounds a bit high. What sort of sail is your 7.5? Maybe you have it rigged too flat? Longboards need a sail with a bit of fullness and less twist than a shortboard. And dont forget to pump and move your weight fore and aft as needed.
wider board, you can run a bigger fin, which gives more lift, plan earlier, bottom shapes have alot to do with it too, my 83litre waveboard planes in the same amount as my 95 freestyle wave, the freestyle wave has a deeper V the wave is single to almost flat through the tail, flater bottom boards have always planed fast i've noticed..
yer that calculator sounds interesting....
ive being going back to older thinner style boards , it gets very choppy here at times and im no rocket scientist and only a begenner but my gps donst lie im now going faster on the old boards than the new ones , but dont forget im well over 100 kgs so i need thin board to cut through the chope cheers bazz
Thanks for the replies. I'll reply further when I have more time.
James' sail calculator http://www.vims.edu/general/sailpaddle/SailCalculator.xls
Here is something I found:
nebs how'd you like your mistral flow? i have one i think a 2000-01 model blue and red stripes 266 custom 95ltrs..an all around board and love it just happened to finally do a 2cm. crack infront of back footstraps..it's out on the pavement baking to get the water out before fixing it
The other thing to factor in is the wind strength. Lift/drag increases with the square of airspeed so 12 kts is a hell of a lot more than 9kts. Once you get down to the sub 10kt region it gets increasingly difficult to create enough lift, obviously.
Interesting, Chris...
Would you say that Savitsky assumes that there is a large amount of planing surface? That is, he only considers the case when the hull is fully planing, and figures out how much horsepower would be needed to keep it going at a certain speed, and assumes that all the area in contact with the water is flat?
This is a different problem to the one we face, that is actually getting planing in the first place.
Do you have a link to the problem we face?
hi nebs,
getting planing in the first place really isn't the problem when talking wide boards. the things plane in hardly any wind because of the extra width. the typical experience with formula sailing is either fully lit or dredging. so in the light winds most formula sailors experience being lit up until the gybe and then a sudden slow down while gybing. followed by another fully lit run once on the plane again. very different to long boards where the narrow width has a low drag aspect.
sure, in less than 8 knots a longboard has better lift due to water line length but anything above that and it's goodbye long board in terms of speed.
for me sailing on an 8.5 m sail on flat water the difference between an 80cm wide board and 100cm wide board for planing threshold is 2-3 knots wind speed. not a lot but in real terms that's huge. i can get my 95cm formula board planing in 8 knots of wind after 3 pumps with the 8.5 and a 85cm board with the same sail needs 5-6 pumps and a little more wind.
tail width is also very important. the new formula boards are now running 80cm tails. that gives them lots of advantages, the extra tail width also creates a straighter rail line which improves upwind performance.
as said previously fins play a very big part.
softer fins plane earlier. my 66cm C3 is great in 8-15 knots (soft). over that and my knee wants to explode.
my 70cm Deboichet custom can be spun out in 8-10 knots (stiffer) but keeps going beyond 20 knots wind.
I think the latest deboichet fins are getting softer too (haven't tried one but i am sure someone can confirm so the 80cm tail would help keep control in stronger winds).
edit"""" i found this laymans description. thought it interesting. http://www.bluejacketboats.com/planing_boat_theory.htm
Thanks Gestalt.
From the article:
Given a good design, it is the bottom loading in pounds per square foot that determines the ease with which it will get on to plane.
I rest my case 
did you read all of the article?
i think you took something out of context there.
Errr yes I did read the whole article. Did you?
Seems pretty clear to me, it's planing surface area, not width that's the critical factor in getting 'over the hump'. Once planing of course it's a different story, and providing you have enough length so that the entire wetted surface is flat then of course width will be the only variable.
But isn't this topic about getting planing in the first place?
Firstly thanks for replies. I'm both interested in the theory to some extent, as well as the real world experiences.
Sailquick said...
