So, Are They Planing?

The theoretical hull speed of a 60-foot sailboat is about 10.4 knots. Assuming a perfect VMG around the theoretical course, the winner of the Vendee Globe actually averaged faster than hull speed. So, add in the extra 2000 miles really sailed and the actual average is higher again.

I lifted this from the Vendee site:

"It looks likely now that the honours for the best 24-hour performance in this Vendée Globe will remain with Bilou. Over a 24-hour period Sill et Veolia clocked up 434.01 miles, averaging 18.08 knots on 12th December 2004 in the Indian Ocean between the Kerguelens and Australia."

So the question is: are these keelboats planing?

 

You could ask yourself "When does a boat plane?" For most powerboats is this phenomenon happening from 17-18 onwards. That's just a general line with - of course - a host of exclusions.
The boats you mention, are going generally much faster than the 10.4 knots you quote. A 16 mtrs VDS Design from the early '70's could sail easily 9-10 knots under a moderate wind, so I doubt a bit if you have used the correct parameters when you made this calculation - but I can be wrong, I am a long way from sailing theory.
When I observe the present boatdesigns than you might see that they are created for long downwind courses, with general speeds exceeding 13/14 knots.
I have no temporarily racing expierience so let somebody chime in who has.......

 

The "theoretical hull speed" figure (around 1.34 times sq rt lwl) is merely the max speed of the hull's wave train; if you don't create much of a wave (light displacement hull) you are not restricted by the speed of the wave train so you can go faster than "hull speed"; sort of like a cat does.

Planing is, I believed, something that technically has nothing to do with speed per se; a boat is "planing" when dynamic lift causes its C of G to rise above its static position.

But I'm no expert...

 

Yes, they are planing.

 

About planning boats.
Hull Speed = 1.34 * (LWL)1/2

LWL: length of the hull at the waterline.
Planning starts at 2,9 -3.0 S/ LWL Ratio.
All depends of the shape of the stern of the boat. A deplacement hull can never
make planning. A semi displacement hull has not power enough to make planning.

 

Hej Kjell, håller inte med dig...

I don't quite agree with you Kjell...
As far as I'm aware there is no fixed threshold for planing (not planning thats different I think ).

Saying that a displ boat can't achive planing is like saying a bee can't fly. The bee flies, and a displ. boat planes IF it has the horsepower/SA (or is being pulled fast enough) to achieve the lift from the hull that lifts it up from it's stable displacing state. But under normal circumstances that does not happen.

AFAIK the stern creates no lift on its own. The wide shape of the stern on a planing boat is an effect of trying to increase the lift-creating area of the hull.
And the best shape for releasing the water from the hull at high speed.

And btw yes, those suckers plane, but my boat don't want to

 

Hej Eric

If you work with Relativ speed instead of S/L Ratio it easy to understand.
Relativ Speed = R = V/ SQR.Root L
V = KM/H L = LWL Meter.
The 4 states of hull speed are.
Drifting = R 0 – 1
Navigating = R 1 – 8
Planning = R 8 – 20
Skimming = R 20 - ------

 

NOW it starts to get interesting...

WHO defines the numbers for the states defining R values?

I have never see planing defined by a formula in any book I have read about yacht design in this way...
Since planing is NOT only dependant on lwl, so just using that in a formula for planing simply can not be correct, in my very humble opinion.

A hull PLANES given that it has sufficient SA/power to achieve lift to reduce her wetted surface area wich will reduce the waves caused by the hull moving though the water and also reduce drag. As long as these changes are significant the speed will increase and the speed will pass over the drag bump and this has nothing whatsoever to do with the formula that you mentioned.

Your formula only takes Length of the hull as a constant and this is simply not enough.

Having your own definition of what you consider is a planing boat is fine, but a lot of other people might not agree with the way that you define planing.

 

<raises hand> Is he related to Frankie-baby?

 

hey now Steve, be nice now he's new around here and he's swedish. Wether that is a redeeming factor... I do not know

sheez i can't spell worth a sh*t...

 

Hmmmm... how about planing happens when the froude number is around 5....Hmmmm if you don't care what a froude number is then how about the boat is planing when the displacement is less than half the weight of the boat. That is that dynamic pressure is responsible for supporting the majority of the boat.

 

What is planing?

ErikG:
"... a displ. boat planes IF it has the horsepower/SA (or is being pulled fast enough) to achieve the lift from the hull ...."
"Since planing is NOT only dependant on lwl, so just using that in a formula for planing simply can not be correct, in my very humble opinion."

You can find Dave Gerr's formula for hull speed here. It includes displacement. If I didn't mess up somewhere, you can also write it like this:
Hullspeed = 1.24 * L^1.433 / D^.311
(D in pounds)

Bethwaite also discusses this subject in observing that triangular hulls don't have the drag hump. They smoothly get more and more lift, until they're mostly out of the water.

But I thought there were hull shapes that never plane. They just keep digging deeper & deeper into the bow wave until they pitchpole or something. I know I read that somewhere, just don't remember where. And of course, they might not be a good shape for a boat.

 

The R values was invented by Juan Baader
His book.” Cruceros y Lanchas Veloces de Juan Baader.” In Spanish is one of the best books you can have. It helps you to understand the most about displacement and fast going hulls.

 

I agree with that - although times have changed.....

 

Hey Skippy

EDIT: didnt look at the formula properly before i replied, and dont have the time to see that you reduced it right. But I'm sure you did.

Gerrs equation gives a lot better result than the old "traditional" formula, but it still does no cater for different hull forms, and we all know that shape AND weight are the fundamentals to create enough lift.

END EDIT;

I know, but my point is that that is also a rule of thumb formula not appliciable to modern sailing craft. its just plain old but it does work reasonably well for boats that DONT have the power to go over the hump @ Fn around 0,40-0,45 in other than surfing larger waves.

But as hull shape changes they way boats actually develop lift and the mentioned formulas don't account for that, they aren't good enough.

But they do both work kinda-maybe, but NOT for all boats and are therefore useless if not used on the right type of craft.

This is how I see things, not that I'm a pro or anything.
I thought I was the beginner around here. still

Well I am