# Big gliders and little gliders, a speed question

Can anyone explain to me why a full sized glider travels faster than a 1/72 scale model of exactly the same density, assuming that they are both on the same glide path?

?

Well,

The air is still the same density (you haven’t scaled that down). When I snowboard with my 44 lb 5-year-old (a 1/4 scale adult!) he will get stuck on a mild slope that I can ride down really fast.

Michael

A little basic physics might give at least a partial answer.

Surface area to volume ratio does not scale in a linear fashion.

The square/cube law dictates that when you double a things size…you have increased surface area by a factor of 4 but its mass/volume increases by a factor of 8. So the bigger something is the less surface area.

This is why a huge pile of snow will not melt for days but a small pile will melt immediately. Also since tensile strength is related to cross sectional area of muscles/bone etc for organics and beams/girders or supports for inorganics you also lose strength as you scale things bigger. This is why a mouse can survive a fall 50 times its body length while a human falling 300 ft will squish and a whale will die under its own weight just sitting on the beach. Also look at the body proportions of similar animals at different sizes. Example-look at a lions legs versus a cats. The lion has proportionately much larger legs to support its greater size. A scaled up house cat at 500lbs would be very weak.

Interestingly (at least to me) another example of the square cube law in action is insects. There is a maximum size of insects beyond which insects wouldn’t be able to breath because they don’t have lungs. They breathe through diffusion through these little tubes in their exoskeletons. That works at little sizes but a 10 foot long bug would have too little surface area in its diffusion tubes and would asphyxiate. Lungs have crazy surface area which is why big creatures have them.

So a big glider should have less proportional surface area than a small glider of the same shape. So that should translate to less wetted area which should mean less friction right?

Interesting question, you got me thinking…

an object in motion tends to stay in motion . . . one thing I remember about longboards is once they are riding you just have to trim and take the highline to build speed . . . but they keep speed . . .

shortboards can accelerate faster but they lose speed faster.

I think tombloke might be getting at something unstated in his post…haha.

Big boards are fast boards…we’ll soon find out.

Thanks, the question was prompted by a windsurfing speed thread, it seems that heavier sailors have an advantage in the speed stakes, because they can handle more sail area, which by itself won’t make them go faster unless they experience proportionally less drag, which I can see is the case now.

If scaling up a glider means that it has a better thrust/drag ratio then this will also theoretically apply to surfboards and their riders !

If so it is good news for large people.

Bigger windsurfers will go faster since they can counterbalance more sail area. More sail area gives more power, more quickly than the change in angle of attack (to support the greater mass) takes it away.

A kid came in yesterday at Diamond Head, we ran into each other in the shower. I was surprised to recognize him, he used to be a little skinny grem, son of a friend who surfs. Now he’s six-six, probably 230, all muscle, riding a home-made tri-fin board that couldn’t be much more than five feet long, and about 15 pounds of carbon-kevlar over a reshaped shortboard. Ugly, but it works - I had watched him flying in and out for two hours as he came from the far side of Black Point. It’s more board than he needs, since the kite gives him more than enough power. All of this means what - that with enough power in your sail, or kite, you can go faster that shite.

As it happens, surfboards are definitely drag limited because of their surface area. Watch an old plywood paipo at Makapuu sometime - tons of speed, light (1/4 inch ply with polyurethane top and bottom) or heavy (5/8 ply with glass job), though not much control.

Reducing surface area, and the means to do that, result in unacceptable compromises. We gotta be able to paddle, so there is a volume constraint. We can’t (effectively) water start like windsurfers or kites.

Also very good news for larger boards and denser boards

At least where speed is concerned

Looks likes speed results will have to be categorised according to rider weight as in boxing. . . I aim to be the Cruiserweight speedsurfing champion of the world !

(Loud chest beating noises)

Interesting stuff Charlie,

Regarding the reduction of surface area, I achieve it by tail riding the board. . . . not all the surface area needs to be contacting the water, the advantage of this method on a longboard is that one can use as much or as little area as is needed, less when going faster.

Bigger and heavier windsurfing boards won’t give more thrust, because the extra weight needs to be used to gain more sail power, and the board can’t do that, but in surfing the heavier board does gain more power, and thus an exactly scaled up setup will go faster.

slower to reach top speed

and slower to come to a stop

and slower to build

and slower to change direction

and slower to redesign

slower to carry out to the end of the peir

slower to catch up to when it is lost toward the reef

slower to ship on a freighter

slower to wash down

slower to sand and regloss

other than that

the damned fastest gull durned

shipshape craft this side of a J-boat

and most ass suredly capable of a commission in any navy round the world

the Flying cloud weighed 1,783 tons

with a length of keel 208 feet

length of deck at 225 feet

and length overall 235feet

extreme beam of 40 feet 8 inches

and of course a full poop deck

of 68 foot length

and the cabins tastefully wainscoted

with satinwood , mahogany , and rosewood , set off by guilded pilasters

on june 3 1851set sail from new york on her record breaking

maiden passage,89 days and 21 hours later…*

every dog has its day.

fast, oh dear god.

how fast is a thirty five mile an hour wave.

real fast

…ambrose…

• the best of ssail by basil lubbock

if the flying cloud was 24’long

how wide would it be

in propper proportion?

was it a full gun?

…ambrose…

Yes it was. Essentially equivilant to a 21 1/2’’ wide x 10’ 6’’ long gun, circa 1958/1966 era.

But has anyone ever built a board with wainscoting? That’s an “entirely different problem”… as monty python would have it…in episode 20…

I thought it was an interesting question, with significant implications for surfboard design.

.

Quote:

slower to reach top speed

False .

I’ve never ridden or ever seen any longer, wider, thicker, heavier board that wasn’t noticeably slower to reach top speed than a smaller, lighter board. Smaller and lighter has less inertia and surface area, but can be a lot quicker and faster if it also has good control. George Greenough proved all this stuff 40 years ago. Ambrose is right.

The laws of physics, (and my experience) say otherwise.

For example a heavy board will accelerate much faster on takeoff, (all else being equal)

I suspect that I am confronting the usual “Lighter is faster” myth, based on the erroneous assumption that that light and heavy boards have the same thrust,which they don’t.

Each to their own though, I acknowledge that you are entitled to your opinion, and I realise that there are lots of different design solutions in surfing, it’s just that it is a physical fact that a larger scale surfboard and rider have a better thrust/drag ratio (All else being equal). Of course all else is not equal, and design differences and rider skill are probably more important, but the fact is that bigger and heavier is always going to have the potential for greater acceleration and top speed.

The 1/10 scale model will not behave like the full size surfboard because the surface tension and viscosity of water have a larger relative effect on the scale model. If you scaled down the properties of water, maybe used whiskey instead of water, then you could do a fair test of the 1/10 scale model.

I’ve seen one of those model surfers at the Hook in santa cruz before. Never thought to try to analyze it’s speed on the tiny waves vs. real surfers on real size waves though.