Wakesurf hydrodynamics

We want to share a little experiment with you. We love to have fun and do some crazy stuff here at Flyboy Wakesurf, but that “stuff” usually has a purpose even if it’s just to have some fun! You may remember the other day we were wakesurfing a small 48″ Victoria. It was a very small board and was able to manage up to 200 pounds on the wake behind our Supreme V226. It got us thinking about shapes and the “footprint” that is in the water of a typical wakesurf wake, but also things like rocker, rails and concaves.

On the small 4′ Victoria, with the pintail, we posed the question, how much of that thing was actually in the water? Obviously not the full 4 feet, but even if it was say…3 feet, what was the actual area of the wakesurf board that was wetted in the wake? Not very much, right? It’s sort of triangular from the wide point to the tail. We roughly calculated that the wetted surface area was around 2 square feet. It was a really rough calculation, because we didn’t have accurate marking for the actually wetted surface area, plus the whole 1/2 x base x height is only rough when the sides of the triangle are curved.

Give us the benefit of the doubt here and lets assume that the area that is actually being wakesurfed for the most part is two square feet. Did that make you go Holy crap! if you’re riding a five foot board? It should, right? If you are wakesurfing a wakesurf board that has four square feet of surface area and you only need two, the extra is just waste or “bling” to impress your buddies? Really? Is it possible that many of us are riding wakesurf boards that are twice as “big” as needed?

So with that in mind we created two new wakesurf boards. A round one and a square one. We ran out of wood for our shapes, but another shape that needs to be tested is a triangle. Here is a picture of the wakesurf boards that we created.

Silly huh? Not so fast there, buckaroo! What if they can be wakesurfed? Who’s silly now, with your 5 foot long log? Our theory was that a general shape that allowed for a wetted surface area of 2 square feet was adequate too allow a wakesurfer to trim in place for at least a period of time that would represent wakesurfing. We’ve all gotten over the whole “push” nonsense associated with wake formation, right? That’s just mareketing hype used by folks that have a vested interest in sell you a new boat, or that are only casually aware of how wakesurfing works. That force is lift coming from the water rushing off the back of the boat after being squashed by the 6 to 9 thouand pounds of water being displaced by our ballasted boats. That lifting force will be stronger closer to the boat and dissipates the further away from the boat you go. For most of us the sweet spot behind our boat is rather close to the boat.

You can elongate that sweet spot with more board surface area or with a more efficient design. That is to say, less drag, more efficient bottom contours and the like. We’ll talk about Newton’s third law of Physics for a moment. That is that every action has an equal and opposite reaction. So when you are wakesurfing and push down on the rails of your wakesurf board, the water underneath you is actually pushing back against you – equal amount and in an opposite direction. What happens when we plow our boat through the water? It’s pushing back and then when it is freed upon as it gets past the transom? WHOOSH! Upward and we get the beautiful wake we love to surface. It eventually settles back down, but you get the idea, the force is UPWARD and THAT is what allows us to wakesurf. When we are wakesurfing we are using the pull of gravity DOWNWARD opposite of the flow of our wake to help us balance on the upward flow of the water exiting the bottom of the boat as it plows through the water.

So all we have to do is capture enough water flow to offset the pull of gravity. We’re sure there is a mathematical formula that would describe that and maybe one day we’ll look at it, but mostly all we wanted to do is test that concept. From our observations, a simple 2 square foot area is what was needed to capture enough of the lifting forces to keep a 150 pound wakesurfer in place. Now you can see how that theory would extend to our everyday observances. Smaller, weaker wake and we need more board – that is more surface area to grab more of that upward flow. We’ve all seen this and no doubt experienced it. We’ve also all experienced how more efficient designs can allow us to ride a smaller wakesurf board, BUT there is a theoretical minimum, right? We don’t really know what that is at this stage, but there is a force of the water flow upward and we know that we have to capture enough of that to support our weight and offset the effects of gravity pulling us down.

Lets assume for a moment that if we could grab a two foot square area it had enough upward force to offset the downward force of gravity reacting on a rider, what would happen? The rider would basically be suspended in that area. This is commonly referred to as trimming. Now that doesn’t mean they could rip up and down the surace of the wake, because once you turn the wakesurf board sideways, or angle it a little what happens to the amount of upward force you are capturing? It slips off the sides and is then diminished, that “angle of attack” would then cause a reduction in the amount of forces that could be captured. It would still capture some and there would be some momentum as we can’t turn instantaneously, but you get the idea. Present a smaller foot print to the upward forces and the less weight that can be supported. It also tells us a little something about forward motion vs trimming in one place doesn’t it?

We can trim in place merely by having enough wetted surface area to support our weight. But what about going forward? That is a relative degree of drag, isn’t it? A design which is more effiecient at reducing drag and also capturing those lifting forces will feel faster down-the-line. We’ll save the entire drag concept for a later date, but what we want to establish today is the concept of minimimum wetted surface area needed to support a 150 pound wakesurfing in one spot.

In the picture above, the square board is probably not really good at the elimination of drag, is it? That does make our testing a little difficult because quite frankly it’s hard to just sit still while balancing in one place. Those huge corners will grab the wake as the rider sort of leans forward which really just stops everything. However, at least for a short period of time, we should be able to see if that two square foot area is adequate to support a 150 pound rider. Plus 2 square feet allows almost no stance adjustment for an average wakesurfer, the centerline between the feet is probably less than 20 inches. That’s really narrow!

So think it is? Think it can be done?

Square

Round

Can it be done? At least for short periods, yes. Is it wakesurfing like we want? Probably not, standing in one place isn’t all that much fun, and standing in a really narrow confined spot isn’t all that much fun. Is two square feet of area enough to allow someone 150 pounds to trim in place? Way more than is needed, actually, as you can see in the pictures, there is lots of UN-wetted surface area.

Here is a short video clip that shows James Walker wakesurfing the 2 foot square section of plywood. We’re trying to embed it from Facebook, so bear with us if it fails!

Fun huh? That is if it worked!

Ok, so we’ll leave you with that for the time being, there are a ton of things going on. One is that there is an efficency concept with every individual wake. Some boats require a TON more weight to develop the lifting forces needed to make an adequate wakesurf weight, those hulls are innefficent. Not worse, it’s not a value judgement, just that the most efficent hull for wake surfing would require the least amount of weight as that requirement goes up, the efficency factor declines. Another is board shape and design has an impact on that efficiency factor. Grabbing more of the lifting forces per square foot is more efficient. Lastly, for our Supreme V226 and with a world champion James Walker riding he is able to trim, atl least briefly, on a 2 foot square foot of pywood.

Thanks so much for following along and we hope to see you back soon!

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