After considerable time spent on the board trying to plane in marginal winds or watching a lighter sailor effortlessly blast past I thought there must be some solution. So far I have been accumulating bigger and bigger sails but they get harder and harder to manage. My local area Perth (Melville) has frustratingly constant winds of 8-10 knots that can go on weeks on end. I have recently joined GPSTC so also interested in just going faster in general.
Researching how to sail fast in low winds revealed that Formula Boards or even long thin style boards that don't plane but slice through the water instead were a way to go.but these involved almost complete new kit.giants sails, huge fins and super wide boards. Since a non planing board's max speed is proportional to the square root of the length it quickly makes a board crazy long which then turns like an Oil tanker. Could go foil approach but ultimately you need bigger and bigger foils in lighter winds to get going which limits places you can sail and they also suffer because the drag goes up with the square of speed.. At speed they also appear to be fairly difficult to control. I have already run out of room to store stuff so I want to minimise accumulating more gear.
Surely there is another way? The solution seems completely obvious, so obvious I thought many would have at least tried it. According to Google though, no windsurfer has. However, just because its not on the internet doesn't mean it hasn't been done so maybe someone out there has some examples or info. Its even more odd because the surfers experimented with this around 30 years ago and were discussing it in the early days of the internet..surely one surfer realised the potential application to windsurfing???
This idea involves getting air under the board to reduce wetted area and reduce board drag. Essentially a hovercraft where the entire board rides on a cushion of air and only the fin touches the water.
Large ocean going vessels like tankers and Cruise liners like the Diamond Princess use what they call "air lubrication" to reduce drag and thus fuel consumption. Depending on shape of hull they use either micro bubbles or air channels running the entire length of ship. Air is injected near the bow and travels aft. Depending what you read they are reporting real world improvements between 5-20% savings in fuel consumption.
For Windsurfing would split the solution into two parts:-
1) Active
2) Passive
1) Active approach would be a hover craft style approach where air is injected via some external power source under the board. Could be as simple as an aquarium bubbler or more advanced like a model aeroplane ducted fan unit. Most would agree this is not in the spirt of windsurfing having an external power supply etc and certainly could not compete with conventional craft in competitions. It would have to be classed as a different vessel, maybe Hoversail. It detracts from the simplicity and also adds complexity and cost. I only found one example of anyone suggesting this: US Patent US20040000265A1 (stick that into google and you can get it) It didn't appear anything was ever made as tech at the time wasn't sufficient. It is still appealing though, especially for ultra light winds but the passive approach is the main reason for this post.
2) Passive approach. Very simply using a Venturi to draw air from the top of the board to underneath. (Not to be confused with the Venturi board designs which channel water in different ways under the board and lots of info and experimentation around for that). I've played around with Venturi's in Aquariums and its super simple way to inject air for the fish (or making underwater sand waterfalls). For a windsurfer the basic idea is drill holes through the board where you want get get air under and make minor indentation underneath with a small slit. As the board moves through the water the indentation causes a pressure drop and draws the air under the board...
Surely someone has tried this before? It turns out Mandurah legend Bert Burger (Sunova founder) made 20 or so of these style boards (only for surfing though). He called them 'ventilated' boards. In a post he describes these boards, quote: "..its without a doubt the fastest thing you could possibly ride..", "this board has holes through it ...the holes are covered on the bottom with just a fine slit allowing air onto the bottom of the board ....as the water rushes under the bottom it sucks the air through the holes ,at a certain speed (just a little faster than paddling speed , just before you take off) the whole board rises and rides on a cushion of air ..... unbelievably fast.....................................
only drama was the air introduced onto the bottom of your board , meant the fins just didnt work when you tried to turn to hard ....i was thinking of cav plates at the base of the fins to keep the air trapped against the board ... but the whole process just got to complicated .."
He also mentions the boards made no spray. He included two photos, one a close up of the Venturi system and another him riding it. Unfortunately the internet has lost these photos.
You can read his post from 17 years ago at
www.swaylocks.com/groups/question-reducing-skin-friction-drag-surfboards (He starts about 7 comments down and makes a total of 4 posts, ending with... "but at the end of it all introducing air onto the bottom of you board causes some complications..... at the time i didnt know enough about design to deal with it ... i think i would be better equiped to deal with those problems now.... its just a time thing...... id sure be happy to problem solve if you guys wanna have a crack at it..... but ive got to much to deal with to go there again.... " Seems like Bert was pretty busy with company changes etc back then so perhaps he has just forgotten about it..I see he makes windsurf boards now though...
I have been trying to get in touch with Bert to discuss further but so far no luck. According the the Mandurah Sunova outlet he lives in Thailand and is active on Facebook.
After a lot of searching I found a couple of photos of surfboards with Venturi's which are attached here. One is I think made by Erle Pedersen and the other unknown. Would be really interesting if anyone has other photos of these boards.
Judging by the spray of a windsurfer the water flow directly under the board is roughly 135deg to the direction of travel.i.e. not down the length of the board so you probably need more than one Venturi but really no clue.
I've made a rough sketch of what a Venturi system for a windsurfing board might look like. Taking it a few steps further you could add basic control to the system by placing these air vents near foot positions. i.e. open or close them with your toes in the front foot straps for cruise control, or closing off air flow by placing the inlets where you put your front foot to initiate the gybe, thus stopping air flow and dropping the inside rail into the water.
Another concept is you could enhance this effect by having a ram air intake somewhere (front of board?) and run the air via tubes hidden in the board to the Venturi's. (For testing you could run the tubes above the board to the venturi's) For real speed you could adjust this air intake towards the apparent wind.
No clue how this would make the board handle, how the fin might behave if the board actually fully lifted out of the water etc.Because board drag dominates in pre planing conditions, even a small amount of air lubrication that drops board drag 10% may help get planing earlier or expand lower wind range. In theory at least the concept should increase overall speeds, reduce noise etc.
Anyway, would be really interesting to know if someone has tried this or have ideas. I have a board I picked off the side of the road that the guy was throwing out that will be the sacrificial first test case. I still have to find an American style fin for it and then learn to ride it to see how it feels before I start drilling holes. Its a Warren Thompson Santosa speed board so won't be an ideal test candidate. Before I do that though I plan to do some tests of Venturi designs.maybe this can help explain why I am building a hydro test tank. I can also see the need to start making my own boards in order to play around much further. Talk about opening a can of worms. However, it seems like it could be a way to get close to an Aladdin's magic carpet ride.
