I believe you are referring to Tomo vanguard 5'5" right? Reason why I mentioned Mr Buzz first short skate for Nathan and his high flying act then he went longer rails bigger with buzzsaw to 5150 surfs now
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Staged vs continuous rocker.
- Thread starter casa_mugrienta
- Start date
The story behind my awesome local custom is that a while back the local sponsored surfer said g'day to me in the water. Which I thought was a nice way of greeting a blow-in. I asked him what board he was on and he said model X. Being a good ambassador he told me that model X would work well for me and suggested I get a custom version. I asked if it was a machine shape, answer was yes which was enough to convince me that it could be faithfully scaled down to elf size. So I ordered a custom model X.
However you did ask the question. So here we are discussing it
I am not sure I always understand Greg, however he definitely uses the convention of apex being the tangent at the centre of the board and has previously said that is where he would place his rocker stick too (something that only needs to be done to de-construct boards now that we have the 3d software). However I think Greg was referring to just lowering the rocker by pushing the apex upwards which would alter the angle of attack - or maybe he meant keep the same overall end/tail rocker but flatten the apex producing more nose/end flip - either way angle of attack is affected although in opposite ways.
Regardless the interesting point is that while one shaper might consider a flat "speed box" in the middle Greg is considering angle of attack Another thing that I have picked up on is that while other shapers might resort to shedding excess lift in the tail via V or V double (for stability), Greg instead keeps it flat but controls it with extra tail rocker and consider how a diagonal path over a flat tail with extra rocker will result in a curved water path in the direction that creates suction as you illlustrated in your inverted air-foil diagram.
Pulling up on convex surface of a foil is responsible for roughly 50% of the lift. Pushing up on the underneath of a foil from deflecting the fluid down provides the other half of a foil's lift.
So we do have an underneath deflection/flow turning surface on the underneath of our boards, however I have come to believe the underneath of a planing surface, surfboard in particular is a very different animal to the underneath of a foil. From reading some of naval architect Lindsay Lord's book on planing hulls it seems the flow on a planing surface is a lot messier than the underneath of foil. A submerged foil's under surface is much more efficient. With a planing hull we have the ridge of high pressure at the spray root. Only some of the surface is actually in contact with the water (as pointed out by Mr Sopa) and there are regions of suction too.
Here is a quote from Greg that manages to encompass my waffle (I think).
I remember MeeCrafty taped streamers onto the underneath of some plexiglass, then put a hose on it and posted some pictures. If someone took it a step further and planed it across a surface of a pool then that is great (I don't remember that, but my memory isn't great). I borrowed Crafty's idea and taped a curly piece of gift wrap underneath my board and noted that it emerged diagonally outwards most of the time. A company called Camber surfboards has taped multiple short streamers all over the underneath of a board with underwater camera and it did show a sort of curved fishbone pattern like you say.
Please notice how large the surface area of the engaged boar is. That's a lot of foil!
One area not discussed yet, water displacement. The rail biting into the wave face, pushes water out of the way. That "pushed away" water has mass. This displaced mass of water means bouyancy applied to the boar.
Look at how much water is wrapping around the rail, I guestimate 4 feet of rail distance is engaged.
Do the spoon experiment.
My understanding, which comes from working in the boat building industry, is that a planing hull is no longer displacing water.
It has to be or there would be no wake. It's just displacing less.
Right?
Right?
Staged vs continuous rocker.
Continuous in good (hh+) waves with push, staged rocker in weaker flatter faced waves.
Continuous in good (hh+) waves with push, staged rocker in weaker flatter faced waves.
A lot less. Go to 2:00 in this video and consdier how little water is being displaced. I'm guessing the boat's draft is less than 6"and oly the back half or less is in the water.
ps- Caca, if you want a thrill thsi is the boat you're looking for.
sbd, you were saying that you have wakeboarded, so does the wake boat travel at planing or sub planing speeds to generate the best wake?
The tokoro is maybe 30litres max. Just looking at the diagonal line where the spray root forms, let's make a rough guess of say one third of that 30 litres submerged. 10 litres supports just over 10 kg weight (salt water). Rider is 70 or 80kg. So overall negative buoyancy.
However that "pushing away" of water with mass from the moving board is flow turning - Newton's 3rd law and all that. So that's where most of the lift comes from, rather than buoyancy.
planing but the biggest wake possible while maintaining enough speed for the rider. You add as much weight to the boat as possible to increase displacement.
The wake is biggest at 6-8 mph but that's too slow to wakeboard, but good for wakesurfing speed.
I think you are missing the depth of the dsplacement. More like 50 liters of water moved.
look like good studies on board bottoms/fins
Even modern keel boats are fully planing. This is one of my favorite vids. Watch about 35 seconds in when they finally get things sheeted in... and the water coming off the stern about 1:00.
Most of the pressure is forward from the driving sail
Surfing the rear is loaded with pressure - VERY Different
Driving pressure from sails is forward directed and not downward causing displacement of water in the fore section of the hull, otherwise I think the plan shape of the boats would be very different. There is not much hull area forward of the mast, even less than the drawing would suggest, given the hull flare. It's almost like the only function of the bow is to break through waves.
But yes, there is not much you can take from racing yachts and apply to surfboards. Very different.
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So for a 30 litre board to displace 50 then it will need to be below sea level with minimal water wrapping on to the deck in order to create a 50 litre cavity. It certainly doesn't look like that is the situation to me, neither from the picture, nor have I witnessed my board below sea level. Even so 50 litres of displacement with an 80 kg rider is still negative buoyancy.
However I have noted your "spoon experiment picture" which I think is intended to model in an inverted way the "ploughing" - my non scientific word for how just a little bit of displacement (as noted by llilibel with his illustrations of racing yachts on the plane) can push water into a wake, plus some whisker spray up front - when the vessel is moving. Speed boats such as for wakeboarding will also have a rooster spray emerging from the transom.
So is the wake, whisker spray and rooster spray the displaced water you are talking about? If this is the case then wake, whisker and rooster spray is a mass of water all pushed above sea level. With above being the operative word, this means a force needed to push that mass of water up. Newton's third law says there will be a corresponding force down i.e suction not lift!
So instead I think the lift comes from the water we can't see that is pushed down:
The spoon trick is to demonstrate how the wrapping of the water around the spoon, contrary to what you may expect, attracts the spoon the in direction of the curve. i.e. the water wrapping around the rail in the long spiral along the rail.
The displaced water is where you can see the rail engaged into the wave face. I wouldn't say "pushed down", more like it just got moved. Push a bucket into a pool of water. The resistance you feel is the weight of the water that was pushed out of the way.
Hence the term "throwing buckets"