Staged vs continuous rocker.

Mr J

Michael Peterson status
Aug 18, 2003
1,981
1,220
113
Regional Vic, Australia
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"
"Push a bucket into a pool of water", then stop and hold that static situation. Even though we have stopped the downward motion of the bucket, there will still be an upward force on our hand. That is buoyancy. It is caused by the difference in density of water and the air in the bucket and the effects of gravity causing the denser fluid to exert more pressure than the air inside the bucket. No velocity involved. Buoyancy is a force and there are calculations for it in Newton's.

"throwing buckets" - that's different. That's accelerating a mass. It creates inertial forces due to a change in velocity of that body of weight. Apparently just changing the direction of mass is also a form of acceleration e.g deflecting water down at the spray root from angle of attack in the entry rocker or water wrapping the rail like you said.

So there will be a combination of a buoyant force and inertial forces on a moving surfboard. We disagree on the magnitude of the buoyant force, I say not much and that most of the lift comes from the inertial forces.
 
  • Like
Reactions: paunch23

Mr J

Michael Peterson status
Aug 18, 2003
1,981
1,220
113
Regional Vic, Australia
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.
View attachment 111529
I watched the vid - fabulous :love: I think what Greg is saying is that the centre of thrust from the sail is somewhere up the mast which acts as a "moment arm" applying torque on the hull with a downward force forward of the mast. However just after 1min we see the bow and just like you said it is on the plane.

While I can see Greg's point on the moment arm effect of the sail and mast I don't see how it is fundamentally different from surfing. It just means that there is sufficient lift up front to counteract the torque. Maybe some suction in the tail too - actually according to Lindsay Lord's planing hull experiments we know there is suction.

Greg, I think there is some suction in the tail of a surfboard - isn't this partly what helps nose riding on a longboard with tail kick? I think there is some suction on a shortboard too - this is why a shortboard with high tail rocker works well in step up conditions - the suction provides us with a bit of braking control. Then when we step on the tail it becomes "loaded with pressure" like you said Greg.
 

One-Off

Tom Curren status
Jul 28, 2005
11,667
6,777
113
33.8N - 118.4W
If I was 18 years old I think I would major in naval architecture and work on sail powered merchant ships. The topic fascinate me. Only problem is hated math in school. even thought mY dad was an engineer. Mr, J, sound like you've read up on the subject. Work related?

One thing I'll add to the discussion, if, as Mr J pointed out, the effective wetted surface of the board is less than half the board's area, the wetted surface of the deck (water flowing over the rail onto the deck) is less than half of that. I searched photos and I'm not sure an of the water is really flowing over the deck? If it did maybe someone would finally invent a smooth alternative to wax.

6932cdc53ce7dcff6e92ba5048499576.jpg
 

000

Duke status
Feb 20, 2003
24,649
4,551
113
i believe a thinner rail is faster, generally
kinda for the same reasons thinner fins are faster than thicker fins
less drag
 

Mr J

Michael Peterson status
Aug 18, 2003
1,981
1,220
113
Regional Vic, Australia
If I was 18 years old I think I would major in naval architecture and work on sail powered merchant ships. The topic fascinate me. Only problem is hated math in school. even thought mY dad was an engineer. Mr, J, sound like you've read up on the subject. Work related?

One thing I'll add to the discussion, if, as Mr J pointed out, the effective wetted surface of the board is less than half the board's area, the wetted surface of the deck (water flowing over the rail onto the deck) is less than half of that. I searched photos and I'm not sure an of the water is really flowing over the deck? If it did maybe someone would finally invent a smooth alternative to wax.

View attachment 111659
The only reading I have done is the NASA educational site on airfoils (quite basic), bits and pieces from the wikipedia and some of Naval Architect Lindsay Lord's book on planing hull experiments. My maths is not good enough for anything complicated, the clever stuff is done by others in my work.

Looking at the last underwater pic in 000's post there is not much of the underneath wetted, although it doesn't look like the normal trim state of affairs, but some tail weighted turn.

Regarding your search of pics which showed that very little water flows onto the deck I agree because I have done a fair bit of staring down at my board while riding. A few years ago I managed to hurt myself while looking down at the water around the bump squash of my CI new flyer - I lost concentration of what the wave was doing and the end section caused the board to fly up the wave and the rail bruised my sternum :bricks:
 

Mr J

Michael Peterson status
Aug 18, 2003
1,981
1,220
113
Regional Vic, Australia
i believe a thinner rail is faster, generally
kinda for the same reasons thinner fins are faster than thicker fins
less drag
I think you might be right about thin fins having less drag - I don't know enough on the subject, however there are other considerations - thin foils stall badly at increased angles of attack - the fluid goes from being attached to detached along most of the chord length, whereas a thick foil tends to lose its attachment more progressively from the trailing edge. We do push our fins at quite angles of attack at times.

Then there is practical construction consideration, a very thin fin will be quite flexy - G10 helps!
 

Senor Sopa

Gerry Lopez status
Mar 11, 2015
1,174
1,788
113
Ponto
I think you might be right about thin fins having less drag - I don't know enough on the subject, however there are other considerations - thin foils stall badly at increased angles of attack - the fluid goes from being attached to detached along most of the chord length, whereas a thick foil tends to lose its attachment more progressively from the trailing edge. We do push our fins at quite angles of attack at times.

Then there is practical construction consideration, a very thin fin will be quite flexy - G10 helps!
Thin fins have less drag at higher speeds. Not as good in shin high dribble.

There's no flow on the deck because the water has released off the rail. You get release when the radius is too tight, or theres's an edge. Sharp rail - lots of release. Fat rail - no release
1624368903794.png
 

PeterDj

Legend (inyourownmind)
Jul 11, 2018
457
325
63
I watched the vid - fabulous :love: I think what Greg is saying is that the centre of thrust from the sail is somewhere up the mast which acts as a "moment arm" applying torque on the hull with a downward force forward of the mast. However just after 1min we see the bow and just like you said it is on the plane.

While I can see Greg's point on the moment arm effect of the sail and mast I don't see how it is fundamentally different from surfing. It just means that there is sufficient lift up front to counteract the torque. Maybe some suction in the tail too - actually according to Lindsay Lord's planing hull experiments we know there is suction.

Greg, I think there is some suction in the tail of a surfboard - isn't this partly what helps nose riding on a longboard with tail kick? I think there is some suction on a shortboard too - this is why a shortboard with high tail rocker works well in step up conditions - the suction provides us with a bit of braking control. Then when we step on the tail it becomes "loaded with pressure" like you said Greg.
There is suction when the board is planing, this is why the AC75 with the foil has a center skeg. It helps reduce the suction by preventing air from being sucked under as fast when getting up on foil. As the boat lifts, the air at the surface of the hull has to fill underneath the boat, so the skeg directs the flow of air and reduces the suction so the boat can lift faster onto foil. Then of course the skeg dampens impact when it drops off foil. https://www.sailingworld.com/story/racing/hulls-of-the-modern-americas-cup/
 

PeterDj

Legend (inyourownmind)
Jul 11, 2018
457
325
63
This video is pretty cool. Skip to about 3:30 to avoid the pre-story. I need to measure my boards now to see if they have about 20degrees of nose and tail rocker.
 
  • Like
Reactions: Mr J

Mr J

Michael Peterson status
Aug 18, 2003
1,981
1,220
113
Regional Vic, Australia
Thin fins have less drag at higher speeds. Not as good in shin high dribble.

There's no flow on the deck because the water has released off the rail. You get release when the radius is too tight, or theres's an edge. Sharp rail - lots of release. Fat rail - no release
View attachment 111688
Lindsay Lord towed vertical foil sections (rudder drag testing) underneath surfboards on outriggers and reached the conclusion you just stated on thickness and drag.

The rudder experiments were done in Hawaii with clear waters and drag was estimated by divers who looked at the amount of bubbles trailing. He did not say whether the foils were dragged at an angle of attack - considering that rudders are meant to steer boats some angle of attack would make sense. If the amount of bubble is an indication of the amount of flow separation then that estimate would work because the flow separation is a "suction wake". The low pressure wake is also turbulence, but must be a different sort of turbulence to the "thin" turbulence of the boundary layer - is that right?

RudderThicknessToSpeed.PNG
 

Mr J

Michael Peterson status
Aug 18, 2003
1,981
1,220
113
Regional Vic, Australia
There is suction when the board is planing, this is why the AC75 with the foil has a center skeg. It helps reduce the suction by preventing air from being sucked under as fast when getting up on foil. As the boat lifts, the air at the surface of the hull has to fill underneath the boat, so the skeg directs the flow of air and reduces the suction so the boat can lift faster onto foil. Then of course the skeg dampens impact when it drops off foil. https://www.sailingworld.com/story/racing/hulls-of-the-modern-americas-cup/
That AC75 foil boat looks different to planing hull suction Peter. The article is written for laymen like us, but loses some of the absolute science which makes it hard to understand. However, that centre skeg seems to be there to provide progressive stability as the board rises up from foil lift and clears the water in a more progressive way to its tapering thickness. Some immersion stability is an advantage in tacking the article says.

The article talks of getting the sail close to the deck to reduce air escape from high pressure windward side to low pressure side. I think it might also be talking of reducing air travel between the elevated underneath hull surface and the deck. They say that the hull is aerofoil shaped, but I am not sure which way vertically/horizontally or both. If it is foil shaped with horizontal surfaces, then it wouldn't make sense that the underneath hull surface is the suction side because it would be counter productive by fighting the lift from the foil.

As llilibel said planing hull is a very different animal to a foil. This is the suction that Lindsay Lord talks of - as model boat is towed the tail sinks lower into the water due to suction. It can be seen that the hull angle of attack (trim angle) is steeper than the towing rope.

WarpedSuction.PNG
 
  • Like
Reactions: paunch23

Senor Sopa

Gerry Lopez status
Mar 11, 2015
1,174
1,788
113
Ponto
Senor Sopa, I found this diagram which adds some context to the one you posted. It shows how the wake is also turbulence. It is low pressure and creates suction drag. So a different sort of turbulence to the slippery, thin laminar turbulence of the boundary layer.

View attachment 111970
If you add dimples, the flow will stay attached for longer. This means a smaller wake profile, which is less drag.
 

PeterDj

Legend (inyourownmind)
Jul 11, 2018
457
325
63
I tried these fins. There was nothing really magical about them. My theory is at slower speeds its adding drag, but once it gets enough speed to build up the boundary layer it should go faster. The dimples are on the curved side of the fin, so if its reducing drag there, then you will get less lift, so it could affect the turn-ability of side fins, but for a center fin it should work great as a stabilizer. I like the graphics, colors and price though. Unfortunately, I went over the falls on a wave my ass hit the board and broke the fin box losing one of these fins, so now I just keep the remainder as backups.