In marine architecture, the usual distinction between whether a hull is acting as a displacement hull or a planing hull is whether the force from the static displacement of water (creating a buoyant support force) is greater than, or less than the support generated by the dynamic downward displacement of water (i.e. through Newton's Second Law: force = time rate of change of momentum). However, to complicate matters, some like to add a third class: semi-displacement, or semi-planing for craft operating near the transition speed.
Hulls that are designed to be most efficient in displacement mode are usually referred to as "displacement hulls" while those that are designed to be most efficient in planing mode are "planing hulls". Certainly a planing hull operates in displacement mode at low speeds...but not all displacement hulls (even with huge amounts of power) can operate in planing mode (some will literally pull themselves underwater).
It's a pretty straight-forward calculation to compute which mode one is operating in if one knows the planform of the wetted area of the bottom,
the trim angle of the board, and either the combined weight of the surfer and board, or the lift coefficient for the hull. If anyone wants an approximate equation, I'll rederive it and post it--it at least gives one a ballpark estimate of the speed one has to be traveling to change from displacement mode to planing mode.
MT
Flex
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hart
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bottom shape
Dan,
Your rounded vees thru the front one-third of your boards are similar to mine..but.
I have found that the 'rounded' shape can have disadvantage..like, water at times will drag over the curve and slow you down.
Probably in line with MT's post above. That is, at the point between displacement and planing.
Lift in the underside of your rail is important..but rather than create it with curve..I lift the rail via a vee-panel..a 'straight' one.
It only sits out near the rail though..because a subtle concave is commencing thru your bottom in the centre..just under your chest as you paddle.
The vee keeps your rail line high..yet the concave invites entry, should your bottom re-engage. A win-win situation.
Oh..and the "transition" point?
Try just behind your knees..that way you can't go wrong. It's all release up front..and all bite down the back.
Now, if our boards were flexible they would create their own transition points..but in the mundane world of rigidity, we have to make the decision, ourselves.
hart
Your rounded vees thru the front one-third of your boards are similar to mine..but.
I have found that the 'rounded' shape can have disadvantage..like, water at times will drag over the curve and slow you down.
Probably in line with MT's post above. That is, at the point between displacement and planing.
Lift in the underside of your rail is important..but rather than create it with curve..I lift the rail via a vee-panel..a 'straight' one.
It only sits out near the rail though..because a subtle concave is commencing thru your bottom in the centre..just under your chest as you paddle.
The vee keeps your rail line high..yet the concave invites entry, should your bottom re-engage. A win-win situation.
Oh..and the "transition" point?
Try just behind your knees..that way you can't go wrong. It's all release up front..and all bite down the back.
Now, if our boards were flexible they would create their own transition points..but in the mundane world of rigidity, we have to make the decision, ourselves.
hart
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Bryan Jackson
- Ripper (more than 100 posts)
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- Joined: Fri Mar 28, 2003 12:14 pm
At least now we're getting a handle on what is really going on with KB "displacement" v. "planing" hulls 
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Beeline2.0
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surfhorn
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- Contact:
Bruce -
The rounded nose vee that I used came from the days riding my 5'6" - 5'8" boards. I would only KB when it was big, cranking, firing down the line conditions; anything smaller I would standup surf.
I wanted my KBs to cut through the wave face on take-off - like the bow of a US Navy destroyer; cut through the water & chop and get up on a plane.
Now that I'm riding the newer, longer shapes, I get to experiment all over again.... gotta' love this new KB era!
The rounded nose vee that I used came from the days riding my 5'6" - 5'8" boards. I would only KB when it was big, cranking, firing down the line conditions; anything smaller I would standup surf.
I wanted my KBs to cut through the wave face on take-off - like the bow of a US Navy destroyer; cut through the water & chop and get up on a plane.
Now that I'm riding the newer, longer shapes, I get to experiment all over again.... gotta' love this new KB era!
kbing since plywood days
Toe-In (long)
It is not necessarily true that toed-in side fins have more drag than parallel ones. Here's the reason why:
For simplicity, consider a flat (no rocker) bottomed hull planing on flat water. A planing hull gets most of it's lift by deflecting the water moving toward the bottom downward and under the hull as it moves past. Since the water had no vertical motion until passing under the hull, it had no vertical component of momentum. At the stern of the craft it's moving downward, as it had to since the hull lies in its original path. So it exits from the hull with downward momentum. Newton's Second Law: Force = time rate of change of momentum, hence imparting downward momentum to the water means an upward force on the hull. This force is proportional to one power of V since the downward velocity imparted to the water passing under the hull is proportional to the speed of the flow toward the hull--the "V" contribution to the momentum. That's increased to a second power of V because the mass of water passing by the hull in a unit time is proportional to the speed of the flow past the hull--the "M" component of momentum.
But...and here's the catch...there is more than one path for the water approaching the hull. Instead of being deflected downward, it can be deflected out to either side. If you had a hull 6' long and only a inch wide, most of the water would prefer that route. On the other hand if the hull was 6' long and 100' wide, only a tiny fraction of the flow towards the board would be deflected to the sides.
A surfboard is pretty narrow (aspect ratio ~ 1, or less) compared with an airplane wing (aspect ratio on the order 7, or more--one of the reasons that hydrofoils are more efficient--in the right speed range--than a planing hull). So a considerable amount of the flow towards the hull flows sidewise (cross-flow) instead of under the board. Of course, the cross-flow doesn't have any downward component to its momentum, so it doesn't contribute to the dynamic lift. That means that the board has to be inclined more with respect to the surface of the water to get enough lift to support the weight of the rider and board. Pressure acts perpendicular to a surface, so that means that the total pressure force is inclined backward more than if all the water went under the hull. The backward component of this pressure force is the induced drag (the induced drag increases as the square of the angle-of-attack of the hull). This is one reason why wide-tailed boards (e.g. fish) are faster on certain types of waves (when you get going fast enough transversely across the face of a wave, the whole bottom isn't "wetted" so the width advantage can disappear, or at least be diminished).
Anyhow...
Even with the high(er) aspect ratio wings on transport planes, there is still a loss of efficiency due to cross-flow. There was a period when engineers studied how this cross-flow might be harnessed to convert part of the momentum in the crossflow to a rearward directed component of momentum. The result was "Whitcomb Winglets" -- small ~ vertical fins (like a small tail fin on the airplane--and actually a pair, the larger on top of the wing and the smaller underneath). When that redirection occurs, then by Newton's Second Law, there's a pressure force directed forward. That pressure force acts against the transverse projected area of winglets to recover some of the momentum, converting it into an additional driving force.
In order to accomplish this, the winglets have to be toed-in a bit (and to minimize the drag of the winglets, they also have camber--i.e. the outer side is more convex than the inner side). So what does this sound like? Sounds like side fins on a surfboard on a surfboard to me. And because of the low aspect ratio of a surfboard, the cross-flow would be greater, and hence there's the potential for even more recovery. So toed-in side fins not only can assist in turning, but can increase the speed of the board in some cases.
The catch is that there is only one speed/angle-of-attack combination for which any specific winglet design will be optimal. For example, as speeds get higher, the induced drag (part of which is associated with the crossflow) becomes less, while the remaining drag (e.g. skin friction drag, fin/hull interference drag, etc.) increases. So there will be some speed at which the additional surface area of the winglets (/side fins) becomes a detriment to the speed performance.
MT
component of the
For simplicity, consider a flat (no rocker) bottomed hull planing on flat water. A planing hull gets most of it's lift by deflecting the water moving toward the bottom downward and under the hull as it moves past. Since the water had no vertical motion until passing under the hull, it had no vertical component of momentum. At the stern of the craft it's moving downward, as it had to since the hull lies in its original path. So it exits from the hull with downward momentum. Newton's Second Law: Force = time rate of change of momentum, hence imparting downward momentum to the water means an upward force on the hull. This force is proportional to one power of V since the downward velocity imparted to the water passing under the hull is proportional to the speed of the flow toward the hull--the "V" contribution to the momentum. That's increased to a second power of V because the mass of water passing by the hull in a unit time is proportional to the speed of the flow past the hull--the "M" component of momentum.
But...and here's the catch...there is more than one path for the water approaching the hull. Instead of being deflected downward, it can be deflected out to either side. If you had a hull 6' long and only a inch wide, most of the water would prefer that route. On the other hand if the hull was 6' long and 100' wide, only a tiny fraction of the flow towards the board would be deflected to the sides.
A surfboard is pretty narrow (aspect ratio ~ 1, or less) compared with an airplane wing (aspect ratio on the order 7, or more--one of the reasons that hydrofoils are more efficient--in the right speed range--than a planing hull). So a considerable amount of the flow towards the hull flows sidewise (cross-flow) instead of under the board. Of course, the cross-flow doesn't have any downward component to its momentum, so it doesn't contribute to the dynamic lift. That means that the board has to be inclined more with respect to the surface of the water to get enough lift to support the weight of the rider and board. Pressure acts perpendicular to a surface, so that means that the total pressure force is inclined backward more than if all the water went under the hull. The backward component of this pressure force is the induced drag (the induced drag increases as the square of the angle-of-attack of the hull). This is one reason why wide-tailed boards (e.g. fish) are faster on certain types of waves (when you get going fast enough transversely across the face of a wave, the whole bottom isn't "wetted" so the width advantage can disappear, or at least be diminished).
Anyhow...
Even with the high(er) aspect ratio wings on transport planes, there is still a loss of efficiency due to cross-flow. There was a period when engineers studied how this cross-flow might be harnessed to convert part of the momentum in the crossflow to a rearward directed component of momentum. The result was "Whitcomb Winglets" -- small ~ vertical fins (like a small tail fin on the airplane--and actually a pair, the larger on top of the wing and the smaller underneath). When that redirection occurs, then by Newton's Second Law, there's a pressure force directed forward. That pressure force acts against the transverse projected area of winglets to recover some of the momentum, converting it into an additional driving force.
In order to accomplish this, the winglets have to be toed-in a bit (and to minimize the drag of the winglets, they also have camber--i.e. the outer side is more convex than the inner side). So what does this sound like? Sounds like side fins on a surfboard on a surfboard to me. And because of the low aspect ratio of a surfboard, the cross-flow would be greater, and hence there's the potential for even more recovery. So toed-in side fins not only can assist in turning, but can increase the speed of the board in some cases.
The catch is that there is only one speed/angle-of-attack combination for which any specific winglet design will be optimal. For example, as speeds get higher, the induced drag (part of which is associated with the crossflow) becomes less, while the remaining drag (e.g. skin friction drag, fin/hull interference drag, etc.) increases. So there will be some speed at which the additional surface area of the winglets (/side fins) becomes a detriment to the speed performance.
MT
component of the
Experience gained is in proportion to equipment ruined.
Beeline,
Have found that the foil on some FCS fins often leaves something to be desired. Boards won't go when there is insufficient foil. Maybe the lack of foil contributes to drag in toed-in fins, where distinct foil helps toed-in fins defy drag.
I believe that foil in the fin tip (i.e. foil that starts to get parallel to the bottom deck) is an important part of the way the fin responds under turning (by "pulling" the rails into the water). I suspect that the curved FCS work on this principle, "lifting" the tail during planing and pushing into the turn when on edge.
Have found that the foil on some FCS fins often leaves something to be desired. Boards won't go when there is insufficient foil. Maybe the lack of foil contributes to drag in toed-in fins, where distinct foil helps toed-in fins defy drag.
I believe that foil in the fin tip (i.e. foil that starts to get parallel to the bottom deck) is an important part of the way the fin responds under turning (by "pulling" the rails into the water). I suspect that the curved FCS work on this principle, "lifting" the tail during planing and pushing into the turn when on edge.