Board "speed"

TomBloke · April 13, 2007, 9:08pm

Hi MTB,

Surely it is obvious that there are significant practical differences between planing hulls and hydrofoils and that these differences do not cease to exist simply because hydrofoils have to operate at a certain depth for maximum efficiency. I am not sure why you insist that there are no significant differences between the two when even a toddler can tell the difference between a hydrofoil boat and a planing hull

Regarding your comment that the efficiency of an underwater foil decreases once it reaches a depth beneath the water surface less than two chord lengths: According to the site quoted below the relevant distance is related to chord thickness, not length . . . .

http://www.hydrofoils.org/Lift/lift.html

“The influence of the free water surface is accounted by function (4), where c is the largest thickness of the foil and h is the foil depth.”

There is a big difference between two chord lengths and two chord thicknesses, on my foils two chord thicknesses is a mere 10mm below the surface whereas two chord engths is 200mm.

Now about those gps readings:

As mentioned previously our Garmin Gecko unit produces track points which include an elevation. . . . and the track recorded is thus IN THREE DIMENSIONS… .

Regards,

Roy

TomBloke · April 14, 2007, 1:52am

Another point regarding the efficiency of a foil as it gets close to the surface of the water is this: with a foil which is used in conjunction with a planing bottom adjacent to the foil proximity of the surface of the water is irrelevant because there is effectively no surface of the water adjacent to the underwater foil. . . if the foil is completely enclosed in this case so much the better.

On the subject of significant differences between underwater foils and planing surfaces:

With planing surfaces an increase in planing area entails either a wider wetted surface or a longer wetted surface, or both. This isn’t the case when underwater foils are added, as it is possible to ‘stack’ the lift area vertically. . … . thus I am able to increase the effective planming area in the tail of my board by a large factor without increasing the width of the surfboard in that area. . . … this gives the best of both worlds: an increase in the available lift area AND a higher aspect ratio in that area while maintaing a narrow tail for control… … . a quantum leap ahead in terms of eficiency, speed, and control

mtb · April 14, 2007, 11:45am

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Surely it is obvious that there are significant practical differences between planing hulls and hydrofoils and that these differences do not cease to exist simply because hydrofoils have to operate at a certain depth for maximum efficiency. I am not sure why you insist that there are no significant differences between the two when even a toddler can tell the difference between a hydrofoil boat and a planing hull

The difference between buoyancy lift and planing lift is like the difference between cats and dogs; the difference between planing lift and hydrofoil lift is like the difference between two species of dogs. The characteristics of a dog (e.g. size, color, facial features, tail, etc.) can be changed by breeding to something intermediate between the two, but the result is still a dog. Same for a planing hull (essentially one half of a submerged foil) and a hydrofoil via proximity to the sea surface. But buoyancy cannot be transformed into dynamic lift (planing, hydrofoil).

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Regarding your comment that the efficiency of an underwater foil decreases once it reaches a depth beneath the water surface less than two chord lengths: According to the site quoted below the relevant distance is related to chord thickness, not length . . . .

http://www.hydrofoils.org/Lift/lift.html

“The influence of the free water surface is accounted by function (4), where c is the largest thickness of the foil and h is the foil depth.”

There is a big difference between two chord lengths and two chord thicknesses, on my foils two chord thicknesses is a mere 10mm below the surface whereas two chord engths is 200mm.

Thanks for the reference. Keep in mind that is one person’s opinion (Harry Larsen’s)–not necessarily the International Hydrofoil Society’s–and differs from the opinion of many others (I will dig up some references for you).

At one time (I don’t know if they still exist) there were some pics on the web of the flow over the Decavitator’s (record holder of the speed record for human-powered boats) main foil as a function of water depth that visually clearly illustrates that there are surface proximity effects at submergence depths better represented by the chord length than by the maximum foil thickness.

If I can get a friend’s permission, I will also dig up and send you some photos that show this “surface effect” extends to depths comparable to the chord of a foil (vs the foil thickness) that I took during the development of his human-powered hydrofoil boat.

I also expect to be visiting with Harry Larsen sometime in June and will try to discuss this difference in opinion with him then.

*** Update: [Tracing back the source of the equations referenced above at the IHS website reveals that the author is not Harry Larsen, but rather one of the co-authors (V. lDubrovsky, K Mateeu, S Sutulo) of the book: “Small Waterplane Area SHIPS”. I apologize for the mistake. Harry Larsen created the EXCEL spreadsheet the implements the equations. Also see my subsequent post in this thread that shows the set of equations discussed at the IHS web site does indeed establish a relationship between submergence depth and foil chord, rather than submergence depth and foil thickness.]

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Now about those gps readings:

As mentioned previously our Garmin Gecko unit produces track points which include an elevation. . . . and the track recorded is thus IN THREE DIMENSIONS… .

We are talking about speeds, not positions. The error in vertical position measurements with consumer grade GPS’s is generally so large as to make decent estimates of vertical speeds by differencing vertical positions over the time scales relevant to surfboard motions worthless.

mtb

mtb · April 14, 2007, 3:22pm

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Regarding your comment that the efficiency of an underwater foil decreases once it reaches a depth beneath the water surface less than two chord lengths: According to the site quoted below the relevant distance is related to chord thickness, not length . . . .

http://www.hydrofoils.org/Lift/lift.html

“The influence of the free water surface is accounted by function (4), where c is the largest thickness of the foil and h is the foil depth.”

There is a big difference between two chord lengths and two chord thicknesses, on my foils two chord thicknesses is a mere 10mm below the surface whereas two chord engths is 200mm.

I apologize (to Harry Larsen, or the author of the equations on the reference web page) for my comments referencing him in my previous post. I was in a hurry to get in a pre-dawn surf and so I didn’t take the time to look up his equations at the International Hydrofoil Society web page (at the link you supplied) before making my response to your post. Now that I am back from surfing, I have examined his discussion at the IHS website and find that I do not have a problem with it or his equations. What I do have a problem with is your interpretation of them. The suspicion that this might be the case appears in the equation you posted from the IHS website (reproduced above). More specifically, the exponential function in the equation requires a dimensionless argument. Hence the mean water depth (h-bar) must be normalized by a length. Obvious candidates for that normalization are the chord of the foil, or the maximum thicknes of the foil section. Examination of Harry’s development of the lift functions reveals that that is indeed the case when he states:

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Reduced values (with overbars) are normalized by a foil chord.

That means that the h(bar) in the equation above is actually the ratio of the water depth to the chord length of the foil. Note also that while the effect of water depth affects the value of the “K” parameter, which, in turn affects the magnitude of the lift coefficient (see equation below), the only dependence of “K” on water depth is via the change in the value of the exponential function, which, in turn, involves only the ratio of the water depth to the chord length. Hence, contrary to your claim, the effect of proximity to the sea surface is only a function of the water depth and chord length–and does not involve the thickness of the foil in relation to the depth of submergence.

PS. Note that in spite of the statement at the IHS website that the depth dependence of the lift coefficient is accounted for via the equation for the “K” parameter (that you quoted), note that the far right term in the denominator of equation (2), above, also depends on the ratio of the water depth to the chord length.

PPS. Tracing back the source of the equations referenced above at the IHS website reveals that the author is not Harry Larsen, but rather one of the co-authors (V. lDubrovsky, K Mateeu, S Sutulo) of the book: “Small Waterplane Area SHIPS”. I apologize for the mistake. Harry Larsen created the EXCEL spreadsheet the implements the equations.

mtb · April 14, 2007, 4:07pm

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Another point regarding the efficiency of a foil as it gets close to the surface of the water is this: with a foil which is used in conjunction with a planing bottom adjacent to the foil proximity of the surface of the water is irrelevant because there is effectively no surface of the water adjacent to the underwater foil. . . if the foil is completely enclosed in this case so much the better.

Yes…perhaps. But, the flow in the presence of the hull differs from that over the foil for a fully submerged foil. Please supply a reference indicating how the total (hull plus foil) lift force changes as the spacing between the hull and the foil changes. Remember, the upper surface of the foil is the low pressure side and it’s presence in proximity to the bottom of the hull may reduce the upward pressure on the bottom of the hull.

Also keep in mind that if the hull is not completely supported by the (submerged) foils, it will be generating a surface wave train and hence there will be more wave drag than if the hull is supported out of the water by the hydrofoil(s).

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On the subject of significant differences between underwater foils and planing surfaces:

With planing surfaces an increase in planing area entails either a wider wetted surface or a longer wetted surface, or both. This isn’t the case when underwater foils are added, as it is possible to ‘stack’ the lift area vertically. . … . thus I am able to increase the effective planming area in the tail of my board by a large factor without increasing the width of the surfboard in that area. . . … this gives the best of both worlds: an increase in the available lift area AND a higher aspect ratio in that area while maintaing a narrow tail for control… … . a quantum leap ahead in terms of eficiency, speed, and control

Stacking the foils vertically is analogous to the difference between a monoplane and a biplane. It is well known (although I don’t have a specific reference) that interference between the flows over a pair of wings reduces the efficiency (lif/drag ratio) on a biplane, compared with that of a monoplane. I presume that by “increasing the aspect ratio” you mean via the attachment of a strut to each end of each foil in the stack (e.g. the same strut for all foils at each end). The problem with that is that junctions between a foil and a strut are significant sources of drag–and the more foils in the stack, the more junctions there are, and the greater the drag.

TomBloke · April 14, 2007, 8:16pm

Hi MTB

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On the subject of significant differences between underwater foils and planing surfaces:

With planing surfaces an increase in planing area entails either a wider wetted surface or a longer wetted surface, or both. This isn’t the case when underwater foils are added, as it is possible to ‘stack’ the lift area vertically. . … . thus I am able to increase the effective planming area in the tail of my board by a large factor without increasing the width of the surfboard in that area. . . … this gives the best of both worlds: an increase in the available lift area AND a higher aspect ratio in that area while maintaing a narrow tail for control… … . a quantum leap ahead in terms of eficiency, speed, and control

Stacking the foils vertically is analogous to the difference between a monoplane and a biplane. It is well known (although I don’t have a specific reference) that interference between the flows over a pair of wings reduces the efficiency (lift/drag ratio) on a biplane, compared with that of a monoplane. I presume that by “increasing the aspect ratio” you mean via the attachment of a strut to each end of each foil in the stack (e.g. the same strut for all foils at each end). The problem with that is that junctions between a foil and a strut are significant sources of drag–and the more foils in the stack, the more junctions there are, and the greater the drag.

The foil ‘stacking’ example which I am interested in is that of having a single non planar tunnel foil working underneath a planing hull. By doing this we are ‘stacking’ the lift surfaces vertically, which cannot be done with planing surfaces alone.

Biplane analogies are not directly relevant in this case because

a) Biplanes do not completely enclose the flow as the tunnel does and

b) Biplanes have struts and wingtips which the tunnel doesn’t have.

By ‘increasing the aspect ratio’ I am referring to the fact that the overall aspect ratio of the surfboard is increased by adding an underwater foil with a higher aspect ratio than the planing surface .

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Another point regarding the efficiency of a foil as it gets close to the surface of the water is this: with a foil which is used in conjunction with a planing bottom adjacent to the foil proximity of the surface of the water is irrelevant because there is effectively no surface of the water adjacent to the underwater foil. . . if the foil is completely enclosed in this case so much the better.

Yes…perhaps. But, the flow in the presence of the hull differs from that over the foil for a fully submerged foil. Please supply a reference indicating how the total (hull plus foil) lift force changes as the spacing between the hull and the foil changes. Remember, the upper surface of the foil is the low pressure side and it’s presence in proximity to the bottom of the hull may reduce the upward pressure on the bottom of the hull.

Please keep in mind that we are using an enclosed tunnel, with an enclosed tunnel the pressure inside the foil is determined by the size of the inlet and the outlet, there is no loss of pressure within the tunnel if the inlet and outlet have the same area.

Incidentally one of the big advantages of an enclosed tunnel is that the pressure within the tunnel tends to equalise, this reduces differences in velocity between pressure layers, which reduces drag.

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Also keep in mind that if the hull is not completely supported by the (submerged) foils, it will be generating a surface wave train and hence there will be more wave drag than if the hull is supported out of the water by the hydrofoil(s).

Yes, that’s true, however this is offset by several advantages gained by having the hull and the foil working together :

The foil will provide a drag reduction (compared with the planing hull alone) at all speeds, there is no ‘hump’ in drag to overcome as there is with craft designed to lift the hull free of the water.
The size of the foil is not critical (unlike craft which lift the hull free, where the size and power of the foil is optimum only for a specific speed)
Continuing from the point above, because there is no drag hump to overcome, an increase in speed results in a smooth increase in the proportion of total lift which comes from the foil, thus increasing efficiency as speed increases (Unlike a non surface piercing submerged foil based craft which lifts the hull free, which loses efficiency soon after speed is increased beyond that which is required to lift the hull free ) To put it another way, with the foil and hull working together the optimum speed for maximum efficiency is always greater than the current speed, but the overall efficiency is always greater than it was at a lower speed. Also, even at very low speeds the foil increases the overall efficiency somewhat compared with a simple planing hull.
Control. With a foil and hull working together some planing surface is always in contact with the water, this means that control is excellent and there is no need to design the underwater foil to take over the job of controlling the craft ( foil based control of a surfboard has several drawbacks, e.g the difficulty of standup control without straps, a high centre of gravity, and the fact that the foil designed for lateral stability and control is not necessarily ideal in terms of drag reduction)

Thankyou very much for your thorough replies, and for taking a good look at those formulae for us.

Regards, Roy

mtb · April 14, 2007, 11:39pm

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The foil ‘stacking’ example which I am interested in is that of having a single non planar tunnel foil working underneath a planing hull. By doing this we are ‘stacking’ the lift surfaces vertically, which cannot be done with planing surfaces alone.

Biplane analogies are not directly relevant in this case because

a) Biplanes do not completely enclose the flow as the tunnel does and

b) Biplanes have struts and wingtips which the tunnel doesn’t have.

By ‘increasing the aspect ratio’ I am referring to the fact that the overall aspect ratio of the surfboard is increased by adding an underwater foil with a higher aspect ratio than the planing surface .

Note: I apologize for the lack of paragraphs to break up the subsequent discussion into smaller, more managable, and logical entities, but the posting editor insists on combining all the paragraphs that I create into just a few large, nearly unreadable, giant paragraphs–even though the paragraphs are displayed when in editing mode. My mistake for misinterpreting what you mean by “stacking vertically”. I assumed that you meant by a vertical stack a series of foils analogous to a stack of records on a record player (but with separation between them, of course). I do not disagree that a typical foil, fully submerged, and intended to contribute to the support of the board and rider will usually have a greater aspect ratio than the wetted area of a planing hull. However, I do not agree that if the fin (or any portion of the fin) that is vertical will contribute to this force since fins/foils generate lift via pressure forces, and pressures act perpendicular to the surface. Hence vertical portions of a foil only generate forces horizontally. Foil areas that are intermediate between vertical and horizontal (i.e. are inclined, in the sense of having cant) divide up the total pressure force into a horizontal component and a vertical component according to the sine of the inclination (cant) angle and the cosine of the cant angle, respectively. The end result (assuming that the foil is aligned so that the angle-of-attack is equal for each hypothetical section comprising the curved foil) is that the effective area of the foil for generating “lift” (to support the rider and board) is not the total area of the foil, but rather the projection of the foil onto a horizontal plane. By way of an example, if a curved foil (say in the shape of half of an ellipse) has a “width” (between the two legs of the ellipse) of 1 ft; and a “height” (from a line connecting the two legs to the end of the foil/semi-ellipse) of 1.5 ft, and a chord of 0.5 ft, the effective area from the standpoint of generating “lift” is 0.5 square feet (i.e. 0.5 ft x 1.0 ft), rather than something between 2 square ft and 1.8 square ft (I didn’t bother to do the exact calculation). Thus using the total area for the effective area would overestimate the “lift” (vertical component) by a factor between 3.6 and 4 times the actual lift. Similarily, the effective area for generating a horizontal component of force would be 1.5 square feet (2 “legs” x 1.5 ft x 0.5 ft = 1.5 sq-ft) (assuming, of course, that the flows over the two “legs” of the semi-ellipse do not interfere with each other, as in a biplane). You are correct that since the ends of the foil (ends of the two legs in our semi-ellipse span-wise curved foil in the example above) terminate against the bottom of the board the effective aspect ratio of the foil will be increased compared with a simple planar foil of the same effective area. How much of a bonus this increase provides depends on the magnitude of the aspect ratio of the equivalent planar foil (if the latter is small, the effect could be significant, if the latter is moderately large, the effect will be small as the beneficial effect decreases as the aspect ratio increases. All the discussion above revolves around the assumptiong of the same angle-of-attack existing for the flow over each section of the curved foil. That is generally not a desirable situation as that means one is generating pressure forces that have a horizontal component on inclined (and vertical) surfaces, with the horizontal component of the force generated by one leg of the foil canceled by those generated by the opposing leg, yielding a net lateral force of zero. But the generation of these pressure forces carries with it the generation of induced drag (relative to a planar section of high aspect ratio generating the same vertical component of pressure force (“lift”) but with no pressure force being generated by the struts (which are typically aligned with the zero lift angle). If the curved foil is “twisted” so that the angle-of-attack becomes zero for the vertical sections, it is also reduced to an intermediate value on the inclined sections. This reduces the magnitude of the pressure force along these sections compared with the “constant angle of attack” case, so the overall lift (vertical component) will be reduced. To make up for this, the base angle of attack (at the horizontal strip of the curved foil) will have to be increased. This, in turn, increases the induced drag contributed by that section of the foil. But I do not know how all this plays out without carrying out the actual calculations (and I don’t have a simulation program capable of handling this type of 3-D geometery). Determining the net effect of all the possible factors that relate to curved/enclosed/etc. “wings/foils” of this type, compared with a simple planar wing/foil, I suspect depends on the specific designs being compared. However, since a variety of wings/foils of this type have been proposed for many years, but only a small number of such craft constructed, a smaller number “successfully” flown, and virtually none (at least that I am aware of) built in quantity for or in commercial enterprises, I am lead to believe that any advantages of foils/wings of this type are minimal, or non-existent, or less efficient than simpler designs. One exception may be design in which the curved section of a wing/foil (e.g. a semi-circle or circle) also serves as a duct for a propellor providing the thrust. However, the best examples of this type of craft that I can think of are all related to improving slow flight (both by reducing propellor losses and increasing the speed of the flow over the foil), with the the “ducts” negatively influencing high speed flight because of the increased skin friction associated with the increased surface area. mtb