'Tis a good article, I learnt something from it.

However here's a counter-quote for you:
Length does play an important role in that the longer boat will have a higher hull speed, which may allow it to get on plane before that speed is reached so that the pilot of such a boat will be completely unaware that such a “hull speed” limitation exists.

Which seems to indicate to me that a length does play a major part in actually getting planing.

I hadn't heard of the 'weight per square foot of planing area' idea before, but it makes a lot of sense and has a parallel in the aeroplane world, namely 'wing loading'. I think this is probably the biggest factor in getting planing, and does so by one number that takes into account:
* Width
* Rocker (or lack thereof)
* Rider weight
all of which (from personal experience) seem to affect early planing.

All good, we learn something new every day. Or if we don't, we're not trying hard enough!

You're dead right about the importance of length in that way, Nebs. The stuff Gestalt and I were saying about the importance of width relates more to the pure planing surface and pure planing effects. As you say, in the real world other factors also arise.

I reckon a U shaped Div 2 board may well be planing earlier than a FW board with an equal size rig but the D2 does that thing of cutting through the water so efficiently below planing speed that you don't feel it "hop" onto the plane and it never does plane as high, therefore it may not seem to plane as early. I actually feel most longboards, well sailed, plane earlier than a FW board BUT when the FW board does start to plane it moves straight into a very efficient planing mode.

A lot of modern skiff and pre-foil Moth design developed has been about going skinnier and having lower wave and wetted surface drag, which allowed those boats to go faster than older boats which were designed to lift onto the plane earlier but suffered so much more low-speed drag that they actually reached their planing speed later.

The longer skinnier hulls also obscure the problem of exactly when they are "planing", which has no clear universally-accepted definition. According to the one used by many naval architects, a sinker sailing waist-deep at 3 knots is planing because it is being lifted above the level of its buoyancy by dynamic lift.

Select to expand quote

Chris 249 said...
I reckon a U shaped Div 2 board may well be planing earlier than a FW board with an equal size rig but the D2 does that thing of cutting through the water so efficiently below planing speed that you don't feel it "hop" onto the plane and it never does plane as high, therefore it may not seem to plane as early. I actually feel most longboards, well sailed, plane earlier than a FW board BUT when the FW board does start to plane it moves straight into a very efficient planing mode.

...

The longer skinnier hulls also obscure the problem of exactly when they are "planing", which has no clear universally-accepted definition. According to the one used by many naval architects, a sinker sailing waist-deep at 3 knots is planing because it is being lifted above the level of its buoyancy by dynamic lift.

Wikipiedia (en.wikipedia.org/wiki/Planing(sailing)) has a good explanation of planing:

"At low speeds, a hydroplaning hull acts as a displacement hull. But, when the speed increases the hull begins acting as a planing hull. However, when the boat begins to plane the formula becomes irrelevant since the boat is climbing its own bow-wave. The bow rises slightly as it starts by mounting its own bow wave. When it reaches the speed where it overtakes the bow wave, the bow resumes its normal attitude. The boat can often be seen to leave its stern-wave some distance behind it. The hull is now planing."

so "...lifted above its bouyancy..." sounds dubious - thats just displacement sailing.

As for longboard vs FW planing - they probably work out to about the same threshold, as they could probably both plane in 9 kts. Given the right conditions (flat water), gear (12m) and sailing ability, a FW can plane in 7 kts. I am unsure what the lower threshold is for longboard planing - has anyone tested this?

So planing is all about surface area? Where does volume come into it?

Two boards can have the same planing surface, but a huge difference in volume. So... somebody please explain, my brain hurts...

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Wet Willy said...

So planing is all about surface area? Where does volume come into it?

Two boards can have the same planing surface, but a huge difference in volume. So... somebody please explain, my brain hurts...

My simple explanation is that volume only matters until you are planing. After that it doesn't matter at all.

If you don't have enough volume then the board won't float you and the board, until you get enough speed to give you enough lift.

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FormulaNova said...

Wet Willy said...

So planing is all about surface area? Where does volume come into it?

Two boards can have the same planing surface, but a huge difference in volume. So... somebody please explain, my brain hurts...

My simple explanation is that volume only matters until you are planing. After that it doesn't matter at all.

If you don't have enough volume then the board won't float you and the board, until you get enough speed to give you enough lift.

By crikey!!

But...but...suppose I got planing in marginal conditions on a high volume board, and another guy got planing on a lower volume board with the same planing area...isn't the other guy more in danger of dropping off the plane???

Select to expand quote

Wet Willy said...

FormulaNova said...

Wet Willy said...

So planing is all about surface area? Where does volume come into it?

Two boards can have the same planing surface, but a huge difference in volume. So... somebody please explain, my brain hurts...

My simple explanation is that volume only matters until you are planing. After that it doesn't matter at all.

If you don't have enough volume then the board won't float you and the board, until you get enough speed to give you enough lift.

By crikey!!

But...but...suppose I got planing in marginal conditions on a high volume board, and another guy got planing on a lower volume board with the same planing area...isn't the other guy more in danger of dropping off the plane???

No. Volume doesn't matter when you're planing. Its just along for the ride.

Gosh.

Then, we could be launched onto the plane by mechanical means (jetski??? Tow windsurfing???) and we'd keep planing even if it was an 80 litre board in 10 knots, provided there was enough sail power????

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Wet Willy said...

Gosh.

Then, we could be launched onto the plane by mechanical means (jetski??? Tow windsurfing???) and we'd keep planing even if it was an 80 litre board in 10 knots, provided there was enough sail power????

Even if it was a 5 litre board!!!!! think of a water ski, that's virtually what you are describing. As long as you are properly planning, there is no lift due to displacement, so volume has no effect, in fact the extra weight is probably slowing you down! It's only when the wind drops you appreciate volume effects.

Well, butter my nipples and call me Slippery

...I'm going right out and buying a BIG 60 litre board!!

One other question: When it comes to high winds and big chop, is it the PLANING AREA or the VOLUME which has the most effect on how comfortable a ride one gets?

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Wet Willy said...

One other question: When it comes to high winds and big chop, is it the PLANING AREA or the VOLUME which has the most effect on how comfortable a ride one gets?

Well as said before once planning volume has no effect.
Ride comfort in chop is all about bottom shape, "v" is better than flat, and flat is better than concave. Rocker also helps, as does nice thick jump pads.
Trouble is, what's good for chop is bad for early planning.

Wet Willy,

You forgot to also ask about:
length
width
nose width
tail width
fin size and flexibility
rocker
vee
pumpinig technique
perceived planing threshold
concaves
board rigidity
etc etc etc...

Since I first asked the question, there has been so much diverse information and experiences presented, I don't know if my concept of the ideal light wind planing board is much clearer. Oh well, it has been an interesting discussion anyway

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nobody said...

>>>>>>>

Since I first asked the question, there has been so much diverse information and experiences presented, I don't know if my concept of the idea light wind planing board is much clearer. Oh well, it has been an interesting discussion...

You'll get used to this forum after a while, "staying on topic" isn't as much an issue here as it is in other forums, it's a bit more like a big conversation amongst friends. If the original topic is really important to you, you have to keep dragging the conversation back to it, very rarely does it stay there for long otherwise.

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nobody said...

Wet Willy,

You forgot to also ask about:
length
width
nose width
tail width
fin size and flexibility
rocker
vee
pumpinig technique
perceived planing threshold
concaves
board rigidity
etc etc etc...

Yeah, but the term "Planing Area" is a general way of covering length, width, nose width, tail width, at least - we can't possibly go over every possible combination; the point is the surface area of the bottom of the board, and whether big = earlier planing.

I "kinda" knew all this other stuff but......whas the story with all this soft fin hard fin bussiness ????

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Richiefish said...

I "kinda" knew all this other stuff but......whas the story with all this soft fin hard fin bussiness ????

Think "soft" is a misnomer, "flexi" and "stiff" would be better.
Back in the 60s, think it was George Greenough started using flex in fins to store bottom turn energy, then release it on exit. A similar effect can occur with a flexi sailboard fin as it's pumped with foot pressure, it helps lift the board.

Hi Chris,

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Chris 249 said...

You're dead right about the importance of length in that way, Nebs. The stuff Gestalt and I were saying about the importance of width relates more to the pure planing surface and pure planing effects. As you say, in the real world other factors also arise.

I reckon a U shaped Div 2 board may well be planing earlier than a FW board with an equal size rig but the D2 does that thing of cutting through the water so efficiently below planing speed that you don't feel it "hop" onto the plane and it never does plane as high, therefore it may not seem to plane as early. I actually feel most longboards, well sailed, plane earlier than a FW board BUT when the FW board does start to plane it moves straight into a very efficient planing mode.

A lot of modern skiff and pre-foil Moth design developed has been about going skinnier and having lower wave and wetted surface drag, which allowed those boats to go faster than older boats which were designed to lift onto the plane earlier but suffered so much more low-speed drag that they actually reached their planing speed later.

The longer skinnier hulls also obscure the problem of exactly when they are "planing", which has no clear universally-accepted definition. According to the one used by many naval architects, a sinker sailing waist-deep at 3 knots is planing because it is being lifted above the level of its buoyancy by dynamic lift.

Interesting about the gradual planing and planing earlier than FW boards. I have noticed the Windsufer hull appears to plane quite early and gradually but doesn't appear to plane relatively fast above 15 knots.

Mr Drake appears to agree with adding length to lower planing threshold:
Mr Drake said...

Select to expand quote

...However, there are secondary effects, primary among them is the influence of board length. The added length of Apollo almost certainly reduces drag in the critical zone where dynamic lift is overtaking static lift. The slight addition of weight is probably more than offset by aerodynamic lift at the nose of the board. This also contributes a higher trimmed angle of attack, beneficial for decreasing planing speed using the above criteria.

www.windsurfingmag.com
Length difference between the Formula 162 and Apollo is 7cm (228 to 235cm)

Earlier you said...

Select to expand quote

Well, no, not according to the classic planing theory of Dan Savitsky, the guru in the field. Width, not area, is the critical measure (all else being equal) for planing efficiency. Induced drag when planing drops by the square of the width. Jim Drake confirms that this is the reasoning behind FW boards.

Does this mean that FW boards should be the theoretical fastest boards outright? (Ignoring fin drag). Is the only reason they aren't due to control issues?

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nobody said...
... Does this mean that FW boards should be the theoretical fastest boards outright? (Ignoring fin drag). Is the only reason they aren't due to control issues?

As you move faster, you will experience both hydrodynamic and aerodynamic drag from the hull. For the sake of discussion, lets assume that you are in control so that you do actually manage to be "flying on the fin", you will still need reduce the amount of air flowing under the board and across its nose.

this is what i have come to understand.

it's the definaition of planing that causes the confusion.

the way i look at it is this.

firstly, semi-displacement hulls (slow). i put longboards in this category. in light winds they aren't flying over the surface of the water but they are actually planing none the less. in this case waterline length is the important factor.

secondly, planing hulls. (fast) ie. flying over the top of the water. with these designs width is a factor. this would include formula boards and all other boards without centreboards.

next thing to look at is the drag aspect ratio. high aspect ratio (wide boards)means faster and low aspect ratio (longboards) means slower.

also the overall drag due to surface area in contact with the water needs to be taken into account. this is a killer for longboards. not so bad for formula boards and really what makes small boards (slalom/speed boards etc) the fastest.

volume. - volume is certainly important as it lowers the planing threshold. the boyant force is part of the overall dynamic lift. a heavy board will actually just have less volume then a board of exactly the same sizre and dimensions weighing less. this is why racers get bundled into weight classes etc. and why manufacturers make super light versions of designs and call them pro models. in effect they ahve more volume for the size so they plane earlier, handle better etc. they stick pro on the label cause they also break earlier. weight is obviously a problem when jumping cause gravity is a drag......(man!)

also don't forget the biggy (that i psersonally think starboard get the kudos for).

"Volume Distribution" buoyant lift is helped through volume distribution, placing the volume at the mast and under the sailor improves the buoyant lift. so no need for a nose in this case. hence short and wide. great for hydroplaning. for sem-displacement hulls length is important so the volume is distributed along the length.

other factors that increase lift is nose width. wider noses help more air under the board which increase lift. but again, long and narrow semi-diaplacement hulls have relativeley narrow noses. problem with wide noses is they fly off the water in the stronger winds. manufacturers reduced the length of the boards further when this was considered an issue as a shorter board has a shorter swing point. shortening the swing point puts the sailor back in control of the board. but the reduced length meant keeping with the wider board shape to keep the volume up and also keep the planing threshold low as the watrerline length was reduced.

freeride boards sit in the middle. ie. not too wide, and longer than a true wide board but not as long as a semi-displacment hull. with these boards the extra length improves the gybing by allowing waterline length to begin to work (keep the board planing) as the board slows during the turn.

fin size. bigger lift due to foils or area is just that.

formula boards aren't the theoretical fastest boards outright because of the drag due to surface area in contact with the water and this includes the fin.

my head hurts.

Select to expand quote

mathew said...

Chris 249 said...
I reckon a U shaped Div 2 board may well be planing earlier than a FW board with an equal size rig but the D2 does that thing of cutting through the water so efficiently below planing speed that you don't feel it "hop" onto the plane and it never does plane as high, therefore it may not seem to plane as early. I actually feel most longboards, well sailed, plane earlier than a FW board BUT when the FW board does start to plane it moves straight into a very efficient planing mode.

...

The longer skinnier hulls also obscure the problem of exactly when they are "planing", which has no clear universally-accepted definition. According to the one used by many naval architects, a sinker sailing waist-deep at 3 knots is planing because it is being lifted above the level of its buoyancy by dynamic lift.

Wikipiedia (en.wikipedia.org/wiki/Planing(sailing)) has a good explanation of planing:

"At low speeds, a hydroplaning hull acts as a displacement hull. But, when the speed increases the hull begins acting as a planing hull. However, when the boat begins to plane the formula becomes irrelevant since the boat is climbing its own bow-wave. The bow rises slightly as it starts by mounting its own bow wave. When it reaches the speed where it overtakes the bow wave, the bow resumes its normal attitude. The boat can often be seen to leave its stern-wave some distance behind it. The hull is now planing."

so "...lifted above its bouyancy..." sounds dubious - thats just displacement sailing.

As for longboard vs FW planing - they probably work out to about the same threshold, as they could probably both plane in 9 kts. Given the right conditions (flat water), gear (12m) and sailing ability, a FW can plane in 7 kts. I am unsure what the lower threshold is for longboard planing - has anyone tested this?

Matthew, the definition I used comes from;

C.A. Marchaj; a scientist and author of "Aerohydrodynamics of Sailing" and other works and probably the best known authority on the physics of sailing;

The UK's National Physics Laboratory, in particular Tom Tanner's measurements of the drag and c of g of an International (sailing) Canoe hull;

An America's Cup designer with a Masters from MIT;

Larsson and Eliasson's yacht design book and/or Pierre Gutelle's similar work;

An academic from an Oz university, well known worldwide for his ship and small-craft velocity prediction work and towing tank studies;

I'll take those guys above Wiki any day.

I have boats and boards where the bow overhangs the bow wave all the time....sometimes the bow wave is further back (when the board or boat is moving fast). Does that mean the craft has "overtaken" its bow wave?

The whole "lifted above its bouyancy" thing is that a hull, when stationary, is supported only by buoyancy ie Archimedes.

When it starts moving, a hull creates waves along the centre of hull, which gradually reduce in number and increase in size as the speed increases to Fn 1.34. The presence of these waves reduces the immersed volume of the centre section of the hull, causing the boat to sink lower in the water. This "squat" is very noticeable in a heavy boat towed fast. The effect is that once a boat starts moving, it tends to actually sink lower in the water; the centre of gravity sinks.

A planing hull is, however, lifted (ie the centre of gravity rises once more) because of dynamic lift as it gains speed. The C of G then rises and eventually rises above the level it was when the hull was stopped. From then on, according to this well known definition, the hull is planing.

This drop in C of G is well illustrated by the Tanner tests, for instance, and by practical experience in craft that don't plane.

Other definitions, like "you are riding only on dynamic lift" don't really apply as every craft displaces some water and therefore there is some element of bouyant lift. Planing theory guru Dan Savitsky of the Stevens Institute says this is true even in a flat plate, and Jim Drake say that this is true for a Formula board.

However, as renowned skiff designer Julian BEthwaite says, planing is a complicated and messy area and the more you look into it, the more you realise how complicated it all is.

In longboards, the move onto the plane is normally so subtle that it would be almost impossible to work out just where it starts IMHO.

Interesting post Gestalt.

Select to expand quote

Gestalt said...

this is what i have come to understand.

it's the definaition of planing that causes the confusion.

the way i look at it is this.

firstly, semi-displacement hulls (slow). i put longboards in this category. in light winds they aren't flying over the surface of the water but they are actually planing none the less. in this case waterline length is the important factor.

secondly, planing hulls. (fast) ie. flying over the top of the water. with these designs width is a factor. this would include formula boards and all other boards without centreboards.

next thing to look at is the drag aspect ratio. high aspect ratio (wide boards)means faster and low aspect ratio (longboards) means slower.

also the overall drag due to surface area in contact with the water needs to be taken into account. this is a killer for longboards. not so bad for formula boards and really what makes small boards (slalom/speed boards etc) the fastest.

I think planing area mainly has to do with (all things being equal) speed and weight. I've wondered why longboards plane slower at the top end. Is it the leverage of the weight of the nose out front that increases the planing area and so the drag? If the board weighed nothing I believe the nose leverage wouldn't be a problem and so would be faster. Maybe this is why manufacturers seem concerned with weight as even short boards would suffer from this to some degree.

The Kona's step tail takes out 34cm and therefore moving the pivot point forward and shortening the leverage length of the nose. The fairly big nose rocker also probably increases lift at speed (from apparent wind) and counters the nose weight to some extent. Maybe this is why the Kona planes, but then takes a few more seconds to accelerate to top speed as the nose is getting lighter with speed (as you have indicated later in this post).

volume. - volume is certainly important as it lowers the planing threshold. the boyant force is part of the overall dynamic lift. a heavy board will actually just have less volume then a board of exactly the same sizre and dimensions weighing less. this is why racers get bundled into weight classes etc. and why manufacturers make super light versions of designs and call them pro models. in effect they ahve more volume for the size so they plane earlier, handle better etc. they stick pro on the label cause they also break earlier. weight is obviously a problem when jumping cause gravity is a drag......(man!)

also don't forget the biggy (that i psersonally think starboard get the kudos for).

"Volume Distribution" buoyant lift is helped through volume distribution, placing the volume at the mast and under the sailor improves the buoyant lift. so no need for a nose in this case. hence short and wide. great for hydroplaning. for sem-displacement hulls length is important so the volume is distributed along the length.

other factors that increase lift is nose width. wider noses help more air under the board which increase lift. but again, long and narrow semi-diaplacement hulls have relativeley narrow noses. problem with wide noses is they fly off the water in the stronger winds. manufacturers reduced the length of the boards further when this was considered an issue as a shorter board has a shorter swing point. shortening the swing point puts the sailor back in control of the board. but the reduced length meant keeping with the wider board shape to keep the volume up and also keep the planing threshold low as the watrerline length was reduced.

How does this lower planing threshold? Was this meant to read "...also keep planing threshold low as beam width is increased."?

freeride boards sit in the middle. ie. not too wide, and longer than a true wide board but not as long as a semi-displacment hull. with these boards the extra length improves the gybing by allowing waterline length to begin to work (keep the board planing) as the board slows during the turn.

fin size. bigger lift due to foils or area is just that.

formula boards aren't the theoretical fastest boards outright because of the drag due to surface area in contact with the water and this includes the fin.

my head hurts.

Select to expand quote

Gestalt said...

this is what i have come to understand.

it's the definaition of planing that causes the confusion.

the way i look at it is this.

firstly, semi-displacement hulls (slow). i put longboards in this category. in light winds they aren't flying over the surface of the water but they are actually planing none the less. in this case waterline length is the important factor.

secondly, planing hulls. (fast) ie. flying over the top of the water. with these designs width is a factor. this would include formula boards and all other boards without centreboards.

next thing to look at is the drag aspect ratio. high aspect ratio (wide boards)means faster and low aspect ratio (longboards) means slower.

I think you could say that longboards are not simply "slow" and shortboards are not simply "fast", without some qualification.

Longboards are MUCH faster in some conditions (put a longboard against a slalom board in the typical puffy 8 knot breezes we see so often, or against an FW board in 5 knots, or a wave board in 10-12, or upwind.....) and a fair bit slower in other conditions.

A Raceboard's top speed is in the low 30s, a shortboard's top speed with a similar sailor is say in the low 40s.

So in conditions that suit a longboard, it's several times faster than a shortboard. In conditions that suit a shortboard, the longboard is about 25% slower. You could look at that as an indication that the longboard is not slow.

It's like the fact that a slalom board with a 3.5 may be faster in ideal conditions (ie down a speed track in 40 bags of breeze) than a Formula with an 11.5, but we wouldn't call the FW board "slow".....it's just fast in different conditions.

I'd reckon that on average, on the typical bit of water in a typical afternoon in Oz, the longboard would not be "slow".

Select to expand quote

Wet Willy said...

Well, butter my nipples and call me Slippery

...I'm going right out and buying a BIG 60 litre board!!

You'll sink at your first gybe.

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NotWal said...

Wet Willy said...

Well, butter my nipples and call me Slippery

...I'm going right out and buying a BIG 60 litre board!!

You'll sink at your first gybe.

No way! I always fall off before the board has a chance to sink.

hi chris,

yep, your edit in my post was what i was getting at. "ie. beam width increased."

i also agree with your comments qualifying speed.

i don't want to sound like i am saying longboards aren't fun to sail. certainly i've had hours of pleasure on longboards. my post was more about describing the differences between the 2 design concepts that are used.

from my own experiences i can say that in light winds the longboard is king! i've seen formula boards with 12m rigs get left in the dust of a longboard on all points of sail. also the seen mistral prodigy's leave formula boards in their wake.

all depends as you say with conditions. even though i own a board for almost every condition i still keep searching for a very cheap rev 2 one design.

Select to expand quote

nebbian said...

'Tis a good article, I learnt something from it.

However here's a counter-quote for you:
Length does play an important role in that the longer boat will have a higher hull speed, which may allow it to get on plane before that speed is reached so that the pilot of such a boat will be completely unaware that such a “hull speed” limitation exists.

Which seems to indicate to me that a length does play a major part in actually getting planing.

I hadn't heard of the 'weight per square foot of planing area' idea before, but it makes a lot of sense and has a parallel in the aeroplane world, namely 'wing loading'. I think this is probably the biggest factor in getting planing, and does so by one number that takes into account:
* Width
* Rocker (or lack thereof)
* Rider weight
all of which (from personal experience) seem to affect early planing.

All good, we learn something new every day. Or if we don't, we're not trying hard enough!

Hi nebs, i didn;t see this when you posted cause the thread started moving very quickly.

for sure the learning curve never ends. it just varies depending on TOW>.

i've also learnt heaps from the forum and those around me. now, to put it into practice.....

Select to expand quote

Chris 249 said...
Matthew, the definition I used comes from;

C.A. Marchaj; a scientist and author of "Aerohydrodynamics of Sailing" and other works and probably the best known authority on the physics of sailing;

The UK's National Physics Laboratory, in particular Tom Tanner's measurements of the drag and c of g of an International (sailing) Canoe hull;

An America's Cup designer with a Masters from MIT;

Larsson and Eliasson's yacht design book and/or Pierre Gutelle's similar work;

An academic from an Oz university, well known worldwide for his ship and small-craft velocity prediction work and towing tank studies;

I'll take those guys above Wiki any day.

Wikipedia provides a laymens definition - if you really want to explain it using physics, lets start another thread and do that.

Just about everyone of these texts describes big-hull boats, where 'big' means efficient displacement-mode sailing for low speeds; they also optimise hull shape so that the displacement-to-planing transition hump is minimised -> some of the mathematics applies to windsurfing displacement hulls, some to planing hulls. In particular, most windsurfing boards are designed for planing efficiency (at its suggested wind range), while ignoring sub-planing conditions and ignoring the transition hump.

Select to expand quote

I have boats and boards where the bow overhangs the bow wave all the time....sometimes the bow wave is further back (when the board or boat is moving fast). Does that mean the craft has "overtaken" its bow wave?

You are taking a simple description out of context - its usually the center of displacement that needs to overtake the bow wave, but even that is not accurate for super-tankers.

Select to expand quote

The whole "lifted above its bouyancy" thing is that a hull, when stationary, is supported only by buoyancy ie Archimedes.

sure - but buoyancy is a constant relative to volume, mass, water density, etc -> it doesn't magically change when you start moving, aka it isn't lifted above its buoyancy, rather "it has a buoyancy". However once moving, other forces are exerted on the hull as you say...

Select to expand quote

When it starts moving, a hull creates waves along the centre of hull, which gradually reduce in number and increase in size as the speed increases to Fn 1.34. The presence of these waves reduces the immersed volume of the centre section of the hull, causing the boat to sink lower in the water. This "squat" is very noticeable in a heavy boat towed fast. The effect is that once a boat starts moving, it tends to actually sink lower in the water; the centre of gravity sinks.

Center-of-gravity is determined by mass/volume distribution - its cant "sink", as it is unrelated to fluid density, etc. Maybe you are thinking that the relative displacement of water-height along the hull, which will vary so that the center of the hull has a higher draft, than the bow or stern?

A boat not only creates waves at/on the center of the hull, but also fore and aft. The correlation of wave position, relative to hull length, is what causes the center of hull to appear deeper in the water. If the half of the wavelength of the wave, were in sync with the length of the hull (which occurs during the displacement-to-planing transition), the draft would definitely increase. **

** For those reading this far, this is why pumping (on a windsurfer) works so well - not only are you increasing thrust, but you are also changing the wave distribution along the hull's length (and changing other stuff too, which helps overcome the transition hump)

Select to expand quote

A planing hull is, however, lifted (ie the centre of gravity rises once more) because of dynamic lift as it gains speed. The C of G then rises and eventually rises above the level it was when the hull was stopped. From then on, according to this well known definition, the hull is planing.

This drop in C of G is well illustrated by the Tanner tests, for instance, and by practical experience in craft that don't plane.

All craft plane, when enough thrust is applied.

The COG doesn't lift -> COG is the center-of-mass in a uniform gravitational field, say the earth. We could take a board on a spaceship the COM is still the same, ie: the COG (aka COM) doesn't change with density/speed/etc.

Select to expand quote

Other definitions, like "you are riding only on dynamic lift" don't really apply as every craft displaces some water and therefore there is some element of bouyant lift. Planing theory guru Dan Savitsky of the Stevens Institute says this is true even in a flat plate, and Jim Drake say that this is true for a Formula board.

agreed - although I have seen slowboat pushing 47+..... the board wasn't touching the water some of the time!

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However, as renowned skiff designer Julian BEthwaite says, planing is a complicated and messy area and the more you look into it, the more you realise how complicated it all is.

amen brother... thats why for the simple wikipedia quote.

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In longboards, the move onto the plane is normally so subtle that it would be almost impossible to work out just where it starts IMHO.

yep - long boards show their heritage with yachts where it the transition bump is (maybe completely?) minimised.