Planing threshold vs width?

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.

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...

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????

Well, butter my nipples and call me Slippery

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

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

Hi Chris,

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.

Interesting post Gestalt.

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.

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.