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fastyacht

Super Anarchist
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I'll concede to being very pedantic here about the technical aspects of planing and I am very much posting about the technical and not challenging your experience. What feels like planing to me has been corrected with a more proper definition hammered into me by two different naval architect engineers I sailed with for over a decade.

When a boat pushes water to the sides and a hole is appearing directly behind the stern with a hump of water behind the hole due to water rushing in to fill that hole, it's surfing and not planing per the physics definition of displacement. The hole and hump show that water was displaced and refilled as the vessel moved through the water. Planing is not only passing the bow wave but also riding up on top of the water. Pushing water to the sides while being faster than the bow wave is partial displacement mode, aka surfing. Very few boats plane. This is why the D/L or more appropriately, the displacement to flat'ish hull surface area is a better way to identify the probability of a planing hull.

Complicated summary is that leaving hull speed displacement mode is not a cliff, more of an exponential model on an X/Y chart. As a boat moves faster it builds hydrodynamic lift and a portion of the hull begins leaving the water further reducing drag and allowing faster speeds. The hole behind the boat shows that the boat is separating from displacement and the stern is beginning to rise up to the static water level, the bow beginning to do so first.

Now consider a small multi-hull that moves at speeds well beyond what is considered displacement mode (Reynolds number calculated based on hull length) before it rides over its bow wave or shows the hole/hump in its wake. How is a multi-hull effortlessly moving into what is called planing mode while displaying none of the common nomenclature?

This is better left to someone with more information so I looked around a little and could not find a decent white paper but did find this new'ish article that in summary declares that planing should never be used as a technical term because it implies a specific behavior when the action of a boat moving through water is much more dynamic and should be considered in phases and the word planing used to describe an experience, not what the boat does. I think I like it much better than what the NA's were banging on about years ago.


There is a long conversation about this in the Australian Skiff thread about what makes a boat a skiff that addresses this topic as well and includes Mr. Bethwaite who has some elegant explanations.
I haven't read your reference yet but yes, the word "planing" in sailing circles is not the same thing as "planing" in naval architecture. Also, the sensation and observation of planing as a distinct stage in the drag curve is more pronounced and important in higher displacement length boats than slender ones. This is part of the confusion people have with the catamaran definitions. By the way, some catamarans develop dynamic lift quite a lot--the F18 has turned that direction as of late. Smyth/Glaser were doing that in the Worrell 1000 many decades ago.

One thing most people don't realize is that a boat that is going faster than "hull speed" but under 2X or so of that spee (depends on some details) is actually "sucking" NOT planing, in other words the center of gravity is appreciably lower than it was at rest. "Fell into the hole it dug." Not until you are really going, do you get back to even and then lifted. With chine hulls and disp/length of typical powerboats (D/(L/100)^3)=250) this is also somewhere near the "chines dry" speed.

I want to point out that you mentioned Reynolds number where what you meant to say was Froude number :-D
 

fastyacht

Super Anarchist
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The next thing I want to point out--about "asym boats" especially J-boats and that ilk of leadmines, is that they are faster "sailing the angles" because, well, they have no choice. Their spinnaker sucks deeper than 135 true. OK slight exxageration but not by much. Note what happened in the J70 class when someone cut a sail that could stabilize better--you saw wing and wing. And deeper angles in the displacement trapped speeds. Well duh!

Even for planing boats such as 505, there are three disntinct regions of downwind: superlight air (usually races are called off sadly in this range) where hot angles are faster; low-moderate where deep sailing is fastest--because the boat is "trapped" in that "hump" range of the speed/pwer curve so "heating up" loses more than it gains, followed by "wire running" where you sail pretty much the same angles down as you do up the course. IT will give you a FRIGHT when you are in the back of the fleet and sailing upwind right into traffic!

And finally, smaller boats are faster than larger boats, on a Froude number basis. Any decent sailing dinghy at its Stability Limit going upwind will be noticeably above "hull speed." Boats such as the classic canoes--WAKE, the one design IC, are rather faster again than trapeze boats such as FD/505 boat for boat, and the New Rules IC are freaking rockets upwind. "Planing" speed as in approaching 2X hull speed--> Fn = 0.8; S/L = 2.7.
All hiking dinghies sail above "hull speed" this includes club 420, Lark, FJ, V15, JY15, all the old classes such as Cottontail, Comet, Enterprise, etc. By the time you get a little bigger, this is no longer true. Pretty much any keelboat--even for instance J24, J22, is notably slower in terms of Froude number than the dinghies.

If you want to know why this is, just play with similitude and note the power to carry sail versus the displacement.
 

Blaise

Member
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Hamilton, ON
One thing most people don't realize is that a boat that is going faster than "hull speed" but under 2X or so of that spee (depends on some details) is actually "sucking" NOT planing, in other words the center of gravity is appreciably lower than it was at rest. "Fell into the hole it dug." Not until you are really going, do you get back to even and then lifted. With chine hulls and disp/length of typical powerboats (D/(L/100)^3)=250) this is also somewhere near the "chines dry" speed.
I am having a hard time processing this. Can you explain the physics that would lead to a lower center of gravity that in the static case?
 

fastyacht

Super Anarchist
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I am having a hard time processing this. Can you explain the physics that would lead to a lower center of gravity that in the static case?
Yeah, it's simple. "digging a hole". Also "suction". It is easy to find the data in the internet era. One of the things you measure on the carriage in the tow tank. As that stern wave falls away behind the transom, the boat is falling into the trough.
 

Autonomous

Turgid Member
4,631
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Hmmm. The boat sits lower in the water but it's CG changes? Naw. More likely the CB changes? GM?
It's been shown a boat on top of a wave can be less stable so does it follow a boat in a hole is more stable?
Is this because the ends are immersed further?
Enquiring minds Et al.
 

Gouvernail

Lottsa people don’t know I’m famous
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Austin Texas
An RS600 in NJ.

Wow!!! Seems like a great choice to buy and sail
In the RS 600 midwinters.
Or maybe one of the RS 600 fleets ought to buy it so it can be sold to an interested newbie.
 

Foredeck Shuffle

More of a Stoic Cynic, Anarchy Sounds Exhausting
An RS600 in NJ.
Huh, 12.14sqm of sail, SA/D of 45.3 with an 87kg driver. That's interesting. Build a slightly bigger sail with a higher angle off the head, a bit more roach, and could maybe get it up to 13.2sqm, SA/D over 49. That would be fun. I wonder how many misfits I can collect before she finds better uses for my body?

Ignore Guv, he used to be fun.

I could potentially drink a beer while racing this and keep sailing when the 29er is too much to singlehand. I'm going to reach out. Bigger sail and that boat has long term potential.
 
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Huh, 12.14sqm of sail, SA/D of 45.3 with an 87kg driver. That's interesting. Build a slightly bigger sail with a higher angle off the head, a bit more roach, and could maybe get it up to 13.2sqm, SA/D over 49. That would be fun. I wonder how many misfits I can collect before she finds better uses for my body?

Ignore Guv, he used to be fun.

I could potentially drink a beer while racing this and keep sailing when the 29er is too much to singlehand. I'm going to reach out. Bigger sail and that boat has long term potential.
finn?
 

Foredeck Shuffle

More of a Stoic Cynic, Anarchy Sounds Exhausting
Sorta. Thinking a boat that needs a trap to carry the sail area and less boat to drag around underneath it all. I live on a beautiful piece of water that for some reason is a bit of a wind hole. It will blow here but not for typical reasons like thermals, sea breezes, etc, that can help make a sailing area more consistent. Sub 6kts is the norm here.

Finn with 87kg sailor:
SA 10.2sqm | SA/D 31.6

Laser (with squarehead sail built for standard rig) with 87kg sailor, I have several things like this now, this is the largest:
SA 8.6sqm | SA/D 31.0

RS600 with 87kg sailor:
SA 12.6sqm | SA/D 45.3

I could buy a VX/One and stop messing about but that means having to schedule with someone to practice and after doing that for almost 30 years I'm enjoying sailing solo. That and I've never sailed/raced skiffs until now and it has turned out to be thrilling in a manner I've never experienced on a sailboat before. Kid just started sailing at 5 and he liked it, so hopefully he'll let me sail with him someday even though he has other choices. I'll keep some carbon something around to tempt him.

Heading up to bring the RS600 home tomorrow. Need a mast butt/base...
 
Sorta. Thinking a boat that needs a trap to carry the sail area and less boat to drag around underneath it all. I live on a beautiful piece of water that for some reason is a bit of a wind hole. It will blow here but not for typical reasons like thermals, sea breezes, etc, that can help make a sailing area more consistent. Sub 6kts is the norm here.

Finn with 87kg sailor:
SA 10.2sqm | SA/D 31.6

Laser (with squarehead sail built for standard rig) with 87kg sailor, I have several things like this now, this is the largest:
SA 8.6sqm | SA/D 31.0

RS600 with 87kg sailor:
SA 12.6sqm | SA/D 45.3

I could buy a VX/One and stop messing about but that means having to schedule with someone to practice and after doing that for almost 30 years I'm enjoying sailing solo. That and I've never sailed/raced skiffs until now and it has turned out to be thrilling in a manner I've never experienced on a sailboat before. Kid just started sailing at 5 and he liked it, so hopefully he'll let me sail with him someday even though he has other choices. I'll keep some carbon something around to tempt him.

Heading up to bring the RS600 home tomorrow. Need a mast butt/base...
Musto? Contender?
 

Foredeck Shuffle

More of a Stoic Cynic, Anarchy Sounds Exhausting
Musto? Contender?
A boat designer, builder, and at the time reseller of Musto's sat at my dinner table a few years ago and said that 87kg was too much weight in a low wind sailing venue, the boat would be sticky and I'd be unhappy.

Contender is a cool boat, but it does not follow the philosophy of less boat. Giving RS600 a go, still sorting myself out on the 29er and dreaming that my kid may someday join me on that with me on the trap. And hopefully he kicks me off someday to sail with friends.
 
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