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Re: The ultimate beach cat, how light is possible? [Re: Wouter] #91890
12/14/06 06:05 PM
12/14/06 06:05 PM
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Stein Offline
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Quote

Stein,

I always considered the "semi-planing mode" to be akin to "a little bit pregnant"

In aerodynamics and hydrodynamics lengthening the object by adding a virtual object is possible, the aerospike rocket engine is using this principle very effectively, but it isn't happening with cat hulls. Look up aerospike rocket for more info.

For such a virtual hull to work it needs to excert a raised pressure onto the vertical stern itself. As in real life the sterns are only experiencing ambient airpressure it results that no vitual hull is effectively present. The theory of virtual hulls applied to cat hulls is now nothing more then fitting the errornous formula to "explain away" conflicting real life data.

I'm sorry.


Semi-planing mode seems to be accepted by all other boat designers. And it has nothing to do with pregnancy (I know, I have a medical degree).
The assumed "needs to excert a raised pressure onto the vertical stern itself" is irrelevant. Why should it?

Ask yourself: Why did the Int 14 sailors move the horizontal rudder wing to right below the water surface?

I think comparing the F16 with the F18 is a little bit unfair. The F18 is a very, very heavy boat, it is 70 % heavier than the F16 (180 kg vs 105 kg). The comparison proves that weight reduction IS important.
And that brings us back to the main theme of this thread....

However, I really do think F16 is a great class, Wouter.
Although I personally would have liked more sailors could enjoy the 19 foot, 140 kg Taipan 5.7.

Stein

-- Have You Seen This? --
Re: The ultimate beach cat, how light is possible? [Re: Wouter] #91891
12/14/06 07:39 PM
12/14/06 07:39 PM
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All the application of formula and theory is fine (and necessary) when applying it to a basic hull shape, but as we all know (or should know) sailing craft are always operating in a continually varying compromise situation between the actions of hydrodynamics, aerodynamics, and their interaction at the boundary layer as well as the effects of movable ballast, its relative position, total ballast weight, sail control, and the overall combined weight of the craft and of ballast.
So many variables, such difficulty in calculating and/or predicting mathematically or practically the combined overall effect in all/most sailing situations.
By the application of JUST the appropriate, two dimensional
Hydrodynamic formulae in the hull design without due consideration to a global formulation of all the other interacting effects that are always present when actually sailing, will generally not produce a very efficient craft. One noticeable example of a compromise design trend seen more commonly in recent years is of building more and more cats with greatly pronounced “tumble“ home in their bows combined with longer finer entry’s BUT still carrying more buoyancy low down in the bow area. Theoretically this should actually reduce their resistance to forward motion when the bows are driven deeper into the water rather than increase the “tripping” effect that bows that become wider at the deck display. This “trend” is not solely the application of hydrodynamics for the “theoretical” best hull shape as seen in a test tank or theoretical formulae applied situation, but mostly it is a practical application response to an unwanted effect when actually sailing. You may “back calculate” the effects of the tumble home in these designs to show why this is so, but why were their predictable effects “overlooked” for so long in the theoretical design of so many catamaran hulls?
My point being that to take in isolation just the theoretical “best” hydrodynamic hull form without compromise to all the other variables applicable to practical sailing, will invariably produce a craft that is generally “a dog”, or at the least no where near it’s optimum.

Re: The ultimate beach cat, how light is possible? [Re: jimi] #91892
12/14/06 10:20 PM
12/14/06 10:20 PM
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I lengthened a cat by 9 feet by adding to both ends with foam and very thin plywood. Carbon was used in key areas for strength and stiffeners. The result is that the main platform is strong; the ends are fragile; the length is 26 feet; it has 350 sq. of sailarea; and weighs 375lbs. I feel that it is safe in that the ends could get destroyed and I'd still get home OK. It sails great. So I've concluded that focusing on making a strong platform to stay and then making ultra-light ends delivers a good boat that is pretty light.

Re: The ultimate beach cat, how light is possible? [Re: johnelliott24] #91893
12/14/06 10:56 PM
12/14/06 10:56 PM
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The amount of rocker can also affect hull speed. Generally the greater the rocker, the slower the boat. http://www.stewartriver.com/special.html

PERFORMANCE: As the amount of rocker increases, the cross sectional area increases, but wave drag is reduced. This sounds backwards. Pushing a larger area through the water should result in more drag (paddles work that way, right?). Wrong. Wave drag on a canoe hull is more sensitive to how "sharp" the waterlines are than it is to cross sectional area. Even though the midships cross sectional area of #4 is 42% more than #1, its predicted drag is 16 % LESS. Canoes #1&2 push the water apart quickly, and pay a drag penalty. Rocker takes volume out of the ends of the boat, which tends to sharpen the waterlines. Sharp, fine ends are common in racing designs, but it's interesting that even a slight amount, like #3, results in 11% less drag than #1, and 9% less than #2. This amount of reduced drag would be very noticeable after a few hours of hard paddling. http://www.johnsboatstuff.com/Articles/canoe.htm

A lighter catamaran requires less rocker?

Re: The ultimate beach cat, how light is possible? [Re: grob] #91894
12/15/06 02:14 PM
12/15/06 02:14 PM
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Wouter Offline
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Gareth.

Those are very interesting plots. For now, I accept for arguments sake the Michlet software produces dependable results. This does not mean that I accept it as such at a later time as well. I've found that results produced by engineering software products can easily be wrong, while the the basic components of the software can be proven to be right in themselfs. I won't go into detail.

So the plots. Actually I don't see much that contradicts the model that I presented. I even see the miss Nylex data reflected in the plots; 15% and 22% drag (of total drag) due to wave-making drag and wetted surface drag at about 10 knots.

The local bulb in the blue plot between 2.5 m/s = 5 knots and 3.75 m/s = 7.5 knots does correspond to the Froude's law prediction that around speed = 1.33 * hull_length^2 the wave-making drag increases significantly.

The wetted surface drag look like power relationship with 1.8 as the factor. I expected a factor very close to 2 and 1.8 is not to far away at all. Also I had to measure the coordinates of the screen.


But I do have some issues with the plots are presented.

-1- We are discussing 105 kg overall sailing weight platforms and not 150 kg platforms. I don't really think that a 150 kg 5 mtr (16 foot) platform qualifies as a lightweight catamaran. It certainly does not qualify as a "shorter extreme lightweight hull"

Can we do this excersize again but now for 105 kg platforms ?

-2- The drag from the daggerboards, a large factor compared to the other two, is not included in the overal drag plots. The immediate effect of including this will be that the green and blue lines will lay significantly closer together in a relative sense.

-3- I never wrote that ANY shorter hull would be less draggy then longer hull with the same displacement. In the past I mentioned that there would be a transition point, thus implying that for one given set of hulls going shorter was better while for another set of hulls going longer is better. The only way to discover this relationship is to plot the drag plots for more then 2 examples. At least 5 examples (different) hull length is needed to go a somewhat go feel for the curved nature of this behaviour.

With only two example it is possible that the lowest drag hull lies between the 5 mtr and the 6 mtr hull for example. Without more then 2 data points one can not tell at all whether the relationship is linear or curved with a possible optimal length.

Is it possible that you work out the plots for 105 kg boats having hull length; 6.5 ; 6 ; 5.5 ; 5 and 4.5 ?



-4- You wrote :"As you can clearly see wave drag is far from insignificant, and is in fact larger than WS drag up to around 9 knots"

However I clearly see the wave-making drag to be smaller then wetted surface drag in the speed ranges : 0 to 3 m/s (= 0 to 6 knots) and 4 to 10 m/s (= 8 to 20 knots). Additionally I never claimed to be insignificant. I said it was much less important that wetted surface drag in these regions. This is also clearly visible in the plots. In the high speed range the blue is significantly lower then the red line; so here it is obvious. But even in the low speed range the difference appears to be noticeable. The lines lay close together that is true but proportionally the red line is significantly higher then the blue line and that ratio is important. It is important because a high ratio says that hull drag is mostly made up out of wetted surface drag. Each component may be rather small in an absolute sense, but RELATIVELY the wetted surface drag is noticeably bigger. By about 20 to 100 %


-5- Also can you produce plots where the individual wetted surface drag is plotted for the 5 different lengths. The same for the wave-making drag.


Thank you.

Wouter


Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: The ultimate beach cat, how light is possible? [Re: grob] #91895
12/18/06 05:42 AM
12/18/06 05:42 AM
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Gareth,

The fact that your simulation finds that wave drag levels off above 12 knots means that the boat is in semi-planing mode, doesn't it?

What was the cross-section area you entred into the simulation?

Does the program take different bottom- and transom-shapes into account?

Stein

Re: The ultimate beach cat, how light is possible? [Re: Stein] #91896
12/18/06 06:11 AM
12/18/06 06:11 AM
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Can somebody define "semi-planing" in more detail ?

Wouter


Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: The ultimate beach cat, how light is possible? [Re: Wouter] #91897
12/18/06 06:50 AM
12/18/06 06:50 AM
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That was quick, Wouter!

Even the definition of 'planing' is controversible.
From "a boat travelling faster than its stern waves",
via "lifted by dynamic forces that are larger than static forces (bouyancy)"
to "lifted to the water surface by dynamic forces".

Most planing boats produce some waves, hence even for planing, there may be some wave drag.

My impression being a non-expert trying to interpret what I find in books and on the web, an ultrabrief version is that 'semi-planing' is "static lift being larger than dynamic lift, with separation of water from the stern/transom" or "the transition between displacement mode and planing mode".

See this thread
http://www.boatdesign.net/forum/archive/index.php/t-2347.html
which contains referances.

If you search "semi-planing" on Google, you will find many ads and stories on vessels and military ships using the term semi-planing.
E.g.:
http://www.globalsecurity.org/military/systems/ship/semi-planing.htm

If 'planing' is imprecise, then it is not possible to produce an exact definition of 'semi-planing'.

Stein

Re: The ultimate beach cat, how light is possible? [Re: Stein] #91898
12/18/06 07:28 AM
12/18/06 07:28 AM
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Stein,

There is no dynamic lift element in the drag prediction used by Michlet, so there can be no account of planing, or "semi-planing". I usually post on Boatdesign.net but have stayed well away from that thread!

Wouter,

I intend to answer your questions more fully but do not have the time to do all the runs requested just yet. In the mean time here is the total drag plot for hull1 5m hull2 5.5m and hull3 6m hulls at 260kg (110kg boat + 150kg crew).

[Linked Image]

Gareth
www.foruhulls.com

Re: The ultimate beach cat, how light is possible? [Re: Stein] #91899
12/18/06 07:35 AM
12/18/06 07:35 AM
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"Semi" in my vocabulary equals something akin to "partial"

To me it appears that "semi" in semi-planing is not really intepreted as "partial" but more like "sub" or "pre" (full) planing.

To me is occurs as a catch all definition. I find it personally unsatisfactory and even unnecessarily confusing.

I followed the links and even there the concensus seems to be that planing and semi-planing are not sufficiently defined.

Several definitions even assume that there is a planing mode for a given hull. That assumption in itself seems errornous to me. It is probably the result of people focussing too much to on standard monohulls. As a result I really do find the concept of "semi-planing" useless when looking a beach catamarans.

Wouter


Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: The ultimate beach cat, how light is possible? [Re: grob] #91900
12/18/06 07:59 AM
12/18/06 07:59 AM
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Gareth,

Please don't feel rushed, of course this is not a life or death matter. I can wait a few weeks if that would be better for you, I'm sure the other can so too.

Quote

In the mean time here is the total drag plot for hull1 5m hull2 5.5m and hull3 6m hulls at 260kg (110kg boat + 150kg crew).



I'm assuming that Blue line = 5 mtr boat, green = 5.5 mtr hull and red = 6.0 mtr. hull. is that correct ?



I also see that you've extended the drag plots to 12 m/s = 24 knots. Something interesting is happening at the far right of the graph as you have mentioned yourself in earlier posts. In the far left the same can be seen. Here the hull with less overall wetted surface area sees less drag. For larger regions at these extremes the differences in drag are to small to favour one hull over another. That is if the colour coding is the one as I described above. But having said all this, it is only found in the extremes.

I'm looking forward to the individual wetted surface drag and individual wave-making drag plots. From the plot for the overall drag is almost appears that Michlet use a base drag level for a given displacement and then adds a wave-drag component.

Is it also possible Gareth that you specify the hull shapes that you have used in producing the plots ? Are the scaled versions of eachother or do they share bow wedge angles. I'm thinking along the lines where the I-20 front of the hull is identical in shape to that of the shorter I-18. Stuff like that.

The difference in hull drag (wave+wetted surface but without daggerboard drag) between 5 mtr hull and 6 mtr hull appears to be 14 % at 10 knots. Including the daggerboard drag (based on miss Nylex data) will reduced the hull drag difference at 10 knots to just shy of 10 % which would be roughly a difference of 5% on the overall drag of the whole boat (based on miss Nylex data).

A 5 % drag difference at this spot would roughly translate into a speed difference of 2.5 % neglecting other limiting factors like minimal angle of attack considerations. A measurement based handicap system like Texel predicts a speed difference of 5 % in such a case, as a direct result of a longer hull line length.

This still seems consistant (in the rough) with real life experiences. It is my personal opinion that the long hulled lightweigt boats are unable to consistantly sail to their Texel handicap numbers. Surely their are more causes for this but this hull drag factor may indeed be one of those.

Assuming Michlet produces dependendable results, these plots show that relatively short hulled lightweight boats don't suffer much at all at very low speeds (light winds) and high speed, but do have more to make up for in the medium range speeds. That is indeed my own experience with my own F16 as well, as I've stated quite a few times in the past.

As a side note. I also suggests how adding a spin to the beach cats helped them to close the gap between short and long hulled boats. By increasing the overall speed of all boats, hull length is significantly less important factor in determining the overall drag of the boat. Maybe this equalizing in overal does also help equalize performance between crews of different weights but with the same skill level. All things that many of us have experienced in reality to be true.

Very interesting, Gareth, thanks for your efforts.

Wouter

Last edited by Wouter; 12/18/06 08:19 AM.

Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: The ultimate beach cat, how light is possible? [Re: Wouter] #91901
12/18/06 08:17 AM
12/18/06 08:17 AM
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It seems to me that in order to create dynamic lift, a boat has to actually be in the water. If the boat is not in contact with the water, you won't get any lift from it (obviously). So, even a fully planing boat displaces a bit of water and so has some component of static lift, so planing cannot be an absolute condition, but rather an arbitrary decision about whether enough of the lift is dynamic.

I think it's quite telling that most of the hits for "semi-planing" that Google turns up are boat ads. This suggests a phrase coined by builders not scientists, and I assume is intended to mean a boat that sits somewhere in between planing-as-in-skiff and displacement-as-in-car-ferry. i.e. there is a significant component of dynamic lift, but not so much that you'd call it planing.

As for cats "semi-planing": I've never really believed it. I find it hard to believe that something that shape is going to produce any significant fraction of the 2.5kN (like 255kg) of lift needed to keep my 2-up Blade afloat. Calling cats semi-planing strikes me as an attempt at explaining why cats go as fast as they do by people who don't understand what Froude's Law actually means. But this is just speculation: I only made it to one lecture on fluid dynamics , and it all seemed much too difficult <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

Paul

Re: The ultimate beach cat, how light is possible? [Re: pdwarren] #91902
12/18/06 08:41 AM
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Good points Paul,

Here some additional comments :

Scientifically speaking a boat is suspended at the water surface by nothing more then a pressure difference between the keel and the decks. It doesn't matter what causes this pressure difference to be present, by displacement (hydro-static pressure difference) or by planing (hydro-dynamic pressure difference). This first force is the result of the water pressure increasing when you dive deeper below the water surface and thus the ambient air pressure as present there. Planing is the result of a pressure build up under the keel line as a result of the onrushing water being slowed down or bend away from its initial direction. Example; as the hull moves through the water to particals are hitting the planing surface and are pushed out of the way to the side or downward. This creates a (dynamic) pressure zone under the keel line that is added the pressures created by displacement. This excess of pressure results in the hull being lifted higher out of the water till this system finds a new stable balance between the forces. But as this example makes clear, there is absolutely a significant amount of water displacement present, if only by the water that is being squirted out to the sides as (white foamed) water. So planing is not a concept that reduces wave-making drag, because it really isn't. We all know that wave system created by planing are very significant. The idea behind planing is to reduce the wetted surface area and its related wetted surface drag. In Gareth plots you can see this potential in the plots. his Michlet plots show that for higher speeds the amount of wetted surface area is much more important in determining hull drag then wave-making drag is.


Wouter


Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: The ultimate beach cat, how light is possible? [Re: pdwarren] #91903
12/18/06 08:59 AM
12/18/06 08:59 AM
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I totally agree with Paul.

The Froude's law at 1.0 just simply states when boat is going faster than the wave it generates, it does not state anything about planing or wave drag amount.

I hope that this doesn't spread as much as at one Finnish forum, there was a 345 post discussion about Froude, planing and sport boat definition. At least let's drop Froude and planing relation with beach cats out of this super beach cat conversation <img src="http://www.catsailor.com/forums/images/graemlins/wink.gif" alt="" />

--
Valtteri

Re: The ultimate beach cat, how light is possible? [Re: Wouter] #91904
12/19/06 07:34 AM
12/19/06 07:34 AM
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Quote

Good points Paul,

Here some additional comments :

Scientifically speaking a boat is suspended at the water surface by nothing more then a pressure difference between the keel and the decks. It doesn't matter what causes this pressure difference to be present, by displacement (hydro-static pressure difference) or by planing (hydro-dynamic pressure difference). This first force is the result of the water pressure increasing when you dive deeper below the water surface and thus the ambient air pressure as present there. Planing is the result of a pressure build up under the keel line as a result of the onrushing water being slowed down or bend away from its initial direction. Example; as the hull moves through the water to particals are hitting the planing surface and are pushed out of the way to the side or downward. This creates a (dynamic) pressure zone under the keel line that is added the pressures created by displacement. This excess of pressure results in the hull being lifted higher out of the water till this system finds a new stable balance between the forces. But as this example makes clear, there is absolutely a significant amount of water displacement present, if only by the water that is being squirted out to the sides as (white foamed) water. So planing is not a concept that reduces wave-making drag, because it really isn't. We all know that wave system created by planing are very significant. The idea behind planing is to reduce the wetted surface area and its related wetted surface drag. In Gareth plots you can see this potential in the plots. his Michlet plots show that for higher speeds the amount of wetted surface area is much more important in determining hull drag then wave-making drag is.


Wouter


Wow. And there I was, thinking that my boat actually just floated. Now I`m too damn scared to go sailing for fear of it being suspended somewhere between the air and the ground, with only a mixture of hydrogen, oxygen and algae holding it all together. You guys need to go sailing a bit more, winter over there, eh ?

Re: The ultimate beach cat, how light is possible? [Re: Steve_Kwiksilver] #91905
12/19/06 11:06 AM
12/19/06 11:06 AM
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Hey, some guys actually do enjoy the science behind sailing as well, it adds up as a additional aspect to sailing. And it is a pretty fun way of learning about hydrodynamics compared to some of the prof.s I have( I'm studying marine technology). But yeah, it's winter.. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Re: The ultimate beach cat, how light is possible? [Re: Wouter] #91906
12/19/06 02:43 PM
12/19/06 02:43 PM
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Quote
"Semi" in my vocabulary equals something akin to "partial"


The latin prefix "semi" literally means "half", not partly.
Semi-planing literally means "half-planning", like semi-annual coupon means a coupon every six months (exactly every half year, not sometime in the 1 to 364 day range).

Regardless the definition of planning and displacement, "semi-planing" is a rare event for the simple reason that it is rather difficult to sail at the exact speed that divides in two the speed range between displacement and planing speeds. "Pre-planing" is a more adequate expression, but that includes displacement speeds too.

A correct indication of the regime when a boat is sailing faster then displacement speed (whatever its definition) but slower then planing speed (idem) would be "post-displacement pre-planing regime".

Take care with those names, though: the existence of a precise name for something transmits a false sensation of scientific accuracy. A precise name lends no precision or accuracy to the phenomena itself. It also doesn't assure that it actually happens.

Last edited by Luiz; 12/19/06 03:05 PM.
Re: The ultimate beach cat, how light is possible? [Re: pdwarren] #91907
12/19/06 03:14 PM
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You do find the term 'semi-planing' on designers' websites and in official military sites.

An example of semi-planing boats: Along the long Scandinavian coast people developed a vessel type looking like short merchant viking ships with fairly deep keel and rather fat round bow and stern. Fishermen used 20-30 foot boats doing the hull speed: 6-9 knots with ca 20 hp engines.

When glass-fibre reinforced polyesther enabled production of such boats for recreational use, the possibility of modifying hull shape to increase speed immediately become important. The designers changed the aft 1/3 and introduced a horizontal flat bottom section with sharp edge between bottom and sides. Keeping the canoe-shape or cruiser-type stern. The deep keel was kept to preserve sea-worthiness. With double engine power, speed increased by ca 50 %.

Hence creating lift of the stern to prevent the sucktion down at hull speed + a sharp termination of the wet surface to enable the water slipping from the stern, this boat type surpassed its hull speed. The boat is not lifted out of the water more than very few %, bouyancy is than main contributon to floating, but there is dynamic lift of the stern.
This type of boat may be considered in a transition between displacement mode and planing mode. Or one may consider this a system in which part of the boat is planing.
In other words: 'semi-planing'.

Consider this:
Almost all high-performance cats have a transom stern to allow water to detach from the stern. Why?

High-performance cats' sterns are not sucked down when surpassing hull speed. Why?

Stein

Re: The ultimate beach cat, how light is possible? [Re: Wouter] #91908
12/19/06 03:47 PM
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Wouter,

You argue for advantages of shorter hulls:
Wetted-surface decreases, hence surface drag decreases.
However, to preserve volume (bouyancy), you need to increase cross-sectional area. This leads to increase in form drag (pressure-drag). Even if your length/width ratio still is low enough to keep wave drag low, there is still increasing form drag.

Hence, reducing length is not automatically a optimal solution.

The consequence of your arguments for shorter length cats, Wouter, would be to cut off a feet or so from the stern of your excellent Taipan 4.9. Or switch to a 12-14 foot class.

But this thread is not about boat length. Let us return to the original question!

What is possible today?
Are the Super Taipan or the M20 the fastest cats around?

Stein

Last edited by Stein; 12/19/06 04:04 PM.
Re: The ultimate beach cat, how light is possible? [Re: Wouter] #91909
12/19/06 07:16 PM
12/19/06 07:16 PM
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Posts: 1,012
South Australia
Darryl_Barrett Offline
old hand
Darryl_Barrett  Offline
old hand

Joined: Feb 2004
Posts: 1,012
South Australia
“Planing” lift occurs when an upward force is generated on an object due to it’s velocity through/on the water and that velocity lift becomes additional to the objects displacement buoyancy, with those two combined influences acting on the object it will move with less of its volume in the water therefore less drag, more potential speed, IE a stone skipped across the water at speed will truly “plane” as it lifts completely out of the water after every early contact with the water, until that speed reduces to the point that for the stone to stay above the water surface it has to displace more weight of water than it’s own weight otherwise it will sink. Semi planing is when an object is under the combined influence of both dynamic velocity lift and buoyancy lift (so in effect all boats, whether considered on full plane or only partial on plane are in reality "semi" planing? if they were truly fully planing with no buoyancy/displacement factor involved wouldn't they be airborne?).
Given enough velocity most objects will “plane”.
Take a snow mobile, at speed it can and does travel across the water surface, when it's speed reduces to a certain point it will sink due to it not being able to displace more weight of water than it's own weight. Displacement (or buoyancy) is always a factor for all “floating” vessels, even the fastest at plane, if it wasn’t then it wouldn’t be in/on the water, it would be flying and therefore airborne instead. My contention is that there is no such thing as a truly "planing" boat and that instead there are only boats that semi plane to one degree or another.

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