Why don't I just send you my ETrex and you can dowload it all and send it back along with an eamil of my track... <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

I'm pretty sure a little bit of poop comes out the first time you are placed in that scenario. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

I was out on this FULLY CARBON 56 foot beast the other day and cruising at 16/17 knots through the islands. Maybe a hundred meters off one I sail a lot.
It was smooth and just felt like rolling along at a good clip.
I went to the side of the boat and just tried to gauge the speed if I was on one of my beach cats. I was happy that I often hit that speed but that it feels WAY faster low down and wet. However the island slips by at the same speed.
I had also sailed an older Alan Write 42 at 16 knots but that felt stressed and fully extended in 40 knots of gusts. Isis was in 12/16 knots that day and was almost always traveling faster than the wind.
One can only imagine what 40 knots feels like in those crazy cats and tris.
Unstoppable, scary and wonderful come to mind.

I love the mast set up on Isis.
It is a carbon, rotating wing section.
No spreaders.
The lowers are set lose but as the mast rotates the windward mast connection moves further way from the chain pate and automatically tensions the lower mast. The connection at mast also rotates to lower stress at that point.

if you've got the old monochrome legend you need the "special" garmin cable to download through a serial port:o(

the newer garmins use a standard mini-usb cable as found on many digital cameras, mp3 players, card readers etc.

but as long as you have some kind of cable for it all you need is free software like easygps

http://www.easygps.com/download.asp

to download it's data on to your computer and then you can send it to anyone as an email attachment if you want

pm me if interested and i'll see what i can do;o)

to download the data from it

Thanks, I installed it on my laptop then afterwards found I don't have a port to connect the gps! I will try it on my desk top later...

this is a video sailing 19kts on a nacra 5.8

don't remember wind speed, but the ocean was very flat that day

bernd

http://www.youtube.com/watch?v=B0U5T9W_pKI

here are the stats from that day

sorry here it is

bernd

good deal on a gps
http://tinyurl.com/6offwk

19.8 knots over 1/2 mile during Miami Key Largo
very gratifying <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Was that on the Tornado Classic?
I just came in from another run with my gps, got it set for knots now, with not as much wind as yesterday, got it up to 17.2 kts. under spinnaker on one good run. Need more wind! <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

The definition of 'planing' is controversible. Either:
1. A boat travelling faster than its stern waves,
2. Lifted by dynamic forces that are larger than static forces (bouyancy),
3. Lifted to the water surface by dynamic forces.

A displacement boat with a vertical sharply cut stern may allow the water to detach from the stern and "fool" the stern wave into assuming a much longer waterline length. This is called "forced mode" and allows the high speeds with huge engine power in naval ships.

With a catamaran sailboat the heeling forces at higher wind strengths submerge the leeward hull, hence there will be wave drag. A planing hull may produce enough dynamic lift to counteract heeling. The net effect may be less wetted surface and lower wave drag. I.e. lower resistance, faster boat.

To my knowledge, planing hulls have not been competitive in lower wind speeds, and with high resistance during transition to planing, they seem uncompetitive in round-the-buoys races. However, several modern monohull dinghys have a chine (e.g. 49er).

Tornados beat planing 18-foot skiffs around the buoys, but the 18-foot skiffs seem to exhibit the highest absolute absolute top speeds.

Hovercraft catamarans, sidewall hovercraft, SES, use an pressurized cushion to lift the boat. This reduces the wetted surface of the hulls substantially.

Ecranolets reduce resistance by flying near the ground, thereby reducing the wingtip vortices that produce drag (and reduce lift).

Our catamarans should be very fast with small engines, hence very efficient compared to most other vessels. But for really high top speed, we probably need planing hulls.

Go for it Warbird!

Stein

1 mile/hr = 1608 meters/hr
1 knot = 1852 meters/hr

Stein

At what point does waterline come into play. Is the terminal speed higher for an 18' cat higher than a 16' cat considering the same amount of thrust and drag?

Stein,

Your comments are correct to the extent of what I've learned over time but I would like to make one comment :




This is a typical example of a statement that is so tortured because the original source wants to cling onto an orginal concept that is so obviously wrong (or misunderstood). In this case that the max hull speed myth.


There is no "fooling" or "assuming of longer waterline length". You can't "fool" a law of physics and neither does such a law "assume" anything. It is not an object with a information processing unit like a brain that can be "fooled".

The foundation of all this torture logic is the misunderstanding that Froude's law says anything about a hull. And indeed, I've have encountered many well educated persons who make this mistake as well while the way out of this situation is sooo obvious.



And now too all :

Again, Froude's law doesn't say anything about a given hull, it ONLY says something about the speed a given wave length travells across the watersurface for a given set of temperature, pressure and water density. That is all.

So why is the Froude formula referred to so often ? Because the pressure distribution around a hull travelling through the water is dependent on the wave system around it and therefor dependent on the wavelength of a wave system that is travelling along the hull at the SAME speed as the hull is travelling.

Obviously, a wave system with multiple crests along the hull will result in a rather symmetric pressure distribution. Such a situation leads to the lost energy (drag) incurred on the front part of the hull to be won back (drive) at the rear part. OFTEN, this leads to lower hull drag. When the hull starts to move faster then there will be fewer and fewer crests along the hull till the last crest moves behind the stern, resulting in a much more a-symmetric pressure situation along the hull where basically there are only pressures working against the movement of the hull (drag) left and there are no more positive pressured (drive) that recouperate some of the loses.

For very heavy boats like frighters and ballasted keel yachts the hull drag (excluding wetted surface drag) makes up a very large portion of the total drag, so when that component increases rapidly then it will quickly use up all available power (drive) from an engine or sails and thus stop accellerating. It will then have reached max. speed. Again for heavy craft this point happens to coincide often with a distanced travelled per second (=boatspeed) that is a fraction longer then the wavelength of the wave travelling alongside the hull (as caused by the bows and sterns). This leads to the misapplying of Froude's law as a formula to calculate max. (hull) speed for any given hull length. Of course for some situations (heavy boats - low power) this works rather well but for many others it is simply wrong. As in the past we only had "heavy-boats with lower power", people began to believe the errornous intepretation of Froude's law.

So what happens on a design like a beach cat ? Exactly the same as descibed above, with only one very important exception. The overal fraction of the hull drag in the total drag of a beach cat is MUCH smaller. For wave-inducing drag it is something like 10-15% of the total where it could be over 50% for a heavy yacht. Now, you can easily double 15% and have the beach cat hardly notice much difference while you can't double 50% or more and expect the yacht to keep travelling at that speed. It will need (alot) more drive then the sails can produce. On a beach cat the increase in boatspeed leads to INCREASES in saildrive that are LARGER then the increases in hull drag when acquiring higher hull speeds. I know this sounds weird but that is exactly what is happening. On a heavy craft this situation is reversed. Here an increase in hull speed increases hull drag more then it increases saildrive. Therefor when surfing off a wave it will pick up speed beyond Froude's inpired max hull speed but then slow down later. A cat however will keep accellerating even without the help of a wave till a point were the aerodynamic limits of the rig stop any further accelleration (increases in sail drive have topped out). And this is why so many forced displacement catamarans and planing dighnies are so close in overall speed. Their real obstacle is aerodynamic performance of their rigs, not so much the drag of the hulls. Just an example, on a beach cat aerodynamically related drag is 25-30% if the total and as good as twice as large as the wave-making drag component of the hull. (Miss Nylex C-class data that using a highly drag efficient wingsail).


So summerizing :

Froude's law equates wave-travelling speed to wave-length for a typical wave on earth on the surface of a body of water with density around 100kg/m3. Nothing more, nothing less. It is certainly not explicetly saying anything about a hull. The fact that Froude discovered this law by looking at ships doesn't change this fact.

Froude's REAL law is : Wave-speed = 1.34 * square root of wave-length

The coefficient 1.54 that is used often is nothing more then designers trick when designing a "Heavy Weight - Low Power" Boat. Remember the earlier comment on how a hull of such a design would typically travell a tad bit faster then its surrounding wave system. The difference between 1.34 and 1.54 is that tad bit. Basically these designers made Froude's law fit their experience with such boats instead of the other way around where the experiences fit the formula. Some people like to use a coefficient of 5 for beach cats but that is just more of the same tweaking of the formula to fit a particular set real life experience. And for each new design you have to redo this, meaning the formula is useless in such roles as it will only reflect what you already know and not predict what you don't already know. And why use any formula when you already know what the result is going to be ?

For only a portion of hull design (heavy craft - low power) this Froude formula coincides closely to the maximally attained speed in real life because in these situations the accompanying wave system (that DOES follow Froude's law in all instances) is so important in the total amount of drag.

In a very large portion of other designs Froude's law still applies to the accompanying wave-system but as its related drag describes only such a small part of the total system that it is of very limited practical use. Beach cats fall into that category.

Wouter

Yes. pintop main, a jib made in the early 80's, 1 trap 8:1 mainsheet(too much work)

WHAT IF...you were to build a catamaran out of two windsurfer type boards-hulls, and some how put a regular cat wingmast and sail (or even Ben Hall's wing) on it, so you would definately be on a plane when the wind was right? How fast could that go?? <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Faster then 45 knots - just split the leeward board in two, increase the distance between the bow and stern parts and add tiny foils here and there.

Its name is "Macquarie Innovation", formerly "Yellow Pages Endeavour".

I love the "pod" the driver sits in!

However, I was thinking more along the lines of a Hobie 16 tramp type platform where you pull the Hobie hulls off, and replace them with windsurfer type "hulls" that would act like giant water skis, then wait for the right wind, power it up and away you go!

Wouter,

My post did not at all mention max hull speed. Please, read the posts carefully before criticizing!

I assume that almost all readers of this forum understand quote marks in the sentence:
“….”fool” the stern wave into assuming a much longer waterline length”.

Froude number: Froude developed his formula from boat-model experiments. The Froude number says something about the wave generated by a hull moving in water.

There are several versions of the formula (dimensionless, densimetric, etc), so stating that “Froude’s REAL law is …” is a simplification at best.
For a ship, the Froude number is defined as: Fr = V/square root of(g*L)
where V is the velocity of the ship, g is the acceleration due to gravity, and L is the length of the ship.
See JIMI’s previous post.

The Froude number relates to the fact that wave velocity is related to wave length.

A displacement boat generates waves. A displacement boat travelling faster than the velocity of the waves it generates, is subjected to large resistance against its movement (wave drag). Hence, a large amount of power is needed to increase the speed further.

With very slender hulls, wave drag is a relatively small component of total drag.

There are ways of altering the stern of a boat to move the stern wave to behind the boat (“fool” the stern wave):
- the vertical sharply cut stern which allow the water to detach from the stern,
- the horizontal wings of the International 14s just beneath the water surface,
- the appendix of the New Zealand’s boat for America’s cup 2003 (the Hula).

Almost all small cats have vertical sharply cut sterns that allow the water to detach from the stern.

Stein