I would just like to state that hydrodynamics is different than aerodynamics. There are so many other factors in hydrodynamics that to compare the two is just silly. Unless you are building your own hulls, don't even consern yourselves with it.

Are speed estimates exaggerated? As long as humans have egos and pride. At our beach we have where we offer rides for the public on Hobie Getaways and free speed measurements via GPS. Here are my personal findings of top recorded speeds on a few different boats on close reaches @ downwind without spin.

Hobie Getaway: 18.5mph
Hobie 16: 22mph
Hobie 18mag: 24.6mph
Hobie 20: 26.8mph
Nacra 570: 24.5mph
Nacra 6.0: 27.9mph

I am sure there are production boats w/spin. that could break 30mph downwind. I personally think the speed of the boat has more to do with who's sailing it and less to do with hull design in everyday use.

Hello Jake & Flounder,

The suction effec which drawn down a deep slender hull is yet a well proven effect that has been shown by tank tests on ORMA 60 tri amas and by CFD ( Computations ).

This a phenomenon which is called "DYNAMIC SINKAGE" by naval hydrodynamicists and that corresponds roughly to an additionnal 20% of the initial weight over 18 kt. Now, if a standard Tornado can hold a 340 lbs Side-Force, the trigonometrically associated Down-Force at 12° heel is 340 x tan( 12° ) = 72 lbs, which means 10% of the 716 lbs of initial displacment of the crew...so Jake, you can see that "suction effect", far from negligible, is TWICE the DOWN-FORCE effect...

About Bernouilli's law, it simply states that in uncompressible fluids like water, when you accelerate flow, pressure falls and when you brake it, pressure rises...relatively to the ambient pressure, which is in most cases the atmospheric pressure at 100.000 Pascals = 14 psi. Bernouilli is not dependant of upper side versus lower side, but a general law...

It's the physics of what I called the 1st domain, immediate vicinity of free surface, which don't behave like aerodynamic stuff, you're right Flounder, suctions dissapeared sometimes and things could plane...other will never...

FRIENDLY,
Frenchie

OK - I'm certainly no hydrodynamicist. If I get a chance this year, I'll tow a sail-less cat behind a power boat and measure the squat of the hulls at various speeds. I've never heard of Dynamic Sinkage but would like to know more about it first hand.

Do you believe that a hull shape like a Nacra 6.0 is capable of planing?

We'll get the spinnaker and the GPS out this Wednesday for what is shaping up to be an extremely windy day in middle SC and see what we can sustain for giggles (although SC lake sailing is typically very gusty). That is...if my ears don't freeze and fall off first!

Hi Jake

Great web site ,-great skiff pic also ,--thanks

One note when you do the towing experiment ,

Just tieing a line on the bows or bridle from the power boat transom will produce a pull up force when towed .

The C E center of effort ,--on the sail has to be taken into account .
Not sure absolutly but you may need to calculate the C E and tow from that mast point to simulate sail forces .
Crew placement becomes another question .
On the wire ,-good time to test out that safety line system ,

Hope the mast insurance is paid up --{joking }
Thanks Jake ,looking forward to the test results .
Happy New Year

Carl

Carl,

The point of towing was to negate the effects of the sailplan to bury the bow. The closer the tow force can be to the water line, the better we can see the squat of the hulls relative to speed....dunno. Will be interesting whatever we find!

I didn't want to get into a debate of formulas since many would not understand the depth and also when they produce false results. Applied mathematical models have their limits. According to basic formulas the regular house fly shouldn't be able to fly (ie: flat wings vs low Reynolds#)
John the analogy in your example is completely false. Water being thrown upwards has nothing to do with a hull being sucked deeper because it is in the opposite direction. This is a fluid subjected to pressure potentials finding the pass of least resistance(lower pressure). For example if I jump in a puddle of water, some water will find its way thrusted upward yet at no time was I being sucked down. Think of it as the seesaw effect in a fluid medium.
A good experiment to show this sucking down effect is tying a line to some ball(has to float well) and dragging it horizontally on the water. The results are quite interesting.

Hi Jake
Should be fun ,-hope you take some pics of the 6/0 hulls at high speeds ,--They will eventually plane ,-though it may be more due to the upward pull from the powerboat .

Experimented with planning hull models ,--been a while ,--
no tank test ,-just towing on the lake with weights , just fun stuff to get insight.
You may be familiar with this site -planning hulls and models ----http://naca.larc.nasa.gov/reports/1936/naca-tm-795/

-Catamaran Design is such a fasinating subject ,--it is so multi-complex and integral that often very knowledgable people will explain or reply in laymans terms so it is easily understood , then another will correct them with engineering data and very in depth design theory ,--
it is still fun .

Happy 03
Carl

Like Marc said, there is a limit to which correlations can be considered accurate. There is a
transitional stage between planing and displacement where you have parts of the hull doing one
thing while other parts are doing something else. There is almost no way that you could
adequately predict the behavior of a hull in this range. That's why tank testing and prototypes are
developed.

I can tell you from personal experience that a spi is not going to produce the fastest speeds on a
cat. The sail is too full to be efficient enough. A beam reach however is a different story. With
the much flatter jib and main combo, eff is much higher. Not to mention you have vector addition
of boat speed plus wind speed very much in your favor.

I'm sure if you were onboard one of the W1000 boats on the last two days, you experienced the
Inter 20 "wheelie". I watched the GPS as we accelerated to 31.x mph (27kts)SEVERAL times.
Usually the problem with sustaining that speed became the wall of water that you had to drive
your boat through. Amazing how fast you can come to a stop...


Will R

I agree that a two sail reach should produce the fastest speed. I have just found a new speed site, check it out:
www.sailrocket.fsnet.co.uk

HELLO to all and HAPPY NEW YEAR !!!!

This testimonies about these blasting Tornadoes & Inter 20 that shooted like 26 kt water bullits pulled up by 40 kt breeze are quite impressive... I have seen many european SPEED WEEKS and neither at Weymouth, Brest or Saintes Maries ( French Camargue ), standard catamarans boosted like that...only 20 to 22 kt maximum, even on 200 m tracks...but american sailors have improved skill so much...

Surely, why not some enthusiastic Catsailors wouldn't put up a regular "Baccardi Blast"-type event like a :

"NORTH AMERICAN DRAGSTER CAT CHALLENGE"

over a 100 m track in very windy places just beside an harbour's dike or over low depth pounds...

what could be the most appropriate place in the U.S.A ?


Friendly,
Frenchie

what could be the most appropriate place in the U.S.A ?


Fro a drag race, it the strong North winds of winter, on the South side of the Texas City Dike. About 5 miles long, shallow water near the dike, a great place for speed runs. The road runs right beside it, I've had people in their cars clock me from the road. and if i'ts strong south winds, use the north side of it!

Hi Frenchie -

Great idea ,-many multihull -skiff -board -and other speed sailors would really enjoy this type of event .

-Thanks also for the "challenge " -
{only 20 to 22 kt maximum, even on 200 m tracks...but american sailors have improved skill so much...}

Todd mentioned Texas ,-that sounds great , the event seems to fit the pioneering independant spirit of the state, so many of us enjoy . Tieing in with a sailing event may get it started .

Many take readings from GPS units ,-but these are not always reliable and are usually in MPH not knots .
For some reason they skip ,-we have read speeds up to 50 which you know are not realistic .

Some of the highest speeds hit have been down the backs of large Atlantic swell and good sized waves with the chute up surfing down the backs of them .

-The Proa configuration from the U K speed sailing project looked fantastic , SailRocket -http://www.whbs.demon.co.uk/sr2/content_fr.htm

Angled small rig for lift and high wind speeds ,-huge lever arm for righting ,-and foils , -If it holds together at 50 they have it ,-The crossbeam--hull connection looks suspect ,-that much leverage with that much force at 50 , it won't take much to snap it ,-they may adapt an extra v shaped connect ,-fasinating how the leeward hull flys .

Angled rig ,-foils ,-planning hull shapes , --we already see them applied to cat design ,--check Wouters post from the Paris boat show ,-Mattia designs ,--and the Ventillo and Cirrus designs with angled boards for lift .

really fun stuff -
take care Frenchie
Happy 03

Carl

Hello to Will R.

I read that you experienced "wheelie Inter 20" surging at 27 kt...and that you were diving through "walls of water"...

Could you tell us if you were pulled off by a spi with a very positive bow-up trim angle or if you manage to put yourselves back as far as possible...because calculations show that an INTER 20 hull need at least 5 to 7° of incidence to be able to plane at such high speeds...

Thank you,
Frenchie

Hello again,

I just forgot...on the marvelous website "The Daily Sail", formerly www.madforsailing.com, there's a whole architectural debate about CHERUB skiff's planing hulls...

VERY VERY INTERESTING!!!!!!

Cheers,
F.

How would this type of event be timed?
By eye?
Radar gun?
or some kind of infrared timing beams?

I think that this would be a great idea for local races, but I need to know how to do it..
Dave
N6.0na #325

In addition to the straight speed runs, it might be interesting and fun to set up a "speed circle" and time people around that, as well. I think that was a Garry Hoyt idea.

Hello to Pitchpole Guy...

It's relatively simple, you call the WSSRC ( World Sailing Speed Record Committee ) of ISAF, England...ask for Sir Reginald Bennett and he will explain you the best way.

Probably, the best and cheaper way could be 2 WEBCAMS ( WC )linked to a single PC's screen, located 100 m away from each other on strong pedestals...a simple soft could start the clock as soon as a blast-cat cross the 1st WC field ( joke... ) and shut it down when she reaches the 2nd...

Friendly Yours,
Frenchie

I was on the wire w/chicken line, back foot on rudder and front foot just aft of rear beam. Skipper was on boat. Jib reaching... the boat just did the "wheelie" on it's own. I was staying back trying to keep the bows up. The speeds were burst, not sustained.

We were jib reaching, you wouldn't be able to reach those speeds w/a spi.

Will R

The original post is a little confusing. Frenchie makes reference to Wouters comment about a beach cat "sometimes hitting 30knts". This has hardly anything to do with average speeds in speed trials. Speed trenches are an unrealsitic example of the conditions we see in an open body of water. A boat will clearly have a lower top speed in flat waters but with the help of waves(and the gulf stream), it's a different ball park. Even high speed power boats prefer slightly wavy conditions for top speeds(for different reasons) over flat water.

High 20s (knts) have been reported many times in history, Frenchie simply needs to get out there and experience it for himself.

BTW Frenchie 1/3 > 1/3.5 ;-)

Marc

Hello Marc,

I have experienced speed on an european Formula 40, on Formula 20, may be 22 kt, hard to say, but as Will R. was saying, the faster, the wetter it has been...

I have seen you post hull lines along MHB, it's your design ?

If so, what do you think of the planing shape of ZIP'O and DYNACATS...would it be the ultimate mean to reach much higher average speeds by surfing at 25kt+ like a ski ?

Friendly Yours,
Frenchie