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Re: How fast can a beachcat go? [Re: Timbo] #140605
04/26/08 05:48 PM
04/26/08 05:48 PM
Joined: Apr 2002
Posts: 805
Gainesville, FL 32607 USA
dacarls Offline
old hand
dacarls  Offline
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Gainesville, FL 32607 USA
Peter Ifju (Prof., Aeronautical Engineering Dept, U. Florida)
sails very wide light windsurfers of his own design. It planes at 6 knots: add 2 under a Hobie tramp 14 structure, add proper Carbon mast, modern sail with foam battens and what else?


Dacarls:
A-class USA 196, USA 21, H18, H16
"Nothing that's any good works by itself. You got to make the damn thing work"- Thomas Edison
-- Have You Seen This? --
Re: How fast can a beachcat go? [Re: Timbo] #140606
04/26/08 06:36 PM
04/26/08 06:36 PM
Joined: Aug 2001
Posts: 1,307
Asuncion, Paraguay
Luiz Offline
veteran
Luiz  Offline
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Joined: Aug 2001
Posts: 1,307
Asuncion, Paraguay
Quote
...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"...


Use a light board to replace the windward ama and two Formula boards to replace the leeward ama. Leave a big gap between the two leeward boards. It will be faster than the Hobie in straight line sprints over flat water, but certainly not worth the trouble.


Luiz
Re: How fast can a beachcat go? [Re: dacarls] #140607
04/26/08 09:23 PM
04/26/08 09:23 PM
Joined: Jan 2005
Posts: 6,049
Sebring, Florida.
Timbo Offline
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Timbo  Offline
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Sebring, Florida.
And you could trap off the side while wearing water skis! <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />


Blade F16
#777
Re: How fast can a beachcat go? [Re: Stein] #140608
04/27/08 05:07 AM
04/27/08 05:07 AM
Joined: Jun 2001
Posts: 9,582
North-West Europe
Wouter Offline
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Wouter  Offline
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Posts: 9,582
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Quote

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



I wasn't critizing you, it was more a post to the general public that I had place somewhere in the thread. This turned out to be you post as you gave a typical tortured reasoning that is often expressed by persons who keep clinging on to Froude's law as a predictor for maximum speed.

Some reply to your new post.

Froude's NUMBER is not the same as Froude's LAW or FORMULA. I did not discuss or refer to Froude's NUMBER which I personally feel is a pretty uninteresting quotient anyway. Someone described it ones to me as the water surface based Mach number which is indeed it most similar cousin.


Quote

There are several versions of the formula (dimensionless, densimetric, etc), so stating that “Froude’s REAL law is …” is a simplification at best.



No it is isn't. Changing the symbols of othering in a formula doesn't change the formula it is still one and the same. Meaning

V = 1.34 * sqrt(L)

is exactly the same formula as.

sqrt(L) = V/1.34

As both describe exactly the same behaviour of a wave.

CHANGING the formula to V = 1.54*sqrt(L) is intentionally altering the formula to make an error and as such is a totally different formula and can not be called Froude's formula. This fast and loose actions are what caused the eternal "maximum hull speed" myth; which is pretty unscientific.

Compare it to this. NO-ONE changes newtons most famous law F=M*A to have it cover different freefalling objects like lead and feathers in the atmosphere better either. We are expected to improved our model and include other effects like aerodynamic forces to be able to use THE ONE AND ONLY ORIGINAL FORMULA.

Only ship designers are so scientifically challenged that they have to change Froude's formula in order to avoid the need to let go of a mental model that is far too simple. I'm sorry but that is just bad science.



Quote

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.



Now you make the same mistake like many other people again. The statement :

A displacement boat travelling faster than the velocity of the waves it generates, is subjected to large resistance against its movement

IS SIMPLY NOT TRUE for a large portion of hull designs. ONLY FOR RELATIVELY HEAVY ship designs. For example a beach catamaran does indeed see a (non-linear) increase in its wave drag component when passing the wave-velocity but that still does NOT mean that the related increase in drag is a LARGE component in the total amount of incurred drag. In fact it isn't. I glarified it as such in my earlier post so you should have spotted that. And indeed you yourself argue again this very statement at the end of your posting.


My comment to ALL readers here is to stop using Froude's law (NOT Froude's number) in ways that are inconsistant with what it described and tell any professors who say otherwise to stop mystifying the situation.

Froude's law (NOT Froude number) does not say anything about any hull design. It only relates the wavelength of a wave on the watersurface above deep water on earth to its travelling speed. In some cases this travelling speed happens to coincide closely with the maximum speed a certain limited group of boats are experienced to achieve in real life.

An analogue example : The fact that some sports car can sustain an accelleration of 1 G in real life does not mean that Newtons law F=M*A = M*G describes the achieved accelleration of all automobiles cars ! Not even when we include some (fictional) Newton Numbers to "Correct" the formula for different automobile designs like. F = Newton number * M*G

So why are ship builders doing to Froude's law ?

Wouter

Last edited by Wouter; 04/27/08 05:39 AM.
Re: How fast can a beachcat go? [Re: Wouter] #140609
04/27/08 09:27 AM
04/27/08 09:27 AM
Joined: Jul 2002
Posts: 539
taipanfc Offline
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Really waiting for that book of yours Wouter to dispel the myths and set us all right.

Re: How fast can a beachcat go? [Re: taipanfc] #140610
04/27/08 10:39 AM
04/27/08 10:39 AM
Joined: Jun 2001
Posts: 64
Sandy, UT
SteveBlevins Offline
journeyman
SteveBlevins  Offline
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Joined: Jun 2001
Posts: 64
Sandy, UT
Wouter, I appreciate your restrained responses on this subject.

Your explanation really strikes a chord from my landsailing and drag from the rig being the primarily limiter of speed. In beachcats I was sure it was hull drag, although I knew it wasn't Froude's thing. Also experiments of others have shown that while increasing max speed, planing hulls aren't the answer for general sailing beach cats. At least until highly sophisticated, very robust cheap systems become available.

So, for just an approximate understanding of what's happening on more recent beach cat hull designs when the crew sense 'planing' are the hulls just experiencing 'forced mode' in different (from, say, a Tornado, Tiger or P19) way?

Re: How fast can a beachcat go? [Re: taipanfc] #140611
04/27/08 01:18 PM
04/27/08 01:18 PM
Joined: Jun 2001
Posts: 9,582
North-West Europe
Wouter Offline
Carpal Tunnel
Wouter  Offline
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Joined: Jun 2001
Posts: 9,582
North-West Europe

Quote

Really waiting for that book of yours Wouter to dispel the myths and set us all right.



That book is not coming. To choice is up to you lot to believe my comments or not. Either you use them to your advantage or you prefer to believe that you can sail to the rim and fall off.

Either way, I not losing any sleep over it.

Wouter


Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: How fast can a beachcat go? [Re: Wouter] #140612
04/27/08 01:22 PM
04/27/08 01:22 PM
Joined: Sep 2007
Posts: 531
Lake Murray SC
FasterDamnit Offline
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FasterDamnit  Offline
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Lake Murray SC
Quote

Quote

Really waiting for that book of yours Wouter to dispel the myths and set us all right.



That book is not coming. To choice is up to you lot to believe my comments or not. Either you use them to your advantage or you prefer to believe that you can sail to the rim and fall off.

Either way, I not losing any sleep over it.

Wouter


There be dragons!

Ever considering posting what you know w/out the attitude?
Good info, tho...

Re: How fast can a beachcat go? [Re: SteveBlevins] #140613
04/27/08 01:28 PM
04/27/08 01:28 PM
Joined: Jun 2001
Posts: 9,582
North-West Europe
Wouter Offline
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Wouter  Offline
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Quote

Your explanation really strikes a chord from my landsailing and drag from the rig being the primarily limiter of speed. In beachcats I was sure it was hull drag, although I knew it wasn't Froude's thing.



max speed is always the result of the play between ALL drag components and ALL drive components, this does always included hull drag. The question that is valueable from engineering viewpoint (improving speed) is to what extent each component is important. It makes no sense in spending resources on lowering a drag factor that only comes 6th in line with respect to importance. If oen does then it will most likely means spending lots of resources on very marginal performance increases. At this time the whole rig (and others) are above wave-making drag on the listing. Actually wetted surface drag (surface area hull) is higher in the listing as well. And that is why planing works on some sailboat design. Planing reduced wetted surface drag while accepting increases in wave-making drag. This is a very important principle to understand by all who want to make sense of this whole thread.

My point to you is that I was argueing that we should not totally ignore wave-making drag but give it the correct placing on the listing of importance (magnitude) and Froude's law intepreted as max hull speed law doesn't do that.


Quote

So, for just an approximate understanding of what's happening on more recent beach cat hull designs when the crew sense 'planing' are the hulls just experiencing 'forced mode' in different (from, say, a Tornado, Tiger or P19) way?


That is my understanding for 99% of catamaran designs.

Wouter


Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: How fast can a beachcat go? [Re: FasterDamnit] #140614
04/27/08 01:32 PM
04/27/08 01:32 PM
Joined: Jun 2001
Posts: 9,582
North-West Europe
Wouter Offline
Carpal Tunnel
Wouter  Offline
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Posts: 9,582
North-West Europe
Quote

Ever considering posting what you know w/out the attitude?



Ahh, this is an interesting philosofical question.

Can it be because of this attitude that I ever undertook the effort to learn these things ?

Would I still have known them if I ever had been a nice, charming, happy to follow others kind of guy ?

I see, ignore my charming self and make maximum use of the information provided. Nobody has to like me in order to max use of what I provide. I guess I've accepted being my own biggest demon ! <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Wouter

Last edited by Wouter; 04/27/08 01:32 PM.

Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: How fast can a beachcat go? [Re: Wouter] #140615
04/27/08 01:32 PM
04/27/08 01:32 PM
Joined: Jan 2005
Posts: 6,049
Sebring, Florida.
Timbo Offline
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Timbo  Offline
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Sebring, Florida.
You know, what we should be talking about here is how fast can you make your catamaran go...(the greater You, as in all of us, not just the Wouter in you.) <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

I'm going out back now and see how fast I can make mine go. See ya! <img src="http://www.catsailor.com/forums/images/graemlins/blush.gif" alt="" />


Blade F16
#777
Re: How fast can a beachcat go? [Re: Wouter] #140616
04/27/08 01:42 PM
04/27/08 01:42 PM
Joined: Sep 2007
Posts: 531
Lake Murray SC
FasterDamnit Offline
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FasterDamnit  Offline
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Posts: 531
Lake Murray SC
Quote
Quote

Ever considering posting what you know w/out the attitude?



Ahh, this is an interesting philosofical question.

Can it be because of this attitude that I ever undertook the effort to learn these things ?


No.

Quote
Would I still have known them if I ever had been a nice, charming, happy to follow others kind of guy ?


Yes.

Quote

I see, ignore my charming self and make maximum use of the information provided.

Wouter



Bingo.

Maximum use? Not sure how much actual boat design will come from any of this, but the intellectual pursuit is interesting.

Re: How fast can a beachcat go? [Re: FasterDamnit] #140617
04/27/08 05:47 PM
04/27/08 05:47 PM
Joined: Jan 2005
Posts: 6,049
Sebring, Florida.
Timbo Offline
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Timbo  Offline
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Sebring, Florida.
17.5 knots this after noon showed on the gps while under spinaker, uni rig, on one hull with the boards half way up, in about 15 knots of wind. Every time I tried to heat it up more, I would stuff the bow and slow down to 15 knots. I think in 20 knots of wind, I could get it to plane if on both hulls boards up. I'll just have to wait for more wind. Off to Bombay tomorrow. I should be passing over Zandvoort about sunrise Tuesday moring. I hope it's clear so I can see ya! <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />


Blade F16
#777
Re: How fast can a beachcat go? [Re: Wouter] #140618
04/27/08 07:40 PM
04/27/08 07:40 PM
Joined: Jul 2002
Posts: 539
taipanfc Offline
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taipanfc  Offline
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Posts: 539
You put so much time and effort (with and without attitude) posting here all this kind of stuff. All you would have to do is copy and paste your numerous posts and you would have a book! But would need an editor to adjust the tone in some instances.

Re: How fast can a beachcat go? [Re: Timbo] #140619
04/27/08 07:47 PM
04/27/08 07:47 PM
Joined: Aug 2005
Posts: 2,921
Michigan
PTP Offline
Carpal Tunnel
PTP  Offline
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Posts: 2,921
Michigan
Quote
17.5 knots this after noon showed on the gps while under spinaker, uni rig, on one hull with the boards half way up, in about 15 knots of wind. Every time I tried to heat it up more, I would stuff the bow and slow down to 15 knots. I think in 20 knots of wind, I could get it to plane if on both hulls boards up.


How far back on the boat were you? I imagine you obviously had your foot in the strap... but were you leaning back also? How much do you weigh? I have hit the same number several times but it is hard to maintain it with any significant chop (more nose diving).

Re: How fast can a beachcat go? [Re: PTP] #140620
04/27/08 08:26 PM
04/27/08 08:26 PM
Joined: Jan 2005
Posts: 6,049
Sebring, Florida.
Timbo Offline
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Timbo  Offline
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I was sitting on the hull at the aft beam, forward foot up against the half raised board, other foot under the foot strap. I have tried trapping but it only nose dives more. I was trying instead to get it up on a plane by quickly bearing off onto both hulls when a good gust would come along, but there just wasn't enough wind for that, so on several runs I just sailed it on one hull to see how fast it would go. 17.5 in that light-ish air was good, but I want more wind! I was trying to keep the bow knuckle up out of the water which seemed to work well in a good gust.


Blade F16
#777
Re: How fast can a beachcat go? [Re: taipanfc] #140621
04/28/08 05:41 AM
04/28/08 05:41 AM
Joined: Jun 2001
Posts: 9,582
North-West Europe
Wouter Offline
Carpal Tunnel
Wouter  Offline
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Posts: 9,582
North-West Europe

Quote

You put so much time and effort (with and without attitude) posting here all this kind of stuff. All you would have to do is copy and paste your numerous posts and you would have a book! But would need an editor to adjust the tone in some instances.



Please go ahead and be my editor !

Wouter


Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: How fast can a beachcat go? [Re: Stein] #140622
04/28/08 06:52 AM
04/28/08 06:52 AM
Joined: Apr 2003
Posts: 1,669
Melbourne, Australia
Tornado_ALIVE Offline
Pooh-Bah
Tornado_ALIVE  Offline
Pooh-Bah

Joined: Apr 2003
Posts: 1,669
Melbourne, Australia
Quote
Tornados beat planing 18-foot skiffs around the buoys, but the 18-foot skiffs seem to exhibit the highest absolute absolute top speeds.


Higher top speeds as quoted by Bethwaite..... Even the Skiffies take what he says with a grain of salt. I believe he also quoted a rediculous max speed for a Tasar once (over 20 knots from memory).

I wounder why the 18s have never done any form of measured speed runs....... Hard to BS timed runs.


Re: How fast can a beachcat go? [Re: Wouter] #140623
04/28/08 07:03 AM
04/28/08 07:03 AM
Joined: Jul 2002
Posts: 539
taipanfc Offline
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taipanfc  Offline
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Posts: 539
Quote

Quote

You put so much time and effort (with and without attitude) posting here all this kind of stuff. All you would have to do is copy and paste your numerous posts and you would have a book! But would need an editor to adjust the tone in some instances.



Please go ahead and be my editor !

Wouter


I have no understanding of what you are saying most of time. Eyes glazing over kind of stuff. Editing the mountainous amount of data/posts would put me to slumber for many a year.

Re: How fast can a beachcat go? [Re: taipanfc] #140624
04/28/08 07:19 AM
04/28/08 07:19 AM
Joined: Apr 2003
Posts: 1,669
Melbourne, Australia
Tornado_ALIVE Offline
Pooh-Bah
Tornado_ALIVE  Offline
Pooh-Bah

Joined: Apr 2003
Posts: 1,669
Melbourne, Australia
Perhaps Doug Lord may be a better choice of Editor.


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