Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 25-08-2008, 12:21   #16
cruiser
 
BigCat's Avatar

Join Date: May 2007
Location: Everett, Washington
Posts: 765
Number crunching's value

Quote:
Originally Posted by Gludy View Post
Big Cat
Thanks for explaining why the St Francis is so much faster than you know what - the mystery for me is cleared up.I can now see why a light slow cat with a short sharp sea and well over 20 knots of wind and all its sails up needs engine power to even tack! There was not enough speed built up to overcome the braking effect of the sea.

I have only one mystery left to solve .... why each thread turns into the same subject even when i do not raise it!!
Hi, Gludy - I was responding to the comment about how manufacturers omit any statistics that make their boats look bad. This is the most egregious example I can think of, because Gideon pushes the claim that his boat is lighter than other boats so very hard in many, many posts on this website. Crunching the numbers makes it clear that this is complete nonsense, which many people do not have the analytical knowledge to see through. Also, several posters have questioned the value of analyzing the statistics of boats' measurements, and I wanted to show a practical example that illustrates the predictive value of the process.

To those who are unintersted in boat speed, I say, that's fine. Some people care about boat speed and some don't. You pays your money and takes your choice. No worries!
__________________

__________________
BigCat is offline   Reply With Quote
Old 25-08-2008, 12:25   #17
Senior Cruiser
 
schoonerdog's Avatar

Cruisers Forum Supporter

Join Date: May 2004
Location: annapolis
Boat: st francis 44 mk II catamaran
Posts: 1,174
Images: 4
Typically people compare the light ship displacement numbers, because what you may decide to carry on the boat is different than the next, but the light ship gives you a baseline from which to work and compare.

So comparing some of the catamarans mentioned:

The fastcat has a D/L of 94 light ship displacement.

The St Francis has a D/L of 103 light ship displacement.

For a legitimate comparison the lagoon 44 has a DL of 141, light ship displacement.

The Outremer 45 has a small D/L of around 80, but her hull living area is completely confined to her very narrow hulls. My PDQ 36 would have been considered more spacious. For a 45 ft long catamaran she only carries 63 gallons of water. She's meant for speed not live aboard comfort. When considering the boat fully loaded, that should be seen as "how much stuff can I carry and not create problems". For outremer, she's anything but a load carrier, so you could pack very very little and her low "fully loaded" D/L reflects that. African cat can carry fully twice as much according to her stats.

BTW, the full displacement of the St Francis 50 is wrong on Multihull Maven (by a large margin), please check angelo lavranos website for a more accurate description.

http://www.lavranosyachtdesign.co.nz/sc_stfrancis48.htm

As to the Lagoon 67 trying to compare a catamaran that is 20 ft longer is completely apples to oranges. Every catamaran when the size goes up the D/L reduces as the cabins are full sized cabins and the designer can afford to have nice longer hulls.

Quote:
Originally Posted by BigCat View Post
Love it !!!
So true, and so true of every statistic, I might add. The so called Fastcat 435, when on its marks according to published figures on Multihull Maven, has the rather high displacement length ratio of 153.5 - a figure you won't find on the Fastcat website! The Outremer 65, to give some perspective, has a D/L ratio fully loaded of 86.5. Smaller D/L = lighter for its size. The Lagoon 67 has a lightship D/L of 82, so if you add, say 12,000 pounds of paylaod, it has a D/L, loaded, of 107. So, the performance of the Fastcat reported by Gludy is no surprise.[/quote]
__________________

__________________
schoonerdog is offline   Reply With Quote
Old 25-08-2008, 12:30   #18
Senior Cruiser
 
Talbot's Avatar

Cruisers Forum Supporter

Join Date: Jun 2004
Location: Brighton, UK
Boat: Privilege 37
Posts: 3,579
Images: 32
Quote:
Originally Posted by BigCat View Post
It's called surfing, but the phenomenon is not to be confused with planing, It takes a huge amount of power to make a boat plane. Vessels under sail just can't summon that much power unless they are very, very light, much lighter than any cruising catamaran.
Whilst I do not entirely disagree. Planing is also a funtion of boat design, and the standard cat underwater profile these days is not really designed to plane. However, the Catalac, that staid old cruiser that doesnt really go welll to windward, does have an underwater profile that is designed for planing, and given sufficient push from the wind, will actually get up and plane. I have achieved it twice (dont normally like to push her that hard) and can assure you that the boat was planing as we were overtaking the waves and on one occassion I was well over 15 kts - cant tell the actual speed as the dial only went up that far.
__________________
"Be wary of strong drink. It can make you shoot at tax collectors - and miss."
Robert A Heinlein
Talbot is offline   Reply With Quote
Old 25-08-2008, 12:50   #19
cruiser
 
BigCat's Avatar

Join Date: May 2007
Location: Everett, Washington
Posts: 765
Planing is dynamic lift, not boat speed per se

Quote:
Originally Posted by Talbot View Post
Whilst I do not entirely disagree. Planing is also a funtion of boat design, and the standard cat underwater profile these days is not really designed to plane. However, the Catalac, that staid old cruiser that doesnt really go welll to windward, does have an underwater profile that is designed for planing, and given sufficient push from the wind, will actually get up and plane. I have achieved it twice (dont normally like to push her that hard) and can assure you that the boat was planing as we were overtaking the waves and on one occassion I was well over 15 kts - cant tell the actual speed as the dial only went up that far.
If you are trying to design a boat to plane easily, you make the aft sections identical, and the flatter they are, the more easily the boat will plane. However, the Catalac is very, very unlikely to plane under sail, as it is heavy and doesn't have a lot of sail area. Just because you are going fast doesn't mean the boat is necessarily planing. Planing is a function of dynamic lift, in which the boat is rising above the bow wave.
__________________
BigCat is offline   Reply With Quote
Old 25-08-2008, 13:11   #20
Senior Cruiser
 
Talbot's Avatar

Cruisers Forum Supporter

Join Date: Jun 2004
Location: Brighton, UK
Boat: Privilege 37
Posts: 3,579
Images: 32
I can assure you that I can recognise planing and what happens, and on both these occasions the boat was planing.

if it has waterproof feathers, walks with a waddle, and quacks, the logical certainty is that it is a duck.

By the same logical process, I am certain that this was planing.
__________________
"Be wary of strong drink. It can make you shoot at tax collectors - and miss."
Robert A Heinlein
Talbot is offline   Reply With Quote
Old 25-08-2008, 13:17   #21
jzk
Registered User

Join Date: Jun 2006
Posts: 639
Quote:
Originally Posted by BigCat View Post
Sailing catamarans do not surf, except for the rare few equipped with hydrofoils. They are displacement vessels. Surfing is like skipping a stone across the water, you are using dynamic lift caused by speed to raise the vessel partly above the surface of the water. Sailing catamarans are fast because they are narrow, and as you might expect, it is easier to push something narrow through the water than it is something wider.
So an Outremer 50 sailing at 20 knots rides just as low in the water as if it were not moving at all? Or has it risen up higher? I can't believe it hasn't risen up.
__________________
jzk is offline   Reply With Quote
Old 25-08-2008, 13:19   #22
cruiser
 
BigCat's Avatar

Join Date: May 2007
Location: Everett, Washington
Posts: 765
You can't argue with faith-

Quote:
Originally Posted by Talbot View Post
I can assure you that I can recognise planing and what happens, and on both these occasions the boat was planing.

if it has waterproof feathers, walks with a waddle, and quacks, the logical certainty is that it is a duck.

By the same logical process, I am certain that this was planing.
Smacks much more of intuition than logic. I seem to recall seeing a video of a comedian quacking, waddling, and decorated with feathers. Did this make him a duck? I won't try to argue with faith.

http://www.harrisinteractive.com/har...ex.asp?PID=618
__________________
BigCat is offline   Reply With Quote
Old 25-08-2008, 13:23   #23
cruiser
 
BigCat's Avatar

Join Date: May 2007
Location: Everett, Washington
Posts: 765
Faith vs. reason

Quote:
Originally Posted by jzk View Post
So an Outremer 50 sailing at 20 knots rides just as low in the water as if it were not moving at all? Or has it risen up higher? I can't believe it hasn't risen up.
Another vote for faith over reason, I would say.
__________________
BigCat is offline   Reply With Quote
Old 25-08-2008, 13:25   #24
jzk
Registered User

Join Date: Jun 2006
Posts: 639
Quote:
Originally Posted by BigCat View Post
Another vote for faith over reason, I would say.
If your position is so reasonable, then rather than throw an ad hominem, how about reasoning it?

Thanks.
__________________
jzk is offline   Reply With Quote
Old 25-08-2008, 13:31   #25
cruiser
 
BigCat's Avatar

Join Date: May 2007
Location: Everett, Washington
Posts: 765
http://www.powermultihulls.com/magazine/articles/displacement%20or%20plane.htm

Quote:
Originally Posted by jzk View Post
If your position is so reasonable, then rather than throw an ad hominem, how about reasoning it?

Thanks.
http://www.powermultihulls.com/magaz...or%20plane.htm

"I can't believe" is not an argument for your views, it is a statement of your limitations. It is you that placed yourself as the argument for your point of view, not me.
__________________
BigCat is offline   Reply With Quote
Old 25-08-2008, 13:52   #26
jzk
Registered User

Join Date: Jun 2006
Posts: 639
"It is, of course, possible to question whether these boats really are displacement craft. Current theory says that for vessels of this length, to go this fast, they must be planing. In fact, if we accept the usual definition of planing vessel, namely: that it has a speed/length ratio of more than 2, then these boats are clearly planing. However, a boat is said to be planing when most of its mass is supported dynamically by the downward directed thrust of the water. A vessel that is planing will typically have a bow out trim and will have bodily risen out of the water. The waters are muddied a little by the fact that there is no sudden jump from displacement to planing. It is a continuum and somewhere in the speed/length ratio range from 1.5 to 2 the craft would be considered to be in a "semi-displacement" mode. We have now designed a large number of displacement power cats exemplifying the "long and slim" approach of powerboat design. "

Thanks for the read. Seems to be saying that they are, in fact, somewhere between displacement and planing.
__________________
jzk is offline   Reply With Quote
Old 25-08-2008, 14:03   #27
cruiser
 
BigCat's Avatar

Join Date: May 2007
Location: Everett, Washington
Posts: 765
Big boat do have a real advantage

"Typically people compare the light ship displacement numbers, because what you may decide to carry on the boat is different than the next, but the light ship gives you a baseline from which to work and compare."

When comparing live-aboard cruising boats meant to cross oceans, I think it is more valid to compare lightship plus a fixed payload, such as 12,000 pounds, as that is how the boats will end up in actual use. This does indeed tend to give an advantage to large boats, but this is reality. Larger boats do have an advantage over smaller ones in this way in the real world, all else being equal. I pointed this out already when I mentioned the advantage of size in this context.

If Gideon wants to compete head to head with large boats for ocean voyaging in his marketing of the 435, he is just going to have to live with the consequences.

No boat carrying two people should, IMHO, carry only 64 gallons of water for an ocean crossing. This is potentially dangerous, and it certainly isn't comfortable.
__________________
BigCat is offline   Reply With Quote
Old 25-08-2008, 14:14   #28
cruiser
 
BigCat's Avatar

Join Date: May 2007
Location: Everett, Washington
Posts: 765
Quote:
Originally Posted by jzk View Post
"It is, of course, possible to question whether these boats really are displacement craft. Current theory says that for vessels of this length, to go this fast, they must be planing. In fact, if we accept the usual definition of planing vessel, namely: that it has a speed/length ratio of more than 2, then these boats are clearly planing. However, a boat is said to be planing when most of its mass is supported dynamically by the downward directed thrust of the water. A vessel that is planing will typically have a bow out trim and will have bodily risen out of the water. The waters are muddied a little by the fact that there is no sudden jump from displacement to planing. It is a continuum and somewhere in the speed/length ratio range from 1.5 to 2 the craft would be considered to be in a "semi-displacement" mode. We have now designed a large number of displacement power cats exemplifying the "long and slim" approach of powerboat design. "

Thanks for the read. Seems to be saying that they are, in fact, somewhere between displacement and planing.

A little knowledge can be a dangerous thing. Current design theory does not say that boats must be planing if they are going over 2x the hull speed. Current theory says that for heavy monohulls of typical cruising monohull width. Narrow boats have a higher hull speed, as you'd know if you read the articles I posted links to in this thread, and the narrower they are the higher the hull speed they have, up to a hull length / hull beam ratio of 15.5.
__________________
BigCat is offline   Reply With Quote
Old 25-08-2008, 14:49   #29
cruiser

Join Date: May 2008
Location: Brecon, Wales
Boat: St Francis 50 on order
Posts: 269
I have always understood that the standard 1.34 ratio for displacement hulls does not apply to narrow displacement hulls. Hence the narrower the hull for a given beam the higher the hull speed.

To drive a hull past its hulls peed requires lots and lots of energy. To have cat engines big enough would be wasteful but as wind is still free sail power can be used to provide a lot more thrust than the engines.

Whilst I do not think that cats can plane I do think they can surf in certain conditions.

I chose a largish boat because for just two of us it provided a balance of comfort with performance.

Some may spend large sums of money making a boat lighter but it seems to me in many real world conditions such a boat can actually go slower. Its simpler to opt for a larger boat.
__________________
Gludy is offline   Reply With Quote
Old 25-08-2008, 14:57   #30
Senior Cruiser
 
mikereed100's Avatar

Cruisers Forum Supporter

Join Date: Dec 2005
Location: Santa Barbara
Boat: 46' custom cat
Posts: 1,573
Images: 2
Quote:
Originally Posted by amelis View Post
Hi guys, I have been told that the Leopards lack the bridgedeck clearance for comfortable passagemaking and are designed for coastal use. This seems t make sense considering where they are most seen but I was wondering if I could have your opinions / experiences please?
I am considering a 470 for a world cruise.
What is the bridedeck clearance? I am told 7% of the LWL is the minimum and that it should be no less than 70cm.
Also, what is the difference between the clearance at the bow and the stern as the Leopard slopes...?
Thank you!
AM
You might try the manufacturer for this info. Keeping in mind, of course, that bridgedeck clearance is just one aspect to consider in a cruising boat.

Mike
__________________

__________________
mikereed100 is offline   Reply With Quote
Reply

Tags
leopard

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Bridge Clearance on the Intracoastal Waterway 2divers Navigation 33 30-12-2015 18:49
Anchor from Bow or Bridgedeck? mikereed100 Multihull Sailboats 12 12-04-2009 09:01
Intercoster waterway clearance? freetime Great Lakes 12 14-08-2008 15:13
Hull clearance from the water KIWI Multihull Sailboats 24 29-01-2008 12:31
Leopard 45, Bridge Deck Clearance isi Multihull Sailboats 4 04-02-2007 22:16



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 16:28.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.