Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 20-06-2010, 07:58   #76
Writing Full-Time Since 2014
 
thinwater's Avatar

Join Date: Nov 2008
Location: Deale, MD
Boat: PDQ Altair, 32/34
Posts: 4,354
Quote:
Originally Posted by tsmwebb View Post
The length of the wave created by your hulls approaches their sailing length at V=1.34*sqrt(LWL). That's true of all displacement hulls and is what I call hull speed, but despite its name it is not in any way shape or form a predictor of boat speed. There is a long and horrible history of arguing this point on the internets and a little googling should uncover a wide variety of opinion.

What I think you may be looking for is a velocity prediction algorithm. The KSP number is about as simple a rule as can be created. It is a rule of thumb and should be treated as such but assuming you don't capsize it says a multihull should go V=0.5*WS*sqrt(SA*LWL/D). You can combine it with the capsize formula WS=9.48*sqrt(0.5B*D/SA*CE) to get an estimate of top speed.

Where:
V= boat speed in knots
WS = apparent wind speed knots
LWL = load water line feet
D = displacement pounds
B= beam between centers of buoyancy feet
SA = sail area ft^2
CE = vertical distance between center of effort of hull and sails in feet

Apologies to all for the hijack.

Tom.
I plugged numbers in for a real-world cruising cat that I know well--the PDQ 32.
Sail Delmarva: Speed Polars

WS calculated to be 19 knots apparent. Nope, that's just where it starts moving well. The manual, a professionally prepared VPP, and my experience are clear that 34 knots apparent is the beam-on capsize point.

The max speed at 34 knots apparent calculates to be about 27 knots. No way. Though I have never taken her to the point, I think 14 knots without surfing might be reached.

I think there is a constant problem.
__________________

__________________
Gear Testing--Engineering--Sailing

Writing full-time since 2014.
Bookstore:http://sail-delmarva.blogspot.com/20...ook-store.html
thinwater is offline   Reply With Quote
Old 20-06-2010, 08:07   #77
Writing Full-Time Since 2014
 
thinwater's Avatar

Join Date: Nov 2008
Location: Deale, MD
Boat: PDQ Altair, 32/34
Posts: 4,354
Quote:
Originally Posted by yeloya View Post
I am an engineer and have a basic understanding of physic and all these "empirical" formulae.
To the buyer of a cat or charterer who wants to enjoy sailing, they are totally irrelevant. You pay tons of money for buying a boat from a reputable manufacturer and you have the right to assume that some naval architects, engineers have made their jobs using whatever formulae and test necessary to make yr "cruising" boat safe enough at the range of 20-25 knots of wind..I know that there is no such a 100% safe boat. In the geography we are sailing, the wind is unpredictable and can change suddenly because of countless small bays, valleys, straits, sharp mountains. The conditions I face with are rare but can happen any time. But, a cruising cat should never fall in such situation with this kind of wind and sea state in even under full sail whatever the trim is. To those who are continiously advocating reefing; the day we sailed the wind varied between 0 and 18 knots max, generally around 12-14 knots. There have been times when I pushed the limits but always with a professional crew with me, not while I had my wife and guests on board enjoying their coffee in a lovely afternoon..
Bottom line;

a-I've got my lesson (don't make assumptions and projections basing on yr previous experience with "apparently" similar boats, get ready to sheet the main out in a second if you see the gust is coming althought it seems a monohull practice..)
b-I have warned you..
c-I will further investigate to find out the exact reasons, talk to FP and if necessary make some amendments on the boat and rigging to make it even safer for charter purpose. I risked my life and I don't want to put at risk any charter customer.

I want to conclude by saying that, I still believe that FP's are great and extremely safe boats. But even big companies can make errors. (remember Mercedes A-160 which was rolling over at virtually any curve. They were recalled and supplied with ESC system later as a standard equipment. Toyota recall, etc.. ?)
If I get any news on that, I will update you.

Thx again for all yr contributions. Happy (and safe..) sailing

Yeloya


I am not blaming FP, they made are still making great and very safe boats.
One compromise we see in many cruising cats is that they keep the rudders shallow to keep the draft shallow. That works when there are 2 in the water. As soon as one gets near the surface and ventilates....

The other problem is that a nice cat convinces the sailor he can sail with full sail in 27 knots true, while the guests sip coffee. I promise, in a similarly rigged monohull (enough sail to go 10-12 knots), you would have been knocked flat, coffee would have been everywhere, and the cabin might have flooded. Monohulls have sunk that way--I saw it happen to a J-24 in a race once near my home--down in moments. If sail for equivalent speed had been carried (a reef) nothing would have happened.

I keep hearing that cats don't give feed back--not true--but it is different. Many think cats are for dummies--I think cats make you think more and listen more closely. No veteran beach cat sailor would have been surprised or had to think what to do or which way to turn.
__________________

__________________
Gear Testing--Engineering--Sailing

Writing full-time since 2014.
Bookstore:http://sail-delmarva.blogspot.com/20...ook-store.html
thinwater is offline   Reply With Quote
Old 20-06-2010, 10:00   #78
Registered User
 
YOGAO's Avatar

Join Date: May 2008
Location: St. Augustine, FL - an unwilling C.L.O.D.
Boat: Maine Cat 41
Posts: 519
Quote:
Originally Posted by yeloya View Post
Bottom line;

a-I've got my lesson (don't make assumptions and projections basing on yr previous experience with "apparently" similar boats, get ready to sheet the main out in a second if you see the gust is coming althought it seems a monohull practice..)
b-I have warned you..
c-I will further investigate to find out the exact reasons, talk to FP and if necessary make some amendments on the boat and rigging to make it even safer for charter purpose. I risked my life and I don't want to put at risk any charter customer.

Yeloya
Yeloya,

This has been a good thread and made me re-think my assumptions a bit.

Really glad to see you start this post with "I've got my lesson."

A couple of things you might want to add. Particularly with first time cat charterers, you might want to suggest starting out with a reef. That way you start with the sail plan rigged for the gusts. True, the boat may sail a bit slower, but it will still move well enough and most charterers aren't expecting race boat performance (okay, they ARE fantasizing about it, but...)

Pay attention to the what the boat is telling you. Weather helm is indicating a trim problem OR a sudden acceleration of the wind. Just like a mono, with weather helm, if the main tell-tales are flying properly, drop the traveler, then ease the sheet. Of course, on the Lipari it might not be convenient for the helmsperson to do this.

If you haven't done so already, you should add tell tales along the body of the main, similar to what you have on a headsail. On a full batten main they are invaluable.

As part of your discussion with FP, I would also seriously explore changing the rudders to give more control. Thinwater has said it and I have experienced it on both monos and multis - boats have a series of compromises, rudder size is probably one of them.

Fair Winds,
Mike
__________________
YOGAO is offline   Reply With Quote
Old 20-06-2010, 12:15   #79
Registered User

Join Date: Apr 2007
Location: Hawaii
Boat: Atlantic 42 Catamaran
Posts: 293
Quote:
Originally Posted by thinwater View Post
I plugged numbers in for a real-world cruising cat that I know well--the PDQ 32.
Sail Delmarva: Speed Polars

WS calculated to be 19 knots apparent. Nope, that's just where it starts moving well. The manual, a professionally prepared VPP, and my experience are clear that 34 knots apparent is the beam-on capsize point.

The max speed at 34 knots apparent calculates to be about 27 knots. No way. Though I have never taken her to the point, I think 14 knots without surfing might be reached.

I think there is a constant problem.
Is your displacement number correct? I used Steve Killing's number for sailing weight (9,679.4) on the PDF and came out with something like 31 knots capsize. I suspect even that is a pretty optimistic sailing weight. Beware published displacements.

Tom.
__________________
tsmwebb is offline   Reply With Quote
Old 20-06-2010, 12:52   #80
Marine Service Provider

Cruisers Forum Supporter

Join Date: Jul 2007
Location: Marmaris
Boat: FP Orana 2010, Lipari 2011, Hélia 2013, Catana C 47 2013
Posts: 1,033
Quote:
Originally Posted by tsmwebb View Post
Yeloya, it is very kind of you to have posed your experience. Thank you.

It is probably just me, but I am still a little confused about the conditions on the day. In your fist post you said That would suggest the true wind was between 27 and 29 knots. That's a lot of wind to be sailing with full sail and no hand on the sheets. But from your last post quoted above it is not clear to me if it was blowing "18 knots max" or 20-25 knots... I completely agree that at 18 knots no boat marketed as a "cruising" cat should fly a hull.

Tom.
Sorry Tom, you are absolutely right.. I've seen at one point in time prior to the accident, clearly 25 knots from 90. That makes exactly 27.12 knots true from 112,78 degree off the bow at 10,5 knots of speed. Obviously as you said, all of this equipment is rarely too accurate, particularly the speed on water.
You agree that at 18 knots no crusing cat should fly a hull. How about 25 knots and flat water ? Pls note that I am referring to flat water every time. To me, choppy sea, breaking or irregular waves, strong currents, confused sea state can totally change the equation. Actually, 27 knots of true should generate much bigger than 2-3 ft waves, but (fortunately) I was in small and protected bay. Similar conditions in open sea could lead to full disaster..

Cheers

Yeloya
__________________
yeloya is offline   Reply With Quote
Old 20-06-2010, 15:27   #81
Registered User
 
Eleven's Avatar

Join Date: Sep 2007
Location: Southampton UK
Boat: Jaguar 22 mono called Arfur.
Posts: 1,220
Images: 3
Quote:
I will further investigate to find out the exact reasons, talk to FP and if necessary make some amendments on the boat and rigging to make it even safer for charter purpose. I risked my life and I don't want to put at risk any charter customer.
Yeloya
I'd suggest a smaller sail set, not mast, for the charter customer. Less likely to get in trouble in the first place, and make your boat(s) more typical.
__________________
Ex Prout 31 Sailor, Now it's a 22ft Jaguar called 'Arfur' here in sunny Southampton, UK.
A few places left in Quayside Marina and Kemps Marina.
Eleven is offline   Reply With Quote
Old 21-06-2010, 01:51   #82
Registered User

Join Date: Apr 2007
Location: Hawaii
Boat: Atlantic 42 Catamaran
Posts: 293
Quote:
Originally Posted by yeloya View Post
You agree that at 18 knots no crusing cat should fly a hull. How about 25 knots and flat water ? Pls note that I am referring to flat water every time. To me, choppy sea, breaking or irregular waves, strong currents, confused sea state can totally change the equation. Actually, 27 knots of true should generate much bigger than 2-3 ft waves, but (fortunately) I was in small and protected bay. Similar conditions in open sea could lead to full disaster..

Cheers

Yeloya
Good question, Yeloya. I'm not sure where I would draw the line between a cruising cat and a performance cat in terms of capsize wind speed. To some extent scale matters -- bigger boats should be more stable. I don't think 25 is an unacceptably low number for some 40 foot cruising cats. But, cruising covers a lot of different tastes and services. I can imagine 25 would not be enough stability for some of them. I think FP should provide guidance so that sailors can make informed choices. My expectations are clearly a bit different than yours but it seems to me it would be easy for FP to put a warning in the user's manual about wind induced capsize. If they had no assumptions would have been made and no lives and property put in danger.

Cheers,

Tom.
__________________
tsmwebb is offline   Reply With Quote
Old 21-06-2010, 02:00   #83
D&D
Marine Service Provider
 
D&D's Avatar

Cruisers Forum Supporter

Join Date: Nov 2008
Location: Blue Mountains, Australia
Boat: now working Syd Harbour charters
Posts: 1,459
Quote:
Originally Posted by tsmwebb View Post
I don't think 25 is an unacceptably low number for some 40 foot cruising cats. But, cruising covers a lot of different tastes and services. I can imagine 25 would not be enough stability for some of them.
We're probably missing (or more likely misunderstanding) something here...or is this thread now seriously discussing 25kn as a capsize wind speed for a 40' cruising cat?!?
__________________
D&D is offline   Reply With Quote
Old 21-06-2010, 03:27   #84
Marine Service Provider

Cruisers Forum Supporter

Join Date: Jul 2007
Location: Marmaris
Boat: FP Orana 2010, Lipari 2011, Hélia 2013, Catana C 47 2013
Posts: 1,033
Fully agree D&D.. That's actually why I started the thread. Otherwise, 35-40 knots, choppy sea with full sail wouldn't have left much to talk about..

Yeloya
__________________
yeloya is offline   Reply With Quote
Old 21-06-2010, 04:44   #85
D&D
Marine Service Provider
 
D&D's Avatar

Cruisers Forum Supporter

Join Date: Nov 2008
Location: Blue Mountains, Australia
Boat: now working Syd Harbour charters
Posts: 1,459
Just so Yeloya...and, as we said back at Post#66, we're grateful to you for the discussion (already up to 85 posts!) thus far.

Your experience sounds greater than ours so no doubt you too were on cruising cats underway with full sails in generally light, flat conditions with occassional gusts pressing 20+kn. We were therefore stunned reading your story and left wondering, given that cats are notororious for giving few clues of distress, how close were we on those similar episodes?!

Now the formulae are appearing -- We again acknoweldge following the technical discussions with interest and even some very limited stabiliy training, but definitely no 'expertise' -- and we were even more stunned to see 25kn suggested as a capsize wind speed...and that without any regard (as we understand it!!) to the relatively unusual position of the Lipari's rudders.

Roll on your discussions with FP. We very much look forward to hearing their comments on both the design brief for the conditions you experienced as well as any anticipated effect (positive or negative) from the rudder position. No doubt FP will say right away "You shoulda reefed!" (or however you say that in French) and we'd agree with that given your description ("25-27 knts just at 90 degree apparent on my starboard, boat speed on water 10,5-10,7 knots") of the basic position. We would be reefed in those conditions, BUT even with full sail that should still be a long way from theoretical capsize conditions...shouldn't it??

So it seems to us there's still a case to be answered from the vessel's designers.

In the meantime, we'll be keeing the main sheet out of the jammers and reefing on boat speed as well as wind speed!
__________________
D&D is offline   Reply With Quote
Old 21-06-2010, 07:40   #86
Registered User

Join Date: Feb 2008
Posts: 2,064
Images: 1
Quite frankly I'm a skeptic. I suspect the original report is flawed... perhaps the wind meter is not calibrated properly?
__________________
SailFastTri is online now   Reply With Quote
Old 21-06-2010, 08:14   #87
CF Adviser

Cruisers Forum Supporter

Join Date: Dec 2007
Location: Wherever our boat is; Playa Zaragoza, Isla Margarita
Boat: 1994 Solaris Sunstream 40
Posts: 2,439
As others have wisely suggested already, what is required for every boat is a plan of action. Yes, you should be familiar with your boat, with the manufacturer's recommendations for reefing and adhere to the mantra 'reef early'; but you should also be prepared to deal with rapidly changing circumstances.

As has also been suggested, when apparent wind is forward of the beam, it is typically wise to head up, rather than bear off. Yes, the centrifugal force of a sharp turn would add to the heeling forces; however, you need not (should not) make a sharp turn and, if you are sailing on a close reach/beat, the sails will start to luff and depower very quickly, thereby reducing heeling forces.

It is also significant that when sailing with wind forward of the beam, the waves are also typically forward of the beam; therefore, to bear off is to expose the beam of your boat to the full force of the waves and this will significantly increase the risk of capsize.

Finally, I find it easier to reef when luffing up into the wind.

Consequently, when sailing with the apparent wind forward of the beam and finding yourself suddenly overpowered:

1. Let off on the main.
2. Gradually head up into the wind while starting your engine/s.
3. Let out the foresail.
4. When heading straight into the wind, reef your foresail, tension your topping lift and reef the main.
5. Once sufficiently reefed, sheet in and bear off.

If and only if the apparent wind is aft of the beam would I bear off and reef downwind.

Brad
__________________
Southern Star is offline   Reply With Quote
Old 21-06-2010, 20:11   #88
Registered User

Join Date: Sep 2009
Location: Florida Keys
Boat: Corsair F31"Susan C" & Sea Pearl 21"Maggie"
Posts: 261
I'd be careful about using apparent wind as an indicator of head up/ fall off. In the F-31 apparent wind is almost never aft of the beam. A spinnaker run DDW is an apparent beam reach. I must be head to wind to reef but will often fall off and roll up the headsail first. Even then the acceleration as the boat rounds up can be alarming. If you can keep track of true wind angle that might be a better indicator. I can't keep track so I fall off first. Dave
__________________
DaveOnCudjoe is offline   Reply With Quote
Old 22-06-2010, 00:31   #89
Registered User

Join Date: Mar 2008
Posts: 92
Quote:
Originally Posted by SailFastTri View Post
Quite frankly I'm a skeptic. I suspect the original report is flawed... perhaps the wind meter is not calibrated properly?
What about this report then?




An 11m cat flipped at Airlie Beach in North Queensland, family on board and below, apparently owner/skipper had everything cleated off and only he was on deck. It was brought in to the marina and with the help of a couple of cranes flipped and put on the hard.

thanks to Sailing Anarchy news.
Attached Thumbnails
Click image for larger version

Name:	cat%20flip.jpg
Views:	165
Size:	22.6 KB
ID:	17263  
__________________
bayview is offline   Reply With Quote
Old 22-06-2010, 02:14   #90
Registered User

Join Date: Aug 2009
Location: Land-locked at the moment :(
Posts: 189
Quote:
Originally Posted by bayview View Post
What about this report then?




An 11m cat flipped at Airlie Beach in North Queensland, family on board and below, apparently owner/skipper had everything cleated off and only he was on deck. It was brought in to the marina and with the help of a couple of cranes flipped and put on the hard.

thanks to Sailing Anarchy news.
Can you toss out the link for this please?

And I think you missed the point of his statement. It wasn't that cats cant flip, it was that they probably wont/cant with so little wind and flat seas (as described by the OP).
__________________

__________________
Event_Horizon is offline   Reply With Quote
Reply

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
Which Is More Forgiving - Cruising Monohull or Cruising Multihull maxingout General Sailing Forum 36 10-02-2010 06:41
Cruising Cat vs. Cruising Mono Performance ssullivan Multihull Sailboats 100 03-01-2010 14:05
on the verge... blove8 Meets & Greets 5 07-10-2007 15:18



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 10:06.


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.