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Old 26-07-2014, 20:36   #331
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Re: At The Risk of Starting WWlll

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Originally Posted by StuM View Post
Wave height is dependent on three factors:
1. Wind strength
2. Wind duration (how long it blows in the same direction)
3. Fetch (the uninterrupted distance over which the wind blows in the same direction)

The third factor is the critical one when talking about ocean waves. A steady 20 knots will build much bigger waves over 1000 miles of ocean than they will over a couple of miles in a lake.
Yes, I never gave thought to the amount of surface area. That stands to reason. Thanks for that StuM. Can you tell me if the wind and seas are a greater problem at times for a mono or a multi? I'm going to guess a multi will handle it a little better just because of the multihulls alone covering the surface area greater to give more stability.
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Old 26-07-2014, 23:20   #332
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Re: At The Risk of Starting WWlll

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Originally Posted by robert sailor View Post
I get a kick out of some of you guys and your discussion of rough seas. Now we have a 19 ft. runnabout that is going to take on some weather....when I read that I know you haven't really been out in anything really big because some of the stuff I've been in a 19 ft runabout would be swamped within a few minutes.

I don't have any experience in big seas and Cats but I expect a Cat would do OK although I wouldn't want to be sitting up on one of those fly bridges.
Valid point made however the discussion point I responded to was scary seas in the Whitsunday Islands and I have been on plenty of vessels there. This area is where I live and I have spent lots of seatime on various charter boats, trawlers as well a small powerboats and two sailing cats. It is a safe area for the many charter vessels of all types just need to watch the tidal rip areas.

Lots of places I don't want to be in a 19ft powerboat but it was fine in the Whitsundays.

Point is it is a safe area by comparison with breaking open seas except for when a cyclone is in the area.

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Old 27-07-2014, 00:28   #333
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Re: At The Risk of Starting WWlll

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Can you tell me if the wind and seas are a greater problem at times for a mono or a multi? I'm going to guess a multi will handle it a little better just because of the multihulls alone covering the surface area greater to give more stability.
It may be apocryphal, but you can come across this story in various places:

"at the time of the famous Fastnet disaster, of which volumes have been written, there were two Prout catamarans in the vicinity. They were shadowing the fleet as unofficial entries. The racing fleet sailed into a serious storm. The carnage caused by the storm was so great that a Committee On Safety From Capsizing was formed. It consisted of the Society of Naval Architects and Marine Engineers (SNAME), the United States Yacht Racing Union (USYRU), and the United States Naval Academy (USNA), and others. The purpose of this committee was to design a minimum stability formula for ocean racing monohull yachts. The weather that was disaster to the Fastnet fleet was considered merely "beastly weather for sailing" by the two adjacent catamarans"
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Old 27-07-2014, 01:40   #334
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Re: At The Risk of Starting WWlll

I can't say either way but "in the vicinity" often isn't the same as being there. I remember many boats "in the vicinity" of the queens birthday storm that had very few problems while some boats that were in the teeth of it were rolled and pitch-poled.
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Old 27-07-2014, 08:03   #335
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Re: At The Risk of Starting WWlll

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..."in the vicinity" often isn't the same as being there...
Couldn't have said it better.
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Old 27-07-2014, 09:12   #336
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Re: At The Risk of Starting WWlll

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Originally Posted by Razoo View Post
Yes, I never gave thought to the amount of surface area. That stands to reason. Thanks for that StuM. Can you tell me if the wind and seas are a greater problem at times for a mono or a multi? I'm going to guess a multi will handle it a little better just because of the multihulls alone covering the surface area greater to give more stability.
It is not as simple as that.

The dynamics of a huge breaking sea interacting at just the wrong moment with a cross sea... will cause a Cat to sit on edge exposing the underside of the bridge deck to incredible wind forces and the powerful rolling forces of wave shape.

This primer explains the hidden forces of a wave.

http://hyperphysics.phy-astr.gsu.edu...s/watwav2.html

Now imagine a 50-90ft breaking wave catching the Cat sideways and burying one hull.

Still.prefer a mono with heavy ballast and a righting moment at all angles in those survival conditions.
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Old 27-07-2014, 09:17   #337
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Re: At The Risk of Starting WWlll

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In analysing the Seas you will face in heavy weather...these are some of the factors to consider.

Wave Period/Height /Fetch/History/secondary swells/topography/ocean current direction

Noting that a wave period the same length as your boat is the most critical and dangerous situation (anywhere).

Worth reading
http://www.stormsurf.com/page2/tutor...vebasics.shtml
Wave period? That doesn't make any sense. A 30 ft boat in a 30 ft wave period is going to experience what 3ft waves? 3 ft waves might be annoying and toss stuff around because the boat is locked in a too and fro rock, but dangerous?

It's the tall breaking waves that are dangerous.

Unless you mean in a following sea, that I could understand somewhat that the "apparent" period while underway with following seas is much longer, therefore would lead to taller and faster waves following in sync with the length if your boat. That could be really annoying and leading to broaching because the boat is pre-loaded to surf down each wave.


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Old 27-07-2014, 10:01   #338
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Re: At The Risk of Starting WWlll

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Originally Posted by Pelagic View Post
It is not as simple as that.

The dynamics of a huge breaking sea interacting at just the wrong moment with a cross sea... will cause a Cat to sit on edge exposing the underside of the bridge deck to incredible wind forces and the powerful rolling forces of wave shape.

This primer explains the hidden forces of a wave.

http://hyperphysics.phy-astr.gsu.edu...s/watwav2.html

Now imagine a 50-90ft breaking wave catching the Cat sideways and burying one hull.

Still.prefer a mono with heavy ballast and a righting moment at all angles in those survival conditions.

But then again, the monohull will trip on its keel in much smaller waves while a well designed multi will slide down the face of the wave. This could be argued over and over.


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Old 27-07-2014, 10:16   #339
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Re: At The Risk of Starting WWlll

You can also have a multhull with ballast, for example..



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Old 27-07-2014, 16:34   #340
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Re: At The Risk of Starting WWlll

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Originally Posted by Scot McPherson View Post
Wave period? That doesn't make any sense. A 30 ft boat in a 30 ft wave period is going to experience what 3ft waves? 3 ft waves might be annoying and toss stuff around because the boat is locked in a too and fro rock, but dangerous?

It's the tall breaking waves that are dangerous.

Unless you mean in a following sea, that I could understand somewhat that the "apparent" period while underway with following seas is much longer, therefore would lead to taller and faster waves following in sync with the length if your boat. That could be really annoying and leading to broaching because the boat is pre-loaded to surf down each wave.


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Scot... The explanation about critical wave period was general

However the interation of 2 or 3 high wave sets from different angled and origind can create that critical wave period in a small vessel
http://www.coastalwatch.com/surfing/...riod-explained
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Old 27-07-2014, 16:59   #341
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Re: At The Risk of Starting WWlll

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But then again, the monohull will trip on its keel in much smaller waves while a well designed multi will slide down the face of the wave. This could be argued over and over.


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I agree, but rather than argue over and over... maybe if everyone would download these 2 Wolfson reports... We could discuss this more intelligently.

Not to prove one better than the other...but to clearly understand the different critical survival scenarios for each design solution in that 'Perfect Storm'

I am more focused in identifying the non linear events in dynamic stability

http://www.wumtia.soton.ac.uk/servic...ltihull-safety
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Old 27-07-2014, 21:32   #342
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Re: At The Risk of Starting WWlll

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Originally Posted by Pelagic View Post
I agree, but rather than argue over and over... maybe if everyone would download these 2 Wolfson reports... We could discuss this more intelligently.

Not to prove one better than the other...but to clearly understand the different critical survival scenarios for each design solution in that 'Perfect Storm'

I am more focused in identifying the non linear events in dynamic stability

http://www.wumtia.soton.ac.uk/servic...ltihull-safety

The link wouldn't download the pdf's.


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Old 27-07-2014, 23:11   #343
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Re: At The Risk of Starting WWlll

Strange... I tested twice with my posted link and then downloaded Phase I and II of the report
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Old 28-07-2014, 02:55   #344
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Re: At The Risk of Starting WWlll

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Strange... I tested twice with my posted link and then downloaded Phase I and II of the report
The links worked for me.
It is great to read a scientific study on the subject.
Essential reading
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Old 28-07-2014, 03:15   #345
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Re: At The Risk of Starting WWlll

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Still.prefer a mono with heavy ballast and a righting moment at all angles in those survival conditions.
Thats Excellent - I don't - still prefer a boat that can't sink (please note i didn't say or suggest that all multis are unsinkable or that all monos are sinkable). But each to his own.
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