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  #121  
Old 12-30-2009, 09:13 AM
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Fanie Fanie is offline
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Vimes, sorry if I duplicated your video. I was looking for a video that I saw before of a cat in big waves. They have not even taken the stuff off the tables.

The guy wasn't sailing, but the waves were substantial. Looking through the ports as the video was taken you can see the cat going down and water all round and then it goes up again.

I'm sure it would become unpleasant if you sail fast to wind.
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  #122  
Old 01-01-2010, 06:11 PM
MikeJohns MikeJohns is offline
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Quote:
Originally Posted by capt vimes View Post
that happens with slow - in this case longkeeled boats - not making, or being able to make, efficient speed through the water...
they get tossed around by the waves like a cork...
Looking at that video clip I disagree. That boat looks safe and comfortable in those conditions and under control.
It's probably moving faster than you credit, and being filmed from an accompanying ship so it's hard to really see what's happening.

It could probably do with a little more sail but the crew are probably worn out after the storm that generated that sea and happy with the speed they are making.
There is less than gale force wind in the video. This is the most uncomfortable phase of heavy weather for a small vessel; when the sotrm initially abates. If all the wind dies but the seas are still rough then there are only passive techniques. Then a lightweight really becomes the cork and with their high GM and low roll gyradius it can be dangerous on deck and more prone to being flipped by a breaking wave.

The other videos show very benign sea-states and conditions and plenty of wind but far from what I'd call heavy weather. I doubt you will see any footage to compare in real storm conditions but that's what's lacking is a true comparison, (like a light and heavy boat going to windward in a heavy chop).

I'd like to see the fast boats in the south pacific trades they are never as suitable for fast transits and that's only with 20 knots continuous but often with a heavy underlying swell.

Longer keels don't automatically produce a slow boat wave making is far more important as to max speed, why should they be slow? And relative to what ? You can't simplify hull design to such reductionist blocks.




Beavrolick you said before "heay masts are always dangerous"
I was wondering what you based that on? There's evidence that heavy masted boats survive breaking beam seas with reduced chance of knockdown much better because of the added roll gyradius.
If a boat is so designed it can benefit from a heavy mast.

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  #123  
Old 01-01-2010, 06:45 PM
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BeauVrolyk BeauVrolyk is offline
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Quote:
Originally Posted by MikeJohns View Post


Beavrolick you said before "heay masts are always dangerous"
I was wondering what you based that on? There's evidence that heavy masted boats survive breaking beam seas with reduced chance of knockdown much better because of the added roll gyradius.
If a boat is so designed it can benefit from a heavy mast.

[/font]
Mike,

The reason that heavy rigs are bad, in my opinion always bad, is that while they do resist a roll initially once the boat starts to roll the heavy rig causes the roll to go much further than a light rig would.

Also, each and every time the boat sails into a wave and tries to get her bow to rise over the wave, and each and ever time a sea comes up on the boat from astern and the boat tries to get her stern to rise over the wave rather than getting pooped, the greater resistance to pitching (which is for exactly the same reason you're citing as a resistance to rolling) increases the chance that the wave with come over either the bow or the stern of the boat.

The basic rule of thumb is that an increased polar moment of inertia is bad.

Polar moment in increased by putting weight in the ends of things, like the bow, the stern, the keel and the mast. Because the Polar Moment increases as the square of the distance from the roll center, NOT in a linear way, this makes weight in the rig particularly evil.

For all the same reasons that you wouldn't park a pair of 200 pound anchors on the tip of the bow during a long beat to windward, and you wouldn't hang a dingy fully of fuel tanks off the stern while trying to run before a bad storm, you shouldn't have anything heavy in the rigging either.

I do agree, that the momentary resistance to rolling caused by a heavy rig, or a large deep bulb keel for that matter, is nice on the one occasion of being smacked by a steep wave from abeam. However, the vast majority of waves are not the sort that will roll a boat over, they are the sort that will just toss a couple of tons of water on the deck if the boat can't get over them. These are far more dangerous because they are nearly infinitely more common. The heavy rig will keep the bow or stern from rising (and will keep the boat from rolling when hit amidships) and thus greatly increase the chances that the sea will come aboard.

Like everything in Yacht design, it's a trade off and I'm claiming that one should optimize for the most common sort of problem and danger, not for the very rare occurrence of a breaking wave from abeam.

BV
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  #124  
Old 01-01-2010, 10:01 PM
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Valid points, BV. However, we should note that, at least anecdotally, the point in many "situations" where things start to get really nasty is when the rig comes down. Large chunks of expensive rig flying around the deck is bad enough, but there are so many reports of boats that developed a dangerous motion and a tendency to capsize repeatedly after being dismasted..... as suggested by the Fastnet '79 investigation, a slightly overbuilt rig can help increase the roll moment of inertia (good for capsize resistance) as well as reducing the chance of dismasting.
I'd rather put up with a bit more pitching and the occasional wave over the deck than lose my rig.....
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  #125  
Old 01-01-2010, 10:55 PM
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Quote:
Originally Posted by capt vimes View Post
funny fanie... exactly the same video i posted earlier on this thread as an example for NOT being in a cat when the shite hits the ven...

mono: turned over but still afloat upright after turning through 360°...
cat: turned over.... end-of-line
Perhaps you should do some research before you comment on things that you admitted you had little knowledge about

Quote:
Originally Posted by capt vimes View Post
i do not like multis and have absolutely no experience whatsover with those types of ships...
.
Here I posted some Info from Loch Crowther where he did studies on wave induced capsize with various vessels with the multihulls very fairing well in same wave heights that rolled monos.

http://www.cruisersforum.com/forums/...ised-5910.html
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  #126  
Old 01-01-2010, 11:03 PM
Frosty Frosty is offline
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If you don't have those silly escape hatches that let the air out I would imagine an upside down cat with mast and sail still intact would be a very stable platform on which to stay till help arrived, and even longer. assuming that it floated flat.
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  #127  
Old 01-02-2010, 04:11 AM
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Quote:
Originally Posted by BeauVrolyk View Post
For all the same reasons that you wouldn't park a pair of 200 pound anchors on the tip of the bow during a long beat to windward, and you wouldn't hang a dingy fully of fuel tanks off the stern while trying to run before a bad storm, you shouldn't have anything heavy in the rigging either.
Same is true with high aspect rigs. Momentum comes from height and mass. So with the same argument we could argue against high aspect ratios.. Staying away from extremes
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  #128  
Old 01-02-2010, 06:18 AM
MikeJohns MikeJohns is offline
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Quote:
Originally Posted by BeauVrolyk View Post
The reason that heavy rigs are bad, in my opinion always bad, is that while they do resist a roll initially once the boat starts to roll the heavy rig causes the roll to go much further than a light rig would.

But the energy in the systems is exactly the same so it doesnt work that way. The light boat rolls faster and further, the heavy boat rolls slower and with a smaller angular displacement. To look at the heavy boat in an even higher energy situation then also apply that to the lightweight !

Damping is also very important.

Several investigations have found that a high roll inertia considerably reduces the angle rolled to following being hit and significantly increases the roll period

Quote:
Originally Posted by BeauVrolyk View Post

Also, each and every time the boat sails into a wave and tries to get her bow to rise over the wave, and each and ever time a sea comes up on the boat from astern and the boat tries to get her stern to rise over the wave rather than getting pooped, the greater resistance to pitching (which is for exactly the same reason you're citing as a resistance to rolling) increases the chance that the wave with come over either the bow or the stern of the boat.
The pitching gyradius is effected far less by heavy rigs than is the roll gyradius and whether the boat is dry has to do with its ratios of reserve bouyancy and rate of change of sections. So it has little effect on a properly designed boat except that in some conditions it will pitch more heavily, but then that approaches oscillatory conditions and we can alter the lines somewhat to compensate .

Pitching is more important in racing boats because it tends to reduce speed and this is one reason that light rigs are very much in vogue.

Quote:
Originally Posted by BeauVrolyk View Post
The basic rule of thumb is that an increased polar moment of inertia is bad.
Whose rule of thumb? Id like to see that substantiated with a decent reference since it's not a rule I have encountered and I'd be very interested in the authors reasoning since it runs counter to modern naval architecture.

Quote:
Originally Posted by BeauVrolyk View Post
Polar moment in increased by putting weight in the ends of things, like the bow, the stern, the keel and the mast. Because the Polar Moment increases as the square of the distance from the roll center, NOT in a linear way, this makes weight in the rig particularly evil......................
Actually polar moment is the section property of a shapes resistance to being twisted not moved, you mean roll inertia. If you take the square root of the sum all the transverse roll inertias divided by the total mass you get the gyradius or radius of gyration which is a common figure in naval architecture. Along with GM its quit informative of the vessels likely behavior.

From what I said before Id re-iterate that a heavy rig is far from evil but can actually make a much safer and more comfortable vessel.

The point AK made before is worth considering, after a boat is knocked down and dismasted it is considerably more prone to being rolled again from the reduction of its gyradius even if it is proceeding under control under a jury rig or under auxiliary motor.
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  #129  
Old 01-02-2010, 07:41 AM
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from Marsja the inertia changes in a seaway, a very interesting subject and ofcourse a trade off yet i lean to BV's side
Attached Thumbnails
Favorite rough weather technique-dynamic-inertia.gif  
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  #130  
Old 01-02-2010, 08:47 AM
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All well and good to talk about design and staying in the bar during blowy weather but....
The heart of this topic must lie in what the heck to do when our caution fails and we are caught out in the boats we in fact own.

Vocabulary:

Inertia: "The tendency of a body to resist acceleration; the tendency of a body at rest to remain at rest or of a body in straight line motion to stay in motion in a straight line unless acted on by an outside force."

In other words, in our application a heavy rig damps the tendency for waves to roll the boat. Something Marsja seems to acknowledge in his pie graph above.

A poorly researched bit i have heard:
Ships of old hoisting heavy anchors aloft to damp roll in hard weather.

Further vocabulary.
I believe the conversation covers two topics:
Heavy weather, where one might still sail, and storm conditions.
I believe many heavy weather tactics will fail in storms.
For instance, I don't think a sea anchor has any chance at all of dealing with a yacht caught in a large breaking wave.
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  #131  
Old 01-02-2010, 09:41 AM
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capt vimes capt vimes is offline
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Quote:
Originally Posted by sabahcat View Post
Perhaps you should do some research before you comment on things that you admitted you had little knowledge about.

Here I posted some Info from Loch Crowther where he did studies on wave induced capsize with various vessels with the multihulls very fairing well in same wave heights that rolled monos.

http://www.cruisersforum.com/forums/...ised-5910.html
very nice PR article....
do not get me wrong...
i know the advantages of multis and i know their disadvantages... and that includes their behaviour in rough seas as well... i do not like them nevertheless and i do not want to get caught on a multi in really bad conditions...

for me it is a wrong 'feeling' being on a multi...
you know the differnce in car driving and motorcycle riding?
both are machines to move one along... on a bike you feel the thing with every fibre of your body - heck, you steer it with all of your body - you feel the tires and every peble on the road... in a car you do not...
i feel lost and detached on a multi... thats why I do not like them... i need to feel the ship, its heeling and motion and this feel for the ship is not the same if you compare multis and monos...

but everything to everybodys liking... don't you think?

@mikejohns
i know that longkeeled does not automatically means slow... never wanted to let it look that way but this boat in mark775s video is making no speed at all... just drifting and geting tossed around by the waves...
and you are right: the worst one may face is heavy seas from a storm and no wind... been in this nightmare more than once - even experienced a knockdown by some freak wave - and did not like it...
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  #132  
Old 01-02-2010, 09:56 AM
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Re. hoisting anchors aloft
Not sure if this particular tidbit is accurate. But some offshore powerboats (Dashew FPB 83 comes to mind) carry ballast tanks on the flybridge, that can be pumped full of water to increase the roll moment of inertia if necessary.
Of course, many powerboats can't handle the weight without creating a dangerously low AVS. And that Dashew boat seems to get by just fine with its oversized active fin stabilizers, I haven't seen any of Steve or Linda's articles yet where they talk about actually having the tanks full.


There have been a lot of good points made on this thread. But it's also clear that each of us has strong personal biases according to what we sail:

- The powerboat types have come out strongly in favour of (a) having enough speed and weather knowledge to stay clear of anything heavy, and (b) if caught in something heavy, relying on careful modulation of the throttle and steering to keep in control.

- The monohull sailors have again said they'd rather stay clear of the bad weather, but have exhibited distinct preferences for particular combinations of boat characteristics that they're comfortable with if the storm does hit. A few seem to prefer running downwind, a few would rather heave to, lie ahull or break out some sort of drogue. Each is probably at least somewhat appropriate for the boat associated with the poster.

- The multihullers again would rather watch the weatherfax and get clear of anything nasty. If bad weather does hit, they seem to have a stronger preference for some form of drag device, not necessarily the same device for all boats though. Perhaps a cat/tri/proa's ability to easily carry a wide bridle, coupled with its inherently high roll moment of inertia, alleviates the nasty motion that some of the monohullers have reported when lying to a sea anchor.

Perhaps the most illuminating thing I've seen come out of this thread so far is this:
No one technique (except avoiding the storm in the first place) will be suitable for all boats; storm tactics must be planned in advance according to the characteristics of the particular boat involved.
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  #133  
Old 01-02-2010, 10:41 AM
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You perfectly summed it up Matt!

Thanks.
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  #134  
Old 01-02-2010, 01:01 PM
TollyWally TollyWally is offline
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Indeed Matt.
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  #135  
Old 01-02-2010, 02:24 PM
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Go out in heavy or even moderate seas in a sailboat without the mast and you will see how violently it rolls.
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