Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 08-04-2008, 16:13   #16
Moderator Emeritus
 
David M's Avatar

Cruisers Forum Supporter

Join Date: Sep 2007
Location: San Francisco Bay
Boat: research vessel
Posts: 10,153
In my stability and trim class, we were taught that a ships trim (its fore and aft "tilt") is dependent on is location of center of buoyancy. You then take the mass times the distance from the Location of Center of Buoyancy (LCB) forwards and also aft and that is how the ship will sit. Think in terms of minimizing the mass times the distance from the LCB and you will do better for yourself. When loading a ship, consideration is taken try to minimize this number because the less a ship pitches in seas, the better. So long as "sagging" does not go over its critical number but that is not something small boats need to worry about.

I will try to explain speed and momentum relating to pitching. The G force at the extreme ends of the vessel is not going to change that much between masses at the extremes and masses closer to the LCB because the length of the moment arm from the LCB times the distance of the mass from the LCB does not vary that much. Does that make sense? In other words...concentrate your mass as close to the LCB as possible because the G forces (acceleration) are not going to vary all that much between weight focused on the ends versus weight focused towards the middle....because the length of that arm does not vary.

Ever notice how the period of a pendulum does not vary regardless of the amount of mass on the end for a fixed length of string? A boat is very similar, but for different reasons. The period varies with the length of the string and not the mass for a pendulum. The moment that causes a boat bow to come back up is the center of buoyancy shifting forward of the center of gravity when you think about it. Its stored kinetic energy stored which 1/2 mass times distance squared. The length of the pendulum (moment arm) matters much more than the weight on the end as far as acceleration goes. Or in other words, your G forces are not going to be appreciably lower by placing mass out on the ends...because mass on the end is only one factor out of two factors that determine acceleration.

Gawd I hope I did not make things worse.
__________________

__________________
David

Life begins where land ends.
David M is online now   Reply With Quote
Old 08-04-2008, 16:17   #17
Moderator Emeritus
 
GordMay's Avatar

Join Date: Mar 2003
Location: Thunder Bay, Ontario - 48-29N x 89-20W
Boat: (Cruiser Living On Dirt)
Posts: 31,596
Images: 240
I think David M is saying that keeping the weight "low" may be more useful than keeping the weight centered (out of the ends).
Although there is merit in this observation, I'm not certain that we can accept is as a universal proposition.
__________________

__________________
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"



GordMay is offline   Reply With Quote
Old 08-04-2008, 16:48   #18
cruiser

Join Date: Nov 2004
Posts: 4,525
These are great posts.

Between going over Gord's PDF link and David's excellent translation of "boat stuff" into "Physics stuff", I think I will be able to work this out.

Thanks for putting that time in, David. Your post helped me bridge the gap between working knowledge and theoretical knowledge. I will re-read and re-review until I can come up with the right answer for this boat.

My COB is tricky, because I have a fairly complex hull shape. Check it out in the pics. I think my COB is fairly far forward due to that bulge in the hulls forward below the waterline. Also, I'm not sure how I could ge the COG for my boat... well... maybe I can approximate all of this to understand it within a margin of error of "moving stuff around" inside the boat.

Here is a useful link as well:

Metacentric height - Wikipedia, the free encyclopedia

BTW: This applies to monos and cats, for those so inclined as to worry about this for their mono.
Attached Thumbnails
Click image for larger version

Name:	SailDrives.jpg
Views:	159
Size:	65.2 KB
ID:	3229   Click image for larger version

Name:	SailDrives2.jpg
Views:	163
Size:	86.6 KB
ID:	3230  

__________________
ssullivan is offline   Reply With Quote
Old 08-04-2008, 19:48   #19
Senior Cruiser
 
Therapy's Avatar

Cruisers Forum Supporter

Join Date: Jul 2007
Location: W Florida
Boat: The Jon boat still, plus a 2007 SeaCat.
Posts: 6,894
Images: 4
I can't make it through that PDF document (at least tonight).
I hope the designers of your cat knew that stuff ahead of building. I would think so.
Good luck moving stuff forward.
__________________
Therapy is offline   Reply With Quote
Old 08-04-2008, 19:54   #20
Moderator Emeritus
 
David M's Avatar

Cruisers Forum Supporter

Join Date: Sep 2007
Location: San Francisco Bay
Boat: research vessel
Posts: 10,153
Quote:
Originally Posted by GordMay View Post
I think David M is saying that keeping the weight "low" may be more useful than keeping the weight centered (out of the ends).
Although there is merit in this observation, I'm not certain that we can accept is as a universal proposition.
Not low. You want it as close to the center of buoyancy as possible as measured from the bow or stern. I did not mean height, as in any sort of vertical distance. Sorry about that confusing jumble of words.
__________________
David

Life begins where land ends.
David M is online now   Reply With Quote
Old 08-04-2008, 19:59   #21
Moderator Emeritus
 
David M's Avatar

Cruisers Forum Supporter

Join Date: Sep 2007
Location: San Francisco Bay
Boat: research vessel
Posts: 10,153
Quote:
Originally Posted by ssullivan View Post
These are great posts.

Between going over Gord's PDF link and David's excellent translation of "boat stuff" into "Physics stuff", I think I will be able to work this out.

Thanks for putting that time in, David. Your post helped me bridge the gap between working knowledge and theoretical knowledge. I will re-read and re-review until I can come up with the right answer for this boat.

My COB is tricky, because I have a fairly complex hull shape. Check it out in the pics. I think my COB is fairly far forward due to that bulge in the hulls forward below the waterline. Also, I'm not sure how I could ge the COG for my boat... well... maybe I can approximate all of this to understand it within a margin of error of "moving stuff around" inside the boat.

Here is a useful link as well:

Metacentric height - Wikipedia, the free encyclopedia

BTW: This applies to monos and cats, for those so inclined as to worry about this for their mono.
Your welcome Sullivan. Sounds like your having a great time on the ICW. I want to do the same thing some day....boring or not.

Metacentric height is something certainly worth understanding for monohulls and for cat owners. For cats, the center of buoyancy shifts radically to leeward as the weather hull leaves the water A cats center of gravity also shifts dramatically as it heels. Once the CG passes over the CB then thats it, your over.

I found a stability curve for a cat versus a monohull here: stability article part 2 by Woods Designs


Look how radically different they are. The initial stability of a cat gives it its speed since instead of heeling, more sail area is left exposed to the wind. But then you see why you reef cats sooner than you would reef a monohull because the cat in this example capsizes if it exceeds about a 75 degree angle of inclination where the monohull in this illustration can go past 90 degrees and still right itself. A catamarans righting moment drops dramatically after the weather hull leaves the water. Thats why you never want to see that weather hull leave the water....sure is fun on a Hobie though.

Unfortunately, this diagram is static but if you imagine the CB and CG moving around as the boat heels, you can imagine that when the CB is at its maximum distance from the CG then this is when a cat has its greatest righting moment and when they get closer together the righting moment decreases...until you get to the point where the CG passes over the CB and the boat capsizes.
__________________
David

Life begins where land ends.
David M is online now   Reply With Quote
Old 08-04-2008, 21:27   #22
Senior Cruiser
 
44'cruisingcat's Avatar

Cruisers Forum Supporter

Join Date: Sep 2006
Posts: 7,459
Images: 69
These graphs are indicative only. How high the initial stability is, and where it vanishes can vary pretty significantly. For instance, on that graph the cat reaches zero righting moment at around 75'. But depending on the hull design it could be beyond 90' where that happens.

http://kelsall.com/images/articles/A...sAndSafety.pdf
__________________
44'cruisingcat is offline   Reply With Quote
Old 08-04-2008, 21:30   #23
Senior Cruiser
 
44'cruisingcat's Avatar

Cruisers Forum Supporter

Join Date: Sep 2006
Posts: 7,459
Images: 69
Quote:
Originally Posted by David M View Post
Unfortunately, this diagram is static but if you imagine the CB and CG moving around as the boat heels, you can imagine that when the CB is at its maximum distance from the CG then this is when a cat has its greatest righting moment and when they get closer together the righting moment decreases...until you get to the point where the CG passes over the CB and the boat capsizes.
From the diagram you can also see that it makes sense to keep the CG low. The lower the CG, the greater the angle of heel would need to be before it moved outside of the CB.
__________________
44'cruisingcat is offline   Reply With Quote
Old 08-04-2008, 22:39   #24
Senior Cruiser
 
maxingout's Avatar

Cruisers Forum Supporter

Join Date: Dec 2006
Location: Fort Pierce, Phoenix
Boat: Privilege 39 Catamaran, Exit Only
Posts: 2,606
Hobbyhorsing can be a problem on a catamaran, especially when sailing into a sea that exactly synchonizes the waves with the movement of the bows up and down at a particular speed.

On Exit Only, when the boat had a tendency to hobbyhorse, we changed the speed of the yacht to minimize hobbyhorsing. What this usually meant was that we turned on one or both engines to push the hulls and bows through and over the waves at a speed that put us out of sync with the wave train. I think that much of the positive effect was the result of having the extra power to push the hulls through the waves rather than riding up and over each wave. It created more forward motion and minimized the vertical motion which meant less hobbyhorsing. Speed does make a difference.

We could achieve the same result by adding sail area to speed up Exit Only as she beat or close reached into choppy seas.

On Exit Only, hobbyhorsing at anchor was never a problem.
__________________
Dave -Sailing Vessel Exit Only

http://SailingUNI.com
http://maxingout.com
http://PositiveThinkingSailor.com
maxingout is offline   Reply With Quote
Old 09-04-2008, 00:18   #25
Registered User

Join Date: Nov 2006
Location: Vancouver, Can.
Boat: Woods 40' catamaran
Posts: 277
Centered weights will of course reduce the polar moment of inertia. That means that the boat will respond quicker to a disturbing influence.

But what is more important is that the pitch motions will be reduced much faster because the damping force is relatively constant (motion of the boats waterplane) and a low polar moment of inertia means that the constant damping force acts to damp the pitching motion faster.

Soooo, keep the ends light as possible.
__________________
Evan is offline   Reply With Quote
Old 09-04-2008, 00:50   #26
Registered User

Join Date: Feb 2008
Location: Oz
Boat: Jarcat 5, 5m, Mandy
Posts: 419
Part of the problem is the shape of the bows. It is messy to change the shape to give deeper forefoot and plumb to reverse bows and take out the flare. It is relatively easy to put plates on the rudders, or extend the stern with steps to damp motion. Some people have added bow bulbs. I had bow bulbs on a little tri I sailed from PNG to Australia and the boat had very little hobby horsing in 6 fathoms of water and sailing SSW in a 25 to 30 knots SE and a long fetch. Similar conditions in my little Jarcat and it is much more uncomfortable. The easiest motion I have seen is on a Harryproa-high prismatic coefficient, no flare on the bows. At anchor , you can throw a flopper stopper into the water to damp the motion. On trawlers we used to sometimes leave a 60L fish box over bow and stern if we were anchored in too much slop.
Robert
__________________
Robertcateran is offline   Reply With Quote
Old 09-04-2008, 01:49   #27
Registered User
 
bill good's Avatar

Join Date: Oct 2006
Location: Brisbane Australia
Boat: sold
Posts: 721
weight

SSULILLVAN

G'day I think you said you are delivering the boat. Are you sailing or motoring? Hobby horsing is a term relating to the vessel under way & is more a function of the buoyancy within the bow. I find under sail the motion is much more damped than under motors. The more the weight is located towards the center the better but if the weight has to be misplaced then keep it out of the front!! The last thing you want is any delay in having the bow lift when you meet a large green thing. The trim is best done by shifting the weight closer to the centre.

Regards Bill
__________________
bill good is offline   Reply With Quote
Old 09-04-2008, 08:19   #28
Senior Cruiser

Cruisers Forum Supporter

Join Date: Nov 2005
Location: Out there doin' it
Boat: 47' Olympic Adventure
Posts: 2,638
Quote:
Originally Posted by David M View Post
For cats, the center of buoyancy shifts radically to leeward as the weather hull leaves the water A cats center of gravity also shifts dramatically as it heels.
A catamarans righting moment drops dramatically after the weather hull leaves the water. Thats why you never want to see that weather hull leave the water....sure is fun on a Hobie though.

Unfortunately, this diagram is static but if you imagine the CB and CG moving around as the boat heels, you can imagine that when the CB is at its maximum distance from the CG then this is when a cat has its greatest righting moment and when they get closer together the righting moment decreases...until you get to the point where the CG passes over the CB and the boat capsizes.
David,

I don't see how a cat's CG would shift dramatically with heel? Unless large chunks of mass are moving around the CG should remain more or less in place. I agree the CB would shift radically as a hull lifts - but then you state that the righting moment drops as soon as that happens I couldn't agree less - IMO, the righting moment would be greatest at just the moment the CB shifts from somewhere between the hulls to the centre of the leeward hull. The GZ (righting arm) would be at its maximum - which ties into the spike in the graph you provided.

As others have said, the graph leaves something to be desired - as a lot of cats have positive stability up to or beyond 90 degrees of heel. Regardless, it can be seen from the graph that a typical cat should have a greater righting moment than the maximum moment generated by a typical mono, up to about 45 degrees of heel. On a cat with 20' BOA, this makes sense - the GZ would be about 6', which would be about the same as the distance between CB and CG on a 40' ballasted keelboat at 90 degrees of heel.

To Sean's question now - I've often heard the dogma of keeping the weight central to reduce hobbyhorsing, but wonder if there is any proof of this? If you look at a catamaran laterally (ie. from the stern) you see the bulk of the mass is generally divided equally between the two hulls, therefore set at the extremes of breadth, with very little mass centred. Yet I have not heard anyone complain about cats rolling excessively (hobbyhorsing sideways?). Personally, I think concentrating all the weight in one area puts greater stress on the structure in that area, whereas judiciously spreading the weight around will not change the CG, therefore will not affect stability. That said, I also think you have to work with the design of your boat. Sean's boat appears to be deeply rockered with the CB just forward of the longitudinal centre. I assume the designer or previous owner(s) had some strategy in mind when they located the engines and batteries. I also assume a sailboat should normally be trimmed by the stern at rest, to offset the forward pitch imparted by the thrust of the sails. I think if Sean experiments with 'moving loads' he should start by putting them as far forward as possible, to see if that has the desired effect. If so, a more permanent solution would be to relocate the batteries closer to the CB. JMO

Kevin
__________________
Lodesman is offline   Reply With Quote
Old 09-04-2008, 16:08   #29
Registered User

Join Date: Feb 2008
Location: Oz
Boat: Jarcat 5, 5m, Mandy
Posts: 419
With the long overhanging bows, you certainly don't want too much weight forward. The more I think about it, the more I think extending the transom is the best long term solution. This effectively brings the cog forward, improves damping, and doesn't increase the chance of the boat nose diving. The other options of converting to plumb bows or bow bulbs would work better. Increasing the damping from extending the waterplane, and allowing the mass to be moved forwards without risk,earlier and gentler entry into waves, but it would be an expensive and messy job. I have seen this done on some commercial catamarans, one a small,, 50', ferry that had atrocious hobby horsing until they added about a 60cm bow bulb, and the other a 60' shark fishing boat that not only added 5' bow bulbs but added a horizontal foil as an under water cross brace as well. This was the most comfortable boat do work on in the whole of the fleet, allowing about an extra 30 days fishing a year. I won't say the solutions looked elegant
Robert
__________________
Robertcateran is offline   Reply With Quote
Old 09-04-2008, 17:44   #30
Registered User

Join Date: Nov 2005
Location: Ohio
Posts: 2,313
We've found when hobby horsing occurs the best remedy was to sag off the rig (power up) and drive off a couple degrees. (ie: straighten the rig, ease the hyd back, a couple clicks on the checks and runners, car forward a touch, ease the sheet, softer entry on the sails, and hang on) The ride will not be comfortable since the motion goes from hobby horsing to more of a accelerate/deccelerate. Power on does the trick but at a comfort cost.
__________________

__________________
Joli 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




Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 10:32.


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.