SailNet Community - Reply to Topic

   Search Sailnet:

 forums  store  


Quick Menu
Forums           
Articles          
Galleries        
Boat Reviews  
Classifieds     
Search SailNet 
Boat Search (new)

Shop the
SailNet Store
Anchor Locker
Boatbuilding & Repair
Charts
Clothing
Electrical
Electronics
Engine
Hatches and Portlights
Interior And Galley
Maintenance
Marine Electronics
Navigation
Other Items
Plumbing and Pumps
Rigging
Safety
Sailing Hardware
Trailer & Watersports
Clearance Items

Advertise Here






Go Back   SailNet Community > On Board > Boat Review and Purchase Forum > Sailboat Design and Construction > Moment curves
 Not a Member? 


Thread: Moment curves Reply to Thread
Title:
  

By choosing to post the reply below you agree to the rules you agreed to when joining Sailnet.
Click Here to view those rules.

Message:
Trackback:
Send Trackbacks to (Separate multiple URLs with spaces) :
Post Icons
You may choose an icon for your message from the following list:
 

Register Now

In order to be able to post messages on the SailNet Community forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
Please note: After entering 3 characters a list of Usernames already in use will appear and the list will disappear once a valid Username is entered.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.



Click here to view the posting rules you are bound to when clicking the
'Submit Reply' button below


Additional Options
Miscellaneous Options

Click here to view the posting rules you are bound to when clicking the
'Submit Reply' button below


Topic Review (Newest First)
09-30-2012 08:35 PM
Classic30
Re: Moment curves

Quote:
Originally Posted by Jeff_H View Post
.......
One observation which surprised me is that at 25 degrees of heel, the rail is nearly in the water. I found that surprising that modern boats with thier greater beam often do not put their rail in the water until they are heeled to 45 or so degrees. When I thought about this a little it occurs to me that one of the lovely features of older designs is that they do not 'jack up' with heel angle, meaning that the vertical center of gravity remains at approximately the same height, while on modern designs the surplus buoyancy in the hull and topsides, combined with their greater beam tends roll the boat vertically upward and actually raises the vertical center of gravity with heel. In doing so, the rotation of the boat means that the rail may be getting lower, but the hull is rising allowing a larger heel angle before the rail hits the water than might be expected otherwise.

I had read up on the Metacentric shelf theory at some point in my life. I believe that Colin Archer and Albert Strange were both proponents of this theory. While the theory has pretty much been discredited, in the days before computer simulations, it provided a way for designers to attempt to develop designs which remained comparatively in trim with heel, and so in theory predict a more forgiving design over a broader heel range.

I also respectfuly wanted to touch on Bob Perry's comment "The ends are probably damn near symetrical and close to the same volume. The change in fore and aft trim would have been at the most minimal." As I look at the body plan, I would tend to agree with that, but when I looked at the moments I saw a trend in the drawing with the moments shifting sharply from 'C' towards 'A' with heel. At 5 degrees C was roughly 75% of A, at 15 degrees C was roughly 52% of A and at 25 degrees C was roughly 41% of A. Similarly the moment bias in 'B' progressively shifts from being closer to C at 5 degrees to closer to A at 25 degrees. I would have to think that all things being equal this would have to impact trim with heel and so would love to hear Bob's comments on this. (Even if the comment is "Jeff, you are way over thinking this".)

Respectfully,
Jeff
After reading the paper AJonsson linked to (thanks! ) it seems to me that the designer must have produced these curves to satisfy all who looked at the drawings at the time that the design should be (and in fact, is) a reasonably well-balanced boat - although the paper does state in conclusion that "it is possible that the shapes of moment curves actually correlate with some other characteristic of a hull which is not directly measured by them."

Jeff, would you mind terribly explaining your take on the effect the slope of the curves has??

(Background: After being re-engined a few times since it was built, the boat "appears" to be squatting a bit at the stern and I'm wanting to improve the static fore-and-aft trim by perhaps shifting some non-engine weight forward.. but if the trim really does change with heel, I don't want to either (a) make things worse or (b) waste my time fixing a problem that isn't really there..)

Thanks,
Cameron
09-28-2012 07:06 PM
bobperry
Re: Moment curves

No, not quite "TIFFANY JANE on steroids". You must be trying to hurt my feelings. Other than being skinny double enders the boats share nothing. But I know that's hard to tell without the hull lines. My hull has a lot more complexity to the shape. I worked very hard to push volume into the ends while not making it awkward looking. Sectional shape in both ends is quite U shaped.

The boat is halfway way through the build now. Strip planked cedar hull with a composite deck and interior. Interior all assembled out of the boat cut with 3D CNC files and then lowered into the boat intact. It was a fun day to be in the shop. You can see an entire thread on the project with copious photos over on CA. It's Kim's boat SLIVER. We are using a Farr 40 rig.
09-28-2012 06:08 PM
Jeff_H
Re: Moment curves

Holy Moely, Tiffany Jayne on steroids. That is so cool! Absolutely lovely.

17,700 lbs on a 55 foot waterline, she would fly if she has enough stability to carry enough sail area. Please tell me she has a fractional rig....

"Under way" Like actually being built?

Jeff
09-28-2012 04:42 PM
bobperry
Re: Moment curves

Jeff:
Thanks for the kind words on the 45'er.

I agree with you that sometimes these theories can appear to work over a narrow range of boats. I do a wide variety of designs. It's asking a lot for a "rule" to work for my new 60 PSC ketch at 60' LOA by 15' beam and also work for my 62' LOA by 9.86' beam double ender underway at Hadlock.

I don't think I am the type to follow other people's design rules. But I also agree with you that it's fun to look back and see what was tried in the past.
09-28-2012 03:51 PM
Jeff_H
Re: Moment curves

Bob,

Thank you for the response. To me something like the Metacentric Shelf Theory is interesting as a piece of history, right up there with parallel rules and planimeters. On boats like the one shown, I could understand why it anecdotally might appear to work. To perform a meaningful calculation it would appear that the boat would either have to heel around the same longitudinal horizontal axis as it heeled from the vertical, or establish a new axis for each heel angle by trial and error. (The OP's drawings appear to assume that the boat rotates around the centerline at the water line.)

With a design like the example, that may be close to right. But with more modern designs the result of the calculation would be way off, and provide nowhere near as useful information as a 'good eye' and a bunch of experience.

The new 45'er for the Swedish client is a very beautiful boat by the way. With her modern foils she should be a joy to sail.

Jeff
09-28-2012 12:47 PM
bobperry
Re: Moment curves

Jeff:
I don't think you are "overthinking" it at all. If the success of a design were based upon having balanced ends according to those curves that theory would be well used today. But I don't base any major design decisions on any one curve and as I have said several times I have never seen the metacentric shelf theory used in a modern design office. I have never heard any of my contemporaries discuss it. I have this idea that the skilled, experienced designer can feel all the nuances of shape changes and how they effect performance and it is that level of "feel" that separates the good designs from the bad while accounting for the variety we see in today's design. (I could argue the other side of that if you like.)

While that curve may be interesting to consider from an academic perspective it is not something I will ever use. It is a graphic display of something I feel intuitively. It's just my way.

Here is the fanny of my new 45'er for my Swedish client. I have total confidence that this will be a well balanced boat. My eye tells me that. And I have a suspicion that it would look pretty good plotted to those moment curves.
09-28-2012 11:44 AM
Jeff_H
Re: Moment curves

A couple quick thoughts here;

That is a really lovely design. As anyone who has read any of my posts over the years would suspect, I really dislike the distortions that racing rules can impose on a boat designed as a racer first and a sailboat second, or even the boats that are styled after those race rule influenced designs. I argue that in any period there are wholesome designs that are only designed to be as good sailing boats as the technology of the day permits.

Seen from the perspective of today, it would be easy to argue that this boat has an enormous amount of wetted surface and and a pretty inefficient rig, but seen from the viewpoint of the period when she was designed, as compared to the RORC influenced racer-cruisers of the era, she would have been pretty fast, and seaworthy, with a lovely motion, and probably tolerate a larger carrying capacity with less negative impact. In other words, absent handicapping corrections on the race course, she would have been (and probably still would be) a lovely boat to own.

As AJonsson pointed out, I had not seen the note on the drawings which indicated which side of the line was windward. He is absolutely right that I was in error in my comments and assumptions about the side of the boat that the moments represent. Visualizing where the center of buoyancy would be with the hull heeled over, it is much easier to see the why the bow and stern curves are to windward and the miships to leeward, and that the shift in moments, is not because of the ends of the boat being submerged as I had surmissed, but because of the hull shape itself.

One observation which surprised me is that at 25 degrees of heel, the rail is nearly in the water. I found that surprising that modern boats with thier greater beam often do not put their rail in the water until they are heeled to 45 or so degrees. When I thought about this a little it occurs to me that one of the lovely features of older designs is that they do not 'jack up' with heel angle, meaning that the vertical center of gravity remains at approximately the same height, while on modern designs the surplus buoyancy in the hull and topsides, combined with their greater beam tends roll the boat vertically upward and actually raises the vertical center of gravity with heel. In doing so, the rotation of the boat means that the rail may be getting lower, but the hull is rising allowing a larger heel angle before the rail hits the water than might be expected otherwise.

I had read up on the Metacentric shelf theory at some point in my life. I believe that Colin Archer and Albert Strange were both proponents of this theory. While the theory has pretty much been discredited, in the days before computer simulations, it provided a way for designers to attempt to develop designs which remained comparatively in trim with heel, and so in theory predict a more forgiving design over a broader heel range.

I also respectfuly wanted to touch on Bob Perry's comment "The ends are probably damn near symetrical and close to the same volume. The change in fore and aft trim would have been at the most minimal." As I look at the body plan, I would tend to agree with that, but when I looked at the moments I saw a trend in the drawing with the moments shifting sharply from 'C' towards 'A' with heel. At 5 degrees C was roughly 75% of A, at 15 degrees C was roughly 52% of A and at 25 degrees C was roughly 41% of A. Similarly the moment bias in 'B' progressively shifts from being closer to C at 5 degrees to closer to A at 25 degrees. I would have to think that all things being equal this would have to impact trim with heel and so would love to hear Bob's comments on this. (Even if the comment is "Jeff, you are way over thinking this".)

Respectfully,
Jeff
09-28-2012 08:28 AM
bobperry
Re: Moment curves

Good call Aaron!

I have heard about the metacentric shelf theory but never seen it in use in a modern office. Not sure those curves tell me anything that I don't know instinctively by eye. I suspect any of my double enders would look pretty good to a metacentric shelf curve.

Looking over those calculations they would be quite arduous to do by hand. We have progressed beyond balancing shapes cut out of aper on knife edges today. A computer design program could do this quite easily but I've never seen it as a feature on any of the programs I am familiar with. I don't think there is a designer working today that uses that theory.

Thanks Hart for giving me the chance to become a little more aquainted with the history of yacht design.
09-28-2012 01:30 AM
AJonsson
Re: Moment curves

Quote:
Originally Posted by blt2ski View Post

That link?!?!?

Read part of it, interesting to say the least. Along with some of us swag'ed it to a degree in some way shape or form.

Marty
Look further down on the google search page for the pdf file. The link you mentioned does not do the theory justice.

Aaron
09-28-2012 01:16 AM
blt2ski
Re: Moment curves

Metacentric moments

That link?!?!?

Read part of it, interesting to say the least. Along with some of us swag'ed it to a degree in some way shape or form.

Marty
This thread has more than 10 replies. Click here to review the whole thread.

 
Posting Rules
You may post new threads
You may post replies
You may post attachments
You may edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On


All times are GMT -4. The time now is 10:43 AM.

Add to My Yahoo!         
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
(c) Marine.com LLC 2000-2012

The SailNet.com store is owned and operated by a company independent of the SailNet.com forum. You are now leaving the SailNet forum. Click OK to continue or Cancel to return to the SailNet forum.