[bolger] Re: Stability/Knockdown Study Bolger Hulls - PRIVATE POSTINGS

Yes, it would. In a 470, in some conditions, you sit together to follow
the waves or spread apart to dampen the pitching motion.

Gregg Carlson

At 02:27 PM 3/16/00 -0500, you wrote:

>I am no expert but....would putting the weight on the edges really slow down
>the rate of rolling? I would liken this to having a mid-engined car
>compaired to one with the engine in either the front or rear. The effect,
>for the same weight is to make the car more stable because of a lower 'polar
>moment of inertia'. (Almost sounds like I know what I am talking about).
>This is like putting the weights on a barbell in the middle as opposed to
>the end. At least that is the theory and practice when in comes to
>automobiles.
>
>> -----Original Message-----
>> From:jsharper@...[SMTP:jsharper@...]
>> Sent: Thursday, March 16, 2000 2:16 PM
>> To:bolger@egroups.com
>> Subject: [bolger] Re: Stability/Knockdown Study Bolger Hulls -
>> PRIVATE POSTINGS
>>
>>
>>
>>
>> > Here's a question for those of you who understand the finer details
>> about
>> > ballasting. If I am going to use a mixture of sand bags and bags of lead
>> shot
>> > for ballasting a flat bottomed sharpie ( not vee bottomed) - is it
>> better
>> to
>> > have the greater concentration of weight on the outside edges - or close
>> to
>> > the centerline? Steve
>>
>> I'm not an expert but I'll take a stab at it ....
>>
>> Your center of gravity will be the same in either case so stability won't
>> be effected by the side-to-side distribution of ballast in a flat bottomed
>> boat. What will be affected is the roll rate. Putting the weight on the
>> outside edges will slow the boat's rolling. That might be a good thing
>> in
>> a sharpie. I'll now defer to the experts...
>>
>> John
>>
>>
>>
>> ------------------------------------------------------------------------
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>
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In a message dated 3/16/2000 3:24:13 PM Eastern Standard Time,
pvanderw@...writes:

<< How about upolstering the seat backs with closed cell foam padding? >>
Been thinking about that!:-) Lots of seat backs.

Increasing the port/starboard division of the weight will certainly
increase the rotational moment of interia around the roll axis. Whether
the difference is noticeable or negligible, I don't know. I think it
would be much less than the difference between mast up and mast down.

Nor do I know whether it would be a good idea or not. In general, a
higher moment implies a longer, slower roll, which may be more
comfortable. But it also means that if a wave starts the boat rolling,
the roll will take longer to stop - which could put you over the edge.

How about upolstering the seat backs with closed cell foam padding?

Peter

> I am no expert but....would putting the weight on the edges really slow

down

> the rate of rolling? I would liken this to having a mid-engined car
> compaired to one with the engine in either the front or rear. The

effect,

> for the same weight is to make the car more stable because of a lower

'polar

> moment of inertia'. (Almost sounds like I know what I am talking about).
> This is like putting the weights on a barbell in the middle as opposed to
> the end. At least that is the theory and practice when in comes to
> automobiles.

Hmmm. I think we're in agreement, here. In the case of a car you want a
lower polar moment of inertia to aid turning, right? In a boat, the lower
polar moment of inertia would make the boat respond quicker to wave action,
hence the faster roll. The weight out on the sides would, conversely,
increase the inertia and slow the roll.

Still daydreaming,
John

I am no expert but....would putting the weight on the edges really slow down
the rate of rolling? I would liken this to having a mid-engined car
compaired to one with the engine in either the front or rear. The effect,
for the same weight is to make the car more stable because of a lower 'polar
moment of inertia'. (Almost sounds like I know what I am talking about).
This is like putting the weights on a barbell in the middle as opposed to
the end. At least that is the theory and practice when in comes to
automobiles.

> -----Original Message-----
> From:jsharper@...[SMTP:jsharper@...]
> Sent: Thursday, March 16, 2000 2:16 PM
> To:bolger@egroups.com
> Subject: [bolger] Re: Stability/Knockdown Study Bolger Hulls -
> PRIVATE POSTINGS
>
>
>
>
> > Here's a question for those of you who understand the finer details
> about
> > ballasting. If I am going to use a mixture of sand bags and bags of lead
> shot
> > for ballasting a flat bottomed sharpie ( not vee bottomed) - is it
> better
> to
> > have the greater concentration of weight on the outside edges - or close
> to
> > the centerline? Steve
>
> I'm not an expert but I'll take a stab at it ....
>
> Your center of gravity will be the same in either case so stability won't
> be effected by the side-to-side distribution of ballast in a flat bottomed
> boat. What will be affected is the roll rate. Putting the weight on the
> outside edges will slow the boat's rolling. That might be a good thing
> in
> a sharpie. I'll now defer to the experts...
>
> John
>
>
>
> ------------------------------------------------------------------------
> DON'T HATE YOUR RATE!
> Get a NextCard Visa, in 30 seconds! Get rates as low as
> 0.0% Intro or 9.9% Fixed APR and no hidden fees.
> Apply NOW!
>http://click.egroups.com/1/2120/5/_/3457/_/953234205/
>
> -- 20 megs of disk space in your group's Document Vault
> --http://www.egroups.com/docvault/bolger/?m=1
>

> Here's a question for those of you who understand the finer details about
> ballasting. If I am going to use a mixture of sand bags and bags of lead

shot

> for ballasting a flat bottomed sharpie ( not vee bottomed) - is it better

> have the greater concentration of weight on the outside edges - or close

> the centerline? Steve

I'm not an expert but I'll take a stab at it ....

Your center of gravity will be the same in either case so stability won't
be effected by the side-to-side distribution of ballast in a flat bottomed
boat. What will be affected is the roll rate. Putting the weight on the
outside edges will slow the boat's rolling. That might be a good thing in
a sharpie. I'll now defer to the experts...

John

Here's a question for those of you who understand the finer details about
ballasting. If I am going to use a mixture of sand bags and bags of lead shot
for ballasting a flat bottomed sharpie ( not vee bottomed) - is it better to
have the greater concentration of weight on the outside edges - or close to
the centerline? Steve

In a message dated 3/15/2000 11:55:36 PM Eastern Standard Time,
dhodges@...writes:

<< I modified a big sea kayak into a small trimaran, >>

Folding amas for martha jane!? Not. Steve

Right on Pippo!

I modified a big sea kayak into a small trimaran, and I gave up after
breaking three unstayed masts (I really didn't want to all-out rig a tri for
every little sail, it wouldn't tack, and so on).

My point is the multihulls are so stiff that the rigs must be really
bulletproof; all the gust loads go to acceleration (or rig failure), not
heeling.

Don Hodges

> A nice thing that I've learned in my amateurish boat design studies is
> that the stiffer one makes the boat, the stronger the rig should be.
> According to Buehler, the stiffer is a boat, the more uncomfortable it
> becomes.
> Best, Pippo
>

Nice discussion. As a few of you know, I've done a pretty extensive
(for me at least) analysis of Micro's hydrostatic behaviour and
seaworthiness which I put on line on my Micro web site. I put the COG
on the waterline plane, and, also taking into account the flooding of
the bow and transom wells, the boat appears to have a fairly
substantial righting moment at 90 degrees (it vanishes at about 120
degrees). I'm (slowly) analytically computing Micro's COG, which is
coming out to be where I estimated it.
A nice thing that I've learned in my amateurish boat design studies is
that the stiffer one makes the boat, the stronger the rig should be.
According to Buehler, the stiffer is a boat, the more uncomfortable it
becomes.
Best, Pippo

ghc <ghart-@...> wrote:
original article:http://www.egroups.com/group/bolger/?start=3710

> I can note from experience that the Micro will recover nicely from a

> degree knockdown.
>
> Gregg Carlson
>
> >The discussion of the ultimate stabilty of box-sectioned boats isn't

going

> to deter me from builidng one, though. My current project, building
> Michalak's IMB has been canceled. It was really too small for my

needs.

> Instead, I've decided to go ahead and build a much more substantial

boat --

> Micro. I sent my order to PCB&F for plans on Monday. I do believe

Micro to

> be a special case of the box-boat, though, becuase of the heavy

external

> ballast. I doubt it has has a negative righting moment anywhere below

> degrees of heel. I hope I never find out first hand what 70 degrees

of heel

> feels like!
>
>

ghc <ghart-@...> wrote:
original article:http://www.egroups.com/group/bolger/?start=3707

> Chuck,
>
> Nice work, but, are you implying internal ballast won't add general

righting

> moment to flat-bottom boats or affect those righting curves?

Hi Greg,

No, but what I am implying is contained in a new set of study drawings
in the vault in a New Folder named Stability and the file named
compare.gif. What this does is compare apples with apples, that is the
same hull, ballasted and UNballasted, (of course, the unballasted hull
floats 3 inches higher than the ballasted hull). Anyway, the heeling
effects can be seen and pondered for both hulls, ballasted and
unballasted.

I'll also work this in to the page on Private Postings
(somehow--probably with a hyperlink).

I must say though that this subject of Stability (and stiffness) has
always been a tough one to verbalize and subsequently visualize, don't
you agree?

Cheers!

Chuck

In a message dated 3/15/2000 1:54:18 PM Eastern Standard Time,
jmbell@...writes:

<< Landroval's capsize could be partially attributed to her captain's
switching from a more forgiving round hulled Sea Pearl to the flat bottomed
MJ and his relative unfamliarity with that kind of behavior. (This is not
intended to be an insult! Please don't take it that way. There was certainly
a lot more to his story that still concerns me about the behavior of MJ.)

>>

No offense taken! There is a lot of difference between the sp21 and a sharpie
- ( both these boats designed for water ballast and leeboards not
withstanding) - The sp often sails with her rail under water and loves it -
The irony, of course - some martha janes have righted from a 90 degree
knockdown - no sea pearl can make that claim. steve

I can note from experience that the Micro will recover nicely from a 90
degree knockdown.

Gregg Carlson

>The discussion of the ultimate stabilty of box-sectioned boats isn't going

to deter me from builidng one, though. My current project, building
Michalak's IMB has been canceled. It was really too small for my needs.
Instead, I've decided to go ahead and build a much more substantial boat --
Micro. I sent my order to PCB&F for plans on Monday. I do believe Micro to
be a special case of the box-boat, though, becuase of the heavy external
ballast. I doubt it has has a negative righting moment anywhere below 70
degrees of heel. I hope I never find out first hand what 70 degrees of heel
feels like!

My experiences definitely confirm this, even with round bilged hulls. For instance, I've got an old, old tandem whitewater canoe which has a decidedly flat bottom even though the bilges are rounded. We've gone swimming plenty of times becuase it would not recover from heeling over too far. Once you get it past a certain point, no amount of leaning would right you. Newer WW canoe designs have much more rounded sections which sacrifices the immense primary stability of my flat bottomed canoe, but greatly enhances secondary stability. For instance, I used to have a great boat designed by my friend Nolan Whitesell that you could put the gunwale under and still have a positive righting moment. (It was also capable of being eskimo rolled, but that is another story.)

My flat bottomed Windsprint sharpie exhibited similar behavior to my old tandem canoe. Heel it past 35-40 degrees and you are in extreme danger of capsizing. My feelings are that it is difficult if not impossible to give an unballasted or internally ballasted flat bottomed hull a large range of positive stability regardless of whether or not you have round bilges or hard chines. For ulitmate stability hard chined v-bottomed boats do much better. And while I have no direct experience, I supect multichined hulls are even more forgiving. We should delude ourselves thinking that our flat-bottomed hulls are best for ultimate stability. Like my old canoe, they have lots of initial (or "form") stability which leads the unaware to believe they are in fact very stable, but they actually have little secondary or final stability which means they can be quite cranky when you get them on edge. My survival technique is to try to keep flat bottom boats to very small angles of heel. I suspect!
!
Landroval's capsize could be partially attributed to her captain's switching from a more forgiving round hulled Sea Pearl to the flat bottomed MJ and his relative unfamliarity with that kind of behavior. (This is not intended to be an insult! Please don't take it that way. There was certainly a lot more to his story that still concerns me about the behavior of MJ.)

The discussion of the ultimate stabilty of box-sectioned boats isn't going to deter me from builidng one, though. My current project, building Michalak's IMB has been canceled. It was really too small for my needs. Instead, I've decided to go ahead and build a much more substantial boat -- Micro. I sent my order to PCB&F for plans on Monday. I do believe Micro to be a special case of the box-boat, though, becuase of the heavy external ballast. I doubt it has has a negative righting moment anywhere below 70 degrees of heel. I hope I never find out first hand what 70 degrees of heel feels like!

John Bell
Kennesaw, GA

Peter V. wrote:

>

I'm sure that a some of the rest of you had the same reaction of
surprise that I did to the result that the limit of stability of the
unballasted Bolger box was only about 45 degrees. However, after some
thought and study of the diagrams, it occured to me that the limit of
stability on dry land was also about 45 degrees. If you were tipping it
on its side on dry land, when it go to about 45 degrees it would flop
the rest of the way onto its side. This seems intuitively correct.

Chuck,

Nice work, but, are you implying internal ballast won't add general righting
moment to flat-bottom boats or affect those righting curves?

We agree the weight of the boat acts through the CG, and the bouyant
force acts through the center of bouyancy (the centroid of the immersed
shape). They equal. At rest, they lie on a common vertical line; righting
moment (force x distance) results as they spread apart during heel. In
a "square" boat, the COB moves sideways pretty quickly.

In my hulls program, I place the CG in the geometric center (centroid) of
the total hull, a good approximation considering a Micro less ballast, for
instance, is roughly a box. (Sure, the bottom's heavier than the deck, but
then add the rig, etc.). Anyway, add some low (internal or external)
ballast and the CG slides down. In your 15 degree example, as the CG
slides lower in the boat with increasing ballast, it slides down and away
from the bouyant force to increase righting moment.

In addition to widening the moment arm, the boat (=bouyant force) is
heavier, the product (force x distance) being a bigger righting moment.

There is no difference statically between internal and external ballast,
except that external ballast can move the cg lower (given the same amount
of ballast).
In fact, there's no guarantee that external ballast would even move the CG
outside the boat (Micro?) - lowering it as much as possible is all you want
without making the boat to heavy to sail fast.

Agreed, a flat-bottom boat has a high initial form stability (due to
shape), but only because it is heavy. No weight - no righting moment.

A sketch is in the vault (ballast2.jpg).

Gregg Carlson

"peter vanderwaart" <pvander-@...> wrote:

> Great job!
>
> I have one question: how did you determine the all-important height of
> the center of gravity of the hull? Obviously, moving it a little lower
> would make a big difference.

Thanks Peter,

The answer is that some lines programs use an average hight of CG based
on a hull of a certain displacement and height of topsides. Location
of ballast isn't factored in but some programs allow the inclusion of
ballast later. In the program I used to do this study (Plyboats) all I
was interested in was a generalization showing how the forces developed
and moved around as the boat was heeled to various degrees--the program
figured the CG automatically. The hull used could be either a sailboat
or a power boat (in this case it's a sailboat). But, you are correct,
changing the CG can make a big difference.

Having said that though, the way to accurately calculate a CG for any
boat, is to add up the weights of the materials that make up the boat
above the LWL and those materials located below the LWL. These
calculations are put on a chart. Usually the weights above are stated
as positive numbers and the weights below are negative. Then, the
moments that are calculated for these weights are combined and if the
resulting distance is a positive number (length), the CG is that
distance above the LWL on the centerline if negative, the CG is located
whatever distance is below the LWL. Once that point is established, it
plays into the rest of the athwartships stability and stiffness
calculations and also into calculating longitudinal stability (trim)

>
> Here is a unrelated question about your private postings that has been
> pending for a while: Are you familiar with the writings on the Xacto
> blade planimeter by Weston Farmer? He published two pieces in National
> Fisherman some years ago. The first is reprinted in 'From my old
> boatshop.' The second, which describes experience and improvements,
> appeared in NF a little later, and has not been reprinted as far a I
> know.

Yes, I'm familiar with the planimeter in Westy's book, but not the
follow up article. However, you might be interested to know that a
year or two ago, Westy's son, (also Weston--he calls himself Wes)
bought plans for one of my boats from me for one of his friends to
build. I was kind of amazed--like with all the boat plans he has
around the house . . .!? Wes is a very nice guy and has one of his
father's old runabouts in vintage condition. Also last I heard,
Westy's wife(Bylo)is alive, nearly 95+ years old and at the time was
still sending out boat plan packages to customers every day.

Cheers!

Chuck

hwa-@...wrote:
original article:http://www.egroups.com/group/bolger/?start=3701

> Chuck - That was extremely impressive - but I confess it lost me.

Right from

> the beginning actually - did I read you correctly that internal

ballast will

> increase stability - but not increase resistance to heeling? I must

> missing something here. If you increase stablity aren't you

increasing

> resistance to heeling? What's the difference? Are you saying that a

boat will

> capsize at a particular degree of heel - and that what ballast ( or

increased

> ballast) is make it harder for the boat to reach that degree of

heel? Sorry

> to be dense on this - Steve

Steve,

Sorry about the confusion. I've done a little rewriting on the web
page and added a summary below the illustration along with a paragraph
mentioning "power to carry sail" calculations. Later I'll also add
these postings from the list to the end of the article.

Stability can only be increased by lowering the center of gravity or
increasing the metacentric height (or both). Lowering the center of
gravity (or raising the metacenter)ultimately lengthens the righting
arm as the center of buoyancy moves outboard when the hull is heeled,
increasing stability.

However, stiffness, the ability to resist heeling can be increased by
moving the ballast down, outside the hull and placing it on the end of
a long plate keel or enclosed inside a deep fin. However, this does not
increase the length of the GM, or the distance that the center of
buoyancy moves outboard at a given degree of heel.

Moving the ballast from the inside of the hull and down increases the
length of the lever. A longer lever increases the force required (in
foot pounds) to move the ballast to the side and upwards in a wider arc
as the pressure of the wind on the sails tries to make the hull pivot
around the center of gravity. The result is a stiffer boat, but
doesn't enhance or extend the characteristics of the stability curve.

This is the effect that designers are trying for with the current
configurations of 12 meter Cup boats, (60 ft. boats with 12 ft. draft,
and all the ballast in a torpedo shaped lead bulb at the end of a long
stainless steel plate (attached to the underside of the deck, BTW).
Also included are and the current Vendee Globe Ocean Racing boats the
likes of which that are designed by Jean Marie Finot.

Interestingly, too much stiffness sometimes can slow a boat down
because the hull is held too straight and upright and not allowed to
heel to the best operating angle (for that hull. Also in a properly
designed hull, heeling usually lengthens the LWL which increases the
potential hull speed. I've seen in years past where part of a lead
keel was cut off and bolted to the deck, or cabin sole depending. The
ballast/displacement ratio didn't change, the stability curve was
fundamentally the same, but the boat was made less stiff by reducing
the weight and length of the lever which in turn allowed the boat to
heel normally.

Does that make things clearer?

Thanks for the input Steve.

Chuck

Chuck - That was extremely impressive - but I confess it lost me. Right from
the beginning actually - did I read you correctly that internal ballast will
increase stability - but not increase resistance to heeling? I must be
missing something here. If you increase stablity aren't you increasing
resistance to heeling? What's the difference? Are you saying that a boat will
capsize at a particular degree of heel - and that what ballast ( or increased
ballast) is make it harder for the boat to reach that degree of heel? Sorry
to be dense on this - Steve

Chuck,
that was both educational and entertaining. Can you run the same
program with a 500lb lead plank keel? I am also curious about Micro's
#s in this area. "Curiosity killed the cat, but it had a good time
doing it."

David Jost
"too many boats, too little time"

> 14. Stability/Knockdown study for typical flat-bottomed square

chined

> plumb-sided hull forms as drawn by Phil Bolger.
>
> The URL for Private Postings is:
>http://members.xoom.com/_XOOM/merrellc1/index.html

Great job!

I have one question: how did you determine the all-important height of
the center of gravity of the hull? Obviously, moving it a little lower
would make a big difference.

Here is a unrelated question about your private postings that has been
pending for a while: Are you familiar with the writings on the Xacto
blade planimeter by Weston Farmer? He published two pieces in National
Fisherman some years ago. The first is reprinted in 'From my old
boatshop.' The second, which describes experience and improvements,
appeared in NF a little later, and has not been reprinted as far a I
know.

Peter

Hello All,

Because of some of the questions raised by the threads on Martha Jane
Stability and Stability and Safety, last week I took a little time and did a
comprehensive study consisting of about ten drawings and the attendant misc.
calculations and have posted them on Private Postings under item:

14. Stability/Knockdown study for typical flat-bottomed square chined
plumb-sided hull forms as drawn by Phil Bolger.

The URL for Private Postings is:
http://members.xoom.com/_XOOM/merrellc1/index.html

Click on Item number 14. Also linked with this study is a short discussion
of hull stability in general.

Rather than go into any of the conclusions reached, I'll let the article
speak for itself and hopefully this will answer some of the questions that
have been posed to date.

Hope this helps.

Cheers!

Chuck

***********************************
CHUCK MERRELL
MERRELL WATERCRAFT
P. O. Box 80264
Seattle, WA 98108-0264
(206) 764-1298
Email:chuck@...
Web Site:http://www.boatdesign.com
**********************************