Re: mj ballast
Guy's,
How would one go about adding a 500lb steel plate to MJ's bottom?
I would think that you could have the plate cut to the exact
shape/outline of the bottom of the hull but curious to know just how
it would it be fastened?
Cheers,
Mrfirkin
How would one go about adding a 500lb steel plate to MJ's bottom?
I would think that you could have the plate cut to the exact
shape/outline of the bottom of the hull but curious to know just how
it would it be fastened?
Cheers,
Mrfirkin
--- In bolger@y..., gertyjandp@f... wrote:
> Steve, Plywood floats (less dense than water) so adding it on the
> bottom would be negative from a ballast standpoint The increase in
> buoyancy would more than offset the lowering of the cg. That's why
> PCB prescribed a steel plate not just a thicker plywood bottom. As
you
> discovered it is still very effective to have the added weight
inside
> the hull, just not quite as efficient as outside. I would be
tempted
> to build MJ with a steel bottom to start with. It would really be
> bullet proof!
>
> --- In bolger@y..., hwal@a... wrote:
> > Just a thought about the new martha janes being built - the
upgrade
> calls for
> > a 500 lb. steel plate to be attached to the bottom of the boat.
> That was
> > beyond my ability so we ended up with extra inside ballast - Just
> a thought - Maybe it's easier to attach a
> > 500 lbs steel plate to the bottom - Steve Anderson ( MJ
> Landroval- upgraded)
>
> Or just put all the steel or (lead which is more dense) inside the
> ballast tanks. 500 pounds of steel in the tanks would still leave
> about 80% of the tank volume available for water ballast (400
lbs?),
> sand or whatever.
>
> John Gerty - MJ Zephyr
In a message dated 2/14/2001 11:36:15 AM Eastern Standard Time,
gunner@...writes:
<< Regarding Steel Plate Ballast - I am still in the planning and part
gathering stages of building but have just priced rolling 4 pcs. of
1/4" x 2' x 4' common plate steel to the MJ bottom Radius taken from
the plans and plan to sandwich the resulting 1/2" of steel, glued to
1/4" plywood with PL400 and covered with a bottom layer of 1/4"
plywood to come out even with the 2 pcs of 1/2" ply fore and aft of
the midships ballast; All covered by fiberglass. >>
I'm not quite sure I follow this - Are you saying here that midship - where
the ballast will be located - the steel plate will only have 1/4" of plywood
above it and below it? This is instead of attaching the steel plate on the
outside of 1" of plywood? Steve
gunner@...writes:
<< Regarding Steel Plate Ballast - I am still in the planning and part
gathering stages of building but have just priced rolling 4 pcs. of
1/4" x 2' x 4' common plate steel to the MJ bottom Radius taken from
the plans and plan to sandwich the resulting 1/2" of steel, glued to
1/4" plywood with PL400 and covered with a bottom layer of 1/4"
plywood to come out even with the 2 pcs of 1/2" ply fore and aft of
the midships ballast; All covered by fiberglass. >>
I'm not quite sure I follow this - Are you saying here that midship - where
the ballast will be located - the steel plate will only have 1/4" of plywood
above it and below it? This is instead of attaching the steel plate on the
outside of 1" of plywood? Steve
Regarding Steel Plate Ballast - I am still in the planning and part
gathering stages of building but have just priced rolling 4 pcs. of
1/4" x 2' x 4' common plate steel to the MJ bottom Radius taken from
the plans and plan to sandwich the resulting 1/2" of steel, glued to
1/4" plywood with PL400 and covered with a bottom layer of 1/4"
plywood to come out even with the 2 pcs of 1/2" ply fore and aft of
the midships ballast; All covered by fiberglass.
Quoted price for cutting and rolling the steel was $120 labor and $120
steel.
gathering stages of building but have just priced rolling 4 pcs. of
1/4" x 2' x 4' common plate steel to the MJ bottom Radius taken from
the plans and plan to sandwich the resulting 1/2" of steel, glued to
1/4" plywood with PL400 and covered with a bottom layer of 1/4"
plywood to come out even with the 2 pcs of 1/2" ply fore and aft of
the midships ballast; All covered by fiberglass.
Quoted price for cutting and rolling the steel was $120 labor and $120
steel.
> > This is mr. bolger's "upgrade" of the design. He recommends the500
> lb steelof
> > plate in addition to the water ballast - for a total of 1000 lbs
> ballast -
> > and the boat is much stiffer with more ballast. Steve
Here is yet another way to look at it. Imagine a boat built with 1/8 inch
plywood. Add the weight difference betweeen 1/8 inch and , say 3/4 inch - and
she settles lower in the water. Think of the outside shape as constant, then
add anything that weighs more than air, and it settles lower (ballast!). If
however, you fill the inside with water, it is another story.
Jim
plywood. Add the weight difference betweeen 1/8 inch and , say 3/4 inch - and
she settles lower in the water. Think of the outside shape as constant, then
add anything that weighs more than air, and it settles lower (ballast!). If
however, you fill the inside with water, it is another story.
Jim
> Think of it like this: The plywood has weight, so it sinks the boatfurther
> into the water. Therefore, it displaces water, not air.In your mind, are you adding the additional plywood inside the boat,
or outside the boat? (It doesn't matter much actually, but it changes
the way you think about it.) I think it is more useful to think of
putting in on the outside. Its buoyancy will lift the boat just a
bit, but when the boat is well heeled and a large part of the bottom
is out of the water, the weight will act as ballast, tending to right
her.
Peter
Here's another way to think of it. Imagine a stack of 48 sheets of
plywood bundled together and floating. It floats pretty high. To
sink it requires real effort i.e. weight. Imagine putting it under a
MJ. She'd float higher, if she 'floated' at all. If she were
actually attached to the plywood on her bottom, she'd likely tip over
becasue the center of gravity was now so high. She wouldn't sink.
She couldn't sink! But she sure wouldn't sail.
Now, put that same 48 sheets inside Martha Jane....
John O'Neill
plywood bundled together and floating. It floats pretty high. To
sink it requires real effort i.e. weight. Imagine putting it under a
MJ. She'd float higher, if she 'floated' at all. If she were
actually attached to the plywood on her bottom, she'd likely tip over
becasue the center of gravity was now so high. She wouldn't sink.
She couldn't sink! But she sure wouldn't sail.
Now, put that same 48 sheets inside Martha Jane....
John O'Neill
--- In bolger@y..., hwal@a... wrote:
> In a message dated 2/9/01 10:12:59 PM Eastern Standard Time,
> StepHydro@a... writes:
>
> << This isn't a rigorous argument, but it is correct as far as it
goes, and
> it
> might set your thinking into a different track.
> >>
> Maybe I should never have brought this subject up... it reminds me
of another
> discussion ( either here or on the seapearl list) as to whether
water ballast
> had any weight when it was underwater. Steve
--- In bolger@y..., "Chuck Leinweber" <chuck@d...> wrote:
original hull would lower the center of gravity (CG) and act as
ballast.Since the external hull geometry is not affected, the center
of buoyancy (CB) which depends on geometry not weight, would remain
the same. Adding material to the bottom OUTSIDE the hull changes the
geometry and CB as well as CG . If the material is less dense than
water the CB will change more than the CG and in the wrong direction.
Stability depends on the righting moment produced by the mass of the
boat acting through the CG, the buoyant force acting through the CB
and the lever arm separating the two forces. The CB changes with
angle of heel (hull geometry) which is what makes stability curve
calculations so complicated.
In either case the total mass (or weight) will increase the same
amount whether the addition is inside or outside the original
configuration. If the boat is still floating the buoyant force will
also increase by the same amount. (The mass of water displaced must
equal the mass of the floating object.) If buoyant material like
plywood is added to the bottom outside the hull its volume will
displace more water than its mass requires for equilibrium, and the
original hull must actually be less immersed than before to
compensate. Does this make sense or just more headaches?
John G.
> > Steve, Plywood floats (less dense than water) so adding it on thejust to
> > bottom would be negative from a ballast standpoint The increase in
> > buoyancy would more than offset the lowering of the cg...<snip>
> >
> > John Gerty - MJ Zephyr
> >
> This ballast thing is hard to get a grip on. It makes my head hurt
> think about it. It seems to me that a double bottom would notincrease
> buoyancy because the extra sheet of ply is much denser than the airthat it
> replaces. It would lower the CG, and the weight of the boat, andthat means
> it is ballast.Hi Chuck, Adding plywood or any other weight on the bottom INSIDE the
>
> Chuck
original hull would lower the center of gravity (CG) and act as
ballast.Since the external hull geometry is not affected, the center
of buoyancy (CB) which depends on geometry not weight, would remain
the same. Adding material to the bottom OUTSIDE the hull changes the
geometry and CB as well as CG . If the material is less dense than
water the CB will change more than the CG and in the wrong direction.
Stability depends on the righting moment produced by the mass of the
boat acting through the CG, the buoyant force acting through the CB
and the lever arm separating the two forces. The CB changes with
angle of heel (hull geometry) which is what makes stability curve
calculations so complicated.
In either case the total mass (or weight) will increase the same
amount whether the addition is inside or outside the original
configuration. If the boat is still floating the buoyant force will
also increase by the same amount. (The mass of water displaced must
equal the mass of the floating object.) If buoyant material like
plywood is added to the bottom outside the hull its volume will
displace more water than its mass requires for equilibrium, and the
original hull must actually be less immersed than before to
compensate. Does this make sense or just more headaches?
John G.
In a message dated 2/9/01 10:12:59 PM Eastern Standard Time,
StepHydro@...writes:
<< This isn't a rigorous argument, but it is correct as far as it goes, and
it
might set your thinking into a different track.
discussion ( either here or on the seapearl list) as to whether water ballast
had any weight when it was underwater. Steve
StepHydro@...writes:
<< This isn't a rigorous argument, but it is correct as far as it goes, and
it
might set your thinking into a different track.
>>Maybe I should never have brought this subject up... it reminds me of another
discussion ( either here or on the seapearl list) as to whether water ballast
had any weight when it was underwater. Steve
In a message dated 2/9/01 9:59:42 PM Eastern Standard Time,
chuck@...writes:
<< This ballast thing is hard to get a grip on. It makes my head hurt just to
think about it. It seems to me that a double bottom would not increase
buoyancy because the extra sheet of ply is much denser than the air that it
replaces. It would lower the CG, and the weight of the boat, and that means
it is ballast.
other layers are essentially inside the boat and so they are just more
ballast - if the bottom was only one layer of plywood thick and inside the
boat you were carrying 1000 lbs of plywood - it seems to me that you have
1000 lbs of ballast....:-) Steve
chuck@...writes:
<< This ballast thing is hard to get a grip on. It makes my head hurt just to
think about it. It seems to me that a double bottom would not increase
buoyancy because the extra sheet of ply is much denser than the air that it
replaces. It would lower the CG, and the weight of the boat, and that means
it is ballast.
>>Maybe only the bottom layer ( the one in the water) is bouyant - and the
other layers are essentially inside the boat and so they are just more
ballast - if the bottom was only one layer of plywood thick and inside the
boat you were carrying 1000 lbs of plywood - it seems to me that you have
1000 lbs of ballast....:-) Steve
In a message dated 02/09/2001 9:<BR59:<BR41 PM
Eastern Standard ,chuck@...writes:
Think of it like this: The plywood has weight, so it sinks the boat further
into the water. Therefore, it displaces water, not air. Now, it weighs less
than the water (per cubic ft), so it isn't ballast, it is anti-ballast.
This isn't a rigorous argument, but it is correct as far as it goes, and it
might set your thinking into a different track.
Cheers/Carron
Eastern Standard ,chuck@...writes:
> the extra sheet of ply is much denser than the air that it replaces.Chuck,
Think of it like this: The plywood has weight, so it sinks the boat further
into the water. Therefore, it displaces water, not air. Now, it weighs less
than the water (per cubic ft), so it isn't ballast, it is anti-ballast.
This isn't a rigorous argument, but it is correct as far as it goes, and it
might set your thinking into a different track.
Cheers/Carron
> Steve, Plywood floats (less dense than water) so adding it on theThis ballast thing is hard to get a grip on. It makes my head hurt just to
> bottom would be negative from a ballast standpoint The increase in
> buoyancy would more than offset the lowering of the cg...<snip>
>
> John Gerty - MJ Zephyr
>
think about it. It seems to me that a double bottom would not increase
buoyancy because the extra sheet of ply is much denser than the air that it
replaces. It would lower the CG, and the weight of the boat, and that means
it is ballast.
Chuck
In a message dated 2/9/2001 4:32:58 PM Eastern Standard Time,
gertyjandp@...writes:
<< The increase in
buoyancy would more than offset the lowering of the cg. >>
Hi John - Saw some pictures of you on the big cruise this summer. But I don't
remember who it was showed me the pictures. I just think that it might be
easier for some to put lead between some layers of plywood than muscle a 500
lb. steel plate into place. But - maybe some people deal with heavy weight
better than I do. So adding plywood would increase the amount of ballast
needed eh? Steve
gertyjandp@...writes:
<< The increase in
buoyancy would more than offset the lowering of the cg. >>
Hi John - Saw some pictures of you on the big cruise this summer. But I don't
remember who it was showed me the pictures. I just think that it might be
easier for some to put lead between some layers of plywood than muscle a 500
lb. steel plate into place. But - maybe some people deal with heavy weight
better than I do. So adding plywood would increase the amount of ballast
needed eh? Steve
Steve, Plywood floats (less dense than water) so adding it on the
bottom would be negative from a ballast standpoint The increase in
buoyancy would more than offset the lowering of the cg. That's why
PCB prescribed a steel plate not just a thicker plywood bottom. As you
discovered it is still very effective to have the added weight inside
the hull, just not quite as efficient as outside. I would be tempted
to build MJ with a steel bottom to start with. It would really be
bullet proof!
bottom would be negative from a ballast standpoint The increase in
buoyancy would more than offset the lowering of the cg. That's why
PCB prescribed a steel plate not just a thicker plywood bottom. As you
discovered it is still very effective to have the added weight inside
the hull, just not quite as efficient as outside. I would be tempted
to build MJ with a steel bottom to start with. It would really be
bullet proof!
--- In bolger@y..., hwal@a... wrote:
> Just a thought about the new martha janes being built - the upgrade
calls for
> a 500 lb. steel plate to be attached to the bottom of the boat.
That was
> beyond my ability so we ended up with extra inside ballast - Just
a thought - Maybe it's easier to attach a
> 500 lbs steel plate to the bottom - Steve Anderson ( MJ
Landroval- upgraded)
Or just put all the steel or (lead which is more dense) inside the
ballast tanks. 500 pounds of steel in the tanks would still leave
about 80% of the tank volume available for water ballast (400 lbs?),
sand or whatever.
John Gerty - MJ Zephyr
In a message dated 2/9/2001 4:55:40 AM Eastern Standard Time,
colmooney@...writes:
<< A good idea, a similar idea was built into a MJ several years
ago. The owner built the bottom as planned, then attached a
sacrificial hardwood bottom. He then felt comfortable driving her up
on rocky beaches when the need arose without causing damage to the
plywood. >>
I solved that concern with extra glass on the forward 2/3 of the bottom (
three layers of 9oz cloth) and then graphite. Very solid. Steve
colmooney@...writes:
<< A good idea, a similar idea was built into a MJ several years
ago. The owner built the bottom as planned, then attached a
sacrificial hardwood bottom. He then felt comfortable driving her up
on rocky beaches when the need arose without causing damage to the
plywood. >>
I solved that concern with extra glass on the forward 2/3 of the bottom (
three layers of 9oz cloth) and then graphite. Very solid. Steve
-
Steve,
A good idea, a similar idea was built into a MJ several years
ago. The owner built the bottom as planned, then attached a
sacrificial hardwood bottom. He then felt comfortable driving her up
on rocky beaches when the need arose without causing damage to the
plywood. Probably not necessary under normal conditions, but the
owner sailed the boat around the top end of Australia where there are
strong currents, not always reliable charts, let alone ports. He
also crossed Bass Straight to Tasmania, where he was knocked over 2
or 3 times in a storm. The extra weight in the bottom would have no
doubt helped to right her.
This could be a very good idea if extended cruising is
envisaged. With the hardwood and a few lead ingots inserted
amidships, the recommended extra 500 lbs would be easily made up. I
would still fibreglass over the second plywood layer to protect
against moisture ingress, then once again over the hardwood. After a
long cruise or a severe grounding, it would be relatively simple to
get under her on the trailer and bog up any "dings".
***********
P.S. - re that message about leeboards on Navigator, Roger Keys of
South Australia's Micro originally was built with Leeboards and water
ballast. He has converted now I believe to the original keel.
Regards,
Col
-- In bolger@y..., hwal@a... wrote:
Steve,
A good idea, a similar idea was built into a MJ several years
ago. The owner built the bottom as planned, then attached a
sacrificial hardwood bottom. He then felt comfortable driving her up
on rocky beaches when the need arose without causing damage to the
plywood. Probably not necessary under normal conditions, but the
owner sailed the boat around the top end of Australia where there are
strong currents, not always reliable charts, let alone ports. He
also crossed Bass Straight to Tasmania, where he was knocked over 2
or 3 times in a storm. The extra weight in the bottom would have no
doubt helped to right her.
This could be a very good idea if extended cruising is
envisaged. With the hardwood and a few lead ingots inserted
amidships, the recommended extra 500 lbs would be easily made up. I
would still fibreglass over the second plywood layer to protect
against moisture ingress, then once again over the hardwood. After a
long cruise or a severe grounding, it would be relatively simple to
get under her on the trailer and bog up any "dings".
***********
P.S. - re that message about leeboards on Navigator, Roger Keys of
South Australia's Micro originally was built with Leeboards and water
ballast. He has converted now I believe to the original keel.
Regards,
Col
-- In bolger@y..., hwal@a... wrote:
> In a message dated 2/8/2001 11:52:38 AM Eastern Standard Time,is it
> giuseppe.bianco@a... writes:
>
> << Steve, I'm curious: would the water ballast still be there, or
> just replaced by the steel plate? >>lb steel
>
> This is mr. bolger's "upgrade" of the design. He recommends the 500
> plate in addition to the water ballast - for a total of 1000 lbs ofballast -
> and the boat is much stiffer with more ballast. Steve
In a message dated 2/8/2001 11:52:38 AM Eastern Standard Time,
giuseppe.bianco@...writes:
<< Steve, I'm curious: would the water ballast still be there, or is it
just replaced by the steel plate? >>
This is mr. bolger's "upgrade" of the design. He recommends the 500 lb steel
plate in addition to the water ballast - for a total of 1000 lbs of ballast -
and the boat is much stiffer with more ballast. Steve
giuseppe.bianco@...writes:
<< Steve, I'm curious: would the water ballast still be there, or is it
just replaced by the steel plate? >>
This is mr. bolger's "upgrade" of the design. He recommends the 500 lb steel
plate in addition to the water ballast - for a total of 1000 lbs of ballast -
and the boat is much stiffer with more ballast. Steve
Steve, I'm curious: would the water ballast still be there, or is it
just replaced by the steel plate? If that plate is just additional
ballast, this seems to me a pretty drastic modification of the design
stability figure. Maybe you remember that some months ago I defined
MJ as an "inherently tender" design in this very group...
Best, Pippo
just replaced by the steel plate? If that plate is just additional
ballast, this seems to me a pretty drastic modification of the design
stability figure. Maybe you remember that some months ago I defined
MJ as an "inherently tender" design in this very group...
Best, Pippo
--- In bolger@y..., hwal@a... wrote:
> Just a thought about the new martha janes being built - the upgrade
calls for
> a 500 lb. steel plate to be attached to the bottom of the boat.
That was
> beyond my ability so we ended up with extra inside ballast - but I
was
> thinking about this in terms of new construction - Here's what I
thought:
>
> Since mj is now recommended to have that extra 500 lbs on the
bottom - I
> wonder if it makes any sense for new builders to add additional
layers of
> plywood on the bottom? If my mj was still under construction and
upside down
> - I would be tempted to use 3 layers of 1/2" instead of 2 and use
lead for
> the extra ballast and sandwich it between the first and third layer
of
> plywood. Or 4 layers? I wonder how much you could reduce the weight
of the
> steel or lead if you had a full two inches of plywood on the bottom
with the
> ballast sandwiched between? Just a thought - Maybe it's easier to
attach a
> 500 lbs steel plate to the bottom - Steve Anderson ( MJ Landroval-
upgraded)
Just a thought about the new martha janes being built - the upgrade calls for
a 500 lb. steel plate to be attached to the bottom of the boat. That was
beyond my ability so we ended up with extra inside ballast - but I was
thinking about this in terms of new construction - Here's what I thought:
Since mj is now recommended to have that extra 500 lbs on the bottom - I
wonder if it makes any sense for new builders to add additional layers of
plywood on the bottom? If my mj was still under construction and upside down
- I would be tempted to use 3 layers of 1/2" instead of 2 and use lead for
the extra ballast and sandwich it between the first and third layer of
plywood. Or 4 layers? I wonder how much you could reduce the weight of the
steel or lead if you had a full two inches of plywood on the bottom with the
ballast sandwiched between? Just a thought - Maybe it's easier to attach a
500 lbs steel plate to the bottom - Steve Anderson ( MJ Landroval- upgraded)
a 500 lb. steel plate to be attached to the bottom of the boat. That was
beyond my ability so we ended up with extra inside ballast - but I was
thinking about this in terms of new construction - Here's what I thought:
Since mj is now recommended to have that extra 500 lbs on the bottom - I
wonder if it makes any sense for new builders to add additional layers of
plywood on the bottom? If my mj was still under construction and upside down
- I would be tempted to use 3 layers of 1/2" instead of 2 and use lead for
the extra ballast and sandwich it between the first and third layer of
plywood. Or 4 layers? I wonder how much you could reduce the weight of the
steel or lead if you had a full two inches of plywood on the bottom with the
ballast sandwiched between? Just a thought - Maybe it's easier to attach a
500 lbs steel plate to the bottom - Steve Anderson ( MJ Landroval- upgraded)