[bolger] Re: Box keels
In a message dated 3/26/00 9:04:04 AM Central Standard Time,
dan.pike@...writes:
<< I was wondering if anyone has designed a box-keeled, water-ballasted
boat. >>
Pippo and Chuck Leinweber put me on to the following essays by Michalak in
response to a query on the subject of water ballast:
http://marina.fortunecitycom/breakwater/274/1998/0228/index.htm#Water
http://marina.fortunecitycom/breakwater/274/1998/0315/index.htm#Water
Ballast Details
Both articles are well worth a read, especially the second one. As a child I
had a disappointing experience with a water-ballasted model sailboat. The
model was a molded plastic sloop, which had delightfully (to my young eyes)
detailed features, and a generously deep and volumptuous hull. It featured a
deep, hollow keel which was filled with water for ballast. This did keep it
from tipping completely over, but its sailing performance was abysmal
compared to other very crude model sailboats of a my acquaintance which
consisted of little more than shaped planks with the benefit of lead keels.
Michalak's second article concisely explains at least part of why such
disappointment could have been expected by someone more knowledgeable than I
was. Basically the ballast did'nt do anything but add a little useful
momentum to forward progress until the model was practically on its beams'
end - while its seductively contoured hull offered much less initial form
stability than the crude, flat plank models.
dan.pike@...writes:
<< I was wondering if anyone has designed a box-keeled, water-ballasted
boat. >>
Pippo and Chuck Leinweber put me on to the following essays by Michalak in
response to a query on the subject of water ballast:
http://marina.fortunecitycom/breakwater/274/1998/0228/index.htm#Water
http://marina.fortunecitycom/breakwater/274/1998/0315/index.htm#Water
Ballast Details
Both articles are well worth a read, especially the second one. As a child I
had a disappointing experience with a water-ballasted model sailboat. The
model was a molded plastic sloop, which had delightfully (to my young eyes)
detailed features, and a generously deep and volumptuous hull. It featured a
deep, hollow keel which was filled with water for ballast. This did keep it
from tipping completely over, but its sailing performance was abysmal
compared to other very crude model sailboats of a my acquaintance which
consisted of little more than shaped planks with the benefit of lead keels.
Michalak's second article concisely explains at least part of why such
disappointment could have been expected by someone more knowledgeable than I
was. Basically the ballast did'nt do anything but add a little useful
momentum to forward progress until the model was practically on its beams'
end - while its seductively contoured hull offered much less initial form
stability than the crude, flat plank models.
Water ballast only works to lower the center of gravity of an existing
hull shape. If you add volume below the waterline to enclose your water
ballast, the volume will add bouyancy equal to the weight of the water,
plus that of the water displaced by the materials used. If the
materials are lighter than water you will end up worse off. If you heel
far enough to get the keel out of the water it's a different story, but
that's awfully far. You can make a tender hull that will never go all
the way over, but just to 80 degrees.
You could do something with steel shot, I suppose, and handle it with
buckets, but it will take some ingenuity to drain and recover.
"daniel pike" <dan.pik-@...> wrote:
original article:http://www.egroups.com/group/bolger/?start=3932
snip
hull shape. If you add volume below the waterline to enclose your water
ballast, the volume will add bouyancy equal to the weight of the water,
plus that of the water displaced by the materials used. If the
materials are lighter than water you will end up worse off. If you heel
far enough to get the keel out of the water it's a different story, but
that's awfully far. You can make a tender hull that will never go all
the way over, but just to 80 degrees.
You could do something with steel shot, I suppose, and handle it with
buckets, but it will take some ingenuity to drain and recover.
"daniel pike" <dan.pik-@...> wrote:
original article:http://www.egroups.com/group/bolger/?start=3932
snip
> I was wondering if anyone has designed a box-keeled, water-ballastedI
> boat. I know Bolger has designed one or two box keeled sailboats, but
> don't think they had water ballast. Seems to me there would be ahanging
> substantial increase in righting force by having the water tank
> down below the main hull. It would also be relatively easy to build, I
> think.
>
>snip
I read with great interest the thread on the self-righting ability of
the Martha Jane, not least because I have had the plans for that ship
for many years and hope to build one someday.
I was wondering if anyone has designed a box-keeled, water-ballasted
boat. I know Bolger has designed one or two box keeled sailboats, but I
don't think they had water ballast. Seems to me there would be a
substantial increase in righting force by having the water tank hanging
down below the main hull. It would also be relatively easy to build, I
think.
I suppose there must be some tremendous disadvantage to this, or there
would be boats built this way. Perhaps the box keel causes too much
drag, and is only good on a planing hull like the Microtrawler? But
then again, these boats are supposed to be very good at low speeds too.
Perhaps someone can comment on this.
Daniel Pike in Tromsoe, Norway, where we hit 2 metres of snow on the
ground today, which is about 6.5 ft for the metrically handicapped, and
I can walk on to the roof of my house, and Spring is but a dream...
the Martha Jane, not least because I have had the plans for that ship
for many years and hope to build one someday.
I was wondering if anyone has designed a box-keeled, water-ballasted
boat. I know Bolger has designed one or two box keeled sailboats, but I
don't think they had water ballast. Seems to me there would be a
substantial increase in righting force by having the water tank hanging
down below the main hull. It would also be relatively easy to build, I
think.
I suppose there must be some tremendous disadvantage to this, or there
would be boats built this way. Perhaps the box keel causes too much
drag, and is only good on a planing hull like the Microtrawler? But
then again, these boats are supposed to be very good at low speeds too.
Perhaps someone can comment on this.
Daniel Pike in Tromsoe, Norway, where we hit 2 metres of snow on the
ground today, which is about 6.5 ft for the metrically handicapped, and
I can walk on to the roof of my house, and Spring is but a dream...