I read on the sailnet site a question about boat weight. According to the specs, my 1994 C320 should displace 11,700 pounds. Each year it has been crane-weighed at about 14,000 lb. This an empty boat with no mast. At our club, the maximum crane weight for a boat is 10,000 lb. If you are over this, commercial cranes come in for the day and launch all the heavy boats. If this is a common finding, someone might think their boat is within local crane limits and might actually be dangerously over the safety capacity of their crane. Anyone else have big discrepencies like this ?
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Catalina owners have reported higher weights
I have seen Catalina 27 owners report higher than stated weights for their boats. I think my Cat is suppose to weight around 6,500; however one chap said the crane weight on his showed around 7,400. He was guessing, but thought Catalina might have weight the boat without motor, sails, cushions, etc. In other words, the dry weight of a bare bones boat, then don't forget all the items you might have added to your boat as well to increase the weight. And was the water holding tank drained?
I have seen Catalina 27 owners report higher than stated weights for their boats. I think my Cat is suppose to weight around 6,500; however one chap said the crane weight on his showed around 7,400. He was guessing, but thought Catalina might have weight the boat without motor, sails, cushions, etc. In other words, the dry weight of a bare bones boat, then don't forget all the items you might have added to your boat as well to increase the weight. And was the water holding tank drained?
are their scales ever calibrated?
It is probably true that the boats weight if probably a dry weight without much aboard (sails, anchors, water, fuel etc.) I am wondering if they actually have the scales calibrated on these cranes?
It is probably true that the boats weight if probably a dry weight without much aboard (sails, anchors, water, fuel etc.) I am wondering if they actually have the scales calibrated on these cranes?
Quick Note:
O'Day listed the sailing displacement for my 25 with five persons @160# each.That is 800#. It listed the sailing displacement at 4782# and the minimum trailering weight at 4007#. You might want to double check Catalina's specs or even call them . If you call them make sure you let us know the findings. You said empty, do you mean nothing at all? I removed 250# from my boat last week end. 1- 150 genoa, 1- bruce anchor with 30' of chain, 1- boom tent, 1- spool of 1/2" nylon line ( about 400' on the spool)and 1- 8'-6" liferaft. I also have canned goods, fire extinguishers, 2 batteries, cooking equipment and alot of stuff for creature and wife comfort. r.w.landau
O'Day listed the sailing displacement for my 25 with five persons @160# each.That is 800#. It listed the sailing displacement at 4782# and the minimum trailering weight at 4007#. You might want to double check Catalina's specs or even call them . If you call them make sure you let us know the findings. You said empty, do you mean nothing at all? I removed 250# from my boat last week end. 1- 150 genoa, 1- bruce anchor with 30' of chain, 1- boom tent, 1- spool of 1/2" nylon line ( about 400' on the spool)and 1- 8'-6" liferaft. I also have canned goods, fire extinguishers, 2 batteries, cooking equipment and alot of stuff for creature and wife comfort. r.w.landau
Displacement IS NOT boat weight
Displacement weight is the estimated weight of the water displaced by your boat when she sits in the water. My 34 has a displacement of 11,200 according to Hunter and weighs in at 15,500 (fully loaded & with mast on) on the crane. My club also gets a commercial crane for heavier boats, but owners would be well advised to check actual weight before getting lifted at a marina or club. Trailers are generally matched to the boat by boat manufacturer, but again if unsure it would be wise to get the boat's actual weight at a scale and then make sure the trailer is rated for that.
Displacement weight is the estimated weight of the water displaced by your boat when she sits in the water. My 34 has a displacement of 11,200 according to Hunter and weighs in at 15,500 (fully loaded & with mast on) on the crane. My club also gets a commercial crane for heavier boats, but owners would be well advised to check actual weight before getting lifted at a marina or club. Trailers are generally matched to the boat by boat manufacturer, but again if unsure it would be wise to get the boat's actual weight at a scale and then make sure the trailer is rated for that.
Displacement does equal weight at rest
If the weight of the water displaced by the boat (the bouyant force) is less than the weight of the vessel and all aboard, then would the boat not sink? If the boat is at rest (not sinking) the sum of all forces equals zero which means displacment must equal weight. If not, Physics ain't right.
If the weight of the water displaced by the boat (the bouyant force) is less than the weight of the vessel and all aboard, then would the boat not sink? If the boat is at rest (not sinking) the sum of all forces equals zero which means displacment must equal weight. If not, Physics ain't right.
Claude's right
Ol' Archimedes said that the weight of an object IN WATER is equal to the weight of the WATER it displaces. Displacement weight is the weight of the water displaced, not the weight of the boat. The weight of the boat is, indeed, different. It floats because of the difference in density per foot cubed between the volume of the boat and the volume of water displaced.
Ol' Archimedes said that the weight of an object IN WATER is equal to the weight of the WATER it displaces. Displacement weight is the weight of the water displaced, not the weight of the boat. The weight of the boat is, indeed, different. It floats because of the difference in density per foot cubed between the volume of the boat and the volume of water displaced.
why do logs float
??? you can't lift it on land, but it floats in water. claude is correct, the difference in density is the key.
??? you can't lift it on land, but it floats in water. claude is correct, the difference in density is the key.
Oops
I feel compeled to chime in here as an engineer. Jared is quite correct in his analysis. The sum of the forces on an object in equilibrium must sum to zero. The weight of a boat in water is the same as the weight of a boat on land (neglecting the very small change in gravity between the two). The weight of the water that a boat displaces is in fact equal to the weight of the boat. We call that displacement. Now there is another term often called the volume of displacement which is not the weight of the water displaced but the volume of water displaced (measured in something like cubic feet instead of pounds). The volume of displacement is clearly very different from the volume of the boat due to the overall density difference. After all boats are mostly air after you account for the fiberglass, wood, and metal. Their average density is much less than that of water thankfully or they would sink.
I feel compeled to chime in here as an engineer. Jared is quite correct in his analysis. The sum of the forces on an object in equilibrium must sum to zero. The weight of a boat in water is the same as the weight of a boat on land (neglecting the very small change in gravity between the two). The weight of the water that a boat displaces is in fact equal to the weight of the boat. We call that displacement. Now there is another term often called the volume of displacement which is not the weight of the water displaced but the volume of water displaced (measured in something like cubic feet instead of pounds). The volume of displacement is clearly very different from the volume of the boat due to the overall density difference. After all boats are mostly air after you account for the fiberglass, wood, and metal. Their average density is much less than that of water thankfully or they would sink.
You can lift logs on land...
if 4 other people help you lift it. However, the weight of the log has not changed. There is just another external force reducing the amount of force you must apply to lift it. If the log is in the water, the buoyant force of the water is assisting you. But I can assure you the weight of the log has not changed. If it had, I know several people who are currently on diets who would be spending much of there natural lives in the water trying to reduce their weight.
if 4 other people help you lift it. However, the weight of the log has not changed. There is just another external force reducing the amount of force you must apply to lift it. If the log is in the water, the buoyant force of the water is assisting you. But I can assure you the weight of the log has not changed. If it had, I know several people who are currently on diets who would be spending much of there natural lives in the water trying to reduce their weight.
Density
The difference in density has nothing to do with the weight of the boat and the weight of the water displaced - only the amount it floats or if it sinks. The difference in the density only determines "how much" it floats. For example, if the boat was a round ball that was 1 cubic foot in volume and weighed 62.4 lbs, then it would just sit there in water (assuming fresh water whose specific gravity is 62.4 lbs/cu.ft). If it weighted 31.2 lbs, then it would only diplace 31.2 lbs of water, thus be half submerged (and still displacing the same amount of water). If you lifted it, initially it would be easy and then get heavier as you reduced the bouyant force by reducing the "helping force" of the water displaced. Sorry for arguing so much, but I am an engineer too and we just can't help it - we are nerds like that.
The difference in density has nothing to do with the weight of the boat and the weight of the water displaced - only the amount it floats or if it sinks. The difference in the density only determines "how much" it floats. For example, if the boat was a round ball that was 1 cubic foot in volume and weighed 62.4 lbs, then it would just sit there in water (assuming fresh water whose specific gravity is 62.4 lbs/cu.ft). If it weighted 31.2 lbs, then it would only diplace 31.2 lbs of water, thus be half submerged (and still displacing the same amount of water). If you lifted it, initially it would be easy and then get heavier as you reduced the bouyant force by reducing the "helping force" of the water displaced. Sorry for arguing so much, but I am an engineer too and we just can't help it - we are nerds like that.
the Royal Pachyderm
I remember reading a story about a potentate of ancient times that wanted to know the weight of the Royal Pachyderm. There was no scale with sufficient capacity. One of Jared’s predecessors came along with this solution: Pull the Royal Barge alongside the Royal Warf, load the beast and mark the new waterline on the hull. Offload the elephant and fill the barge with stones, which could be weighed, until the new waterline was reached again. Offload the stones, weigh them individually, add the tally and voila - - - the Royal Answer!!! Now, is Claude going to tell His Highness that they got it wrong? Paul s/v The Lord Nelson
I remember reading a story about a potentate of ancient times that wanted to know the weight of the Royal Pachyderm. There was no scale with sufficient capacity. One of Jared’s predecessors came along with this solution: Pull the Royal Barge alongside the Royal Warf, load the beast and mark the new waterline on the hull. Offload the elephant and fill the barge with stones, which could be weighed, until the new waterline was reached again. Offload the stones, weigh them individually, add the tally and voila - - - the Royal Answer!!! Now, is Claude going to tell His Highness that they got it wrong? Paul s/v The Lord Nelson
More info
The only way displacement would differ from weight is in the case of any object with a density greater than water. In that case the object would sink and we would not know its actual weight. We would just know that it was more dense than water. Displacement and boat weight for a FLOATING boat have to be equal. Yep. I'm also an engineer.
The only way displacement would differ from weight is in the case of any object with a density greater than water. In that case the object would sink and we would not know its actual weight. We would just know that it was more dense than water. Displacement and boat weight for a FLOATING boat have to be equal. Yep. I'm also an engineer.
One other note.
Yep, an object weighs as much as the water that it displaces, unless it sinks, and then it weighs more than the water that it displaces, that's why it sinks. A solid bar of lead or gold weigh much more than the water that they displace.
Yep, an object weighs as much as the water that it displaces, unless it sinks, and then it weighs more than the water that it displaces, that's why it sinks. A solid bar of lead or gold weigh much more than the water that they displace.
Larry
Funny one Larry, your post was not there when I submitted mine (notice the times are the same) I too, noticed that one factor that had not been addressed. Nope, I am not an engineer, nor do I play one on the internet, but I did take a lot of physics classes.
Funny one Larry, your post was not there when I submitted mine (notice the times are the same) I too, noticed that one factor that had not been addressed. Nope, I am not an engineer, nor do I play one on the internet, but I did take a lot of physics classes.
thank you all
It's great to post a question that has stimulated the interest of so many qualified sailors. That's what these forums are all about. The humour is also appreciated !
It's great to post a question that has stimulated the interest of so many qualified sailors. That's what these forums are all about. The humour is also appreciated !
rocks and elephants
neither the elephant or the rocks were in the water, but it does beg the question of how much the barge weighed and what was the displacement. i am also and engineer. i may not be a very good one, but i think what has not changed is the mass of the boat. the defining elements here are mass, density, and gravity. if i were suspended from a load scale and reached into the water to lift a rock just off of the bottom, the resulting load on the scale would be "x". if i then continued lifting the rock until it was out of the water, the resulting load would be greater than "x" due to the difference in density of air and water. go figure. it still is a little confusing. rt
neither the elephant or the rocks were in the water, but it does beg the question of how much the barge weighed and what was the displacement. i am also and engineer. i may not be a very good one, but i think what has not changed is the mass of the boat. the defining elements here are mass, density, and gravity. if i were suspended from a load scale and reached into the water to lift a rock just off of the bottom, the resulting load on the scale would be "x". if i then continued lifting the rock until it was out of the water, the resulting load would be greater than "x" due to the difference in density of air and water. go figure. it still is a little confusing. rt
How empty is empty
Are you saying the boat is completley empty, no fuel, water, pot & pan, batteries, lines, clothes, cleaning supplies, sheets, ect.?
Are you saying the boat is completley empty, no fuel, water, pot & pan, batteries, lines, clothes, cleaning supplies, sheets, ect.?
Engineers have it right!
Yep, another engineer chiming in ... displacement weight should equal the weight lifted by the crane. So, why is there a discrepancy? Maybe some lift operators unscrupulously have erroneous scales so that they can overcharge (if they are paid by weight). Look for certification to be protected. Maybe boat manufacturers are notoriously inaccurate? IS there and answer? Sorry jtaylor, Whisky II, and Claude, your arguments are misleading. The weight of an object submersed in water is its actual weight MINUS the weight of water it displaces. My mooring block is a cubic yard of concrete and it weighs about 4,050 pounds (150 x 27) because concrete weighs 150 pounds per cubic foot. It displaces 1,650 pounds of water (62.4 x 27). Therefore it weighs 2,365 pounds as it is resting on the bottom of the lake. A cubic yard of iron would be far heavier on the bottom of the lake, not the same as the concrete, as your arguement implies. A log, on the other hand, has air nearly locked in vessels that run throughout the fiberous cellulose. Cellulose has a specific gravity that is only slightly heavier than water, though it varies depending upon the species of wood. I say the air is nearly locked because it takes a long time for the air to escape. When the wood is dry, its buoyancy is pretty close to neutral because the total volume of the wood (and air locked inside) weighs slightly less than 62.4 pcf. When the log becomes waterlogged, the air in the vessels is replaced with water and the log sinks. You won't be able to lift a big log in water then ... not without a crane. (I studied Forest Products, too)
Yep, another engineer chiming in ... displacement weight should equal the weight lifted by the crane. So, why is there a discrepancy? Maybe some lift operators unscrupulously have erroneous scales so that they can overcharge (if they are paid by weight). Look for certification to be protected. Maybe boat manufacturers are notoriously inaccurate? IS there and answer? Sorry jtaylor, Whisky II, and Claude, your arguments are misleading. The weight of an object submersed in water is its actual weight MINUS the weight of water it displaces. My mooring block is a cubic yard of concrete and it weighs about 4,050 pounds (150 x 27) because concrete weighs 150 pounds per cubic foot. It displaces 1,650 pounds of water (62.4 x 27). Therefore it weighs 2,365 pounds as it is resting on the bottom of the lake. A cubic yard of iron would be far heavier on the bottom of the lake, not the same as the concrete, as your arguement implies. A log, on the other hand, has air nearly locked in vessels that run throughout the fiberous cellulose. Cellulose has a specific gravity that is only slightly heavier than water, though it varies depending upon the species of wood. I say the air is nearly locked because it takes a long time for the air to escape. When the wood is dry, its buoyancy is pretty close to neutral because the total volume of the wood (and air locked inside) weighs slightly less than 62.4 pcf. When the log becomes waterlogged, the air in the vessels is replaced with water and the log sinks. You won't be able to lift a big log in water then ... not without a crane. (I studied Forest Products, too)
Not being an engineer,,
doesn't really matter here. (BTW, this is a fun thread) The boat as it floats displaces water. The weight of the water that it displaces is equal to the weight of the boat FLOATING. To me, that's self-evident. A court would take it under 'judicial notice'. (no proof needed) But then too, being an engineer was my first choice for a career in high school. Life got in the way.
doesn't really matter here. (BTW, this is a fun thread) The boat as it floats displaces water. The weight of the water that it displaces is equal to the weight of the boat FLOATING. To me, that's self-evident. A court would take it under 'judicial notice'. (no proof needed) But then too, being an engineer was my first choice for a career in high school. Life got in the way.
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