How Heavy is Your Boat?

[Kings Gambit]

Just conceptually translate everything to force of gravity. The force of gravity (pounds) trying to push an object beneath the water is resisted by the force of gravity trying to push the water being displaced back to whence it came. If the forces are equal, then the thing floats. So, it’s a perfect correlation. Whatever the boat weighs in air, i.e., pounds of force; it will be the same pounds of force in the water. The variable is the pounds of force acting on the amount of water that it displaces. If it floats, they match., etc.

Of course, pounds of force is related to the mass of the object. A liter of seawater has more mass than a liter of freshwater. So, a vessel has to displace less seawater to say afloat than freshwater; less cold water than warm water, etc. But, the total balance of forces remains the same.

I remember questions in the written exam for a captain’s license about maximum load “restrictions”, or related, on vessels entering fresh waters.

 

[Terry Cox]

Just quoting the travel lift operator. No argument here whether or how precise the scale. Different hull than the 40.5. Both keel and hull heavier. Again, what does the spec weight include? And no CE certificate. Boat handles and sails just fine, fast and stiff. No worries.

 

[Terry Cox]

For those concerned about the weight of our boat, it is appreciated. BTW, when fully loaded, as it is currently, the waterline is below the original boot stripe. How that translates, again, no worries here.

 

[smokey73]

Just quoting the travel lift operator. No argument here whether or how precise the scale. Different hull than the 40.5. Both keel and hull heavier. Again, what does the spec weight include? And no CE certificate. Boat handles and sails just fine, fast and stiff. No worries.

Terry, I wasn't saying "the same hull" but just the same general hull shape. I"m kind of surprised the Passage 42 doesn't have a CE Certificate in the Owner's Manual. I had one for my 1994 40.5? Agreed, the 42 hull is heavier and the keel is heaver (my keel is 7000 lbs and the 42 is 7,700 lbs, only 700 lbs difference). If the shapes are about the same, then the relationship to increase in draft for a given load would be comparable (not exact, but comparable). By the way, if Sailboatdata is consistent, that 24,000 lbs is the Light Load Displacement (since mine lists 20,000 lbs as the displacement and that is almost exactly my published Light Load Displacement on my CE Certificate. That is why I used 2.5 inches (conservative) increase in draft at full load over light load (mine is 2.67 inches) and using 11,000 pounds a about 2 times the additional load of my boat. Measuring with micrometer, marking with a piece of chalk and cutting with an axe as we used to say on Submarines in Radcon Math. Just using some reasonalbe comparative estimation, not absolutes.

I still can't imagine in my wildest dreams for a stable sailboat that your additional load is 1 1/2 times your keel weight. Just doesn't make sense from a stability standpoint. With that difference your righting moment would disappear and you would turtle too easily. Unless all that load is below the center of bouyancy you'd be dangerously unstable past a certain point of heel and I think a Passage 42 has a capsize ratio of 1.95 or so which is reasonably stiff (below 2.0). My 40.5 has a capsize ratio of 1.83. Those numbers are based on calcuations based on hull shape, displacement, keel weight and shape, center of gravity and center of bouyancy. By the way, I'll trade you if you'd like a "stiffer boat" I"ve always liked the Passage 42 and the Admiral really likes it!

I just can't accept 11,000 allowable load on a 24,000 sailboat with a capsize ration of 1.92

 

[jon hansen]

my whole point is the scale must be off.

 

[smokey73]

my whole point is the scale must be off.

Mine too. West winds at 15 knots tomorrow, partly cloudy with high about 80 - going sailing tomorrow! Hopefully withh the Admiral but Solo if she doesn't feel like going. Just a day sail but a sail none the less.

That being said, the discussion is important because every boater, but partularly sailors, should understand stability, loading and how it effects stability. I recall doing a full set of calculations when our sub came out of drydock and as the Engineer Officer signing off on the loading and stability calcs. Every modification and piece of equpment changed or added had to be accounted for in the calcs. I also had to enter the ballast tanks (along with the snakes residing there) to make sure there wasn't anything left in there that would rattle around. Those two things always scared me more than the "pre-critical" preparations for reactor startup.

 

[Terry Cox]

my whole point is the scale must be off.

Ditto our bathroom scales. One day I weigh X and the next day I weigh Y by several pounds. Go figure.

 

[Paul Cossman]

Just chiming in here with my experience. I have a Hunter Passage 42, about the same vintage as Terry's (1991 model--hull #65). We have her rigged for distance cruising as well. Last time I had her weighed on a travel-lift she came in at 30,000 lbs. We were so low in the water that I raised the waterline to the top of the boot stripe. She still generally sits in the water well below the new waterline--I just got tired of cleaning the slimeline above the original waterline. I've crossed the Pacific a couple of times since then (N. America to NZ) and she's handled just fine. I'm actually pretty sure she's heavier now than when I last weighed her.

 

[Terry Cox]

1991 model--hull #65).

Hi Paul, thanks for chiming in. Ours is hull #40. Perhaps Hunter decided to take off some weight between our boat and yours. I'm assuming Hunter weighed our boat before all the rigging, Genset and related equipment was installed, which would account for some of the weigh. It also had our dinghy and outboard on as well. It all just adds up, I guess.

Hope you're doing well over there in hinterland. Just a strange time.

 

[JamesG161]

Travel lift scales are not precise.

Hunter weight specs exclude mast, rigging, etc.. as other suggested previously.

Displacement vs. weight distinctions are a moot point in this discussion.

Most of us have nothing better to do than debate this.

For fun trip down SBO thread lanes....
Boat weight?

Humorous and Full of science.
Jim...

 

[All U Get]

So how big of a travel crane to lift this?

 

[All U Get]

Let me ask y’all this. A full bucket of water to the brim, now you add a brick and it sinks forcing water out. The amount of water equals the volume of the brick. Now with the same full bucket add a lead brick of the same size and it sinks displacing an equal amount of water, not more because it’s heavier.

 

[smokey73]

Let me ask y’all this. A full bucket of water to the brim, now you add a brick and it sinks forcing water out. The amount of water equals the volume of the brick. Now with the same full bucket add a lead brick of the same size and it sinks displacing an equal amount of water, not more because it’s heavier.

So what's the question? Your statement is true.

 

[Terry Cox]

View attachment 177712

So how big of a travel crane to lift this?

Hi Tom, looks photo shopped to me. Can't imagine a boat weighing that much. Must be a misprint on the sign.

 

[All U Get]

So what's the question? Your statement is true.

Hi Tom, looks photo shopped to me. Can't imagine a boat weighing that much. Must be a misprint on the sign.

I already had my coffee so here goes. If I start adding weight to get near the displacement volume, lets say 40,000 lbs, would that be when my toe rail is just above the surface?

 

[smokey73]

T

I already had my coffee so here goes. If I start adding weight to get near the displacement volume, lets say 40,000 lbs, would that be when my toe rail is just above the surface?

Tom, Can't tell if you are kidding or not so I won't answer that question. As they used to say on a quiz show "Could you please rephrase that question?"

 

[All U Get]

T


Tom, Can't tell if you are kidding or not so I won't answer that question. As they used to say on a quiz show "Could you please rephrase that question?"

If the design says 45k and I add 40k, the boat should still float? Submarines have this figured out because they use it to travel. I might be getting rid of a 1000 lb generator so the water line would change, just thinking about how much.

 

[All U Get]

Maybe I can ask this differently. A boat with 2 feet of freeboard and displacement of 10k lbs. If you add 5k lbs would the waterline be at 1 foot?

 

[JamesG161]

If you add 5k lbs would the waterline be at 1 foot?

No, unless you have a rectangular hull.

The hole in the water, that your boat makes, is shaped like your, stationary hull.
Jim...

 

[DrJudyB]

Unless all that load is below the center of bouyancy you'd be dangerously unstable past a certain point of heel and I think a Passage 42 has a capsize ratio of 1.95 or so which is reasonably stiff (below 2.0). My 40.5 has a capsize ratio of 1.83. Those numbers are based on calcuations based on hull shape, displacement, keel weight and shape, center of gravity and center of bouyancy. By the way, I'll trade you if you'd like a "stiffer boat" I"ve always liked the Passage 42 and the Admiral really likes it!

The “capsize screening formula” has nothing to do with stability, stiffness, hull shape, keel weigh and shape, center of gravity or century of buoyancy.

The formula is
CSF= Beam ((Displacement/64.2)1/3) , with displacement measured in pounds.

Any two boats with the same beam and displacement will have the same CSF.

ON EDIT:
@JamesG161 noticed I had a type in the formula. It was missing the division symbol. The correct formula is
CSF= Beam / ((Displacement/64.2)1/3) , with displacement measured in pounds