Displacement and ballast

[Polux]

Originally Posted by Steady Hand

Does the design displacement value of a new boat include mast, sails, winches, running rigging, Diesel engine, toilets, plumbing, tanks?

Or does it only mean the empty hull with nothing else?

There are many displacements but they never include only the hull. The two more frequent are lightship that includes all that but not tankage and the Minimum sailing condition that includes half tanks, engine oil and everything that is necessary to sail the boat. The nomenclature can vary but regarding Europe, because the boats are evaluated and certified all those names have a precise meaning defined on the RCD.

[transmitterdan]

If we want to be pedantic the term "displacement" is a factual value that can only be determined by direct measurement. The term "design displacement" is what the NA considered for certain calculations. They are never the same value.

[Paul J. Nolan]

Paceship 29 was designed by C&C. Most of the boats designed and built under C&C's own name had balsa cored hulls. This is the first thing I'd check. Look at the inside of the hull where the topsides and bottom meet the transom. Does the hull appear to gain about 3/4" thickness several inches from the corner? You should also check the inside of the stem, just below the rail and at the garboards. Some boats, though not C&Cs to my knowledge, were cored only above the waterline, so I'd also look around the waterline. (Though if cored only above the waterline it probably would not be the source of your problem.)

In the 60s and 70s fiberglass was regarded as waterproof (it is actually not perfectly waterproof) and boats were not given epoxy barrier coats. If your boat has a cored hull, it is more than likely the source of the discrepancy.

Paul

[Don C L]

Pint's a pound the world around, my dad always said. (for fresh water, salt's a little more) So if I do the math, 8 lbs per gallon, so approximately 300 gallons (>2400#,) let's say, has soaked into the core? Something isn't adding up. I have always assumed that displacement is as Archimedes said, the weight of the liquid being displaced. For this particular boat, even if you took out the engine, the rig, mast, the fuel and the water, it still doesn't add up to a ton and a half does it? I am still thinking the travel lift scale is a bit off. Also, I have a letter from Olin Stephens in my album where he comments, among other things that as the architect he has little control over what the builders and owners do in construction of the boats so that displacement can vary from his original plans. Still I doubt the builder added over a ton more than called for...

edit: oops, just checked and I did not post that letter from Stephens. I'll take care of that later!

[Paul J. Nolan]

I don't know how much water a balsa cored boat can soak up. I once had a Mueller Lightning that gained weight alarmingly fast and she was dry sailed. Selling that boat (to a nonracer) was a happy day for me!

The Congressional Cup was raced in Cal 40s to which some attention had been given to equalize speeds--same bottom jobs, same props, weight of gear aboard, etc. Most of the boats were donated for use year after year and it soon became apparent that some were relative dogs and some were relative superboats. I don't know that the causes were extensively investigated or if the boats were even weighed. It wouldn't surprise me to learn of a significant discrepancy in weight between 15,000 lb. Cal 40s, but 3,000 lbs of builder error in a 6,500 lb. boat seems unlikely to me.

Wouldn't the simplest thing to do would be to check the calibration of the scale? There are so many industrial scales in daily use in any large city that someone is in the business of maintaining and calibrating them.

Paul

[Steadman Uhlich]

Thanks to those nice members who answered my question.

As I read more posts, I thought about how much water weight a saturated cored boat could retain. It made me think about the old style kapok PFD or a natural sponge. Some things, when saturated, can get very heavy, despite the original design or original buoyancy.

[Steadman Uhlich]

I was looking at a boat yesterday. It had fuel capacity of 400 gallons of fuel and 150 (I recall) of water tanks. That is a lot of variable weight in a boat that has less than 5 feet of draft on 40 feet of LOA. It is not more than some, and may very well be appropriate, for the hull design, but still strikes me, a light boat sailor, as a lot of variable weight.

[transmitterdan]

Regardless of what the nice yard owner says the accuracy of a travel lift strain gauge is to be regarded with great suspicion. Even if tested the repeatability is not that good. There are too many variables in the rigging of a travel lift. And small boats in a big lift are even more prone to giving inaccurate results.

[Jim Cate]

Originally Posted by transmitterdan

Regardless of what the nice yard owner says the accuracy of a travel lift strain gauge is to be regarded with great suspicion. Even if tested the repeatability is not that good. There are too many variables in the rigging of a travel lift. And small boats in a big lift are even more prone to giving inaccurate results.

Seems to me that the lift guesses the weight of the suspended object from tension in the lifting straps, via strain gauges or hydraulic pressure or such. The tension in the straps will vary considerably, depending on the beam of the vessel being lifted: narrow beam, big angle to the hoisting gear, wide beam, narrower angle. I doubt if the operator takes this into account in his reports.

My experience with my previous boat, lifted in many different yards, was that the reported weight was never the same, often with considerable discrepancy.

Finally, one must remember that even the fibreglass skin absorbs water over time, not just any coring therein. More weight added over the years... (same thing seems to have happened to my body... must be water absorption??)

Jim

[Adelie]

Originally Posted by UNCIVILIZED

Keep in mind that displacement and weight aren't the same thing too. As, for example a cubic foot of fiberglass (50% glass, 50% resin) weighs 96lbs on a scale on land. Yet in theory, when you put it into the ocean, & hooked it up to a scale, it'd only "weigh" 32lbs. As the cubic foot of water which it displaced weighs 64lbs.

It's the same reason why a PFD with 15lbs of bouyancy is plenty to keep most adults afloat under benign conditions, even though they weigh a lot more than that.

Try again, the weight of water displaced by a boat is equal to the weight of the boat measured in air.

You seem to be confusing buoyancy and displacement.

[Don C L]

Just as a matter of historical interest and to the point of displacement considered in an architect's calculations, I posted the letter from Olin Stephens regarding metacentric height of a Columbia 29 in members images. Click on below my info at left.

[jimbunyard]

Just for grins, a completely saturated (all the wood replaced by water) deck, originally cored with 1/2" balsa, 9' X 29' (x.66), would add 283.6056972 lbs, versus a deck cored with dry 10 lb./cu.ft. balsa; 448.378425 weight of water - 164.7727272 weight of balsa = 283.6056972 lbs.

I realize all boats are different but 6500 lbs for a 29 ft internally ballasted sailboat seems a little light. For instance, mine is 33 ft, listed at 11000 lbs with 5000 lbs ballast. Last time, travel-lift said it was closer to 14000.

[valhalla360]

Originally Posted by UNCIVILIZED

Keep in mind that displacement and weight aren't the same thing too. As, for example a cubic foot of fiberglass (50% glass, 50% resin) weighs 96lbs on a scale on land. Yet in theory, when you put it into the ocean, & hooked it up to a scale, it'd only "weigh" 32lbs. As the cubic foot of water which it displaced weighs 64lbs.

It's the same reason why a PFD with 15lbs of bouyancy is plenty to keep most adults afloat under benign conditions, even though they weigh a lot more than that.

You are describing bouyancy not displacement.

No one builds solid fiberglass boats...and by solid I mean the entire volume of the hull leaving no space for a cabin or mechanicals, just a big block of solid fiberglass.

While there is a technical difference between displacement and weight, for practical purposes they are the same. The displacement will never exceed the weight as the boat will float before displacing more water (ignoring dymanic forces).

Where the technical difference comes in is an object can weigh more than it displaces. If you drop that screw over the side, it weighs more than it displaces and thus sinks to the bottom. Other than with submarines, I don't know any boat builders who design for this case, so it's safe to assume the design displacement and the weight are pretty much the same thing.

[jimbunyard]

Originally Posted by valhalla360

You are describing bouyancy not displacement.

No one builds solid fiberglass boats...and by solid I mean the entire volume of the hull leaving no space for a cabin or mechanicals, just a big block of solid fiberglass.

While there is a technical difference between displacement and weight, for practical purposes they are the same. The displacement will never exceed the weight as the boat will float before displacing more water (ignoring dymanic forces).

Where the technical difference comes in is an object can weigh more than it displaces. If you drop that screw over the side, it weighs more than it displaces and thus sinks to the bottom. Other than with submarines, I don't know any boat builders who design for this case, so it's safe to assume the design displacement and the weight are pretty much the same thing.

Not sure you can have a discussion about displacement without including buoyancy.

Hopefully a naval architect or at least someone who has a better understanding and explanatory skills than me will come along, but as I understand it, the volume of water that the underwater body of the boat takes the place of, dis-places, has a specific weight, and is listed as the boats 'displacement'. The ballast overcomes the inherent buoyancy of the hull to bring the hull to its' designed water line. The on land weight has much less (nothing?) to do with it.

Imagine two boats, identical in every way, one floating in the ocean and one floating in a pool of mercury. The boat on the ocean floats on its' proper waterline, but the one in the mercury pool would float on it side....even though their respective weights on land are the same.


Interestingly, it the ocean were mercury, since lead floats on it, you'd have to use gold, platinum, tungsten or uranium as ballast.

[valhalla360]

Originally Posted by jimbunyard

Not sure you can have a discussion about displacement without including buoyancy.

Hopefully a naval architect or at least someone who has a better understanding and explanatory skills will come along, but as I understand it, the volume of water that the underwater body of the boat takes the place of, dis-places, has a specific weight, and is listed as the boats 'displacement'. The ballast overcomes the inherent buoyancy of the hull to bring the hull to its' designed water line. The on land weight has nothing to do with it.

Imagine two boats, identical in every way, one floating in the ocean and one floating in a pool of mercury. The boat on the ocean floats on its' proper waterline, but the one in the mercury pool would float on it side....even though their respective weights on land are the same.

Displacement is simply the weight of water that the boat will displace when put in water (or you could use mercury but hard to find a pool of mercury big enough). In terms of a boat that is not sinking, that's the same weight as if you hung if from a big scale on land, so yes, it very much has a lot to do with it.

The only time that doesn't work is if the boat is sinking. If you start filling the bilge with water, you increase the displacement until eventually the hull can't displace enough water to stay afloat, since most boats rely on a combination of the density of the materials they are made of and the density of air trapped in the hull which has negligible weight, once you replace air with water most boats will sink.

Mercury works just the same except 1 gal of mercury weighs far more than 1 gal of water, so the displacement weight will be the same but the displaced volume will be less. (this assumes the boat doesn't fall over and fill with mercury because it is sitting so much higher in the water but that's the sinking scenario described above). The difference in volume displaced will be proportional to the ratio of the density of water to the density of mercury.

Ballast is not there to overcome the inherent bouyancy. It's there to stop the boat from falling over on it's side. Remember weeble wabbles when you were a kid. Same principal. Balast puts weight down low. If you roll the boat on it's side, that heavy weight pulls down and turns the boat back upright.

For a normal boat floating safely, the displacement and the weight out of the water are the same.