How do I properly read these specs?

Been reading some boat specs on boattraders.com, got quite confused.

Dry Weight: So I guess this means hull, engine, machineries, structure, but no fuel, water nor people?

Displacement: What does that mean? Full displacement? Hull displacement? Is there any convention people follow or do everyone just put whatever they want there?

Draft: Is this the max draft? I guess it has to be, right?

One critical piece of information seems to be missing from almost all online post is the certified capacity. I was told most boat has a plate with that info on it, but looks like no one bother to include it when they post on line? Is there any general guide like for a 50000lbs dry weight boat, roughly how much can it carry?

Not much consistency in the boat world...especially older boats say pre 2000.

Capacity plates are only mandated on boats 20 feet and less but I believe a few larger boats might have them...probably no boats over 30 feet, yet some do give limits for upper decks/flybridges.

Boat carrying capacity is all over the map for any given length due to great variations in design...so no...not even a rule of thumb is applicable for larger (say over 30 feet with cabins) boats.

chicagoq; said:

Dry Weight: Is derived from actually weighing the empty vessel as though it was needed for shipping purposes. Now, I don't know at what size that becomes impractical.

Displacement: Is the weight of water displaced when the vessel is set, again empty, into the water. It is the weight of the boat as mathematically calculated by the designers.

Draft:Is this the max draft? I guess it has to be, right? Right.

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What I would like to see in listings, along with the specs, is ER headroom from a usual standing, squatting, kneeling position.

I'm sure weight-displacement/people-deployment has lots to do with a boat's stability and performance. Put 12 people (over a ton of weight) on the flying bridge or one side of the boat could have unwelcome consequences.

markpierce said:

I'm sure weight-displacement/people-deployment has lots to do with a boat's stability and performance. Put 12 people (over a ton of weight) on the flying bridge or one side of the boat could have unwelcome consequences.

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He's talking specs, Mark, not special effects.

Hawgwash said:

He's talking specs, Mark, not special effects.

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Theoretical or practical?

Rather than a single person aboard, how well would a dozen of "in the wrong place" people do in this situation?



I'd have serious concerns.

OK, Thanks! If there's no general guidance I can follow, could someone tell me for a boat like Hatteras 53 MotorYacht(or similar boat), how much weight can it carry? Including water, fuel, everything on top of dry weight? Not looking for anything precise, just a rough number.

Chicago-can't give you precise #'s on the 53 Hatt, but can give you some comparison. Our boat is 58', full displacement, weight loaded is around 52 tons or so. Per design specs, it takes just over 10,000 lbs to lower the boat 1" in the water. Now, that assumes even weight distribution and is not a measure of stability. As has been noted in previous posts, where the weight is can, and often is, more important than the weight itself. But in general, overloading our boat has not only never been an issue, we have never even thought about it. When it comes to people on the boat, our measuring stick is simply comfort-how many can be comfortably accommodated for what we are doing, i.e. a short 1 or 2 hour cruise? or a daylong excursion? or an overnight? In order, we have had up 20 on the boat, in the second instance we have had 10-12 and overnight 4 (except for my daughter's birthday we had 6).

Just to be clear - displacement is the same as weight. As Archimedes pointed out, a floating vessel displaces a weight of water equal to its own weight. I see "half load" quoted a number of times. I'm not sure if that means tanks half full? On my boat the combined weight of full tanks of fuel and water total over 13,500 lb.

One of Tollycraft's gurus... who goes 1970's and 80's decades deep inside the Tolly company as lead purchasing agent runs this site... has a pretty good stat referral block; model's pictures listed by year on page top.

Tolly Classified - Model Selection

THD said:

Chicago-can't give you precise #'s on the 53 Hatt, but can give you some comparison. Our boat is 58', full displacement, weight loaded is around 52 tons or so. Per design specs, it takes just over 10,000 lbs to lower the boat 1" in the water. Now, that assumes even weight distribution and is not a measure of stability. As has been noted in previous posts, where the weight is can, and often is, more important than the weight itself. But in general, overloading our boat has not only never been an issue, we have never even thought about it. When it comes to people on the boat, our measuring stick is simply comfort-how many can be comfortably accommodated for what we are doing, i.e. a short 1 or 2 hour cruise? or a daylong excursion? or an overnight? In order, we have had up 20 on the boat, in the second instance we have had 10-12 and overnight 4 (except for my daughter's birthday we had 6).

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10,000lbs is just one inch? Woo, that's much bigger than I had in mind. Thanks!

Wait, I am still confused, how wide is your beam? Even if I count your boat as a square box:
58' x beam x 1/12 x 7.48(CuFt to Gallon) x 8.34(water weight per gallon) = 10000lbs
If my formula is correct, the beam needs to be at least 33'?

Is my formula wrong?

Greetings,
It's my understanding that the weight of the boat is NOT the displacement. Displacement has to do with cargo carrying capacity or some such.

Our vessel "weighs" aprox. 25 tons (according to travel lift #'s). Gross displacement is registered as 41.43t. Net displacement is registered as 31.07t.

chicagoq said:

OK, Thanks! If there's no general guidance I can follow, could someone tell me for a boat like Hatteras 53 MotorYacht(or similar boat), how much weight can it carry? Including water, fuel, everything on top of dry weight? Not looking for anything precise, just a rough number.

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There is the question of how many people can a fully fueled, watered boat hold? In that calculation you need to figure that everybody on that boat will be on the upper deck and run to one side when the New Years Eve fireworks start. That scenario should cut the number of people you deem safe on your boat, in half.

RT Firefly said:

Greetings,
It's my understanding that the weight of the boat is NOT the displacement. Displacement has to do with cargo carrying capacity or some such.

Our vessel "weighs" aprox. 25 tons (according to travel lift #'s). Gross displacement is registered as 41.43t. Net displacement is registered as 31.07t.

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https://en.wikipedia.org/wiki/Displacement_(ship)

Displacement or displacement tonnage is the weight of water that a ship displaces when it is floating, which in turn is the weight of a ship (and its contents). It is usually applied to naval vessels rather than commercial ones, and is measured when the ship's fuel tanks are full and all stores are aboard.[1][2]
Displacement should not be confused with other measurements of volume or capacity typically used for commercial vessels such as net tonnage, gross tonnage, or deadweight tonnage.

Larry M said:

https://en.wikipedia.org/wiki/Displacement_(ship)

Displacement or displacement tonnage is the weight of water that a ship displaces when it is floating, which in turn is the weight of a ship (and its contents). It is usually applied to naval vessels rather than commercial ones, and is measured when the ship's fuel tanks are full and all stores are aboard.[1][2]
Displacement should not be confused with other measurements of volume or capacity typically used for commercial vessels such as net tonnage, gross tonnage, or deadweight tonnage.

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Thus my post #2 basically saying that the recreational boating community has been fed the trash boat companies call brochures and have created "word monsters" for so many nautical terms...oh...say...like trawler?

chicagoq said:

Wait, I am still confused, how wide is your beam? Even if I count your boat as a square box:
58' x beam x 1/12 x 7.48(CuFt to Gallon) x 8.34(water weight per gallon) = 10000lbs
If my formula is correct, the beam needs to be at least 33'?

Is my formula wrong?

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Your formula is correct and does bring into question the statement that 10,000lb only lowers the boat 1".

For reference...


Our 34' tri cabin Tolly weighs 17K lbs. as she comes out the factory door. See stats link for Tollycraft on post 11. With filled 200 gal fuel tanks, 77 gal water, and all sorts of extra "stuff" I calc she weighs 20K + lbs.

The "design" displacement of a Krogen 42 is 39,500 lbs at "half load". Our cruising weight, with stores, full water and fuel is just over 45,000 lbs. It also takes 2000 lb/inch immersion. We have raised the water line.

Pgitug said:

There is the question of how many people can a fully fueled, watered boat hold? In that calculation you need to figure that everybody on that boat will be on the upper deck and run to one side when the New Years Eve fireworks start. That scenario should cut the number of people you deem safe on your boat, in half.

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There is a noted place on Lake Travis near Austin, TX called "Hippy Hollow". It is the only registered nude beach in Texas. Well a Gay/Lesbian group rented a houseboat on Splash Dayand put 60 people on it. As they neared Hippy Hollow everyone went to one side of the boat hoping to catch a peek of the nudity and over the boat went!!!

Details below:

http://www.nbcnews.com/id/4890829/n...sinks-after-nude-sighting-texas/#.VuHf05MrInM

Tonnage is volume, and displacement is weight, maritime wise.

Yes, tonnage refers to volume - with very obscure, tax-code logic applied to the measurements - and displacement will be equal to the weight of the boat, regardless of load condition (full, empty, half, etc.). Unless you've sunk, and then it wasn't equal to the weight.

Regarding the math behind the 10,000 lbs per inch... That's referred to as the 'weight to immerse' and is usually given in lbs per inch of immersion for a specific hull at a specific draft. It's just the cubic volume of a 1" thick slice of the hull, multiplied by the density of water. Usually stated for SW, obviously a little different in fresh or brackish. I too find the 10k number hard to replicate; I think there's something amiss in the specs being read. Which is not unusual at all, much published data for boats is 'squishy' at best.

A 'typical' planing boat will add about 30% to its dry / empty / lightship (that's the proper term) weight when in the fully-loaded condition, i.e. tanks full, some stores on board, and as many people and gear as are reasonably accommodated in the cabins. That's a big increase, but note that the bulk of that added weight goes into the tanks, which are usually located low in the boat. The center of gravity (CG) goes lower as it goes from empty to full in that case.

That generalization holds true for planing and semi-planing (semi-displacement) hull shapes and pleasure boat load cases. Cargo and passenger carriers will be different. Full displacement hull forms will also, but I doubt the difference is drastic when loaded like a yacht.

After the tanks are full, as you start to add more people and gear - which generally go onto the decks - the CG starts to rise again, which is bad. And, as the hull sinks lower into the water it generally becomes less 'stable' hydrostatically, too. Things can get sketchy quickly at that point if no one is paying attention.

Or, you can add six months' worth of canned goods in the bilges and improve the situation by lowering your CG. It all depends, and managing it requires understanding basic physics.

Fortunately or unfortunately, depending on whom you ask, the pleasure boat industry in the US very loosely regulated in terms of stability and loading. Never mind licensing operators to ensure they understand the physics. So the OP and all the rest of us are well advised to think about these things, consult whatever specifications you can find, ask questions, and ask again if you don't understand or believe what you find. There's no need for this to be a dangerous hobby, but don't assume it's inherently safe, either. And don't take the captain's responsibilities lightly.

Wow, sermon over....

@ Chicogog: You mention capacity. Are you trying to figure this out for passenger capacity? As in passengers for hire? That's a different animal. You will have to hire an architect to determine that if you go over 6 passengers for hire.

For simple private boat use the amount of people is determined to some degree of how comfortable people are, and that you have enough life preservers for all. Practical loading of a boat is like an eye chart test. How it look to you may be different than how it looks to others. If you submerge the waterline you have gone too far. Add up all the stuff to bring, and full tanks and there is a definite pax weight to consider. Bring less 'stuff' and water and fuel and you can take more pax.

Cappy - all due respect, but no. It's not how it looks to you vs me vs him. It's physics. And please don't advise anyone to bring less fuel and water so they can load up the people and stuff on deck. That backfires quite easily.

Chicago-I was down at the boat today and checked the figure I gave you. I had posted it from memory. I was in error. Checking the hydrostatic data, the 10,000 lbs is the moment to trim one inch (actually 10,070) and the immersion data is 1 inch immersion per 3,425 lbs.

Tnanks Cappy, Chris and THD!

Nah I am not looking for to load a lot of people. Just me and the Admiral that's it.

I figured I will be a LRC kind of guy, so just trying to get a sense how much fuel, supplies etc can I load onto certain boat.

chriscritchett said:

Cappy - all due respect, but no. It's not how it looks to you vs me vs him. It's physics. And please don't advise anyone to bring less fuel and water so they can load up the people and stuff on deck. That backfires quite easily.

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Listen to Chris!

I believe displacement of a boat is when its floating exactly at the waterline marks the naval architect designed his specs from; including draft, drag, hull lenght at the waterline, stability etc etc....

You can see the displacement line drawn on the original boat plans. Of course the NA has to include normal load conditions in his calculations.

Unless running an inspected vessel or one that comes with a complete stability study...it IS by eye and gut for the recreational vessel.


Physics? Well sure..but reality is if you don't have numbers to start with to do any useful calculations...all the physics in the world wont help unless you do your own stability test...or a boatload of experience where you can make educated guesses ad do a few calculations from those. Few boaters I know are there.


This is the problem with boating these days...to many skippers of larger boats that haven't learned the tricks along the way of determining safe boat handling. Sure...even the pros make mistakes all the time...but for overloading larger vessels...both the pros and recreational skippers manage to roll very few and the ones that do roll are usually followed by the "can you believe that!!!!" cries within seconds of it happening.