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Old 03-10-2016, 03:59 PM   #21
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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.
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/ns.../#.VuHf05MrInM
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Old 03-10-2016, 09:13 PM   #22
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Tonnage is volume, and displacement is weight, maritime wise.
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Old 03-10-2016, 10:12 PM   #23
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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....
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Old 03-10-2016, 10:27 PM   #24
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@ 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.
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Old 03-10-2016, 11:12 PM   #25
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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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Old 03-11-2016, 02:07 AM   #26
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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.
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Old 03-11-2016, 10:16 AM   #27
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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.
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Old 03-11-2016, 11:04 AM   #28
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Originally Posted by chriscritchett View Post
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.
Listen to Chris!
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Old 03-11-2016, 12:03 PM   #29
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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.
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Old 03-11-2016, 05:05 PM   #30
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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.
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Old 03-11-2016, 07:46 PM   #31
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Assessment of the situation for the recreational skipper is by eye and gut feel, that's almost universally true. But the laws of physics govern, whether we know how to do the math or not, and the inputs are rarely available or terribly accurate.

My point was simply to realize these facts and give them some thought before deciding to forego filling the fuel tanks in favor of inviting another few guests on the fly bridge to watch the fireworks. You're in charge, and you're responsible.

So I'm not sure whether any of us has answered the OP, but I bet we've made him nervous...
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Old 03-11-2016, 07:59 PM   #32
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However...in some cases leaving consumables and packables ashore to take passengers isn't all bad.

You still have to step back and think...not all have the instinct or experience to do it.... there are operators...and everyone else.

Physics and possibly laws of nature that exist but haven't been discovered yet might govern....but try to pass that on here or even to your most experienced boating friend in person...mot likely much will actually change.

So the best we can do here is pass along some experiences we all have had and say to the OP...take a stab at it...unless you feel really comfy...best to bring along someone that knows the tricks or can do the math ahead of time.
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Old 03-11-2016, 09:31 PM   #33
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Originally Posted by chriscritchett View Post
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.
My experience over the last 35 years of both commercial boating and yachting is that boats accumulate tons (literally) of junk on the boat over time. This extra weight is NEVER taken into account as far as Pax load is concerned (until afterwards). Every boat has a weight limit that it was designed to operate at. This is reflected in the 'normal waterline. Getting weight OFF the boat helps return the boat to float at the designed waterline.
The people being unable to understand that 'just because you can physically fit on 10 more pax does not mean you should. Some people just keep adding pax until they have the 'whole guest list' aboard. And, no boat I have ever run has the pax count depending upon FULL fuel tanks. Quite the opposite. Empty is worst case scenario, and that is how pax count is determined.

People operating boats and trying to decide on how many is too many often don't get the whole scheme of things not because they don't want to know, but the answers are very tough to come by unless your vessel has an occupancy plate. (good luck finding that over 20') or unless the vessel has or had a stability letter and documented pax capacity some time in the past.
Here's an interesting read about how many people fit on a 34' silverton.
This seems to be a point of interest for Chicagoan as general information about running a boat.
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Old 03-11-2016, 09:36 PM   #34
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How is the capacity calculated? Seemingly, the experts assume a hundred-pound (or a bit more) person in their calculations.
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Old 03-12-2016, 12:18 AM   #35
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Sorta like a pissed off mule - chucks its loot and keeps on a goen!

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Old 03-12-2016, 05:01 AM   #36
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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.
Ah...are you sure about that Britt. By my reckoning, (@ 2.2 pounds per kg, your boat would only weight about 6140kg = 6.140 tonne, and my 34ft CHB weights about 8.5 tonne, and your vessel is A Kadey Krogen 54, is she not..? I would say she weighs about 28-30 tonne, i.e. about 60,000 lbs

Sorry, my bad, I see you were just talking about the fuel and water weight - that does feel much more correct...but how close was I in the overall boat weight estimate anyway, just out of interest..?
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Old 03-12-2016, 09:16 AM   #37
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Ah...are you sure about that Britt. By my reckoning, (@ 2.2 pounds per kg, your boat would only weight about 6140kg = 6.140 tonne, and my 34ft CHB weights about 8.5 tonne, and your vessel is A Kadey Krogen 54, is she not..? I would say she weighs about 28-30 tonne, i.e. about 60,000 lbs

Sorry, my bad, I see you were just talking about the fuel and water weight - that does feel much more correct...but how close was I in the overall boat weight estimate anyway, just out of interest..?
Hah! I do recognize with some awe that the weight of my fuel alone is about the same as the weight of my first sailboat - a Catalina 30!

Krogen lists my boat as 67,800 lb displacement. So yes, you were close. Full cruising weight it probably closer to 85,000 lb. I'll make sure I get the weight next time she's hauled.

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Old 03-12-2016, 11:40 AM   #38
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Hah! I do recognize with some awe that the weight of my fuel alone is about the same as the weight of my first sailboat - a Catalina 30!

Krogen lists my boat as 67,800 lb displacement. So yes, you were close. Full cruising weight it probably closer to 85,000 lb. I'll make sure I get the weight next time she's hauled.

Richard
Geeeezzz - Talk about being weight conscious! - LOL Don't you guys know BIG girls are in these days! Curves... got luv them curves!!
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