how does hull weight effect capacity?

[tincanman]

Showing my ignorance here. I was looking at a friends boat the other day and I noticed his total capacity was about 500lbs more than my boat. Both boats are simiar design and deadrise. Both 19 feet with similar beam. My boat is alluminum, his is fiberglass. Got me thinking.

Explain this to me. His boat is weighing in at almost 800lbs more than mine. These hulls should displace close to the same amount of water, so shouldn't his boat loose capacity because of the extra hull weight? Or am I way off??

 

[tpenfield]

Re: how does hull weight effect capacity?

No, capacity is the net of what the boat can hold. It is figured out by testing a flooded boat in prescribed conditions. So, the amount of flotation built into the boat can effect it, as well as the overall dimensions of the boat, beam and length.

So that is why two similar boat may have widely differing capacities.

 

[Ned L]

Re: how does hull weight effect capacity?

"Displace close to the same amount of water"? - Nope, you answered much of your own question. His displaces 800 lbs more, which literally translates to his is a 'bigger' boat of more displacement & can therefore carry more.

 

[Ned L]

Re: how does hull weight effect capacity?

Ah, so I have to ask, ... How is carrying capacity defined by testing a flooded boat? I am certainly no expert on small boat design these days with all the regs, but I would still expect that the carrying capacity is determined through load calculations involving stability testing, center of gravity, center of bouyancy, metacentric height, & reserve bouyancy, and then test to verify that the installed permanent floatation can support that load after flooding. Generally simple carrying capacity is directly related to displacement, and the fiberglass boat in this example displaces 800 lbs more (= a bigger boat).

 

[tincanman]

Re: how does hull weight effect capacity?

ok, let me simplify this for my tiny mind. Say we have 2 identical boat hulls. Both from the same mold. One hull the trigger on their chopper gun gets stuck and ends up 800lbs more. lol. Would thier carrying capacity be the same? Explain. I think I know the anwser, but I want to see if I'm right.

 

[tincanman]

Re: how does hull weight effect capacity?

No, capacity is the net of what the boat can hold. It is figured out by testing a flooded boat in prescribed conditions. So, the amount of flotation built into the boat can effect it, as well as the overall dimensions of the boat, beam and length.

So that is why two similar boat may have widely differing capacities.

but I don't care what my boat can carry when its flooded, I would rather not find that out. lol, jk.

 

[tincanman]

Re: how does hull weight effect capacity?

see what happens when I start thinking. I could build you a house from ground up, but this stuff makes my brain hurt. Then I reply 15 times in a row to my own post. lol

 

[Ned L]

Re: how does hull weight effect capacity?

Oh, nothing wrong with asking questions. I understand your analogy here, unfortunately there isn’t a simple answer. The correct answer really could be “it depends”. If we take your example & the chopper gun ‘got stuck’ and the guy shot way to much material along the keel and all the extra weight was right down low, centered on the keel then it is possible that the calculated carrying capacity for that one specific boat could increase. We lowered the center of gravity and raised the center of buoyancy so the metacentric height increased, which means greater stability, which is also calculated into carrying capacity. If the chopper gun got stuck as the guy was shooting along the gunwales, then the center of gravity would be raised, which reduces the metacentric height and reduces stability. If the metacentric height were reduced to ‘zero’ you could have a boat that rolled over just by putting it in the water.
Think of two 35’loa x 10’ beam x 6’ draft sailboats, one a modern ‘fin keel skimming dish’, and one a full bodied Collin Archer design pilot cutter. The Colin Archer design will have a greater displacement and much greater carrying capacity

 

[tincanman]

Re: how does hull weight effect capacity?

ok, I didn't know the anwser. Once I looked up metacentric height, your reply made much more sence.

Here I thought they just threw some bricks into a boat hull until it sinks. Pull a few out until it floats and weigh the remaining bricks in the hull and thats the capacity. Well, atleast I can make myself laugh. See, I'm a simple man. :facepalm:

 

[Texasmark]

Re: how does hull weight effect capacity?

Might look up BIA certification and what testing leads to certification. Going to go along with the flow that the amount of flotation dictates the max weight rating. Forrest Wood did a cut away of a Ranger bass boat years ago and did it to show us how much foam was in the boat and how it could float with whole sections cut out of it. That was a neat "trick" and did show the insulation, but when your boat is sinking it fills with water so you have to add the weight of the boat full of water to the displacement that the floatation has to displace.

The total weight of the boat displaces it's weight in an equal weight of water. That's how some large things are weighed. The item is submerged in a tank of water that was filled to overflow and if in equilibrium, the volume of water that is displaced when it is inserted is measured and at 7.xxx lbs per gallon, (don't remember exactly) the volume is converted to pounds.

For a boat, with light weight and a large cross sectional (wetted) area, it will ride high in the water but still displace it's weight in the amount of water displaced. A heavy boat with a small cross sectional area will do likewise but ride deeper. If both boats weigh the same, regardless of hull shape, they will displace the same amount of water, just ride at different heights while doing so.

Been a long time, getting rusty, but used to do that on some large items.

HTH,
Mark

 

[Ned L]

Re: how does hull weight effect capacity?

“when your boat is sinking it fills with water so you have to add the weight of the boat full of water to the displacement that the floatation has to displace” Sorry, but this is incorrect. When a boat is ‘flooded’ or swamped, the “floatation” (foam) must displace ONLY the mass of the boat, motor, gear, etc. that is in excess of a specific gravity of one (density of water) (plus the mass of anything that is above the water at that time, plus it’s own mass), and has nothing to do with the water inside the boat.
Again, load calculations have nothing to do with the flotation (foam). You would run your load calculations and then determine if you can realistically install sufficient floatation (foam) to keep things afloat if swamped. If not then you would reduce the allowable load until things do stay afloat.
Your second paragraph holds true only for objects with a specific gravity of one or less (that float). This does not work for objects with a specific gravity greater than one (that sink).

 

[tpenfield]

Re: how does hull weight effect capacity?

but I don't care what my boat can carry when its flooded, I would rather not find that out. lol, jk.

Many of us don't care what a boat can carry when it is flooded and even more never want to find out. It is the US Coast Guard that cares and it is the USCG certification label that prescribes what a boat can carry. There are all kinds of procedures for testing and certifying. You can google the information if you are really that interested, but, unless you have a solid background in physics or engineering, it may not be worth figuring out. Just take it as it is.

So, if you are comparing two similar boats with intent to purchase, you may want to choose the one over the other, if it had a substantially higher capacity rating.

 

[Texasmark]

Re: how does hull weight effect capacity?

“when your boat is sinking it fills with water so you have to add the weight of the boat full of water to the displacement that the floatation has to displace” Sorry, but this is incorrect. When a boat is ‘flooded’ or swamped, the “floatation” (foam) must displace ONLY the mass of the boat, motor, gear, etc. that is in excess of a specific gravity of one (density of water) (plus the mass of anything that is above the water at that time, plus it’s own mass), and has nothing to do with the water inside the boat.
Again, load calculations have nothing to do with the flotation (foam). You would run your load calculations and then determine if you can realistically install sufficient floatation (foam) to keep things afloat if swamped. If not then you would reduce the allowable load until things do stay afloat.
Your second paragraph holds true only for objects with a specific gravity of one or less (that float). This does not work for objects with a specific gravity greater than one (that sink).

Are you addressing this answer to my remarks? If so, you need to check your physics.

Mark

 

[Ned L]

Re: how does hull weight effect capacity?

If that were true then you would literally have to full the hull from gunwale to gunwale with foam and the boat still wouldn't float. Think about it.
As an exanmple, if we were to build a simple 'barge boat' that is 20' long x 7' wide x 4' deep, and weighed (we'll say) 3000lbs. our 'boat' has a volume of 20'x7'x4'= 560 cu ft. At 62lbs per cubic foot for water that is 34,720 pounds of water. Now add the 3000 pounds for the boat and we have a total of 37,720 pounds.
Your statement “when your boat is sinking it fills with water so you have to add the weight of the boat full of water to the displacement that the floatation has to displace” says that the foam has to displace 37,720 pounds. Sorry, that is not correct. If this were true our boats would have to have more volume in foam than we have in boat in order for them to float when swamped.

 

[Texasmark]

Re: how does hull weight effect capacity?

This is my final input on this topic. From the PE training manual, Professional Publications, 8th edition, page 15-14, subject: Buoyancy: This is copyright material so rather than scan it and let you read it I will iterate something I initially said. I don't have Greek symbology handy so I will omit the equations that support this theorem.
---------------------
Per Archimedes' Principle, the "Buoyancy Theorem", "the buoyant force on a submerged (partially submerged) object is equal to the weight of the displaced fluid".

"An equivalent statement of Archimedes' principle is: a floating object displaces liquid equal in weight to it's own weight" "In the case of stationary (i.e., not moving vertically) floating or submerged objects (at equilibrium as I said) the buoyant force and object weight are in equilibrium." "If not in equilibrium, object will either rise or fall until equilibrium is reached......that is the object will sink until it's remaining force is supported by the bottom of the 'tank' or will rise until the weight of the liquid is reduced by breaking the surface."

So, if I fill up a vessel with foam, it's ability to fill with water will be reduced and it can instead, fill with people and gear and still remain at some level of buoyancy that causes it's point of equilibrium to be at the surface of the water upon which it is floating.

Therefore, the boat with the higher BIA rating, must have a larger volume of foam!

Mark

 

[Ned L]

Re: how does hull weight effect capacity?

I certainly understand Archimedes' principle, and I agree with that. Yes, two identical boats, one with a higher rating. The one with the higher rating would have more floatation foam.

**************************After reviewing the USCG requirements I have to revise my statement here. Two identical boats will carry the same carrying capacity regardless of the amount of foam floatation. (Lots of foam or not they both carry the same rating). You see, the capacity rating for a boat is derived by calculations & testing and then once you have the capacity numbers then you build in foam to keep the boat afloat. Extra floatation does not increase your rated capacity.

 

[Frank Acampora]

Re: how does hull weight effect capacity?

Ahh yes! You guys are right on BUT you are forgetting "reserve capacity" which the Coast Guard also takes into account. When a boat is filling with water in the real world, it does not sink straight down. So for example: In a deep vee hull, the shape of the sides determines how much the boat can settle before swamping. A boat with higher sides (freeboard) and a curved side (similar to a Glastron) will have a higher weight carrying capacity than a straight sided low freeboard hull. The CG does check this by loading only one side of the hull.

And the chopper gun example does not hold water. --no pun intended. Coast Guard tests a representative hull or representatve sample of hulls for certification. Thus: a single heavier hull delivered from the factory to a buyer WILL have the same rating--even though in the real world it may not be able to carry the same load.

 

[Ned L]

Re: how does hull weight effect capacity?

Yes, you are talking about reserve buoyancy, and a hull with flare in her design will have greater reserve buoyancy than one with tumblehome. Reserve buoyancy is interrelated to, and has an effect on, but is separate from stability, which is what you are referencing when you mention ?loading only on one side of the hull?. A Part of stability calculations includes loading weigh off center (to one side). The C.G. has tremendously simplified the stability calculation tests that result in the numbers for the capacity tags on small craft to that ?add weight to one side until water comes over the gunwale, then to the other side until water comes over the gunwale?.
We also need to understand that a boat with ?more freeboard? will not necessarily have a greater ?carrying capacity? than one with ?less freeboard?. If we take a beamy flat bottom inboard boat with low freeboard, that will be a much more stable platform than a fairly narrow deep-V with a big outboard and high freeboard. You will put the rail of the deep-V underwater sooner when adding weight to one side.
My ?chopper gun? example was simply meant to explain what happens to one specific hull design if you add the exact same amount of extra mass to one location of a hull vs. another location. Certainly the capacity plates are marked for a ?standard? hull, will ALWAYS be marked the same for a given model boat, and would never reflect a single manufacturing anomaly of too much glass/resin in one area. Sorry if I was misleading there.
It is also good to keep in mind that the ?carrying capacity? information is not typically defined by the amount of floatation. Rather the other way around. A boat builder designs & builds his boat, then he runs his tests and calculations (which includes stability testing &/or calculating) to determine the carrying capacity information that he wants to put on the tag. Then the amount of floatation is determined that is needed to support 1) the submerged boat, 2) the submerged propulsion equipment, 3) a portion of the passenger load.

I made an error in one of my comments above - sorry about this. I implied that the metacentric height is the distance between the center of gravity and the center of buoyancy. It is actually the distance between the center of gravity and the vessel?s metacenter (which is a function of the vessels center of buoyancy). .