How Heavy is Your Boat?

JamesG161

JamesG161

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Feb 14, 2014
7,425
Hunter 430 Waveland, MS
If your fully loaded boat is floating at berth...
What does the boat weigh in water?

Correct answer wins a SBO beer :beer:

Jim...
 
JamesG161

JamesG161

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Feb 14, 2014
7,425
Hunter 430 Waveland, MS
Clarification on question...
Your fully loaded boat @All U Get, on hoist weight = 40k pounds.
Jim..

PS: sorry @DrJudyB I missed your post, when I asked the question.:biggrin:
 
Will Gilmore

Will Gilmore

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Oct 19, 2017
7,747
O'Day 19 Littleton, NH
All U Get said:
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.
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All U Get said:
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?
Click to expand
The two numbers that you are talking about are not related. Displacement refers to how much fluid is displayed if floated on a liquid medium. It is a direct correlation to the mass, of the floating object. With a fairly consistent gravitational pull by the Earth, that mass is convertible to weight, which is an expression of force.

For boats, displacement comes is two basic numbers, three here. One is the weight, or displacement of the boat when just hull and basic equipment, the design displacement. The second is the cruising displacement, the weight of the boat when fully outfitted and ready to sail. This should be close to what the travel lift scales weigh, minus what had been removed to take home. The third displacement is closer to what you're thinking of, the loaded displacement. I may have the exact terms wrong, but this is the capacity, in cargo, that the boat is certified to hold. It is not the displacement required to sink the boat.

As JamesG pointed out. A simple linear relationship like what you are asking about would occur only if the bottom of the boat is flat and the sides were plumb. Then, every foot of boat below the waterline would displace the same amount as every foot of boat above the waterline.

It is my guess, that the use of displacement for boats came about because it is easier to calculate the volume of boat design than the weight when you don't know the materials you are building it out of are consistent in their density. English oak doesn't have the same density as mahogany or pine, for example.

-Will (Dragonfly)
 
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Kings Gambit

Kings Gambit

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Jul 27, 2011
5,004
Bavaria 38E Alamitos Bay
All U Get said:
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.
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Good question. However, neither of the bricks are designed to float. If they were, the heavier one would have to displace more water. I guess this supports the “no correlation” statement. Two objects of different weights have the same displacements, etc. So, one has to resort to speaking only in terms of objects heavier than water that float!
 
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DrJudyB

DrJudyB

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Jun 25, 2004
1,108
Corsair F24 Mk1 003 San Francisco Bay, CA
Terry Cox said:
Hi Tom, looks photo shopped to me. Can't imagine a boat weighing that much. Must be a misprint on the sign.:p
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Take a look at some 50+ foot Swans. Or boats built by the Cheoy Lee Boatyard or Tayanas.

A Hunter 54 has a reported displacement of 20,500 pounds. It weighs less than half as much as its more expensive sisters of the same length. It costs way less than half as much.
 
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JamesG161

JamesG161

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Feb 14, 2014
7,425
Hunter 430 Waveland, MS
Will Gilmore said:
if the bottom of the boat is flat and the sides were plumb. Then, every foot of boat below the waterline would displace the same amount as every foot of boat above the waterline.
Click to expand
The bottom could be like a sailboat, the sides are the key for a linear relationship.
Jim...
 
smokey73

smokey73

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Oct 26, 2010
1,904
Hunter 40.5 Beaufort, SC
DrJudyB said:
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.
Click to expand
I stand corrected. Thank you Judy.
 
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JamesG161

JamesG161

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Feb 14, 2014
7,425
Hunter 430 Waveland, MS
DrJudyB said:
The formula is
CSF= Beam ((Displacement/64.2)1/3) , with displacement measured in pounds.
Click to expand
I think it is this...
Capsize Screening Formula = Beam / (Displacement / 64)^1/3
Displacement is in Pounds
Beam is Measured in Feet
_________
The divisor was missing in the CSF formula.

The 64 is used to convert cubic meters of displacement volume to pounds.
Jim...

PS: My boat CSF= 1.93
 
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All U Get

All U Get

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Oct 2, 2008
3,807
Pearson/ 530 Strafford, NH
Will Gilmore said:
It is my guess, that the use of displacement for boats came about because it is easier to calculate the volume of boat design than the weight when you don't know the materials you are building it out of are consistent in their density. English oak doesn't have the same density as mahogany or pine, for example.

-Will (Dragonfly)
Click to expand
I thought it was to carry more stuff with less wood.
 
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