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Old 28-07-2019, 00:13   #31
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Re: Boat lower in water

I have not read all the posts so don't know if my bit of silliness is unique in this thread.

You obviously have the boat parked above a strong gravity anomaly. Move it away and it will rise back up.
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Old 02-08-2019, 18:50   #32
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Re: Boat lower in water

The difference in draft between fresh and salt will barely be noticeable.
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Old 02-08-2019, 18:54   #33
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Re: Boat lower in water

It is surprising, but the US lost the first two or three “S” boats in or just after WW1 moving from the Great Lakes to the Atlantic. Glad my grandfather was in a later one.
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Old 02-08-2019, 18:59   #34
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Re: Boat lower in water

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Originally Posted by GrowleyMonster View Post
The difference in draft between fresh and salt will barely be noticeable.
Working from memory (Which is very fallible these days) the SG of sea water is around 1.025 which is 2.5% more than fresh. So guessing at the prismatic volume of say a displacement sail boat I'm going to propose somewhere between 0.5 to 1".
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Old 02-08-2019, 19:07   #35
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Re: Boat lower in water

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Originally Posted by RaymondR View Post
Working from memory (Which is very fallible these days) the SG of sea water is around 1.025 which is 2.5% more than fresh. So guessing at the prismatic volume of say a displacement sail boat I'm going to propose somewhere between 0.5 to 1".

An inch is about right for most of the boats the forum members here will own, yeah. At least in my empirical observation. Like I said, barely noticeable. Get up to 50 or 60 foot and it will be a bit more but still not much. Compare the FW draft difference to the molded depth of the hull and it isn't much at all.
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Old 03-08-2019, 20:18   #36
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Re: Boat lower in water

My boat was 70 ft,and the hull was 2inch thick strip plank mahogany, it would weigh in at about 128,000 lbs at haul out after a couple of years in the water. Following the haul out , after a month or more on the hard, it would always weigh a lot less since it would partially dry out. Sometimes as much as 8000 lbs less. The difference was not the growth since I would dive on it regularly and scrape thr hull.
I did notice a slight change in draft but not a significant amount
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Old 03-08-2019, 20:32   #37
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Re: Boat lower in water

Again my concern is - remember the boat is now 55 years young (old) that when we slipped her in Seattle, cold fresh water, I asked how long since she had been anti fouled, as i wiped a bit of scum/sheen off the hull - below the water line.

'5 years!'

Sheeeet! I replied, you would have a hull like that in 3 months back home!

I am roughly ten miles up river with a tidal flow of up to 3' and we get weed - salt water and high ultra violet - plus some years (??) a coral like substance clinging to the boat.

You can definitely tell the boat to be lighter after slipping, as she seems to bob like a cork! Am soon to slip so when she's back in the water - the 'Plimsoll Line' will be of interest!

Cheers
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Old 03-08-2019, 20:55   #38
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Re: Boat lower in water

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Originally Posted by RaymondR View Post
I have not read all the posts so don't know if my bit of silliness is unique in this thread.

You obviously have the boat parked above a strong gravity anomaly. Move it away and it will rise back up.
Ha ha! The force of gravity affects the buoyancy as well as the weight of the boat. They cancel! The displacement would be the same on the moon as on Jupiter and on Earth, if the temp on Jupiter and the moon were the same as on Earth.
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Old 03-08-2019, 23:27   #39
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Re: Boat lower in water

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Ha ha! The force of gravity affects the buoyancy as well as the weight of the boat. They cancel! The displacement would be the same on the moon as on Jupiter and on Earth, if the temp on Jupiter and the moon were the same as on Earth.
Yeah but because of global warming the oceans are now a lot hotter and have expanded thereby lowering the density.

I was obliged to research the subject of Mean Sea Level during my offshore oil drilling days and was surprised to find the magnitude of gravity induced variations over fairly short distances. I vaguely recall figures up to a metre.
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Old 03-08-2019, 23:33   #40
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Re: Boat lower in water

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Originally Posted by gbmacca View Post
'..., my boat is sitting 1 - 2 inches lower in the water!

...is it the fresh water...?

Gbmacca
Here is the math: The answer has nothing to do with the prismatic or block coefficient, but just to get an idea of the possible effect of a change in density of the water, I looked at my boat, Hunter 405, displacement 20,000 lb, with water line length of about 35 feet, max beam of about 12 feet, and average beam of about 8 ft, per eyeball. Just to keep it simple, replace the hull shape with a block with the same WL and average beam. Since we are looking only at small changes in draft at the waterline, the details of hull shape are irrelevant. Then, the draft (depth below the surface of the water), for the block shape, h, is as follows:

h = wt of boat/(density of water X waterline area)

If we have density 1 and density 2, say, 62.4 lb/ft^3 (nominal fresh water) and 64.2 lb/ft^3 (nominal salt water), then the depth difference due to the change in density is

h2 - h1 = wt of boat/(WLA X density 2) - wt of boat/(WLA X density 1)

A little bit of algebra gets us to:

h2 - h1 = (wt of boat/(WLA X density 1)) X (1 - (density 2)/(density 1))

We can see that a small change in draft due to a change in water density depends only on the ratio of waterline area to the displacement, and has nothing to do with the details of hull shape below the nominal waterline. Using 64.2lb/ft^3 for salt water and 62.4 lb/ft^3 for fresh water, we get:

h2 - h1 = 20,000 lb/((35 ft X 8 ft) X 62.4 lb/ft^3) X (62.4/64.2 - 1)

h2 - h1 = -0.032 ft = -0.4 in, where the minus sign indicates that the draft got shallower for the transition from fresh to salt.

So, the change from salt to fresh can pretty much be ruled out as the explanation for a change in waterline height of more than about half an inch, at least for boats of modest size and draft. As ship length increases, displacement goes up as the cube of length, whereas the waterline area increases as the square of length. So, change in draft from fresh to salt will be proportional to the length of the ship, all else being equal. Or, more intuitively, the change in draft going from fresh to salt is proportional to the draft.
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Old 03-08-2019, 23:50   #41
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Re: Boat lower in water

yes, variations in gravitational forces will change the height of the water surface, as in tides, but still shouldn't affect the draft. Incidentally, I read an analysis that stated that gravitational effects on the water surface due to the sun and moon are purely horizontal. The vertical effects cancel out! So the drawings in all the books that I have seen explaining tides where the oceans are bulging supposedly due to the pull of the moon and sun are B.S. Bulging obviously doesn't explain why tides are highest in more northern latitudes, rather than at the equator.
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Old 03-08-2019, 23:50   #42
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Re: Boat lower in water

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Originally Posted by RaymondR View Post
Yeah but because of global warming the oceans are now a lot hotter and have expanded thereby lowering the density.

I was obliged to research the subject of Mean Sea Level during my offshore oil drilling days and was surprised to find the magnitude of gravity induced variations over fairly short distances. I vaguely recall figures up to a metre.



Currents alone can make a difference of a meter or so in MSL. Gravity makes a difference of well over 100 meters around the globe


https://ocw.mit.edu/courses/earth-at...e_notes_3b.pdf
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Old 04-08-2019, 00:23   #43
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Re: Boat lower in water

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Originally Posted by Rothblum View Post
Bulging obviously doesn't explain why tides are highest in more northern latitudes, rather than at the equator.

That's actually a fallacy. It's a lot more complicated than that:Tides

"Since the TOPEX/Poseidon satellite has been measuring surface height data, the oscillating surface of the oceans due to moon tides could be measured and mapped. Note that the nodes correlate with areas of no tide change, except where these rotate around islands such as New Zealand and Madagascar. Highest tidal ranges are found where continental coasts distort the tide wave."





For a more detailed explanation of the Dynammic Theory of Tides (Declination, Coriolis and Land forms) and those nodes above see
https://www.encyclopedia.com/earth-a...nography/tides
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Old 04-08-2019, 08:17   #44
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Re: Boat lower in water

The encyclopedia.com analysis still clings to the bulge idea. It is true that tides are actually waves -- tidal waves, in fact. As the article points out, they are also shallow water waves. Their behavior is incredibly complicated. So much so, that there is no comprehensive predictive theory of tides. Localities rely mostly on historical records to predict tides, modified by some empirical correlations. I was surprised to learn this. Tide tables are only approximations. I guess nothing is certain....
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Old 04-08-2019, 10:33   #45
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Re: Boat lower in water

A boat that got rotten sinks in before proper sinking.


Haul out and check thoroughly with a screwdriver ...


As others said boats float lower in fresh water, it is quite some difference too.


Cheers,
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