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Old 27-06-2009, 19:32   #1
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Draft in Fresh Water?

I'm about to go through the trent severn canal in my pearson 365. the max draft in the canal is 5', and my boat draws 4'6"....however, that is likely the draft in saltwater. Does anyone know how much of a change there is in fresh versus salt water? one percent, five, ten? Barring that, does anyone have a good way for measuring the draft while the boat's in the water?
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Old 27-06-2009, 19:41   #2
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I bet GordMay or DavidM will have an answer. I wonder if the canal has been uniformly dredged, if max canal draft is 5' what is minimum?
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Old 27-06-2009, 20:28   #3
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Saltwater is roughly 3% denser than fresh, so adding 3% to your draught would give you a rough, totally unscientific guesstimate.
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Old 27-06-2009, 20:43   #4
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another uninformed, but possibly slightly more scientific opinion:

it would add 3% to the volume of water you displace, which means that the added depth would be closer to the cube root of 3%.. about 1.5%, although it'll be less than 1.5% since the size(at the waterline) increases as you submerge the boat.

Assuming the above figure of 3% more dense is correct
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Old 27-06-2009, 20:59   #5
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Technical...

Here's the ‹ber Technical 'Low Down'... I'm sure there's a simpler answer out there somewhere though. Like adding 3% as Lodesman suggested. This quandary is likely something with a great deal of empirical precedence from ships that make Great Lakes to Ocean transits though.
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Old 27-06-2009, 21:16   #6
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trent canal

Aside from the depth which is not always as advertised, the weeds in the shallower sections can be a real pain. They build up on the keel and rudder to the point where you can hardly make headway. The later in the season the worse it gets. They usually get washed off in locks but don't count on it. I have been told that because there are fewer power boats locking through early in the season they are not getting chopped up as much.
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Old 28-06-2009, 05:36   #7
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Objects (including boats) are more buoyant in salt water, than in fresh water because salt water is denser than fresh water. The salinity of the ocean varies, but the generally accepted average amount is 2.5%; so salt water weighs 2.5% more than the same volume of fresh water.

A cubic foot of fresh water weighs 62.4 pound (density of 1.0); whereas a cubic foot of salt water weighs about 64 pounds (density of 1.025) - So there is 1.6 pounds more buoyancy in a cubic foot of salt water than in fresh.

Due to the shape of a sailboat, the volumetric displacement increases slightly with immersion (as it sinks lower), so the actual difference in buoyancy will be less than 2.5% (only a symmetrical cube, with a consistent waterplane area, will feel a 2.5% difference between salt & fresh water.

See also Ted Brewer on POUNDS PER INCH IMMERSION (PPI): The weight required to sink the yacht one inch. It is calculated by multiplying the LWL area by 5.333 for sea water or 5.200 for fresh. The PPI usually increases as the hull sinks into the water as the LWL area is also increasing due to the shape of the hull above water.
Ted Brewer Yacht Design

Accordingly, I would expect less than a 1-1/3 inch increase in draft (between salt & fresh water), for your boat.

Note: Itís not correct to say that salt water is more buoyant than fresh water. Objects (boats) in salt water are more buoyant than objects in fresh water. The buoyant force is exerted on a boat, not the water itself.
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Old 28-06-2009, 14:02   #8
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Come on guys, the information you present is great but the ballyhoo over the semantics is not becoming. Actually the "Max Draft" (to be able to pass) "is 5' is acceptable. The minimum depth would be 5'. And since the forces are in equilibrium who really cares if you say boat or water.

Sorry, but I think that that kind of harassment on a questioner in intimidating and just shouldn't happen. I know you are all better than that.

Have a good day

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Old 28-06-2009, 14:46   #9
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Actually I enjoy GordMay explanations I guess it's the dweeb in me wishing I was a more certifiable dweeb . Anyway I don't take it as harrassment just a good technical explanation offering an answer to the question.

The concern I have is the 5' stated canal depth reliable? 6" or less clearance would concern me.
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Old 28-06-2009, 17:36   #10
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Not that scientific

Quote:
Originally Posted by bencoder View Post
another uninformed, but possibly slightly more scientific opinion:

it would add 3% to the volume of water you displace, which means that the added depth would be closer to the cube root of 3%.. about 1.5%, although it'll be less than 1.5% since the size(at the waterline) increases as you submerge the boat.

Assuming the above figure of 3% more dense is correct
If I may borrow Gord's symmetrical cubic boat say 1 metre in length, 1 metre in breadth, perfectly square waterplane that remains constant down to the keel. It also weighs exactly 1030 Kg. 1 cubic metre of saltwater weighs 1030 Kg, so it's easy to determine the draught of the boat - 1 m.

As Gord alluded to, sea water is not uniform; variations in salinity and temperature give it a range of density (1020 - 1030 kg/cubic metre) - I chose 3% as the worst case scenario.

If I took my cubic boat into fresh water (which weighs 1000 Kg/cubic metre) it would still need to displace 1030 Kg of water. 1 Kg of fresh water = 1 litre = 1000 cubic centimetres. 1030 Kg = 1030000 CC of water. Since length and breadth remain the same at 1 m or 100 cm, the draught calculates as 103 cm or 1.03 m; a 3% increase in draught for a 3% difference in density.

As has been pointed out, boats tend to have a more complex form than my cubic boat, but contrary to what Gord stated, a boat needn't be a perfect cube to have a consistent waterplane area. TPI or PPI is tied directly to the waterplane area at the actual waterline. The OP's boat with 4'6" draught at the max change of 3% will only sink by about an inch and a half. There are many boats that would not show a significant change in waterplane area from their waterlines to 1.5 inches above. Assuming the boat doesn't have a reverse bow or significant tumblehome (which would cause PPI/TPI to increase), one could assume that the full 3% might represent the worst case scenario.

So for a rough estimate, I stand by my 3% figure.
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Old 28-06-2009, 18:14   #11
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Hey, these guys are what makes this forum tick. When I see a post like this, I usually wait a day or two for the smoke to clear, come back and see what the result is. I'm glad we have the "dweebs" we have. Don't think its coming off mean spirited just having fun with a conundrum.

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Old 28-06-2009, 18:33   #12
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Actually the frequent focus on semantics and not the bottom line is why I personally don't post more. I do wonder how many more people would post thier ideas if not fearful of the critique and criticism from others on what they post and if it is phrased "just so". After all, the interest in the forum for me is knowledge and experiences of others- I can use google and search online with the best of them for the book answer. I still find the forum a fantastic source of collective knowledge, which is why I keep coming back. (sorry for the thread drift)
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Old 28-06-2009, 18:46   #13
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Quote:
Originally Posted by Lodesman View Post
corrections
Hey! you're totally right, percentages can't convert like that. Ignore my previous message, my apologies!
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Old 28-06-2009, 19:25   #14
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I don't mind anybodies input......sheesh...the people who really make me chuckle are the ones who start with...."I was reading about blah-blah-blah and it sez I should have a Bonefarb Frammulator plumbed in to my Primary Phlogiston Pistonizer....will that interfere with my Ultrasonic Algae-Q Barnacle Biter Circuit?
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Old 29-06-2009, 05:03   #15
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Quote:
Originally Posted by svquest2 View Post
Come on guys, the information you present is great but the ballyhoo over the semantics is not becoming. Actually the "Max Draft" (to be able to pass) "is 5' is acceptable. The minimum depth would be 5'. And since the forces are in equilibrium who really cares if you say boat or water.

Sorry, but I think that that kind of harassment on a questioner in intimidating and just shouldn't happen. I know you are all better than that.

Have a good day

Joe S
I apologise to anyone who felt harassed or intimidated by my little elucidation on “buoyancy”.

Since nobody actually misapplied the term
buoyancy to water, it wasn’t directed to (or “at”) any particular individual. The lexigraphic note was intended merely a trivial aside.

I care about semantics*, because the use of precisely correct language communicates technical ideas (meanings*) better than does sloppy semantics.

In this case, understanding that buoyancy is a property of (or a force acting upon) the boat, rather than the water, is useful in understanding the concepts of displacement and (water) density.

* Semantics is the study of the meaning of words.
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