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Discussion Starter #1
I know there is a difference in buoyancy between salt and fresh water. Some cocktail napkin math says that the difference on a generic 35-40' boat might be a draft difference of .25" to .5". Still a little curious what manufacturers use.

Also is displacement advertised as a "dry" weight with water and fuel tanks empty? A cruising boat can easily have over a 1,000# of water and fuel.
 

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Goodness.... here's a hairy question!!
Salinity really only affects large commercial boats and they must have the markings.
As for dry weight / displacement / light weight / half weight etc etc but yes, my boat displaces 8 tonnes... with fuel and water and bodies its another tonne..... add cruising junk and its a large addition percentage.
Where is comes into play, in reality, is many new boats or ones only used in flat water, is when you go cruising and a heavier boat and healing will put a dirt line far above the antifouling line. I have increased mine about 5 inches to help.
 

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Discussion Starter #3
I'll admit that I've been looking at too many numbers the last few days and am going off the deep end a bit. Part of my wandering mind stemmed from another discussion I had. As part of the cruising grounds that I want to cover in my eventual boat, there is a fresh water area that is theoretically maintained at 6' but they make you sign a waiver if you draft more than 5'. I can easily see adding an inch for tankage and an inch or two for people, provisions, and other cruising gear. Just trying to think about what boats I may need to eliminate from consideration.
 

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Good academic question. For practical purposes, I don’t believe the manufacturers numbers are that exact anyway. They have no idea what will be permanently installed aboard, let alone might manufacturing differences add up. Ours, for example, comes in four different configurations, which clearly have different mass issues, but they only publish one draft number. I think each vessel is likely different by more than the salt/fresh difference.
 
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Good academic question. For practical purposes, I don’t believe the manufacturers numbers are that exact anyway. They have no idea what will be permanently installed aboard, let alone might manufacturing differences add up. Ours, for example, comes in four different configurations, which clearly have different mass issues, but they only publish one draft number. I think each vessel is likely different by more than the salt/fresh difference.
I'm sure that's true. Not to mention that they have no control over how much crap we all pile on board. FWIW I am about an inch deeper on my 33 in fresh water.

And remember that the boat is going to be moving up and down with the water motion, so you don't want to cut it THAT close anyway.

What cruising area has an entry guard that checks for waivers? That seems unworkable.
 

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I think the short answer to the question is that there is no standard yardstick for calculating the draft of a boat; although there may be a rule of thumb that used as a starting point, every manufacturer probably does it differently. Moreover, the big manufacturers often modify their designs as they go through production; sometimes from year to year, sometimes from boat to boat. I'm referring to changes in location of water tanks, batteries, machinery, etc. I've never seen the sales literature reflect any changes in weight or draft as a consequence. In any case, I doubt these kinds of changes impact the draft by more than an inch or two. If an inch or two makes that much difference to you, be very careful about what boat you buy. I wouldn't trust any published data to give you more than a number within 6 inches of the actual draft.
 

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I wouldn't trust any published data to give you more than a number within 6 inches of the actual draft.
Funny, that's exactly the fudge I put in my depth transducer offset.
 

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Coming out of the Amazon on my freighter, I had some concern about that, as the fresh water can extend out over 100 miles.
At first I met every confluence bow on, but that was very tiring, even with a toggle steering. There was a definite dip as she hit fresh water from salt, etc., but it wasn't enough to worry about capsizing.
 

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Discussion Starter #9
What cruising area has an entry guard that checks for waivers? That seems unworkable.
End goal with the boat is to complete the great loop using the Trent-Severn waterway through Canada from Lake Ontario to Georgian Bay. Several people in great loop discussion groups have said they had boats with 5'6" drafts but they were glued to their depth sounder the entire way.
 

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End goal with the boat is to complete the great loop using the Trent-Severn waterway through Canada from Lake Ontario to Georgian Bay.
Ahhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh! An explanation that will make it far easier to help! :)

What I did a few years ago, and you could do once you are in fresh water, one a nice warm day, no wind, and clear water, onto a gently shallowing sandy beach: Lassooed my girlfriend when she was trying to escape and she slowly dragged the boat ashore until I could have the keel just touch the sand. Then I could set up my depth guage exactly, and also, my leadline - a coke bottle with a lump of weight at the bottom and tied a knot in it exactly at keel depth.

It certainly made me much, much more comfortable sailing around. And I still use that leadline today, same knot :)

Mark
 

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Typically displacement and draft are calculated for fresh water, but no guarantee. However, the difference in draft in high salinity sea water vs fresh is 2.5 %. So on a 5 ft 6 inch draft vessel a difference of ~ 1.65 inches. Measurable yes, but not terribly useful for navigation purposes.

Trent Severn lock sills are 6 feet btw, not 5ft 6 and Parks Canada is pretty good at hitting that target. Depth challenges are more likely to be outside of or at the edges of the channel on the way to marinas or getting squeezed to the outer edges of the channel by boat traffic than in the locks themselves.
 

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So here is the deal. At least when I was designing boats, displacement was calculated two ways and the numbers had to match each other. In designing a boat, the designer is the one who sets the shape of the boat that they want. From experience and rough sketching, the designer is starting out from a collection of parameters. The length and beam are often roughed out from a sketch of the desired interior layout. The waterline is roughed out from that length on deck and a profile sketch, The amount of the displacement is chosen from a mix of past experience for the type of boat being drawn and tested against a desired D/L ratio. Then the lines are drawn and tweaked to get the hull shape, buoyancy distribution, and displacement that is desired.

In laying out the lines, the designer sets where they want waterline to be, and the displacement is calculated as a volume of the hull, keel, rudder etc. from that waterline down. Traditionally that volume is then multiplied by the weight of seawater (63.9 lbs per cu. ft. which the old timers rounded to 64 lbs per cu ft.) And that gives the designer the displacement at the DWL (Design Waterline).

Then the designer calculates the weights and positions of everything on board and the weight of everything on board has to equal the design displacement and buoyancy distribution if the boat is going to float on its design waterline.

Draft is then measured from the design waterline to the bottom of the keel, and mostly that is what is published.

"But wait" you say, "Is that with the tanks and storage empty or full, with crew and gear?" And that is the part that is variable. To begin with, if the designer figures the weight of the hull, deck, interior fit out, hardware and rigging and all of the expected fixed components of the boat, minimally subtracts that from displacement and uses the delta for ballast, that would be the dry weight of the boat, But with sloppy workmanship, or teak decks, a bigger engine and battery bank, pretty quickly the designer's dry weight displacement is quite a bit lower than the actual 'dry weight' displacement of the boat and the draft gets deeper.

There was a time when the design waterline was drawn at the 'lightship' displacement and that was the published displacement, Lightship assumed some partial filling of the tanks and some storage and so on. But in my lifetime, the published dimensions for boats was shifted to match the way that racing rules measured boats, which for displacement meant dead empty. And the published draft matched a boat sitting on that dead empty design waterline. Some cruising boat manufacturers who somehow saw heavy displacement as a virtue would publish a fully loaded displacement (tanks and storage lockers full,) But they saw shoal draft as a virtue so would publish a draft based on the waterline with the boat loaded to lightship or the race rule. (It has been alleged that Island Packet did tended to do that as well as jiggering their sail area calculations from the standard measurement methods.)

So when you look at the published numbers for a performance oriented design, you can be pretty sure those were the intended weights and draft for the boat dead empty. When it comes to cruising oriented designs, all bets are off.

And the other part of this is water plane, (the area of the boat that is at the waterline.) That controls submersion inches (the amount that the displacement increases as the boat sits an inch lower in the water). Light weight boats for their length tend to have larger water planes relative to their displacement and so tend to sink less per a given percentage of their displacement that is added. The result is that light displacement boats can carry a larger percentage of their dry weight displacement in excess carrying capacity, But while heavier boats for their length may only be able to tolerate adding smaller percentage of their overall displacement, they of course start out with more displacement, so may actually still have more surplus carrying capacity overall.

And so while the difference in the weight of a cubic foot of fresh water is 62.47 lbs and a cubic foot of salt water weights roughly 64 lbs. (maybe adding 200 lbs and a 1/4" submersion on a 10,000 lb boat) the bigger difference in draft will come from how full your tanks are, how many AGM's you added, what's in your tool box and spare parts kit, and how many weeks of can good you brought aboard,

Jeff
 
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Perhaps what Jeff means to say is that if you're worried about being .5" to 1" deeper in fresh water, you need to put the crew in the dinghy and tow them behind. Getting their weight off the boat should more than compensate for the difference.:)
 

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Years ago, when I was shopping for a used boat, the Bristol brokerage told me, as a rule of thumb, to add 4” to the published draft to allow for added extras.
 

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Discussion Starter #15
Thanks Jeff for the engineering/design input. The more I learn, the more I want a boat with an advertised draft of 5' or less. Between manufacturing tolerance, fluids, gear, and people, we may add a significant amount of draft. I'd rather not be measuring the amount of water under the keel in fractional inches.
 

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Thanks Jeff for the engineering/design input. The more I learn, the more I want a boat with an advertised draft of 5' or less. Between manufacturing tolerance, fluids, gear, and people, we may add a significant amount of draft. I'd rather not be measuring the amount of water under the keel in fractional inches.
I will point out that there is nothing magical about 5 feet or less. That number gets tossed around like it is a magic wand that will prevent groundings. The only thing that minimizes groundings is careful navigation and accurate and up-to-date charts and truly up-to-date charts almost do not exist. A few inches in draft won't keep you from staring at the depth sounder, and staring at the depth sounder won't prevent you from hitting that uncharted hump, rock, of debris on the bottom.

When I lived in Savannah, and in South Florida and when I moved to the Chesapeake, I always heard that the maximum draft boat that made sense was a 5-0 draft (Soth Florida the magic depth was 4-0). I sailed boats with 8 foot draft in Savannah, I owned a nearly 6-0 draft boat when I lived there that I ran up and down the ICW on a regular basis. In Sarasota we owned a 5-6" draft boat and did not have a depth sounder other than a lead line. I have cruised most of the Chesapeake with a roughly 6'-6" draft.

The point being that while there are certainly places where less than 5 foot draft might be useful, some arbitrary depth should not rule your life to the point that you buy a boat that does not sail worth a darn just to so you can have some theoretically magic draft.

But the other thing is that in tidal waters or larger bodies of non-tidal waters, the depth can vary by far more than the difference that loading can make. I live in a creek where the normal shallowest spot in the channel is around 8 feet at low tide. I go out in my Kayak and measure it periodically and the deep spot in the channel moves and changes depth almost annually, but even so, when there has been a strong north wind, the depth in the channel can drop to around 7 feet. (Pretty skinny for a 6-6 draft boat,) but we have had gale force north winds that lasted for several days that drove so much water out of the channel that the depth dropped to less than 5 feet for a few hours.

Jeff
 
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