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Old 10-06-2019, 05:41 AM   #1
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Understanding Draft

I'm struggling to understand the huge differences in draft on what I'd consider to be similar full displacement trawlers. I've recently been lusting over a SeaSpirit Queenship and its draft is listed at under 5 feet. I took a look at a similar sized Seaton trawler and its draft was over 9ft! All other things being equal it appears a Nordy has significantly more draft than a KK. And for reasons that really perplex me a KK with twins drafts a foot less than a KK with a single (I do understand why there might be some savings in draft but a foot??)

All other things being equal, are boats with more draft inherently more stable?

I'm obsessed with draft because I envision a significant amount of our boating to be in the Bahamas.

Thanks as always!
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Old 10-06-2019, 06:07 AM   #2
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First of all, KK's brochures are probably accurate for their boats. As you look at used boat listings, you may find mistakes either based on owner or broker's information. When it comes to this information, you need to verify it for yourself.

Draft is also relative to load and fresh versus salt water. Boats will draft a little more in fresh water as it weighs less than saltwater. A boat fully loaded for an expedition can draft significantly more than the same boat doing weekend cruising. 600 gallons of fuel and 300 gallons of water adds around 5,700 pounds and inches to the displacement.

I purchased my boat with the intention to do inland cruising and the Great Loop. My goal was to have less than a 5' draft and a 15' air draft. The reality was that certain boats wouldn't meet that requirement and I just had to eliminate them from my list and move on.

Ted
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Old 10-06-2019, 06:24 AM   #3
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If you plan to do significant cruising in the Bahamas, especially the Abacos try to keep your draft to 5' or less. There are lots of harbors and inlets where 6' will be tough. You can do it with 6' or more, but you will often be anchoring outside in less protected waters.

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Old 10-06-2019, 06:42 AM   #4
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Compare the beam with the boats. Wider beam MAY affect the draft too.
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Old 10-06-2019, 09:20 PM   #5
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Quote:
Originally Posted by GraniteStater View Post
I'm struggling to understand the huge differences in draft on what I'd consider to be similar full displacement trawlers...



...All other things being equal, are boats with more draft inherently more stable?



I'm obsessed with draft because I envision a significant amount of our boating to be in the Bahamas.



Thanks as always!

You could spend a semester or three studying stability. I did, and I forgot the vast majority of it. The short version is that stability is how the center of gravity relates to the center of buoyancy.

Draft on its own doesn’t determine stability. The RoRo on it’s side in Brunswick, GA definitely had a deeper draft than any of our boats. For whatever reason, her center of gravity moved to a place where her center of buoyancy couldn’t counter it, and she rolled.

The draft is really just a function of hull form and weight. If two boats are identical in size and shape, but one weighs more, the heavier one will have a deeper draft.

If that extra weight is added down low, it will likely have more stability than the lighter boat. That’s not always necessarily a good thing. That might make her rolls and recoveries too fast, and too violent, and it can break things and hurt the crew. That’s referred to as a ‘stiff’ ship.

If the extra weigh is all added up at the top of the mast, then her draft will still be the same, but her stability will be much lower. Her rolls will be slower, and she’ll take longer to recover. A slow rolling, wallowy ship is a ‘tender’ ship.
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Old 10-06-2019, 09:50 PM   #6
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If a were buying a boat for significant use in the Bahamas, I'd be looking for less than 5', and if possible less than 4'. I was there with 6.5' and about half of it was inaccessible.
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Old 10-06-2019, 10:14 PM   #7
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The length is not the only thing to look at. Breadth, Hull shape, actual boat weight, type of keel, single or twin engine.
A rounder bottom will typically be deeper than the same weight boat with a flatter bottom. A heavier boat will typically be deeper than a similar but lighter boat.
A single engine will typically be deeper than a twin engine boat.
Some boats use a much deeper keel than the same length of another boat.
A wider boat of the same length and weight will have less draft.
Some boats use cored fiberglass which gives great stiffness and cuts weight, other build in steel which usually is heavier size for size.

My 32 ft has a 5' draft, all right 58-59", a good deal more than many larger boats. It has a narrow but deep keel of fabricated steel, a single engine with a relatively large prop, a heavier built hull that many similar sized vessel than were built as pleasure boats only. Mine was a commercial hull on which they designed and built a pleasure cabin so they were typically a bit thicker thus heavier. It's breadth or beam is about one foot or more less than later builds. Mine is 11'4" whereas many 32's are 12' or more.

Even my engine/gear contributes to the ballast as it is 2,200 lbs , it's old design, where newer boats use engines that are as or more powerfull but as much as 700 - 800 lbs lighter.

So you need to have all the specs. AND understand the builders intentions and building ideas.
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Old 10-06-2019, 11:29 PM   #8
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If you like the Bahamas, you may want to add a few catamarans to your list of boats to view. They are generally quite shallow for the same length, but have their own particulars including higher initial cost and sensitivity to weght. Still, they provide a stable (if not a bit snappy) ride, a good turn of speed when you need it, and economy at a greater range of speeds.

One thing to keep in mind though, they may only be three feet or so in the water, but it’s three feet over here, and three feet way over there, which on modern catamarans can be more than 20 ft. away.
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Old 10-06-2019, 11:53 PM   #9
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Originally Posted by GraniteStater View Post
I'm struggling to understand the huge differences in draft on what I'd consider to be similar full displacement trawlers. I've recently been lusting over a SeaSpirit Queenship and its draft is listed at under 5 feet. I took a look at a similar sized Seaton trawler and its draft was over 9ft! All other things being equal it appears a Nordy has significantly more draft than a KK. And for reasons that really perplex me a KK with twins drafts a foot less than a KK with a single (I do understand why there might be some savings in draft but a foot??)

All other things being equal, are boats with more draft inherently more stable?

I'm obsessed with draft because I envision a significant amount of our boating to be in the Bahamas.

Thanks as always!
Just looking at your simple example. A KK 52 has 47'6" waterline length and a N 53 has 48'3". KK Beam is 17'9" and N is 16'1", so significantly narrower. KK weighs 70,000 lbs and N weighs 90,000 lbs. So, Nordhavn is narrower and much heavier, nearly 30% heavier. KK Draft with half load is 5'5" with single and 4'6" with twins. N Draft is 5'11". It makes sense that KK draft is less based on weight spread over the bottom area. Then it makes sense that draft with twins is less based on their location in the hull.

One other number seldom published though is minimum and maximum drafts and we're left to assume that it's maximum. However, KK uses a term Designed Water Line with half load and that isn't maximum. So, assume safely with a full load that the KK draft is likely 2-3" more.

A lot of factors go into it.
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Old 10-07-2019, 04:39 AM   #10
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Do not confuse "draft," depth below the water line, with "hull depth" a measurement term for boat documents.
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Old 10-07-2019, 02:45 PM   #11
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Then it makes sense that draft with twins is less based on their location in the hull.
This doesn't make sense to me at all. I may be missing something, tho.

Weight in the hull lowers the waterline the same no matter it is located, unless there is some heel.
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Old 10-07-2019, 02:48 PM   #12
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This doesn't make sense to me at all. I may be missing something, tho.

Weight in the hull lowers the waterline the same no matter it is located, unless there is some heel.
Depending on engine size for the same boat with twins vs single, the twins may be spinning smaller diameter props. And the props aren't under the lowest point of the hull. And they likely have 2 smaller rudders instead of 1 big one. So depending on keel design / size / placement, it's possible for the running gear to be tucked up tighter than on a single.
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Old 10-07-2019, 03:01 PM   #13
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This doesn't make sense to me at all. I may be missing something, tho.

Weight in the hull lowers the waterline the same no matter it is located, unless there is some heel.
The shafts and props are at a higher point of the hull. They are higher than the deepest part of the V. Single engine is in the deepest part of the V. So, the shaft and props of twins may be 3" to 12" higher than those of a single.

Note that this is only true when the shaft and props are the most extended parts of the boat into the water.
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Old 10-07-2019, 03:17 PM   #14
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The shafts and props are at a higher point of the hull. They are higher than the deepest part of the V. Single engine is in the deepest part of the V. So, the shaft and props of twins may be 3" to 12" higher than those of a single.

Note that this is only true when the shaft and props are the most extended parts of the boat into the water.
Got it. Thanks.
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Old 10-07-2019, 08:17 PM   #15
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First of all, KK's brochures are probably accurate for their boats. As you look at used boat listings, you may find mistakes either based on owner or broker's information. When it comes to this information, you need to verify it for yourself.

Draft is also relative to load and fresh versus salt water. Boats will draft a little more in fresh water as it weighs less than saltwater. A boat fully loaded for an expedition can draft significantly more than the same boat doing weekend cruising. 600 gallons of fuel and 300 gallons of water adds around 5,700 pounds and inches to the displacement.

I purchased my boat with the intention to do inland cruising and the Great Loop. My goal was to have less than a 5' draft and a 15' air draft. The reality was that certain boats wouldn't meet that requirement and I just had to eliminate them from my list and move on.

Ted
I learned something today! Thanks Ted. Didn’t realize the difference in fresh water verses salt water!
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Old 10-07-2019, 09:20 PM   #16
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I learned something today! Thanks Ted. Didn’t realize the difference in fresh water verses salt water!
In my 44,000 lb boat, the difference in bouyancy going from fresh to salt water is like losing 1/2 a load of fuel
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Old 10-07-2019, 09:53 PM   #17
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It can also depend on owner choice. I was talking to a Linssen yacht dealer about one of their boats. I asked what the draft was and he said "It's a steel boat, we can make it whatever you want between [this] and [that]." Don't remember the exact numbers but the difference was measured in feet.
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Old 10-08-2019, 07:06 PM   #18
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Thanks all - this has been incredibly helpful!!!
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Old 10-08-2019, 10:00 PM   #19
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Raised on fishing boats in the North Sea.
Can't bring myself to spell it "draft"
DRAUGHT !
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