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Old 29-03-2011, 16:58   #1
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Minimum Draft for Blue Water?

Hi. I'm still fairly new to the forum and have an inquiry; It seems the more I research, the more confused I get! I know a longer draft is better for ocean sailing, but I keep running across apparent BW boats with anywhere from 5-6' drafts that seem to be capable in the open water (as in: "crossed the Atlantic twice" - "capable offshore cruiser" - "bluewater boat" - "take you anywhere you want to go").

Any input?

Does anyone have any experience with BW sailing with a shoal draft? Is it recommended? It has been my understanding that anywhere from 6'6" and deeper is usually desired.

I know there are many other considerations for deep water, but I would like to know about draft depth.

Thanks in advance for your input!
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Old 29-03-2011, 17:07   #2
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Re: Minimum Draft for Blue Water?

My $0.02:

The lower the center of gravity and the higher the longitudinal axis, all other things being equal, the more stability you're going to have. You can still sail the boat so poorly as to mitigate a good naval architecture but the general idea is the lower the better for stability.

A vessel rolls from side to side along its longitudinal axis (metacenter). The lower the center of gravity is the better. The center of gravity isn't usually in the keel with all the lead or iron, because it's the "center" of gravity. So the batteries, people, water, engine, decks, and rig all cause the center of gravity to up from the keel.

Good naval architecture, a properly loaded and operated vessel is really the key beyond absolutes.

Metacentric height - Wikipedia, the free encyclopedia
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Old 29-03-2011, 17:26   #3
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Re: Minimum Draft for Blue Water?

numerous allied seawind's have been around the world at 4'6" draft....

you simply cannot correlate draft and blue water ability....

any boat without a full keel will have a deeper draft then an equal displacement boat with a full keel... my vote for blue water goes to the full (encapsulated lead) keel... everytime
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Old 29-03-2011, 17:30   #4
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Re: Minimum Draft for Blue Water?

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Originally Posted by KJB View Post
I know a longer draft is better for ocean sailing, but I keep running across apparent BW boats with anywhere from 5-6' drafts that seem to be capable in the open water (as in: "crossed the Atlantic twice" - "capable offshore cruiser" - "bluewater boat" - "take you anywhere you want to go").

Any input?
Complex question, and you are right that there are many boats with very shallow draft that have had very successful and safe ocean voyaging records. The flat bottom centerboard Ovni's come to mind, with only 1m draft with the board up, and have been safely to both the arctic and antarctic.

There are four separate concepts of stability and it helps to examine each to understand the complexity.

(1) Initial righting moment - this is how hard it is to tip the boat the first 1 degree from flat in the water/vertical mast - usually referred to as how "stiff" the boat is. I deep keel, with the lead on a bulb at the bottom, with minimal weight in the rig, is the way to accomplish this. Its desirable for blue water cruising because (a) it makes the boat able to stand up to gusts better and less need for lots of reefing and unreefing and (b) it makes the boat more powerful and faster.

(2) Limit of positive stability - how far over the boat can be knocked and still come right back up the same side, if you go further the boat will continue and go 180 degrees. Righting moment helps here also but hull shape (narrow beam) and cabin trunk shape (big round cabin trunk) and mast floatation are as important. This is obviously desirable in blue water because it gives you the ability to bounce back from a knock down.

(3) Energy required to re-right the vessel once upside down - if the boat does get knocked over and goes 180 degrees how big a wave is required to knock it right side up/how fast is it likely to come back up. This factor is very rarely calculated and depends on whether water has gotten inside the boat and if the sails are up. But a deep keel will help.

(4) Tripping factor - when a boat is pushed sideways how much rotational force is created vs allowing the boat to slide side ways. A deep keel hurts on this factor - creating more tripping/rotation and less sliding.

So, deep keep is almost essential for #1, useful for #2 & #3 but other factors are as important, and negative for #4. So you can see it's a bit of a design trade-off and there is no simple answer.

I know that's not so helpful for you, but I think its realistic. We have sailed round the world with 4' 3" of draft on one boat and 7' of draft on another and they sailed completely differently and we needed to treat them differently but they were both good blue water boats.
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Old 29-03-2011, 17:32   #5
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Re: Minimum Draft for Blue Water?

1st RTW 28' 50's built boat 6' draft

2nd RTW 36' boat built 1975 4' draft

3rd possible RTW coming up, 1976 built, 5' draft.

Catamarans, under 2'

All depends on the design....
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Old 29-03-2011, 17:54   #6
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Re: Minimum Draft for Blue Water?

Great advice all! I really need to dust off my old algebra books for that wiki article - seriously though, it is good reading and provides a good deal of information.

And yes, I agree, a lot has to do with ability and experience!

The four concepts is also very helpful; I have actually been looking at a boat with a 7'2" fin keel with a bulb (#1). Sounds like it could be a very sturdy vessel.
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Old 29-03-2011, 18:05   #7
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Re: Minimum Draft for Blue Water?

I may also wish to add that we will be short-handing the vessel. The crew entire will be my wife and two sons. Does anyone think that would play any role in the decision - shoal vs deep?
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Old 29-03-2011, 18:24   #8
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Re: Minimum Draft for Blue Water?

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I may also wish to add that we will be short-handing the vessel. The crew entire will be my wife and two sons. Does anyone think that would play any role in the decision - shoal vs deep?
It probably depend more on your experience level.

It could be argued that a deep keel generates higher forces and loads and is thus better for experienced sailors who know how to handle those loads/forces and a shallow keel is better for less experienced sailors. I know the BT challange sail training boarts were specifically designed to be a bit tender to keep the forces down. But on the other hand a deep fin keeled boat is usually more fun to sail . . .
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Old 29-03-2011, 18:30   #9
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Re: Minimum Draft for Blue Water?

having sailed once around the world on a 5" draft boat then another on and a half circumnavigations on my present yacht with 11'6" draft i would really reccomend a yacht with a shallow draft,ideally beachable.

this has nothing to do with the sea worthyness of the vessel, more the practical aspects of modern cruising,95% of the time is spent on anchor,all popular anchorages are full these days, a shallow draft vessel will allow you to go lots of places others can not,do you want to be part ot the 2% or part of the 98% cant go there?
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Old 29-03-2011, 18:34   #10
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Re: Minimum Draft for Blue Water?

Only in the decision as to amenities. Some boats that look really nice at the dock are not the best at sea because their hull design has been determined by how much room is needed for a spacious galley, head, etc. Level of comfort may be in inverse proportion to seaworthiness. If going offshore, choose a proven design before worrying about the color of the Formica. There are really good boats, such as the 70s vintage P40, that have retractable keels which enable them to get into more places yet retain seaworthiness. My A35 draws a fixed 5 1/2' which is a PITA at times. Remember, "When your draft exceeds the water's depth, you are most assuredly aground."
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Old 29-03-2011, 18:36   #11
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Re: Minimum Draft for Blue Water?

Without trying to get overly technical (although this seems to be a somewhat technical thread), My primary concern with this question is safety. With that in mind, could anyone address the following:

According to the wiki article mentioned above - all monohull sailboats built since 1970 are designed to have a positive righting arm of 120 degrees, although 90 degrees is sometimes the engineering standard.

I know this may sound rather unknowledgeable, but does that mean that a specific boat with a deep draft will be built to the same righting moment as a specific boat built with a shoal draft?
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Old 29-03-2011, 18:42   #12
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Re: Minimum Draft for Blue Water?

think we might need gord on this but catamarans fail to right after about 92 degrees.......................
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Old 29-03-2011, 18:50   #13
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Re: Minimum Draft for Blue Water?

Here's a good site if you really want to get out the old slide rule.Hull Shape and Performance - resources for cruising sailors
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Old 29-03-2011, 19:01   #14
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Re: Minimum Draft for Blue Water?

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According to the wiki article mentioned above - all monohull sailboats built since 1970 are designed to have a positive righting arm of 120 degrees, although 90 degrees is sometimes the engineering standard.
That's complete BS - different boats have different limit of positive stability. Just for instance . . . my boat was build 1998 and has positive stability to 140 degrees.

If you want to see the stability numbers for a range of boats, the last column in this spreadsheet has the "ORR stability index" - this is positive righting arm with a couple adjustments - you can see there is rather a wide spread. ORR stability spreadsheet

Here is another spreadsheet that shows righting moment, pure positive stability and the IMS stability index for a range of boats IMS stability spreadsheet
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Old 29-03-2011, 19:02   #15
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Re: Minimum Draft for Blue Water?

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The four concepts is also very helpful; I have actually been looking at a boat with a 7'2" fin keel with a bulb (#1). Sounds like it could be a very sturdy vessel.
It's all relative. On a 40' boat, that 7'2" bulb keel would be considered quite deep. On a 60' boat, that same keel could be considered to be "shoal draft."

One thing you want to consider is the displacement/ballast ratio.
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