Minimum Draft for Blue Water?

[KJB]

Wow. So basically, when I see a boat I like - do the homework! Nothing beats hours of mind-enlarging research huh? Seriously, I'm getting a lot of good information here and appreciate the responses.

Displacement/Ballast ratio sounds like a term to become familiar with.

For the record, the boat w/ the 7'2" bulb keel is 45'. Nice site smurphny!

[smurphny]

One other thing to think about is the angle of heel the boat normally attains when close hauled or reaching. My A35 heels very quickly and stays there with the rail almost in the water. Having to operate with the deck at a steep angle for extended periods is not comfortable for a lot of folks. There must be some sort of ratio for this but narrower hulls with short waterlines relative to overall length have this tendency. They are designed to put more hull in the water and increase wetted surface as the angle of heel increases.

[KJB]

Originally Posted by estarzinger

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

Thanks for the spreadsheet! A great evaluation tool.

[Bash]

Originally Posted by KJB

For the record, the boat w/ the 7'2" bulb keel is 45'.

Hmmmm. Bulb keel. 7'2" on a 45' boat.

This wouldn't be a retired racer, would it?

[KJB]

Originally Posted by Bash

Hmmmm. Bulb keel. 7'2" on a 45' boat.

This wouldn't be a retired racer, would it?

Originally owned by a racer - but a two owner boat owned by same couple since 1980. Off subject but, needs some work, nothing unusual I know of.

[KJB]

I should add - it's just one I'm looking at. My criteria are more along the lines of safety and condition, with comfort (although important) lagging behind the former.

[barnakiel]

I do not see any magic nor complexity here. Ovnis are very shallow and they make long offshore voyages on regular basis. My own boat has only 3 ft draft. Draft alone does not say anything about seaworthiness nor about stability. I do not think there is a limit here.

Too deep a boat is a problem though - if you are going to visit any shallow places.

b.

[KJB]

I really appreciate all the replies and advice. I've learned quite a bit already as I've read through some of the indicated material. Also, just starting this thread and thinking about the responses has forced me to reevaluate some things.

One of those things is that i don't think I want a fin keel - for several reasons; bad to hit things with, cannot beach, bolted onto hull, not integrally a part of, etc.

There is one thing I keep thinking on though; If a shoal draft is just as sea-worthy as a deep-draft (as some have supposed), then why make one?

I understand I don't know nearly as much as many of you about boats and sailing, however, a draft that is deeper seems more stable than a draft that is shallower - otherwise, why would they make them?

Just a thought.

[atoll]

throuroughbreds,steeplechasers and hacks,horses for courses.......

[KJB]

Originally Posted by atoll

throuroughbreds,steeplechasers and hacks,horses for courses.......

I guess that means "whatever floats your boat".

[Adelie]

Originally Posted by KJB

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?

Somebody contributing to the Wiki is passing off a reccommendation from ABS or some other boating authority as accomplished fact.

Case in point would be the J24 (mid-70's design) which reaches it's limit somewhat over 90 degrees from the info I could find in a casual search. Recalling photos of crew standing on their boat's keel trying to right them leads me to believe this is approximately correct.

In addition to the items Evans listed above the big functional use for deep draft is improved upwind efficiency. On the otherhand owners on one of the recent Centerboard threads indicated the boats did as well with board up or missing on all points of sail except close hauled where there was some loss of efficiency. Some of these centerboarders draw as little as 3'6" board up and would be generally suitable offshore with appropriate upgrades.

While range of stability and energy required for a capsize (area under the curve) play into capsize resistance, so does roll moment of inertia. Contrary to expectation a heavier mast can help capsize resistance. At what point it stops helping and starts contributing to the problem is complex. So just because the boat has a shallow draft doesn't preclude it from going offshore, other factors are also at play in a very complex fashion.

A

[KJB]

Thanks Adelie. That actually helps me figure things out.

[Adelie]

Originally Posted by KJB

I really appreciate all the replies and advice. I've learned quite a bit already as I've read through some of the indicated material. Also, just starting this thread and thinking about the responses has forced me to reevaluate some things.

One of those things is that i don't think I want a fin keel - for several reasons; bad to hit things with, cannot beach, bolted onto hull, not integrally a part of, etc.

There is one thing I keep thinking on though; If a shoal draft is just as sea-worthy as a deep-draft (as some have supposed), then why make one?

I understand I don't know nearly as much as many of you about boats and sailing, however, a draft that is deeper seems more stable than a draft that is shallower - otherwise, why would they make them?

Just a thought.

A lot of popular boat design is driven by whatever the current rule is governing handicapping of boats so that different designs can race together. The idea is that by handcapping the results the only thing affecting the outcome is the relative skill of the skippers and crews. In reality the designers looked for loopholes in the rules and gained advantages for a year or 3 before the rules were modified to close the latest round of loopholes. I believe this situation has abated a bit with some makes and models being designed more with non-competative sailing in mind, though racing is not completely ignored by most designs.

As far as draft goes, draft and beam can be interchanged up to a point while maintaining stability. The righting moment curve will change shape some while doing this. Taking either to extremes will have negative consequences.

[KJB]

Example: Boat A and B are the same model, but boat A has a draft that is longer than boat B. Do both handle the open ocean equally well? I have come across this in my searches.

Any ideas?

[downunder]

One of the good books on the subject is

Seaworthiness : The Forgotten Factor by Czeslaw A. Marchaj

which is available at Amazon - should be a good libaries.