How much does a Shoal Draft Keel affect Blue water sailing?

[IslandHopper]

Originally Posted by Jim Cate

Odd that none of the gurus have brought this up, but a shoal draft MAY influence the AVS adversely, and this means that it is easier to capsize the boat in big seas (gross simplification of a complex issue). It is certainly possible to design a shoal draft boat with a big AVS, but I suspect that many mass producers simply fit a shorter keel with a bit more weight than in their standard draft models and know that very few of their customers will ever sail in conditions where it matters. Perhaps not a concern, but you were asking about such issues.

Jim

I'll just drop these here for you to study Jim, both are the HR310 deep and shoal, good eg i think.....

https://www.hallberg-rassy.com/yacht...erg-rassy-310/

[NYSail]

My last bot was a pearson 36-2 with shoal keel and centerboard. 4’2” board up..... 8 with it down. It was a great setup. Loved the 4’2” draft..... never thought twice when entering thin water. And with the centerboat added to the pointing ability. New boat is 5’6”.... still not so bad

[Suijin]

Your buying a boat to live aboard. Don’t base your choice on how it performs for a few weeks out of 10 years of ownership.

[Over Budget]

To me, “Shoal Draft” means a shortened keel with an appendage the makes the keel look like an inverted “T”. Nevertheless,.

You might be interested in this month’s issue of Cruising World with an article describing 10 well built cruising boats that can be found for under $50K, a number of which are blue-water capable. I believe one has a centerboard which could address your blue-water-shallow-draft question.

[Seabeau]

It appears that many are unaware or have forgotten, that in extreme conditions i.e. breaking seas, it is the boat with the least amount of underwater appendage, that is the safest. Its not wind that destroys boats but water.

[boatman61]

Originally Posted by Seabeau

It appears that many are unaware or have forgotten, that in extreme conditions i.e. breaking seas, it is the boat with the least amount of underwater appendage, that is the safest. Its not wind that destroys boats but water.

Which brings us to the Smaller is Safer arguement..
But then theres members who think a crossing to the Bahamas is ocean sailing.

[MartinR]

Originally Posted by Jim Cate

Odd that none of the gurus have brought this up, but a shoal draft MAY influence the AVS adversely, and this means that it is easier to capsize the boat in big seas (gross simplification of a complex issue). It is certainly possible to design a shoal draft boat with a big AVS, but I suspect that many mass producers simply fit a shorter keel with a bit more weight than in their standard draft models and know that very few of their customers will ever sail in conditions where it matters. Perhaps not a concern, but you were asking about such issues.

Jim

AVS is not influenced by the draft, but by the VCG. Normally, in modern boats, the VCG of the deep and shallow draft versions are almost identical. This due to the fact that shallow draft keels are either winged or bulb, keeping the VCG low.
Also, the AVS is only important once you are knocked down, and then actually the area under the negative stability curve is decisive. Big area, and the boat will be very stable upside down. Beam affects this very much, so a beamy boat will be much more stable in the inverted position, all other factors being equal.

[WingRyder]

Originally Posted by IslandHopper

I'll just drop these here for you to study Jim, both are the HR310 deep and shoal, good eg i think.....

https://www.hallberg-rassy.com/yacht...erg-rassy-310/

Forgive my ignorance, but could someone explain what this diagram means? I assume that the smaller, negative bell curve, is a bad thing.

[MartinR]

It shows righting moment vs heel angle. Where the curve intersects the x-axis is the angle of vanishing stability, as you see around 130 degree heel on these diagrams. At this point, the boat will turn turtle. The area under the positive part of the curve is basically the total positive stability. The area under the negative part of the curve is the total stability when the boat is upside down. You would like this area as small as possible.

[WingRyder]

Originally Posted by MartinR

AVS is not influenced by the draft, but by the VCG. Normally, in modern boats, the VCG of the deep and shallow draft versions are almost identical. This due to the fact that shallow draft keels are either winged or bulb, keeping the VCG low.
Also, the AVS is only important once you are knocked down, and then actually the area under the negative stability curve is decisive. Big area, and the boat will be very stable upside down. Beam affects this very much, so a beamy boat will be much more stable in the inverted position, all other factors being equal.

That makes sense. I assume there is some ratio to consider, Beam/draft that is factored against displacement/keel weight? These numbers are new to me, so I am just thinking out loud here.

Most cruising boats are 'beamy' so this would have a negative effect in a capsize scenario. Does a shoal keel have a MAJOR effect on this or is it marginal? From the diagram provided by IslandHopper, the shoal draft version of the HR310, seems pretty stable upside down... If I am reading it correctly.

[WingRyder]

Originally Posted by MartinR

It shows righting moment vs heel angle. Where the curve intersects the x-axis is the angle of vanishing stability, as you see around 130 degree heel on these diagrams. At this point, the boat will turn turtle. The area under the positive part of the curve is basically the total positive stability. The area under the negative part of the curve is the total stability when the boat is upside down. You would like this area as small as possible.

EDITTED: Thank you. What affect does the mast, rigging and sails UNDER WATER play on this? Is this data calculated on a computer? Or is there some sort of "crash test" performed? Is this information widely available for different boat models?

Perhaps a ridiculous thought, is there some sort of inflatable device that could be hoisted to the top of a mast during such conditions, that would prevent a >90 deg moment?

[WingRyder]

I thank you all for your input! This has given me yet another aspect to explore and study. I don't want to overly complicate the decision process with too many variables, but this seems a fairly important topic for me to consider, early on! Even if only comparing shallow draft boats.

Even if I end up with a boat with mediocre AVS numbers, it is still an great piece of information to know.

For the most part, I think this thread has helped me to better understand that the main 'fair weather' consideration for a shoal draft is, upwind performance penalty vs shallow(er) water abilities. BUT, for a blue water, there are some serious considerations as well. Since I don't plan on a circumnavigation, or crossing the horn, and 95% of my (planned) sailing will be Atlantic seaboard and Caribbean, I think that a shoal draft boat is the direction I will take.

As Suijin said though, it's not a deal breaker, if the right deal came along on a fin keel, well I'd consider that too! In fact, for some strange reason, I can't put my finger on, The Bavaria 42 Match, really calls to me! Even though it drafts 7 feet, and is a racing boat. I love the layout, large forward facing Nav Station, spartan interior, etc. but I digress...

I am now engaged in this subject and I'm researching this and 'STIX', ... which seem to vary widely even among similar style boats!

Thanks again, all for the great info.

[robert sailor]

Originally Posted by Jim Cate

Odd that none of the gurus have brought this up, but a shoal draft MAY influence the AVS adversely, and this means that it is easier to capsize the boat in big seas (gross simplification of a complex issue). It is certainly possible to design a shoal draft boat with a big AVS, but I suspect that many mass producers simply fit a shorter keel with a bit more weight than in their standard draft models and know that very few of their customers will ever sail in conditions where it matters. Perhaps not a concern, but you were asking about such issues.

Jim

Good points Jim. Depending on the builder the shoal draft models will often feature a heavier keel to keep the AVS similar to deeper draft. Others will concentrate much more weight at the bottom of the keel without adding more overall weight and then as you say many of them keep weights the same and simply produce a boat with less overall stability.
We ended up with a shoal draft and personally I'd prefer a deep draft boat, our last boat was a Tartan 44 and we drew close to 8 feet, that boat sailed like a witch upwind. For those who think winged shallow draft boats are equal to their deeper draft brothers...delusional might be too strong a word but go out on the race course and you'll find the best shoal draft designs probably give up 5 degrees pointing upwind.

[MartinR]

Originally Posted by WingRyder

Most cruising boats are 'beamy' so this would have a negative effect in a capsize scenario. Does a shoal keel have a MAJOR effect on this or is it marginal? From the diagram provided by IslandHopper, the shoal draft version of the HR310, seems pretty stable upside down... If I am reading it correctly.

Stable, but not very stable. In a real situation, with heavy seas, she would right herself in a relatively short time.

All of these figures are calculated, nobody tests the boat to upside down condition. Often this is verified by a heeling test to some reasonable heel. I am not sure if anybody knows how mast and rigg affect this while they are underwater. I would normally expect to loose the rigging when being rolled.
So, I always try to stay upright

Todays boats are beamy, because it gives the biggest interior room for the bucks, not the most seaworthy boat. All boats are compromises, but personally I would go for a few feet longer and moderate beam. Gives the same interior space, but a much better motion comfort factor.

[MartinR]

Originally Posted by robert sailor

Good points Jim. Depending on the builder the shoal draft models will often feature a heavier keel to keep the AVS similar to deeper draft. Others will concentrate much more weight at the bottom of the keel without adding more overall weight and then as you say many of them keep weights the same and simply produce a boat with less overall stability.
We ended up with a shoal draft and personally I'd prefer a deep draft boat, our last boat was a Tartan 44 and we drew close to 8 feet, that boat sailed like a witch upwind. For those who think winged shallow draft boats are equal to their deeper draft brothers...delusional might be too strong a word but go out on the race course and you'll find the best shoal draft designs probably give up 5 degrees pointing upwind.

I agree more or less. The wing is not actually helping very much, a little maybe, but probably a bulb would have more or less the same effect. On a race course, the shallow draft boat will always loose, but in real life, that matters little.
If a sea is running, you will anyhow fall off a few degrees for comfort, and then there is little in between. And most cruising boats do not do much upwind sailing anyhow.

I was just looking at how this translates to handicap. For example Bavaria 42, full draft has a SRS of 1.0061 while the shallow draft version has 1.0043. These are partly empirical values. This translates to the shallow draft version being about 2% slower around a race course, probably you loose 4% VMG upwind and nothing downwind. This is a lot when racing, but not much when cruising, as most of the time the boat is not sailed optimal anyhow.