Understanding the Ratios

[sailorboy1]

I don't feel any of the ratios take into account a boats shape etc. Boats are way too complex as far as "ratios" to really determine any real number beyound light, medium, heavy using the simmple to use ratio formulas.

Take the keel, none use a formaul take factors in how the keel weight is located and assume it is even. But is long with a bug bulb at the end it is alot different than a full keel as far as the keels performance.

I used to spend a lot of time looking at te ratios with I was looking for a boat. But I finally just decided I didn't know more than the person do designed the boat. If the designed says it's a racer it's a racer, if they say it's a cruiser it's a cruiser.

[barnakiel]

Originally Posted by swagman

With respect, if you only choose an offshore boat by the number you see out there 'doing it' then you're going to choose a production boat like Jeanneau / Beneteau because they would easily be in the majority who cruise safely round the globe. (...)

John,

When was it? We sailed 2003-2007 and the a/m makes were nearly absent in 'our' RTW fleet. There was one Bene (not sure they made it or got stuck somewhere in the Pacific). There was one Jeanneau too - a big, brand new 53', they made it fine, for sure.

I think most boats sailing round the globe are quite a bunch of various makes, vast minority of them being things like Bava/Bene/etc. (generic use, not meant to offend anyone).

b.

[callmecrazy]

Originally Posted by barnakiel

I am not sure if by seaworthiness we understand (higher) ability to recover from a 180' knockdown. I would suspect, seaworthiness may have more to do with the boat's resistance to be knocked down in the first place.

A seaworthy hull is one thing but when you think of a seaworthy sailing boat one would allow for the presence of such appendages like masts and sails. Would you agree?

Cheers,
b.

I agree with this. but I'm talking entirely about the older style boats in the sub 34 foot range, coupled with the older ideals of what seaworthy is. Not my own personal opinion.... I personally prefer a little initial stability along with relatively deep ballast and a high ballast/dispacement ratio, coupled with self-righting hull shape. (see the T30 numbers compared to a alberg 30).

The idea is, a boat with a higher initial stability, that's difficult to knock down past it's righting moment, will also be difficult to self right IF it is ever knocked down past 180'. That's why the skinnier, more 'tender' hull shapes have a better capsize ratio. They remain 'stable' well past 90' (despite being 'tender' to sail). A flatter, wider hull, with he same ballast and displacement, should have a higher initial stability, but also a higher moment of vanishing stability. at some point it goes over, then becomes stable once again while upside down in the water.

And this is also why its important on smaller boats, because the chance of getting rolled over by a breaking wave is a function of the size of the boat as well as it's stability performance.

For example, if you were planning on sailing around cape horn in the winter in a 30 foot boat. or maybe a non-stop circumnav or something like that, you can pretty plan on getting knocked down by breaking waves at some point. You want a boat that's going to come back up as soon as the very next (similarly sized) wave hits the keel.

Now, if you're a normal cruiser, sailing across oceans on the normal weather routes, you would probably prefer to sail as flat and fast as possible, carry a heavier load without detriment, and still retain some level of motion comfort and righting ability 'just in case'....

[barnakiel]

Originally Posted by Don Lucas

I don't feel any of the ratios take into account a boats shape etc. Boats are way too complex as far as "ratios" to really determine any real number beyound light, medium, heavy using the simmple to use ratio formulas.(...)

I think some do.

But aside from this, the specific shape must be reflected in some physical property that can be measured so that we can build a formula out of it. If two boats with different shapes show the same properties then what difference does it make if they are round or square?

Then again, nobody denies us the right to build our own formulas that reflect specific features that we find desirable or not. There is no limitation for anybody seeking a specific feature in a boat to build a spreadsheet and 'find' the one that fits their needs best!

barnie

[barnakiel]

Originally Posted by callmecrazy

(...) at some point it goes over, then becomes stable once again while upside down in the water.(...)

It is possible to built a flat and initially stiff boat (of the kind many will claim un-seaworthy) that will come back each and every time. The fact that many stayed inverted was not related to lack of seaworthiness but rather to the design of their decks.

You will notice too that today the new boats of this type go thru actual (not calculational) reversal tests before heading offshore. Also, having a canting keel, they are more like to come back (much as some will say canting kell=un-seaworthy!).

Personally, I would strongly prefer a boat that is next to impossible to knock down from one that is easier to roll over (and will come back every time). From reading about many deep water mishaps I take it that once you get rolled over that's actually it. Especially if you lose the rig which will almost guarantee that you will get rolled over a couple of times more.

An initially stiff, hard to roll boat that would also come back from a 180' might be the golden means. But does such a thing exist?

b.

[callmecrazy]

Originally Posted by barnakiel

It is possible to built a flat and initially stiff boat (of the kind many will claim un-seaworthy) that will come back each and every time. The fact that many stayed inverted was not related to lack of seaworthiness but rather to the design of their decks.

You will notice too that today the new boats of this type go thru actual (not calculational) reversal tests before heading offshore. Also, having a canting keel, they are more like to come back (much as some will say canting kell=un-seaworthy!).

Personally, I would strongly prefer a boat that is next to impossible to knock down from one that is easier to roll over (and will come back every time). From reading about many deep water mishaps I take it that once you get rolled over that's actually it. Especially if you lose the rig which will almost guarantee that you will get rolled over a couple of times more.

An initially stiff, hard to roll boat that would also come back from a 180' might be the golden means. But does such a thing exist?

b.

Sunfish?

[barnakiel]

Originally Posted by mitiempo

(...)

Able's Apogee 50 is a great example of an offshore boat as well.

I think later developed into Bougainvillea. Fantastic boats. IMHO archetypal. Given a sudden lucky strike at Lotto I would be hard pressed to chose between one of such and a good catamaran. Then again, given such a luck, I would probably get both.

Cheers,
b.

[swagman]

Originally Posted by barnakiel

John,

When was it? We sailed 2003-2007 and the a/m makes were nearly absent in 'our' RTW fleet. There was one Bene (not sure they made it or got stuck somewhere in the Pacific). There was one Jeanneau too - a big, brand new 53', they made it fine, for sure.

I think most boats sailing round the globe are quite a bunch of various makes, vast minority of them being things like Bava/Bene/etc. (generic use, not meant to offend anyone).

b.

Barney,

I was not referring to any specific RTW 'fleet' and I've never completed a circumnavigation myself in any year, but we've met quite a few foreign registered boats cruising ourselves in Australia, the Med, the Carib. There was always a variety of makes, but AWB numbers continue to grow.

In 1990 you'd hardly see any AWB's arriving on their own keels, but certainly the majority of the East Med Rally Fleet in 2006 and the ARC Fleet in 2007 both of which we did take part in, were production plastic boats.

IMHO it's the skipper who needs to be able to handle the oceans and not necessarily the make of boat.

Cheers
JOHN

[Adelie]

Originally Posted by swagman

IMHO it's the skipper who needs to be able to handle the oceans and not necessarily the make of boat.

Cheers
JOHN

Extending that statement to cars, any normally competent driver should be able to finish the Paris-Dakar in a Mini, a Scion xB or any other stock street car.

The reality is a good skipper can make up for the deficiencies of a mediocre boat when crossing an ocean. Likewise a good boat can make up for the deficiencies of a mediocre skipper. Nothing in the long run is going to be able to overcome the problems of a truly poor skipper or poor boat.

A boat like the Pogo 10.5 has compromised a lot in order to achieve speed. For a very good skipper is would be a reasonable boat to take offshore. But not for the run of the mill skipper.

[barnakiel]

Originally Posted by swagman

(...) IMHO it's the skipper who needs to be able to handle the oceans and not necessarily the make of boat.(...)

John,

That skipper is not swimming, are they?

Thumbs up on skippers' abilities, and a good boat definitely goes a long way.

Alas, we have seen all kinds of skippers in all kinds of craft: also complete 'ignorants' who made it from South America to the islands in boats with no external ballast/keel and rig that should have fallen down under its own lack of quality. And yet.

So, apparently, it ALL is relative! Skipper's ability inclusive.

Them be sailors.

b.

[swagman]

Originally Posted by Adelie

Extending that statement to cars, any normally competent driver should be able to finish the Paris-Dakar in a Mini, a Scion xB or any other stock street car.

The reality is a good skipper can make up for the deficiencies of a mediocre boat when crossing an ocean. Likewise a good boat can make up for the deficiencies of a mediocre skipper. Nothing in the long run is going to be able to overcome the problems of a truly poor skipper or poor boat.

A boat like the Pogo 10.5 has compromised a lot in order to achieve speed. For a very good skipper is would be a reasonable boat to take offshore. But not for the run of the mill skipper.

Only having seen this boat on the youtube clip, I'd say any skipper seriously contemplated 'cruising' a Pogo 10.5 off on a world cruise would definately have his whole village out searching.

John

[boatman61]

Originally Posted by swagman

Only having seen this boat on the youtube clip, I'd say any skipper seriously contemplated 'cruising' a Pogo 10.5 off on a world cruise would definately have his whole village out searching.

John

Looks good to me.... great 'P' Station at the stern...

[Polux]

Originally Posted by callmecrazy

it's an older design concept that predates fin keels, so if you're looking for a boat with the best capsize ratio, it's probably gonna be an older full keeled boat. Which also happens to be heavy because of older building practices... Ultimately people can find all sorts of good reasons those older boats are still considered 'seaworthy'.

I'm not advocating them...

the capsize ratio only is meaningful among boats with the same characteristics and even so....

The formula does not take into consideration the CG of the boat. If you use the formula on a light boat with deep draft and a bulb and on a heavy full keeler, it would give a very good value to the old boat even if the modern boat with the bulb and the deep draft has a better AVS, more righting moment at 90º and a much better dynamic stability.

[Polux]

Originally Posted by goboatingnow

Portugal I beleive does restrict sail area by boat design. But as a general answer is that RCD categorisation has no effect on sailing destination. It merely " attempts" to provide you with a design guide.

Dave

Hum, I would like to see an insurance company in Europe insuring for offshore work a boat that is according to RCD categorization not designed for that.

In Portugal they restrict sail area by boat design? what do you mean? A boat approved as Class 1 (no restriction) by RCD is a Class 1 boat independently of the design or sail area.

[Adelie]

Originally Posted by Polux

the capsize ratio only is meaningful among boats with the same characteristics and even so....

The formula does not take into consideration the CG of the boat. If you use the formula on a light boat with deep draft and a bulb and on a heavy full keeler, it would give a very good value to the old boat even if the modern boat with the bulb and the deep draft has a better AVS, more righting moment at 90º and a much better dynamic stability.

The trick is to design a lightweight boat that has those qualities.

The only design I've seen that might even come close to that is the POGO 10.5, a 35' boat with a 9' draft. Even then I think the only place it will have the advantage is righting moment at 90º. Without a significant coachhouse, the AVS is just over 120*. As far as dynamic stability goes it's beam of almost 13' gives waves a lot of lever arm to act on, the fin is high aspect minimizing water entrained during rolling and decreasing damping and given that significant effort has been made to minimise mast weight so roll inertia is also minimised.

No, the formula is still pretty good, it has limitations, but nobody has started designing boats that seriously pushing those limitations.

The limitation that the formula has is not the disregard for CG, but for distribution of mass and that it completely disregards lateral area.

The fact is that since designers that make light boats have all, to the best of my knowledge, also gone to great efforts to minimise mast weight and wetted surface area. So weight has continued to be a good proxy for roll inertia and it turns out lateral area needed for roll damping.

If you want to build a boat that's a formula buster here's the way to start:

Take a Cal 40 mold, build a hull using all the tricks you can to minimise weight: Balsa core, Kevlar and carbon fiber, vacume bagging, nomex bulkheads or whatever is lightest, undersized engine. Modify the mold to raise the cabin aobut 3-4". Instead of molding the ballast into the hull, bolt is to the bottom of the keel in a bulb. If you want to really go overboard, use depleted uranium or better tungsten. Don't change the fin shape. Going with the bulb allows you to decrease the mass of the ballast without decreasing static stability and the change in roll inertia is probably a wash. The mast is slightly taller, 2-4', but section stays the same.

Now you have a boat that is 4000-6000lb lighter, roll inertia is about the same, and dynamic damping is a bit higher with the increased draft of the bulb.

Given the decreased weight, immersed shape is going to change, but this is a good starting point. You've drastically decreased weight, significantly increased sail area, without changing the things that contribute to capsize resistance. AVS may have increased too.

Now you have beat the formula. Nobody is doing that these days.