Swain BS_36 Stability curve

Sounds good. When I get the rest of the steel on (only the aft cabin sides & bulwarks plus maybe a couple internal tanks to go) I'll cut the tacks off the steadying props and then see where she balances. Easiest way will to use a jack with a bit of heavy angle iron on top and lift at various spots along the keel. Using all due care of course, it'd be pretty stupid to lift the hull a bit far and have it fall on me.... before I plated it, the keel was *very* stable, it wanted to stay upright. I have a very good idea of the weight of the plate that's gone on.

See photo - the pictured support is the fwd one, there's another one aft plus the 4 corner steadies to stop the hull falling over.

I built the cradle as it is basically so I could jack it up, put rollers under it and pull it out the front doors of the shed. Easy access at that point for a crane to lift onto a low loader.

We're diverging again from the thread subject so if this is worth pursuing it should be in another thread either here or steel boat building.

PDW

 

pdw

nah we are still related to stabilty, sort of
And you can bandy numbers around til you are blue in the face
But at the end of the day, us boatbuiders do not need one thousand pages of b/s we did it and they did it by seat of pants for eons
I,ll bet my last Zac that half the Boffins here have never sailed a boat,built a boat or savoured the ambience of a boat
bon nuit

 

PDW
I have several sets of plans I have bought and collected over the years, They came from well known designers that have been around a long time.
Only one set came with stability charts and in reality they are off on displacment. I never figured stability was an exact science in a smaller cruising sailboat, way to many varables, we can choose stiff, tender or inbetween but the way we sail it and load it is a huge factor.
Personaly I would be more conserned with hatches. ports. windows. and vents flooding the boat rather than weather it has vanishing stability at 125% or 140% but thats just me.
Tom

 

No, most plans don't come with lots of info on stability, in my own experience. But, certain info. is often supplied to enable the builder/owner to balance the boat when out-fitting it, such as VCG. Personally, I do like to know how tender/stiff a boat will be at various angles of heel & find value in knowing if I'm going to be sailing on my ear most of the time, or not. Of course, as you've pointed out, downflooding is an important aspect for operating the boat in unprotected waters and is considered in the stability analyses. Still, while you as a builder or owner may not want to delve into the design this deeply, you should want the designer to have considered the stability aspects of the design, right?

In the ideal situation, the designer knows what he/she is doing, same for the builder, and the result is a great boat that performs to expectations. Assuming the builder knows what he/she is doing & builds to the plans and that the owner chose a design that suits the way in which the owner will use the boat, the only issue is whether the designer's description of the boat's capabilities/characteristics is accurate.

Suppose you started building a BS36 tomorrow. You stick to the plans, do a beautiful job of the build, yet, when you go for a sail, under full canvas, the toe rail is buried at less than 20knots wind on a reach. As you look at mama & the kids clinging to the windward combing, do you feel confident that this boat will take you comfortably & safely to far off places? who wants to spend days, weeks or a month at sea, at a 30 degree heel?

But, even these points are moot as the thread was started with an attempt to prove Brent's often-offered statement regarding the stability of his design. So, the fact that the boat can't meet established standards is probably more interesting to the designers than the builders and owners, however, it is a further indictment of the designer's lack of knowledge and/or honesty. When someone is searching for info. on building steel sailboats, invariably, he/she comes across posts of Brent's from various sites, wherein he's slagging another designer. What this and other threads on this site establish is that Brent doesn't know his own "designs", let alone another designer's and, therefore, deserves no more respect of his opinion than the many people who build or sail a boat. Bluntly put; Brent can't hold a candle to someone fresh out of design school, let alone the senior designers with decades of experience, not because his boat designs are good or bad but because he himself didn't/couldn't do the work to ascertain whether they are good or bad.

So what if a few of his boats have made it across an ocean, or two. I fully expect for my Cal to end up in Asia, yet, I recognize the compromises of such a boat. When in Asia, should I start a thread on the site about how the Cal is superior to other boats? No, because some will be of the mind that the boat was adequate & some will tell me that I was bloody lucky to make it there on a 40 year old coastal cruiser. But, at least I have a good idea of the characteristics & capabilities of the boat & can gather more info. if desired. Would it be good to know the VCG? It'd sure make it easier/safer when loading the boat with gear.

 

Knowledge is power....the more you know about the real stability characteristics of your boat, the better prepared you are to deal with eventualities.........Commercial vessel skippers are armed with a stability booklet so that they (the skipper) can (at least in theory) calculate actual stability even with the vessel in a damaged condition.......This is very useful should the boat be up on a rock with the tide out......you need to know if there is adequate stability (positive righting arm) as the tide returns.....

Even 20 years ago a complete curve of righting arms was rare in small yacht design.....there would be an estimate of GM and a rough WPC (Wind pressure coefficient) or Dellenbaugh heeling angle.........only the advent of personal computers changed that.......

I've just been commissioned to write a stability letter (study) of a well known production cruising boat designed 30 years ago. This is required for her to enter the Victoria to Maui race. I'll do an inclining, measure freeboards, and create a computer model from the hand done lines drawing......It's a few days work and arms the skipper with additional knowledge in the safe operation of his vessel.......

Weighing a vessel while ashore will tell you little about her stability....it will help you decide if the designer was accurate in his displacement estimate.....you can also locate the LCG....but it doesn't give you a VCG, and measuring flotation after launching (with a complete vessel) can give you displacement and LCG as well. That's assuming you have a reasonably accurate hull lines drawing........

 

It's posts like this, where you choose lipping people off over contributing to the issue of the thread, that has gotten you banned in the past. I'll never understand why you always choose that course. A "good enough" philosophy, to me, is nothing more than an excuse for lazyness, however, so we - obviously - would disagree in that regard.

 

I suspect, as Tad points out, perhaps few calculations were done then... I hated doing them by hand myself way way back. It was probably more trial and error and “experience” that fed their hands in creating what they did. Took it to sea, if it “felt” ok..that was good enough.

But that was before the days of the personal computer and even today freeware. This freeware can provide you with the very basic and value information, so just from that point of view, there is no excuse not to provide such data today. Also, with so many taking to the water these days, accidents inevitably are on the rise. Whether this is through the sheer weight of numbers on the water today compared to say 30 years ago, or because of poor design, or some other reasons(s), is for another thread, and requires more factual evidence. However, regardless which it may be, such accidents have caught the attention of Coastguards around the world, and also insurance companies. Thus legislation and risk analysis is a fact of life..

If you bought the plans in 2003, there should be no excuse not to supply stability information. If you bought in 1963, I could understand, but not this century. The person selling you those plans, should know that too, since they become complicit in any accident that may occur owing to full-disclosure of a design they are selling.

I think you hit the nail on the head here, most amateurs don’t ask, because it is not in their frame of reference when buying, nor do they consider it a “selling point”.

You should be, without knowledge of the vessels stability, you are literally putting your life’s in the hands of luck and hoping the designer has looked at all possible scenarios that could occur. This aspect drifts into professionalism. Is the person a professional, by that I mean, a member of any one of the world wide institutions. Since one is “bound by a code of conduct”, rather like a Lawyer or Doctor, and this provides a measure of “peace of mind” at least.

Whether it meets a criterion or not, is not the point here. The point is, what the limits of safety of the vessel are; when is she safe and when is she unsafe…whatever method one uses to establish this. You can argue once you have the data, if a KG of 1.2m is better than 1.3m, which means you can’t put your extra bunk high up…but you cannot state this if you have zero data to begin with.

You also need to realise that the stability of a yacht is measured in more ways than normal non-sailing boats. Some of this is because of what is the definition of “stability”…..or made more simply, is she ‘stable’….but what does that mean, stable???

When you walk across the deck from port to stbd, does she roll?…or when you sail into the wind does she keeps a good course?..or when in heavy seas she doesn’t roll/pitch too much?…if hit by a gusting wind how does she behave?.. does she tack well, etc etc and so on. Take your pick.

This is half the problem. Stability has many definitions to a naval architect, but to most non-NAs, it is how she ‘feels’. Thus a conversation with different definitions is occurring.

Again, Tad hit the nail on the head…knowledge is power

One of the measures of Stability is the simple relationship between the CoG and CoB. And when the vessel is heeled through ever increasing angels, what happens? It is this which we are referring to, what happens and what influences the final behaviour? Whether the vessel has an avs of 160, 170 or 180…so what?...means little if the area under GZ curve is next to nothing, or if the downflood angle is submerged under a heavy gusting wind, thereby allow progressive flooding, and the vessel has no reserve of dynamic stability to recover.

This is not helpful.

Weighing your boat, again, as Tad has pointed out, doesn’t give you the information you need. If you have the lines, get someone one (I suspect Mike is already doing this) to input them into a stability program and then once launched, incline her, you can obtain the displacement and KG, LCG. Then you will know the stability characteristics of your vessel. This is very very valuable information.

And this is the kind of comment that gives professional boat builders a bad name. This is a typical rank amateur comment, no respect for safety nor quality nor those that have the intelligence to debate real issues pertinent to those that wish to know and explore them. The comment is no better than if it can from BS himself, all subjective and an opine based upon your thoughts on other people’s ability compared to yours.

Precisely.

Maybe that is because to us naval architects, Stability, is a very very big issue and should never be taken lightly. Thus, our terms of reference are different for different reasons.

 

It's significant that it was the insurance companies that introduced sensible strength regulations for many vessels. Other people like Plimsoll saved the lives of hundreds of sailors each year with loading and stability regulations. The masters often got the blame but iron men could never prevail in unstable or weak vessels, then or today.

Smaller boats simply evolved sensible design and scantlings over hundreds of years and you just built them like the last success rather than the last failure. Sometimes the successes were imagined and whole fleets of boats would be lost with the first real storm when boats broke or foundered and the design would be changed.

Bit by painful bit the 'boffins' put together enough science to predict accurately whether a one off vessel is strong enough and stable enough and with adequate performance for the odds to be in the operators favor.

Some boat builders and shipwrights I work with have had some very confused notions firmly held and quite wrong, it's almost impossible to change some peoples deeply held convictions.
Others have been designers in their own right and they are really good to work with and they still seek professional advice and understand where they need it. I think the more you learn the more you look for at least peer input and preferably someone better versed in the subject. That betters the chance of success.

Very few designers have not been involved closely with the building of their boats. As a budding designer you learn a lot from the yard about how to design for practical fabrication, but never about fundamental NA that's something you really need to absorb from a different source.

Being able to follow (eg) Lloyds register class rules, makes a lot of builders able to design strong enough boats but not necessarily stable enough or even to meet performance oriented goals. We have strict regulations for strength and stability for any commercial use for good reason, the same standards should be applied by anyone to a prospective design. It's their lives they are risking.

 

Come on! It's still not "pounding" in 12 to 16 foot surf for 16 days is it !

And as I said it's very common for a small sailboat to survive such a beaching even on rocky shore at times they can come of just scarred but intact.

So all this anecdote would prove is that the boat was at least as strong as any other boat that survived such a beaching. That includes lightweight foam cored plastic racers, plywood , ferro cement, trad timber....

The very fact that the mast broke on the way out in more clement waves should say something about the beating it didn't receive whilst lying on the beach all that time.

 

Variation on the theme BS style and theory....

Since a 50 ton superlink truck and trailer at speed cannot destroy a small concrete divider wall between highways or total a concrete bridge / pillar, and in fact, the concrete always destroys the steel made truck and cars it goes without saying a ferro cement boat will then be stronger than a steel boat when they collide...

Steel reinforced concrete is the same as ferro cement since it is also steel reinforced Let the demolition derby start and see how the ferro cement boat destroys the steel boat

Sorry Mike, I could not help myself

 

As I said, the design was done in 1964. Some 150+ have been built. I know of some quite old ones still sailing out there.

The designer is in his 80's as near as I can work out. He's emphatic about building as per design, not adding a lot of weight high up, not going for a towering rig on a shoal draft hull designed for a multi-mast low aspect rig. Not to play games with the tankage etc. If it's built to the design, it's known to be ok for its intended purpose. If you screw with the design, it's on your head. I can live with that, I've asked before I've done any material substitution and I'm willing to bet that my as-built hull is within millimeters of the drawn design lines. I'd like the data and may ask about it but frankly I can't get too excited about it. If I was commissioning a new ice breaker, sure.

In a previous job I had a 30' workboat designed by a naval architect. Lovely plans & details, great computer analysis. The build was done under his supervision and one of my most senior staff.

It hit the water 20% overweight and we had to upsize the engines which led to cooling issues and so it went on....

I take your point but frankly I'm a lot less concerned about building hull 150+ of an old design than I would be building hull number 1 of a new design regardless of calculations etc.

I am only a little curious about the dry ship weight and balance as I doubt it'll bear much relationship to the ballasted point of balance. If it's easy to do, I'll probably do it.

I've got all the lines & offsets but have no software to load them into and I'm not willing to ask someone else to do such a job for me just to satisfy my curiosity. I don't have the designer's permission to put the data into digital form or give a copy to a 3rd party and while I've a spreadsheet where I converted the imperial measurements to metric, I've not let a copy out of my possession.

So I haven't asked Mike to do any analysis and while I'd like the information for my own purposes, I don't plan to ask either. It's a bit of an imposition on someone to get them to do this really. Next time I write to Tom I may ask him about it.

When I started down this road there was a lot I didn't know to ask. I've learnt a great deal from people like you, Tad, Mike and others and I greatly appreciate the time and effort you guys put in to this forum. So don't think I'm dismissing the importance of this, I'm not. If I can get more data on the hull characteristics, well and good, I'll be happy to have it and am open to suggestions on how to proceed.

PDW

 

It may not be everyones cup of tea, but perhaps you could follow the methods and procedures in ISO 12217-2, in the annex's at the back. It would anwser any questions you may have or have though of, in the abscence of any other hard data on your boat. Once completed of course...

ISO is by no means perfect and probably not to your liking, but it at least gives you a way of establishing basic data.

 

Tom, I agree with you. There's a lot of value in empirical data, the trick is to realise its limits and not extrapolate past them. That's what I'm trying to do.

WRT opening ports etc, yes, agree completely. And big windows asking to be bashed in by that rogue wave.

My real objections WRT the BS 36 design is the *inaccurate and misleading* data like the initial stability curve put forward as fact, and the 4000 ft-lbs righting moment due to mast buoyancy, and the myriad of other stuff that's just plain WRONG yet keeps on getting said by Brent and people like junk2lee.

If Brent just stuck to saying that hey, the boat sails, nobody has reported losing one at sea, a number have crossed oceans, it's fast, cheap and simple to build, go for it at your own risk, I'd say fine, no worries, I know what I'm getting into. Mike (welder/fitter) is planning on taking something a lot less strong structurally across the ocean. Might be more stable, I don't know, but as long as you know the limitations you can decide for yourself what risk level you're comfortable with.

PDW

 

The most reliable way by far to determine how she will float. Put the bar under the LCB and move things around il she balances on that point. Great if you are making changes which affect the weight distribution.

 

A BS 36 would make short work out of a cement boat. I've seen how easily they break up in surf, like that in the pictures posted on the origamiboats discussion. It takes them only minutes to break up in those conditions.