jpcarter30
Well-Known Member
Can somebody please explain in simple terms what balast ratio is and what ratio makes a boat safer? Many thanks.
balast ratio....
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Three kings
Well-Known Member
Ballast divided by displacement to give you percentage. In other words total displacement is xxxxKgs of which xx% is ballast.
What ratio makes a safer boat is a hotly contested debate but I believe that a ratio over 35% is considered good.
My boat is 12500kgs and ballast of 4990kgs so ratio is just shy of 40%.
What ratio makes a safer boat is a hotly contested debate but I believe that a ratio over 35% is considered good.
My boat is 12500kgs and ballast of 4990kgs so ratio is just shy of 40%.
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Salty John
Well-Known Member
Ballast ratio: Ballast x 100 / Displacement.
This is the relationship between total displacement and ballast; it tells you how a boat might stand up to her canvas. Heavily ballasted boats, those with a ratio of 40 -50%, stand up well to their sail area and allow more sail to be carried in heavier conditions.
Lightly ballasted boats rely on their hull form to provide some resistance to heeling and generally need to be reefed earlier to remain under control.
That's a little simplistic because ballast carried lower or higher - within an encapsulated keel or as a bulb at the bottom of a deep keel - will also influence how stiff the boat is.
This is the relationship between total displacement and ballast; it tells you how a boat might stand up to her canvas. Heavily ballasted boats, those with a ratio of 40 -50%, stand up well to their sail area and allow more sail to be carried in heavier conditions.
Lightly ballasted boats rely on their hull form to provide some resistance to heeling and generally need to be reefed earlier to remain under control.
That's a little simplistic because ballast carried lower or higher - within an encapsulated keel or as a bulb at the bottom of a deep keel - will also influence how stiff the boat is.
Tranona
Well-Known Member
High ballast ratios used to be considered desirable as they allowed narrow beamed hulls with slack bulges to carry sufficient sail area to move the weight. However the trend towards hull forms with greater inherent resistance to heeling has lead to a reduction in ballast and more of the displacement of the boat allocated to other purposes than just dead weight and stability enhanced by placing ballast at the bottom of the keel.
However there is no such thing as a free lunch and lower ballast ratio can lead to a more lively boat that needs more attention to matching sail area to conditions to keep the boat flatter. You often find such boats have relatively greater sail areas and efficient means of adjustment.
As to safety some will argue that higher ballast ratio is safer but what they really mean is that they can cope with a wider range of conditions with working sail and are arguably more comfortable in heavy weather. On the other hand thousands of boats with lower ballast ratios are use all over the world without any so called safety issues.
However there is no such thing as a free lunch and lower ballast ratio can lead to a more lively boat that needs more attention to matching sail area to conditions to keep the boat flatter. You often find such boats have relatively greater sail areas and efficient means of adjustment.
As to safety some will argue that higher ballast ratio is safer but what they really mean is that they can cope with a wider range of conditions with working sail and are arguably more comfortable in heavy weather. On the other hand thousands of boats with lower ballast ratios are use all over the world without any so called safety issues.
Koeketiene
Well-Known Member
A high ballast/displacement ratio does not necessarily indicate that a boat is 'safer', it is just an indication that the boat will be able to carry more canvas longer than a boat with a low(er) ballast/displacement ratio.
Also, these figures require some interpretation.
According to this we have a ballast/displacement ratio of 42.4%.
Fully laden, our displacement hovers around the 13,000kg mark.
This would bring our ballast/displacement ratio down to around 36%.
Is our boat less safe when fully laden? Of course not.
Ballast/displacement ratio gives you an indication of how a boat will behave in a breeze, nothing more.
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Three kings
Well-Known Member
Very well put and a good retort to the perennial MAB vs AWB debate.
I've owned both types and my personal preference is for the higher ratio heavier boats for any serious offshore work. But the others do their job just as well and have many happy owners.
Pleiades
Well-Known Member
Talking a load of ballast
The ballast ratio by itself is no guarantor of safety - that depends on so many other factors. Different boats need different ratios - and no particular ratio is "safer".
This dingy for example carries 0% fitted ballast - the crew are the ballast:-
The Hurley 22 however has a whopping 60% ratio :-
While the scrummy Saltram gets by with "only" 46%:-
All three can be regarded as safe boats in their own right so the ratios in themselves do not tell us that much about safety - they do however give an indication of important thngs such as sea kindliness, self righting ability etc.
I would argue that most designers get the ballast ratios about right for the intended use of the craft - dangers do arise if ballast is not put where it should be or is allowed to move about - or it falls off suddenly!
So research the boat type in question, find out the design brief, check out if the ballast is where it should be and that you propose to use the boat in the way that the designer intended.
Robin
Pleiades of Birdham
MXWQ5
The ballast ratio by itself is no guarantor of safety - that depends on so many other factors. Different boats need different ratios - and no particular ratio is "safer".
This dingy for example carries 0% fitted ballast - the crew are the ballast:-
The Hurley 22 however has a whopping 60% ratio :-
While the scrummy Saltram gets by with "only" 46%:-
All three can be regarded as safe boats in their own right so the ratios in themselves do not tell us that much about safety - they do however give an indication of important thngs such as sea kindliness, self righting ability etc.
I would argue that most designers get the ballast ratios about right for the intended use of the craft - dangers do arise if ballast is not put where it should be or is allowed to move about - or it falls off suddenly!
So research the boat type in question, find out the design brief, check out if the ballast is where it should be and that you propose to use the boat in the way that the designer intended.
Robin
Pleiades of Birdham
MXWQ5
Pleiades
Well-Known Member
We Three Kings
Dear Three Kings, are you the guys that brought the gold, Frankincense and Mhyrr?
That was very kind of you - hope you sail home safe - watch out for that Herrod chappy - he has no respect for his wash or the Rules of the Road.
Robin
Pleiades of Birdham
MXWQ5
Dear Three Kings, are you the guys that brought the gold, Frankincense and Mhyrr?
That was very kind of you - hope you sail home safe - watch out for that Herrod chappy - he has no respect for his wash or the Rules of the Road.
Robin
Pleiades of Birdham
MXWQ5
Tranona
Well-Known Member
Interesting that the latest Ron Holland designed Discovery 57 tougher as the ultimate for bluewater voyaging has a ballast ratio of 34 percent.
Koeketiene
Well-Known Member
With a displacements of 29tons, who cares about a couple of percentage points either way?
Tranona
Well-Known Member
Bit more than a couple of percentage points. 20 years ago. 45 was the norm a nd anything under 40 was considered light. You will find the same trends in smaller boats and maybe 35 is the new norm.
Three kings
Well-Known Member
Interesting indeed and further proof that the myriad of ratios devised in sailboat design, which in theory are supposed to tell us a lot about the boats abilities, don't really tell us what we would like to know. There's just too many variables I guess.
Tranona
Well-Known Member
Yes. If you go back pre 1960 or so you find lower ballast ratios much more common, partly because of the difficulty of hanging heavy weights on the bottom of a wooden structure. Stiffness was helped by spreading the ballast out and having lower longer sailplans. GRP structures made use of heavy cheap ballast easier. The IOR also had an influence as it penalized stability so high ballast ratios were used to stand pup to the large sail areas. However the ballast was high up in the keel to reduce the effect on stability.
While a single ratio tells you something it usually prompts more questions rather than providing an unambiguous answer.
While a single ratio tells you something it usually prompts more questions rather than providing an unambiguous answer.
dunedin
Well-Known Member
I think this is an over simplistic view - or possibly simply wrong.
The ability to carry sail without heeling excessively is driven by a combination of
(a) hull shape - and how far the centre of buoyancy moves laterally when heeling (or in simplistic terms wider is normally stiffer)
(b) centre of gravity height and hence righting lever - the positioning of the ballast can be at least as important as the amount. Some boats with high "ballast ratios" may have internal ballast (like some Southerly's) which is far far less effective than weight in a lead bulb at the tip of a deep keel.
mjcoon
Well-Known Member
Why should that make any difference? Surely the position of the centre of gravity of the ballast is the only relevant attribute of the ballast's geometry to stiffness? Distribution will make a difference to dynamic behaviour, though.
Mike.
PeterWright
Well-Known Member
Spreading the ballast keel along the length of a boat (i.e. long keel design) reduces dramatically the stresses in the garboard strake (where the keel joind the hull) In the days of wooden yachts, it was not normal to see a steel matrix transferring the loads from the top of the keel into the hull.
The first time I saw such a structure was when Maurice laing's Loujaine (sp?) sister ship to Heath's Morning Cloud retired from an Ostende race leaking badly with both garboards cracked, just through sailing her hard. S&S flew a rep out who recommended a repair in the original cold moulded construction plus a stainless matrix extending halfway up the frames and fore and aft well beyond the keel to distribute the keel loads more widely in the hull. Apparently it worked as I never heard of her suffering the same problem again. However, when Cloud was lost off Brighton, Pictures of the wreckage showed half the hull still joined onto the keel with the other half broken away, right along her garboard strake!
It's simple really: for a given righting moment, the shortter the keel hull attachment the higher the stresses.
The first time I saw such a structure was when Maurice laing's Loujaine (sp?) sister ship to Heath's Morning Cloud retired from an Ostende race leaking badly with both garboards cracked, just through sailing her hard. S&S flew a rep out who recommended a repair in the original cold moulded construction plus a stainless matrix extending halfway up the frames and fore and aft well beyond the keel to distribute the keel loads more widely in the hull. Apparently it worked as I never heard of her suffering the same problem again. However, when Cloud was lost off Brighton, Pictures of the wreckage showed half the hull still joined onto the keel with the other half broken away, right along her garboard strake!
It's simple really: for a given righting moment, the shortter the keel hull attachment the higher the stresses.
Koeketiene
Well-Known Member
This is not 'Yacht Design 101'.
As a rule of thumb there's nothing wrong with what I've said.
jpcarter30
Well-Known Member
Thanks people.
So, if the boat balast is above 45% then this allows you options with the sail plan. What difference then is the keel on a boat that has 'good balast'?
For example,given a boat with something like 45% balast of its overall weight would a fin keel perform better or a twin bilge keel?
So, if the boat balast is above 45% then this allows you options with the sail plan. What difference then is the keel on a boat that has 'good balast'?
For example,given a boat with something like 45% balast of its overall weight would a fin keel perform better or a twin bilge keel?
mjcoon
Well-Known Member
That's all fair enough; you were just using "stiffness" in a sense not connected with yacht behaviour. Strength and stiffness are not the same things...
Mike.
Simondjuk
Well-Known Member
It's a lever. The lower the ballast, the greater its effect. A given hull shape, of a given displacement will be much stiffer if you dangle the majority of a given amount of ballast a few feet below the the hull than it will if you dump the lot in the bilges. The mass of the ballast is only half the story.
