Ballast Ratio - is this the best indicator?

[carrswood]

Boat design is surely a complex science with many variables affecting such commonly quoted phrases as seaworthiness, seakeeping ability, comfort, stability, stiffness or tenderness.

But after all the counter discussions (eg. draft V's how deep a yachts bulb is etc) - would it be fair to say the best method of gauging a yachts "comfort motion, seakeeping, stability and stifness" is fundamentally to look at a yachts ballast ratio?

With a lot of French and German production manufacturers producing yachts with Ballast ratios below 32% I know this can be a formula of much discussion. Hence the question!

What sparked the above is reading about the X-412 in this months YM and a quoted ballast ratio of 47% - which I thought was high for such a performance orientated manufacturer.

 

[Courageous]

For stability quite possibly yes although this would not be the case in a vessel that is over rated on sail area or tallness of rig of course. In any case a high ballast ratio will help.

For comfort certainly not. It is a factor but whether it is a big player in this league I would personally doubt. Imagine a boat with a bluff bow head onto a big sea and you will understand where I'm coming from.

 

[Lakesailor]

But the placement of the ballast has a great bearing on the stiffness. Putting pig iron in the bilges won't have as great a moment as a similar mass on the bulb on the bottom of a fin keel.
So I would say you are correct to question a simplistic ratio.
Other considerations such as the buoyancy of the turn of the bilge, the surface area of the topsides, as well as the buoyancy of the bow sections must have some bearing on the seakeeping and comfort.

 

[carrswood]

For stability quite possibly yes although this would not be the case in a vessel that is over rated on sail area or tallness of rig of course. In any case a high ballast ratio will help.

For comfort certainly not. It is a factor but whether it is a big player in this league I would personally doubt. Imagine a boat with a bluff bow head onto a big sea and you will understand where I'm coming from.

Your quite right - afterall a yacht with the hull shape of a breeze block but high ballast ratio doesn't make for a comfortable ride???

 

[30boat]

One of the things that really affects comfort at sea is speed.A boat traveling fast will obvioulsly do so in a bumpier way than one that's going slower.A lighter boat will logically be faster and light boats may have a lower ballast ratio with a deeper keel which in itself can be a factor for uncomfortable motion.This applies mainly to racy boats as modern cruising boats may have the low ballast ratio together with shoal draft.They can still sail fast on a reach but not so often to windward when the motion would be more unpleasant.

 

[Bosun Higgs]

As LakeSailor has said, its where you put the ballast not just how much it weighs. So the key measure is the maximum righting moment and the angle at which it is achieved. Even that can be misleading to a non engineer since weight for weight, the max righting moment and therefore the notional ballast ratio ( repeat notional) of a cat will almost always be higher than a similar mono. But the angle at which it is achieved will likely be 40 odd degrees not 90 odd.

But " comfort motion, seakeeping, stability and stifness" are mostly subjective measures anyway. About asll you can say with any reliability is that bigger is better. Forgetting any question of construction integrity, a 40 ft Bav will be more comfortable in a seaway than a 30ft HR. And will have better seakeeping and be less likely to invert. All because its bigger and heavier and has more inertia.

 

[Coaster]

Clearly, size and hull shape are very important factors in motion comfort. I'm less certain of the importance of ballast ratio to comfort.

This subject has puzzled me since we bought our current boat last year, a 1980s Westerly.

The builder's stated weight is 5.7 tons and the ballast 2.6 tons, giving a ratio of about 45%.

Lifting the boat off road transport, the crane guage recorded a weight of 7.1 tons, without the mast and with no fuel, water or stores. Assuming the guage measured correctly, I presume the difference in weight resulted primarily from the generous use of materials during construction. The boat is certainly very robustly built.

In use our boat is well laden with fuel, water, tools, stores, books and lots more. Let's say a displacement of 8 tons. Assuming that the keel was cast to the designed weight, that reduces the ballast ratio to 32%, which seems rather weedy.

What puzzles me is how this compares with another version of the same model. If Westerly subsequently reined in their employees' generosity with materials, a later boat might actually weigh 5.7 tons and have a ballast ratio of 45%. Would that boat be more or less comfortable than ours? (I'm not concerned about stiffness as we are cruising sailors and reef early.)

My tentative conclusion is that the heavier boat would be more comfortable at sea, despite the lesser ballast ratio. However I'm by no means sure about this and would be interested to read other opinions.

 

[charles_reed]

Boat design is surely a complex science with many variables affecting such commonly quoted phrases as seaworthiness, seakeeping ability, comfort, stability, stiffness or tenderness.

But after all the counter discussions (eg. draft V's how deep a yachts bulb is etc) - would it be fair to say the best method of gauging a yachts "comfort motion, seakeeping, stability and stifness" is fundamentally to look at a yachts ballast ratio?

With a lot of French and German production manufacturers producing yachts with Ballast ratios below 32% I know this can be a formula of much discussion. Hence the question!

What sparked the above is reading about the X-412 in this months YM and a quoted ballast ratio of 47% - which I thought was high for such a performance orientated manufacturer.

Simplistic - a light boat with a high ballast ratio, will be anything but sea-kindly.

Total weight does make for a much more comfortable ride in a seaway.

So it's more likely to be total weight/LWL.

Than ballast/total weight.

Most of the AWBs to which you are referring rely on high form-stability, which makes them good performers in light winds and seas. Just what their owners want!

 

[Salty John]

The designer Ted Brewer produced a formula to try and define seakindliness. He called it the Motion Comfort Ratio. He claims it was a bit tongue in cheek, but it does at least try to factor in relative contributions to motion from weight, length and beam. The formula is:

MCR = DISP / (.65*BEAM4/3(.7*LWL+.3*LOA))

 

[Salty John]

I should have added that the MCR ranges from 5 at one end of the scale to over 50 at the 'heavy' end, with the well known 40ish foot blue water cruisers rating in the mid 30's.

 

[carrswood]

Thanks everyone for all the replies - interesting stuff
In conclusion it would seem Ballast Ratio is an excellent indicator of a yachts stability but there are many other factors which play a big part.

I wish the production yards did increase there overall weight and Ballast ratio and perhaps didn't place such an importance on 'volume stability'. But then thats what the production market is largely about - the race for more sumptuous interior volume and sadly sea kindliness is of lesser importance to the owners

Shame, - many will never know how much improved a yachts sea keeping can be (im my opinion of course)

 

[Little Rascal]

Ballast ratio is only one half of the story.

The other half is form stability - the biggest factor in which is beam, but also flare, lines at the turn of the bilge etc.

There is also a formula called the Capsize Screening ratio that assesses beam and displacement but not ballast ratio.

Some Motion Comfort ratios are given here for 34 footers:
http://www.boatdesign.net/forums/stability/ballast-dsplacement-ratio-20939.html

 

[Bosun Higgs]

Thanks everyone for all the replies - interesting stuff
In conclusion it would seem Ballast Ratio is an excellent indicator of a yachts stability

Pay attention there at the back ! You havent been concentrating. There is a world of difference between a 5 tonne boat with 2 tonne of internal ballast and the same boat with 2 tonne of ballast some 5 foot below the hull. Both have the same ballast ratio at 40% but the latter has far higher stability . And thats before you take into account form stability best illustrated by the difference between a mono and a cat - the latter having way more form stability.

 

[Bosun Higgs]

This subject has puzzled me since we bought our current boat last year, a 1980s Westerly.

The builder's stated weight is 5.7 tons and the ballast 2.6 tons, giving a ratio of about 45%.

Lifting the boat off road transport, the crane guage recorded a weight of 7.1 tons, without the mast and with no fuel, water or stores. Assuming the guage measured correctly, I presume the difference in weight resulted primarily from the generous use of materials during construction. The boat is certainly very robustly built.

.

Taking your figures at face value, the workers at Westerly would have had to make the hull more than 50% heavier than normal. Now there may be variations in boat weights but no way would they have made everything that much thicker.

 

[Tranona]

Thanks everyone for all the replies - interesting stuff
In conclusion it would seem Ballast Ratio is an excellent indicator of a yachts stability but there are many other factors which play a big part.

Don't know how you can draw that conclusion from the views posted here. Seems that you want it to be the case - but as most have pointed out, yours is a very simplistic view.

As for people not knowing what they are missing, I think you will find that many owners of modern boats with lower ballast ratios have previously owned "old fashioned" boats with higher ballast ratios - simply because that was what was available at the time.

If you follow this topic over the years on here, one of the constants is that owners of "modern" boats don't regret their choice and there is no rush to go back to earlier designs.

In fact one could argue very strongly that those who cling to older designs rather than modern ones don't know what they are missing!

 

[Little Rascal]

... would it be fair to say the best method of gauging a yachts "comfort motion, seakeeping, stability and stifness" is fundamentally to look at a yachts ballast ratio?...

No, not really. You cannot isolate one ratio for comparison of very different types of yacht.

Like for like, a boat with a high ballast ratio will have some benefits when it comes to ultimate stability and will probably be able to carry more sail.

A high ballast ratio does not give initial 'stiffness' which is more to do with beam and waterplane area. At 15 or 25 degrees of heel it may well contribute to a stiffer feel.

A higher ballast ratio may even contribute to a jerkier feel and a worse motion on some types of boat!

If by seakeeping you mean the ability to survive a gale mid-atlantic, then ballast ratio does become important. But so do many other things!

There are just too many factors to be able to judge by ballast ratio alone. It's possible to compare similar types like this, but it's just one ratio of many that you need to compare and not a magic wand. It's much better to develop an understanding of all the factors and understand the compromises the designer has chosen.

New boats or old boats, they all have compromises and are optimised for different uses. If you understand these compromises and match your boat to your use you'll be happy!

 

[William_H]

Comfort in a boat

Assuming you mean comfort in a big sea. Not comfort in a stiff boat resisting heeling from wind sail pressure.
If you picture a beamy catamaran in a lumpy sea coming in on the beam it will roll as one hull lifts and the other drops then the opposite as the wave passes. So in the same way a beamy monohull with generous chine volume will roll with the wave passing from the side. by comparison a narrow deep vee bottomed boat will tend not to roll so much. The ballast will have more chance to keep the mast vertical.
I have noticed my light weight TS is very susceptible to beam seas. It depends on the buoyancy of the chine area for stability hence a wave higher on one side than the other makes it roll.
Perhaps in a similar way a hull with a lot of bow buoyancy will rise to oncoming waves (or stern to departing waves) while a narrow fine entry/ exit will tend to dive into the waves rather than rise to them.
So it might be said that narrow boats which might be described as old fashioned will tend to have less motion in a seaway.
By contrast the beamy boat will manage with a smaller ballast ratio or use a larger b/r more effectively to enable it to carry more canvas. (so go faster). and of course the newer beamy boat will have more room.
So as said b/r is only part of the story. I believe a boat should have enough ballast to ensure recovery from at least a 90 degree knock down so there should be minimum b/r regardless. This of course has to take into account the location of c of g. ie depth of ballast.
All very complicated. But yes I think I prefer modern boats to your older narrow long keels. olewill

 

[Ubergeekian]

If you follow this topic over the years on here, one of the constants is that owners of "modern" boats don't regret their choice and there is no rush to go back to earlier designs.

There's also a recurring theme that "the best fun I ever had sailing was in my Corribee / Leisure 17 / Jouster (no reasonable offer refused) / Silhouette / Pioner 10 / whatever". Mind you, I'm inclined to take a slightly different view on that: I think it's quite true, but that as we get older, "fun" in that sense diminishes in importance as a factor for sailing. I had huge fun in the Jouster (n.r.o.r) in my 20's, but now I'm enjoying something with rather more room to rest and relax.

In fact one could argue very strongly that those who cling to older designs rather than modern ones don't know what they are missing!

It would be interesting to know how many of those who bought a 34' AWB starter boat and worked up have ever tried something a little more ... traditional? Do they know what they're missing?

 

[Wansworth]

Eric Hiscock relates in his book"Cruising Under Sail" that he noted that his friend Dr Pye always arrived more relaxed and rested after a passage.He put thisdown to the fact that dr pyes old wooden cutter had inside ballast and as it had a working boat hull form rolled less and never to windward,whereas his more modern yacht rolled to windward and was uncomfortable.Wanderer had outside ballast whilstMoonraker s ballast was all inside.Both boats where simular in size.In the event of a knock down I assume Hiscocks might fair better.

 

[Lakesailor]

I think it's quite true, but that as we get older, "fun" in that sense diminishes in importance as a factor for sailing.

Don't know really. It probably depends a lot on the use you put your boat to.
Being landlocked I get great pleasure from going out for a couple of hours and sometimes ghosting about, or if it's more lively having a good thrash.
But either way it's not passage-making and it's not serious.
The age thing is only relevant in that I creak a bit (lot) more when I get home!


Yesterday was a stiff breeze day........

These two guys were having fun on a Cat