freeboard, how much is enough?

Hi all,

People say it isn't hard to make a sailboat look good with low freeboard. But how much freeboard is enough? What are the seaworthyness considerations that must be taken into account when deciding on freeboard? Is it a matter of convention, or simply setting the rail just above the waterline at 30 degrees of heel?

cheers, dionysis

 

Adequate freeboard depends largely on the expected use of the boat and its overall shape. For example, a hull with lot of flare needs less freeboard than one that's slab-sided. Surfboats have low freeboard, but their bow and stern are very high. Trawlers have low freeboard in the aft part but a very high bow and forecastle for reserve flotation. Each boat type is a study in itself.

 

Freeboard affects the stability of a sailing boat, as shown in this picture of righting arms curves. The boat analized (continuos curve) is 10 metres long, Boa 3 m, freeboard 1 m, while the other curves were made increasing and decreasing the freeboard of 0.3 m. As you can see the difference is not much, but the boat with higher freeboard has a better behavior especially in the negative zone. In powerboats and larger ships, freeboard has got a more importante influence on stability, because those boats has less weight stability than sailboats. Anyway, there are rules that have to do with freeboard for small boats too, like the ISO/TC 188 (small craft). It also talks about flooding and the minimun heights of apertures on the deck, taken from water level.

 

Seems like the hull with moderate freeboard has an overall better behavior. It convinces me more that extreme designs don't work well except in very narrow conditions.

 

Take care how you interpret the curves. We are told that the hull form is identical up to the depth of the lowest deck. So the curves are greatly influenced by the vertical centre of gravity and we can see the effect.

At the start (from zero degrees) until the deck edge immerses on the lowest depth hull we see how the three curves are not the same. The change is only due to the KG but the curves to this point would be identical if the KGs were the same for the three hulls.

These points are very important and a high freeboard may be necessary at times.

Don't get caught by the tendency to "design" something according to what you think it ought to be - read the rules.


Michael

 

what about windward performance

thanks to you all for your replies,

I will study ISO/TC 188 (small craft) when I get my hands on it.

From the graph you can see the advantage of high freeboard beyond maximum GZ and for ultimate stability, yet you pay the price of extra windage and a higher centre of gravity, when beating to weather in stiff conditions.


From Baader's discussion in "The Sailing Yacht" overall windage accounts for some 20 to 30% of resistance to windward. High freeboard contributes a sizable proportion to this figure.

cheers dionysis.

 

Tumblehome

Nemo:

Please inject some tumblehome and show what the resultant curve would be.

Thank-you.

 

Tumblehome changes the charcteristics. This is a point I strongly emphasize: all aspects of design must be considered together. Experienced designers and buyers compare a new design to existing similar ones. Of course, once in a while, there is something so radical that no close comparison is possible. Most times, though, we can use a typical model to predict a boat's behavior.

 

I'll try to change the curves after adding a tumblehome, even if it will changes things at very high heel angles. Those diagrams were only intended to show how they are influenced by freeboard only. When optimizing a hull, you have to choose what parameters you want to analyze, and then change them one by one. We saw that freeboard doesn't change radically the boat's stability, even if we'd also take a look at stability of equilibrium. We know that the boat is in stable equilibrium when the derivate d(GZ)/d(°)>0. Look at 60°: while the boat with more freeboard is stable, the one with less is unstable, and could tend to heel more.

 

humble pie

hi all,

I am suitably humbled Mike.

What are the rules and where does one get them from?

cheers dionyis.

 

Freeboard & GZ Curves

Freeboard to an extent will be driven by accomodation requirements, and the proportions will be driven by length, and perhaps if standing headroom is required, or whether sitting headroom is required under the deckhead.

Low freeboard can look great, but not if it results in an overly large coachroof or deckhouse.

Also a high B/T ratio will force up the freeboard, in order to fit in the accomodation.

The GZ study is interesting, but bear in mind that a coachroof should improve the stability curve at large angles.

Look at the ISO's, they will have some influence on the cockpit geometry, notably the sole height and companionway sill height, will not give you a freeboard.

However the proportions will influence the STIX rating, on this subject there is an excellent article by Rolf Eliasson in the latest Professional Boatbuilder on the whole subject of STIX and stability.

Hope this helps.

 

dionysis

Better to eat humble pie than say "If only I'd...." as you go under

Every maritime nation issues its own code or rules and regulations as part of its statutory law. Most countries have some sort of department or ministry of Transport that controls this aspect of the law. Some others have what is termed the Coast Guard. Under international law these groups are termed the National Authority.

The N.A. itself usually applies all the rules of safety of people, stability, fire-fighting etc and mandates Classification Societies to apply the rules of strength, scantlings etc.

These laws etc were put in place for new vessels many, many years ago and they applied to commercial vessels greater than a certain size so that recreation boats etc were excluded. It is only in relatively recent years that rules were introduced for these smaller craft but not all countries have them. However, due to the increase in yacht size and international racing, interest rose and some standards were introduced. Then the boating. craze really took off and the predictable happened – casualties soared. The New Zealand authorities published a very interesting report, find it here http://www.msa.govt.nz/Publications/publications/PBSAG2000.pdf

So it is up to you to find out if there is any legislation in Australia that applies to you. I did a quick search and here are two Aussie sites that may help. At least you can contact them and get their assistance,
http://www.dpi.wa.gov.au/imarine/rec_boating/index.html
http://www.amsa.gov.au/sd/BOAT/ANZSBEG.pdf

These sites will help you but they are not Australian so they are only guidance, not mandatory.
http://www.uscgboating.org/regulation.htm http://www.uscg.mil/d8/mso/louisville/WebStuff/comdtpubp16761_3b.pdf
http://www.ccg-gcc.gc.ca/obs-bsn/
http://www.imci.org/english/index.php?menue=1&main=WhoIsImci
The first two are American, the US Coast Guard. The third is Canadian, Canadian Coast Guard. The last covers the European Union. You already have the ISO site.

If there are no regulations that apply then you must study enough naval architecture to understand what is going on. The large, professional books are not geared to yachts and boats so you should find textbooks particularly about yachts and sailing boats.

A major difficulty in naval architectural design is that there is not a single, definitive answer to most problems. There is rather a compromise to be found and instead of re-inventing wheels good text books guide you in the right direction.

Happy hunting.

Michael

 

hmm...ask a simple question...get a complicated answer!

I'm onto it .

You can design for any reasonable stability curve you like, taking into account the numerous attendant compromises from aesthetics and accomodation requirements, to ultimate stability etc. Freeboard is just one of the contributing design factors.

Over the years freeboard has steadily grown higher and higher. The question is, how much of this growth is driven by accomodation requirements and how much by questions of safety? If your canoe draught is deep enough, why all that height above water?

These are the delights and challenges of yacht design.

But Mike has put the wind up me with talk of "the rules".
Looks like I got to be a yacht broker, insurer, surveyor...as well as an electrician, plumber... etc.


now where was I? Ah yes...Safety of Small Craft...


thanks all, dionysis

 

In Australia the rules for commercial craft are the "Uniform Shipping Laws" or USL. These are in the process or have already been upgraded and ammended. Each state has the power to control these commercial craft, and is usually done with the Department of Transport. I think the aim of the new rules is to get a more uniform code across the states.

For pleasure craft its another story, I don't recall that there are any specific laws governing non-commercial vessels in Australia (I might be wrong), the exception is the state controled laws regarding minimum safety equipment and licencing.

For larger vessels and ships, the international rules of the IMO, and the relevant classification society rules are overiding. In australia the IMO rules are controled by AMSA.

Back to the subject of freeboard. For me the deciding factor for freeboard is the ability of the ship or boat to stay afloat after sustaining damage. Basically the rules for ships is that the ship must have sufficient freeboard with two (adjacent) compartments flooded. There are other situations to analyse but I don't remember the details. For a true seagoing boat or yacht, I would apply similar principles.

Michael

 

Dionysis,

I see you are in "Sunny" Tasmania, are you studying at the AMC?

If not the library there has loads of books and information (thesis etc). As far as I know it is open to the public, although you probably can't borrow any books.

Michael