Thursday, December 1, 2011

Internal or external? Lead or iron?

Except for the smallest ones, monohull sailboats (you multihull readers can skip this post) universally have a keel on their bottoms.  They carry this device around with them because:
  • It helps to keep the boat from sliding sideways when the wind is not taken directly astern, and
  • it keeps the boat from tipping over when the sails are full of wind.
Today, I'd like to talk about the second item: keeping the boat from tipping over.  In naval architecture, the boat's ability to resist being tipped is called her "righting moment". 

Old sailing ships used rocks, held in the bottom of the bilge, as ballast.  When they took on cargo, some of the ballast was jettisoned because the cargo held below the waterline would provide the righting moment, and because the ships were designed to carry cargo, after all, not rocks.  (This led to the ballast spoils areas in many harbors that were heavily used in the 18th and 19th centuries.)

Modern sailboats have moved the ballast outside the boat, lower, where it can have a greater effect.  Like the big kid and the little kid on the teeter-totter (have these all been removed from playgrounds?  What will we use for the illustrative analogy in the future?), the more weight and the deeper it is suspended below the boat, the better: the greater the righting moment.  This started with shaped blocks of lead bolted to the bottom of the keel in full-keeled boats.  But it has evolved from there.

Internal or external?

For a fin-keeled boat, there is really only one choice here:  the keel is a shaped piece of metal bolted onto the boat's bottom.  It is an external keel.

But for a modern fiberglass hull which is designed as modified- or full-keeled, a second choice is possible:  internal ballast.  In this design, the shape of the keel is part of the fiberglass molding of the hull - the keel is molded as part of the hull.  And then the ballast is added to the inside of the molding.  Typically, this is as metal shot combined with resin in a cement mixer and then poured into the keel.  But it can also be done using shaped pieces of metal laid up in the interior of the keel and then encapsulated with resin.
  • Internal keel pros:
    • There are no keelbolts to worry about.  Short of complete destruction of the hull, the keel simply cannot drop off the boat as some external keels have famously done.  
    • Maintenance of a water-tight seal between the keel and the hull is unnecessary.
    • In a collision with a hard object, there is no fear of loosening the hull-keel joint.
  • External keel pros:
    • In a collision with a hard object, it is the keel which makes contact, not the hull.  If the keel is a soft metal like lead, deformation of the keel at the point of impact will absorb a portion of the energy.
    • Because the void space of uniform-sized shot is 33%, the density of the poured shot keel will be considerably less than the solid metal.  Therefore the solid metal keel of equivalent shape will have a greater righting moment than an internal keel made from poured shot.  This advantage is considerably less if shaped metal pieces are used. 

      (Question:  Has any manufacturer explored the use of a range of shot sizes, designed to reduce the void percentage?  Concrete manufacturers have been doing this with the aggregates they use for centuries.  Seems like a simple improvement.)
Until some more exotic material is used (depleted uranium anyone?), keels will be made of either lead or iron. Both have advantages:
  • Iron keel pros:
    • Iron is a lot cheaper than lead
    • For a steel boat, there is less galvanic potential between an iron keel and the hull than there is with a lead keel.
  • Lead keel pros:
    • Lead is denser than iron. This means that for equivalent shapes, the lead keel will have considerably greater righting moment.
    • Lead does not corrode in seawater;  tedious and frequent scraping, sandblasting and painting of the keel is not needed.  In fact, other than for protection against freeloading sea life, painting of a lead keel is completely unnecessary.
    • Lead does not corrode in seawater;  iron does.  In fact iron swells as it corrodes - that is, iron oxide is greater in volume than the iron from whence it came.  Therefore leakage is a serious problem for internal iron ballast, since the incoming seawater will corrode it and eventually cause the keel structure to burst.
    • Already mentioned above:  An external lead keel will absorb a portion of the impact energy in a collision with an underwater object.  An iron keel will convey essentially all of that energy to the keelbolts and the hull at the top aft end of the keel.
This is a lot to consider.  When we were in our boat search, I boiled it all down to an ordered list of preferences:
  1. Internal lead
  2. External lead
  3. External iron
  4. Internal iron
As it turns out, Eolian's keel is internal lead.

But as any of you who have gone thru a boat search already knows, the emotional tug that a boat has on you overrules almost any of the analytical studies that you might have prepared beforehand.

At least it did for us.  So I guess we are just lucky...  but hey, this makes a nice rationalization, don't you think?
Share/Bookmark

4 comments:

Serah said...

When we were looking at buying our boat, this was one of the few design considerations that was non-negotiable. We had started making a list of desirable features; an aft cabin, C shaped dinette, adequate galley space, all those lovely things. But at the end of the day, we found it was in general restricting our search. We settled on finding a boat with an internal keel, as we knew from reading books from boaters far wiser than ourselves, chances were very very high that we'd ground ourselves, especially as we're planning on going offshore. Apart from size (from 32' - 37') this was our only "absolute must".

We found our Douglas 32 a few months later, a full keel with a cutaway forefoot, with a poured lead keel. We've been living aboard (and sailing as much as possible) for almost a year now. When we're out in really snotty weather, we're so thankful for the stability this helps provide, while also a nice bit of peace of mind when it comes to rocks.

bob said...

Serah -

Congratulations on having the strength of your convictions. I must confess that during our search, we were considering boats with both external lead and even (in one case) an external iron keel. I'd like to think that my analytical self rules, but unfortunately, that is wishful thinking...

bob

Alf said...

Having read this, I'm a little baffled why my 36 year old external iron keel doesn't have any rust at all. And it has never been given any more care than the fiberglass hull -- just occasional copper based bottom paint during the 25 years I've owned it.

Maybe it is because iron reacts with oxygen to form rust, not "sea water".

And that's a lucky thing since virually all boats bigger than about 50 feet are made from iron.

The main reason lead was commonly used originally: It is a lot easier to melt and pour into a mold than iron. You can melt lead on your kitchen stove at 621 deg. F. Melting iron requires 2795 deg. F.

The main reason it is used now is so many people, who don't know any better, believe iron will just rust away. The internet is full this dogma.

bob said...

Alf -

You seem to be implying that being underwater, the keel is in an oxygen-free environment. (Shhh! Don't tell the fish)

And finally, is your anchor galvanized? Why?

Thus ends this thread.

bob

Related Posts Plugin for WordPress, Blogger...