AVS for narrowboats

[Little Rascal]

Max,

Here is a chart done for kayaks of constant waterline beam (from Guilemot Kayaks) - it's an attempt to isolate the effect of hull form...

It shows that initial stability (stiffness/tenderness in the first ten degrees of heel) is pretty much the same, but the AVS is the lowest (only 40 deg)

It has to be said that in reality it's not possible to isolate one factor, the max beam of the 'better' hull forms it much greater.

Basically there are two things you can change to improve stability, metacentric height and hull form. Lowering the metacentric height (deeper keel, higher ballast ratio) helps. Hull form is a matter of beam, and the shape of the sections/topsides. The wider the waterline beam, the higher the initial stability. Secondary stability can be improved by making sure the CB moves well outboard as heel increase (flared topside for example.)

Once past 90 degrees heel however, other forces come into play, particularly the buopyancy of the cabin top etc... Like on self-righting lifeboats.

 

[maxcampbell]

Thanks, Little rascal, that does surprise me, actually, but there you are.

Re ballast, most NB's are ballasted with about 3 tonnes of paving slabs or bricks, lying on the bottom plate. Obviously, you would make sure these were well fixed somehow, and you'd have to make many other amendments - as well as those mentioned, you'd probably snorkel the air intake, fit duel fuel supplies, etc, etc.

The overwhelmingly unattractive feature of a NB for seagoing is its beam of 6' 10", but I'd still like to get a theoretical handle of what sort of sea state would cause one (assume all the necessary amendments, including well fixed ballast) to roll.