Sailing stability and safety stability

After having owned three ‘serious’ keelboats and two cb. boats, I have started to divide a boat’s stability into two categories: Useful sailing stability (max 30° heel) and safety stability (knockdown).

My present boat (Marieholm IF, Ingeborg), with narrow beam and 58% ballast, has very little initial stability, and one must let her heel to 30° to get max power out of her rig. I have no stability curve for her, but I bet her max righting moment is found at closer to 90° heel, and is then huge. One could probably launch an IF upside down, and she would be likely to self-righten at once. The concept gives a very weatherly and easily-handled boat, but all that ballast will  slow her down when reaching and running.

In total contrast, my last boat, the 6.5m Jollenkreuzer, Frøken Sørensen had no ballast at all. She only had a flat, wide bottom and then 740kg displacement to provide sailing stability. Her righting lever topped at 43cm when heeling 25°, and from 83° she would capsize. Frøken Sørensen coped surprisingly well, and was powerful enough to windward, much thanks to her low resistance ( She touched 5kts with a 2.3hp Honda), but in strong gusts, it was clear that there was little safety stability in spare. What saved us, was my fast sheet work, and that the cb. lifted out of the water, so we side-slipped when blown over to 45° heel.

I happen to have Bolger’s book, “Boats with an open mind”. His trick to get maximum sailing stability (= power to windward), is to make square frames, and then add enough inside ballast to give enough displacement: Righting moment = displacement x righting arm.

Since this internal ballast doesn’t bring the total CG very low, he instead made the topsides taller. This way the centre of buoyancy moves rapidly ‘upwards’ and ensures self-righting in a knock-down. The drawback is higher windage of the hull, but the AS29 has shown that the concept works.

An alternative is to use Bolger’s double chine designs as on his 20’ Chebacco, 25’ Red Zinger, and also used on Annie’s SibLim. This brings the cabin sole lower, and the shallow ballast shoe will ensure plenty of safety without too tall superstructure.

Arne

Graham Cox wrote:

I was looking at Bob's website and saw drawing for Bolger's proposed 11m sharpie.  It looked very exciting.  Then I saw the words on the drawing, beneath the cabin sole, water ballast.  That made me gulp.  I'd find it hard to go to sea without a bit of lead under my feet.  Perhaps I am just not open-minded enough! 

The 11m sharpie with water ballast was an old "cartoon" of Phil's and I don't believe the design was ever finished. The design brief for this boat was as a maximum trailersailer able to be towed by a compact car which makes the water ballast seem like a good idea in this case.

That said, Phil's larger Breakdown schooner (http://boatbits.blogspot.com/2012/12/just-good-but-not-great.html), which also used water ballast, was built and cruised successfully .so there does seem to be some proof that the concept actually works. I do agree that aa heavier ballast medium would be better if for no other reason that water ballast just takes up an unacceptable amount of interior space in a cruising boat.

I suspect that, if I were going to build a sharpie for pelagic cruising based on the 11m sharpie cartoon in question, I'd ballast it with a combination of a lead shoe on the exterior land movable lead shot or ingots in ballast boxes in the interior if budget permitted or concrete and steel punchings if money was a factor.

Anyone who wants  to check out Morejohn , here´s the link

chrismorejohn.blogspot.dk

yes, quite the leap from IF to Box boat.

This particular one  H-32 is 32ft, by 8ft with 18" draft board up disp 13000 lbs light,

18000 lbs loaded.

I'm just very intrigued by Morejohns designs.  

lots of interesting stuff on his blogs 

Just a quick note that Bolger's sharpies are very able and seaworthy beasts. We've sailed thousands of blue water miles without problem. Our last sharpie (Loose Moose 2) only drew .33 meters in cruising mode and our collective experience has convinced me that a very shallow draft is far safer when things get nasty with nothing underneath to catch and trip on.

Also I should add that when we had Phil design Loose Moose 2 for us as our Bolger "Jessie Cooper" was just a bit too small and we really needed more tool storage as well as a dedicated office space, the boat design brief was for a boat able to do ocean crossings as well as to be non-problematic for areas like the European canal system and areas of very skinny water.

Obviously a "box boat" is not to every ones taste but when you tally up the positives of a quick build (Our Jessie Cooper took 3 1/2 months to build and LM2 took us just a kiss over 6 months), a frugal cost, excellent internal stowage/livability, and performance under sail that surprised a lot of people, it is an option that deserves at least a bit of thinking outside the box to the advantages of sailing/cruising a box.