Historical multihulls

Sadly Ppalu hit the bricks recently but the photos of her rebuild give more insight into her construction method.

https://www.facebook.com/media/set/?set=a.273606469467601.1073741864.170355479792701&type=1

 

Video on the sinking and recovery of Ppalu and a bit of history on this interesting Peter Spronk designed catamaran.

http://youtu.be/IeMVCL6t8vQ

 

Wow! What a mess one rock can make!

Reinforces the old adage to "look before you anchor" and not just directly underneath the boat, but all thru the swing area.

It also highlights how necessary 'positive floatation' is in cats - foam of some kind, not simple reliance on 'buoyancy compartments' - as all of these were breached and flooded.

And shows why a trimaran is so much better than a cat. If a tri had been holed in the main hull, the amas would have kept it afloat.

Hope they can fix it and get it sailing again, what a beautiful and historic cat.

 

Comparison with foam sandwich

Here's a photo of my boat when the previous owner ended up on the rocks. Every compartment is breached in the port hull, but the construction is foam sandwich, and no bridgedeck weight. It's floating higher, but still substantially submerged.

 

And this from my 1992 newsletter

From a fax from Pete Benjamin, of Heritage Manufacturing, the S-African builders of the Sagitta:

" My charter Sagitta Bojangles was put on the rocks at a place called Rooi Els. The charterers claim the anchor rope parted at 3am and they were awakened by horrible sounds of the boat grinding on the rocks.

To cut a long story short, when the tide went out they effected emergency repairs and the Sagitta was refloated on the following high tide by crowds of people - minus a keel and a badly damaged starboard hull as well.

They motorsailed to Gordon's Bay and although the modules were awash the bloody thing didn't sink!!!!' It made national TV and I think that the safety aspects of the watertight compartments are not to be overlooked as is the incredible design feature that led her to being such a safe boat - Thanks folks!'

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

You can have as many buoyancy tanks full of air as you like but if they all get punctured ie, Titanic, Hump101s cat or ppalu (assuming they had them) they may or may not save the boat, if you are going to sacrifice the storage space in favor of buoyancy chambers they had better have foam in them. Obviously if you only compromise some of them, air will work but I wouldn't want to roll that dice.

 

Mine has five separate compartments in each hull, one filled with foam in the bow, but it is so fine the volume is tiny. The other four are sealed from each other, but all were punctured. I sail with children on board and fit permanent buoyancy on the trampoline (inverted inflatables) so the boat will always stay afloat, and will always keep the tramp above the waterline even when inverted, so anyone trapped underneath can breath and swim out. I have four separate inflatables on the tramps, each enough to support the entire boat.

However, at least we aren't towing a lead weight underneath.

 

I used to own a Macgregor 36 cat that had a lot of floatation in the form of blocks of polystyrene foam in sealed compartments,which of course took up a lot of space, however, sunk for a mac 36 is a mere inconvenience and would not prevent you from continuing sailing. BTW, that's a nice looking cat you have there, what is it?

 

Given the availability of CAD and the more recent invention of CAD-controlled 'shapers' - or 3D-sculptors, it makes sense to have the bottom 1/4 at least of cat hulls 'shaped' from XPS foam, then 'planked' in foam (or ply), and fibreglassed over the top. Say foam up to the sole level.

This would fix the problem with hump's boat, for example, as the floatation would be along the whole length of the hull.

With ocean-going cats and charter cats even if the bow was lunched by a half-awash shipping container, or one of the hulls lunched on a rock (a la Ppalu), there would still be enough floatation built into the hull, and no (or minimal) loss of storage and usable space.

OK, so it's maybe a bit expensive for DIY just yet, but I'm reasonably sure a clever DIY'er could figure out how to 'hot-wire' cut the foam blocks.

As I see it, you'd make aup a strongback with the bulkheads and 'sole' in place, but only a temporary 'tacked on' keel stringer. Fit rectangular blocks of foam between the bulkheads, having pre-notched them to take the keel stringer, attach the keel stringer.

Then make up a hot-wire foam cutter, and run it along the hull using the keel stringer and the edge of the sole as guides.

You may need to change the shape at the bows, but, in principle, I can see that working.

Final shaping with Surform and blocks, then fill and sand prior to glassing to get a smooth surface.

Then either foam or ply the rest of the hull and glass over, using vacuum infusion technique, and a relatively simple process would see a far superior strength and floatation-equipped hull.

What do people think?

 

Apart from the build issues, I'm not sure losing the bilge is a good thing.
If the hull is damaged the only way of locating that damage is to go over the side or beach the boat, further I don't trust foam to stay waterproof, I've seen too much sodden foam in boat hulls to have faith in it.
My preference for buoyancy in compartments is sealed plastic bottles or bladders glued together so they don't float out of a breach.

 

rr
My guess is the foam you've seen that was sodden was polyurethane. It's well known that this breaks down over time and becomes swater-logged.

EPS and especially XPS (extruded) styrofoam does not.

Hence why I suggested XPS foam, as it is the better oif the two with the smallest and tightest 'air bubbles' and least voids for any water penetration.

The bottom line is, would you rather be afloat after hitting a container, or awash?

The problem with the bilges is that often you can't get to where the damaged area is, as it's usually inside a sealed buoyancy chamber, or completely covered by gear and crap, and by the time you can get to it, the hull is awash, eg Ppalu.

If you hit something offshore, you need to be able to float in order to sail or motor to shore and get hauled out. There's bugger all you can do to repair a badly hole dhull at sea, short of old fashioned 'fothering', although doing so with modern materials like caulking and plastic sheeting is more effective than tar-smeared canvas.

You will need to haul out regardless of where the damage is.

On lead mines, with full length bilges, with a removable sole, having access makes sense, but multis are constructed differently.....making the 'desirability' of an accesible bilge less important than overall floatation ability once holed.

Lead mines go to the bottom, regardless of where they are holed, but a small hole, or a leak, might be able to be stopped or plugged, if you can get at it, hence the need for accessible bilges.

If the foam is contigous, epoxied to the bulkheads and to itself, it forms a solid lump of foam, effectively a giant 'airbag' at the bottom of the hull, where that floatation effect is most needed.

 

The pour in place urethane buoyancy foam is like a sponge and I would never trust it even though the Coast Guard here approves it anymore, I have used it before on a design of mine built 27 years ago, it fills all the areas below the vee berth and afterberth and makes the boat totally unsinkable but the boat has never leaked a drop so its been a non issue and will do its job if ever called upon. I would however trust my life to 1lb density polystyrene, contrary to what some might say, it does not absorb water at all, it is sold here in the land of 10000 lakes as dock billets used to support floating docks , just strapped under the metal dock directly in the water, all summer long, year after year.

 

Polyurethane and pvc, polystyrene doesn't get waterlogged it just falls to bits until it is bean bag filling.

 

In block form, in a sealed compartment as in the Mac 36, falling apart is a complete non issue. Actually, ive often thought one could recycle packaging peanuts stuffed into fabric bags into sealed airchambers, you've already given up the space.

 

Block form lightly glassed in epoxy would make me happy, tissue or veil as you call it would make me happy . And glued in place with silicone.
On it's own any movement or vibration and it's all over, and that psychopath inducing squeak !