Positively buoyant catamarans

[UNCIVILIZED]

Originally Posted by lindabarzini

Maybe those front parts of the forward cabins and storage under the beds could be filled with empty 2-liter bottles with the cap on tight. I read about some Gemini owners doing something like this.

The trick (problem) with this is, keeping them in place, & inside of the boat, for long enough to make it to safe harbor. As the wave action & hydrostatics definitely give them lots of incentive to move around, & try to escape.

[barnakiel]

Originally Posted by UNCIVILIZED

The trick (problem) with this is, keeping them in place, & inside of the boat, for long enough to make it to safe harbor. As the wave action & hydrostatics definitely give them lots of incentive to move around, & try to escape.

Remember PlasTiki?

They placed the bottles in a netting, so that say every 100 bottles become one unit. Now they are much easier to control, tie down or immobilise in some other way.

Should work. ?

b.

[JohnT]

Originally Posted by MikeFergie

Linda I have. Some pictures of a leopard 44(sad case) that once inverted travelled upside down from almost Australia back to Cape Town over a full year with only. With human intervention. Sadly it ripped. Apart and sank.

So for Mcel it's clear that they are positively buoyant


Sent from my iPad using Cruisers Sailing Forum

Mike, I sent you a PM regarding the above statement you wrote. I would like to know how you know the Leopard 44 was ripped apart and sank? If you can point me to where you found this information, it would be greatly appreciated. Sorry for the OT question.

[UNCIVILIZED]

Originally Posted by barnakiel

Remember PlasTiki?

They placed the bottles in a netting, so that say every 100 bottles become one unit. Now they are much easier to control, tie down or immobilise in some other way.

Should work. ?

b.

Actually, scenarios & setups like the above are why I posed the question. Since the efficacy of such setups, & how long they last when actually loaded up & they're supporting a vessel, is quite poor with some designs. So one needs to ensure that both a good design, & good materials are employed if you choose to go that route.

In the past, some high speed racing powerboats, have used netting to contain ping pong balls in order to meet class floatation requirements. Since ping pong balls provide the lightest form of floatation for a given amount of floatation. But there have been some shall we say, awkward goofs with keeping them in place, & inside of the boats, over the years. A few of them being quite comical.

So at times it can be cheaper & more reliable to go with more conventional methods of floatation. Especially if you have to do things such as resort to using Spectra netting for the above. So that things are both strong enough, & have enough wear resistance when subject to loading & chafing. Since if the netting gets cut at all, or develops holes, then you're sunk (pun intended).

[barnakiel]

Upside down does not imply positive buoyancy. It implies air pockets.

Floating hulls with no rig does not imply positive buoyancy. It implies positive buoyancy of a bare hull.

You want positive buoyancy of the boat upright, with full rig on and all gear in.

Or do you sail boats without rig, upside down? You must be mad then.

b.

[SV DestinyAscen]

Originally Posted by barnakiel

You want positive buoyancy of the boat upright, with full rig on and all gear in.

And that boat would be the most space inefficient spartan boat with minimal tankage, tons of wasted flotation voids - but I guess you can admire the boat while all the few things aboard are sitting in a meter and half of water while waiting for rescue. Not really sure what the appeal is - seems like beyond simple bow and rudder bulkheads, everything else claimed is better for marketing than enjoyment and safety.

I've always wondered why with catamarans, builders don't just offer a simple inflating skirt/bladder under the bridge deck - if you unluckily holed in longtitutal way that collusion bulkheads won't save you from imbalanced flooding in one hull, a bladder displacing about 60 cubic feet would keep that hull out of deep water and you'll be able to stuff the breach and manually pump out.

[44'cruisingcat]

Originally Posted by SV DestinyAscen

And that boat would be the most space inefficient spartan boat with minimal tankage, tons of wasted flotation voids - but I guess you can admire the boat while all the few things aboard are sitting in a meter and half of water while waiting for rescue. Not really sure what the appeal is - seems like beyond simple bow and rudder bulkheads, everything else claimed is better for marketing than enjoyment and safety.

Actually it's not hard to build a cat with positive buoyancy, that would float quite high even with both hulls flooded and the rig still on.

Cat's generally have fine bows and sterns, which are very suitable spaces for buoyancy without major impact on living or storage space. (The ends of the hulls shouldn't really be used for storage, especially of heavy items.)

[admiralslater]

Originally Posted by SV DestinyAscen

And that boat would be the most space inefficient spartan boat with minimal tankage, tons of wasted flotation voids - but I guess you can admire the boat while all the few things aboard are sitting in a meter and half of water while waiting for rescue. Not really sure what the appeal is - seems like beyond simple bow and rudder bulkheads, everything else claimed is better for marketing than enjoyment and safety.

I've always wondered why with catamarans, builders don't just offer a simple inflating skirt/bladder under the bridge deck - if you unluckily holed in longtitutal way that collusion bulkheads won't save you from imbalanced flooding in one hull, a bladder displacing about 60 cubic feet would keep that hull out of deep water and you'll be able to stuff the breach and manually pump out.

SV My PDQ 36 is positively buoyant , and does not suffer from the failings you describe. I do realize that it is not a deep sea cruiser.
Last winter in Martinique we where in the large marina where the charter bases are. One of the operations lifted the entire back end of an Outremer 55 far enough out of the water to take out both saildrives for service. It was done with a heavy-duty bag and a shop vac. Very cool

[river251]

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[mikedefieslife]

The strange thing I find is that all of the buoyancy is low down. On my boat, it's in the four lower corners. I think making the roof sections more buoyant would have a positive affect if it were to ever be inverted. Like-wise sealing the upper corners.

On more fun note. Don't pool noodles hold 150kg. So ten of those is 1.5 tonnes. Easy to store under floors or ceiling panels in a cat.

[Cheechako]

Originally Posted by barnakiel

Upside down does not imply positive buoyancy. It implies air pockets.

Floating hulls with no rig does not imply positive buoyancy. It implies positive buoyancy of a bare hull.

You want positive buoyancy of the boat upright, with full rig on and all gear in.

Or do you sail boats without rig, upside down? You must be mad then.

b.

That's right. a cubic foot of air gives you about 64# of bouyancy. So for a 15000# boat you need 4 ft x 4 ft x 16 ft of enclosed air.

So for example, a Lagoon 42 the dry weight is about 25000 lbs. Figure 30000 lbs loaded?
You need 470 cubic feet of air. 4 ft x 4 ft x 29 ft long!

[Chotu]

Originally Posted by Cheechako

That's right. a cubic foot of air gives you about 64# of bouyancy. So for a 15000# boat you need 4 ft x 4 ft x 16 ft of enclosed air.

So for example, a Lagoon 42 the dry weight is about 25000 lbs. Figure 30000 lbs loaded?
You need 470 cubic feet of air. 4 ft x 4 ft x 29 ft long!

And this all depends a LOT on what your boat is made from.

I can cut any piece out of my hull and throw it in the water to see it float very high out of the water.

I could also cut holes in the bilges of both hulls and not see the boat sink down much at all. There’s still more weight to add, but 1” (25mm) foam core with a thin glass skin makes for an extremely bouyant boat, aside from watertight bow and stern compartments.

There’s a lot that goes into this other than displacement. You need to know when the submerged displacement equals the submerged buoyancy. That changes non linearly as the boat sinks and is highly dependent on how the hull was built.

Pretty complex problem actually.

[river251]

Does anybody know if a Prout Snow Goose, or Snow Goose Elite, has positive buoyancy? In other words, is unsinkable? I haven't found information about their construction materials.

They are attractive because they might have the cat stability, but be narrow enough in the beam (15', 16') to be friendly to Mediterranean marinas, which I've read are difficult or expensive for newer, larger cats with 20' beams, and the Mediterranean has less option for anchoring--forcing you to the marinas.

I've gotten this from YouTube.

Thanks.

[mikedefieslife]

Originally Posted by river251

Does anybody know if a Prout Snow Goose, or Snow Goose Elite, has positive buoyancy? In other words, is unsinkable? I haven't found information about their construction materials.

They are attractive because they might have the cat stability, but be narrow enough in the beam (15', 16') to be friendly to Mediterranean marinas, which I've read are difficult or expensive for newer, larger cats with 20' beams, and the Mediterranean has less option for anchoring--forcing you to the marinas.

I've gotten this from YouTube.

Thanks.

Earlier Snowgooses like mine are solid fibreglass throughout. That is hulls, bridgedeck, coachroof, everything. From then on they went solid glass down below, and cored decks and coach. No foam is used. I severely doubt the construction materials alone give them any buoyancy.

They all have sealed compartments in the four corners. There's are around 1.5m3 on my boat. Probably more on the larger boats.

Considering the huge numbers produced, very few have been lost or flipped. One that was lost a year or so ago had the buoyancy chambers cut open to be used as storage (unbeknown to the new buyers).

As to Med marinas, the beam doesn't mean anything. A 7m wide lagoon will be charged the same rate multiplier as a 4.67m wide Prout. There are a few marinas where they use total area, but not many.

The static and dynamic stability (the latter just being a division of the former), it pretty high despite the narrow beam, due to the low COE.

Where is it that you think the Med has less options to anchor than? In my experience so far, the Med has far more anchorages than the Caribbean by a large margin, but those anchorages are much smaller, and you need to watch the weather as one day you might have a mistral, and a few days later a sirocco (wind, not the boat). You'll have no need to visit a marina at all during the spring and summer. Most people winter in a marina, and those with cats choose ones that don't charge any extra.

[Mr B]

Putting sealed 2 litre bottles in the floatation tanks on a Gemini is good value,
They have to be put in thru small round holes that has a plate that screws on after,
The bottles cannot come out of the floatation tanks under any circumstances,

I have bought a double sized blow up Queen sized bed for mine, It sits under the mattress in the main bedroom,
Its fully inflated in 3 minutes with its own inbuilt pump,
Its about 2 cubic metres blown up in area, Thats 2 Tons of displacement,
I will be buying two double sized double bed blow up mattresses to go in my back bedrooms, With their own blow up pumps, That will give me another approx 2 Tons of displacement,
I weigh 4.5 Tons, With my four floatation chambers built in,
I should be unsinkable,

I might be floating on the water line, But I wont sink,
That will give me time to figure out how to effect repairs at sea,
Or get to a beach where I can sit my Cat on it to effect repairs,

I have also moved my batterys up high so they dont short out if I do get water inside the hulls, ,
Previously they were 4 inches above the bilge, Or the bottom of the hulls,
so any water inside the boat above 16 inches from the bottom of the hulls,
I had dead batterys, They got flooded and no power for any thing, Inc pumps,

This is from Experience, I did sink once, Chucked up on a rocky beach in a violent storm,
Anchored,

Made me think a lot on how to prevent sinking in future, and what I could do to prevent it happening again,

This is for a Gemini 105 MC,