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Old 04-10-2011, 03:10   #1
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How Many Bulkheads Do I Fit for Flotation ?

hello i was wondering how to divide up the yacht to achieve positive flotation or contain damage after hitting a reef?

1 bulkhead material - marine ply? strength?
2 doors - positive locking steel, or timber?
3 how many compartments?

ideas and sources welcome~

Cap'n John
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Old 04-10-2011, 03:14   #2
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Re: how many bulkheads do I fit for flotation?

Need more info?

Yacht construction?
Size?
Sailing Area?
How many reefs do you need to hit?
Can we talk you out of hitting reefs (what have they done to you anyway)?
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Old 04-10-2011, 05:25   #3
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Re: How Many Bulkheads Do I Fit for Flotation ?

Consider a crash tank forward (typically the area under a v-berth) and aft around the rudder tube.

If you're really smashing on the sides, it's not going to hold. The keel, bow, and rudder are the key vulnerabilities. Not my thoughts alone; many boats are designed this way.

Material and hatches? Generally cored FRP very well tabbed in. Only small inspection hatches.
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Old 04-10-2011, 07:17   #4
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Re: How Many Bulkheads Do I Fit for Flotation ?

Positive floatation requires captured air space greater than the displacement of the vessel. This can be roughly estimated by dividing the displacement of your boat in lbs by 64 (the weight of a cubic ft of sea water). This will give you a guide as to the volume of air you will need trapped in watertight compartments so as to have the boat just floating awash. You will want more to have the deck above water.

For example, if your displacement is 28,000lbs, then you will need 438 cubic feet of trapped air - about a 7.5'x7.5'x7.5' area.

This is all very rough - you will want to calculate floatation you already have on board (wood, coring material, etc), as well as calculate the distribution so that you don't float on end. Those calculations become more complex because they require compensation for density and stability.

Containing damage is easier to accomplish - you only need to seal off small areas around potential damage sites like the bow.

Mark
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Old 04-10-2011, 10:17   #5
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Re: How Many Bulkheads Do I Fit for Flotation ?

For inspected vessels, the requirement is that any single compartment can be completely flooded and no point of the deck can be underwater. What this means is that you end up with small compartments in the ends of the boat and big ones near the middle. On the commercial sailing vessels that I used to work on, 5 compartments was pretty common. If you know the location of the center of mass and the displacement, it isn't too hard to figure out how much volume you need in certain areas.

Another way to do it is to protect around the areas that are most likely to have problems. This usually means a collision bulkhead forward and a way to seal off around the rudder post and the keel attachment.
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Old 04-10-2011, 10:24   #6
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Re: How Many Bulkheads Do I Fit for Flotation ?

I think air provides 64 lbs of floatation per cubic foot. You can run the calcs, but you are going to need to offset nearly the full weight of your boat, some of your boat weight is offset if it's in water. For a 20,000 lb boat you could need about 300 cubic feet of floatation.. whoops, I see columj already said it!
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Old 04-10-2011, 10:35   #7
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Re: How Many Bulkheads Do I Fit for Flotation ?

See Titanic's Bulkheads & Watertight Compartments
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Old 04-10-2011, 11:57   #8
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Re: How Many Bulkheads Do I Fit for Flotation ?

Quote:
Originally Posted by Shanaly View Post
hello i was wondering how to divide up the yacht to achieve positive flotation or contain damage after hitting a reef?

1 bulkhead material - marine ply? strength?
2 doors - positive locking steel, or timber?
3 how many compartments?

ideas and sources welcome~

Cap'n John
What you're looking for is single compartment subdivision and it's usually a job for a naval architect. You need to consider flotation and trim, the permeability of the flooded compartment, as well as freeboard and metacentric height after flooding. The bulkheads have to be strong enough to withstand the pressure imposed on them by the flooded compartment.

A simplified means of locating watertight bulkheads in boats less than 65 feet long can be found in the Code of Federal Regulations for passenger vessels, 46 CFR 179.220, which can be downloaded from the CFR website.
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Old 06-10-2011, 06:36   #9
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After seeing the new Morris Yachts Daysailer models, most of which are open concept with only the head compartment closed off, this thought crossed my mind too. But then again, knowing Morris Yachts reputation for excellence in construction, I quickly forgot about it, as I assumed the naval architect has already made provisions for positive floatation in the event of grounding upon a reef. If your boat is from a reputable builder I would think that those things are already taken care of.
However, I was thinking, wouldn't an inflatable air bladder, made of the same material used in Zodiac boats and using a CO2 cylinder for inflation, work in your situation. It could be stored in a compartment under a settee or below the v-berth and set up to deploy automatically, much like an airbag in a car. I think this could work. Just a thought.
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Old 06-10-2011, 09:50   #10
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Re: How Many Bulkheads Do I Fit for Flotation ?

In the U.S. positive flotation is only required for inboard powered boats under 20 feet in length. Positive flotation or watertight subdivision for boats over 20 feet is extremely rare because the amount of flotation required or the close spacing of watertight bulkheads will reduce the interior space to the point of making the boat un-marketable.

The air bag concept can work if planned properly. 35 cubic feet of air is required for every long ton (2,240 lb) of displacement, and only the portion of the inflated air bag that is submerged below the flooded waterline will be effective. The air bags should be placed so the boat floats close to level after flooding and so the boat has enough reserve stability to keep it from capsizing after flooding. If air bags are only fitted forward under the vee berth, for example, the boat may float with just the tip of the bow breaking the surface of the water after being flooded.
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Old 06-10-2011, 11:27   #11
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Re: How Many Bulkheads Do I Fit for Flotation ?

An airbag trapped anywhere inside a vessel will at least help keep it afloat, even if it is not level. It would be advisable to have more than one and have them placed in strategic locations. Having more than one provides not only balance but also redundancy.
I don't know if anyone has tried this idea but I think it has merit and ought to be explored further. Anyone with any suggestions about this concept?
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Old 06-10-2011, 15:18   #12
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Re: How Many Bulkheads Do I Fit for Flotation ?

I know there was at least one company that sold air bag systems for boats about 15 years ago. I looked into it when I was thinking of buying a classic wooden sailboat.

I'm building a boat for myself now, and it will be fitted with watertight bulkheads.
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