Adding foam compartments for flotation

[w00dy]

I'm working on a list of top priority modifications to make my boat more seaworthy and at the top of the list is the addition of enough foam to keep the boat afloat in case of capsize or collision.

Although my boat was built without any foam in it, the original plans and design call for two sealed foam compartments, one forward and one aft. I have an original set of plans with detailed instructions for the installation of the foam. The design of the boat is such that it includes the "plug" into its structure. Instead of removing the plywood cross sections, they are included in the eventual structure as a series of even bulkheads which add strength and effectively compartmentalize the boat.

The plans call for 30 cu/ft of foam to be divided up between the two compartments. They recommend sealing the adjacent wood with cuprinol to prevent future rot, but I would just use epoxy on the spots which aren't already sealed. Then I'd just cover with well fastened plywood and fiberglass.

It's seems pretty straightforward and I doubt I'll have any problems, but here's what's been bugging me:
The Aleutka displaces 5500 lbs. Add gear and supplies for a long voyage, and that number increases substantially, right?
So the pour foam I found online will support 60lbs per cubic ft.
Now, who am I to doubt the arithmetic of lord high potentate of small boat sailing, John Letcher, Ph.D of Engineering???
But 30 cu/ft x 60 lbs/ cu/ft = nowhere close to 5500 lbs.

What am I missing? Did the almighty John Letcher make a mistake? If so, how the heck am I supposed to fit 100! cu/ft of foam into a 25 ft x 7ft boat?

Btw, new blog entry with pics of our trip to the Tortugas here:
http://peanutbutterdiet.blogspot.com

[CharlieJ]

Hey James- David told me you are coming home for a short while. When will that be? Hope to catch you but we may be sailing to Belize for a few weeks doing a delivery first part of Feb.

Tough I know-but somebody has to do it ;D

[Captain Smollett]

Here's my understanding of the problem: 

The foam does not have to displace enough water to support the full weight of the boat.  Think in terms of the amount of water the hull can hold.  You just have to displace enough of THAT water for the boat to buoyant.

Say your 25'x 7' boat averages 5' deep, so we have a cube...not realistic, I know, but now we are talking about a volume of 875 cu ft. (just to put a number on it).  Now, that 30 cu ft lowers that to 845 cu ft,

What Letcher is saying is that THAT is enough of a reduction to keep 'er floating, maybe just barely.

The volume of your hull is significantly less than the number I used (it is smaller than a 25x7x5 cube), but I think that's the idea.  Letcher probably used more accurate hull volume numbers to arrive at the 30 cu ft., to the difference between water weight gained without to with flotation will be a greater percentage than my simplistic example.

It comes down to the lbs per inch calculation from the shape of the hull.  How much water can the hull hold and still remain afloat vs how much can it hold and sink?

If I'm wrong about this, I welcome the opportunity to be set straight.   ;)

[skylark]

Displacement is the same as the weight of the boat and contents.

If your displacement is 6000 lbs, you would need to have trapped air in the amount of 97 cubic feet to equal your displacement.  6000 lbs/ 62 lbs/cf = 97 cf

Note that if you add foam, the foam has weight so you would have to increase your total displacement amount.

The hull and interior of your boat also have volume, which could be counted in the 97 cubic foot total.

So 30 cubic feet is not going to keep you above water if the boat is full of water and submerged. 

[Captain Smollett]

Displacement is the same as the weight of the boat and contents.

If your displacement is 6000 lbs, you would need to have trapped air in the amount of 97 cubic feet to equal your displacement.  6000 lbs/ 62 lbs/cf = 97 cf

Note that if you add foam, the foam has weight so you would have to increase your total displacement amount.

The hull and interior of your boat also have volume, which could be counted in the 97 cubic foot total.

So 30 cubic feet is not going to keep you above water if the boat is full of water and submerged. 

I don't think this is quite it.

The boat is displacing 6000 lbs (97 cu ft) of water when she is sitting on her lines.  She can hold a bunch of water and still be afloat.  Presumably, if she fills completely, she will sink.  If you displace enough water from what the hull can hold, she will stay on top.

I could not find lbs per inch numbers for the Aleutka, but I do have them for my Alberg 30.  So, let's put some numbers on it (and assume the lb per inch submersion is linear):  A-30 lbs per inch submersion from her design lines = 678 lbs.

She has roughly 18 inches of freeboard at the lowest point.  If we can keep that above the water, she should still be floating, right?  It would take 12,304 lbs to push her down to the gun'l at that point (again, assuming the lbs per inch is linear, we know it's not and that number is high).

Let's only push her down to 15" to allow for trim issues, pitching and rolling and some wave action.  Now, we get 15" down with 10170 lbs, or 2134 lbs less than full "hull at surface" to leave our 3" margin.

2134 lbs = about 34 cu ft of water.  So, about 30 cu ft of foam displacing interior volume would, in principle, keep enough water out to keep the deck from submerging.  The numbers are a little off because I've made some simplifying assumptions, but that's the gist as I understand it.

We need Auspicious Dave to give us the definitive answer on this one, I think.

[skylark]

I would start the calculations assuming the boat is under water and is full of water.  Does it rise to the surface or does it do a Titanic?

[Captain Smollett]

I would start the calculations assuming the boat is under water and is full of water.  Does it rise to the surface or does it do a Titanic?

Ok. Either way, the calculation is the same.  If you squirt out x cu ft of that water she is full of, she risrs. You don't have to empty her completely.

If you pump out enough water so she is only displacing 6000 lbs (for the Aleutka) of water, she is sitting on her lines, not just barely afloat.

This is Archimedes 101...the weight of the boat = the weight of the water displaced. If she takes on water into her interior volume

[Captain Smollett]

Sorry...got interupted.

If she takes on water, she still floats - to a point. But she can hold a certain amount of water and still float. The difference between that and her total internal volume is the required volume of flotation.

Let's try it from a different angle.

As you said, one can add cruising gear and she can still float, but lower in the water.  Let's say we put 3000 lbs of gear aboard and she sinks down x inches (about 4.5 inches on the Alberg, but x for the Aleutka). 

Now, what's the difference if that 3000 lbs is "gear" or water taken aboard?  None.  Mass is mass and it changes the density the same.  She won't sink until the density is greater than the density of water.

Conversely, if she is under water, on the bottom, she will begin to rise as soon as her density is below that of water.  You don't have to displace her full "hull weight" worth of water for that to happen.  This is because of the volume of hull, and how much weight that volume can "support" and still have a density less than that of water.

Hppe this clarifies...

[skylark]

I think we are saying the same thing...

However, remember that any water above the surface level of the ocean acts as weight and is not neutral displacement.

[Captain Smollett]

I think we are saying the same thing...

We just might!  Grog to you, sir.

The difference, or clarification, lies in the original poster's question.  He asked if he needs to add 100 cu ft of ADDITIONAL floatation.  I don't think so.  I agree with Letcher's number, that the value is closer to 30 cu ft.

Saying he needs 100 cu ft of floatation may represent some total based on the dry weight of the hull, but he does not need to add that volume.  There are already displacement volumes present in the form interior structure, cushions, etc.

As I look around my boat, I can exceed (or at least get very close to) the 'required' displaced volume by making all main lockers (lazarette, cockpit lockers, chain locker, cabin sole and two main cabin setees) watertight compartments; in theory, I would not have to add any, or hardly any foam. 

Perhaps Letcher was starting with a similar baseline.  He designed the boat, so I'm sure he understood her engineering requirements, and 30 cu ft was the number he calculated.

[w00dy]

Thanks for your earnest brain work gentlemen. I spent all my library time arguing with customer service people today, so I didn't get to sit down and read your posts in depth, but I'm saving them to read tonight and will post my questions tomorrow after I've had some time to mull.

[w00dy]

Ok, so let me attempt to sum up my understanding of what's been said, and please correct me if you see anything weird:

Density = Mass/Volume. d=m/v

As long as my boat remains watertight, the volume should remain static and the density of the boat (and space contained) will vary with mass.

As water leaks through the hull, however, the volume of the space inside the boat is reduced. As more water comes in, the ratio of mass to volume (density) increases until it becomes greater than that of the medium (seawater) it is in. It sinks :(.

This is where the foam/watertight compartments come in. They ensure that no matter how much the boat's volume is reduced by incoming water, there is always enough volume to keep the density above that of the seawater surrounding it (afloat).

A fairly precise calculation of the boat's mass (weight) would be necessary to ensure that you did indeed have enough trapped air to maintain the necessary volume. That's the sort of calculation that I would trust Letcher to make. Still, I would want to err on the upward side of the foam needed in his estimation. Although his boat and mine were built from the same design, there might be enough differences to warrant an allowance for some extra weight.

Of course, this brings to mind a whole other slew of questions, such as:

Even though "afloat", on what lines would the boat ride if partially submerged? Although it would be better than sinking completely, a lone mast sticking up out of the water doesn't give me a whole lot of comfort. If she didn't at least keep her sheer line above water, how could I ever expect to repair a hole and pump her out again? It would take some pretty advanced calculus to come up with an equation that would effectively model the lbs. per inch submersion of a boat. Is there a standard equation for this, or does it vary by boat?

It would seem that watertight compartments would support more mass, and also allow for storage of supplies. What about condensation and mildew though? Has anyone here had experience with watertight compartments on a small boat?

Also, I've been reading about foam on the boatdesign forum, and it seems like some people there have had problems with the closed cell foam breaking down and becoming waterlogged over time. Anyone have any advice about what kind of foam to use, if not the pour kind? Letcher recommends getting the blue styrofoam sheets used in construction and cutting/stuffing them into the compartments, then filling the gaps with pour foam. I would just as soon pay to have someone shoot it in with a gun, but not if the stuff is liable to break down and soak up water. I'd be right back where I started then, except with some extra ballast ;)

[Captain Smollett]

As long as my boat remains watertight, the volume should remain static and the density of the boat (and space contained) will vary with mass.

The volume of the boat does remain constant whether the boat is watertight or not.  Adding water fills up that volume and adds mass, but the volume of the hull does not change.

As water leaks through the hull, however, the volume of the space inside the boat is reduced. As more water comes in, the ratio of mass to volume (density) increases until it becomes greater than that of the medium (seawater) it is in. It sinks :(.

This is essentially correct.  The volume does not change, but the mass does, until the density is greater than that of the water.  As it takes on water (mass), the sinks lower and lower in the surrounding water.

This is where the foam/watertight compartments come in. They ensure that no matter how much the boat's volume is reduced by incoming water, there is always enough volume to keep the density above that of the seawater surrounding it (afloat).

I think by "volume," you are using the term to mean "volume if AIR."  If so, this statement is correct.  You have to displace enough of the hull's volume with something less dense than the water she is floating in to keep the overall density less than that, too.

But just to be pedantic, again the volume actually does not change.  What is changing is what you are filling that volume with.  A floating boat has that volume filled largely with air (less dense than water).  Your floatation is to fill the volume something less dense than water that also cannot be displace (removed) by inrushing water.

The ordinary air that is filling the hull's volume is pushed out as water comes in; in this case, you replace the air (less dense) with water (of sufficient density) to the ultimately sink the boat.  The foam is not displaced by  the inrush of water, so you keep some of the volume of the hull less dense that the surrounding water she is floating in.  If there is ENOUGH of that volume that cannot be occupied by the water coming in, she remains less dense and thus floats.

A fairly precise calculation of the boat's mass (weight) would be necessary to ensure that you did indeed have enough trapped air to maintain the necessary volume. That's the sort of calculation that I would trust Letcher to make. Still, I would want to err on the upward side of the foam needed in his estimation. Although his boat and mine were built from the same design, there might be enough differences to warrant an allowance for some extra weight.

Maybe either way.  Being an engineer, Letcher undoubtedly allowed for a certain margin of safety and his boat WAS a "long distance" cruising boat.  I don't know this for sure, but I kinda doubt he cut the corner too closely on his floatation calculation.

Even though "afloat", on what lines would the boat ride if partially submerged? Although it would be better than sinking completely, a lone mast sticking up out of the water doesn't give me a whole lot of comfort. If she didn't at least keep her sheer line above water, how could I ever expect to repair a hole and pump her out again? It would take some pretty advanced calculus to come up with an equation that would effectively model the lbs. per inch submersion of a boat. Is there a standard equation for this, or does it vary by boat?

Each boat's lbs per inch function is different; it is completely dependent on the hull shape - the entire hull shape, both below and above the normal waterline.  And though it would be hard to compute by hand for a given hull, the designers calculate this as a matter of course; it is a 'routine' calculation by modern design procedures.

You can download and play with Free!Ship and get an idea about how this works.  The program comes with some sample hull models, and you can compute waterlines/drafts as a function of 'displacement.'  You could also play around with putting some weight here and there to change fore-aft trim (for example) and recompute the submersion function to see how it changes.

The point is, though, for any professionally designed boat, this function should be known.  It's out there somewhere for the Aleutka, so I'm sure you could find.  Might take some digging though.

I feel nearly 100% confident, though, that Letcher had that in hand and it was part of what he used to determine the 30 cu ft floatation to keep her up in a flooding situation.  I'd be VERY surprised if he did not.

It would seem that watertight compartments would support more mass, and also allow for storage of supplies. What about condensation and mildew though? Has anyone here had experience with watertight compartments on a small boat?

The maximum volume of the 'empty' hull can support a maximum mass.  As you start to fill up the hull with furniture, gear, foam, or water, it all depends on how you distribute that mass into the volume.  If you fill your 'watertight compartments' with pigs of lead, they are not doing you any good for staying afloat.

So, filling them with "supplies" may give a benefit or may not.  Canned food sinks, so is contributing negatively to your 'floatation.'  Keeping the water out of those compartments is still better than letting them fill with water (the spaces between the cans, etc).

But, we are thinking of the watertight compartments as displacing flooding water.

As an alternative, the watertight compartments serve as potential to contain a leak moreso than keep water out of a flooded hull.  For example, if the chain locker is water tight and we sustain damage at the bow, the chain locker can flood, but the rest of the hull is the floatation (rather than the other way around).  As I look around my boat, I that's how I 'envision' my watertight compartments...dual role.  Keep water OUT (hull flooding) or keep water IN (compartment flooding).  See Dave Martin story below.

One other point I thought about in this discussion.

All of this 'required floatation' assumes that no damage control is being done.  It would be nice to have a boat that even if flooded to the decks still remains afloat (and that would be my ultimate goal), but...

We fother or otherwise plug holes to stem the ingress and

We pump to get rid of what came in already.

Keeping her on top is #1 ultimate safety consideration, so I see "Belt n Suspenders" here in more ways than one.  Adequate (even if only "close" to "required") floatation, watertight compartments, reasonable pumping capacity and useful repair materials on board all contribute to that goal. To that end, I think we (1) fight the urge to make only one component of that system that "main" component and (2) realize that there is always a point where we just cannot save the boat - a big enough hole, for example, in the right place just is not going to be savable.

For example, we are limited in the size of the pumps we can carry and operate, and we are limited in the repairs we can conduct at sea.  The key, in my mind, is to design a damage control system that allows enough floatation, repair and pumping to save the boat in {insert some high percentage} of cases.

Neal Peterson, for example, sustained severe damage from a collision in the Atlantic near the bow; his first job was to stem the leak as much as possible (I think he stuffed in lifejackets and maybe fothered with a sail from the outside), and even then he had to pump 20 minutes out of every hour.  But, he made it in.  A TON of work on his part, but his overall "system" was up to the task.

Also, Dave Martin, while sailing his steel hulled boat (with his wife and three children aboard) from Bermuda to Iceland noticed water coming in.  When tasted, he found it was salt water and he began looking for the leak.  He found a crack in the steel low  in the hull, which he temporarily repaired by constructing a small box and using waterproof epoxy to glue it around the crack.  When ashore, he welded the crack for a more permanent repair.

This is such a fascinating (and important) thing to think about and plan for.  I'd rather put design effort and money into my damage control "system" than into so many other "systems."  Keeping the hull floating is more important than keeping the rig up.  Too many mag articles talk about spares for the engine and rigging components; though they do give a nod to damage control (carry some wooden plugs), but overall "system" design is almost NEVER delved into deeply.

Thanks for bringing it up.  All this reminds me of MORE projects to make this boat what I want her to be.  Will she EVER be "done?"

[Frank]

Before foam flotation...I think it was Blondie Hasler when asked what he would do if his boat was sinking during the race that reponded "I am resolved to drownd like a gentleman"   ;D

[Grime]

James,
From what I have been reading your boat it was designed with flotation, one in the bow and one in the stern.  Wouldn't the designed have taken in to account that this would keep the boat afloat so no additional foam would be needed?

[w00dy]

True, Grime, if the foam had been installed in the first place. Unfortunately, it was not (in my boat), and I got a little extra stowage instead. Still, it shouldn't be too hard to add on afterwards. I just want to make sure I do it right, so I don't have to drown like a gentleman ;p

Capt. S, I think we were saying the same thing, though using different words. Luckily, you were able to understand where I was going with it. Isn't it funny how easy it is to mis-communicate?

At any rate, I will put some more thought into damage control, roleplaying different situations, and coming up with some strategies for separate compartments. My boat is already divided up into 16" stations and is over-compartmentalized IMHO. I should at least take advantage of it, eh?
The only problem with that is that there are limber holes and tons of "air holes drilled in every single bulkhead. It would be a pain to seal off all the compartments amidships. Still, sealing off the chainlocker and perhaps the aft lazarettes would go a long way toward making the boat safer.

Thanks for your help and please let me know if you have any advice about foam installation. GROG!!

[w00dy]

Had another thought:
Making watertight compartments at either end of the boat seems to make sense, but if you were going to section off each part of the boat, wouldn't you need a separate bilge pump system for each compartment? How would you handle that problem?

[CharlieJ]

Blue foam would be my choice. It's what I used in floatations compartments in the boats I built.

Also, looking at Lecher's bio, I would tend to trust his calcs

To whit-


John S. Letcher, Jr., RG Inventor/Senior Software Engineer
Ph.D., aeronautics and applied mathematics, California Institute of Technology, 1966
M.S., naval architecture, University of Michigan, 1984

Dr. Letcher has pioneered many applications of computers in the design of boats and ships, including hull surface definition, fairing and lofting; hydrostatic and hydrodynamic analysis; sailing yacht performance prediction; aeroelasticity of sails; optimal routing; optimal design; and simulation-based design.

He was the Senior Scientist for the design team of Stars & Stripes, the America?s Cup winner in 1987, and a member of design teams for the 1988 Cup defense and for America?s Cup contenders in 1992 and 2002. He also has made fundamental contributions to aeronautical engineering, in particular to computational methods for prediction of induced drag.

[Captain Smollett]

Had another thought:
Making watertight compartments at either end of the boat seems to make sense, but if you were going to section off each part of the boat, wouldn't you need a separate bilge pump system for each compartment? How would you handle that problem?

My thoughts are: a 'portable' pump that can be used in any compartment, leave it till you get to port, a bucket/sponge, etc.

If your chain locker or laz are big enough that if they flooded it would seriously and adversely effect trim/handling (and thus ability to MAKE port), perhaps they could have a dedicated pump?  I don't know; I've thought about that, too, but so far, like the portable pump idea the best I think.

My boat right now has two electric bilge pumps (smaller one low, bigger one up higher) with float switches and I also have one "canister" style Thirsty Mate.  I have purchased a Whale Gusher that I have not installed yet, but it will go to the bilge.

I guess in the case of flooded chain locker, the Thirsty Mate will be it?  Haven't worked it all out yet.