A lift raft for an offshore boat with no foam, or a foamed in aluminum hull with no lift raft?

[Bullshipper]

Before starting to build a 24' aluminum catamaran, I am trying to decide over 2 options:

Foam vs no foam in under deck Bulkheads plus a Lift Raft.

Foam would seem at first glance to be an ideal solution to avoid carrying a lift raft., but

Foamed inProblems
Expanding mixed foam will put extra outward pressure on confined decks and gunwales.
Non computerized mixing and temp. foam guns present mixing problems that make foam questionable interms of its actual ability to fend off water intrusion.
Foam will break away from side surfaces and start to crumble, jamming bilge pumps
Cheap Type 1 foams will absorb gas even when properly applied
It is difficult to create drain channels down the the bottom of the centerline channel that water can drain away from foam to prevent water intrusion. And if the centerline drain passage gets plugged with foam or debri it never gets to the bilge pump.

Airtight Bulhead Problems
Gas fumes accumulate underdecks unless bulkheads are ventilated, making sparks from bilge pumps or other wiring very dangerous
Unventilated spaces still accumulate condensation and water from above when welds or seals are compromised, making the term airtigt short termed.
Wet surfaces rust or create mold and foul smells
Holing the non foamed boat will allow it to sink if the bilge pump can't keep up on compartments that allow water and air travel from one to another.

Option 1- Hanging foam blocks
Fiberglassing blocks of foam attached to the bottom of the deck will allow drain spaces around the blocks, and prevent water or gas intrusion. But on an alloy boat with welded decks, the foam can burn unless applied after welding. Welding chambers to recieve the foam is not practical as there are stringers to support the deck. Filling the volumes with enough foam to garantee flotation is very labor intensive.

So the only way I can see to do this is to attach the foam blocks to the stringers with straps, then glue, not weld, the aluminum decks onto the stringers and just seal weld the outer edges to the sides of the hull. Exit air ventilation would be achieved by 2 - 3" diameter risers that also carry wire up through the gun whales, and air would be forced down under decks by scoops on top to the ttop legs going under decks. But water will fill around the foam on a capzided hull.

Option 2 - Lift raft in Place of Foam Flotation
Foregoing foam makes a better metel structure, saves on construction time, labor and materials that are a greater cost than the raft.

A raft presents some storage and maintenance problems but add another option to offshore safety over just a holed or a capsized boat. But the cost is the less than the foam, so the question is,

What would you rather have?

A lift raft and large bilgle pumps in a boat with no foam?

Or a foamed in hull with or without a raft?

Thanks for your input.

[salty_dog]

No foam and a good raft. Sinking is one thing. Burning is another.

[A Few Dollars]

I would rather have the foam. It will make a quiter more solid structure. Could you foam just the hull side walls. Would this be enough flotation? It would prevent the foam from sitting in water.

[Loose Crew]

salty_dog - 3/18/2006 1:09 PM

No foam and a good raft. Sinking is one thing. Burning is another.

Ditto. Fire is a very real issue and that raft will also contain the essentials for survival at sea which you might lose if your foam boat we to capsize but stay afloat. Don't foget an EPIRB as well.

[vikingham]

I hate to answer a question with a question, but what sort of use do you foresee for the vessel? Are you planning mostly day use with family and friends, or will it be a cruiser?

If you'll be using it more socially, with many people aboard who aren't experienced sailors, I'd opt for the flotation. If you hole a pontoon, you'd at least all have the hull to hang onto, with no worries about deploying the raft and getting everyone aboard. And help would probably be relaitvely close.

If you're going cruising, it will probably be mostly you and one or two other people aboard (being that it's a 24), and presumably you and they will be more serious about water activities and better prepared than your non-boating friends. So deployment and boarding of a raft would be less of a problem, especially if you do a drill every now and then. It would still be nice to have the flotation, but most boats that size don't, so....

In either case, an EPIRB is probably a good idea....and if it were me, I'd get a bunch of inflatable PFDs, wear one at all times, and "strongly encourage" my guests to do the same (this is what I do anyway on my 19-footer with full flotation!).

[Bullshipper]

This hull will be used offshore 50 miles, warm water and climate, Mexico.

[auguste]

Foam will provide about 50 lbs of floatation per cubic foot . . . once you subtract the weight of the foam and water infiltration.

I am guessing your boat will weight in at 3,500 lbs meaning you need 70 cubic feet of foam to be neutrally boyant and 80 to 90 cubic feet of foam to be comfortable . . . there are a few other factors such as major water penetration into the foam and you have lost your buoyancy . . . and water penetration also makes your boat heavy to operate during normal operation . . . I know the arguments about closed cell foam but with the chaffing I expect in an aluminum hull, I expect some deterioration of the foam

And 90 cubic feet is a lot of room to fill in a boat . . . you lose a lot of below deck space etc.

I have a life raft because my boat would require over 220 cubic feet and I don't have the space

.

[Bullshipper]

A Few Dollars - 3/18/2006 2:10 PM

I would rather have the foam. It will make a quiter more solid structure. Could you foam just the hull side walls. Would this be enough flotation? It would prevent the foam from sitting in water.

The hull requires a lot of foam to stay level without the motors going under. Because the sponsons are narrow this means 90% full of foam to keep 7800 lbs of flotation against a 6500 lb loaded hull.

[Bullshipper]

Loose Crew - 3/18/2006 2:13 PM

Ditto. Fire is a very real issue and that raft will also contain the essentials for survival at sea which you might lose if your foam boat we to capsize but stay afloat. Don't foget an EPIRB as well.

This is an alloy aluminum hull.

[auguste]

6,500 lbs of floatation . . . 130 cubic feet minimum

.

[Bullshipper]

auguste - 3/18/2006 2:42 PM

Foam will provide about 50 lbs of floatation per cubic foot . . . once you subtract the weight of the foam and water infiltration.

I am guessing your boat will weight in at 3,500 lbs meaning you need 70 cubic feet of foam to be neutrally boyant and 80 to 90 cubic feet of foam to be comfortable . . . there are a few other factors such as major water penetration into the foam and you have lost your buoyancy . . . and water penetration also makes your boat heavy to operate during normal operation . . . I know the arguments about closed cell foam but with the chaffing I expect in an aluminum hull, I expect some deterioration of the foam

And 90 cubic feet is a lot of room to fill in a boat . . . you lose a lot of below deck space etc.

I have a life raft because my boat would require over 220 cubic feet and I don't have the space

Smart answer Gus. You are very close on the hull and engine weight, but with fuel, people, and other accessories I will be closer to 6500 lbs fully loaded and will have to move people farther forward to counter balance the weight of the motors when dead in the water.

[auguste]

Bullshipper . . . is your plan that the head of the motors stay above the water level IF your boat fills with water?

Assuming your hull does not roll over.

.

[solarfry]

Bullshipper - 3/18/2006 12:31 PM

Before starting to build a 24' aluminum catamaran, I am trying to decide over 2 options:

Foam vs no foam in under deck Bulkheads plus a Lift Raft.

Foam would seem at first glance to be an ideal solution to avoid carrying a lift raft., but

Foamed inProblems
Expanding mixed foam will put extra outward pressure on confined decks and gunwales.
Non computerized mixing and temp. foam guns present mixing problems that make foam questionable interms of its actual ability to fend off water intrusion.
Foam will break away from side surfaces and start to crumble, jamming bilge pumps
Cheap Type 1 foams will absorb gas even when properly applied
It is difficult to create drain channels down the the bottom of the centerline channel that water can drain away from foam to prevent water intrusion. And if the centerline drain passage gets plugged with foam or debri it never gets to the bilge pump.

Airtight Bulhead Problems
Gas fumes accumulate underdecks unless bulkheads are ventilated, making sparks from bilge pumps or other wiring very dangerous
Unventilated spaces still accumulate condensation and water from above when welds or seals are compromised, making the term airtigt short termed.
Wet surfaces rust or create mold and foul smells
Holing the non foamed boat will allow it to sink if the bilge pump can't keep up on compartments that allow water and air travel from one to another.

Option 1- Hanging foam blocks
Fiberglassing blocks of foam attached to the bottom of the deck will allow drain spaces around the blocks, and prevent water or gas intrusion. But on an alloy boat with welded decks, the foam can burn unless applied after welding. Welding chambers to recieve the foam is not practical as there are stringers to support the deck. Filling the volumes with enough foam to garantee flotation is very labor intensive.

So the only way I can see to do this is to attach the foam blocks to the stringers with straps, then glue, not weld, the aluminum decks onto the stringers and just seal weld the outer edges to the sides of the hull. Exit air ventilation would be achieved by 2 - 3" diameter risers that also carry wire up through the gun whales, and air would be forced down under decks by scoops on top to the ttop legs going under decks. But water will fill around the foam on a capzided hull.

Option 2 - Lift raft in Place of Foam Flotation
Foregoing foam makes a better metel structure, saves on construction time, labor and materials that are a greater cost than the raft.

A raft presents some storage and maintenance problems but add another option to offshore safety over just a holed or a capsized boat. But the cost is the less than the foam, so the question is,

What would you rather have?

A lift raft and large bilgle pumps in a boat with no foam?

Or a foamed in hull with or without a raft?

Thanks for your input.

place 1 1/2" sched 40 pvc pipe with 1/2" holes every 12", wrapped in cloth, in bilges from bow all the way to stern drain hole. Place 4" hatch holes every 36" on deck. Pour foam thru hatch holes. Allow foam to exit hatch holes and dry up. Wait 2 days and then cut foam level with hatch holes. place hatch holes plugs in holes even with deck. Seal with epoxy cloth and epoxy resin. Paint with good paint. That should solve your problem. Water that penetrates foam will drain out pvc pipe. Fuel lvapors will exit thru pvc.

[ubettcha13]

? If you need to repair a breach can you weld where the foam is or do you have to remove it? I like foam for piece of mind Solar fry said what I was going to say though I would just use screw in covers in the Aluminum double the deck under the access holes and thread them with pipe dope also lay down double chases for all wiring either welded al tubing or epoxied pvc
I was just informed that my hull at 23' and fully foamed allows me to fish tuna in the gen cat without a raft if I so desired one less thing to purchase If I was going to get started again which at this time I don't plan on it

[bob v]

Bull, I would rather be in a liferaft in rough seas than trying to hang onto a foam filled boat which has sunk to the waterline. Think about it. How would you stay attached to a boat that is floating at the waterline with 5 foot seas breaking over you? Just a thought. Bob

[solarfry]

ubettcha13 - 3/18/2006 5:39 PM

? If you need to repair a breach can you weld where the foam is or do you have to remove it? I like foam for piece of mind Solar fry said what I was going to say though I would just use screw in covers in the Aluminum double the deck under the access holes and thread them with pipe dope also lay down double chases for all wiring either welded al tubing or epoxied pvc
I was just informed that my hull at 23' and fully foamed allows me to fish tuna in the gen cat without a raft if I so desired one less thing to purchase If I was going to get started again which at this time I don't plan on it

You cut a hole, remove some foam, cover remaining foam with aluminum foil, then weld an aluminum plate to cover hole. I really don't know how Heliarc would affect foam. I have read that if it is covered it will not burn. I don't think you would easily get a hull breach if it is foamed in. The foam pressure makes that aluminum tougher than steel. Remember that Caterpillar running over the small Boston Whaler?

If you use screw in covers you will let water in! You can use screw-in covers but you would have to seal it all around with parrafin or wax. I have new screw in covers with new o rings and they frikkin leak! They leak more than I like..

There are pockets you can stick in access hatches to allow storage. I've seen a manufacturer use 8" pipe with end cover allowing foam to get inside and then removing foam from inside. I guess they could be plumbed to drop water into that stern to bow pvc pipe for drainage, with some preplanning. then you would have an 8" x 20" hole for storage of fire extinguishers or what have you. YOu could go to a 10" hole and plate.

You can plumb sched 40 2" pvc pipe to required locations in bow and sides and just fish wire through.

For stern to console I would use 2 - 3"+ pipes. One on each side of hull? It seems we never have space to run wires to console with all electronic stuff we stick on console..

You can place boat on its side and then inject foam so it is only on hull sides and not bottom. This is another option.. In this case, Access hatches would not be on deck but on sides of boat.

[Chaps]

Bull, my 28 MaxCat had subdecks (horizontal bulkheads?) welded in to enclose the lower spaces in the hulls. These areas were foamed in thru 6" dia holes in the subdecks then alum plates were fastened over the holes after the foam stubs were cut off. There were a couple of areas aft that allowed access deep into the hulls for thru fittings but even in there Andy sprayed all exposed aluminum with about a 2" thick foam coating so I had to chop it out wherever I did a thru-hull. It for sure didn't have enough foam to keep her afloat in a worst case scenario but the boat was so solid feeling, quiet and condensation free that I couldn't imagine building an alloy cat without it. Cats make a lot of hull noise while underway and the foam, particularly in the tunnel, cuts that way down. The space I had between the decks and the subdecks was available for storage, plumbing, inverter, water tanks, etc. Bottom line, foam enough of the boat to gain the benefits of it but unless you jam every cubic inch of space with foam you're still going to want the raft IMO.

[skypoke]

You guys need to rethink your calculations a bit....based on these figures of approx 50 lbs of flotation per cubic ft. foam a life vest for a 200 lb man would need to be 4 cubic feet. Now we are not trying to provide a life vest for a man to stand on and be completely out of the water because it would be #1 unstable, #2 impractical so we make it about .75 cubic feet which allows the head and upper torso to be out of the water.

This is exactly the situation you want for your boat. You absolutely want it to partially sink, in a level fashion, if the dreaded day comes. This will prevent it from turning turtle and also answer the eternal question of whether or not you want foam in the bilge. No, you don't for lots of good reasons, this being the main one. It will provide plenty of flotation alright which will serve to roll the boat over. Another good aspect of calculating partial submersion is your hull material will then have a weight of a fraction of its dry weight....gasoline provides positive bouyancy, even in metal tanks, even the portion of the outboards which will be submerged needs to be factored in. You'll probably find your 6500 lb boat looking more like 3500 lbs when you do this.

Foam the outboard hull sides extending all the way up to under the gunwales. This will provide stability where you need it, and an increasing flotation level as hull tries to submerge. Bull I'm sure Roger shows a watertight bulkhead about 2' or so in front of engines. Mine worked out to be about 12 cubic feet each side and since this is an area of high loading (batteries, engines) I filled it to the rim with removable foam. Most of the foam used in my boat is in the form of removable panels...2" builders foam, foil backed, tested for gas resistance. Under the gunwales I was able to install foam 8" thick and about 48" tall, with cutouts where the rod holders are. These panels extend down to the lower spray chine, keeping them well up out of the bilge. Each side has about 20 cubic feet of foam in these otherwise useless areas. Inside the cabin I cut these same panels to fit between ring frames, sitting on spray chine, and installed against the stringers. Since these are exposed I covered them with a couple different surfacing, one is plain ol fabric backed vinyl in areas like headliner....ceilings (sidewalls for the lubbers) and wrapped with 3/4" closed cell brightly covered neoprene material. This gives me...flotation where I need it....sound deadening....removability....appearance....cushion ed surfaces instead of those brutally hard alloy edges,and a cabin insulated to around R30! In addition, I could care less whether my foam is open or closed cell, it's all installed in a vertical fashion, above wet areas, and even if it does get wet from condensation or such it is free to drain and dry out.

Take a look at the USCG site dealing with calculations for level flotation, factor in the proper submerged weights of the different materials, realistically figure just how high she needs to float and you'll be amazed at how easy it is to achieve level flotation with a variety of materials.

Chuck

[jobowker]

Being realistic, most hulls past a certain size cannot realistically have enough foam to make a difference. There is flotation, and then there is level flotation. Sufficient flotation means you can stick it anywhere, as long as the buoyancy exceeds the weight of the boat. The problem is that even though it won't sink to the inky depths, it will most likely float upside down with the bow sticking up! It's awfully hard to hold onto the bow of a deep v boat. Level floation means that even if you swap it, the boat will stay level. The problem here is that not only do you need to find room for a whole lot of foam, but most of it needs to be towards the stern and near the gunwhales. This often goes beyond the physical limits of the boat.
I would rather have a life raft with a properly supplied ditch bag. There was a case of a single person surviving on the water alone for 76 days in a liferaft. No one has ever survived even close to that while clinging onto a swamped boat.

[Bullshipper]

Wow, great answers from everyone.

Lots to think about but its late.

I was kind of leaning to no foam, insurance + a life raft and a transmitter, but I'm too fuzzy to decide now. Will get back to this tomorrow.

Thanks for the input.