Positive flotation in 'bigger' boats . . .

I have been thinking about the idea of positive flotation for my new (to me) boat and wanted to see if anyone out here in iBoats land has dealt with this in the past. (nothing really comes up in the search pertaining to ‘bigger’ boats . . . )

My previous boat (Formula 242) supposedly had “Positive Flotation” per the product literature, but my ‘new’ boat probably does not, and the corresponding literature is a bit vague. My ‘new’ boat – Formula 330SS - has “foam filled chambers” along the length of the hull and foam up under the cabin floor, etc., but I am thinking that it is not enough to actually keep the boat afloat in the case of an ”unplanned event”. Bigger boats are generally ‘sinkers’, and I believe that my boat would sink as there is probably not enough flotation to keep it afloat.

I did some measurements and calculations:
( I used the weight of fresh water, even though I boat in salt water – slightly heavier)

Weight:
Boat weight = 8,900 lbs (per specs)
Fuel weight (max) = 160 gallons, so 160 gallons x 8 lbs/gallon = 1,280 lbs
Total boat + fuel = 10,180 lbs . . . add some people and gear . . . figure 11,000 lbs.

Buoyancy:
I considered the boat’s buoyancy in 3 aspects - (iron(engines), fiberglass, flotation). To be conservative, I figured everything that was not ‘engine’ or ‘flotation’ to be the weight of fiberglass, even though some of the stuff is lighter.

Engines = 2,000 lbs / 500 lbs/cu. Foot = 4 cubic feet
Fiberglass = 8,900 lbs – 2,000 lbs = 6,900 lbs . . . 6,900 lbs / 125 lbs/cu ft. = 55 cubic feet
I did some measuring of the foam compartments throughout the boat and figured them to add up to a total of 56 cubic feet.

So, total displacement if submerged would be:
4 + 55 + 56 = 115 cubic feet, 115 cubic feet x 62.4 lbs/cubic foot = 7,176 lbs

So, it looks like it comes up a bit short . . . 11,000 lbs – 7,176 lbs = 3,824 lbs

I would need about 3,800 lbs additional buoyancy. . . which would be:
3,824 lbs / 62.4 lbs/cubic foot = 61.3 cubic feet.

That seems like a lot, and not really practical to add to the boat. So, I am wondering if I goofed in the measuring or calculations somewhere. (I emailed Formula Tech Support to see if they have any more info regarding the flotation designed into the boat)

If it were a minor amount of flotation needed, I might consider adding some as there is plenty of space under the cockpit floor and in other areas. There used to be a company/product called “Yacht Savers” than sold inflatable bags to keep a large boat afloat during a crisis . . . but they went out of business. (Maybe those with big boats would rather have them sink. . . :noidea: )

Anyways, thoughts, ideas ??? maybe a life raft instead ??? :noidea: TIA.

Re: Positive flotation in 'bigger' boats . . .

Well..... it's not even required in bigger boats as per the USCG Regs.....

You might be over thinking the whole concept. If flotation is provided it will keep the boat at swamped level so it will not vanish into the depths. If your boat has flotation it will "probably" sit just under the surface when swamped.

Boats float higher in salt water too....... I guess I can compare it to holding your significant other in your arms when you are both in the water......easy when in water but not easy when on land.

I guess if you are really concerned you could add 2 part foam to the hull. 1 gallon of foam will support 60 lbs in the water. with those twin 704's I imagine it will sink ......

Re: Positive flotation in 'bigger' boats . . .

Bob_VT said:

You might be over thinking the whole concept. If flotation is provided it will keep the boat at swamped level so it will not vanish into the depths. If your boat has flotation it will "probably" sit just under the surface when swamped.

Boats float higher in salt water too......

Click to expand...

That is exactly it! The OP needs to find the density of his fiberglass used in his boat. If the density is 30 pounds per cubic foot for example, the actual weight of the fiberglass above can be chopped in half. Fresh water at standard pressure/temperature weighs about 62 pounds.

Also, fuel is less dense than water, it becomes a +, not a -, when you are talking a submerged vessel. (in reality don't consider fuel weight at all, because its close enough to neutral to ignore, and you don't know how much fuel you are going to spill/leak, or how much water the vent is going to allow in.) The density of metal isn't really that big of a factor, you can safely assume it doesn't change (much)

Re: Positive flotation in 'bigger' boats . . .

In terms of safety, it would probably be a lot easier and more practical to invest in an inflatable lifeboat than to try cramming in enough foam to keep the boat afloat.

In terms of saving the boat itself... Picturing a situation where the foam would come into play and the amount of water damage the boat would suffer, I'd question whether or not I'd want the boat back anyway. There's a strong argument to be made for just making sure you have really good insurance, which you'll want anyway to cover the cost of raising and retrieving the boat if you sink in an area where you're on the hook for that. In the lakes around here, just leaving it on the bottom isn't one of the options you have available to you.

Re: Positive flotation in 'bigger' boats . . .

Roger that on the lifeboat idea vs adding foam. If my boat ever swamped that bad I don't WANT it back!

Re: Positive flotation in 'bigger' boats . . .

your boat weighs less submerged as water is heavier than air

Re: Positive flotation in 'bigger' boats . . .

Even though it's not required in boats over 20', some manufacturers do build their 20'+ boats with positive floatation:


Key West Boats Inc. - Your Key to Performance and Quality

Re: Positive flotation in 'bigger' boats . . .

smokingcrater said:

That is exactly it! The OP needs to find the density of his fiberglass used in his boat. If the density is 30 pounds per cubic foot for example, the actual weight of the fiberglass above can be chopped in half. Fresh water at standard pressure/temperature weighs about 62 pounds.

Also, fuel is less dense than water, it becomes a +, not a -, when you are talking a submerged vessel. (in reality don't consider fuel weight at all, because its close enough to neutral to ignore, and you don't know how much fuel you are going to spill/leak, or how much water the vent is going to allow in.) The density of metal isn't really that big of a factor, you can safely assume it doesn't change (much)

Click to expand...

Hey thanks folks for the replies so far . . keep them coming

I used 125 lbs per cubic foot as the density of fiberglass . . . (i.e. heavier than water). If it were 30 lbs/cu. ft, I'd be all set as the boat would float all by itself.

I included the volume of the fuel tank in my 'bouyancy' number, so I also included the weight of the fuel in the weight side of the equation. I guess if you are going to have a sinking incident, it would be good to do it on a near empty tank, as it would provide about 1,000 lbs of extra bouyancy.

What has me scratching my head is that dispite the foam flotation that is in the boat, it seems like no where near enough and therefore it would not lie slightly beneath the surface, but rather go straight to the bottom. I think the foam may be more for structural reasons than bouyancy. :noidea:

However, if for some reason, the boat would float just at the surface of the water (or slightly below), then it may be beneficial to add a little bit of extra foam, just to get the boat to sit a bit higher in the water during a crisis.

I'm thinking of safety, just being able to have the boat stay up until help arrived, etc.. . . As far as preserving the boat, if the boat were to become majorly swamped/sunk, I'd be shopping not fixing.

Re: Positive flotation in 'bigger' boats . . .

as for inflatable bags, that's a common tool for salvaging sunk or swamped larger vessels--stuff them in, inflate and displace the water, get it gunwale to the water and pump. Doesn't seem practical to carry a flat bag and compressed air for an emergency; use a compressed air life boat instead.

Re: Positive flotation in 'bigger' boats . . .

coolbri70 said:

your boat weighs less submerged as water is heavier than air

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Yes, by my calculations, my boat would weigh about 3,824 lbs if it were under water.

Re: Positive flotation in 'bigger' boats . . .

A 6x10x1' area isn't much more than the size of one of those king size luxury bed mattresses.

You can get 80 cu. ft. of 2 lb. density urethane foam for about $600.

Keeping a 33' boat afloat during a crisis... priceless!

Re: Positive flotation in 'bigger' boats . . .

Using your numbers and you are responsible for the accuracy of your assumptions/calculations, (11,000 lbs.) to support without sinking, making no mention of floating above the surface, just not sinking, I see it this way:

But first some laws of physics:

If you want to measure the weight of an object in water, you put it in a tank of water and measure the weight of the water that was removed from the tank to support the object from an initial reference point, fully submerged, but immobile, i.e. it is completely beneath the surface but is moving neither up nor down.

If the object continues to sink, it's volume displaced did not represent it's actual weight. It weighs more than it displaced.

If it floated with part of it above the surface, it's volume displaced exceeded it's actual weight. It weighs less than it displaced.

So sir, you gave me 11k#. My reference data gives water (pure) at 8.34 #/gallon at STP. (Standard Temperature and Pressure...US National Bureau of Standards). To support 11k#, in equilibrium, fully submerged but immobile, would require 11k/8.34 = 1319 gallons of water displaced.

My reference data continues with the volume of pure water at STP to have a volume of .1337 ft (exp3)/gallon which amounts to 176 ft (exp 3) of flotation.

So, my calculations say that with 176 ft (exp3) of flotation, the boat would fully submerge but not sink.

Sounds to me like you are chasing a ghost. Increasing the buoyancy by 50% which statistically would amount to half the weight of the boat above the surface of the water would be a volume of 176 x 1.5 = 264 ft (exp3) of foam which is 6.4' x 6.4' x 6.4'.

That's the way I see it.

Mark

Re: Positive flotation in 'bigger' boats . . .

Texasmark said:

Using your numbers and you are responsible for the accuracy of your assumptions/calculations, (11,000 lbs.) to support without sinking, making no mention of floating above the surface, just not sinking, I see it this way:

But first some laws of physics:

If you want to measure the weight of an object in water, you put it in a tank of water and measure the weight of the water that was removed from the tank to support the object from an initial reference point, fully submerged, but immobile, i.e. it is completely beneath the surface but is moving neither up nor down.

If the object continues to sink, it's volume displaced did not represent it's actual weight. It weighs more than it displaced.

If it floated with part of it above the surface, it's volume displaced exceeded it's actual weight. It weighs less than it displaced.

So sir, you gave me 11k#. My reference data gives water (pure) at 8.34 #/gallon at STP. (Standard Temperature and Pressure...US National Bureau of Standards). To support 11k#, in equilibrium, fully submerged but immobile, would require 11k/8.34 = 1319 gallons of water displaced.

My reference data continues with the volume of pure water at STP to have a volume of .1337 ft (exp3)/gallon which amounts to 176 ft (exp 3) of flotation.

So, my calculations say that with 176 ft (exp3) of flotation, the boat would fully submerge but not sink.

Sounds to me like you are chasing a ghost. Increasing the buoyancy by 50% which statistically would amount to half the weight of the boat above the surface of the water would be a volume of 176 x 1.5 = 264 ft (exp3) of foam which is 6.4' x 6.4' x 6.4'.

That's the way I see it.

Mark

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yes, that is what I figured is that the boat would need to displace 176 cubic feet in order to float. The problem lies in that my estimations based on weight and density, plus measuring the actual size of the "flotation" chambers yields only 115 cubic feet of displacement versus the 176 that is needed. So, it is no where near being able to float if fully submerged and would be about 60 cubic feet short ( or 3,800 lbs). So, it would be a sinker not a floater.

I may have underestimated the displacement a bit, by using the density of fiberglass for everything, other than the engines and the flotation, but I'm thinking it would still not make up for the shortfall of 60 cubic feet.

Re: Positive flotation in 'bigger' boats . . .

I got word back from Formula Boat's Tech Support group this afternoon regarding the flotation in my boat. Apparently the boat DOES have positive flotation, but they are still pulling together the details of how much foam is used to achieve it, etc. So, maybe I'll get some more details in a day or two.

I'm guessing that there is probably a bit more foam in the boat than what I was able to estimate, AND the stuff that is not fiberglass (i.e., wood, cushions, panels, etc.) is a lot lighter (less dense) than I had estimated.

Re: Positive flotation in 'bigger' boats . . .

tpenfield said:

I got word back from Formula Boat's Tech Support group this afternoon regarding the flotation in my boat. Apparently the boat DOES have positive flotation, but they are still pulling together the details of how much foam is used to achieve it, etc. So, maybe I'll get some more details in a day or two.

I'm guessing that there is probably a bit more foam in the boat than what I was able to estimate, AND the stuff that is not fiberglass (i.e., wood, cushions, panels, etc.) is a lot lighter (less dense) than I had estimated.

Click to expand...

Ayuh,.... The Dry stringers, 'n any other coring materials need to be counted too,...

Get yerself a self-inflatin' life raft, 'n deploy it in the cabin,....

That'll easily hold her nose up, 'n probably outa the water....

Re: Positive flotation in 'bigger' boats . . .

It wouldn't hurt a thing to put some more in, you can never have too much floatation foam. It could mean the difference between floundering on top of the water and staying safe and dry should there be an emergency.

Re: Positive flotation in 'bigger' boats . . .

Yup, my boat probably has 'basic' flotation. So, maybe some foam in the mid-ship and stern areas (where ever it will fit) would make it float a bit better and more level.

I did find some USCG calculations and testing method for "basic flotation", and it looks like they use around 90 lbs/cu ft for the density of fiberglass. I was using 125 lbs/cu ft. Plus there is other stuff that is a lot lighter. . . I cranked thru some of their numbers and got a figure of needing about 62 cubic feet of flotation, which is pretty close to the 56 cubic feet that I had measured & estimated.

I think that I can add about 20 cubic feet of foam fairly easily in the dead space that is throughout the boat. This will give it about 1,200 lbs of extra buoyancy.

Re: Positive flotation in 'bigger' boats . . .

tpenfield said:

I included the volume of the fuel tank in my 'bouyancy' number, so I also included the weight of the fuel in the weight side of the equation. I guess if you are going to have a sinking incident, it would be good to do it on a near empty tank, as it would provide about 1,000 lbs of extra bouyancy.

Click to expand...

The tank can't be considered either way. The tank isn't sealed, if it sinks the vent is most likely going to let out most of the trapped air pocket.

Re: Positive flotation in 'bigger' boats . . .

I guess you could do it. But the main reason that larger boats don't have level floatation at all and most don't have basic flotation is simply that if you put enough flotation in to keep a couple of big blocks and of course everything else afloat there won't be enough room left for using the boat, or having any storage on board.

Re: Positive flotation in 'bigger' boats . . .

Georgesalmon said:

I guess you could do it. But the main reason that larger boats don't have level floatation at all and most don't have basic flotation is simply that if you put enough flotation in to keep a couple of big blocks and of course everything else afloat there won't be enough room left for using the boat, or having any storage on board.

Click to expand...

Yes, it definitely seems to be a matter of useable space and storage with larger boats. . .

If it is a matter of the "flotation status" of the boat being close to positive (i.e. if will stay at/above the surface), then it may be feasible to add some additional flotation to improve the flotation capabilities. If the boat was far from being able to float itself, then there would not be enough space to add flotation foam without significantly taking away from the regular use and storage space.

Since Formula has indicated that the boat has positive flotation, but it does not appear to be overly 'positive', it seems that it may be useful to add a bit more just to improve things.