floatation formula

I have a 12 foot aluminum row boat that needs positive floatation, The boat with all gear and motor less people weighs around 250# . I would like to know if there is a formula that would allow me to calculate the amount of foam to add . The boat is used sometimes to cross Barnegat Bay, so I'd like to keep afloat in an emergency.

Thanks in advance

Scott

[duluthboats]

One cubic foot of salt water weights about 64 lbs. So add enough cubic feet of foam to exceed your total displacement (people included).

Gary

There IS a formulae - and If i could find my copy of it - I'd tell you what it is.

It's quite a complex one.

You see, foam bouyancy displaces more or less weight of water depending on whether your in fresh or salt water, then - theres the things "in your boat" that have some bouyancy themselves for example, your portable fuel tank is a case in point - the fuel being lighter than water, when swamped - the fuel tank supports itself. Likewise yourself and passenger/s - their bodies are ~90% warter so tend to be partially bouyant.

Thus in terms of how much water you have to displace with foam to support the weight of these people and fuel etc the 'formulae' from memory only allows enough foam to displace about 10% or so of the people and fuel, while it WILL have to displace 100% of the weight of the hull and outboard etc.

In addition to having enough foam - is the need for the boat to float level when swamped - not sink transom first leaving only the bow above water for you all to 'sit on' awaiting rescue. (Ouch!)

So - where the foam is placed becomes as critical as having enough foam.

You need the ullage calculations (volume of a complex shape), from the area underfloor as well as thwart seat internal dimensions to determine how much foam you can fit to the hull and whether thats enough to keep it afloat when swamped.

You also need - the all up weights of whats actually IN your boat including fuel and passengers - AND you then need the formulae (which I can't locate...right now).

I'm sure someone here will have it, you may need to get the volume (ullage) figures for you hull from it's designer.

You also need the right TYPE of floatation foam - it must be high density closed cell fire retardant foam. Styrene foam for example is no good - one cup full of spilt fuel underfloor and you no longer have flotation foam - you have a sticky black goo that won't float squat.

Your foam SHOULD be encapsulated in heavy plastic, wrapped with waterproof gaffa tape, to prevent it crumbling with pounding, and floting down to any bilge pump - blocking the screen and burning out the bilge pump.

Hopefully someone will post the formula for you..

In the meantime, I'll TRY and find my copy.

Cheers!

I saw this while doing some research and was wondering if you ever found that paper with the formula for calculation floatation .....if so, could you send me a copy of it? Would save me some leg work .......best regards ..Greg Lombardo

Originally Posted by trouty

There IS a formulae - and If i could find my copy of it - I'd tell you what it is.

It's quite a complex one.

You see, foam bouyancy displaces more or less weight of water depending on whether your in fresh or salt water, then - theres the things "in your boat" that have some bouyancy themselves for example, your portable fuel tank is a case in point - the fuel being lighter than water, when swamped - the fuel tank supports itself. Likewise yourself and passenger/s - their bodies are ~90% warter so tend to be partially bouyant.

Thus in terms of how much water you have to displace with foam to support the weight of these people and fuel etc the 'formulae' from memory only allows enough foam to displace about 10% or so of the people and fuel, while it WILL have to displace 100% of the weight of the hull and outboard etc.

In addition to having enough foam - is the need for the boat to float level when swamped - not sink transom first leaving only the bow above water for you all to 'sit on' awaiting rescue. (Ouch!)

So - where the foam is placed becomes as critical as having enough foam.

You need the ullage calculations (volume of a complex shape), from the area underfloor as well as thwart seat internal dimensions to determine how much foam you can fit to the hull and whether thats enough to keep it afloat when swamped.

You also need - the all up weights of whats actually IN your boat including fuel and passengers - AND you then need the formulae (which I can't locate...right now).

I'm sure someone here will have it, you may need to get the volume (ullage) figures for you hull from it's designer.

You also need the right TYPE of floatation foam - it must be high density closed cell fire retardant foam. Styrene foam for example is no good - one cup full of spilt fuel underfloor and you no longer have flotation foam - you have a sticky black goo that won't float squat.

Your foam SHOULD be encapsulated in heavy plastic, wrapped with waterproof gaffa tape, to prevent it crumbling with pounding, and floting down to any bilge pump - blocking the screen and burning out the bilge pump.

Hopefully someone will post the formula for you..

In the meantime, I'll TRY and find my copy.

Cheers!

[tspeer]

I submit you don't need to calculate the required buoyancy very closely. A swamped boat that has exactly enough buoyancy is fully submerged and quite miserable to deal with when swamped. If you don't have a reasonable amount of freeboard when flooded, you can never get it bailed out.

Instead, get a rough cut at the minimum, then put in as much flotation as you can fit, making sure it's well above your minimum. Between the area under the seats and in the ends, you can put in a surprising amount without interfering with the useable space.

If you want to spend time fine tuning the flotation, I recommend you work on where it's distributed. The heavy weight, like the motor, is in the stern. But it can be hard to fit in enough flotation there. Flotation forward and weight aft means you'll have a spar buoy on your hands if you swamp it.

[dereksireci]

You guys are scaring me.

I suggest to do it right or don't do it at all.
First use ABYC H-5 - Boat Load Capacity
Next ABYC H-8 - Buoyancy in the Event of Swamping

Both guidlines cover about 32 pages and give nice diagrams and examples. I haven't worked with boats under 20' in a while but to summarize in general:

Capacity: Imagine the boat in the water with the scuppers sealed. Then add as much weight as possible, distributed properly, to submerge the hull just to the point where water is about to come over the sheer. This is called the static float plane. The amount of weight you added will determine the capacity. The book has formulas to estimate capacity at static float plane.

Floatation: You have three things you need to float, the boat, the engine and the person capacity (includes gear).

The calculations are tedious but not difficult. Just follow it step by step. At the risk of a copyright violation, from ABYC H-8:

H-8.7.2 Calculations - Basic Flotation
H-8.7.2.1 Perform the following steps to calculate the volume of flotation material required to support the swamped boat (Fb), the submerged propulsion equipment (Fp), the persons capacity (Fc1), and the dead weight (Fc2).
H-8.7.2.1.1 Step 1: Determine the flotation needed to support the swamped boat (Fb).
Formula: Fb = (Wh x K1 + Wd x K2 + .69 We) ÷ B
Where
Wh = dry weight of hull, i.e., everything below swamped water line
Wd = dry weight of deck and superstructure
We = dry weight of factory installed equipment, hardware, and accessories
K1 and K2 = conversion factors for materials used from Table I (See Note 1.)
B = buoyancy of flotation material in pounds per cubic foot
(NOTES DELETED)
EXAMPLE
Calculate the flotation material needed to support the swamped boat (Fb).
Assume
Wh = weight of hull (fiberglass) = 600 lbs.
Wd = weight of deck and superstructure (plywood) = 120 lbs.
We = weight of equipment, hardware and accessories = 50 lbs.
Fb = ((Wh x K1) + (Wd x K2) + (0.69 x 50)) ÷ B
Fb =((600x0.33)+ (120x-0.81) + 0.69x50)) ÷ 60.4*
Fb = (198 - 97.2 + 34.5) ÷ 60.4
Fb = 135.3 ÷ 60.4
Fb = 2.24 cu. ft.
*The figure for a two pound flotation material.

Next there are formulas to calculate the floatation for submerged propulsion equipment, persons capacity and dead weight (gear).
Good luck.
djs

[Thunderhead19]

Dereksi-
Really, ABYC H-5 and H-8 are kind of rough too. Wouldn't it be better to use Simpson's formula?

[dereksireci]

I wasn't aware that Homer had a formula.

Seriously, just follow it one step at a time and it's not bad. Small boat (<20') designers do it for every new model. These formulas were developed from years of research and testing. They cover either basic or level floatation. This is not something I'd want to wing. Imagine the consequences if the vessel were to sink or capsize.

[SailDesign]

Originally Posted by Thunderhead19

Wouldn't it be better to use Simpson's formula?

Which of Simpson's Rules gives the amount of flotation needed? The First (1,4,1), the Second (1,3,3,1), the Third (5,8,-1) or the Radial Integration rule (whose coefficients I can't remeber off the top of my head)?

Steve

[Thunderhead19]

Simpson's first, or one-third rule (1424241), gives you the total displacement, making a curve of areas below the waterline leads you to the CB, which should align with your CG, and be a good place to put foam. The problem is that eveything inside the boat displaces water and or floats to a greater or lesser degree.

[Arrowmarine]

i believe the us coast guard site has the flotation formulas online.