Author Topic: Aft buoyancy to avoid a capsize  (Read 196 times)

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json

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Aft buoyancy to avoid a capsize
« on: June 08, 2021, 02:41:47 PM »
Brian (or anyone else who wants to chime in) - I am trying to build in some chambers to hopefully make my boat have the tendency to roll upright in the event of a capsize. I am pretty sure it's been discussed here but not sure where, something to the point that adding buoyancy under the aft shelves would help avert the full upside down unrecoverable scenario. Approximately how much displacement would help in this? I am building a euro transom similar to grady300's, and have the triangular spaces to both sides open (under and behind the shelves all the way aft), but not sure if they are big enough. I could probably get creative if they were anywhere close to adequate. Any feedback about this? Very rough guess is those spaces are about 3000 cu in each, could be made somewhat bigger, and I should be able to seal them to be air tight.

ghelland

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Re: Aft buoyancy to avoid a capsize
« Reply #1 on: June 08, 2021, 10:25:33 PM »
Sounds like we are building the same transom.  I also plan to have an area back there for flotation.  Each cubic inch of space will lift .038 lbs.  So 3000 cubic inches will lift 108 lbs. 
My plan is to use foam in a sandwich type construction for my roof.  If I lay her on her side the roof will not allow it to complete the roll.  So now I just need get the motors high enough not to fill with water. 
Are you going to run twins too?

json

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Re: Aft buoyancy to avoid a capsize
« Reply #2 on: June 08, 2021, 10:45:38 PM »
I am going to be running a single yamaha f300, so all the weight will be to the center. 108 pounds doesn't sound like a lot, however maybe it's enough in water and exerting force on that corner to push it back upright. I am not sure how to know though haha. My roof is straight okoume, so while light, it's not really thick and it's probably not gonna float that high. I could probably build in more under the sides of aft cockpit thingy (not really a transom right?). Maybe I just do my best and hope I never have to find out. And if I do find out I can hopefully high five myself for a job well done in an upright boat...

Todd j

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Re: Aft buoyancy to avoid a capsize
« Reply #3 on: June 08, 2021, 11:18:33 PM »
Do you have a self bailing cockpit?

Brian.Dixon

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Re: Aft buoyancy to avoid a capsize
« Reply #4 on: June 09, 2021, 06:47:19 AM »

Yes, seawater weighs about 62# per cubic foot.  Subtract from that the weight of the foam per cubic foot, say 2# per cubic foot.  Buoyancy per cubic foot of foam-filled flotation is then about 60# per cubic foot.

If the boat is on it's side ready to roll over, about half the weight of the boat is above and below the CG and the motor CG is about 12" 'above' the CG.  Rough estimate to right the boat is that you'd need a moment-arm of about 800-900 pound-feet to do it reliably.  If your flotation chambers are about 1.5 ft above the CG, then you'd need about 560# flotation (850 lb-ft divided by the 1.5 ft arm that the flotation has).  That's more or less 9 or 10 cubic feet of flotation right up under the gunnels to counter the tendency of the boat to capsize when it's on it's side (theoretically rolling over).  Flotation only matters in a flooded boat ... prior to flooding, the hull design versus the CG location is what matters.

The bottom line is that while flotation sounds pretty good, it's generally not helpful in turning a boat upright until it's too late.  It's better thought of as a way to keep the boat high enough in the water for you to hang onto while you wait on a rescue.  The boat's crew definitely doesn't weigh enough to turn a boat upright once it has turned over, regardless of flotation.  NO boat is big enough to be bigger than the ocean.  Buy a mustang suit (or jacket), EPIRB, and waterproof VHF marine handheld. 

Some boats are more stable upside down than right side up (many sailboats, catamarans etc).  The Great Alaskan is more stable right side up, and the flared sides really help it resist rolling over, and if it does try to roll over, the sides are high enough to keep water out of the boat for a higher number of degrees of heel than most other boats.  The Great Alaskan is very seaworthy and hedges your bets much better than most boats out there ... but the ocean is always bigger than any boat.  Don't count on being able to keep it upright if the conditions are overwhelmingly large versus the boat.  Head into the waves, pray, get your radio hanging off your belt, pray again, and put on your Mustang suit if it's that bad!

That's my story and I'm-a stickin' to it!

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json

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Re: Aft buoyancy to avoid a capsize
« Reply #5 on: June 09, 2021, 08:13:58 AM »
Do you have a self bailing cockpit?

Yep... Lots of floatation under decks too, so guess I just need to count on that holding me high enough for the rescue boat to find me if I get into that situation. It's probably unrealistic to think that I can plan against any scenario, so I think I am just gonna call the 4-years-of-college-fund in pool noodles under the deck good and move it forward.

Todd j

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Re: Aft buoyancy to avoid a capsize
« Reply #6 on: June 09, 2021, 10:25:02 AM »
I bought mine at the dollar store.  Like 250$ worth.  This is one of the reasons I built the GA.   Remember wood floats even when it is not in the shape of a boat!

Brian.Dixon

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Re: Aft buoyancy to avoid a capsize
« Reply #7 on: June 09, 2021, 03:31:33 PM »

I think flotation nearer the stern will help keep more of the boat out of the water .. if it does get capsized.  Might be more to hang onto.  Might enable climbing out of the water, or to hang onto the transom or motor .... maybe easier than hanging onto other parts of the boat?  Skegs and keel also make the hull easier to hang onto.

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ghelland

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Re: Aft buoyancy to avoid a capsize
« Reply #8 on: June 10, 2021, 10:26:47 PM »
Has anyone tried placing plastic bottles under the deck?  I figure if I glue the lids and use pool noodles to lock them in place they should last a very long time.
I would also like to note that I believe Brian has designed a great boat and all our discussion about flotation will most likely be a complete waist of time. 

Brian.Dixon

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Re: Aft buoyancy to avoid a capsize
« Reply #9 on: June 11, 2021, 06:54:29 AM »
Has anyone tried placing plastic bottles under the deck?  I figure if I glue the lids and use pool noodles to lock them in place they should last a very long time.
I would also like to note that I believe Brian has designed a great boat and all our discussion about flotation will most likely be a complete waist of time.

Plastic soda bottles are an option.  The PEP plastic is pretty indestructible and should last a long time.  I know people are using them, but am not aware of any long term testing, e.g. water, solvent, petroleum product exposure.  I did my own testing of closed-cell polyethylene (the 'ETHYLENE' part of that is important!) ... I kept a chunk of this stuff submerged for a year, once every few weeks dousing it in gasoline and oil, including light penetrating oil (WD-40).  After a year, I let it surface dry in an almost-closed ziploc bag then inspected and weighed it.  It was dirty looking but in good physical condition, not deteriorating or rotting or showing any type of damage.  I weighed it ... it had gained only 3% in weight, likely due to the 'closed cells' around the cut edges getting violated by the cutting.  My vote is A-OK ... wherever you get it, closed-cell polyethylene is the stuff you want ... far out-performing polyurethane "flotation foam" (totally NOT recommended - it absorbs water, rots, and becomes nothing but extra weight).

I'd feel fine with PEP bottles too.  Close them tightly ... caulk the lids on?  Probably not.

Using drain-plug sized scuppers that can be closed when underway and lots of chambers is good too, although most have pipes and things going through under deck bulkheads, so hard to seal except where stringers drain into the bilge.  This is covered in the manual.

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Brian.Dixon

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Re: Aft buoyancy to avoid a capsize
« Reply #10 on: June 11, 2021, 07:16:12 AM »

BTW, for the record, the best way to avoid capsize is to a) keep water out of the boat, and b) provide a great way for water to get back OUT of the boat.  The #1 cause of capsize is the free-surface affect of water in the boat, e.g. water is heavy and sloshes to one side or the other, and it does this out of sync with what the hull is trying to do ... in other words, if a hull is trying to right itself, the water is usually (at that time) moving in the opposite direction and resisting the righting of the hull.  The #1 way people get water in the boat is having a heavy stern (4 fat boys fishing, couple hundred pounds of ice, all the weight in the stern) followed by a sneaky wave coming in over the stern.  Once some water gets in, the stern becomes even more heavy and more likely to take on more water.   I saw a boat capsize in Puget Sound in conditions where there was only about a 2-ft chop ... they played out the scenario above perfectly and they all went swimming.  Check this out:  Let's say you have a cockpit that is about 6 ft by 7 ft and you took on water that was only 1 ft deep in the cockpit ... that's 6 * 7 * 1 * 62 lbs/cu. ft of weight ... 2600# !!!   You think that'd have much force when it sloshes against the side of the boat?  How much more likely will it be that another wave can make it in when you've got an extra 2600# in the stern?  It happens in seconds...

SO, The Great Alaskan was designed to allow as many degrees of heel (roll) as possible and to strongly resist heeling ... more than most other boats in its class.  The high flared sides help a lot.  Keeping water out by using coaming around the cockpit and bulwarks that help prevent water draining into the boat and good seamanship are key.  The icing on the cake is to provide LARGE scuppers that allow water to wash back out ASAP.  Do the calcs ... your bilge pump is for incidental water, NOT getting 2600 lbs back out.  Large rectangular scuppers, even (especially?) on the sides is a great idea ... the hard part is that you'd like them to be close-able until you need them.  But when water gets in, the key to success is getting it back OUT and doing it FAST, before worse happens.  Research those scuppers ... I do not advise on exact requirements because they vary around the world and even within some areas (USCG v. ABYC), the requirements contradict each other.  Just study the info above, read available standards, and look at commercially-made boats in the same class as a Great Alaskan to see what they are doing then do the best you can ... and you're the captain, responsible for wise boat management that keeps you out of trouble... unlike that guy in Puget Sound.  NOTE: If planning on a captain's license and 'inspected' status, it's the USCG rules that matter in the USA.

Brian

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kennneee

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Re: Aft buoyancy to avoid a capsize
« Reply #11 on: June 15, 2021, 08:47:49 PM »
This is an interesting topic for me. I don’t have a GA but a Outer Banks 26. It has quite a lot of waterline beam (7’ 4.5”) and is very stable in most conditions I have had her in which is almost never more than1-2ft chop. I also know at some point I will encounter more nasty seas. I wonder what the sweet spot is for waterline beam (if there is one)? I know there are many other factors which determine a boats resistance to capsize such as flare and deadrise, metacenter, weight distribution, etc. Will a boat such as mine resist a beam seam to a point and then quickly turn turtle or does the large flare of my Carolina style bow help to prevent that?  I say this in comparison to a boat such as the GA which has less beam at the waterline and a more dory type flare. I see that most stitch and glue hulls are narrower at the waterline than some hulls which are diagonally planked such as the OB26 and wonder if that is totally design choice or partly due to necessity for hull formation with a plywood sheet or both?  Does a boat with a narrower waterline beam yield to a wave but roll back to center more easily even with a similar maximum overall beam? I love discussions like this and would like to hear any of your thoughts.
Ken

Brian.Dixon

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Re: Aft buoyancy to avoid a capsize
« Reply #12 on: June 16, 2021, 05:33:07 AM »
This is an interesting topic for me. I don’t have a GA but a Outer Banks 26. It has quite a lot of waterline beam (7’ 4.5”) and is very stable in most conditions I have had her in which is almost never more than1-2ft chop. I also know at some point I will encounter more nasty seas. I wonder what the sweet spot is for waterline beam (if there is one)? I know there are many other factors which determine a boats resistance to capsize such as flare and deadrise, metacenter, weight distribution, etc. Will a boat such as mine resist a beam seam to a point and then quickly turn turtle or does the large flare of my Carolina style bow help to prevent that?  I say this in comparison to a boat such as the GA which has less beam at the waterline and a more dory type flare. I see that most stitch and glue hulls are narrower at the waterline than some hulls which are diagonally planked such as the OB26 and wonder if that is totally design choice or partly due to necessity for hull formation with a plywood sheet or both?  Does a boat with a narrower waterline beam yield to a wave but roll back to center more easily even with a similar maximum overall beam? I love discussions like this and would like to hear any of your thoughts.
Ken

There are a few basic components here ... side height - It's just a physical thing.  Higher sides means water has to climb higher before it comes in, right?  Simple.  Second, the side flare (amidships much more important than bow area) effectively 'makes the boat wider' as water climbs up the side, and this makes the boat more self-righting .... and the boat will heel, bob up and over waves, naturally keeping the water further below the sheerline ... more below means more safety margin. 

Finally, the waterline beam is the end result of a few factors coming into play.  First, for the boat to plane in a stable manner AND for it to exhibit good stability when adrift, you select an appropriate 'design waterline' or DWL. Actually, you select an acceptable range for the DWL to be in when loaded in various ways.  If a boat is lighter, such as most stitch-n-tape boats, making the waterline beam too wide will float the boat higher and then the waterline lands below the "good DWL" range.  There is an optimal length/width ratio for boats that have different intended usages too ... ala Lindsay Lord and Daniel Savitsky.  A high speed lake boat has a different answer than a high speed ocean boat.  Finally, you want the boat to be responsive to swells or waves coming in from various angles ... but not TOO responsive, e.g. 'snap rolling'.  Being less responsive than ideal, the boat becomes late to respond to water movement, develops a 'sickly motion', and gets out of sync with the water (danger, Will Robinson, danger!).  If a boat is TOO responsive, then it 'snap rolls' with water movement ... that wears out the passengers in the boat and it's hard on the boat structurally but is at least safer than an unresponsive boat.  Again, for boats with different intended usages, ranges of displacement, and boat size, there is a different answer.  The two numbers that have to be optimized are the Longitudinal Metacentric Height (Gml) and the Transverse Metacentric Height (Gmt).

Boat design for boats that go offshore is an iterative, time-consuming, process ... and MOST boats out there were NOT optimized, at least nowhere near as much as what I put into the Great Alaskan.

Hope that answers a few questions...
« Last Edit: June 16, 2021, 05:37:09 AM by Brian.Dixon »
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kennneee

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Re: Aft buoyancy to avoid a capsize
« Reply #13 on: June 19, 2021, 06:39:32 PM »
Brian- Been out on Rosie for a few days and this is my first chance to say thanks for the thoughtful reply.
Cheers,
Ken

Brian.Dixon

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Re: Aft buoyancy to avoid a capsize
« Reply #14 on: June 20, 2021, 07:19:47 AM »
Brian- Been out on Rosie for a few days and this is my first chance to say thanks for the thoughtful reply.
Cheers,
Ken

Hey, no problem!

Another interesting observation is that for a given Gmt (resistance to rolling/heeling), heavier boats have to have a wider waterline beam and vice versa.  As a boat gets heavier, the Gmt value lowers.  It's natural for a lighter, more efficient, boat like the Great Alaskan to need a narrower beam than a heavier boat that's full of frames (or aluminum, or polyester fiberglass).

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