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Buoyancy calculation?

A forum for discussing topics relating to older MacGregor/Venture sailboats.

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Re: Buoyancy calculation?

Postby warren631 » Sat Dec 30, 2017 12:07 pm

I'm saving up all my empty plastic bottles and then stuff them in any spare spaces I can find. If they don't save me then at least this is good for the environment.

I figure I will need about 19 cubic feet of bottles:

26D weights:
Boat 1650 lbs = 748 Kg
Outboard 120 lbs = 54.4 Kg
5x150 people 750 lbs (with their own buoyancy jackets)
Misc 200 lbs = 90.7 Kg

Australian standard: 1.2 x ((M x K) + F) divided by (1000 minus D)
where:
M = hull and deck weight
K = alum 0.62, GRP 0.375, Steel 0.87
F = weight of machinery and fittings
D = density of buoyancy material (foam approx 35kg/cubic meter).
1.2 = safety factor of 120%

so: 1.2 x ((M x K) + F) / (1000 - D)
1.2 x ((748 x 0.375) + 145 ) / (1000 - 35)
1.2 x (280.5 + 145) / (965)
1.2 x 425.5 / 965
1.2 x 0.441
= 0.53 cu meters
= 18.7 cu ft.
= 150 foam blocks each 12"x6"x3"
= 140 US gallons

Check my calculations above before you rely on them.
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Re: Buoyancy calculation?

Postby warren631 » Thu Mar 01, 2018 4:10 pm

I don't know how many foam blocks I have or how good they are. So I am thinking of using air mattresses for buoyancy. Each one would displace a lot of water and provide lots of buoyancy - 62 lbs per cubic foot. A queen extra thick in the back and two twins in the sides in front. Also some air cushions. What do you think?
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Re: Buoyancy calculation?

Postby Ixneigh » Thu Mar 01, 2018 6:01 pm

Don't these boats already have foam inside?


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Re: Buoyancy calculation?

Postby warren631 » Thu Mar 01, 2018 6:49 pm

As I said - I don't know how many foam blocks I have or how good they are.
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Re: Buoyancy calculation?

Postby Ixneigh » Fri Mar 02, 2018 7:50 am

Oh. Has the boat been butchered? The foam blocks I took out of my v22.2 when I butchered her were in fine shape.
I would recommend reusing the foam instead of throwing it away thinking you'll find something better when putting the boat back together. I used trash bags filled with peanuts. However my dog chewed one bag open, and another one got damaged by stowing an anchor. Personally I don't think plastic bottles are a good alternative. If your boat hasn't been tampered with there is no reason to think the factory foam isn't any good. If you've added a lot of weight, certainly adding foam is a good idea.
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Re: Buoyancy calculation?

Postby dlandersson » Fri Mar 02, 2018 7:57 am

Ok,

Why are you doing this? :?:
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Re: Buoyancy calculation?

Postby warren631 » Fri Mar 02, 2018 12:23 pm

dlandersson - I hate it when my boat sinks and I drown. :P
Ixneigh - Its an old boat 26D and I think most of the foam was thrown out for storage space long ago. What is left is black and shrunken. :)
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Re: Buoyancy calculation?

Postby RussMT » Fri Mar 02, 2018 12:47 pm

warren631 wrote:dlandersson - I hate it when my boat sinks and I drown. :P
Ixneigh - Its an old boat 26D and I think most of the foam was thrown out for storage space long ago. What is left is black and shrunken. :)


Do you plan on sinking it? Do you do a lot of boating near reefs or rocks? Offshore blue water sailing?

I never understood the comfort of believing your boat won't sink. I've never owned a boat that I didn't expect would sink, so this concept is weird for me. Nor have I ever heard of a Mac capsizing and implementing the floatation.
I'd invest more in a good handheld VHF and good PFDs.

Your water bottle idea is a good one. Why not?
Just rinse them out well and dry them so they don't grow mold and funk. Stuff 'em everywhere. I just wonder if they will rattle around and shift during transit.
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Re: Buoyancy calculation?

Postby Herschel » Sat Mar 03, 2018 10:46 pm

RussMT wrote:
warren631 wrote:dlandersson - I hate it when my boat sinks and I drown. :P
Ixneigh - Its an old boat 26D and I think most of the foam was thrown out for storage space long ago. What is left is black and shrunken. :)


Do you plan on sinking it? Do you do a lot of boating near reefs or rocks? Offshore blue water sailing?

I never understood the comfort of believing your boat won't sink. I've never owned a boat that I didn't expect would sink, so this concept is weird for me. Nor have I ever heard of a Mac capsizing and implementing the floatation.
I'd invest more in a good handheld VHF and good PFDs.

Your water bottle idea is a good one. Why not?
Just rinse them out well and dry them so they don't grow mold and funk. Stuff 'em everywhere. I just wonder if they will rattle around and shift during transit.


Granted, we hope we never have to use the positive buoyancy of our boats, but as a sailing instructor for novice sailors, I do sense a noticeable easing of apprehension when I tell them about the positive buoyancy. And, at 73 y/o, and mostly a fair weather sailor these days, I have to admit I find it comforting, as well. To add that proverbial "extra bit" I have taken three heavy duty Styrofoam coolers and taped them shut and thrown them up under the cockpit where I have just empty space. With the trapped air inside that gives me a block of flotation larger than just the Styrofoam itself and makes use of empty space that would just sit there. I try hard to avoid ever being in an accident and wishing I had done something I could have reasonably accomplished to either prevent or ameliorate the situation. :)
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Re: Buoyancy calculation?

Postby sailboatmike » Sun Mar 04, 2018 6:28 am

I would of thought there is a bit of space under the front V berth forward of the opening that is used for nothing that you could shove a heap of foam in, however I thing with the X and M models carry that heavy motor, a bit more floatation at the stern could be handy, I mean who uses the last 2 feet just forward of the stern anyway
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Re: Buoyancy calculation?

Postby Herschel » Sun Mar 04, 2018 9:58 pm

sailboatmike wrote:I would of thought there is a bit of space under the front V berth forward of the opening that is used for nothing that you could shove a heap of foam in, however I thing with the X and M models carry that heavy motor, a bit more floatation at the stern could be handy, I mean who uses the last 2 feet just forward of the stern anyway

My thoughts exactly, mate! That is where I put my Styrofoam coolers. Sure don't want to lose the engine, either! 8)
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Re: Buoyancy calculation?

Postby slugbug » Mon Mar 05, 2018 11:54 pm

My S was pulled off the mooring during a storm with 70 mph winds and wound up on the rocks with huge holes big enough for a small child to crawl through and never sank. In fact it stayed high enough in the water to protect the motor and anything stored above the seats on the inside. The storm was huge and the boat was dashed against a bridge and rip rap on the shore and it stayed together and I was able to haul it out on its trailer and take it home. I loved that boat.
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Re: Buoyancy calculation?

Postby Herschel » Tue Mar 06, 2018 10:49 pm

slugbug wrote:My S was pulled off the mooring during a storm with 70 mph winds and wound up on the rocks with huge holes big enough for a small child to crawl through and never sank. In fact it stayed high enough in the water to protect the motor and anything stored above the seats on the inside. The storm was huge and the boat was dashed against a bridge and rip rap on the shore and it stayed together and I was able to haul it out on its trailer and take it home. I loved that boat.

Wow, what a testimonial. Bravo for your boat. 8)
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Re: Buoyancy calculation?

Postby Herschel » Sun Mar 11, 2018 10:19 pm

A shot of my added buoyancy via Omaha Steak coolers taped shut.
Image
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Re: Buoyancy calculation?

Postby dlandersson » Wed Mar 14, 2018 10:40 am

Ok, this shows a serious issue. I don't recall being invited to help empty those Omaha steak coolers. :?

Herschel wrote:A shot of my added buoyancy via Omaha Steak coolers taped shut.
Image
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