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Volume question - how much float is need? #239800
11/07/11 10:32 AM
11/07/11 10:32 AM
Joined: Oct 2002
Posts: 1,226
Atlanta
bvining Offline OP
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bvining  Offline OP
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Atlanta
I turtled the HT in a race a couple of weekends ago, it went over slowly and I had time to get weight on the righting line, but my inexperienced crew had his foot wrapped up in a line in such a way that I had to get him untangled before I got to righting the boat.

By the time I got to the task of righting the mast had filled with enough water and the boat was on its way to turtling.

Long story to ask a question of the engineers. How much float would be needed to keep a mast from turtling? Assume a 34 foot mast, with a strong wind blowing on a 8 foot wide tramp.

Any way to calculate the volume needed for a mast head float? I want to go out by myself and need a safety device for a distance race at the mast head. Thinking some kind of carbon Hobie Bob type device.

Bill

-- Have You Seen This? --
Re: Volume question - how much float is need? [Re: bvining] #239805
11/07/11 11:38 AM
11/07/11 11:38 AM
Joined: Jan 2004
Posts: 330
S
srm Offline
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srm  Offline
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Posts: 330
Originally Posted by bvining
Any way to calculate the volume needed for a mast head float?


Yep, pretty simple in theory. The bottom hull acts as the fulcrum. The mast float has to counter all the moments that are rotating the boat into an upside down position. This would be the weight of the rig, the weight of the top hull, and the wind force acting on the bottom of the boat.

So, multiply the weight of the rig (assume 100lb) times the horizontal distance of the rig's center of mass from the bottom hull (assume 17ft) plus the weight of the upper hull (assume 100lb) times it's horizontal distance from the bottom hull (say 1 foot) plus the wind force on the tramp (say 50 lb) times the vertical distance of the center of effort of this force from the bottom hull (say 4ft) and this gives you 2000 lbft. Since the mast float is located 34ft horizontally from the bottom hull, the required bouyancy is 2000lbft / 34ft = 59lb. At 8lb/gal water density (bouyancy), this would equate to about 7.5 gallons for the mast float.

sm

Re: Volume question - how much float is need? [Re: bvining] #239807
11/07/11 11:54 AM
11/07/11 11:54 AM
Joined: Oct 2011
Posts: 217
Palm Harbor, FL
daniel_t Offline
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daniel_t  Offline
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Palm Harbor, FL
Instead of all the math that srm came up with, I suggest capsizing the boat on land and resting the masthead on a bathroom scale (Gently, you don't want to break the mast doing this.) Whatever the scale reads, divide it by 8 to figure out how many gallons you need for the float.

Remember, the float itself weighs something and since the float will be on the masthead, its full weight will have to be added to the scales reading before dividing by 8.


Daniel T.
Taipan F16 - USA 213
Re: Volume question - how much float is need? [Re: bvining] #239809
11/07/11 12:24 PM
11/07/11 12:24 PM
Joined: Sep 2007
Posts: 571
Hamburg
Smiths_Cat Offline
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Smiths_Cat  Offline
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Hamburg
Even simpler: seal the mast. Lighter, cheaper, less drag, less work to do.

Cheers,

Klaus

Re: Volume question - how much float is need? [Re: bvining] #239810
11/07/11 12:24 PM
11/07/11 12:24 PM
Joined: Feb 2005
Posts: 4,118
Northfield Mn
Karl_Brogger Offline
Carpal Tunnel
Karl_Brogger  Offline
Carpal Tunnel

Joined: Feb 2005
Posts: 4,118
Northfield Mn
or..... you can skip the float and tell your crews you sail under Alaskan fishing rules.


I'm boatless.
Re: Volume question - how much float is need? [Re: bvining] #239814
11/07/11 12:44 PM
11/07/11 12:44 PM
Joined: Jul 2004
Posts: 125
Clinton, Mississippi
rattlenhum Offline
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rattlenhum  Offline
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Clinton, Mississippi
I'm betting srm is in the ballpark and that if you follow daniel t's advice then multiply by a safety factor of two you'll get about the same result. It's not really a static problem since you need to consider gusts, waves, and people falling in the wrong spot and/or getting hung up in lines (thus the safety factor). Before I built anything from scratch based on theory and assumptions, I'd do a little empirical research. Take it out in calm conditions/shallow water and practice capsize with a few gallon jugs tied to the mast to get an idea. Work up from there (more jugs) and practice capsize in more the conditions you want to simulate. As a reference the Hobie Bob and Mama bob (for the Getaway) are about 60 lbs. (7.2 gal. or almost one cu. ft. of water) of flotation. The Baby Bob is about half that.

Do consider that the required flotation amount is net (after subtracting the weight of the device and hardware). Also consider that the weight of the float on the end of a 34 foot moment arm will actually add to the weight required to right the boat.


Jerome Vaughan
Hobie 16
Clinton, Mississippi
Re: Volume question - how much float is need? [Re: rattlenhum] #239815
11/07/11 12:49 PM
11/07/11 12:49 PM
Joined: Oct 2002
Posts: 1,226
Atlanta
bvining Offline OP
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bvining  Offline OP
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Atlanta
Thanks guys I knew I could count on you all to help me with this.

Its going to be carbon, and wing be shaped, and have flames....

Re: Volume question - how much float is need? [Re: bvining] #239817
11/07/11 01:00 PM
11/07/11 01:00 PM
Joined: Nov 2005
Posts: 141
M
mini Offline
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mini  Offline
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Posts: 141
Originally Posted by bvining
Thanks guys I knew I could count on you all to help me with this.

Its going to be carbon, and wing be shaped, and have flames....


No amount of flames is going to make a bob on a performance cat look anything but gay. It's like buying a trials bike and putting training wheels on it.

If there is a significnat amount of wind on the tramp and the mast is down wind you need a HUGE float. If the mast is upwind, it will right itself.

If you just make sure your mast is sealed all is good. Also note a violent pitch witha big bob is not so good on the life of your mast. About the same force as pitching onto land, a dock, or a course mark. Un-turtling a broken mast is hard

Re: Volume question - how much float is need? [Re: mini] #239818
11/07/11 01:08 PM
11/07/11 01:08 PM
Joined: Feb 2005
Posts: 4,118
Northfield Mn
Karl_Brogger Offline
Carpal Tunnel
Karl_Brogger  Offline
Carpal Tunnel

Joined: Feb 2005
Posts: 4,118
Northfield Mn
Originally Posted by mini
Originally Posted by bvining
Thanks guys I knew I could count on you all to help me with this.

Its going to be carbon, and wing be shaped, and have flames....


No amount of flames is going to make a bob on a performance cat look anything but gay. It's like buying a trials bike and putting training wheels on it.



Exactly what I wanted to say. lol


I'm boatless.
Re: Volume question - how much float is need? [Re: bvining] #239824
11/07/11 02:05 PM
11/07/11 02:05 PM
Joined: Mar 2002
Posts: 284
Norfolk, VA
Dan Berger Offline
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Dan Berger  Offline
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Posts: 284
Norfolk, VA
Why don't you just seal the mast? The mast is wide open at the bottom, but you have to worry about the halyard that runs through a hole in the side of the mast. There is already a small plastic tube through the foam plug at the top of the mast, so if you extend that all the way down to the exit hole on the side of the mast, then you can seal off the inside of the mast above that hole. You'll have 30 feet of empty mast for volume.

Granted, it will be a pain in the butt because you'll have to take apart all the downhaul stuff inside the mast, but it beats trying to right a turtled boat with a mast full of water!



Dan Berger
Norfolk, VA
A Cat USA139
Supercat 15
Re: Volume question - how much float is need? [Re: bvining] #239825
11/07/11 02:21 PM
11/07/11 02:21 PM
Joined: Oct 2011
Posts: 217
Palm Harbor, FL
daniel_t Offline
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daniel_t  Offline
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Palm Harbor, FL
What about stuffing a swim noodle into the mast? I haven't tried this, but I've often wondered...


Daniel T.
Taipan F16 - USA 213
Re: Volume question - how much float is need? [Re: daniel_t] #239851
11/08/11 09:23 AM
11/08/11 09:23 AM
Joined: Nov 2002
Posts: 465
FL
sail7seas Offline
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sail7seas  Offline
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Posts: 465
FL
I have not seen a big cat with a bob at the top.
Here's a big cat in Oz (Firebird) with one:
http://www.youtube.com/watch?NR=1&v=-FhEFCGdc0o

Re: Volume question - how much float is need? [Re: bvining] #239878
11/09/11 01:19 AM
11/09/11 01:19 AM
Joined: May 2006
Posts: 1,380
Kingston SE South Australia
JeffS Offline
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JeffS  Offline
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Kingston SE South Australia
If you have an internal halyard its probably the water entry point, I had an internal jib halyard on my Taipan 5.7 with a similar length mast and that let just enough water in to stop me righting the cat, so I put the halyard external, sealed the mast and comes up straight away. Your external halyard may not be as sexy as internal but nowhere near as funny as a float


Jeff Southall
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