Re: Water flow rate through drain holes (corrected)

From: Jim Nolan (panache426@hotmail.com)
Date: Thu Feb 03 2000 - 19:40:57 PST


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>From: SolarFry@aol.com
>To: wwpotter@tscnet.com
>Subject: Re: Water flow rate through drain holes (corrected)
>Date: Thu, 3 Feb 2000 21:23:17 EST
>
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> West Wight Potter Mailing List maintainer
> dfarrell@ridgecrest.ca.us
> List hosted by www.tscnet.com
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>{:^)
>
>So..
>
>You need to know how many gallons of water are needed to flood cockpit in
>order to find out how much time till the lady drowns.. She really doesn't
>know how to swim. Did you know that? :^)

I didn't know she couldn't swim. If I had known that I would have done the
calculations for bunker oil. She surely would have floated in that and
potentially enjoyed calm seas.

>
>If the 1 inch drain fitting has an inner diameter of 1/2" we would expect
>an
>exhaust rate of roughly 2.5 gallons per minute? That is assuming no back
>pressure from water on other side.. How many gallons can you stick inside
>cockpit? Say for sake of discussion you would get 125 gallons inside
>cockpit.
>It would then take 50 min to exhaust that much water? So you would have to
>sit in water for 50 minutes? If you went to a 1 1/2" drain that has an
>inner
>diameter of 3/4" you would cut that time in half? 25 minutes sitting in
>water?
>
>Did I get this right?

One thing to remember is that as the head of the water decreases, so does
the flow rate. So as the water goes down it flows out more slowly according
to the equations provided. Since the flow varies as the square root of the
head, when the head level is 1/4 of the starting level the flow rate will be
1/2 of the starting level. So in reality it will take longer than the 50
minutes to drain the cockpit.

To get at a precise answer the shape of the flooded volume has to be known,
to get the head as a function of the flooded volume dimensions. Then you
integrate the formula to get the total time to drain. For instance an
inverted cone will drain much differently than a non inverted cone even
though they have the same initial head . For the same volume the inverted
cone (point down)will drain faster because the higher head is associated
with the larger volume portion. Likewise an inverted Potter will drain
faster than a non inverted Potter.

The one thing the table does illustrate is how effective drain size is.
Obviously the bigger the better but it gives some idea of how much.

Jim Nolan P-19 #426 Panache :)

>
>SF
>
>In a message dated 2/3/00 2:15:49 PM Pacific Standard Time,
>panache426@hotmail.com writes:
>
><< Subj: Water flow rate through drain holes (corrected)
> Date: 2/3/00 2:15:49 PM Pacific Standard Time
> From: panache426@hotmail.com (Jim Nolan)
> To: wwpotter@tscnet.com
>
> Recently there's been discussion of drain hole sizes for flooded boats.
> Here's some data that may help you decide about drain holes.
>
> This is the water flow rate through various drain hole diameters. The
>flow
> is in gallons per minute, drain diameter in inches. This is for 6 inches
>of
> water head. In other words if your boat had 6 inches of water above the
> drain hole, this is the rate it would flow out into the air. This also
>works
> for water flowing into the boat (sinking). From the chart you can see
>that
> the water flowed into the P-15 in the brochure at 45.4 gpm (two 1.5"
>holes
> at bottom of hull). Question - after how many minutes would the lady be
> swimming?
>
> 0.25 inches .63 gpm
> 0.5 inches 2.52gpm
> 0.75 inches 5.67 gpm
> 1 inch 10.08 gpm
> 1.5 inch 22.7 gpm
> 2 inch 40.34 gpm
> 2.5 inch 63 gpm
> 3 inch 91 gpm
> 3.5 inch 123 gpm
> 4 inch 161 gpm
>
> The formula used is:
>
> Flow rate (cubic inches/second) = 0.74 x Area x square root (2 x g x h)
>
> g = 386 in/sec/sec
> h = head in inches at any instant
> gallon conversion: 235.2 cubic inches to the gallon
>
> Multiply flow rate by 60 and divide by 235 to get gpm.
>
> Simplifying:
>
> Flow rate (gpm) = .1889 x Area x square root (772 x h)
>
> This formula DOES NOT take into account length of drain pipe. More
>viscous
> fluids, such as oil, would drain much slower. Someone please check the
> calculations and formula. Thanks,
>
> Jim Nolan P-19 #426 Panache
> >>
>

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