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Drain Plug Science

7/10/17 @ 12:56 PM
Timmothy A
MEMBER since 5/17/09

My sons and I were debating a few concepts relating to a missing drain plug, waterflow in and out of a moving boat and bilge pump performance.

Concept 1:  If you launched without the drain plug and took on water, would water run out the boat if you kept the speed of the boat up?

Concept 2:  On our boat, the drain plug hole is bigger than the bulge pump expeller hole, but is the velocity the bilge expels the water out of the boat a higher velocity than it runs in through the drain hole? (when not moving).

8/20/19 @ 12:35 AM
User since 9/24/03

This was a random bump of a two year old thread.  

Not forgetting the drain plug is easy.  If you have a bungee on your transom saver hang it from a hook on the bungee strap.  If you do this and still forget to put the plug in then you shouldn't own a boat.  

8/19/19 @ 4:49 PM
User since 6/16/03

Did a doozie last july 4th not putting in plug. Thought i could drain it going faster but too much weight. Quickly put it in drain hole but realized it was livewell hole. Back to shore slowly to plug in drain hole and 15 minutes bilge pump and good to go. Bilge pump good as dry when pulled plug at landing when leaving. I probably could have drained using speed without 5 others in boat. I now now which hole is what going in blind....

8/19/19 @ 3:54 PM
MEMBER since 6/22/01

I don't know about your concepts, but I can tell you this:  My boat fills with water fast! Over the weekend I parked the at the dock and had my friend and my son hold the boat while I went to park my vehicle and trailer.  2 minutes later I got back to the boat and noticed water coming up onto the floor.  Oops, I knew what happened right away!  Well dummy me instead of turning on the pump I waited til I got back and by then I had 2 inches of water on the entire floor bottom! So we got the boat out of the water and let the water drain and I turned on the pump.  Took about 5 minutes to drain it that way, then we went back out fishing.  Just to make sure I would get all of the water out when I got home I removed the drain plug again and to my surprise it still took fifteen more minutes before all of the water was gone!  

So I sure as heck hope scenario 1 or 2 never happens because water gets into my boat quite fast don't think I'd ever be able to get the boat on plan or drain all of the water while on the lake!

8/5/17 @ 3:27 PM
User since 3/26/15

I doubt this thread will ever help me in a real life situation but excellent scientific research guys! ??

8/5/17 @ 12:40 PM
User since 12/19/06

I hope you refrain from snarky comments with your students.

8/4/17 @ 8:54 AM
User since 6/25/02

Come on guys, this is 9th grade science.  The reason water drains from the drain plug is inertia.  Better known as Newton's First Law of Motion,  "An object in motion stays in motion, an object at rest stays at rest".  The water in the boat wants to stay at rest but the boat is moving causing the water to move the the back of the boat and out the drain.  I empty my live well this way if I forget to put the plug back in.

The second question isn't comparing similar concepts.  The bilge uses a motor to force water out of the boat while water comes into the drain hole based on the downward pressure of the boat (Archimedes principle).  All things being constant if the same amount of water is moving through two holes it will have to move through narrower one faster to compensate.  That is a concept in fluid dynamics.

Yes I am a teacher.

8/4/17 @ 7:58 AM
User since 12/19/06


The laws of affinity do support your statement about same amount coming out, just at a faster rate, to a point.  If the discharge piping is choked enough, the discharge flow speed increases significantly, but the volume will start dropping as the choke becomes more obstructive.  The psi on the supply-side of the choke will rise (the pressure rises immediately with any sort of choke...even a little).

Pressure control valves work in this scenario, allowing a wide range of volume at a constant pressure (on discharge side of the choke), while the supply-side of the choke varies greatly.   We call these "poor-man's constant pressure systems" in the waterwell industry....


8/3/17 @ 6:43 PM
User since 3/22/10

I can touch on scenario two a little bit.

Like was said, pumps are based on a flow rate (Q) (GPH-gallons per hour or cubic feet for second). The formula for flow rate is Q=V/A or Flow Rate = Velocity of flow divided by cross sectional area of output (in this case a pipe). Assuming the input (water in the boat) is constant, the output will be constant. If you decrease the area of the pipe, the velocity goes up, if you increase the area the velocity goes down. Think about putting your thumb over a garden hose. The same VOLUME of water comes out, just faster.

Simply put, for the boat the to stay afloat, this expression must hold:

Outflow >= Inflow ... Q(out) >= Q(in)

7/14/17 @ 3:38 PM
User since 12/19/06

Regarding concept 1, take a look at your transom while on plane.  The transom should be completely out of the water...meaning the water level should be at the bottom of the transom.  If your drain plug is stern-facing, it will be above the water level and draining.  Assisting this is the force of gravity pushing the water in the bilge toward the stern.  Even if your plug is below the water level, it will still drain if you are moving fast enough, gravity forcing water out of the bilge.  The point between water filling or draining is a function of speed and gravity (inertia actually) on the bilge water...different for every boat and plug position.

I was on a 20' glass boat years (I say this to protect the guilty  captain ;) forgot to put the plug in.  Several hours into the excursion (lake MI) we noticed we were riding a tad low in the water... it took us a mile or more to get on plane.  As we continued toward home, the bilge drained as expected, and the boat planed better as we continued on.

This is assuming you have a motor powerful enough to get your boat on plane of course...

And all of this was already said.  Diarrhea of the fingers I guess.

7/12/17 @ 8:29 PM
User since 3/26/15


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