Drain Plug Science

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).

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.  

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....

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!

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)

Thank goodness for all the people willing to be scientists in this matter. Technology is advancing at warp speed due to all you dedicated men.

Getting the boat on plane is the key. It will pull the water out. Pumps are rated in gallons per hour, so the bigger pump will put out more volume, but they do require larger diameter plumbing. I have seen them as high as 4000 you and they require 2" plumbing. This pump is also multi voltage, so it will so up to 36 volts. The biggest issue would be how much power the pump uses. If the mood is running it should not be a problem, but pumping out a bilge with the motor off would drain a battery fairly fast.

It all depends on the boat.  I have a 14' boat with a 25 horse that easily drains the water out when on plane to the point it does it better than using the pump.  My 21' glass boat, not an option because the plug is on the outside.  However, I have 3 pumps on that boat and can keep up no issue with the plug not in. 

I used to have an old 15.5' aluminum boat with a 20 hp on it.  The boat leaked a little.  All I had to do is run on plane and pull the plug and the water was sucked out at a pretty fast rate.  

I have drained a boat by pulling the plug while moving. I have also sunk a boat with out a plug and with the bilge pump running. Thankfully it sunk onto the trailer. It was crooked, but after it was pulled out and it drained it was easy enough to straighten out.

Maybe they make a bilge pump that will keep up. This one did not.