Being a seafarin' man Bruce, I'd think you know that's not a duck. Looks to be a cormorant to me -- not too popular a critter in these parts.

Interesting statement GFC. I'd be interested in a brief explanation, por favor.

a cormorant. yeah, that's what i meant. although at first, i thought it was a tiny periscope with a snout, err bill.

excuse me while i cormorant and hide.

edited because I hit Submit too soon.

OK Phil, you asked for it, but if you fall asleep in the middle don't blame me....

As a boat moves very slowly through the water it creates a wave at the bow (logically called a "bow wave"). This wave is the boat's effort to push water out of its way. As it does this, there is a built up kinetic energy in that wave. The faster the boat moves through the water in displacement mode the larger the bow wave and the more kinetic energy that is created in the wave.

Sounds good eh? Well it isn't. As that bow wave grows, and the energy in the wave is increased, this takes away from the kinetic energy of the boat, taking power away that could be used to propel the boat forward and costing the boat efficiency. The result of this is lower fuel economy.

There is also a wave created that moves backward along the hull as the speed increases. This wave (which I’ll call a stern wave) is clearly visible in the photo that Troop posted. Look at the stern of that tug and you’ll see the wave. More on that in a minute.

Every boat has a Hull Speed, often called its Displacement Speed. This is the most efficient speed that that particular hull can move through the water. To find a boat’s hull speed, you take the square root of the water line length times 1.34.

Square Root of LWL * 1.34 = Hull Speed

To make the math simple, suppose a boat has a 36’ waterline length. The square root of that is 6, so the formula would be 6*1.34 = 8.04 kts

At that speed, the boat is moving efficiently through the water, creating a manageable bow wave and stern wave.

There is another number that comes into play at this point. It’s called a Froude Number which is a relationship between the speed and the length of the hull, and used in the comparison of boats of different sizes. As the boat’s speed increases, since the Froude Number is speed/length, the Froude Number will increase. When this ratio gets to about .94 the boat starts to outrun its bow wave which has moved backwards along the hull and is at the stern. This stern wave actually supports the weight of the stern of the boat.

As we outrun the stern wave and the stern wave moves behind the boat, the stern squats down in the water. We’ve all experienced this as we accelerate and our boat makes that transition from displacement speed to planning speed. When the boat outruns the stern wave and the stern squats in the water, the bow is pointed toward the sky and we’re running at our LEAST efficient attitude.

As the boat continues to increase speed and makes the move toward planning speed, we continue to outrun the stern wave. At this point it doesn't matter any longer because we have transitioned out of displacement mode. The bow starts to come down and when the boat is on plane we have completed that transition from displacement speed to planning speed.

So Phil, are you still awake? In the next chapter we'll discuss what effect water depth has on the amount of speed that is lost to the bow wave. We'll also take a look at how bulbous bows work to decrease the bow wave and increase efficiency.

Ding. OK class, there's the bell. See you tomorrow.

But can you tell us what that round, spinny thing under the boat is?

Good explanation, gfc. Too much math, but still good.

If my aim was good, it's the remaining parts of a PWC!

(My best Emily Litella) Never Mind.

JK, GFC. Thanks, that was actually understandable. Wink, wink, nudge, nudge, know wut I mean?

Good place to learn, this.

All I know is that if there are dolphins riding your bow wave, you are in the right place going the right speed.

Hey GFC --- where did you take your hydrodynamics - marine engineering courses -- (leading question of course) just curious --

Dale, no regular courses, just a lot of reading over the years about boating topics.

This spring I considered buying a 60' custom designed boat that was built with a bulbous bow. It's unusual to find a 60' boat with a bulbous bow so that got me started doing some reading on bulbous bows, their designs and purposes, and their effects on hull efficiency, etc.

That led to the reading on bow waves and what happens to the bow wave as boat speed picks up and the boat transitions from displacement speed up to planing speeds.

That led to more reading on the effects of bow waves in different water depths, etc., etc., etc.

Just my natural curiosity about how things work.