What is Hydrodynamic Effect?

Our friend Arman asks us to clarify a term that was used in the BoatSafe.com contest. The situation was that a large passenger ferry was overtaking a pleasure yacht and, because they were in a narrow channel, the ferry passed very close to the passenger vessel. Because of the “hydrodynamic effect” and the suction of her propellers, the ferry pulled the passenger vessel into her side causing damage.

Hydrodynamic effect can be broken down into its word elements which may help explain what it means. Hydro means water, dynamic means energy or physical force in motion and effect means anything brought about by a cause or a result. So…if we put this all together, hydrodynamic effect means the result of water in motion.

On vessels of any size, but especially on large vessels, as the boat moves through the water the friction of the water on the bow and on the hull causes the water to create turbulence. This turbulence, as the force of the water moves toward the stern (back) of the boat, tends to pull floating objects into the side of the boat. In addition, as the large propellers (which are also toward the stern of the boat) turn, they, too, pull water down and toward the center line of the boat. So, if a large boat passes too close to you in a small boat, you could be caught in this hydrodynamic effect and pulled into the side of the larger boat. Not a good idea.

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To demonstrate the effect of hydrodynamics next time you are out in the family boat, ask the responsible adult in charge if you can try an experiment. With the boat moving forward at a slow speed, drop a small ball that floats just off the stern of the boat. Actually, just let it roll down the transom until it hits the water. You will notice that the boat does not run away and leave the ball behind but actually pulls the ball along behind it as if it were attached. This is hydrodynamic effect. Don’t forget to stop the boat before retrieving the ball with your fish net. If you imagine that your small boat were the ball and you go too close to a large ship, you too would be pulled along and into the ship.

Be careful out there. There are strange and mysterious forces at work. But, you can avoid this one by staying far away from large boats.

Daniel asks “Why is it hard to back the boat trailer?”

For many boaters, the hardest part of trailering is backing up. You donÂ’t have to be a master truck driver to do this. You do need to remember a few simple rules. The most important of which is; you have to be able to see the trailer. This might mean putting extended mirrors on the tow car. These are available at automotive stores and can have either permanent or temporary mountings. For very low trailers you may want to add a couple of bright fiberglass poles so you can see it when the boat isnÂ’t on it. These are available from bicycle stores for a few dollars each.

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To back the trailer, remember to GO SLOW. More people get into trouble trying to rush than for any other reason. Place both hands at the bottom of the wheel and move them in the direction you want the rear of the trailer to go. With this method, it doesnÂ’t matter if youÂ’re looking out the back window or in the mirrors. Get some practice by taking your car/truck and trailer to an empty parking lot and practice backing it into different areas. Most single-axle trailers turn and back easier than double- or triple-axle rigs. The two-wheel trailers do have a tendency to turn very sharply once they start, so back them even more carefully. Above all, donÂ’t be reluctant to pull forward and start over if a backing operation is going badly. Trying to turn a bad start into a good finish often results in dents or other disasters.

Stuff to Have on Your Small Boat

Even if your boat is very small and moves by oars or sail,
you want to be prepared to help yourself or others.
Here is a minimum list of stuff to take with you on your boat.

pfd.gif (1745 bytes) Required!
At least one PFD (life jacket) for everyone onboard. Always wear your PFD when in your boat.
whistle.gif (995 bytes) Required!
A whistle – make sure it is one that works if it gets wet. If the weather turns bad (or it gets dark) and you can’t see very well (or can’t be seen by bigger boats), blowing your whistle will let other boaters know where you are. If you find yourself in trouble, blow it to attract the attention of anyone who may be in the area. This is a recognized help signal, so only use it when you really need help
flash.gif (3902 bytes) A flashlight – you may need it so that other boats can see you if the weather turns bad. Also, you can use it to signal in the dark. The Morse code signal – SOS (HELP!) – looks like this: sos2.gif (4228 bytes)
oar.gif (1555 bytes) Two oars and a way to keep them in the boat if it tips. When not using the oars, be sure they are properly in the oar locks.
pail.gif (2733 bytes) A bucket or can to bail out any water that gets in the boat. You can also use it to signal SOS morse.gif (363 bytes) by tapping out 3 quick-3 slower-3 quick taps.
aid.gif (1398 bytes) A First Aid kit that you have learned how to use.
blanket.gif (5037 bytes) A blanket. If your clothes get wet, you can easily (and very quickly) lose heat from your body, a condition called hypothermia. Removing your wet clothes and wrapping yourself in a blanket will help you stay warm.
line.gif (1647 bytes) Two ropes – called lines on a boat. You can use them to tie up the boat and also to help a person overboard get back to and in the boat.
mirror.gif (1183 bytes) A mirror or shiny object for daytime signaling.
blackbag.gif (1492 bytes) Two garbage bags for rain ponchos or shelter from the weather. You can “make a tent” over your boat to protect you in a rainstorm.
orangebag.gif (1039 bytes) A plastic bag to keep these items dry and secure. An orange plastic bag is especially good because it can also be used to signal for help.

Okay . . . ready to go? Ooooops, one more thing! Before you go anywhere, tell an adult where you are going, where you plan to boat and when you expect to be back. This way, if you don’t return on time, they’ll know where to start looking for you! This is called a float plan .

Required Equipment for Boats


Less than 16ft/4.9m

16 to less than 26 ft/7.9m

26 to less than 40 ft/12.2m

40 to not more than 65 ft/19.8m
Devices (PFDs)
One approved Type I, II, III or V(must be worn) PFD for each person on board or being towed on water skis, tubes, etc. One approved Type I, II or III PFD for each person on board or being towed on water skis, etc.; and one throwable Type IV device. ( A type V PFD may be used in lieu of any wearable PFD, if approved for the activity in which the boat is being used. A TYPE V HYBRID MUST be worn to be legal. )
Check state laws for PFD wearing requirements for children and for certain water craft and sports.
Bell, Whistle reqequ2.gif (17124 bytes) Every vessel less than 39.4 ft (12 meters) in length must carry an efficient sound producing device. Every vessel 39.4 ft (12 meters) or larger in length must carry a whistle and a bell. The whistle must be audible for 1/2 nautical mile. The mouth of the bell must be at least 7.87 inches (200mm) in diameter.
Visual Distress
(Coastal Waters, the Great Lakes &
US owned boats on the high seas)
Required to carry approved visual distress signals for night-time use. Must carry approved visual distress signals for both daytime and night-time use.
(Must be Coast
Guard approved)
One B-I type approved hand portable fire extinguisher. (Not required on outboard motorboats less than 26 ft in length if the construction of the motorboat is such that it does not permit the entrapment of explosive or flammable gases or vapors and if fuel tanks are not permanently installed.) Two B-I type OR one B-II type approved portable fire extinguishers. Three B-I type OR one B-I type PLUS one B-II type approved portable fire extinguishers.

When a fixed fire extinguishing system is installed in machinery spaces it will replace one B-I portable fire extinguisher.

(Boats built on or after 8/1/80)
At least two ventilation ducts capable of efficiently ventilating every closed compartment that contains a gasoline engine and/or tank, except those having permanently installed tanks which vent outside of the boat and which contain no unprotected electrical devices. Engine compartments containing a gasoline engine with a cranking motor are additionally required to contain power operated exhaust blowers which can be controlled from the instrument panel.
(Boats built before 8/1/80)
At least two ventilation ducts fitted with cowls (or their equivalent) for the purpose of efficiently and properly ventilating the bilges of every closed engine and fuel tank compartment using gasoline as fuel or other fuels having a flashpoint of 110 degrees or less. Applies to boats constructed or decked over after April 25, 1940.
Flame Arrestor
One approved device on each carburetor of all gasoline engines installed after April 25, 1940, except outboard motors.
Note: Some states have requirements in addition to the federal requirements. Check your state’s boating laws for additional requirements.

Joshua asks, “I was wondering if it was a coincidence that port is 4 letters long and left is 4 letters long? Or was it planned? I am 11 years.”

It is a coincidence but it’s a good way to remember which is port (left) and which is starboard.

ship899.jpg (10963 bytes) The actual origin of the term “port” is speculated to be because the left side of old merchant sailing ships had a loading or entry port. The right side had a steering board that hung over the side of the ship (before the invention of rudders) which is where the term starboard comes from. So if the steering board hung over the right side, the boat would need to dock on its left side, or put into port on its left side.


How do you save someone who falls overboard in a boat that is moving quite fast? If you don’t know how drive.

Lauren, you have posed a very important question. I only hope that you are asking as a preventative measure and that you were not presented with this problem.

First, let me emphasize some things to remember.

  1. If a boat in which you are riding is going fast enough that a person could potentially be tossed overboard, it is going way too fast.
  2. If you are being bounced out of your seat, then the boat is going too fast.
  3. If you are getting uncomfortable, then the boat is going too fast. You should politely ask the operator to slow down.
  4. You should wear a life jacket and try to get others boating with you to always wear their life jackets.
  5. Never stand up in the boat when underway, you need to keep a low center of gravity to keep from falling.
  6. Always slow down if you are approaching a big wake created by other boats.
  7. Be sure to keep one hand for yourself and one for the boat. This means don’t try to hold too many things in your hands that would prevent you from holding onto the boat.

Following some simple steps should help you in your assessment of how to react if a non-preventable emergency should arise.

Ask the adult or adults who operate the boat in which you are riding to teach you the following basics:

  1. How to pull back on the throttle so the boat slows down, and how to take it out of gear so it stops.
  2. How to find the required floatable cushion, that should be readily accessible, and throw it to someone in the water .
  3. How to find the required sound signaling device and how to use it to signal for help.
  4. How to use the VHF radio to summon help in case of an emergency.

I hope that this helps. And please make sure when you go boating that you are riding with a responsible adult who has completed a safe boating course.

Parents get help at BoatSafeKids

Parents get help at BoatSafeKids

dive_helmet.gif (8651 bytes) Frank writes: My son is reading 20,000 Leagues Under The Sea and asked me what distance a league is. Also, he wanted to know what the water pressure per square inch per foot of depth under water is. Your help would be greatly appreciated. A league is a measurement of length used in estimating sea distances. Its length varies among different nations. In Great Britain, France, the US and Spain the league has a recognized length of 6,075 yards. (5,554.9 meters)

As for the second part of your question, the water pressure increases the deeper you go. At the surface the atmospheric pressure is 14.7 PSI (pounds per square inch). For each foot of water depth the pressure increases by .445 PSI. You then have to add this to the atmospheric pressure to get the absolute pressure you are experiencing. In diving, each 33 feet adds an additional “atmosphere” or adds an additional 14.7 PSI to your body.

As an example, at a water depth of 33′ what would the pressure be?

33′ depth X .445 PSI/ft. = 14.685 (round to 14.7) you then must add this to the 14.7 atmospheric pressure and you get a total pressure of 29.4 or two atmospheres.

At 66′ of depth X .445 PSI/ft = 29.37 (round to 29.4) plus 14.7 atmospheric pressure gives you a total experienced pressure of 44.1 or 3 atmospheres.

Very good questions.

Capt. Matt

cruiseship.jpg (5037 bytes) Hi. My son, (11), and I have seen the movies, “TITANIC”, and, “SPEED II Cruise Controll”. He wants to know how fast say 17 “knots” is if he were traveling in a car (MPH). Can you answer or guide us to somewhere that could?

Thanking you in advance for your help in this matter, Jami & Josh

Statute miles are used to measure distance on land and on inland lakes, rivers and intracoastal waterways. Nautical miles are used to measure distances on the oceans of the world. If you assume the earth is perfectly round, (it is slightly flatter at the poles), there are 360 degrees around it at the equator. There are 60 minutes in each degree and each minute is one nautical mile. Doing the math, 60 minutes X 360 degrees = 21,600 nautical miles around the earth at the equator.

A statute mile is 5,280 feet in length.
A nautical mile is 6,076.11549… feet in length.

To convert distance use the following formulas:

statute miles x 1.15 = nautical miles

nautical miles x .87 = statute miles

To convert speed use the following formulas:

MPH X .87 = Knots
Knots X 1.15 = MPH

So… 17 knots would be approximately 19.5 MPH.

This is not completely accurate because of rounding but should suffice.

Joshua asks: Where does the term figure head come from?

The figure head is the carved ornamental and painted figure erected on the bow of ships. The origin of the figure head in the early days of seagoing was twofold; a mixture of religious symbolism and the treatment of the ship as a living thing.

Early sailors thought the figure head would please the sea gods and bring protection to the vessel at sea. They also felt that a ship needed to find her own way across the water, and could only do so if she had eyes.

figurehead.jpg (12049 bytes) The ancient Egyptians provided both protection and eyes by mounting figures of holy birds on the bows; the Phoenicians used the heads of horses to symbolize both vision and swiftness; Greek ships had a boar’s head for both its quick sight and ferocious reaction; Roman ships often carried a carving of a centurion to indicate their fighting quality. In the thirteenth century one of the favorite figure heads was the head and neck of a swan, possibly in the hope that the ship would possess the same mobility and stability as that bird upon the water. Other figure heads over the years have been lions, leopards, antelopes, dolphins, unicorns, dragons, tigers, eagles and many men such as St. George slaying a dragon and King Edgar on horseback.

With the advent of the clipper ship, with her graceful lines, the figure head blossomed, usually into a single figure. Figures of women were more popular than men or animals and began to replace them almost exclusively. Although women were thought to be unlucky on a ship, it was thought that a woman as a figure head could calm the seas.

(I don’t make this stuff up, I just report it!)

(FYI – Figure head can be written as one word, figurehead, but according to my nautical dictionary figure head is the preferred use.)