Propellers are made from various different materials. Most outboard props are made from stainless steel, aluminum, or type of composite. Most inboard props are bronze, nibral, or stainless steel. Nibral is a very strong material made from NIckel, BRonze, and ALuminum (NI-BR-AL).
To determine the diameter of a propeller -- you must measure the distance from the center of the hub to the tip of a blade and then multiply that number by 2. When describing a propeller size, the first number is always the diameter.
The propeller pitch is the theoretical distance the propeller travels forward through the water during one revolution. When describing the propeller size, the second number is always the pitch.
Propeller rake is the angle of the blade in relation to the hub. Forward rake helps to bring the bow of the boat down. Aft rake helps to lift the bow upwards.
The cup is a curved lip on the trailing edge that allows the prop to get a better "bite" on the water by holding the water on the blade longer. It will typically reduce the amount of slippage and ventilation that occurs.
Boat Propeller Cavitation
Cavitation is caused by tiny, microscopic bubbles collapsing due to a reduction of pressure on the back of the propeller blade. It can cause physical damage to the propellers blade surface. There are many causes of cavitation like : damaged blades, wrong pitch, incorrect propeller, etc.
Ventilation occurs when air or exhaust gases are pulled into the propeller blades causing high RPMs and loss of speed. Possible causes are an over trimmed engine, a motor that is mounted to high on the transom or very tight cornering.
How to Determine the correct propeller size
1. Determine the rpm range your motor is rated to run. This information can sometimes be found in your owners manual. If not, consult the manufacturer they should be able to tell you.
2. Now test run your boat at wide open throttle with an existing propeller being careful not to exceed the manufacturers recommendation. If your rpm's are too high, you need to go up in pitch. If they are to low, you need to go down in pitch. Every 1" increase in pitch typically results in a 150-200 rpm reduction. Similarly, 1" decrease in pitch will gain about 150-200 rpm. With this information you can determine or approximate the correct size.
Some propeller repair Q & A
Q: How much pitch can a propeller be changed?
A: To avoid damage to the propeller usually no more than 2".
Q: Can the rotation of a propeller be changed?
A: Sorry, it can not.
Q: How can I tell if my hub is bad?
A: Usually if your hub is bad, you will have high RPM's and your boat will not go anywhere when you try to accelerate. If it seems to be intermittent you may want to remove the prop and mark the end of the hub and the propeller housing with a center punch keeping the marks in-line with each other. Then replace the prop and try it out. If the hub is slipping the marks will be out of line.
Q: Are all propellers repairable?
A: Most of them are but unfortunately not all propellers are repairable. Especially the composite or plastic props, props with severe electrolysis, props made from poor material or a prop that is just to thin to work with.
Propeller Safety -- Preventing Propeller Accidents
(1) Boat propeller injuries have been described:
"Motor propellers can inflict severe, devastating injuries that result in death, loss of extremities, and severe permanent deformity, disfigurement, and/or disability. These the injuries surpass by far those seen in other motor vehicle accidents, and some authors (Hummel) compare the resulting wounds with those seen on the battlefield."
(2) The United States Coast Guard (USCG) Office of Recreational Boating Safety (RBS) has looked at this problem since 1978 when a statistician suggested a propeller guard could provide protection in 600 to 3000 propeller accidents per year.
(3) The l989 National Boating Safety Advisory Council (NBSAC) sub-committee report looked at the universal application of propeller guards and felt they were not suitable for all boats, and therefore recommended them for no boats. Instead they recommended "the most rational approach to the problem is to educate boaters, especially operators." Seventeen years later, there is no education standard to teach propeller accident prevention.
(4) The USCG Office of RBS statistics - fatalities and injuries.
2002 - 239 struck by propeller reflected a 19% probability of fatality
2003 - 266 struck by propeller reflected a 12% probability of fatality
2004 - 186 struck by propeller reflected a 17% probability of fatality
(5) The National Association State Boating Law Administrators (NASBLA) comment in 2002 on NPR 2001-10163 supported education and legislative efforts in lieu of mandatory guards. This standard would address this alternative.
(6) Since l978 a disproportionate number of children have been victims to propeller strikes and an increasing number of rental boats (13%) are involved. According to the USCG 2002-2004 statistics, houseboat and pontoon rentals represent 39% of all rentals involved.
(7) The USCG statistics, 2002-2004, determined the most at risk boats: outboards (51%) and stern-drive (30%) and targeted the most at risk boat lengths from 16 to 26 feet.
IT IS WHAT YOU KNOW .... sometimes
1. Know that a 13" propeller blade can travel from head to toe on an average person in less than one tenth of a second. A typical three-blade propeller, running at 3200 rpm can inflict 9600 impacts in one minute or 160 impacts in one second.
2. Know that too many children die of this accident or are disfigured for life. Your children are supposed to outlive you. According to SPIN research only, in the summer of 2004, more than 50% of the deaths/injuries by propeller were children 17 and under. In 2005, more than 35% of the deaths/injuries by propeller, from data available to SPIN research, were children 18 and under.
3. Know what propels your boat. As in Carbon Monoxide (CO) deaths, what you cannot see can harm or kill you or your loved ones. Show ALL your passengers where the propeller is located. Indicate the location with bold warning labels. The State of Utah has the most dramatic label message for propeller and CO and generously provides them, upon request (Utah State Parks and Recreation 801-538-7341). SPIN also distributes these labels. Locate the propeller in relationship to the boarding ladder.
4. Know that once you are in the way of the spinning propeller, it is no longer what you know, it is TOO LATE.
WHO IS AT RISK?
Experienced and inexperienced, educated and uneducated, old and young, swimmers, boaters, operators and passengers, divers, PWC riders. EVERYONE!
FIRST LINE OF DEFENSE - EDUCATION
1. Get an education from a National State Boating Law Administrators Association (NASBLA) approved Boating Course (refer to www.nasbla.org). Courses are offered by the United States Coast Guard (USCG) Auxiliary; Power Squadrons; many States, and the Marine University of Fort Lauderdale Florida.
2. Insist on thorough training at the point of rental.
3. Research and obtain additional training before you purchase or operate a boat.
4. Make sure propeller safety is thoroughly understood by all before boarding a vessel.
SECOND LINE OF DEFENSE - AVOID PROPELLER THREATS
1. Bow Riding - especially on pontoon boats
2. Swimming in or around any boat engine (Carbon monoxide also poses a huge danger)
3. Sliding off Houseboat slides
4. Sitting on the swim platform or ladder
5. Falling Overboard listed as #1 cause of deaths and injuries again in 2005, according to the USCG. Stay in or with your BOAT! Wear personal flotation devices or life jackets. Drowning is still number 1 consequence of accidents/incidents. Look into the inflatable type devices that also allow you to get plenty of sun and movement while you remain safe should you fall overboard.
6. Jet Skiing/personal water craft (PWC) - Be very alert to traffic and wakes. PWC riders are often victims of propeller strikes. Make certain your PWC is equipped with off throttle steering controls to help avoid collisions and encounters with boat propellers.
7. Teak-surfing, tubing, water skiing. Do not teak surf. This puts you at close proximity to the propeller and in danger of CO asphyxiation. Tubing and skiing ropes may catch into the propeller and pull you into the blades.
8. Boating Under the Influence. No one deserves the consequences of impaired judgment and poor reflexes caused by sun & alcohol. Drink lots of water.
Knowing about these dangers are important. However, be aware of the safety equipment which is available. Installed safety technology/equipment provides the highest level of security when judgment and education or circumstances fail.
THIRD LINE OF DEFENSE - SAFETY TECHNOLOGIES
1. Kill Switches also known as emergency shutoff switches or lanyards - Many boats since 1995 provide lanyard engine shut off, or "kill", switches as standard equipment. Older boats can be retrofitted with them. In September 2004, NASBLA passed the Model Act for all states to adopt a law mandating the use of the lanyard or remote wireless kill switches on boats such equipped. 2006 resolutions by the NBSAC have recommended that the USCG proceed with two regulations: (a) mandatory wear of the shut off switch; and (b) make it mandatory that all new boats manufactured be equipped with a cut-off ("kill") switch.
2. Wireless Shut off switches. A remote system now available to give you more freedom. Shuts down the engine and eventually stops the propeller if ANY passenger goes overboard.
3. Ladder/Gate Interlocks to prevent common boarding accidents - propeller is frozen until the ladder or gate is unlocked, giving your assigned "lookout" the opportunity to check around the stern, reducing accidental engaging of the engine.
4. Anti-feedback steering greatly reduces the threat of the circle of death.
5. Propeller Guards- Cage-Type Guards or Ring-Type Guards, as applicable to type of vessel and environment. Please note that on a pontoon boat, if one falls from the bow of that boat there is no escape from the propeller. If you must have a pontoon boat, a cage, a ladder interlock system, and a remote shut off system-combo is your best defense.
6. Jet Drives have NO PROPELLER.
REVIEW OF COMMON SENSE STANDARD OPERATING PROCEDURES
Do a head count before start up.
Assign a responsible lookout.
Assign a seat to each occupant.
Never bow ride or sit on gunwale, transoms, seat-backs, etc.
Always engage your Lanyard Kill Switch between the Ignition and the Operator's person or Always engage the remote shut off system and have each occupant wear a sensor.
Never start a boat with the engine in gear.
Never board or disembark a boat when the engine is on, i.e. idling. The propeller is still rotating.
Be aware of extreme waters or weather conditions.
Be aware of congested areas and designated swimming zones.
Know how to recognize divers' flags or buoys.
Be smart about high risk activities: water skiing, inner tubing or riding a PWC (the "rapid transit" into a propeller)
NEVER operate a boat under the influence of alcohol or drugs.
PERSON OVERBOARD - PROPELLER SAFETY PROCEDURES
Turn the bow of the boat toward the person in the water; keep the prop to the rear.
Slow down, circle around, and keep the individual in sight at all times
Take the engine out of gear and turn off the engine at least a boat length from the victim.
Tie a line to a float and pull the person toward the boat.
Practice this procedure, then practice it again.
Remember turn the boat opposite of a car to keep the prop away from the in-water person.
Have the designated LOOKOUT alert and in position watching for any sign of danger.
Seems like a cake walk until you are in rough waters and under stress, tired, overheated or the person overboard is the most important person in your life.
PREVENT -- PREVENT -- PREVENT
BOAT SMART -- BOAT RESPONSIBLY
YOU'RE IN COMMAND