On The Escape of Tigers: Haddon & Bass Boat Propeller Safety
On The Escape of Tigers: an Ecological Note.
by William Haddon, Jr.
American Journal of Public Health.
December 1970. Vol.60. No.12. Pages 2229-2234.
The the article has since been widely republished and reprinted.
The thesis of the article is that per Mr. Haddon, by 1970 The United States and most developed nations had made significant progress against “living environmental hazards” (medical issues caused by living organisms such as germs, bacteria, etc.). However, similar progress had not been made against “non-living hazards” (accidents leading to injury or death).
The field of Medicine had many strategies and processes by which to attack emerging problems causes by living organisms (such as the Zika virus in current times). While science may not immediately defeat the Zika virus, the basic processes and methods to use to begin to develop a means to combat such a threat are well known.
That same basic structure of processes by which to develop a means to combat an emerging health risk caused by a living organism did not exist to combat a health risk caused by a non-living structure (such as some new type of accident that injures or kills humans).
In his paper, Haddon tried to bring structure to the process of mitigating or eliminating accidents resulting from the sudden release of energy.
In order to better marshal resources against these “non-living hazards”, William Haddon, Jr. suggested that many of them result from “the transfer of energy in such ways and amounts,and at such rapid rates, that inanimate or animate structures are damaged. (Like a bass boat outboard motor striking a submerged object, breaking off, flipping into the boat, and striking those on board with its still rotating propeller). Haddon goes on identify some harmful “non-living hazards” that interact with people and property as: hurricanes, earthquakes, projectiles (like an outboard motor), moving vehicles (like a boat), ionizing radiation, lightning, conflagrations, and notes the cuts and bruises of daily life illustrate our interaction with the rapid transfer of energy.
The paper’s title derives from one example of the quick release energy, the release of tigers.Large outboards on bass boats striking submerged objects can pounce on those on board as quick as a tiger.
Haddon’s paper culminates with ten strategies to reduce losses from the quick release of pent up energy. Examples are cited for each strategy.
The first 4 are applied “before the event” to stop the event or minimize damage caused by it, the second 4 protect the person or thing during the damaging event, and the last two have to do with minimizing damage after the event.
William Haddon, Jr.’s Ten Strategies
1. Prevent the marshaling of of the form of energy in the first place. Such as preventing the manufacture of gunpowder.
2. Reduce the amount of energy marshalled. Such as reducing the size of bombs or firecrackers, or not storing large quantities of gunpowder or munitions at on site.
3. Prevent the release of this energy. Such as preventing the discharge of gunpowder, prevent the falling of elevators, prevent the escaping of tigers, etc.
4. Modify the rate of or spatial distribution of release of energy from its source. Such as slowing the burning rate of explosives.
5. Separate in space or time the energy being released from the susceptible structure whether living or inanimate. Such as the use of sidewalks and the phasing of vehicular and pedestrian traffic. Haddon says this strategy has as its hallmark, the elimination of intersection of energy and susceptible structure.
6. Separate the energy being released and the susceptible structure whether living or non-living by a barrier. Such as safety glasses, thermal insulation, shin guards, helmets. armor plate, boxing gloves, etc. Note, some barriers such as fire nets and impact guards attenuate or lessen the energy. They do not totally block the energy from the structure (living or non-living) to be protected.
7. Appropriately modify the contact surface, sub surface or basic structure in eliminating, rounding and softening corners, edges, and points which can eventually come in contact with people. Such as use of a large radius of curvature and softness on the object that will contact people.
8. Strengthen the structure, living or non-living, that may be damages by the energy transfer. Such as the physical fitness training of athletes or soldiers.
9. Move rapidly in the detection and evaluation of damage that has occurred or is occurring and counter its continuation and extension. Fire sprinkler systems and fire alarms.
10. All measures taken between the emergency period following the damaging energy exchange and the final stabilization of the process and long-term reparative and rehabilitative measures. Such as returning to pre-event status or stabilization of structurally or functionally altered states. Basically making things save again.
Haddon notes there is no order of priority to the strategies above. They can be applied in any order.
Applying Haddon’s Strategies to Bass Boats Striking Submerged Objects
Forty-five plus years ago William Haddon, Jr. wrote a guide to preventing living or non-living structures from being damaged by the rapid release of energy. While his paper has very broad applications, it also applies directly to preventing large outboard motors from striking submerged objects, breaking off and flipping into bass boats (and other boats as well).
We strongly encourage those trying to address this problem to closely study the article for inspiration.
Note – actual impacts with submerged objects are very dynamic. Statements below are generalizations.
If there were no log strike system and the outboard motor could freely swing up, the kinetic energy of impacting a submerged object with the outboard motor would be 1/2 times its mass time its velocity squared. If the outboard motor was locked down and not allowed to flip up, the kinetic energy of the impact would be calculated using the mass of the boat plus the mass of the outboard motor. If the log strike system holds down the motor too much early in the strike and prevents the outboard from raising fast enough to clear the object as the boat goes across it, the mass of the boat can add to the impact force.
Just a quick look at Haddon’s Ten strategies immediately identifies several opportunities, some of which have been explored and some of which have not.
We are not saying all these solutions are practical or would be accepted. The purpose of this exercise is to show the ability of Haddon’s ten strategies to quickly inspire some approaches.
The table below heavily draws from a similar table for combating Ebola in Wes Chapman’s Blog titled, Considering the Ebola Outbreak in Haddon’s Matrix and Reason’s Swiss Cheese Model. October 21, 2014.
|Strategy Number||Haddon General Strategy||Bass Boat Strategy|
|1||Prevent creation of the hazard||Use in hull jet drives, use tunnel drives, use shaft drives, limit maximum speeds to below break off speeds|
|2||Reduce the amount of the hazard||Use smaller twin outboards that total to the same horsepower (smaller mass, lower energy impact with submerged object), limit maximum speeds, run aluminum propellers (lower weight and less upward rotational energy after impact)|
|3||Prevent release of the hazard||Mark location of floating and submerged hazards, properly mark dredge pipes. A system alerts operators to submerged objects in time for the collision to be avoided. If submerged objects are not struck, most large outboards will never enter the boat. The use of warnings could prevent the release of energy in some instances. Educating bass boaters of the hazard and the capability of their boat would prevent the release of energy in some instances. Increase minimum break off speeds by design.
See our Design Chart.
|4||Alter release of the hazard||Design the lower portion of the outboard to shear off in high speed collisions (like Volvo Penta IPS and Brunswick Zeus pod drives). Drives sheared off low do not enter the boat.
See our Design Chart.
|5||Separate person and hazard in time and space||Make the rear deck of bass boats a few feet longer (or move seating further forward) so the outboards cannot reach seating area when they break off. Or us a standoff box to put the outboard still further back (like a 2.5 foot setback jack plate). Consider trim tab(s) that auto-deploy when the outboard rips off to keep the boat on trim so it does not slow down as rapidly and the outboard may not catch up with the boat.|
|6||Place barrier between person and hazard||The Leash does this by tethering the outboard to the transom. Mercury Marine, Outboard Marine Corporation, and Suzuki have used large metal fences / barriers to prevent outboard motors from flipping into boats during log strike testing. Similarly, existing cables (steering and shifting cables) could be modified for use as tethers. Some have ran a cable around the back of the outboard and tied it to stern eyes on each side of the outboard. They run the cable through a hose to prevent it from scratching the outboard.|
|7||Modify basic qualities of the hazard||Use a safety propeller such as the Australian Safety Propeller, inflate an airbag around an outboard once it breaks off, inflate an airbag around passengers once the outboard breaks off.|
|8||Strengthen resistance to the hazard||Passenger wears a helmet capable of protecting their head against a rotating propeller|
|9||Detect quickly||That is what traditional log strike systems do. They could do better if they could detect when the outboard is about to break off before it has cleared the object and then allow the outboard to clear the submerged object with minimal resistance, then apply the forces needed to retard its upward swing before it reached maximum rotation of the outboard.
See our post on Active Trim systems.
Lure more doctors, nurses, and paramedics on the water (offer them discounts on boats, marina slots, fuel, buy their lunch, etc.). More medical personnel on or near the water can help keep the injured alive till officially summoned first responders and paramedic crews arrive, likely with more tools and resources at their disposal.
|10||Repair the damage||Have more doctors on the water, major trauma centers near lakes, improve the ability to more quickly find boats on the water involved in accidents, improve communications from boats to paramedics en route, helipads near the lake, life flight service coverage of major lakes, provide first aid training at boating events that includes what to do for severe boating injuries.
Making sure the accident is reported to the state boating law authority and the verbal description as reported by the state includes the outboard breaking off and entering the vessel while under power helps safety professionals better identify, monitor, and investigate these accidents.
Mental, personal, and economic damages associated with propeller accidents include: mental stress, trauma, personal injuries, grief, prosthetics (adapting to use, cost), rehab, medical costs, lost wages, rent, the mortgage, insurance issues, depression, failed marriages, continued surgeries, physical limitations, etc. Many of these challenges take a long time to repair, if ever.
The William Haddon Jr.’s article is said to have been originally published in:
On the Escape of Tigers: An Ecologic Note
William Haddon, Jr.
Edited by Massachusetts Institute of Technology
Vol.72. NO.7. May 1970
as originally published it includes additional examples of loss-reduction tactics and the strategies they represent.
Another version featuring the same ten strategies:
Energy Damage and the Ten Countermeasure Strategies.
William Haddon Jr.
The Journal of Trauma
Vol.13. NO.4. (1973)