As outboard motors began to increase in horsepower, speeds went up, and striking submerged objects became more dangerous. Manufacturers designed systems to handle the loads created from striking logs or other submerged objects, and ways to test those systems.

We (PGIC) cover log strike testing because the industry often uses log strike tests to evaluate propeller guards, most notably as a defense against the use of guards in propeller injury legal cases.

Part 2 of this post, Log Strike Testing Part 2 covers the testing of these systems at Mercury Marine.

Before we cover the history of log strike testing (in Part 2), we will first explain:

  • Variables and Dynamics of a Log Strike
  • Log Strikes With Manual Trim Systems
  • Conventional Shock Absorbers as Log Strike Systems
  • Hydraulic Trim Systems Are Challenged by Log Strikes
  • Trim Cylinder Design for Absorbing / Cushioning Log Strikes
  • Trim Cylinder Relief Valve Spring Rates and Preloads
  • Trim Cylinder and Outboard Shock Absorber Patents

Note, some people spell log strike as logstrike.


Variables and Dynamics of a Log Strike

Boaters strike all kinds of submerged objects. Log strikes look simple, but they are complex, high speed impacts. Some of the variables / dynamics surrounding a log strike or similar incident are:

  • Speed and mass of the boat, passengers, and gear
  • Mass and center of gravity of the drive
  • Size, dimensions, and shape of the log
  • Mass of the log including absorbed water
  • Added mass of the log – mass of water that must be displaced when the log is quickly accelerated by the drive
  • Height on the drive leg where the log was struck
  • Where the log was struck along its length and depth (with respect to its center line)
  • Hardness / stiffness of the log
  • How squarely the log was struck (glancing blow, obliquely, or straight on)
  • Attitude of the boat and trim of the drive

Log Strikes With Manual Trim Systems

Early outboards were small, lower horsepower units without hydraulic systems for trim and tilt. Some used a series of pin holes in the outboard mounting bracket (often a series of 5 holes) and a pin to set the trim. The pin actually sets the lower limit of trim. If the unit hit something at speeds typical of small outboards, the drive could ride up over the object and fall back down to the pin setting.

A trim locking bracket was either manually or automatically engaged in reverse on some models to prevent the drive from raising when in reverse. If a boater forgot to disengage the reverse trim lock and struck something while going forward, some systems included a spring that could be overridden and still allow the drive to swing up over the submerged object.

Similar approaches are still used on today’s small outboards.


Conventional Shock Absorbers as Log Strike Systems

As speeds edged up and outboard started to get bigger, designers turned to conventional shock absorbers, including hydraulic shock absorbers to dampen log strikes (slow the upward swing of the drive). Mercury (Carl Kiekhaefer) and others turned to normal shock absorbers designed to provide minimal resistance when the outboard started to swing up, then resistance became stronger as the outboard cleared the water. Similarly, they provided minimal resistance when the outboard first started its downward swing (to allow it to get to the water faster).


Hydraulic Trim Systems Are Challenged by Log Strikes

As outboards continued to gain horsepower and weight, trim systems became hydraulic for convenience, and to handle the increased weight of larger outboards.

When an outboard or stern drive with hydraulic trim strikes a submerged log at even moderate speeds, the trim cylinder cylinder rod(s) tries to extend and is immediately placed under tension, tremendous pressures are almost instantly developed in the rod side of the trim cylinder as the cylinder rod tries to extend. Without some means of relieving these pressures and allowing the drive to swing back, up, and over the log, either the boat would be stopped in an instant (ejecting everybody), some drive components would fail allowing the drive to at partially or fully break loose, the transom of the boat would be partially or fully ripped off, or some combination of disastrous alternatives.

With available options resulting in catastrophic failures and horsepower (speeds rising), designers searched for a way to absorb these loads. They began to incorporate some of the dampening methods of shock absorbers directly into trim cylinders.


Trim Cylinder Design for Absorbing / Cushioning Log Strikes

Today, outboards and stern drives typically use their hydraulic trim system to absorb the shock of impacting logs and other submerged obstacles. A common method is to build relief valves into trim cylinders.

Trim cylinders are damped by relief valves that allow their cylinder rods to extend (drive to swing up) when submerged objects are struck, then check valves allow the fluid to return to the other side as the cylinders retract as the drive comes back down.

Some of Mercury’s drives build the relief valves and check valves directly into the cylinder shock piston. The relief valve function is accomplished by classic ball and spring relief valves. Several of them are built into the piston to handle the high flow rates. Similarly, much lighter springs and balls are used as return flow check valves and built into the piston as well. Mercury also includes a “memory piston”, a floating piston that normally cycles back and forth with the shock piston. During a log strike, the memory piston stays put to mark (remember) the position of the shock piston before impact.

Memory piston is sometimes called a stop piston. Hydraulic cylinder is sometimes called a ram. Check valve is sometimes called siphon valve.

Trim Cylinder Design for Log Strike

Trim Cylinder Design for Log Strike

Over the years many designers have tried to minimize the additional length created by adding log strike protection, reduce system costs, and improve performance. Their efforts have resulted in a host of alternative designs, but they all try to accomplish the same tasks performed in the example illustrated above.

If a drive using the trim cylinder log strike system of relief valves and check valves illustrated above strikes an underwater obstacle:

  1. The resulting forces try to extend the cylinder rod but fluid cannot escape the rod end. The rod side pressure begins to rapidly spike (increase).
  2. The rod side pressure reaches the cracking pressure of the relief valves. Relief valves begin to crack and the rod begins to extend (fluid flows from the rod end to the cavity between the shock piston and the memory piston).
  3. As pressure continues to build, the memory piston stays put, the relief valves open more fully allowing the drive to quickly swing up (more fluid passes from the rod end to the now enlarging intermediate cavity between the shock piston and the memory piston as the rod extends and the shock piston moves along with it).
  4. When the propeller clears the water (breaks into the air) on the way up, the engine begins to rev up (no load on the propeller) and it continues to rev up until it hits the water again, or some governing system prevents it from over revving the engine.
  5. Although the drive quickly swings up, relief valves absorb much of the energy of the strike and slow the rate of upward rotation to prevent drives from breaking off when they hit the upper stop (however the designers chose to limit the upward rotation of the drive during a log strike).
  6. The drive reaches the end of its upward swing and the weight of the drive begins pushing on the cylinder rod (that had been in tension during the upward swing) and pressure begins to build in the cavity between the memory piston and the shock piston. Fluid is forced from that cavity through check valves in the shock piston back to the rod end of the cylinder where it came from just moments earlier).
  7. When the propeller re-enters the water, the engine RPM begins to slow down due to the increased load on the propeller.
  8. The drive begins to settle back down and comes to rest when the shock piston comes back in contact with the memory piston.

Theoretically the drive is right back where it was before the log strike and none the worse for wear.

Plus most of the events above happened in just fraction of a second. The speed and mass of the boat require the outboard to fly up in an instant or suffer catastrophic failure.


Trim Cylinder Relief Valve Spring Rates and Preloads

Trim cylinder relief valves are set by a combination of the spring constant “k” lbs/inch and the amount of preload (how many inches the spring is compressed when the system is at rest). The relief valve balls begin to crack open when the force on the portion of the ball exposed to pressure equals the amount of preload of the spring (how much many inches it was compressed times its spring rate). Per Teleflex’s U.S. Patent 7,722,418, trim cylinder relief valve springs typically have a spring constant between 200 and 400 lbs per inch. The springs are significantly preloaded (compressed) resulting in a high cracking pressure.

The Teleflex patent goes on to explain their approach of using a higher spring rate with lower preload minimizes variability and is more economical (fewer relief valves are needed). It also adds protection at slower speeds when the conventional approach (low spring rates with high preloads) might not result in enough pressure to crack open the relief valves, resulting in an instant stop and people being ejected. We spoke to one of the inventors at Teleflex when the patent was issued. He said they were not sure because they had not designed or tested it for this purpose, but it seemed to him like the high spring rate / low preload approach could reduce blunt trauma injuries to those struck by propeller guards. We have previously encouraged the U.S. Coast Guard and the industry to test Teleflex’s approach as a means of reducing blunt trauma to those struck by propeller guards (relief valves begin to crack open at lower pressures reducing blunt trauma or increasing the speeds before blunt trauma becomes an issue). So far our requests have fallen on deaf ears.

If the log strike system does not work properly, or key components mechanically fail (like outboard motor brackets, transom clamps, or swivel brackets), disaster may result. The tremendous forces induced by log strikes at higher speeds can actually rip drives off the transom, and catapult them into the air. They may even land back in the boat with the drive still running and the propeller still turning.


Trim Cylinder and Outboard Shock Absorber Patents

Many of the early trim cylinder log strike system patents were filed by Mercury (Kiekhaefer Corporation or Carl Kiekhaefer) and Outboard Marine Corporation (OMC). A few examples of log strike trim cylinder patents are listed below:

  • U.S. Patent 2,953,335 issued to Carl Kiekhaefer September 20, 1960 – conventional shock absorbers designed with minimal resistance till the outboard skeg is about free of the water. Similarly, little damping is provided at the start of the downward swing.
  • U.S. Patent 3,003,724 invented by Carl Kiekhaefer October 10, 1961
  • U.S. Patent 3,246,915 issued to Kiekhaefer Corporation April 19, 1966 – conventional shock absorber that provided minimal resistance at start of upward swing and and start of the downward swing
  • U.S. Patent 3,285,221 issued to Kiekhaefer Corporation November 15, 1966
  • U.S. Patent 3,434,449 issued to OMC March 25, 1969
  • U.S. Patent 3,722,455 issued to OMC March 27, 1973 – claims to be first hydraulic trim system to settle back down to the same trim after a striking a submerged obstacle. Uses a combination of trim and tilt cylinders to accomplish the task.
  • U.S. Patent 3,999,502 issued to Brunswick December 28, 1976 – includes “trail out” feature for low speed impact and a memory piston
  • U.S. Patent 4,050,359 issued to Brunswick September 27, 1977 – includes “trail out” feature for low speed impact and a memory piston
  • U.S. Patent 6,176,170 issued to Brunswick January 23, 2001 is the patent we took the illustrations above from. An attempt at being more compact.
  • U.S. Patent 7,722,418 issued to Teleflex May 25, 2010 for high spring rate, low preload relief valves to also cover low speed impact

Part 2 of this post, Log Strike Testing Part 2: A History of Log Strike Testing at Mercury discusses the history of the testing of log strike systems at Mercury (Kiekhaefer Corporation), at Kiekhaefer Mercury, and later at Mercury Marine, a Brunswick Company.


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