Modeling Boat Motor Log Strikes – A Senior Design Project

The boating industry has long been aware that marine drives can strike submerged or floating obstacles. In the early days they conducted on water log strike tests. In more recent times, some have moved to dry land impact tests (log strike test stands), and even considered virtual log strike tests (computer simulated log strike tests).

Old Mercury Outboard Log Strike Test

Old Mercury Outboard Log Strike Test

They moved from on-water testing to the dry-land impact testing shown below.

Log Strike Test

Mercury Marine Log Strike Test

As we understand it, the progression from on-water testing to dry-land impact testing was primarily verified by adjusting the dry-land impact masses to achieve similar failures to the on-water tests with the standardized logs at similar speeds. Dry-land impact facilities cannot be quickly dialed up to achieve a collision with a different kind of log, length of log, diameter or log, geometry of log, log submerged to a different depth, etc. The actual force X time profile may be quite different for a dry-land impact test than for an on-water log strike.

This progression (on water testing to test stands, to considering using simulations) was made without a total understanding of the science and mechanics behind log strikes.

We encourage Senior Design Projects, Sr. Thesis, Masters Thesis, Capstone, and Masters Degree projects on the science behind log strikes (the striking of driftwood, stumps, dredge pipes, and other floating or submerged obstacles by recreational boat outboards and stern drives). A better understanding of this science will allow more accurate designs and testing, resulting in safer boating.

We recently came across a technical paper that lays a tremendous amount of groundwork in this field, however it focuses on bow strikes of large driftwood by larger high speed vessels. The boating industry could greatly benefit from a rework of the paper to focus on recreational marine drive strikes of driftwood (logs).

The paper by is highly mathematical and needs someone with a good background in math, engineering, and design to convert its findings to a recreational boat perspective.

We suspect the topic could support a series of three separate projects:

  • Directly convert the Yasumi Toyama’s drift-wood paper (reviewed later in this post) to a marine drive / recreational boat log strike perspective.
  • Take those findings and integrate them with a generic log strike system design (show how log strike forces would vary from just hitting a log with minimal give in the system, to hitting one with a log strike system with relief valves to absorb the energy of the log strike).
  • Compare those results with the forces developed in today’s dryland impact tests (log strike test stands).

The Science Behind Log Strikes

Most modern larger outboards and stern drives include a log strike system to absorb the energy of striking a log, drift-wood, or other underwater obstacle.

The amount of energy to be absorbed depends on several variables including:

  • Size and mass of the log
  • Speed of the boat
  • Mass of the boat (and its contents including people)
  • Inertia of the boat
  • How far the drive extends down into the water
  • Angle of the leading edge of the drive
  • Center of gravity of the drive
  • Mass of the portion of the portion of the drive that rotates (swings up)
  • Rotational Inertia of the drive
  • Current planing angle of the boat (angle of bottom of boat with respect to surface of the water) which may also be partially due to waves or wakes
  • Submergence of the log (percent submerged, or depth below surface for fully submerged logs)
  • Stiffness / Flexibility of the log
  • Length of the log
  • Diameter of log
  • Perpendicularity of log to oncoming boat
  • Geometry of the log (major branches) and orientation of that geometry to the oncoming boat
  • Amount of water soaking of the log (increases its weight and submergence)
  • Where log was struck along its length (near one end, in the middle, etc)
  • Type of wood (oak, pine, ash, etc)
  • Crushability of the log. As the drive crushes into (indents / cuts into) the log, the contact time is extended, reducing peak forces
  • If boat is at or very near surface, the boat may ride up over the log, raising the drive with respect to the log
  • Virtual mass or added mass of the log (the additional water that must be accelerated if the log is accelerated)
  • If the log breaks or not (breaking logs limit peak forces)
  • Propeller thrust at time of impact (is said to be of negligible importance because it pales in magnitude against the momentum of the boat)
  • Mounting structure of the drive (geometry and rigidity)
  • Flexibility of the transom
  • Coefficient of Restitution for log – drive collision. The ratio of differences in post collision speeds to differences in pre collision speeds, (Vboat post- Vlog post)/(Vboat pre – Vlog pre)
  • Behavior of any log strike systems involved

While many variables are involved, the overpowering variable for most situations in which the log does not break, is boat velocity because the energy of the collision is directly proportional to the square of boat velocity.

Logs come in all kinds of sizes and shapes, some may still have lots of limbs and smaller branches attached, while some may be predominately shaped like a log / telephone pole.

In the early days of log strike testing at Lake X, Mercury used 12 foot long, 12 inch thick machined logs for consistency in test results. Much of today’s testing is done on test stands called Dry Land Impact Test stands. They propel a lead “log” at a stationary transom with a drive mounted to it.

While the exact specifications of early day log tests and present day log tests are a bit murky, especially to outsiders, we suggest many insiders think a drive that passes a typical dryland impact test should still be operational after striking a fully submerged 12 foot long 12 inch diameter log in the middle at about 35 mph.

If the drive was rigidly mounted (no log strike system) the amount of energy exchanged in such a collision would depend on several of the variables mentioned earlier.

As long as the log does not break, impact forces rise as:

  • The log becomes more perpendicular to the oncoming drive
  • The more towards its linear center the log is struck
  • Log diameter increases
  • Length of the log increases up to a certain point, then additional length fails to increase impact, primarily due to flexibility of of the log
  • The deeper the log is submerged as long as it completely engages the drive (has to move more water out of the way when it is accelerated)
  • The less the log is crushed on impact (crushing extends contact and reduces acceleration)

We reviewed the literature and found an excellent paper on larger high speed vessels striking logs with their bow:

  • Drift-wood Collision Load on Bow Structure of High-speed Vessels. Yasumi Toyama. Marine Structures. Vol.22. (2009) Pgs. 24-41.

Toyama investigated the impact forces of driftwood against the bow structure of larger high speed vessels. He estimated impact loads for different sizes and craft speeds while allowing for whipping motions of the driftwood and local crushing near the hitting region. The strong similarities of this situation with a drive striking driftwood led us to create this post.

Review of the Toyama Paper

A review of “Drift-wood Collision Load on Bow Structure of High-speed Vessels” by Yasumi Toyama in light of its application to outboards and stern drives.

Toyama notes at speeds over 30 knots, impact loads can become substantial because they are proportional to the square of boat velocity.

He uses finite element analysis in the time domain to to see if the bending moment is large enough to break the driftwood when struck by the 45 degree slope of the bow.

Larger vessels do not slow down when they strike driftwood because their mass is so much larger than that of the driftwood resulting in the driftwood being pushed forward with velocity V accompanied by an added mass of water. This assumption does not directly hold for stern drives and outboards as their log strike system will begin the engage after striking larger logs, allowing the drive to swing up before the log is moving forward at boat velocity (at speeds below the log strike system design speed). In addition, a small boat may begin to slow down during striking a large log or other obstacle. Testing would need to be done to gain a better understanding of the dynamics for outboards and stern drives.

Toyama assumes the bow is rigid and does not get crushed itself. That should apply to both stern drives and outboards (the drive is an incompressible casting).

Toyama derived a simple formula for the prediction of impact loads based on assuming the driftwood dynamically responds like a loaded beam. The reduction of section modulus due to the log being partially crushed by ship is taken into account. Dynamic whipping of the driftwood (beam) is important to the distribution of the bending moment and to its maximum value. Toyama conducted a Finite Element Analysis (FEA) in the time domain to identify when and where the bending failure takes place (if the beam breaks).

Toyama also investigated the effect of added mass varying with time as the driftwood whips up and down while in contact with the 45 degree bow of a vessel. The added mass coefficient varies as a function of the submergence of the log. It gradually increases from zero to one as the log goes from floating entirely out of the water (not possible) to being submerged with the top of the log being twice its diameter below the surface. A log floating to where you can just see its top breaking the surface of the water has an added mass coefficient of about .6.

Historically, the defense in propeller accident trials argues the submerged object has a coefficient of added mass of one, but this is not true in many situations (less water has to be moved because the object is so close to the surface of the water).

Toyama then developed nondimensional curves showing the projected crushed area vs. crushed depth. Then he went on to explore the natural vibration modes of a cantilever beam fixed at the midpoint (where it hit the bow). He then compared the results from an FEA analysis with those of his modal analysis and found fairly good agreement.

As a result, Toyama was able to plot maximum impact loads as a function of vessel velocity. When maximum impact load is less than the critical bending moment (point at which the log breaks), the log may experience some crushing, but will just be pushed along by the vessel until it eventually goes on around or under the vessel.

One very interesting and very applicable finding was that maximum impact load is almost independent of length when log diameter is less than or equal to .5 meters (about 20 inches). These logs are flexible enough that localized bending absorbs the impact v. trying to push a much thicker stiffer log.

This is an excellent paper and we strongly encourage the industry to join us in trying to encourage a masters student to re-work this paper with an emphasis on outboard and stern drive applications. The paper is highly technical and very insightful, is just needs to be redirected to this specific applications, then accompanied by a less technical “applications of this paper” narrative.


In addition to the Toyoma paper, there are several other excellent references that could assist the work in better understanding the science behind the forces generated by log strikes. Tsunami driftwood flows and iceberg strikes also bear striking similarities to log strikes so we listed some of their papers here as well.

  • Drift-wood Collision Load on Bow Structure of High-speed Vessels. Yasumi Toyama. Marine Structures. Vol.22. (2009) Pgs. 24-41. The paper is available from Elsevier on ScienceDirect
  • Crash Prediction for Marine Engine Systems. Arden A. Anderson. Mercury Marine. Presented at the 2008 Abaqus User’s Conference. Details Mercury Marine’s efforts to test computer models of drives for log strike impacts.
  • Method For Estimating Collision Force of Driftwood Accompanying Tsunami Inundation Flow. Hideo Matsutomi. Journal of Disaster Research. Vol.4. (2009) No.6. Pgs. 435-440. This paper also lists several Japanese papers in this field.
  • Impact Forces from Tsunami-Driven Debris (In-Air Impact Study). Naito, Riggs, Kobayashi, and Cox. Presentation and technical paper.
  • Load Estimates for Ship Damage Due to Impacts With Icebergs. Prepared by Westmar Consultants for National Research Council of Canada. PERD/CHC Report 20-60. May 2001.
  • Log Strike Testing Part 1
  • Log Strike Testing Part 2

We encourage students with solid math, engineering, and design backgrounds to consider making Modeling Boat Motor Log Strikes their Senior Design or Capstone project. Please contact us if you are interested.

We also encourage any boat or drive manufacturers that might be willing to assist these projects by the donation of funds, components/parts, use of a boat, use of some test equipment, use of a test facility, project mentoring, additional reference materials, branded clothing (college students always like caps, jackets, t-shirts, etc.), a plant tour, or anything else that might help to please contact us so we could put interested college students in contact with you.

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