Research Paper

Mechanical Security for
Outboard Engines


This section is a general overview of the requirements for, and types of, Outboard Motor /Engine security locks.
The response by the author’s company in product design is illustrated in Outboard Motor Lock in Products.
The products have been developed for, and with the input from, the engine manufacturers, dealers and security test centres throughout Europe.

What is a good lock? A good lock consists of three elements:

(1) The lock mechanism fired by turning a key should be substantial and manufactured by one of the companies stated below:

Assa Abloy Finland/Sweden

Abus or Alko Germany

IFAM Spain

Squires UK

Most of these companies produce CEN6 top rated padlocks, the technology of which has been transferred to radial lock mechanisms that are drill and screwdriver attack proof.

In a marine salt water environment it is desirable for the lock mechanism to be created from either high nickel/chrome stainless or brass. A cocktail of various metals will produce GALVANIC ACTION once seawater is introduced. The mechanism will then seize up in as little as a month.

cross section of lock heads to show galvanic actioncross section of ock head ABUS

Further investigations with Abloy & Abus mechanisms in outboard motor/engine locks has shown that the materials used in their padlocks is not suitable in a marine salt water environment, observe the above grain structures!

(2) The material used in the lock structure should be 316 Marine Grade Stainless Steel. Any other material will either corrode or rot, especially coated steels!

(3) For our company's designs please refer to PRODUCTS.

So spend that little extra and buy a properly made lock that is easy to fit and gives you PEACE of MIND.
Outboard Motor Loc’s aim is to provide you with a reliable and effective deterrent that will last a lifetime.

An Investigation into radial lock mechanisms

Basic Knowledge

The metallurgical properties of the materials lend themselves to galvanic action if their contents are dissimilar. It is known that ferrous metals, treated or otherwise, corrode in salt water. Only when these (iron and steel) are allowed with corrosive resistant materials, such as chrome or nickel, do they resist salt water damage. The marine engineers prefer a grade of Stainless Steel to 316 standard for components used above the waterline. Stainless Steel to this specification is not magnetic.


The majority of quality marine locks used in Europe are made by Abloy, Abus or IFAM. The Alko small brass lock is not reviewed as it has a good track record for an economical item. The samples used were those recommended, or in use, and these were cast in clear resin as shown in the photographs below.

ABUS, ABLOY & IFAM lock cross sections

  1. The Abus mechanism used in STAZO/YAMAHA products is nearly all ferrous material that makes it very strong when new, but is highly susceptible to salt water corrosion.

  2. The ABLOY CL105 unit with an OML cam has a number of defects that lend it to be attacked by both salt water and galvanic action. It has differing grades of ferrous materials, with a lot of brass and unlike the other two, lots of space for water to lodge. The result as seen is that a key is seized in the mechanism after just three months of use in the West Indies.

  3. IFAM make their DC9 unit mostly of brass with an anti attack plate in the top that is an alloy steel and a 316 SS segment plate in the bottom. This is now the preferred lock of the author’s company and the modified version is fitted to OML products from 2009. Slight galvanic action occurred on previous models that used a ferrous bottom segment plate.


It is unlikely that a complete mechanism will be manufactured from 316SS as it will not be economically viable. In the meantime IFAM Spain produces the best alternative.

Percussion Caps for OML HSH Locking Heads

We have had a couple of reports that our locks have been attacked though their keyholes. This is a common activity, especially to the models that originate in Asia or the USA.

To combat this, we are manufacturing an interference fit cap that is now available on request. It will fit ATLANTIC, BALTIC, PACIFIC & VOLCANO products. It is aimed at the commercial customer as it requires drills and tooling to remove it once fitted.

The photographs below show the cap and where it is supposed to fit i.e. between the outer face of the lock mechanism and the inner face of the lock housing.

Lock heads and percussion cap

The data from members of the EEC and the Far East shows that outboard engine motor theft is continuing to be the most destructive manmade threat to both leisure and commercial boats. To combat this increasing trend, a number of companies have designed solutions that fall into three main categories. The locks should be purpose made which is the usual requirement for most insurance companies.


These are lengths of tube of either square or round section that have a slot cut into them. They are designed to fit over the clamping screw toggles once the engine is secured to its fastening surface. The cheaper versions have one size that fit all locks. As clamping screws vary in diameter from 8 -16mm it can be assumed that a 16mm slot on a 8mm screw thread is very loose and can be levered unless spacers or padding is provided. The advantage of the slot lock is that it can be quickly and easily fitted and removed needing the minimum of knowledge. Its disadvantage is that additional protection is required to safeguard attacks against the exposed threaded portion of the clamping screw. Further it may be used as a surface face to lever the outboard engine off using the top of the transom as the other fulcrum. 


These devices are usually built around a small shell like cartridge, the bottom of which has a hole in it and acts as a washer in the common bolt, nut and washer assembly. The top of the shell has a specially designed locking cap. Some preferred models are made of marine grade stainless steel that is either machined or cast. The outboard lock is attached on the inside of the transom by one of the hexagonal bolts supplied in the engine rigging kit. The nut used, must be a STIFF/NYLOC type, otherwise the outboard engine lock can be used to undo the assembly. This type of lock can be successfully used on most motors of 8hp and above and is the most cost effective and secure of the three types of designs. A few tools are required to fit the shell of the lock and cut the rigging bolt to the correct length, so this is best carried out by your engine dealer as part of the engine delivery service.



Thread guard in the position of clamp lock
Outboard engine lock honda


The clamp outboard engine lock was created to eliminate attacks to the one or both sides of the clamping assembly. They are usually considerably larger and more agricultural in their construction and hence cost nearly twice as much as the other designs. Shape wise they mostly resemble rectangles or squares.

Recently their popularit
y has increased because a number of engine manufacturers have increased their toggle size resulting in the fact that some slot locks no longer are large enough in cross section area to accommodate the toggles, hence useless.

All the designs necessitate that part of the lock remains fixed to the transom. This could result in it going missing if the owner does not remove it with the engine. As these locks are substantial structures the only area where additional protection can be achieved is if the bolt/screw that is provided to fix the body to the transom be made of 6mm diameter material and be captive as an integral part of the transom.

Clamp Lock Assembly
Outboard engine clamp lock


ideas on transom plates

Slot locks by their very design offer a thief a lever point for a crowbar between the lock and the transom to lift off the motor. To overcome this a dormer drill can be used to countersink the washer area on the transom to a depth of 5mm. An alternative is to get a 4mm thick steel plate made that picks up on the clamping washers and cut two large holes in it, then bolt or screw the plate to the transom.

Dormer drilled recesses
kit 1
Commercial Kit 1
for small outboard engines

Another attack point is the exposed clamping screw threads between the lock inner face and the transom. If one measures the exposed thread when the engine is in-situ and tight against the transom. Then unlock and cut a clearance stainless steel tube to a slightly smaller length than the measured length , remove the washer, fit the tube, then repeat the process to secure the engine.


Most manufacturers rigging kits are supplied with hexagonal headed bolts. Some have stiff nuts and previously stated bolt/nut locks must be secured with a stiff or nyloc nut. Further, it is desirable to protect the exposed bolt head on the outside of the transom as these can be ground off by modern tools. It is suggested that other types of bolts be used as once the hexagonal head has been ground away, the attacker has simply to punch the shaft of the bolt through its hole with the lock still attached to the other end. The deterrent is to use a coach bolt, step bolt or countersunk bolt.

Recently it has been reported that well organised gangs are starting to attack the bolt locks with parallel sides by using jemmy tubing techniques. This results in the shearing of the manufacturers rigging bolt.Two ideas to improve existing outboard engine locks would be either to weld a large washer just below the top of the lock housing or alternatively weld a tri-angular shaped web to one side of the housing.
These modifications would prevent tubes being placed over the lock. Other solutions are firstly to increase the base area of the bolt lock shell by welding a large washer to it, so that far more leverage will be required for the base to penetrate the surrounding structure. The alternative to this additional engineering is to make a lock shell with sloping sides like OML's VOLCANO to be marketed in 2006.
Volcano bolt lock

       VOLCANO shell in-situ.


The types of outboard engines that benefit from the solution described below are those that are too small to bolt initially to the transom. A situation exists where hire boat companies generally use engines of 4-6hp on their rowing boats or pleasure boats. Some of these boats have electric motors that do not use conventional clamping styles that mate readily with current slot locks. The method suggested is to weld a plate across both clamping brackets. This forms the basis for a coach, step or countersunk bolt to be fitted. On the nut end of the bolt there is either a substantial 50mm diameter washer of 5mm thickness with a standard nut tightened and welded, or a BOLT LOCK.

alan key

The other area of attack is the bolt that attaches the clamping bracket to the outboard engine. Its nut end can either be welded or a small bolt/nut lock can be used to deter attack. The problem with welding is that the manufactures warranty will probably be broken, but hey, this is minimal compared with engine theft!


Much has been written about how good or bad various locks are. Little if nothing has been suggested for transom protection. A few instances have occurred where transoms have been chain sawed, or the rubber structure of inflatable dinghy has been cut with a shape knife for the removal of the outboard mounted there upon. The author prefers a clandestine approach by the boatbuilder. During moulding/ laminating they should insert strands of piano wire across the transom to form a grid. Once finished the boat builder should ensue a notice is placed on the transom Armed Transom to cover any liability. Another solution that is more common is to bend a U channel made of 3mm stainless steel plate and place it across the top of the transom. Ensure that it is bolted and the bolts are countersunk and welded.

kit 2

Written by Tom Snook
Darrian Lilley, Managing Director of Outboard Motor Lock Ltd. 2010      916-740-4733