Marine Forensic Technicians
Marine Forensic Technicians, Stevensville, Maryland - James R. (Randy) Renn, SAMS® AMS® Marine Surveyors
Stevensville, Maryland

James R. (Randy) Renn, SAMS® AMS®
Principal Marine Surveyor
Damage & Insurance Loss Specialist

Worldwide Service




Flexible Couplings, Love And Logic


"Good Vibrations" we can't stop the music, maybe we can turn down the volume"

Hello very busy boaters! The weather person said this morning that the lower 48 would have temperatures between 78 and 109 degrees. With that data we can bet spring commissioning is pretty well over and you have had your favorite water craft out on the pond. Ready with sun block, bottles of water and loaded with enthusiasm for the upcoming summer. Motoring along you say to your boating partner "would you please turn up the music I can't hear the radio" "what did you say? I can't hear you over the music and rattling hatches" is the response. Well now what has happened now you think. Has something changed? Hearing aid batteries low? Can I blame it on ethanol? In past articles we have touched on several specific subjects and procedures. In this article we will address a specific item that was very rarely installed in older boats-mostly aftermarket and not seen often in new production craft. This article will discuss some versions of the "flexible coupling for propeller shafts" and what they can do, in some cases will do and some cases can not do. Properly installed "flex" couplings can be a silent, nearly maintenance free, positive amendment to your boating experience and we love them and what they do but logic must be employed as this is not the simple fix some may believe.

What are "Flexible Couplings"? you ask. The short story is that flex couplings are unions between propeller shafts and transmissions that absorb some shaft/engine movement in order to reduce wear and mechanically borne vibration. Some flexible systems actually eliminate thrust against the transmission by transferring it to the hull thereby removing rear force upon the engine altogether. Flex couplings in general are not a single panacea for the myriad of propulsion issues that lead to vessel sound levels but can greatly improve motoring conditions by their installation.

flexible coupling The reason you are now having to crank your sound system up to danger is not likely a single cause but more an confluence of conditions and components that are all aging in a parallel manner. Let us discuss for a moment some areas that are sources of sound aboard and what can be done about them mechanically as well as how a Flex coupling can help.

With the boat out of the water and properly blocked and grounded we begin an inspection for possible sources of vibration and whether a Flex Coupling Installation should be a single (almost never) activity or part of a greater project. First let us gaze upon the rudder or rudders. A single power or sailboat rudder should not have bearing or bushing wear to the point where the rudder stock can be made to "clunk". If the rudder will "clunk" then bushing wear is evident. If twin rudders are fitted then the rudders should be made to steer by grabbing and twisting if clanking is heard from inside the hull the tie rod between the rudder quadrants is worn at it's ends and should be repaired. Worn bushings can result in rudder fishtailing in the propeller wash and produce in hull vibration. Moving to the propeller we can ask ourselves when was the last time that this prop was measured or serviced. Propeller blades being bent, even very slightly can lead to vibrations that end up being felt in the hull so ask your local yard worker to measure the propeller and shaft for being bent by checking "run out".

Next we inspect the cutless bearing. Cutless bearings whether in a strut or keel are considered "fixed" bearings and wear to the point of side to side or up and down movement is unacceptable. Cutless wear is very common and bearings should be replaced as part of flex coupling installation. The shaft is supported at one end by the transmission coupling and at the other end by the cutless bearing in most cases. Well, you are already looking at the cutless bearing and you notice a half inch shiny band on the shaft on the propeller side of the strut. What is that you wonder is this about. What that band indicates is how much the engine is moving fore and aft in the hull as it bounces on the motor mounts. It is difficult to imagine the engine actually moving that much but there is the visual evidence. A point to consider is if the engine is moving that much fore and aft is it moving that much side to side…and yes it is. These movements result in hull vibration, locker rattles etc that we are hoping to reduce with a flex coupling. Moving along we arrive at the strut or keel and another source of possible vibration. As the propeller blades pass the strut or keel a pressure pulse is built up that wants to add resistance to the blades negative pressure path and can put a pulse against the cutless that can translate into the hull. As part of your Flex coupling installation it is impotent to consider that ABYC P-6, 6.5.5.4 states "The distance between the forward end of the propeller hub and the aft end of the last strut bearing shall be limited to one shaft diameter". As an aside if your vessel has been or is being re-powered be very careful that the shaft material is of the proper type and has a yield strength great enough to withstand any power changes.

Now take a look at the shaft as it enters the hull. Turn the propeller with the engine out of gear-do not turn the propeller if you are turning the engine! The propeller should turn easily. Is it straight as in enters the hull? Is it shiny at hull entry point or is the shaft log edge cracking suggesting the shaft is impacting the hull resulting in real powerful source of noise and vibration. Is the shaft true in the center of the shaft tube? You may have the shaft out of the hull during the coupling installation so that would be a good time for inspection, cutless replacement, propeller measurement, rudder bushing and stuffing box service or replacement.

stuffing box Inside the hull we take a good look at the stuffing box or Dripless seal. Is it worn to one side, has the packing burned? Or is it cocked out of alignment. You will be in a grand position in most cases to service the stuffing box, hose and clamps as the shaft will be out to have some types flex couplings fitted to the shaft by a machine shop. Taking a close look at the shaft coupling at the transmission you should make a few observations. First, is there enough space to install the flex coupling? Some flex couplings are considerably larger that the steel couplings they replace so measure twice and have no regrets. Also be certain that the flex unit will not move the shaft aft too far to be outside ABYC recommendation. Is the coupling loose on the shaft? Are the fasteners loose between shaft coupler and the transmission coupler. If the steel coupling has been loose the shaft then the shaft is worn and must be replaced. If the fasteners have been loose the couplings then the transmission coupling bolt holes are wallowed out and that coupling should be replaced as well. As another aside if the transmission is dripping fluid in any amount under the transmission coupling the rear seal should be replaced. If however fluid has been leaking between the transmission coupling and the shaft coupling then the transmission coupling fastening nut must be resealed or replaced.

Now take a look at those motor mounts, the fastenings and adjustment nuts. We discussed in another article that motor mounts wear out, get soft or fail so plan on mount replacement as part of the flex coupling activity if they are original. If engine mounts are aged simply replace them for a start fresh and a good result. Engine alignment will be required as part of your flex coupling install and must be performed in the water. An area often overlooked are the motor mount mounting frames-those plates to which the mounts are attached that are bolted to the engine. If the motor mount mounting holes are worn or wallowed out the frames must be replaced as the mounts will loosen quickly and destroy your alignment. It is often assumed that with flex couplings that engine alignment is not required, that the nature of the couplings is that they are self aligning. An initial alignment must be made to position the engine as closely as possible to best alignment.

What how does a flex coupling work anyway? You may not know it but is most likely you already own several flex couplings in the drive train and steering of your cars. Certainly if it is front wheel drive there are four flex couplings called Constant Velocity Joints on the front axles You may have rubber flex joints in the transfer case of your four wheel drive or plastic waffle plates on the rear suspension of your commuter car. It was a matter of taking the type of technology you see spinning under the local delivery van and making it adaptable to a boat propeller shaft. Directing the inbalances of several rotationaly affected items to a positive end.

Vetus flexible coupling So what do we have for choices in way of flexible couplings? The choices run from a very simple hard plastic ring called a Driversaver that needs little more than hand tools and a ratchet to an Aquadrive system that requires some serious engineering, large investment and hull additions. The middle ground is held by a type of flex coupling that involve reinforced "rubber" inserts or plugs in metal housing as in the Vetus Bull Flex and Uniflex types. Prices run from $100+- for the Drivesaver, Vetus type units starting around $550 and the Aquadrive systems in the thousands. As in many things the investment balance is a point to ponder. Further the performance expectation of flex couplings varies rather greatly closely resembling costs.

What will I get for my investment? Because flex couplings are absorbing shock and loading you can expect, with all other systems up, that engine vibration will be reduced along with some accompanying noise level. Shifting may be smoother, transmission wear will be lowered and some risk of impact damage to the power plant may be reduced. Greater comfort should be easily recognized even with the simple easily installed hard plastic yet flexible Drivesaver type units. The more flexible type Vetus couplings will immediately result in a more quiet drive line and the Aquadrive system will completely change the vessels feel as all propeller thrust is applied to the hull and not to the transmission tremendously reducing hull vibration.

Well, just how difficult is it to install a flex coupling ?. For the Driversaver type and Vetus and other types installation should be within the scope of your average Bimini Shade mechanic. The Drivesaver requires the most basic of hand tools and the Vetus style while perhaps some light machine shop work can be installed nearly anyone who knows how to work a hand drill and tie a square knot. The Aquadrive type requires some real skills, heavy tools and good drive line knowledge. A fair amount of science if not a lot of art. All types/styles can produce pleasant and in ways surprising results. Regard that you will be adding to the original design so you addition will be an improvement over what the vessel was on it's first day.

If your propeller shaft is fixed-that it has a bearing at each end-strut and stuffing box and the engine is on flexible motor mounts then the transmission must have a true flexible coupling. If it does not then add one. There were some boats built without them but one should be installed to account for engine movement. It is a good bet that your vessel has a non-rigid stuffing box and flexible mounts which is the most common installation. The Flexible Coupling manufacturer or dealer can tell you what size/style coupling you will need according to horsepower and shaft size.

We are almost home now so please bear with me. Remember that replacing just a coupling may not cure any problems, solve any dilemma, in fact poorly prepped a flex coupling may make matters worse. Should you have a system that uses u-joints and metal truck type drive shafts it is time to consider changing up to "Rubber" type couplings requiring very little care and are much less likely to fail. Flex Couplings are one of those things that you did not realize you would love so much until it is installed.

All the best boating... hope to be seeing you out there. If anything, anywhere has fogged your boating day just blame it on ethanol.

Randy Renn, SAMS, AMS-YSCE, MIIMS

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