How concerned should I be?

[Manny]

The backstory: A few weeks ago I replaced my old 8 HP four stroke outboard with a Honda 9.9 HP Four stroke extra long shaft that weighs about 30 pounds more. The weight is below the max rating of my mount. Also the new outboard has considerable more "length" - not talking height. I can probably use it as an emergency rudder!
Cut to this past Saturday: I took a break from working on the boat and went for a very quick sail. I was cruising along on a beam reach at 5.5 knots on jib only when I looked back and noticed that the outboard was rocking slightly . I had left the motor down out of laziness and was getting broad sided with gentle rollers. I had never seen my old outboard move at all. I immediately raised the outboard which lessened the movement (the shaft is so long it still was submerged a few inches).
Back at the dock I tried pushing on power head to determine if the movement was from the mount or the transom. It turns out it is the transom flexing. I spent some time inspecting the transom to see if could find any evidence of stress cracks but all looked well. I have starboard backing plates larger than the mount footprint on the outside of the transom and also on the inside. One side of the inner backing plate butts up against the reinforced area for the stern ladder and the bottom butts up against a glassed in shelf that runs the length of the transom. If I had to guess the transom fiberglass thickness at the area of the mount is about 3/16 inch.
Has anybody experienced some flexing of the transom with an outboard?

Manny

 

[caguy]

Manny, was the outboard moving or the mount? Mine rocks sliightly but it is the hinging mechanism that has some play in it. There is also some play in the hinged mounting bracket it self. Add it all up and it is going to rock noticably.

 

[sailingdog]

If the transom is flexing, you're in trouble... you need to repair the transom and beef it up to support the outboard. If you flex fiberglass repeatedly, it will fatigue and the glass fiber will break—eventually, it will break.

Starboard is a lousy material for the plates on the transom, as it isn't anywhere near rigid enough to add any real support unless it is very thick.

Your best bet is to add additional fiberglass on the outside, and possible glass in some marine plywood on the inside. At a minimum, I'd add a 1/2" marine plywood epoxied and glassed in to the inside to spread the load of the engine.

 

[sailormanbigd]

I would have to go with both replies. The starboard is probably flexing, the bracket is flexing and the outboard is flexing alittle too. You pushed on the power head? why not push on the foot of the motor? More leverage. Is the motor mount bracket too low? Should be able to tilt motor out of the water. I would check it again except next time have someone else push on the motor so you can be free to investigate closer. Starboard dents pretty easy, maybe the mounting bolts for the bracket have come alittle loose?

 

[Ross]

The loading involving just the weight of the engine is minor compared to the loading you will get when the engine is pushing the boat. If the transom is not flat then the size of the piece of plywood that you can add is too small to support the load but several narrow staves glassed in side by side would allow you to reinforce a rather large area. As I remember the layout of the stern of your boat, access to the inside will be difficult. Remember that 3/4 inch plywood is twice as stiff as 1/2 inch plywood. Rounding all of the corners of the outer edges of the reinforcement will allow better glass work.

 

[jsalley]

My old Catalina 25 had the same problem, though not to the extent you describe. I found a supplier that sold heavy stainless steel L-channels that were cut and drilled to fit the motor mount. These solved the problem. I think SD's suggestion to beef up the inside of the transom with epoxy-and-glassed plywood is good as well, since it would spread out the point loads of the motor mount even more.

 

[BobM]

I agree with Ross. It is far more likely due to the longer lever (longer shaft) pushing on the transom than the additional weight. I'd opt for 3/4 inch plywood glassed in. If it proves necessary to better match the curvature, consider laminating multiple sheets of 1/4 inch on the fly with epoxy or a good construction adhesive then glass over it later.

 

[Ross]

There are several factors involved in causing fatigue failure in any material. The first and most important is the loading of the member.
Second is the frequency of the loading.
The third is the type of material and its elastic limits.
Automobile springs flex millions of times during the service life of a car without failure because the loading is within the elastic limits of the steel. Fiberglass has the same charictaristics but with different numbers. Everything on a boat flexes to some degree. It results in cracked gell coat when the elastic limits of the gell coat is exceeded. I am sure that someone, somewhere has taken a section of glass laminate from a boat and mechanically flexed it to failure.
I wouldn't be concerned that the tramsom will fall off but at the same time I would make plans the get it taken care of.

 

[SamLust]

Well spoken (or is that well written?) Ross. A voice of reason. While not an emergency you're going to want to beef that installation up. While pretty, Starboard is not very stiff compared to it's weight.

Manny; It is likely your transom is closer to 1/4 inch thick based on what I saw drilling about 10 holes in the transom of my 1983 Hunter 33 for my outboard mount.

 

[Randy Burns]

I had a 9.9 Honda on my 76 H25. It replaced a 9.9 Evinrude. I happened to have a 1/2" thick piece of aluminum plate, (it may have been two seperate pieces) that installed as backing plates. I never noticed any flexing. When I bought my 27 it was improperly set on stands with a lot of oil-canning. It is amazing how it could flex like that and pop back again. I miss the Honda, it could be ideling and was so quite you would forget it was running.

 

[Benny]

We have a 9.9hp 2 stroke Nissan hanging on the back of our Starwind 223. We see quite a bit of flex on the transom usually when under power in heavy seas but no cracks on the gelcoat. What Ross explains is correct, a degree of flex is acceptable in fiberglass as long as it does not exceed the limits. We have had had some violent forces working on that out board when at times I had thought it was going to ripped out of its mount with not a single stress crack. I think the h27 has a stronger transom than the Starwind 223.

 

[Ross]

I am not comfortable with an engine mount that moves under load. I ascribe to the adage "that nothing too strong ever broke". If you are in a situation where you are trying to claw your way off a lee shore the last thing you need is a worry about how durable your engine mount is. If you have any doubt, even just a tiny bit, do something about it besides hoping and praying.

 

[Manny]

Okay, I feel a little better now

Thanks for all the replies. My main concern was sudden and catastrophic failure without warning but it sounds like that is unlikely. I went to the local big box store and bought some oak that I will glass in this weekend as reinforcement. I actually looked for a piece that had a bend in it and found one that had a slight curve to match the transom. How often do you look for a board that isn't straight?
The amount of movement is probably about a 1/4 to 3/16 inch total at the power head which translates to a bit more at the prop. I'm sure the outboard has a little movement just because it is so long, and the only movement on the mount is when it is at it's highest position, I definitely noticed flexing at the transom. A couple of years ago while on the hard another boat owner that was installing a new outboard mount on his Centaur asked me how much flex I had on the transom, so I proceeded to hang off the mount and we didn't notice much movement (I am about 50 pounds heavier than the outboard).
The reason the outboard doesn't fully come out of the water is that the mount only has a little over 9 inches of travel. If I had set it too high the outboard would pop out in choppy conditions. I can tilt it to get it completely out if necessary. If I have a few bucks this winter I'm getting a mount with more travel.

The picture shows how much surface area the outboard has.

Manny

 

[jibes138]

A few comments on Fatigue

Elastic limits are interesting in a tensile test as the point beyond which you have plastic deformation. In fatigue you usually work with a designed load limit of about half the elastic limit. Since tensile testing is a single load application for short duration the material stretches (elongates) until it enters the plastic deformation zone then continues to stretch up to the ultimate strength point (broken part).
Fatigue is more intersted in the small surface defects or subsurface defects along with any stress risers in the vacinity (drilled holes or fillet radii or other discontinuities). Looking at the surface of a metal that is highly polished on a scanning electron microscope and you will think you are seeing craters on the moon or some alien landscape. The microscopic defects can be tiny cracks in the surface and flexing of the material can cause localized stresses, again microscopically, that will start to propogate and eventually result in macroscopic cracks. There are two types of fatiue, low cycle and high cycle. Many structural parts are subjected to high vibrations in thousands of herz (cycles per second) such as a jet engine running at 20,000 RPM or a propeller blade spinning at 1200 RPM or the propeller on your outboard. These mini defects can propagate amazingly fast under those conditions.
So for good fatigue life stay at about 50% of the elastic limit for cyclic loading of the part. As far as how thick to make the transon on my dinghy I laminated three sheets of 1/4 inch plywood for the transom as required in the plans and this is a very rigid structure. Of course this can be modeled as a beam so flexing is a function of distance to the supporting points. So width of the transom plays a part in how much flex. The transom may be thick enough but not have the supports (sides of the boat and deck) close enough to the mounting area. If you simply support a two by four and stand on it with your feet next to one support it will flex a little, if you stand in the middle of the board, it will flex a lot. So another approach and a lighter on might be to add support structure to the transom in the area of the mount by running some braces from the hull to the transom or the deck to the transom to increase rigiditywithout addind a lot of mass to the stern which will increase pitching and hurt performance.

 

[Ross]

Re: A few comments on Fatigue

Different materials have vastly differing responses to cyclic stress loading. Wood can be loaded to 90 percent of its rupture strength and then flexed millions of times without failure because of its structure. Lead on the other hand is practically devoid of elasticity but is so ductile that it may be formed and reformed in ways that would result in work hardening and fracture in must other metals. It is for these and many other reasons that the selection of material is such an important part of the design process. Half a century has demonstrated that fiberglass reinforced plastic, when properly engineered is an ideal material for building boats.
The problem in this case is the this boat was designed and built without an engine and without a reinforced mounting point for a future engine. As with any modification this can be done well or it can be done make shift. If it were my boat and problem I would sandwich a large piece of durable wood in the area of the engine mount and extend the fiberglass overlay for several inches beyond the wood in all directions so that I didn't have any abrupt changes in the stiffness of the transom.