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Old 28 August 2011, 16:05   #1
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What is the transom thickness on your SIB?

Just wondering about transom strength having made up a bridle to tow other boats... thankfully not needed on our hols last week.

Be really interested to hear the exact measurement of your transom thickness. My Zodiac will come out of the bag tomorrow to wash off the salt so will post it's dimension as soon as I have it.
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Old 28 August 2011, 20:40   #2
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~2 3/8" (60mm)



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Old 29 August 2011, 07:44   #3
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Transom thickness plays a part in towing, but the attachment to the boat fabric is where you will run into troubles. The leverage from the motor thrust attempts to pull the transom away from the boat. The force of towing increases that pressure. The easy way to visualize this would be to think what effect putting a too large engine on the boat and restricting its movement across the water would be.

The distance from the attachment points of the tow line to the motor bracket is not too far, so the transom itself would not have to be too thick to take the loading. The thrust from the engine added to the towed load would be the problem.

If you overdo the towed drag, the transom mounts to the tubes may fail starting from the top of the transom. I believe this is the limiting factor. Keep your towing speed low.
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Old 29 August 2011, 08:18   #4
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Yes appreciate any tow needs to be very carefully considered. The transom thickness came to mind because I was suprised by the movement I could see in the outboard when on the plane in a moderate chop. Mine is only 24mm thick!

I know the thread hasn't really grabbed folks attention but I'd really appreciate any more examples.
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Old 29 August 2011, 08:32   #5
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My own boat transom is 3cm thick,
and a couple of pics of my lifting eyes ,which i also use for towing or being towed,
Different stresses ,,crane launch its a vertical load as towing is a horizontal one,

on some boats if fitted the thrust board will take most of the load from the engine power as well as the top tube mounts ,
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Old 29 August 2011, 08:44   #6
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Quote:
Originally Posted by frankc View Post
Transom thickness plays a part in towing, but the attachment to the boat fabric is where you will run into troubles. The leverage from the motor thrust attempts to pull the transom away from the boat. The force of towing increases that pressure. The easy way to visualize this would be to think what effect putting a too large engine on the boat and restricting its movement across the water would be.

The distance from the attachment points of the tow line to the motor bracket is not too far, so the transom itself would not have to be too thick to take the loading. The thrust from the engine added to the towed load would be the problem.

If you overdo the towed drag, the transom mounts to the tubes may fail starting from the top of the transom. I believe this is the limiting factor. Keep your towing speed low.
The motor is pushing the transom. It doesn't matter how big a boat you tow, as long as the towline is attached to the transom at the right place and the transom can take it. The rest of the boat is under no extra load compared to using it at full throttle with someone sat far enough forward to stop the bow coming up.

Keeping your towing speed low is sensible anyway though.
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Old 29 August 2011, 10:12   #7
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The prop thrust is under the boat, so the thrust is not in line with the transom. You have a distance between the fixed point on the transom and the prop which causes the transom to be under a bending stress. The prop causes thrust which must be offset by the weight of the boat pulling on the connection between the tubes and transom.

It the thrust were on the center of the transom, you would have no bow rise on acceleration. Put a long shaft motor on a boat with a transom designed for a short shaft motor and you increase bow rise.

To help with understanding this, assume the prop thrust is 1.5 ft below the fixed point on the transom and developing 100 lb of thrust. The transom is subjected to a bending stress of 150 ft.lb but the boat only receives 100 lb of force in the forward direction. The joint between the transom and tubes has to handle both the horizontal force and the bending force. The lower part of the motor bracket offsets the bending stress somewhat, but does not cause thrust to be in the center of the transom.

The mention of the thrust boards is good information. This helps offset the load on the bottom of the transom to keep the transom from being twisted in the attachment between the tubes and board. Without going into a lot of math, surfice it to say that this lessons the twisting of the transom but does put most of the load at the top of the transom.

If the transom is thick enough not to bend, the entire force is applied to the tube attachment. The force used up in bending the transom eases the shocks applied to the tube joint similar to a shock absorber so while it is disconcerting, it does not hurt anything so long as the bending is within the limits of what the transom can take. This may be why some boats have such thin transoms, but that is a pure guess.

Our Avon Rover has a thin, lively transom, but after 20 years with no problems, I guess the designer knew what he was doing. I would give you a slow speed tow with no worries.
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Old 29 August 2011, 10:30   #8
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frankc, ignoring the fact that I think you've made some incorrect assumptions about the origins of "bow rise" how does this mean that towing something is going to increase the force on that joint?
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Old 29 August 2011, 12:10   #9
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I just measured my transom more closely (previously stated thickness was based on that old photo I had posted). The marine plywood portion is 2 1/4" (57mm). There is also a 1/4" aluminum outboard mount plate for a total of 2 1/2" (64mm). The outboard mounting screws have to be turned almost all of the way out to fit the outboard over the transom.

I agree with the point that Frank makes that the strength transom-fabric connection is extremely important and is a common area of failure. I don't believe that having a flexible transom is a proper way to compensate for a weak transom-fabric connection. The transom should be as rigid as possible and the transom-fabric connection should be as strong as possible.
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Old 29 August 2011, 12:22   #10
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Polwart, it would be nice to hear your take on where the forces are exerted. You have a cantilever situation, so motor thrust cannot be centered on the transom. Maybe some structural guys might want to draw up a schematic or force diagram for help in understanding this.
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