What is the transom thickness on your SIB?

Fenlander · 28 August 2011, 21:05

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.

prairie tuber · 29 August 2011, 01:40

~2 3/8" (60mm)

frankc · 29 August 2011, 12:44

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.

Fenlander · 29 August 2011, 13:18

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.

m chappelow · 29 August 2011, 13:32

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 ,

Nos4r2 · 29 August 2011, 13:44

Quote:

Originally Posted by frankc

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.

frankc · 29 August 2011, 15:12

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.

Poly · 29 August 2011, 15:30

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?

prairie tuber · 29 August 2011, 17:10

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.

frankc · 29 August 2011, 17:22

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.

Poly · 29 August 2011, 17:48

Frank to my simple mind the forces which give "bow rise" come from two places:

(1) the boat lifting up over its own bow wake.
(2) the downward component of the engine thrust (based on its trim angle) which attempts to force the bouyant stern downwards, which cantilevers the hull about a point further forward.

Once on the plane, increasing revs (therefore thrust) at a fixed trim position doesn't normally make a significant difference to the bow attitude, which contradicts your theory (as the boat has become no heavier so more thrust should lift the bow).

Nos4r2 · 29 August 2011, 18:07

Quote:

Originally Posted by frankc

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.

Even if you're right:-
1) in your hypothetical example your motor is never going to give you more than 150lb of thrust-so it can't exert enough force to cause damage.
2) The point you'd be towing from in a sib is never going to be above the top of the transom bracket of the outboard so the forces on the tube joint aren't going to change appreciably.

frankc · 29 August 2011, 18:21

Very interesting thoughts on this matter, but it is not worth pursueing

I am always up for agreeing to dissagree.

m chappelow · 29 August 2011, 18:49

Biggest problem when towing with a lighter boat is snatch loading where perhaps the towed boat is going into a wave or down sea at the back of a wave then it starts surfing down the wave until the wave passes then snatches the tow ,,though carefull use of the throttle ,having a long tow line and keeping to a reasonable speed is the key.

I dont think when towing i have ever gone above a good 1/4 throttle except perhaps to momentry just keep the line taught if the other boat surfs a bit .

kitten · 31 August 2011, 06:36

frankc

ferryden · 31 August 2011, 11:29

Came on to check out whether a transom carrying maximum engine weight is capable of also pulling an inflatable... and am now more than confused... pulling an inflatable with a body (even with ´snatching´stress´onvolved in such an activity is surely putting lower stress on than towing a boat at lower speed due to the weight differences involved ?
Hope so...and will let you know how it goes!

frankc · 31 August 2011, 15:32

This has taken a life of its own, and the answer is to just tow a boat and be done with it. I started to calculate the strength of a plywood transom for you, but that would entail going up in the attic to find if we still have the tables needed for properties of plywood. A lot of calculations would need to be made and it is not worth the effort. Certainly not worth getting into an argument over.

Cut a piece of plywood 1 ft wide and the same thickness as your transom and clamp it to something as wide as the motor bracket. Now place your feet the same distance apart as the towing bolts you will use to tow and stand on it. You may see some bending, but probably not much.

Now jump up and down on the board and see the result. That is the snatch load m chappelow is talking about. The plywood does not have to be too thick to withstand this, but if it fails, you have a problem. The attachment of the plywood to the tubes in a real world test on the boat will prove to be the problem.

When a snatch takes place, all the power developed by the motor will be delivered to the boat plus the motor weight. There will be no power used up by moving the boat through the water, so it will be a much greater towing load than when the boat is moving through the water. When we designed things in the past, we would consider the load to go to infinity in this case so think about it before subjecting your boat to snatching. Bad things happen when a load such as this is applied to any material.

Imagine applying power to accelerate the boat to top speed in an instant. A snatch is the equivalent to this but in the opposite direction and the time factor could be measured in milliseconds. The transom could possibly tear away from the boat.

Bottom line, tow away without worrying about transom thickness unless your transom is extremely thin. Avoid too much power and avoid snatches at all costs. Watch the transom for bending while towing and keep the speed slow enough to minimize any distortion of the transom.

If you are really worried about transom thickness, fashion a support structure you could install to beef up the transom. That would take care of transom stress, but you would have the same problem with the tube to transom attachment.

Let us know how it works out in a real world test. If you do not snatch, you will probably be just fine. Remember that snatching subjects the mechanism load exceeding anything the boat is designed for.

Nos4r2 · 31 August 2011, 18:23

Quote:

Originally Posted by frankc

This has taken a life of its own, and the answer is to just tow a boat and be done with it. I started to calculate the strength of a plywood transom for you, but that would entail going up in the attic to find if we still have the tables needed for properties of plywood. A lot of calculations would need to be made and it is not worth the effort. Certainly not worth getting into an argument over.

Cut a piece of plywood 1 ft wide and the same thickness as your transom and clamp it to something as wide as the motor bracket. Now place your feet the same distance apart as the towing bolts you will use to tow and stand on it. You may see some bending, but probably not much.

Now jump up and down on the board and see the result. That is the snatch load m chappelow is talking about. The plywood does not have to be too thick to withstand this, but if it fails, you have a problem. The attachment of the plywood to the tubes in a real world test on the boat will prove to be the problem.

When a snatch takes place, all the power developed by the motor will be delivered to the boat plus the motor weight. There will be no power used up by moving the boat through the water, so it will be a much greater towing load than when the boat is moving through the water. When we designed things in the past, we would consider the load to go to infinity in this case so think about it before subjecting your boat to snatching. Bad things happen when a load such as this is applied to any material.

Imagine applying power to accelerate the boat to top speed in an instant. A snatch is the equivalent to this but in the opposite direction and the time factor could be measured in milliseconds. The transom could possibly tear away from the boat.

Bottom line, tow away without worrying about transom thickness unless your transom is extremely thin. Avoid too much power and avoid snatches at all costs. Watch the transom for bending while towing and keep the speed slow enough to minimize any distortion of the transom.

If you are really worried about transom thickness, fashion a support structure you could install to beef up the transom. That would take care of transom stress, but you would have the same problem with the tube to transom attachment.

Let us know how it works out in a real world test. If you do not snatch, you will probably be just fine. Remember that snatching subjects the mechanism load exceeding anything the boat is designed for.

Can't argue with that!

TBH, I hadn't figured snatching into my argument-nor had I factored in that the rest of a sib is 'massive' as well as the transom... I stand corrected.

frankc · 31 August 2011, 19:51

Nos4r2, it is really almost impossible to calculate just what forces will be encountered, so your remarks are good info. I would just try it with my own boat and be very watchful while making a tow. The weakest link breaks first, and without a whole bunch of data, it would be hard to say what that may be.

Just like two porkipines, be careful and don't worry yourself too much about "what ifs". Sometimes taking a calculated risk is necessary for safety's sake.