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Old 29 August 2011, 12:48   #11
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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).
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Old 29 August 2011, 13:07   #12
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Originally Posted by frankc View Post
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.
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Old 29 August 2011, 13:21   #13
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Very interesting thoughts on this matter, but it is not worth pursueing I am always up for agreeing to dissagree.
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Old 29 August 2011, 13:49   #14
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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 .
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Old 31 August 2011, 01:36   #15
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But I may be talking Rubbish.
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Old 31 August 2011, 06:29   #16
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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!
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Old 31 August 2011, 10:32   #17
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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.
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Old 31 August 2011, 13:23   #18
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Quote:
Originally Posted by frankc View Post
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.
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Old 31 August 2011, 14:51   #19
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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.
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