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Old 18 June 2008, 22:13   #1
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transom strength for towing

Hypothetical question....

I was reading the article on towing in the last RIB International and particularly the two suggested ways of rigging a long tow - a bridle or a line with a bowline in from one side and the tow line through the loop to centre it and then onto a towing point on the other side, which means it is easier to vary the length of the tow line.

But ... if you have an eye bolt through the transom (recommended as a good towing point and I have two fitted to replace the old ski hooks) what sort of towing load can this take before there is a risk of damage to the transom? As far as I can see it, using the second method of rigging, virtually all the pull load would be taken by a single eye bolt with maybe a 2 inch washer on the back of it. Now I know that the engine is basically held on with four of these but ultimately how strong is a couple of inches of timber and GRP?

As I say it is a hypothetical question because I will only ever be towing anything heavy in an emergency but it would be nice to have some idea of what I might be able to get away with before there is a serious risk of a loud crack and an eye-bolt disappearing backwards in the mirror...

Also just out of curiosity what sort of "bollard pull" (if I had a bollard!) would a typical 115hp outboard achieve? I guess it depends a lot on the prop pitch and revs achieved but I just wondered what a typical figure might be - I'm guessing probably only a few hundred kilos given that the engine is bogged down at low revs by the prop pitch? I'm mainly thinking about what sort of breaking strength any tow line would need to be, and I realise it would also have to deal with snatch loads in rough sea conditions, which comes back to the transom strength again. My mooring lines are 10mm nylon which is rated at 2000kg breaking strain which seems to be that it would be enough though the article says 12 to 16mm for tow lines but without really saying what size RIB that would refer to. At the end of the day I would rather have the tow line as the weakest link rather than the towing point on the transom.

Would towing say a 40ft yacht with a 5.8m/115hp be a practical proposition in an emergency? It would be particularly interesting to hear from anybody that has broken a boat in these conditions so as to know what not to do
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Old 18 June 2008, 22:35   #2
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You could always beef up the towing points with a steel plate rather than washers.

The little props outboards have would hardly overtax the transom. The RNLI often tow some very big boats. The biggest danger comes from sudden snatch loads. Having a stretch rope and brisle will help - another good idea is to attach a weight or length of chain to the tow rope to introduce some sag - this will act as a great shock absorber.
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Old 19 June 2008, 01:44   #3
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Stephen;

Your point of failure is well taken, but a worse scenario involves the eye on the towed vessel failing, and your rear-view mirror showing a recoiling line with a mass of stainless steel aimed at your head.

To wit, I have no idea what the limits are for towing with respect to transom strength, but I do know that nylon rope plus heavy loads equals a surprisingly effective slingshot.

jky
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Old 19 June 2008, 01:57   #4
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Originally Posted by BogMonster View Post
Hypothetical question....

I was reading the article on towing in the last RIB International and particularly the two suggested ways of rigging a long tow - a bridle or a line with a bowline in from one side and the tow line through the loop to centre it and then onto a towing point on the other side, which means it is easier to vary the length of the tow line.

But ... if you have an eye bolt through the transom (recommended as a good towing point and I have two fitted to replace the old ski hooks) what sort of towing load can this take before there is a risk of damage to the transom? As far as I can see it, using the second method of rigging, virtually all the pull load would be taken by a single eye bolt with maybe a 2 inch washer on the back of it. Now I know that the engine is basically held on with four of these but ultimately how strong is a couple of inches of timber and GRP?

As I say it is a hypothetical question because I will only ever be towing anything heavy in an emergency but it would be nice to have some idea of what I might be able to get away with before there is a serious risk of a loud crack and an eye-bolt disappearing backwards in the mirror...

Also just out of curiosity what sort of "bollard pull" (if I had a bollard!) would a typical 115hp outboard achieve? I guess it depends a lot on the prop pitch and revs achieved but I just wondered what a typical figure might be - I'm guessing probably only a few hundred kilos given that the engine is bogged down at low revs by the prop pitch? I'm mainly thinking about what sort of breaking strength any tow line would need to be, and I realise it would also have to deal with snatch loads in rough sea conditions, which comes back to the transom strength again. My mooring lines are 10mm nylon which is rated at 2000kg breaking strain which seems to be that it would be enough though the article says 12 to 16mm for tow lines but without really saying what size RIB that would refer to. At the end of the day I would rather have the tow line as the weakest link rather than the towing point on the transom.

Would towing say a 40ft yacht with a 5.8m/115hp be a practical proposition in an emergency? It would be particularly interesting to hear from anybody that has broken a boat in these conditions so as to know what not to do
Used to tow 22 ft yacht with a 4m ish sib that Had a 40 2 st on + a bag of sand in the bow as the sib was only rated for 30hp but we only had a 40. never had a problem. We used to to put the tow line (bow line from yacht) around a transom eye then onto a cleat to get a bite then hold the end of the line so you could let it go quickly if needed
Have also towed a 6.5 m rib with my old 4m searider their bow line around my aframe just above the transom, we could only make about 3-4 knts but no problems with the transom

Jim
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Old 19 June 2008, 02:01   #5
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Stephen;

Your point of failure is well taken, but a worse scenario involves the eye on the towed vessel failing, and your rear-view mirror showing a recoiling line with a mass of stainless steel aimed at your head.

To wit, I have no idea what the limits are for towing with respect to transom strength, but I do know that nylon rope plus heavy loads equals a surprisingly effective slingshot.

jky
Was always told to pass the tow line around the mast etc then to the bow when towing

Jim
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Old 19 June 2008, 03:57   #6
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My Avon sea rider with a 450 hp inboard jet has something like a ton bollrd pull but has happily towed a 30 ton vessel. So duno what the bollard billof a smaller rib is but it wont be much.

As a rough guide you can quite comfortably tow a vessell 2 to 3 times you boat size


My son, Igor, when he was 15 towed a 30 foot yacht 2 miles down the Hamble with his rowing dinghy, strangely enough the yacht owner was keen to pay him afterwards.

It's worth practicing your towing refine your techniques andlaern how tomanouver when under tow. If I'm towing something and have strapped on the tow boat If I cant make a 90 degree turn to port easily then I'll turn 90 degrees in reverse to starboard to get my heading right.

Some golden Rules about towing

Never tow from an a frame,
Never fasten to Rib handles
Never use a snap hook or Caribena use a suitable shackle and split pin
Never use Frayed rope
Never upset Igor
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Old 19 June 2008, 04:02   #7
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Originally Posted by BogMonster View Post
Also just out of curiosity what sort of "bollard pull" (if I had a bollard!) would a typical 115hp outboard achieve? I guess it depends a lot on the prop pitch and revs achieved but I just wondered what a typical figure might be - I'm guessing probably only a few hundred kilos given that the engine is bogged down at low revs by the prop pitch? I'm mainly thinking about what sort of breaking strength any tow line would need to be, and I realise it would also have to deal with snatch loads in rough sea conditions, which comes back to the transom strength again. My mooring lines are 10mm nylon which is rated at 2000kg breaking strain which seems to be that it would be enough though the article says 12 to 16mm for tow lines but without really saying what size RIB that would refer to. At the end of the day I would rather have the tow line as the weakest link rather than the towing point on the transom.

Would towing say a 40ft yacht with a 5.8m/115hp be a practical proposition in an emergency? It would be particularly interesting to hear from anybody that has broken a boat in these conditions so as to know what not to do
Pulled a 70 foot canal barge sideways off a mud bank on the Thames one year with suprising ease. The Yam 115 was given some stick and the barge moved, engine had enough power to ventilate the prop which raised the revs.

16mm rope? HMS is having a laugh, were are you going to put 50m of that on a small rib. Stick to the 10mm it's more than strong enough to lift your rib with let alone tow someone else. If its a yacht I wouldn't bother with the mast you will just damage the wires for the lights and instruments, instead you will find they have cleats at the sharp end and if the yacht is properly equipped may even have a bridle to take the strain across two cleats.

Pete
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Old 19 June 2008, 04:22   #8
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Stephen - I would have thought a 1"+ penny washer wasn't going to rip though 18 mm of solid wood too easily. I would have thought repeated shock loading was more likely to stress the transom where it joins to the rest of the boat... ...but this is all from a laymans view and no doubt this will stimulate a discussion between the experts on breaking strain of steel, and the correct way to make a U bolt!
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Old 19 June 2008, 05:06   #9
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Maybe this is too simplbut consider:

The transom carries the engine. The 'push' comes from the engine.

Taking the load onto the transom is the best option becauce it doesn't go from boat to transom to engine?? So there is no extra force on the transom to hull joints??

Does that make sense.

I agree that a penny washer, provided it is thick enough not to bend will take a huge force.
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Old 19 June 2008, 07:35   #10
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Transom Strength

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Originally Posted by JABS View Post
Maybe this is too simplbut consider:

The transom carries the engine. The 'push' comes from the engine.

Taking the load onto the transom is the best option becauce it doesn't go from boat to transom to engine?? So there is no extra force on the transom to hull joints??

Does that make sense.

I agree that a penny washer, provided it is thick enough not to bend will take a huge force.
Makes sense to me
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