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Old 18 June 2008, 21:13   #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, 21:35   #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, 00:44   #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, 00:57   #4
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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, 01:01   #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, 02:57   #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, 03:02   #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, 03:22   #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, 04:06   #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, 06:35   #10
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Transom Strength

Quote:
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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Old 19 June 2008, 06:49   #11
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Ta for the replies, all interesting stuff

Yes I agree that the scenario of a bit of metal heading towards me at speed is not welcome, I have seen the outcome of that using 24mm nylon rope snatch towing Land Rovers and it can be lethal and will take somebody's head off.

There aren't really any other boats round here to practice with reading the article just set me thinking about what I would do "if" something happened so I had some ideas in my head rather than standing there with a bundle of rope hopping from foot to foot thinking "ooh F where am I going to tie it on er um bugger b***ocks" while somebody drifted gracefully onto some rocks.

I can easily get 10mm and 24mm three strand nylon rope here so I was thinking about carrying maybe 30-50m of 10mm under the seat "just in case" - it isn't very expensive about 50p a metre I think. I have several 316 stainless shackles I bought last time I was in the UK so I think I might keep a "towing bag" kept stowed away somewhere with a bridle made up to spread the load between the eyes and a good long length of rope plus shackles, carabiners etc. not only for towing but in case the worst happens and I need a tow!

I don't want to spend time reinforcing the boat for something that may well never happen, really just wondering what the limits are of what is there. As JABS said the towing load should never really leave the transom, I wouldn't tow off the cleats as I have had enough problems with the blow up bits already one reason I replaced the ski hooks with eye bolts.

Thanks for the replies
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Old 19 June 2008, 07:01   #12
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I wouldn't worry too much about towing, anything large enough to break a boat will just leave you hanging by the rope going nowhere.
The power your outboard can produce will be less than that required to break the transom so its self limiting. The transom and motor are also all one unit so the rest of the hull is not being strained. The motor is pushing the transom which is pulling the load.
As far as pulling the bolts out is concerned this is how I have always recovered grapnels on ONE U bolt rather than tying across the two.
The grapnel hooked into the wreck is tied to a U bolt and once slack is taken up, to prevent shock loading, the motor is given some welly to bend the tines and pull the grapnel out of the wreck. For a moment the boat is straining on just about full power till it "pings" and I have never worried about popping the bolts out or seen any sign of it.
You don't need much power once something is moving and if you are heading towards WOT and it isn't moving then its too heavy
You can tow a suprising amount of boat with even quite a small RIB, just avoid shocks and jerks as this is what breaks things.
I don't keep a specific towing rig but would just use the 60m of 11mm anchor line if I needed to and tie rather than shackle onto the boat.
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Old 19 June 2008, 07:09   #13
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Quote:
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.
I like simple, and that makes sense. Especially, if I try to pull a very large load, gradually appling power so there is no shock loading. But if my boat is moving and then the rope goes taught it effectively tries to stop the boat - the transom stays where it is whilst the inertia of the hull tries to rip the transom off?
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Old 19 June 2008, 07:24   #14
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its ok towing from the transom but when it comes to trying to steer its hard as the towing boat keeps getting straighted up if the towed boat is heaver or deeper in the water, a way around it is to have a bridle from the transom and then have a sliding ring or shackle to the tow line , if you are going to tow other than a straight line and you need to steer the best tow point is midships ,as another member said earlier the rnli atlantic ribs tow big stuff but they tow from a samson post in front of the roll bar and when setting a tow up they have to rig a bar over the engines to stop the tow rope getting caught on the cowls, another safety point is dont use nylon rope when recovering a boat or trailor with a vehicle on a slipway or beach i once saw a young teenager loose an eye from a fitting on a speed boat breaking out whilst been pulled off a beach after getting swamped with a 4x4 years ago, health and safety at our club banned nylon rope for out of water use on the slip after that.
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Old 19 June 2008, 08:06   #15
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Nylon Rope banned why? Surely the ban should be on attaching to dodgy fittings

Tow from midships - how you gonna do that without compromising your console etc. If the boat is set up for it fair enough but if it's not it's all gonna get a bit tricky.

One other thing if you want to have stering control when you are towing isn't the best way to do a sideways tow
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Old 19 June 2008, 09:22   #16
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Nylon Rope banned why? Surely the ban should be on attaching to dodgy fittings

Tow from midships - how you gonna do that without compromising your console etc. If the boat is set up for it fair enough but if it's not it's all gonna get a bit tricky.

One other thing if you want to have stering control when you are towing isn't the best way to do a sideways tow
Hi R.W , we the boat club were told to use poly rope for lowering or recovering boats or the type of rope that goes dead if it parts with no whipback,nylon stretches up to 50 % before parting its great for anchors ,another reason was we had a boat and trailor run over the end of the slip and bottomed out the wheels over the sill the tractor had hell of load on it ,when it did pop out the whole lot shot up the slip , anyone in its way would have been killed ,granted the rope was long as to keep the tractor off the weed , and yes if towing in confined space its best to lash alongside to the other vessel.
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Old 19 June 2008, 09:33   #17
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Use a tow bridle - it means you still have steerage regardless of the angle of the tow, and it also shares the load between 4 fixing points!

Snatching is solved by a weight on the tow line - I have an olde rusty shackle off the QE2's anchor (slight exaggeration, but it's around the 1Kg mark) which if put on the tow line with a retaining / recovery string reduces the snatch a lot. Ironically the stretch of Nylon 3- strand which is good for absorbing shocks is the very same stretch that when the fitting fails will propel said fitting towards you at high speed!

As for towing yachts (from the beginning of the thread) I have moved an 8m with an Suz25 /SR4. Took a couple of mins to get going, but once on the move there was no easy stopping it! Bruce is abolutely right. Weight is slightly irrelevant when it comes to shifting through water at low speed - think of the umpteen tons of coal towed by one horse along a canal 200 years ago........
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Old 19 June 2008, 09:48   #18
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One other thing if you want to have stering control when you are towing isn't the best way to do a sideways tow
Actually, it's easier to steer both vessels with a sideways tow than a trailing tow. You ideally want the towed vessel's stern ahead of the midpoint of the towing vessel (i.e. thrust and steering well back of the towed vessel.)

You still get some steering offset from increased drag and asymmetrical mass, but not as much as you'd think.


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Old 19 June 2008, 09:54   #19
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Nylon rope is a huge no-no for any sort of towing or recovery - just as it is never to be used as a waterskiing rope. Use polyproplene or polyethylene rope.
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Old 19 June 2008, 10:14   #20
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It's possible I may be thining of the wong stuff when I call something Nylon rope I mean the blue 3 strand polyprop rope. I don't see much give in that and I certainly don't have a problem towing with that!

To go back to the tractor and trailer freefalling down the slipway to go subse a submarine state then reentering the atmosphere like a leviathon from the deep then I hope the launchee was banned along with the nylon rope. It strikes me as a case of very bad practice that the rope wasn't set up to be controlled properly. Must have been funny after the fact though!
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