View Full Version : Bow tow eye ( U bolt ) size

23-09-2008, 08:25 PM
I'm just after some advice on the size of the bow eye U-Bolt ( Tow eye ) I will need on the boat I'm building. I want to use one made of 316 stainless the few that I have found on the net quote the size M8, M10, M12, but they don't give the safe working load that these size U-bolts can pull. The boats all up approximate weight fully rigged will be around 1,100kgs. I would like a fair sort of safety margin but at the same time I'm not to keen on having something that looks like a bent railway spike sticking out of the bow of the boat. The trailer for the boat will have keel rollers and bunks, any help I can get on this would be greatly appreciated.


23-09-2008, 09:41 PM
You have alive issue there, & I will not be surprised if some are hesitant to give you a figure.

The term "safe working load" is a very specific thing and only truly relates to items intended, tested and rated for industrial lifting.

I think you will be hard pressed to find someone who will give you a rating on a "U" bolt of any sort. The problem is, there is such a great variance in the way the item will be used and mounted.

But the simple answer is........much bigger than you may think.

If you look at rated shackles in for example and in relation to hardware shackles.

Rated shackles have quite low SWL in comparison to chain of the same aparant size
The diameter pin in a rated shackle will be significantly larger than the the diameter of the rest of the body.

Chain is 3 to 4 times stronger than a shackle of similar size and material, and a ubolt in its self will be weaker than a shackle.

Ubolts have a couple of inherent engineering flaws.
firstly you have to derate the shank size to the root diameter of the threaded portion..... so if you have a bolt made of 10mm rod with a 10mm metric coarse thread you would have to derate that to about 8mm. About 20% in diameter which probaly equates to half the original strength.

A "U" bolt does not support its self against off set loads, combine that with there being a stress razor presented by the end of the thread...."U" bolts are enclined to break off at the end of the thread when stressed from the side.

then your biggest issue is how that "U" bolt will be fastened to the hull and how the load will be distributed.

As you can see this is a complicated engineering question

"U" bolts are not used at all in overhead lifting.

quite problay a more appropriate fitting wold be a shouldered eye with a bolt thru.

or some sort of " engineered fitting".

What is the material and what measure have been taken to distribute the load?

Sorry to be a wet blanket:(


23-09-2008, 10:07 PM
G/day oldboot,
Mate the boat is a fibreglass composite build and will have a 75mm hardwood timber block laminated in behind the bow as well as a stainless backing plate in front of the nylock nuts and washers, so the load will be pretty well evenly distributed. It's just that most production glass boats seem to just have a U bolt stuck in the bow which often looks all out of proportion to the size and weight of the boat they are pulling.

23-09-2008, 10:50 PM
If you want to get an idea of real proportion go down to a lifting gear supplier and look at the hardware and the SWL stamps.

If the production boats have a big lump of steel hanging out of them there is probaly a reason.

Lets talk about the shackle you are going to connect to this.

It will need to be at least a 1/2" aloy steel rated lifting shackle.
stanless will be bigger, in mild steel it will be huge even then you just don't know.

On a boat over a tonne I would not be surprised to see a lump of 16mm stanless hanging out the front.

No also remember this loop has to be big enough to provide for.

winch hook, tie forward and tie down.


24-09-2008, 05:22 AM
Thank's oldboot,
I will give a couple of the major glass boat builders a ring and have a yarn with them about it, I may be able to buy a tow eye from one of them that is suitable for the weght and size of the boat. Thank's for taking the time to help out oldboot your'e first post was a real eye opener for me I appreciate your'e time.
thank's again

24-09-2008, 10:18 AM
Thats what these forums al all about.

happy to help.


24-09-2008, 10:41 AM
I have a Whittley Cruiser 660, which weighs in at over 2 tonnes, and the bow eye would be about 10mm diam Stainless.
I recently replaced it with a modified one, supplied as part of a boat retrieval device, which is the same gauge, but with an extra piece projecting from the underside, which is part of the catch device.
It is difficult to relate the usual concept of 'safe working load' to this application, as although the boat weighs over 2 tonnes, the bow eye never has to withstand the boat's full weight, as it is only pulling it horizontally (and retaining it there on the trailer, together with other tie-downs.)
I would have thought that the weakest point in the system, is the fibreglass through which it is fitted, which you are going to re-inforce greatly by the details in your post.
Interesingly, when I changed mine over, I could not remove the original one by pulling it out of the bow (after removing the nuts) - I had to cut it into two with an angle grinder!


24-09-2008, 11:19 AM
A 10mm eye also doesn't surprise me.........and your argument is quite reasonable also.

It realy comes down to engineering philosophy.

In lifting applications the standard safety margin is 4, move into racing yacht rigging and the safety margin can be as little as 1.2. But how often do racing yachts break.....hmmm

So if i was setting an eye into something for overhead lifting it may be 16mm diameter stanless, to take the same load in a racing yacht it might be 6 or 8mm.

An argument for a lighter eye

If said "U" bolt could be fixed so that it could be reasonably expected that each leg of the "U" took a more or less equal portion of the load.
the bolt was set in such a manner that the weak point at the end of the threaded section was not subject to off set loading
the thread was applied on a manner that didn't reduce the diameter significantly... such as rolling the thread
the item in question was of known good manufacture
the load the eye was being put to was not the total weight of the vessel
there was no substantial shock loading expected

the size of the "U" bolt could be significantly reduced.... very significantly

I notice that there are "turned" '"U" bolts available, that have a sholder turned so the center section of the U can bare on a plate or washers then the thread is cut on the reduced shanks.

I note also that in the past bow eyes on aluminium boats appeared to be significantly lighter than common now.
I also note that tie down expectations for boats are quite a bit higher.

In the past it was the done thing to pull you boat onto the trailer with the winch and secure it there with the winch rope, throw a strap over the top... and off you go......the gung ho types didn't even hook up the token effort "safety chain"

These days in polite circles.... the boat isn;t expected to be secured by the winch, the front of the boat is tied froward and down with chain....there is still the strap over the top and there are two rear tie downs.
And all beefier than in the past........ and probably a good thing.

Have you seen any of those "boat on car" pictures.


24-09-2008, 12:08 PM
re: 'Boat on Car',
My best mate had such an incident in the middle of a country town many years ago, where he was involved in a minor accident where a car turned across in front of him, and he collided with it at no more than 15-20kph. The picture made the front page of the local newspaper.
We used to use the picture when teaching boating safety (Coastal Patrol Melbourne), to show what can happen. The boat was secured to the trailer with all the usual tie-downs, but the impact sheared off the u-bolts securing the winch post, and the result was a lovely V-shape dent in the roof of his Falcon wagon.
We recommended the use of what I call a 'check chain', between the bow-eye, and the frame of the trailer back under the bow in a rearward direction to stop the boat moving forward in the case of such an incident. This chain on my Whittley is 10mm heavy gal chain with a 12mm stainless turnbuckle to keep it bar-tight.
If you allow any slack, it allows a 'snap tight' effect if the boat comes loose, which can break the chain.


24-09-2008, 06:53 PM
G/day Topaz,
Mate the boat retevial device sounds handy, I have been talking to a few people about U bolt size and grade and they said the same thing as you which I hadn't really taken into account and that the boat is being pulled along and not hanging by the U bolt. So the way i figure if I set my trailer up correctly it should take a lot of the stress of the U bolt , plus I will be making sure it is strapped down properly and not just relying on the Bow to keep it secured to the trailer. Thank's for your'e input I appreciate it.

24-09-2008, 07:04 PM
I bought a couple of m12 316 collared eye bolts as rear tow points today, they are 10mm thick, still dunno what the m12 stands for.

cheers fnq

24-09-2008, 07:19 PM
Here you go http://www.stainlessfastener.com.au/pdfs/Hardware/eyebolts.pdf

Not identical to what you need but might give some scope.

cheers fnq