Why would you own/use your own yacht cradle in a boat yard?

[dunedin]

No boats have fallen over. They do the job.

Let's hope they don't allow masts to stay up - as that would make the mechanical leverage issue massively worse than it already is.
Also, let's hope none do fall over - as could be an interesting stand off between the yacht insurers and the yard insurers over liability (negligence?) which would be a disaster for the poor owners affected

 

[FWB]

Let's hope they don't allow masts to stay up - as that would make the mechanical leverage issue massively worse than it already is.
Also, let's hope none do fall over - as could be an interesting stand off between the yacht insurers and the yard insurers over liability (negligence?) which would be a disaster for the poor owners affected

Most boats have their masts up. That shot is of my boat after repainting just before launching. Don't criticise a yard you know nothing about, they've been storing boats in this way for years with no problems.

 

[Deleted member 36384]

Let's hope they don't allow masts to stay up - as that would make the mechanical leverage issue massively worse than it already is.
Also, let's hope none do fall over - as could be an interesting stand off between the yacht insurers and the yard insurers over liability (negligence?) which would be a disaster for the poor owners affected

If you look at Spi D's video you will see very similar cradles but with light weight angle supports. Both Spi D and FWD cradles work exactly the same way, nearly all the load is taken on the keel and the uprights are nearly all in 100% compression, all nearly vertical, with very little resultant force. This is how these cradles work and is a basic principle where small diameter things can hold up very large loads. Yes, they will not be capable of supporting great side loads but there are no great side loads, the yacht is trying to compress the uprights when a side force is applied to the yacht by pivoting on its keel. If the yacht was struck by a force which moved it bodily sideways, such that the keel shifted on the base plate, then the uprights would not have the resilience to resist that. The design principle is that the uprights are acting like columns, not beams. Clearly the principle works as just about all building scaffold structures work this way. The scaffolders go to great lengths to ensure that the vertical tubes are vertical. I am pretty sure that that this is a standard structural principle when used like this. Now, I am moving out my knowledge level but if the column is pinned at the top and bottom i.e. the hull pad can pivot and there is a pin holding it in place at the bottom, then the upright will always be 100% in compression as that is a feature of pinned joints. It would be great to have one of our structural engineers comment.

 

[doug748]

The Mylor set up is the most carefully thought out arrangement that I have seen and used. Genius to have largely identical cradles and shift them around the yard with a tug. Was it done in house? I always wondered if it was a commercial system, if not it should be.

However it is a very sheltered site, on the Clyde I have no doubt there would be the odd pair of braces (literal and metaphorical) added here and there. Triangulated legs and, particularly, something to anchor the keel and prevent the boat rotating in the cradle.

 

[Spi D]

Just to add a little info on the side load subject.

The company supplying our solution offers an extra support
http://www.lp-yacht.dk/marina-udstyr/mobile-badstativer/lp-korestativer2010-186-800x600

All cradles btw tested and certified for their nominal load

 

[RichardS]

Here in Mylor most boats ashore are in steel cradles. Apart from anything else it makes it easy for the yards transporter to pick up the boats and move them around.

An accident waiting to happen I'm afraid. It is inevitable that someone at Mylor will be reading this and cross-bracing of some sort will soon be appearing.

Please let us know when the revised cradles start to appear. :encouragement:

Richard

 

[Even Chance]

Agree that those cradles just havent been thought out at all. Luck is definately on their side...... so far.
Angle supports are needed to support the sideways loading on the legs initially, as are keel supports to stop the keel moving sideways. Ive seen failures in both respects, and its not pretty.
The loading moments can be calculated if proof is required from some simple measurements of the cradle.

 

[Deleted member 36384]

... The loading moments can be calculated if proof is required from some simple measurements of the cradle.

Go on then, its easy to estimate the dimensions from the pictures and apply some loads, so make the calculations prove just how lucky everyone has been, or not. Assume 4130 steel and Schedule 40 tube dimensions.

I think the cradles are strong and safe and are designed to be so. I looked around Kip Marina today and they too have lots of cradles like this style. In my working life, I drill oil wells, the drilling rigs are mobile and have very slender derricks that sit vertical on two pins in teh raised position. They support over 500 tons (US tons) of vertical load, both pins are about 9" in diameter, nothing fancy, the rest of the structure is just 8" OD tube.

I think there is a lot of misunderstanding about how the loads are being applied to the cradle uprights as clearly they work, like scaffold towers work on flimsy wee tubes. Another example, today I watched a small yacht get towed away on its trailer, supported exactly like the cradle, sitting on its keel with support from its uprights. I guess that the yacht would see side force as it went round corners. Looked safe to me.

Sure there are better designs around in cradles but clearly this system works and there are modern versions of this system in use today. In a previous post on this thread, a poster has even put a link to video of the system and quoted the codes that it is designed to, what more do you want to see?

 

[NormanS]

Scaffold towers invariably have cross bracing.

 

[Deleted member 36384]

Scaffold towers invariably have cross bracing.

They are not stiff columns at the height they get to and the slim tubes of scaffolding cannot withstand significant side loading. The cradle arrangement is a stiff structure in comparison to the scaffold tower, the yacht is pretty rigid and will behave in a predictable way under loads from wind i.e. the yacht is not prone to deforming with parallelogram motion, at least not to any noticeable extent. In an earthquake the cradle would likely fail very easily as the yacht shifted sideways in the cradle.

In the picture of my cradle in this thread, the uprights supporting the side of my hull are steel channels formed from about 3.5 mm steel,held up with Arco type screw jacks. They need a wider base because the push against the side of the hull. In both cases the loads are quite small because the yacht is sitting upright on its keel. It requires very little force to keep it upright and my yacht sat through some winter storms. As long as the yacht does not deform, and it pivots on its keel, teh uprights are pretty strong in compression and they are not seeing any significant bending moment.

A Force 12 produces about 630 N/m. 2" Schedule 40 Pipe, 6' long can resist about 30,000 lbs (133kN) of force before failure in buckle. You can do the arithmetic on hull dimensions / mast dimensions to calculate the expected loads. Hence, as long as the columns remain in compression, the cradle will not fail, they remain in compression because the yacht is pivoting about the keel.

 

[LittleSister]

I agree that the Mylor cradles are dubious.

The supporting posts need to be in compression when under load, not have a bending force put on them. They are not supporting the hull off the ground (that is the job of the keel), but stopping it tipping over. Ideally, the supporting posts should be at right angles to a line from the bottom of the keel to the point where the supporting post meets the hull.

In practice that is hard to achieve, because that gives either a very wide base to the cradle (space hungry and expensive to make). or supports the boat only low down (relying more on the strength of the hull to transmit the load). If that right angle isn't achieved, then braces are needed to stop the supporting posts bending out (or tearing the weld or fitting that holds them). I would imagine that on a fin keeler they would also need bracing fore and aft.

 

[geoid96]

.......... Hence, as long as the columns remain in compression, the cradle will not fail, they remain in compression because the yacht is pivoting about the keel.

But they are not under compression. Virtually all the weight of the boat is on the keel and, if balanced, there will be almost no loading on the columns to keep her upright. However as soon as a lateral wind loading is applied the initial force on the supports is entirely horizontal.

Imagine if the supports were free to pivot at their join with the base and a lateral loading was applied. . . . they would just swing out, the pad would slide up the hull and the boat would fall. The only thing that stops this happening is that the supports are fixed rigidly to the base.

The major weakness of the Aeolus cradle design is that it relies almost entirely on the quality of the welds between the support and what is now a rusty base.

 

[NormanS]

They are not stiff columns at the height they get to and the slim tubes of scaffolding cannot withstand significant side loading. The cradle arrangement is a stiff structure in comparison to the scaffold tower, the yacht is pretty rigid and will behave in a predictable way under loads from wind i.e. the yacht is not prone to deforming with parallelogram motion, at least not to any noticeable extent. In an earthquake the cradle would likely fail very easily as the yacht shifted sideways in the cradle.

In the picture of my cradle in this thread, the uprights supporting the side of my hull are steel channels formed from about 3.5 mm steel,held up with Arco type screw jacks. They need a wider base because the push against the side of the hull. In both cases the loads are quite small because the yacht is sitting upright on its keel. It requires very little force to keep it upright and my yacht sat through some winter storms. As long as the yacht does not deform, and it pivots on its keel, teh uprights are pretty strong in compression and they are not seeing any significant bending moment.

A Force 12 produces about 630 N/m. 2" Schedule 40 Pipe, 6' long can resist about 30,000 lbs (133kN) of force before failure in buckle. You can do the arithmetic on hull dimensions / mast dimensions to calculate the expected loads. Hence, as long as the columns remain in compression, the cradle will not fail, they remain in compression because the yacht is pivoting about the keel.

Yes, but your cradle is entirely different from the dubious one supporting Aeolus. The Accro props on yours are in pure compression, because they have ties down to the base, giving the triangulation which is missing from Aeolus's My home-made cradle is very similar.

 

[KREW2]

An accident waiting to happen I'm afraid. It is inevitable that someone at Mylor will be reading this and cross-bracing of some sort will soon be appearing.

Please let us know when the revised cradles start to appear. :encouragement:

Richard

Totally agree.
A few years ago I was concerned that the boat next to me was not sat properly in its cradle, basically it wasn't cradled to my mind.
The acrows were vertical, as they were cranked up, the pads just slid up the topsides. The cradle just wasn't wide enough for the boat to allow any inward angulation.
To me this is wrong, the strength of a support is to have some down pressure, an acrow is not designed to take the strain on the side.
I was told that I was not an engineer, and not to worry about it.
Luckily for me when it fell over my boat was upwind of it.

 

[FWB]

They've been using these cradles for over 20 years without incident.
In the case of Aeolus she will stand upright on her keel without the cradle. I answered the OP's question giving one reason that it was easy for the yard to transport the boats. If the yard think that additional support is needed then it's given. Don't criticise what you haven't seen first hand.

 

[RichardS]

Don't criticise what you haven't seen first hand.

I wouldn't want to see those cradles first hand .... too high a risk of being crushed!

Richard

 

[Deleted member 36384]

Here is my yacht being transported with uprights and no side bracings / ties in place. She was moved 160 miles this way, around corners, leaning over on cambers and did not fall over. The principle is exactly the same as the cradle in the yard. There are also many small yachts on trailers that are towed around with very similar narrow base cradles and uprights that don't fall over.

As I have said I am not a structural engineer but I believe that the uprights are loaded nearly in 100% compression and with any resultant force being minimal. If the geometry of my cradle and the cradle in question are compared, then it is obvious why the designs are different and why both work, at least it is to me. Provided that the sockets in the cradle that support the props are strong, that the prop is strong, then I think that the cradle can support the load.

 

[doug748]

For those who don't know it, the yard at Mylor is very long and thin and HUGE. A great place to look at boats, you walk on and on and on. Here it is:

https://www.google.co.uk/maps/@50.1767195,-5.053727,133m/data=!3m1!1e3

600 boats in there in the winter.

 

[NormanS]

A major difference between Aeolus's cradle on BoB's lorry supports, is that on the lorry the supports are pretty much at right angles where they bear on the hull, so there is no outward force. Good to see that they also used loadbinders.

 

[Clydewanderer]

If the forty footer that fell on my boat destroying my mast and rigging a few years ago was in a cradle it would not have happened. Then again i wouldent have a new modern mast today!
Now selling my cradle as current yard supplying one.
Cw