Sling Positioning- How Critical??

[spimik]

How important is sling positioning with respect to where the sling indicators are marked? I was told that as long as you are "close" or within a reasonable distance, you are fine.

It would stand to reason that there is some margin here, but I would think that 'dead-on' is preferred.


The theory is that on a 2 sling boat you never want to use 3- this is rumored to have happened to a dock mate and the story is that substantial damage to the structure was accomplished.

 

[Henry Boyd]

I'm not sure there is a blanket answer to your questions. For example, I am guessing that sling position would be more critical on a longer boat than a shorter one. Travel lifts come in a finite size range, a 560 DA is going to have more mass extended beyond the slings, than a 280 DA will. As a result, it will be more critical to stick to the factory sling points on the longer boat to ensure that the COG is centered.

Another thought would be that bigger boats are heavier than smaller boats. Sling straps are also roughly the same size. Therefor the load exerted on the hull by the strap (weight of boat divided by strap contact area) will be greater on the heavier boat. Granted there may be slightly more contact area on the heavier boat due to the wider and deeper hull of a larger boat, a boat that is twice as heavy as a 280 will probably not have twice the beam and draft.

Three straps might not be a good idea. The loads on the hull with two straps can easily be calculated by an engineering school sophomore. The process is called statics and is just a zero sum calculation of the forces and moments. For example, assuming a block of uniform mass and a strap on each end with no overhang,then each strap is carrying 50% of the weight of the block. With a more complicated strucure like a boat the hard part is not calculating the loads, but determining the information about the boat, centers of gravity, dimensions, point loads etc. With three slings it gets harder to calculate the loads. If you remember your algerbra, when you have the same number of variables as equations, algerbra works, if you have more variables than equations its time for calculus! While we don't really need to calculate the loads in real life, the fact that three slings are very hard to analyze tells us that it would be very easy to put unpredictable loads on the hull with equally unpredictable results.

The recent post of the Youtube video of the boat dropping out of the slings is a good example of the unpredictability of three point loading.

Henry

 

[hack4alivin]

I have a friend of mine that had is boat pulled once by the marina and the sling straps where not in the correct position. Well as it happened the strap was over one of the transducers in the hull. All that weight and pressure on that transducer caused it to break, and they did not find out about it until the boat went back in….. Leaky boat!

 

[fwebster]

Correct..........

There is more than COG and weight distribution involved in where Sea Ray puts the sling marks. Part of the layup schedule on larger boats and on boats with cored bottoms is a planned location for transducers and intakes. Those areas are solid glass, not cored, so the decision locating the sling marks takes into account avoiding lifting on the transducer/intake area of the hull as well as the weight and balance issue.

As far a larger boats, the number of slings is doubled so there are 2 forward or 2 aft slings located at the sling marks. I've never seen a 3 sling travel lift in our area, but we sometimes lift a boat using 3 of the 4 slings instead of all 4 when the added capacity isn't needed.

So I'll answer the original question with a question.......Sea Ray tells us where to lift their boats. They build them, they know where the danger points are so why use some other lift point?

And, I know what happens......the travel lift operator puts the slings in the water, however they were set on his previous lift, rather than either measuring the distance between the marks or pulling the slings back out of the water and re-setting the travel lift correctly for your boat. I never hesitate to tell the travel lift operator to reposition his slings when they are not right for my boat. I figure I wrote the check that bought the boat and I'm paying for the haul out service, so the guy running the travel lift who rides a bike to work because he has too many DUI's to keep a drivers license isn't going to make that decision for me.

 

[Blueone]

http://www.break.com/index/failed-launch-capsizes-yacht.html

I think this is the video you were referring too

 

[spimik]

Some great points- thanks guys. My issue exactly. I want the slings where Sea Ray wants them too. The lift today went well. Sling on the back was dead on. Sling up front was back about 1 foot or so from the mark. I felt better that it was this that was back a bit- less chance of it slipping forward and no gear there to crush or catch on.

Thanks for the comments.

 

[Robski97]

The boat in the video clearly made contact with the larger ship as they continued to lower it . I saw a better video of this . Ill see if i can find the link.

Rob