Are there any engineers that can tell me if this will work?

[rbc]

I have a 15' boat that I wish to hang from davits. I don't want to use conventional spreader bar as they are heavy cumbersome,etc. I want to use 5/8 dock line which is plenty strong for a 1300 lb. boat. Bow is easy with bow hook in bow for lifting. If I use stern davit hooks that are on boat the rope would form a triangle from hooks up to davit cable hook. I think this would be very unsteady and boat could tip easily.
My idea is to use a piece of sched. 40 PVC to act as a spreader in order to spread hang lines out to boat hooks. I would drill hole thru each end of PVC and pass line thru it then up to davit hook. There would be no upward pull on PVC as lines just pass thru it. But my question is would there be a lot of compression on the PVC that just acts as a spreader? The PVC is fairly strong as I can hardly put any bend in it by leaning hard on it.
What do you think?
thanks
RBC

 

[Bondo]

Re: Are there any engineers that can tell me if this will work?

Ayuh,... Sch.80 maybe,...

 

[scoutabout]

Re: Are there any engineers that can tell me if this will work?

Not an engineer, but I'd say anything deflecting tension on a line between two points is going to be subject to whatver forces are acting on the line to keep it straight. In this case the weight of the back of the boat, including engine and depending on the centre of gravity, etc.

I agree that going with aluminum or light steel tubing is going to provide a better margin of safety from failure.

The first thing I would do is test it without the spreader by hoisting it up from the three points and rocking it a bit. Because you already have widely spaced rear anchor points it should already have a certain stability. Now, if you were hoisting from a single at the rear as well as the front, then I could see it being super unstable.

 

[Frank Acampora]

Re: Are there any engineers that can tell me if this will work?

A spreader bar at the rear will give no adavntage since the stability is determined by the distance apart of the stern eyes. In fact, since the spreade may allow shifting of the ropes used for lifting, it may actually decrease the stability. The only thing the spreader will do is allow more room for the engine and other gear to fit between the lifting ropes. Try it without the spreader bar first. Agree that PVC will become sun damaged and brittle.

 

[smokeonthewater]

Re: Are there any engineers that can tell me if this will work?

EVERYTHING frank said is spot on..... I'll add that you need two separate ropes to the stern and not one passed through both sides.... I also recommend that you step up to at least 3/4" rope because it will wear and the extra margin of safety will be cheap... it is also easier on the hands and slightly stiffer thus less prone to tangle.... Last.... use metal lifting hooks rated at much higher weight that they will have to hold with the rope permanently fastened instead of simply tied to the boat

 

[Bob_VT]

Re: Are there any engineers that can tell me if this will work?

I would not use the bow eye or the rear hooks. I have done it with truck tow straps (very cheap) and used them as slings. I have seen way too many bow eyes and rear hooks fail since you are not placing straight pressure on them with the load. That is from an engineering point of view.

 

[Dick Sorensen]

Re: Are there any engineers that can tell me if this will work?

Agree with Bob VT.....the hooks on your boat are not designed, engineered nor installed to handle the stresses you are thinking about applying (if you haven't you should crawl up in your bow compartment and inspect the backing plate...be surprised if there was one .... same with your transom)....My experience (limited) with small boat hoisting is that the deck hooks are bolted into the keel...somewhat inboard from the bow and stern.....for what it's worth....

 

[DBreskin]

Re: Are there any engineers that can tell me if this will work?

The stability of the boat and it's resistance to tipping is not determined by how many points are used to hoist the boat. The stability is determined by the vertical distance between the hoist point(s) and the boat's center of gravity (CG). If the CG remains below the hoist point the boat will be stable.

Think about it this way: if you hang yourself from a bar by your hands, you are very stable; all your weight is below the attachment point. If you stand on top of the bar as if you were on a balance beam, you are very unstable and at high risk to fall off (tip over).

The stability of your boat depends on how low the bow and stern eyes are located, and the distance between the stern eyes. Lower/closer is less stable.

 

[zagger]

Re: Are there any engineers that can tell me if this will work?

Everything mentioned above plus you should know the PVC will become softer and loose it's rigidity during hot sunny days.

 

[engrgrad]

Re: Are there any engineers that can tell me if this will work?

I am an engineer. The PVC is way too brittle and will fail without warning. Follow the others' advice and use a piece of A36 steel. If you know anyone in the machine shop field, see if they have a piece of scrap stainless.

Also the long term loading on the loops is questionable as well

 

[5150abf]

Re: Are there any engineers that can tell me if this will work?

Agree, especially with not using the bow and stern eyes, those are designed to hold thebaot at mooring not to hold the weight of the boat and most are pretty low grade castings, you can get stainless but here again it is only as strong as what it is attached to.

So you can probably do what you want but you will need to make sure you bow and stern eyes can take the load.

It is a little more complicated but spreaders and straps will never fail and it won't ever tip over or hurt the hull.

 

[V153]

Re: Are there any engineers that can tell me if this will work?

So long as the transom under the stern eyes isn't rotten, and the bow eye is reinforced sufficiently, lifting from those points should not be a problem. We are talking a 15' boat here right? I see it all the time. Sometimes lifted from a single davit.

Most folks use flat straps a la tow straps with a locking hook at the boat end & an eye at the davit hook end. Or sometimes just a loop sewn in the strap to go over the davit hook(s).

The best setups are with aft straps that are roughly the same length as the distance between stern eyes. ie giving you an equilateral triangle. However sometimes the straps must be longer to clear an outboard motor.

I'm in the canvas & cushion business so I spend a lot of time in, on, and around boats. The vast majority being in the water, on a lift, or swinging from davits. On davits being my least favorite. But I can tell you I would much rather work on a boat with straps vs one with a spreader bar. Not an engineer but it seems to me a spreader bar only multiplies & exacerbates the pivot points?

Truth be known simply the thought of climbing into a boat suspended from anything gives me the heebie jeebies. And yes if it were my boat & I intended to store it that way. I'd replace the factory hardware with the biggest baddest stainless U bolts I could find.

Carry on.

 

[Ned L]

Re: Are there any engineers that can tell me if this will work?

Something else you should consider is that the load on you aft lifting harness will be noticably greater than the weight you are lifting with the harness in an inverted "V" as I think you are describing. I forget the calculations (they shoud be easilly looked up), but the wider the "V", the greater the load on each leg of the "V". The "V" doesn't need to be very wide before you have more than doubled the weight you are lifting as a load on each leg (meaning if you are lifting 750lbs, the load on each leg of the "V" can exceed 1500lbs pretty quickly as the "V" getts wider). Just something to think about. -- Off the top of my head, I'm thinking that at something like a 155? angle at the "V" you are doubling the load on each leg.

 

[rbc]

Re: Are there any engineers that can tell me if this will work?

Thanks for all the thoughtful replies.
Ned, not lifting with inverted V. 2 points on boat to 1 point on davit hook.
The boat is a Key West 15 and the bow and stern eyes are designed to be used with davits. It is pretty much standard in Southern Florida. My Key West 20 I lift and leave it on the dock and leave the 15 on davits for quick use.
The comments about PVC being brittle under sun make sense to me. i had some aluminum tubing but it was a few inches short and the PVC was cheap to experiment with.
The rear lift eyes on the boat are almost 70 inches apart and only about 4 inches from outer edge of boat. I think I will try the suggestion of just the lines from boat to davit hook and see what stability is like. I am using the standard lift hooks tied to line with a bowline.
I will let you know results.
Randy

 

[rbc]

Re: Are there any engineers that can tell me if this will work?

Ned, maybe I am using inverted V and misunderstood what you mean. Doubling the load not a good thing, still as I remember, line was rated at 8,000 lbs.
Randy

 

[jeeperman]

Re: Are there any engineers that can tell me if this will work?

You want a inversed "Y" spreader bar.

Not a plain, straight spreader bar that has the Davit hook point in the middle of the bar.

 

[Ned L]

Re: Are there any engineers that can tell me if this will work?

Randy, Yep - I think we are on the same page now. It would be those upper legs of the "Y" sreader bar that jeeperman posted that I am refering too. Those carry a deceptively large load. It sounds like you should be fine with your 5/8" line (use Dacron (low stretch) & not Nylon). My comments were as much for others that may not think things through as well.

 

[Home Cookin']

Re: Are there any engineers that can tell me if this will work?

Considering that we are discussing a 15' boat, I'd think he's OK on the eyes and with PVC. he can always put a large dowel or a 2x2 inside the PVC for strength and protection from sudden failure. The front eye might acutally have more strength, pulling up, than pulling straight out like when it's winched onto a trailer.
However a boat hung from the eyes for a long time can end up with a warped hull, so if it were me, I'd use straps.
That being said, straps will slide up the bow so have something to prevent that, or back-up.

 

[rbc]

Re: Are there any engineers that can tell me if this will work?

The front eye is not the towing eye, but an eye on the deck that I am pretty sure goes thru hull keel and actually part of front tow hook.
The rear transom eyes are designed to haul boat with davits.
Yes inversed spreader bar is what I was shooting for with no upward pull on spreader bar as line would just run thru it. This way when I put in water I just disconnect upper rope from davit, pull line thru spreader and leave rear hooks attached to boat and coil rope in motor well.
Randy