13-foot and 17-foot Lifting Harness

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jimh
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13-foot and 17-foot Lifting Harness

Postby jimh » Mon Dec 11, 2017 9:34 am

A new drawing showing a design for a lifting harness for 17-foot Boston Whaler boat hulls created by Wm. Fontaine has been added to the REFERENCE section. The link for the drawing appears in the REFERENCE article on drawings at http://continuouswave.com/whaler/reference/drawings.html

The drawing is located at

http://continuouswave.com/whaler/reference/graphics/17LiftingHarness.pdf

FISHNFF
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Lifting Harness Construction, Load Rating, Clearance, and Suitability for other Models

Postby FISHNFF » Mon Jan 01, 2018 8:13 pm

[This post was made in a separate thread. The two threads are combined now--jimh]

Seeing [the above] post on a lifting harness for a 17 Montauk has me wondering if such a harness could be assembled from nylon lifting straps with appropriately rated hardware.

If so, would one-inch 3200-lbs-working-load lifting straps be adequate?

A three-point harness would distribute the lifting weight and give the harness a 9600-lbs capacity. A two-inch-strap rated at 6400-lbs would have thicker and larger end loops.

Without doing the math, by how much does the three-point dawned ring clears the console?

Would a 8 x 8 x12-foot harness work with an 18 Outrage?

jimh
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Re: 17-footer Lifting Harness

Postby jimh » Mon Jan 01, 2018 8:24 pm

The designer of the lifting harness just advised me via email that the drawing is not appropriate for the standard layout of a MONTAUK 17. Improved drawings will be posted shortly.

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Re: 13-foot and 17-foot Lifting Harness

Postby jimh » Tue Jan 02, 2018 10:38 am

Two new diagrams have been added to the REFERENCE section list of on-line drawings. These new diagrams show designs for a lifting harness for 13-foot and 17-foot hulls. An earlier drawing for the 17-foot hull has been deleted; that drawing showed a design for a non-standard console location.

jimh
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Re: Lifting Harness Construction, Load Rating, Clearance, and Suitability for other Models

Postby jimh » Tue Jan 02, 2018 10:47 am

FISHNFF wrote:...would one-inch 3200-lbs-working-load lifting straps be adequate?...A three-point harness would distribute the lifting weight and give the harness a 9600-lbs capacity...


Your supposition that use of three cables rated at 3,200-lbs load will create a lifting harness rated for 9,600-lbs is not a good analysis.

The tension in the straps due to the vertical load of the boat weight is only a portion of their load; there are considerable horizon loads because the straps are not pulling directly upward on the boat.

For example:

liftingForceAnalysis.jpg
Analysis of lifting forces with 1,000-lbs load and 30-degree angle
liftingForceAnalysis.jpg (8.96 KiB) Viewed 10799 times


Assume a load of 1,000-lbs is suspended by two cables of equal length that attach to form a 30-degree angle to the load as shown above. The vertical load of each cable must be 500-lbs. But the tension in each cable would be 1000-lbs. Each cable would exert a horizontal compression force of 866-lbs on the load, for a total compression on the load of 1,732-lbs.

In addition to designing the lifting harness to be made from elements with appropriate strength, the attachment points at the hull must also have sufficient strength to withstand both the vertical and horizontal loads created by the lifting harness.

padrefigure
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Re: Lifting Harness Construction, Load Rating, Clearance, and Suitability for other Models

Postby padrefigure » Tue Jan 02, 2018 6:09 pm

I guess the drawing is just theoretical, but in practice, wouldn't you want a four point harness? One leg for the loop connected on the inboard end of the bow eye, and one fore each of the lifting eyes of the transom going to a single lifting point? In the drawing shown, would the boat tilt to port or starboard depending on weight balance?

gheenoe
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Re: Lifting Harness Construction, Load Rating, Clearance, and Suitability for other Models

Postby gheenoe » Wed Jan 03, 2018 1:21 am

The drawing [that is the formal drawing or drawings which have been made available for download--see link above--jimh] is not theoretical, I designed the lifting harness assemblies and use them to pick up 13-foot and 17-foot Boston Whaler boats with a jib crane. A typical 13-footer with a 40 hp engine weights approximately 600-lbs. A 3/16-inch-diameter stainless wire rope cable has a safe working strength of 650-lbs. With the loops, reduce by 15-percent, thus 550-lbs per cable. The factor of safety for the wire rope is 5-to-1. Each cable can carry over 2,760-lbs in tension before breaking. If you don't like 3/16-inch-diameter wire rope, use 1/4-inch-diameter wire rope.

A typical 17-footer whaler with a 70-HP engine weights approximately 1,300-lbs. A 1/4-inch-diameter stainless wire rope cable has a safe working strength of 1,000 lbs. With the loops, reduce by 15% which is 935-lbs per cable. The factor of safety for the wire rope is 5-to-1. Each cable can carry over 4,675-lbs in tension before breaking. If you don't like 1/4-inch-diameter wire rope, use 5/16-inch-diameter wire rope. A 5/16-inch-diameter wire rope is the largest diameter that can be hand crimped using Nicopress sleeves without going to hydraulic swaging.

Don't get in the boats while lifting and don't put any dynamic loading on the cables.

Remember, these are three part harness assemblies, not two part harness assemblies. The Boston Whaler boats have three eyes which make them very easy to pick up with these harness assemblies since they are very light and their eyes are designed to be picked up.

You can also make up the harnesses using Nylon slings; I have one that I use. There are many rigging shops that will sew custom sling harnesses.

jimh
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Re: Lifting Harness Construction, Load Rating, Clearance, and Suitability for other Models

Postby jimh » Wed Jan 03, 2018 11:26 am

padrefigure wrote:I guess the drawing is just theoretical...

Which drawing are you referring to? If you refer to my sketch above showing the distribution of forces for two lines to lift a 1,000-lbs load, the theory and practice are the same. The sketch shows how the tension in the lifting lines will be greater than the lifting force each is exerting on the load. This occurs whenever the lifting line is not pulling directly vertically on the load.

padrefigure wrote:...in practice, wouldn't you want a four point harness? ...

I doubt that a four-line lift harness would be as effective. Recall that in geometry it takes three point to establish a plane. A three-line lift creates a stable plane for the boat to lay in. If you have four lines, unless their length were absolutely perfect, one of the four lines would tend to be slack and would not carry any load. Also, the boat would rock back and forth between the several planes that combinations of three of the four points would determine.

Also, Boston Whaler boats have been made and continue to be made with three lifting points. To lift them with a four-line harness would require installation of new lifting points onto the hulls of the boats. I cannot see how that would be advantageous.

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Re: Lifting Harness Construction, Load Rating, Clearance, and Suitability for other Models

Postby jimh » Wed Jan 03, 2018 11:41 am

gheenoe wrote:...I designed the lifting harness assemblies...

Thanks for joining the discussion, for the design of the lifting harnesses, for the excellent engineering drawings that describe them, and for contributing them to be available for download.

SS130
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Re: 13-foot and 17-foot Lifting Harness

Postby SS130 » Thu Jan 04, 2018 8:02 pm

I have a new 130 SUPER SPORT and [want] to use a balance bar off the davit.

Have you any experience with this method?

Would you be able to give me some guidance to the length of the three cables?

Thank you.

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Re: 13-foot and 17-foot Lifting Harness

Postby jimh » Thu Jan 04, 2018 9:19 pm

What is a "balance bar off the davit"?

ConB
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Re: 13-foot and 17-foot Lifting Harness

Postby ConB » Thu Jan 04, 2018 9:38 pm

The lifting harness that came with my 1969 13' is put together like the drawing in the reference section with the exception of the 2 short cables are 60" long and the single cable is 96" long.

I have not used the harness my self.

Con
!987 Outrage 18 / 2011 Yamaha F150
1969 13 / 30hp Johnson tiller

SS130
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Re: 13-foot and 17-foot Lifting Harness

Postby SS130 » Fri Jan 05, 2018 2:10 pm

A balance bar is similar to a spreader bar. Its about three-feet-long and has several holes spread out on the top of the bar for the davit to attach. The three cables connecting to the boat pickup points are shackled to the ends of the bar. This allows you to use shorter pickup cables.

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Re: 13-foot and 17-foot Lifting Harness

Postby jimh » Fri Jan 05, 2018 3:30 pm

Sound like "balance bar" is similar to a strongback single-point lift.

If the advantage to using a strongback single-point lift is shorter cables, the disadvantage is the much higher weight added by the strongback. Also, predicting where to attach the single hosting line to the davit sounds impossible. You'd have to do a lot of calculation so that the strongback was in balance fore-and-aft and rode horizontally.

Considering the added weight, which must also mean added cost, and the added complexity of predicting the cable lengths, I don't see why using a strongback single-point lift would be more advantageous than just using three cables (as the harness in the drawing does). The only reason might be having a very limited range of lift in the davit. Perhaps if the strongback were very well designed, you could lift the boat higher with a strongback than you could with a three-cable lift without a strongback.

A further problem: designing the strongback. You'd have to figure the bending moment on the strongback and construct it with material that had sufficient strength to resist the bending moment.

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Re: 13-foot and 17-foot Lifting Harness

Postby Jefecinco » Fri Jan 05, 2018 7:19 pm

A balance beam has the advantage of allowing the user to relocate the lifting point on the beam in the event that it's desirable to adjust the attitude of the hanging load. This could be useful to ensure complete draining of the cockpit immediately after boating or preparatory to washing down a hull. It could also be very useful when painting a hull bottom.

The beam would not require an engineering analysis to determine the design. For the 13 to 17 foot boats under discussion a simple beam of 3/8 inch thick steel plate approximately 5 inches wide should suffice. For larger heavier loads some engineering should be considered.

Steel pipe is often used to construct a strong back but I believe plate is preferable to avoid hidden areas of corrosion.
Butch

jimh
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Re: 13-foot and 17-foot Lifting Harness

Postby jimh » Fri Jan 05, 2018 8:00 pm

Another advantage of the strongback beam: more weight on the winch so it might be easier to lower the rig without worrying about the cable being overrun on the spool.

You can adjust the plane of the boat on the harness with the three-point cable lift.

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Re: 13-foot and 17-foot Lifting Harness

Postby Jefecinco » Sat Jan 06, 2018 9:44 am

Certainly the added pounds of a plate steel strong back can be a concern if using a light duty davit and or winch and the lifting harness cables/straps can be modified or adjusted to change the plane of the boat when lifted. A benefit of a balance beam is the effort and time to adjust the boat angle is greatly reduced. A balance beam is not superior to a strong back but it is more versatile.
Butch

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Re: 13-foot and 17-foot Lifting Harness

Postby jimh » Sat Jan 06, 2018 9:56 am

Drilling a lot of holes into the lifting beam or strongback or balance bar in order to permit movement of the attachment point would tend to weaken the beam.