The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.
In a previous post I inquired if anyone had any luck with setting up their Catalina 250 Wing Keel for single-point lifting. No one responded so I did my research, looked at other installs, designed one for the 250 and tried it out. I mounted a custom lifting plate to the two aft keel bolts. The plate accepted a lifting shackle that allowed me to install an continuous loop dymeema rope with an approximate 24,000 pound capacity.
My efforts paid off and it all worked. Today we lifted our 250 WK from the water and onto its trailer. The following link has a set of pictures for reference.
In the pictures there is also a picture of the draft of our initial plan.
We did the weight with the 5 HP engine mounted. It came in at 4,750 pounds. That allowed us to use the club winch which has a limit of 5,000 pounds. We calculated the boat weight by weighing the trailer and boat first and then the trailer only. Simple substraction got us the boat weight.
It took some trial and error lifting over the water where we got the balance point right. With that figured out we now have a harness that we can quickly put on in the future. The length from the shackle on the keel bolts to the other end is 8 feet.
You will also notice that we needed to pull the purple lifting line (Dyneema) aft just to the rear of the open hatch. We did that with standard ropes/lines attached to the cleats in the rear and the two winches. All ropes go to a shackle the purple Dyneema slides through it.
All we have to do now is slide the hatch open, pop the top, install the lifting harness and hook the Dyneema rope to the hoist.
From my research a 5/8 stainless steel keel bolt can hold up to 14,400 pounds- and I am using two of them. I could not find the strength of the weld on the stainless steel lifting plate but it is likely dependent a lot on the experience and expertise of the machinist/welder- I have complete faith in the individual who make mine. The shackle attached to the lifting plate is rated for 12,400 pounds. The Dyneema lifting sling is rated for over 20,000 pounds. Together the entire setup easily holds up a mere 4,750-pound boat.
I will say that it is a bit strange to see the big boat held up by that purple Dyneema rope. I know that it specs out stronger than steel cable but it seems counter intuitive.
quote:Originally posted by DavidCrosby
Thanks for sharing. I have always been amazed how much weight can be lifted, pulled, etc. from tiny threads of a bolt.
William, you made the lift but the concern is really about the SF. Lifting a boat like this with components combined need to have a safety factor for each component to find the weakest link and point of failure. Boat designers always use a safety factor, which can vary. From your post this is what I saw: Keel bolts in tension = (14,400+14,400)/4750 = 6.1 as a S.F. totally acceptable Hand made plate = unknown rating or S.F. Shackle = (12,400)/4750 = 2.6 as a S.F. pushing the envelope, but it is your boat
Before we look at the dyneema, I have attached a link to the rope rescue safety factors page and would recommend anyone attempting a lift such as this review and stay within their recommendation of a 5 to 15 static safety factor. Rope Rescue Safety Factors https://roperescuetraining.com/physics_safety_factors.php
Dyneema = this is where it gets tricky, it seemed that you were using a “continuous” line up and back down and then doubling the strength from 10,000 to 20,000 because it is two lines (this only applies if you strand, yellow lines in the diagram). This is what I think you had with the limited information (10,000*.66 knot reduction)/4750 = 1.4 S.F. on a dyneema line. This well below what is recommended for static load, just imagine if the lift had a sudden jerk. William, I just want to bring up the importance of Safety Factors when making decisions such as this. Every component you chose exceeded the weight of the boat, but you didn’t give yourself wiggle room for error. Strand your dyneema and keep that SF up. My two cents.
Regards, John Westlawn Institute graduate Yacht Design and Naval Architecture 04 Catalina 250 WK Standard rig w/wheel steering Yanmar 9hp diesel
I don't think that I would try this with my boat. I'm much happier letting the professionals lift the boat out of the water. Especially professionals with insurance in case something goes wrong. I will concede that your engineering skills are better than mine.
Notice: The advice given on this site is based upon individual or quoted experience, yours may differ. The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.