Davit cable tension

[Dan Mapes]

It would be easy to assume what the tension is providing the lift is straight up. However with these transom davits, we have a winch pulling up on an angle, and hinge points that make it all more complicated. I swagged some of this and if the dink is 800 lbs, it could be as much as 1400 lbs going up the cable to the transom hard point or cable anchor.

Does anyone have a handle on this sort of work? I'd like to add pulleys to make it easier on the winch which is buried in the davit but without knowing the loads it's difficult to make a call.

 

[gbharrington]

Every installation is different but 2X the dinghy weight will be pretty close for figuring the load at the transom attaching point (or points) for the cable. If you use cable blocks as I do you spread the load to 2 areas of the transom and reduce the point loads by half, as well as reducing the stress on the winch and cable.
BTW, I have used 3/16 Amsteel rope on the last 2 installations and so far it's working really well.

 

[tonytrakovich]

Yes this can be calculated. If you provide the drawings / pictures and measurements we can calculate the tension on the cable.

As Dan said, that Amsteel is great stuff. Won't rust or corrode. Stronger than cable. Light. If exposed to sunlight, it should be replaced after a few years.

 

[Dan Mapes]

I had one vendor tell me I had to use wire blocks if I went with the dyneema. I'll take some measurements tomorrow and try to provide enough detail.
I have been thinking that I am looking at twice the load because of the angles involved. I have a second hard point of the transom now so as long as the blocks have an overall length less than say 5 or 6 inches I may be able to get them in place.
Film tomorrow...

 

[Dan Mapes]

Ok here are some numbers. They are all approximate. Note that when fully retracted the top of the pulley is a bit lower than the hard point on the transom. The angle of the davit in the photo is about 45 degrees, which if trig serves me at all, should be about the max force on the cable.
Red numbers are 36 for the davit leg length, and 38 inches for the height above the hinge bolt. The green line is the distance from the hard point to the pulley center and the yellow line is the distance out to the vertical drop to the dink. The angle between the cable and the davit arm at this point is about 0 degrees.


I figure the dink weighs in around 800 lbs., and if I take that at 800x1.414 the force at the cable end is going to be around 1150 lbs.?

If I go with 2 blocks I realize that cuts this in half, and I like that idea, but given the room I have at full retraction is about 6 inches, that leaves me with something like a Ronstan RF103 or RF104? If anyone has a smaller block that will work with wire, chime in!

 

[gbharrington]

I use RF104s with a 1/4" shackle for the shortest length.

 

[Dan Mapes]

Greg I am not surprised, there isn't much else out there at this size that is up to this kind of load. Do you happen to know the over all length of the block?

 

[tonytrakovich]

First, I will say that I am not an engineer. The loads on the cable are the greatest when the dink is just leaving the water. I think your max load could ba as much as 1600 lbs. The load on the cable greatly decreases as the dink gets higher out of the water. I think typical safety factor is 4X max load. So a 6400 lb cable will do it. 1/4" 7x19 wire is 6400lb. 1/4" Amsteel AS-78 rope is 64000 lb. The rope must be properly spliced to maintain its full strength. Splicing is relatively easy on 12 strand rope. I learned on YouTube.

You can make a 2:1 purchase on this load. Load on cable will decrease in half. The hardware on the transom will have to be rated for the full load 6400lb. You will need a pad eye on the cross member that can be rated for half the load 3200 lb. Make sure there is enough travel on the winch or ram to handle the extra purchase.

 

[Dan Mapes]

The original cable was 7/32 steel cable and the davit is originally rated at 1000 lbs.. Right now it has 1/4 7x19 ss aircraft cable installed since it snapped over in the Bahamas. It was about the closest thing I could get.
I doubt the 7/32 has a WLL that high. Breaking strength is close to that value.

I am a little leery of pad eyes, because if it is installed on the vertical of the cross member I do not think there is enough gauge there.

I'll probably make up my mind when I find out the over all length of the RF104's. I haven't found anything else that looks compact enough and handles the WLL I am looking at.

 

[Werner Schoenknecht]

Hi, I have been struggling for years to get the dinghy off of our 1986 43 MY. I am just getting to old
to climb ut on the top.So my question is what system are you using and how did you arrive at your
choice?Any info would be greatly appreciated.
Werner Schoenknecht "MIRAGE"

 

[Dan Mapes]

Werner, this is an RS-1 check out boatdavit.com. It came with our boat. It suits our purpose well enough but the installation was not the best hence all of the dialog. I would say it is ok for a 400 lb. dink on our boats but above that you had better reinforce the transom where the hinges mount and beyond.
In fact, you should do that anyway. Ours has some plywood back there but I do not believe it is correctly installed flush to the curve of the transom.
If you go this route, consider that you want radio control with a key fob. I have done that retro fit with an all terrain winch controller and it has made the chore much easier!

It does push you out of the finger more and as a result short fingers will make it difficult to get on and off the Hatt.

I was just on the phone, and the RS-1 is a 7500 dollar davit. Again this is another reason I will remove this so it can be powder coated.

 

[Dan Mapes]

Werner, when I mentioned what I was using, I was not advocating that particular brand. I will say I think the feet that attach to the transom are small with regard to the rating when applied to our boat. I agree having it on the aft deck lid is cumbersome for our boats.

Where we have ours is the best solution, but don't make the mistake of small foot prints at the hinges, or a less than useable control location.

As I said it has served us well, but the upgrade to our dink has forced a rethink and some changes. Look around here I believe there are others that have pulley systems that are much easier on the transom.

And this is the direction we are headed in because if I can have this poweder coated, add some pulleys and reinforce the transom behind the feet, I am thinking we are good to go. If the feet are wider and more spread out, I am thinking that is a much better solution. I would not use the swim platform as a reinforcement though.