Stern tie line

[Airnet]

I've been searching for months to find an answer on appropriate stern tie line size. I'm pretty sure the yellow polypropylene line I had for our last boat was undersized. We upsized boats last summer, so now I need a larger stern tie line. The question is: how strong (breaking strength) is correct? There must be a simple formula for boat weight vs breaking strength. I love it when someone says, "Figure out your weight and windage, and there you go". Great, how do you calculate windage? What do you do with those numbers vs line strength? I've talked to Anacortes Rigging, and they said 5/8 Samson MFP "but that won't be strong enough if it gets windy out." He also said don't use Dyneema. Why not??? A buddy with a 92' loves Dyneema for stern tie line.

The only answers I've been getting are more 'feelings'. "That should be good" or "That will work if it doesn't get too windy."
The only concrete info I can gather is using our anchor chain strength. I know the stern tie line doesn't need to be that strong.

Anyone know where to find the official formula, boat weight vs stern tie line breaking strength?

We are 75k lbs, 58' water line, 63' overall.

Any info would be helpful.

Thanks,
Curt

 

[stevemitchell]

I have been doing the same thing - moved to a bigger boat about 15 months ago, and have been searching for data to make a choice. As far as I can tell, there isn't a good formula for all of this, and a lot is conjecture or info from a single person and their experience.

I brought the Samson MFP from my previous boat, and just used it 6 times over the last few months, several times in windy conditions, and it is definitely undersized.

I'm 58' LOA, 50' LWL, 70,000lbs and a lot of windage.

I also explored higher tensile line, namely Amsteel Blue because of how well it floats. It would definitely wear better at the shore side, but it has almost zero stretch, which is important with a stern line. That could be mitigated with a snubber or two, but it will never be as stretchy as MFP.

Just like anchors, I think there is a lot of personal preference here, and not a lot of formulas that you can go plug numbers into.

I don't intend on stern tying in super windy conditions, so stretch isn't as important to me. The size of 600' of 5/8" MFP is considerable, and managing it with the reel and on deck is likely to be a pain. Doing the same with 600' of Amsteel Blue would be a lot easier, but it is also 2x the cost.

It seems like a breaking strength between 5000-8000 lbs is what folks with boats our size seem to have the best luck with. That's about the only consistent set of data I've been able to find after chatting with a lot of people.

Here are some gratuitous pictures of stern tying from the last couple of weeks....

 

[Airnet]

Thank you for the info. I'm glad to hear I'm not the only one trying to solve this puzzle?

 

[SteveK]

It seems like a breaking strength between 5000-8000 lbs is what folks with boats our size seem to have the best luck with. That's about the only consistent set of data I've been able to find after chatting with a lot of people.

That would be a 5/8-3/4 inch line. OR since the line is returned to the boat a 3/8-1/2 inch. Maybe these links will help. I use 3/8", the weak link is the turn point chaffing.

Understanding Rope Strength: Breaking Strength and Working Load

Best materials

 

[guy with a boat]

There is no hard answer to this one because a stern tie is a compromise regardless of the line size. Do you want the stern tie to hold if the wind swings and blows hard enough to make the anchor drag? That might be worse than the stern line parting first.

Consider your anchor rode and dock line sizes and go from there. You know those will hold the boat’s weight in most reasonable conditions, so those should be the upper end of the size range. Go down in size as far as you dare, depending on your confidence that the wind won’t blow hard in the times and places that you would stern tie.

I’ve thought a few times about the possibiity of a midnight fire drill with unexpected wind on the beam of a row of stern-tied boats. Then I go out where there is swing room and drop a big anchor.

I know some places are tight and almost require stern ties. If they are also sheltered from the wind and you are a summer cruiser, then a smaller line is almost always fine.

If I had to pick an answer, I would say 1/2” is probably a good and reasonably conservative choice, and 5/8” isn’t too much but probably on the conservative end. Just keep in mind that its not an anchor or primary line, it is just to keep the boat from swinging.

As long as we are over analyzing this, here is another point to consider. Stern tying in that cove in the back of Prideaux Haven is tight and pretty sheltered, and lots of boats get by with relatively light stern lines. On the other hand, if you are stern tying in a more exposed place where the bottom drops off quickly and you want to keep the anchor stuck in that drop off, then a heavier stern line is in order.

FWIW, I have avoided both scenarios in several seasons of cruising BC an SE AK. Our boat is around 75 tons and I don’t want to carry around the size of stern line that would make me sleep well. Lots of others do it regularly and successfully so I’m not passing judgement at all.

 

[rslifkin]

As far as sizing, don't forget that something like Samson MFP is weaker than an equivalent size nylon dock line.

 

[bkcorwin]

Super interested in following along with what people do on heavier boats. We stern tied last year in teakern arm but I was never really happy with the setup we had. I was leaning towards something like 5/16 amsteel blue with snubbers. But like everyone I haven't found any kind of consensus particularly up near the 130k lbs we site at.

 

[Moonfish]

Our Ultraline Dyneema flat rope has a breaking load of 7,050 lbs. The equivalent nylon rode would be 1/2-inch (7,300 lbs). But then the Dyneema is flat, rolls up very tightly on the reel, and it floats.

BTW, love those gratuitous pics, Steve!

 

[MYTraveler]

I'm pretty sure the yellow polypropylene line I had for our last boat was undersized. He also said don't use Dyneema. Why not??? A buddy with a 92' loves Dyneema for stern tie line.

We are 75k lbs, 58' water line, 63' overall.

Any info would be helpful.

My boat is just a little bit bigger than yours, and both are big enough to put a huge load on a stern line. The stretch of polypropylene really scares me -- all that energy loaded up and ready to go, instantly and without warning. That literally has the potential to be deadly.

It has been several years since my boat was in the PNW (and therefore haven't needed the stern tie since), and I don't remember the line I bought (may well have been dyneema, and was not typical three strand nylon), but will see if I can figure it out next time I am on my boat.