Mooring rope - strength

[n4585k]

Quick question...

46 ft / 10 tonne displacement. Is 4800 kg breaking strength enough for mooring rope?

Thanks!

 

[sarabande]

it is, I suggest, more a question of the overall technical specification than simply BS.

e.g. to a buoy, or a pontoon, or an anchor; all of which may need rope with different characteristics.

 

[jdc]

Quick question...

46 ft / 10 tonne displacement. Is 4800 kg breaking strength enough for mooring rope?

Thanks!

Almost certainly, but ropes need to be oversized not so much for strength but for margin against chafe. The wind force end-on on a 46' boat will be around 1100 kg in a steady 50 knot wind, and a 4x factor is probably ok.

However what do you mean by "mooring rope"?

If the strop to a swinging mooring, then I'd say that 4800 kg breaking strain is marginal as far as load is concerned, and implies far too small a diameter as far as chafe is concerned (assuming your rope is polyester, nylon or polyprop). It's also rather important to use either very oversized 3-strand with over-long splices, or, much better, 8-plait lines. This is because if the swivel jams then the boat moving around can unlay 3-strand surprisingly rapidly.

If you mean the lines used as breast ropes and springs when tied up alongside, then 3 strand is fine and 4800 kg is probably fine also as the loads will be shared between several ropes. imho the guide sizes, eg from Jimmy Green, are over large given the strength of modern ropes and tend to over-fill cleats and fair-leads. I use 14mm diameter 3-strand polyester for this purpose on a 13 tonne, 42', boat. But when leaving her for any extended period, I double up by adding 20mm dia lines: not for strength (~7 tonnes BS) but for added margin in the event of chafe.

 

[n4585k]

Thanks for the replies so far. How do you come up with 1100 kg in a steady 50 kn wind for a 46' boat?
The use would be in marinas in the Med (stern to), not always that protected from swell and wind. The rope is double braid, 16 strand braided HT polyester core, 24 plait HT polyester cover. I'm asking because I have seen the diameter sizing charts for mooring rope, but these don't seem to take the quality/breaking strength of more modern rope into account...

 

[jdc]

I don't care for double braid ropes for mooring as I find it impossible to assess how much strength has been lost if at all chafed, especially in a splice as one really doesn't know what proportion is taken by the inner braid and what by the outer. I also think splices can't readily be inspected or assessed without destructive testing - ok on a commercial production line but rather implausible if home made. BUT I have to admit that the above is not at all quantitative, and so may have strayed into prejudice: other's experience may be different.

As far as the force is concerned, I have it from three sources:

1. Measured data for my boat, extrapolated upwards for wind velocity as v^2
2. Data from the American Boat and Yacht Council on forces on anchor. These over-estimate in my opinion, but provide a useful relationship betwen LOA and wind force
3. An excellent discussion on the web site of Alain Fraysse (http://alain.fraysse.free.fr/sail/rode/forces/forces.htm) who has studied the matter in some detail.

The graph below uses Alain's formula.

 

[Tranona]

Thanks for the replies so far. How do you come up with 1100 kg in a steady 50 kn wind for a 46' boat?
The use would be in marinas in the Med (stern to), not always that protected from swell and wind. The rope is double braid, 16 strand braided HT polyester core, 24 plait HT polyester cover. I'm asking because I have seen the diameter sizing charts for mooring rope, but these don't seem to take the quality/breaking strength of more modern rope into account...

Just considering the ultimate BS in this situation is misleading. First the effect of the wind on the boat varies according to its bulk rather than its weight. A high flybridge cruiser will react differently to a low freeboard yacht. The biggest enemy is snatch loads from different directions leading to chafe. So first consideration is to ensure mooring lines can take the loads from different directions and there is some form of damping in the lines. There are various commercial devices that allow this, or you can double all lines with one a bit slacker than the other. Minimising chafe by using fair leads and chafe protectors also helps. Like jdc I am not keen on braid on brain for mooring warps as you can't tell what is going on inside and it is the core that provides the strength and not the outer sheath.

 

[Mistroma]

Interesting figures. I did my own calcs. in 2009 when having my mooring upgraded. I knew that chain and rope side were well up to the job but wanted to check approx. drag the ground would need to resist. I was only looking for a ball park figure and calculated surface area of boat, mast & rigging with wind on the bow and also side on to the wind. I applied drag coefficients to all surfaces (list of approx. values for wire, tube, cube, sphere etc.) and used an average figure for air density.

Figures came out at 490kgs bow on and 1400 kgs side on. I realised that the boat would veer from side to side and think I assumed a max. angle of 45 deg. but can't remember. I also made an allowance for moderate surge as my mooring is well sheltered.

I produced a graph and final figure was 980kgs at 50kts for my 13m boat. This isn't miles away from the figure on your graph (appears to be around 950kgs for 13m boat).

I also ran calcs. using Prof. Knox's formula and it gave a lower figure of around 850kgs. At the time, I assumed that my figures might be OTT but was happy as my mooring did appear to be oversized. Can't remember where I found Prof. Knox's calculations as it was about 5 years ago.

I came up with 14.8 and 29.8 sq. m for a Jeanneau 42DS (bow into wind and side on to wind) but can't find the calcs. I used to estimate dynamic forces due to boat swinging and surge from waves.

 

[n4585k]

jdc and Mistroma, great to read how you have made your calculations. Gives me a very good idea on how to look at this. Thanks!

 

[n4585k]

Mistroma, I just looked at your website and saw that you are from Glasgow. Which is where I am at the moment. What a coincidence!

 

[J Neeves]

Interesting figures. I did my own calcs. in 2009 when having my mooring upgraded. I knew that chain and rope side were well up to the job but wanted to check approx. drag the ground would need to resist. I was only looking for a ball park figure and calculated surface area of boat, mast & rigging with wind on the bow and also side on to the wind. I applied drag coefficients to all surfaces (list of approx. values for wire, tube, cube, sphere etc.) and used an average figure for air density.

Figures came out at 490kgs bow on and 1400 kgs side on. I realised that the boat would veer from side to side and think I assumed a max. angle of 45 deg. but can't remember. I also made an allowance for moderate surge as my mooring is well sheltered.

I produced a graph and final figure was 980kgs at 50kts for my 13m boat. This isn't miles away from the figure on your graph (appears to be around 950kgs for 13m boat).

I also ran calcs. using Prof. Knox's formula and it gave a lower figure of around 850kgs. At the time, I assumed that my figures might be OTT but was happy as my mooring did appear to be oversized. Can't remember where I found Prof. Knox's calculations as it was about 5 years ago.

I came up with 14.8 and 29.8 sq. m for a Jeanneau 42DS (bow into wind and side on to wind) but can't find the calcs. I used to estimate dynamic forces due to boat swinging and surge from waves.

Mistroma,

I have done similar calculations and come to the same sort of results. John Knox had a series of articles way back in the 90's (I think PBO) - so to find the formula might be hard work. I considered worst case scenario and assumed the yacht would be side on to the wind. Which could be valid in the OP's case as he seems to be talking of med mooring and the boat will be fixed but presumably the wind variable. I also used the 90 degree on the full beam as in some anchorages yachts do veer and the wind does come as bullets (at sometimes more than 90 degrees). But its about snatch loads, not simple wind and surface area, and in any mooring the snatch loads are much higher - for example if there is any sort of sea running. You need good snubbing, whatever protection you can devise for chafe and you must assume mooring lines are consumables.

Another big imponderable is the likelihood of the 50 knot wind.

Jonathan

 

[Mistroma]

Mistroma, I just looked at your website and saw that you are from Glasgow. Which is where I am at the moment. What a coincidence!

Boat is registered with Glasgow as the home port. Although our mooring is in Holy Loch, we live mid-way between Glasgow and Edinburgh. However, Mistroma hasn't been near Glasgow since early 2012 and we are currently in Galicia.

So unfortunately not as close as you might have thought.

 

[Mistroma]

Mistroma,

I have done similar calculations and come to the same sort of results. John Knox had a series of articles way back in the 90's (I think PBO) - so to find the formula might be hard work. I considered worst case scenario and assumed the yacht would be side on to the wind. Which could be valid in the OP's case as he seems to be talking of med mooring and the boat will be fixed but presumably the wind variable. I also used the 90 degree on the full beam as in some anchorages yachts do veer and the wind does come as bullets (at sometimes more than 90 degrees). But its about snatch loads, not simple wind and surface area, and in any mooring the snatch loads are much higher - for example if there is any sort of sea running. You need good snubbing, whatever protection you can devise for chafe and you must assume mooring lines are consumables.

Another big imponderable is the likelihood of the 50 knot wind.

Jonathan

Strong winds aren't that unusual. I think it was summer 2010 when I took fright at a forecast and moved the boat into a marina. We stripped off the sprayhood, fitted tubing over doubled etc. Glad we did as the wind was over 80mph (poss. knots). We spent most of a day trying to persuade a tree to keep upright but gave up (chain, Tirfor etc.). At least the boat was fine.

I won't be home for several months and only have the graph I made so don't have the calcs. I suspect I've lost "Prof. Knox's" formula but the results were interesting vs. my own rather rough calcs. Pity, as it would be interesting to look at the formula he used.

Wind speed(kts)....10....20....30....40....50....60
Own figs. (kg).......39...156...352..625..977..1407
Prof. Knox (kg)......34...135...304..540..844..1215

 

[westernman]

Snatch loads are what will break a warp.

Chafe can be a major problem in Med stern to berths which will reduce the breaking strain - but in the end it is the snatch load which will cause it to part.

You must have something elastic somewhere in the connection. I use 22mm 3 strand Nylon warps and old fire hose for protection. I can easily see the chafe and they are elastic enough such that the use of an additional damper is not required. The more elastic the warp, the lower the maximum snatch load compared to the steady state load.

When I knew less than I know now, I used am 18mm low stretch braid on braid for a warp and it lasted less than 4 hours before breaking when tied up alongside a dock with a F8 blowing off.

Where I used to berth in the Med we have had winds of over 160km/h through the marina in the winters.

 

[J Neeves]

The more I read the more I realise why its called 'The Lucky Country'

160km/hr - that's 80 knots!

To get elasticity (or its not elasticity but the ability to store energy) you need longer lengths. Nylon is basically the most elastic of the common and cheap fibres. In terms of a mooring warp then construction does not matter (but do not rely on cheap cordage). The thicker it is the less elastic, less ability to absorb energy (or snatch loads). Its a balance between too, thick (will not store energy) and too thin (will not take the load). If you can balance a number of thinner pieces of cord it would be better than relying on one, or a few, thicker pieces.

For 80 knots the rubber snubber devices would be a complete waste of money, malt whisky would be better (at least you would forget the problem short term).

For chafe - firehose is good (and cheap if you can find discarded product)

Jonathan

 

[fishermantwo]

The more I read the more I realise why its called 'The Lucky Country'

160km/hr - that's 80 knots!

To get elasticity (or its not elasticity but the ability to store energy) you need longer lengths. Nylon is basically the most elastic of the common and cheap fibres. In terms of a mooring warp then construction does not matter (but do not rely on cheap cordage). The thicker it is the less elastic, less ability to absorb energy (or snatch loads). Its a balance between too, thick (will not store energy) and too thin (will not take the load). If you can balance a number of thinner pieces of cord it would be better than relying on one, or a few, thicker pieces.

For 80 knots the rubber snubber devices would be a complete waste of money, malt whisky would be better (at least you would forget the problem short term).

For chafe - firehose is good (and cheap if you can find discarded product)

Jonathan

Actually Jonathon, the best material for chafe is new firehose. The blue flat hose made from synthetic stuff. Comes in various widths, I'm using 5" stuff wrapped around the riser and secured with cable ties. Its about $10 for half a metre. I have used leather, steel reinforced rubber hose etc in the past and nothing matches the fire hose.

The US coast guard did tests on different materials and found the synthetic fire hose far better than the rest.

 

[J Neeves]

I'm a Scot - it never crossed my mind to buy new fire hose. But, in this instance, I'll amend my parsimonious ways and find some new stuff. As you are relatively local, being within 800km!, - where did you buy it?

Thanks

Jonathan

 

[fishermantwo]

Probably about a 100 km away! I bought it locally at a hose specialist. Comes in various widths and is cheap.

 

[J Neeves]

100km, almost next door!

I'll go on a search.

Thanks

Jonathan