load

[adriano meis]

about you

how heavy is the load on every single lashing of the beams?

I mean howmany kg has every single lashing to sustain?

[Rory McDougall]

Hi Adriano,

It is hard to calculate how much force each turn of a lashing takes. Some turns are tighter than others. I would guess that the inner lashings take most load to begin with until the lashing has 'worked' itself into an even loaded bundle of fibers.

On Cookie I first used 4mm pre-stretched 3 strand polyester line as advised in Wharrams plans and used 6 turns on the beams. I had no problem with this taking the load or coming loose.  I now use 6mm braided line for lashing mainly because I want a bit more safety factor in case of chafe to the lashings over the top of the beams or high amounts of UV degradation from the sun.

For most lashing purposes I have found that the smaller the line and the more turns of the lashing will give you a tighter and more easily tied lashing system than if you use larger line and fewer turns.

 

Cheers

Rory

[Ann and Neville Clement]

We try extra hard to get the tension on each lashing turn about equal if we can, so they will all be working together and not just one or two tight turns and the others relaxed.  Hope this is clear.... 

[Galway Bay]

On my PAHI 31 many many moons and even years ago James specified each lashing as equal to 2 1/2 ton which would equal boat at 1 1/2 ton plus full load of 1 ton.

Based on this it would seem that each lashing should be equal to the weight of the laden boat. If you divide by the number of turns you should get a breaking strain for the rope used.

These are all designed loads of course actual loads are very different.

If you want to go all nerdy and mathematical about it bear with me a minute -

because James follows beam/length/mast height ratios fairly closely from boat to boat it is not difficult to draw a general overview. As an approximation the measurement of the height of c/e of the rig works out about equal to the beam so they cancel one another out when comparing heeling and righting couples - that means the force in the sails at the point of lifting a hull is approx equal to the weight of the boat. This is no more than many sailors would intuitively feel.

This is resisted by an equal and opposite force by the underwater hull. Again since the depth of this below the beam works out approx equal to the beam of one hull these cancel one another out when we compare the couples involved leaving the total strain on the lashing as approx equal to the weight of the boat.Thus if you have three beams the max load is weight/3 and if four it is weight/4 etc....again most sailors probably guessed this already.

The exception among James' designs is the Pahi range. Since the beams are pivoting on a central bolt the lever is only 1/2 one hull beam so despite having 4 beams the force per beam is doubled at weight/2 not weight/4 as you might expect.  Probably most sailors have guessed most of this already and the only value to this is that you can now hold your own with the know-it-alls at the bar [ that's club not harbor ]

In reality since most sailing is done -by me at any case- with a light boat at no more than perhaps 1/4 max power the strain is never very high.

May the wind be always at your back.....

 

[adriano meis]

Galway Bay said:

 if you have three beams the max load is weight/3 and if four it is weight/4 etc....

 

this means: if my tiki has 1000 kg load and three beams, the maximum load for each lashing is nothing more than 150-160 kg? (3 beams = 6 lashing)

and what about the leva law?

[adriano meis]

Based on this it would seem that each lashing should be equal to the weight of the laden boat. If you divide by the number of turns you should get a breaking strain for the rope used.

 

actualy wharram's suggest is 5 turns of 5mm prestreched rope: if the maximum load of a polyester 5mm rope is around 5-600 kg= 550X5  2750 kg for each lashing

[Galway Bay]

There are a number of separate issues here.

The spec. of each lashing being equal to the weight of the boat is as given to me by James when I was building in the late '80's

The point of max load for a cat. is at the point of lifting a hull. At this point the full load is on the lee lashings of the lee hull  all others being slack so on a 3 xbeam boat only 3 lashings carry the strain. If each can do the job on it's own then the safety factor is 3x.

 The nerdy maths bit is my own observation based on my boat and how wharrams approx follow the proportions of 2x beam = length = mast height.

The observations on the resolution of forces is based on opposing couples. A couple is a force x lever. So there's your law included. My point is that on Wharrams these lever lengths tend to be very similar - eg. on a boat of say 30ft c/e of sail is 10ft up mast so approx. 15 ft above w/l which is approx the same as the beam measurement so that these cancel each other out in the calculations.

My observations are meant as a general comment calculations will vary for each boat. I have the calculations for Pahi 31 your boat will be different but similar, just as the boats are similar but different.

 Boats that are very different eg. low 2x mast rigs extra wide xbeams etc will give very different calculations.

I expect that people who do not bother with math and just sail by the seat-of-the-pants intuition will not be surprised at the idea of each lashing being equal to boat weight.

In the real world of building and sailing as opposed to speculating while a lashing could be one rope such an arrangement would be impossible to tighten and would quickly saw into the beam. Likewise a rope of less than about 5mm is an effective saw. So while we may start with the math we must add in practical experience.

All of this is only wind induced forces the measurement of a breaking sea is probably impossible to calculate, certainly I would not try, practical experience is probably the only answer here.

Again we are talking max. calculated loads in a long time sailing I do not believe I have ever met this level of load and I hope I never do.

I am happy with the lashings as specified by the plans both the calculations and the experience of a huge number of sailors confirm this.

The only reason I looked into this for myself is that I wanted to replace the 4 full plus 1/2 beam on my boat with 3x full beams and eliminate the central pivot bolts.This has been a success.

Fair winds and safe seas be yours.

 

[kim whitmyre]

Great info, GB! Thanks for putting it out!

[adriano meis]

I'm geting  marryed today!!

. than we are ready for our honey moon. thanks for your suggest, i will study it tomorrow

[Galway Bay]

Congratulations !!!! Adriano

And thank you too Kim although in this case the congratulations are premature as I am only half right.

At the point of lifting a hull the force in the sail is only half the boat weight [it only has to lift half the boat] so the safety factor is higher than I gave being 4x for pahi and 6x for tiki.

I really should have checked my facts rather than relying on memory from so long ago.

James specified a lashing of 5 turns of 6mm as equal to 2 1/2 ton. I never questioned this but just today I checked with the American boat and yacht building site who rate it at 5 ton max/breaking and 1 ton safe working load.

 For sure the safety factor in these boats is high.

And Andriano no posting on honeymoon !! I thought Italians were supposed to be romantic !!

[Geminidawn]

There are three types of stress in this case the stress would be tensile. To calculate the stress you would use E = f/a.

Where E (stress) is equal to f (Force) devided by a (Area) in this case it will have be found using the value of Pi of the overall diameter.

We would test the material by pulling it apart until we find it's point of failure, tests would be taken on many strands of the same material to get an average. Once we know it's failure point we'd know how much of the material is required to do the job needed or what alternatives are better.