The revisionist mythology of Wharram

To answer Fallguys call out. In simple terms Wharram and co started at a flexible mounting system of beam with external bolts on rubber tyre offcut mounts and straps over the beam. That was changed to the "rope" tie down system over the beams. Later web straps were used on smaller designs. But folks there is a variation on larger deigns. There is a 19 mm stainless steel rod through the beam and bulkheads on EG the Tiki 46 with lashings externally to tie the beams "down". The rod is placed in the centre section of the beam at a central point between the gunnels. The jpeg betlow shows a 46 being built with a doubler in the centre section of the beam at each end of the beams in the jpeg where the rod goes through.

Wharram, like all early designers, started at one point and developed things. But at the start of the multihull journey all the early guys (Wharram, Choy, Piver) were 60% marketing, 20% design, 20% engineering. Now the world of multihulls is 40% design, 40% engineering, 20% marketing. Be very glad Wharram sold 1000's of people on the concept of sun, females and escape, be glad Rudi Choy "guessed" (his words not mine) that a 40 foot cat should be build from 9 mm ply and be glad Piver told people and demonstrated simple home built tris could cross oceans. In each case they built a ground swell of support for what we now have today. Very seriously designed racing and cruising vessels capable of safe fast ocean crossing.

As to those people who still build Wharrams please remember the words of Jeff Schoinning, "I sell dreams". Some people buy into the technical side but the majority buy into the lifestyle promise and don't really care about how they achieve it.

 

It doesn't matter how many ropes there are, the elongation to break does not change.

If the breaking load is 2000 pounds and the elongation to break is 4%, then more of the same rope increases the tensile strength of the connection, but not elongation to fail. Two ropes break at 4000 pounds and 4% elongation. Nothing is halved from a mathematical perspective only. Probably some nuance to skip.

So the same load reduces the stretch of more ropes, but the reason for 8 lashings in the connection is one is not enough. This is where engineering is required and I shut up.

But, if say, you expect a force of 10,000 pounds, then 5 lashings would break, the 6th lashing provides a safety margin of 2000 pounds and perhaps added tensile strength crudely guessing at 4000, and so on.

So, the connection stretches based on the load and the tensile strength, but it does stretch.

If you assume (ouch), a load of 10,000 pounds, tensile strength of 8x4000 or 32,000; you can probably start to calculate or make some good guesses about the amount of stretch.

Ropes also stretch and don't return to their original length, so this is why we see loosened lashings and shims used on long passages as explained on a few youtubes. If a spacer is needed, then the loose lashing will certainly allow movement that is more slop than stretch.

It'd be fun to hear from someone who knows this issue well, versus a math hack approach like mine.

I know David Halliday has built a few of these, so he probably could answer well, but I don't want to pester him.

 

Fallguy,

I just realized there is a very easy way to calculate the load. The Australian method for beam strength is to assume one hull is fixed to an object (quay, wall etc) and the other one is left floating. Richard Woods (sorry to repeat his name) says these are loads you would not encounter on the water (edit: this is not correct, he said that about jacking up a catamaran from opposite corners). For the lashings, the inner hull lashing point (towards center of the craft) on the fixed hull is the fulcrum, while the outer lashing of the fixed hull takes the load. Here it is easy to calculate the tension on the lashing.

The Tiki 21, for example, has 3 beams. If we assume only the outer beams, take the load (not mast beam), then each beam takes one half of the floating hull's weight. The empty weight of the boat is 360kg, so let's conservatively assume each hull is 180kg. In the Australian method each beam must hold half of that, or 90kg. On the tiki 21 the forward beam has the narrowest mounting of 2 feet, so that is the most stressed. The beam spacing is 9ft, to so the lever arm is 2ft on the fixed side and 8ft on the floating side. That's a 4:1 ratio, multiply that by 90 kg and we have 360kg of load on the most loaded lashing.

Roger of Tiki 21 little cat says "The plans specify 5 full turns of 3/16 low-stretch," so that's 10 lines of 3/16 holding a force of 360kg. So 36kg per 3/16 strand? I have to get ready for the day, so I'll leave that here, but I assume there is some chart somewhere where we can get the stretch from the load.

The other issue is I believe you don't want your boat to flex because you need it to be stiff to maintain rig tension. If the lee shroud goes slack, that can result in higher rig loads on vessel. And that makes me realize that the lashings, if flexible, could potentially create more stresses on a catamaran than a rigidly fixed beam.

I'm not a boat designer or structural engineer, so this is all speculation. If you know better, please correct me!

 

I am Hanneke Boon, mentioned in the original post. I have been James Wharram's partner and co-designer for 50 years. I am appalled to read various people's speculations and prejudices in this thread and felt compelled to join the group in order to write my answer.

I think we all know that what is written in newspapers often has to be taken with some skepticism. You are lucky if they spell your name correctly, but often the facts given in articles are far from the accurate truth. This was the case in the 1950s, just as it is to this day, when you read the sensationalist Red Top news papers, like the Daily Mail or the Sun. It was the Daily Express that reported the untrue story of the wrecking of Tangaroa on arrival in Trinidad and the sharks in the bay. It greatly worried James' parents as this is the first they heard of their arrival. An unscrupulous Trinidadian newspaper journalist, who met them on first arrival, had sold the story without permission and had not sent the promised telegram of their safe arrival to his parents (this is all written accurately in Two Girls Two Catamarans).

Wharram's story as written in James' and my autobiography People of the Sea is the true story. James was greatly inspired by the voyage on double canoe Kaimiloa by Frenchman Eric de Bisschop. James studied Canoes of Oceania in depth to gain knowledge of Polynesian canoe types, but he ultimately based 23'6" Tangaroa on a model in the South Kensington Science Museum of a canoe from the Society islands. The model is still there, I photographed it some years ago. James had an original B&W photo in his archive, which he bought at the museum in the early 1950s. This photo is printed in Two Girls Two Catamarans. James could not build the curved hull shape out of the wood available to him, so simplified it into a flat bottom dory style shape. He would be the first to acknowledge that this first design was crude, but it served him to show a double canoe was seaworthy. He never planned to sail round the world on Tangaroa, that is a newspaper myth/exaggeration. He did later plan to sail round the world on his second catamaran the 40ft Rongo he built in Trinidad. I have every newspaper article from the 1950s in our archives and it is amazing how many facts they got wrong.

All Wharram catamarans are connected flexibly in order for them to 'give' to large ocean waves without stressing the structure. On the early Classic Designs this was done with rubber mounts on metal brackets that acted as shock absorbers. The later designs from 1979 are fastened with rope lashings (metal fabricating has become very expensive). The flexing of these first prototype lashings was carefully calculated with the help of English Braids in 1975 for the first Pahi prototype, so they gave the same amount of flex as the rubber mounts. One must ignore the comments of people writing on this forum who have probably never sailed on a Wharram catamaran. The flexing of the structure is relatively small, but it focuses the stresses in the ropes and saves stress on the rigid wooden structure. Ropes can be replaced making for a very durable structure, avoiding the fatigue cracks that often appear in older rigid catamarans.

Our small designs under 30ft like the Tiki 21 and the earlier Hinemoa can be bolted or fixed with short tight non-flexing lashings or webbing straps as the stresses are so much less in these boats.
Read more about how we design here: Articles & Blogs | James Wharram Designs https://www.wharram.com/articles And anybody who believes that Wharram catamarans are slow should watch this video of a remarkable record Atlantic crossing on a Tiki 38, where they averaged 6.5 knots over the whole 4500Nm 31 day voyage from New York to Marseille (averaging 150Nm/day) and had a peak speed of 17.1 knots

 

Hi Hanneke, welcome to the forum. Thanks for your detailed reply. It must be irritating to have some "rando" on the internet pick apart your designs, and your life to some degree, and for that I apologize. I should not have created this thread, but it's a bit late for that.

I do agree that newspapers can be an unreliable source of information, and your claims that James never intended to circumnavigate, or that Tangaroa never broke up are plausible.

I've also previously seen the very nice video of pilgrim, including the part where they hit 17 knots. Additionally, Roger, the owner of the Tiki 21 "little cat" has posted VMG figures on his blog which are respectable. At one point Roger was sailing his Tiki 21 800 NM per year on the pacific coast and the San Francisco bay, which is a testament to the design as much as anything.

Wharram Tiki 21 "Little Cat": Can Wharrams Go To Windward? (tiki21littlecat.blogspot.com)
site:tiki21littlecat.blogspot.com VMG - Google Search

When I said "somewhat substandard" in reference to the engineering and performance, I was referring to structurally inefficient features of classic designs, such as stringers that lay flat instead of on end, and solid beams. I am also referring to the deep v hulls without daggerboards, which are the least efficient type of hull for a catamaran and put your designs at a disadvantage to other multihulls in light airs and to windward. Anyone who has sailed a classic Hobie 14 or 16 knows this. Does this mean the disadvantages of these design features outweigh the benefits, such as ease of construction? No, it does not.

Regarding the photo of the model in Kensington, I assume you're referring to the 2001 reprint of "Two Girls Two Catamarans." My 1969 copy does not contain that photo, so I'll assume it was added later.

 

Fallguy,

To continue my thought process on the lashings, we were at 36kg per 3/16 strand of lashing. But this of course is an extreme load that would be not seen on the water according to RW (edit: again, RW didn't say this see post above). So let's assume a safety factor of 2:1, which puts the load at 18kg max, and probably much less under normal conditions. (Edit: "safety factor" here refers to a hypothetical factor assumed baked into Australian rule. The assumption was this rule has some relationship to loads seen on the sea, and that the rule has some margin of safety. You can assume a factor of 4 and get lower stretch. You can also assume 1, which would mean the rule has no margin of safety, and get higher stretch)

Sta-set, which is a popular brand of dacron line, has a break strength of 1,400 lbs in 3/16, which puts our 18kg load at 2.8% of break strength.

If we assume the line is pre-stretched, and the lashings are pre-tensioned, then the rope stretch (when I say stretch I mean reversible) should be linear with load. Say the line is pretensioned with frap to 50kg, then the load would oscillate between 50 and 68kg. Assuming the figure I previously listed of about 1% stretch per 10% of breaking load is correct for sta-set, we have 0.28% stretch.

I'm guessing the total loop length of the lashing on the beam is about 20 inches, which makes each side a guestimated 10 inches. So 2.8% of 10 inches is, drum roll, 0.0028 inches or 0.71 mm.

Again, we have a 4:1 ratio with the beam, so that would allow the opposite hull to move a total of ~3mm when the rope stretches 0.71 mm. Edit: According to RWs page on crossbeam design, I believe this would be something like 12% - 24% of the movement caused by the bending of the crossbeam, assuming a beam of 1-2% stiffness.

 

When I said "somewhat substandard" in reference to the engineering and performance, I was referring to structurally inefficient features of classic designs, such as stringers that lay flat instead of on end, and solid beams. I am also referring to the deep v hulls without daggerboards, which are the least efficient type of hull for a catamaran and put your designs at a disadvantage to other multihulls in light airs and to windward. Anyone who has sailed a classic Hobie 14 or 16 knows this. Does this mean the disadvantages of these design features outweigh the benefits, such as ease of construction? No, it does not.

And this is why new designs have been added to the portfolio, with new innovations and improvements.

 

I've decided I've generated my fair share of bad karma for the year and am going to stop posting on this thread. There was nothing constructive to be gained by starting this thread, and I regret doing so.

 

Sorry to hear that waterb.
Happy Holidays, hope to hear from you again in the new year.

 

I beg to differ.
One can never assume extreme loads to a degree of magnitude of any absolve value. Only that, an extreme load will at some point in the life of the vessel, occur. When and where...is another matter.

And therein lies the issue. With an unknown load, is a FoS of 2 sufficient?
If one is putting ones life into such 'trust' of no extreme load case and an "assumed" lesser load...i would say a FoS of 2 is rather low!

 

My only comment on Wharram was that he did quite a bit of trial and error. I'd hardly say that is untrue and is supported by many comments. Pretending a designer is above criticism is rather silly. Wharram has brought many people enjoyment on catamarans. One of my favorites is this fellow.

 

One can ignore the comments of those that do not understand structural engineering.

Define small, and what is a "safe" amount of deformation?
Ropes only work in tension.
When ropes are lashed they lose approximately 35-50% of their tensile strength.
The general rule of thumb is that the working load is some 20-25% of the tensile strength.

That is a lot of rope one must content with...

A rope cannot provide any absorption of the energy, as it only works in tension, unlike a shock absorber - as that is its MO, converting kinetic energy.

How many chandlers selling rope, in the middle of the ocean, do you know of?

With the amount of unknows in a seaway being imposed on a vessel, and the actual working load of rope, that is a very bold statement to make, when lives are potentially at risk.

Fatigue is a well known mechanism of failure, and like most things in engineering, there are do's and don'ts that either enhance or retard this mechanism. The principal ones being poor fabrication and structural design.

Flexible structures are used the world over...but like everything, they have their limitations and must be fully understood and quantifiable before advising others of the design procedures and/or benefits. So the engineer and make an informed choice.

 

Thoughts on Warren boats a in the form. No point covering it again.

I have learned to use a computer by speaking in case anyone is interested. Still difficult.

Interesting YouTube forum.

 

Isn't there also, in ropes, some inelasticity? So, when a rope is stretched, it does not come back all the way? Of course, this doesn't mean a rope stretches an inch and return halfway, but all the Wharram's need to be relashed. So, the unknown would be the extent of the elasticity and then if a rope has been stretched; it certainly must also lose some tensile strength. And then, when a wave hits the boat on a loosened lashing, I may be wrong, but the looser lashing allows the same wave to apply more torque to the vessel as the lever arm length has changed. Over many wave cycles, this incremental increase in torque and incremental change in tensile can lead to failure. So, while the system is decent; it has clear drawbacks due to the nature of rope.

Alleviating stress on the hull sounds like a misnomer to me as well. A well designed hull should never fail at a connecting point.

Again, no designer is above critique, just as no builder is above critique. Pretending otherwise results in things like the Titanic and Oceangate's Titan. The manufacturing videos of the Titan led us to horrifying observations of construction malfeasance of dirt in the filament windings, handgluing of critical elements to name a few. Failing to criticize the vessel operator, design, and even the CG in the MV Conception disaster would be malfeasant by all of us. The idea 33 souls died and had no chance to escape ought to haunt all boat designers and the CG as well. The passengers had zero chance.

Criticism is what makes boats better.

 

I used to own a 34' Tangaroa. It had the metal brackets over the beams instead of the later lashed type. In general, I sail boats hard. I once left New York harbor at the tail end of a hurricane. It was blowing at about force 9, but the seas were larger because of the hurricane. I had a rogue wave break over the side and completely bury the boat. I climbed the mast and saw it dissapear. Nothing broke and the boat handled fine. The next morning entering Barnegatt inlet the tide was comming out. The waves were really steep and breaking. I surfed one wave all the way in (not by choice). My experience with Wharrams designs is that they are seaworthy.