Multihull Structure Thoughts

Larry Pollock had the Running Cloud at Tarpon Springs Marina back in 89, he was dismantling it to take to Lake Tahoe; was surprised to see it back racing; your pics look a bit different, could just be the angle, b&w etc, here's more recent one:

 

Nice to see Running Cloud still sailing.
I remember seeing her here in Barbados after she screeched across the Atlantic in the first ARC Rally in 1986.
I think that she was the first boat across the line on elapsed time.

 

The three 50 x 34.5 foot open bridge deck designs from Shuttleworth are good cats and due to their weigh studies taught us all a bit. The Tektron 50 weighed 10,000lbs, The Dogstar 50 ranged from 9400 lbs to 8800 lbs depending on core type and fabric choice. But the Dogstar 50 had the greatest weight reduction when it redesigned to reduce the hull surface area and remove unnecessary bulkheads and furniture etc. The Dogstar 50 weighed 7950 lbs. The Pulsar 50 was designed over 20 years later than the Tektron 50 and with the resulting improvement in materials and understanding of construction techniques etc it has slightly more accommodation but only weighs 7800 lbs.

The most important lesson from this design sequence is good initial design to minimise weight is the most important feature. Reducing surface area, minimise unnecessary items, do you really need 2 extra double bunk cabins, are chart tables needed in this modern electronic world, can you reduce or simplify the rig or beam width etc.

The next major step is the choice of materials and structural design. Thin prepreg carbon fibre skins and thick Nomex honeycomb cores may produce the lightest results but can you build with the materials? Do you have the resin infusion equipment, the ovens or autoclaves to get the optimum strength and weight? Thin skin prepregs require very good surface finishes to prevent moisture ingress to the honeycomb core etc. A high density PVC foam core with thin skins may initially appear heavier but after a few year’s service may end up being a lighter boat.

But the real issue for most people is the use of the boat. A true cruiser will carry a lot of water, fuel, anchor equipment, spares etc and also human food, clothes, books etc. Can any of this be reduced? If you want air conditioning, hot running water, sound proof engine rooms, a boat capable of 20 knots under power, electric winches etc you are in a different situation. The reason for this part is that a “light weight” boat structure is a little futile if you are going to carry the equivalent of your house around with you.

Second issue for a cruiser is how bullet proof do you want the boat to be. Thin skin multi’s have lower puncture and knock resistance. If you are prepared to repair the boat as required then thin skins are structurally alright. Most people don’t want to continuously repair a boat. In short match your structural design for your usage requirements.

Be honest with yourself about what intend to do with the boat and then be honest with the designer about what you need and you will often get a good long term boat. Most stock designs are over designed and often over built by home builders so it “will be strong”. If you want a light fast boat start with a design mindset of minimizing the requirements of accommodation, payload etc. This will allow the designer to reduce the structure and structural material weights for a lighter boat. Just be prepared to treat it kindly and not sit it on rocky beaches.

 

Moderator, could you please place this table of contents page 31 to 45 on the front page.

 

Done. Thanks!

 

A small story of a man who had a dream that took 20 years to grow to a real boat. His dream started when he went to a boat show and looked at nice cats with big price tags. He wanted a cat but could not afford one. He searched found a design and spent 15 years figuring out how to build it. He and his father finally started in 2011. He choose a Suttleworth 32 cat. Its 32 x 22.5 and weighs 5000 lbs carrying 650 square foot of sail on a 49 foot mast.

They opted to build with resin infusion which required full moulds of the deck and hull to start with as shown in the jpegs. They initially did dozens of vacuum tests of the laminate schedule and every single one was a complete disaster. They sent samples to their vendors who also couldn’t get the resin infusion to work either!

They didn’t give up though because they had already built the first mold and it was beautiful. In desperation they tried something a little radical, against the advice of nearly everyone they spoke with. Sure enough, it worked and they infused two perfect decks off of this mold. Then “Unfortunately, I had been laid off but was given a severance package that allowed me to spend the next six months doing the final assembly. This was when the true scale started to really show. I got a great pic that I was showing off to all my friends when I had parked my car almost entirely under the boat.”

They spent the summer of 2014 assembling all of our prefabricated pieces in the Houston humidity. A commercial spar maker was employed to make the mast, a beautiful 49 foot tall Selden aluminium rig. Quantum sails measured, constructed and deliver the sails. Stix obtained a 2.1 meter carbon bow sprit and they have also built us a custom carbon radar mast. Almost exactly 4 years after they started, they were finishing the interior and awaiting the sails which have been ordered. Based on the fact that she weighs very little (less than 6000 pounds), is 32 feet long and 22.5 feet wide, the boat flys.

This is about finding solutions to difficult problems. The attached materials list for a Shuttle 32 (31) which will give an idea of what is involved. There was about 8000 build hours with effectively the construction of 2 boats, one a complete set of very good molds and the real boat. This design sails really well and can be built in strip plank cedar, foam glass or cold moulded ply. Rig options are aluminium or a carbon fibre wing. It can and has crossed oceans.

 

The Shuttle 32 mentioned yesterday was a resin infusion build where the builder said he had to experiment with various methods to finally understand how to get completed infused hulls, decks and parts. The builder has a website about the build which has several photo’s (samples below) and several very good video’s of the resin infusion process. He has a simple infusion set up that appears to work well. The completed boat “Josephine” was available for sale in the US for $US130,000 after several years of sailing.

Also, when you see the final hull shot prior to painting you will notice around the bow the reason that a female mould is better than a male mould. The interior of the hull is very smooth but the exterior of the infused hull requires some fairing before final painting. This is a good boat with a builder who solved problems as he went. A lot of extra people to help at the right time also helped enormously.

The website is a little slow in loading (warning has a high number of ad trackers associated with it). Look under projects for the build photos and video’s. The address is: https://www.packsail.com/

 

I have a lot of time for the mathematical nous of John Shuttleworth but I don't think the Tektron 50 - Dogstar cats can be called cruisers in any real sense of the word.
One reason is the design brief to reduce windage with the Dogstar. In this Shuttleworth reduces freeboard and rounds the topsides to encourage wind flow over the hulls. This is in direct contravention to most liveaboard ideals - where wind flow, with its carried fine spray, is reduced as much as possible.
Then there is the finely computed calculation of weight. Almost all cruising boats are overweight and one very important number Shuttleworth does not include is a thing called the immersion rate. In this you get to see how much the boat sinks with increased weight. The immersion rate skyrockets on the Dogstar hull type.
I have designed and built two boats similar to the Tektron shape. I used Shuttlworth's articles to assist me. But these hulls are wickedly unsympathetic to overloading or overbuilding. Whereas a "normal" cat hull will sit down in the stern and maybe require a stern extension to return to its previous hydrodynamic qualities, the heavy flare shape does not. It will require a total rebuild of the immersed sections of the hull. Every cm in extra depth increases beam quickly. When going to windward, this effect is marked, as the leeward hull is pushed down in the water. This hull shape can be a good choice and it does increase interior room, but I don't think it should be used in a cruiser with no reserve buoyancy when launched.
Interestingly, Shuttleworth was looking at his trimaran with its flared hulls and thought that the low flare could be used on a cat. There is a big difference however, a tris main hull only ever is lifted when sailing, whereas a cat's lee hull is pushed downwards by heeling forces. So don't just look at these hulls when at anchor, in your minds eye, picture the lee hull when overloaded and when going to windward. The difference in drag between the lee and windward hulls will be large. I see low flare on a multitude of cruising cats now and I struggle to see how a responsible designer can use this feature without catering for its potential to dramatically change hydrodynamic properties when hard pressed to windward.
This feature can be done well, Mark Pescott's Whitehaven's are a case in point, but the feature is softened, and as a cruiser and racer, Pescott puts enough volume in hulls to ensure that the worst aspects of the flare are reduced.

 

2015 Shuttleworth 32 Catamaran for sale - YachtWorld https://www.yachtworld.com/boats/2015/shuttleworth-32-3464781/

 

And the man's name is ???

 

I think the mans name is Will Pack. The 32 cat is Josephine please refer to the following website page for further detail. https://www.packsail.com/news

 

Yes his names William Pack and we saw Josephine during the fairing/painting stage and also after completed. She is a stunning catamaran and priced very well.

 

Catsketcher comment about Shuttleworth hull shapes not tolerating overloads very well, is accurate. But any hull shape that does not have much flair above the waterline has the same problem if they have fine length to beam ratios. So, lets analyse this a little further. I will try and keep this simple and not use much maths. There are 3 aspects of a multihull hull shape which dictate its load carrying ability. The length to beam ratio. Its fullness in the ends controlled by the prismatic coefficient which is the relative fullness in the ends relative to the centre section. Finally, the block coefficient which basically means a square hull shape can carry more than a semi circle can carry more than a V shape. For the purposes of this analysis we will use a fixed prismatic coefficient and block coefficient. Both of these coefficients can apply to any size of hulls.

What stays the “same” for this analysis is the length of the waterline, the relative fullness of the hull ends and the general shape of the semi circle below the water. What will vary is the width of the semicircles or the length to beam ratio (L:B) of the hulls and the hull shape above the waterline.

For a “cruising“ 50 footer like a Lagoon the hulls L:B is about 7.5:1. It displaces 50,000 lbs and can carry a very big overload due to it fat hulls. The shape above the water is pretty irrelevant. 6 to 8 knot averages, 12 to 14 knot peaks.

A higher performance cruiser the hulls L:B 9 to 10:1. It will displace about 40,000 lbs and again will be able to carry an overload irrespective of the shape above the waterline. These boats will average 6 to 9 knots and peak at 15 to 18 knots and if it has an efficient rig and boards be able to go well upwind.

At about a L:B of 11:1 a 50 foot cat will displace about 30,000 lbs and now will start to depend apon its shape above the waterline as to what overload it can carry. If these cats are built to weight and are loaded sensibly, 10 knot averages are possible with 20 knot peaks possible.

Now we get to the point Catsketcher is making. If the hulls have vertical sides from the waterline to the gunnel any overloads will add wetted surface fairly quickly which slows a boat, but having finer waterline shape allows the boat to drive through a seaway faster. Translation in flat water and stronger winds the added wetted surface will slow the boat, but in a seaway the narrower hull will allow it to push through the seas better than a fat hull.

If the shape above the waterline has some flair as in a Wharram hull or a chamfer between the bridgedeck and hull side you will pick up less wetted surface in an overload but you will have a relatively fatter L:B ratio as the hull is depressed. The boats speeds will slow in flat water and waves. This is where you hope your designer has a good understanding of the total hull shape and how it acts in a seaway over a load range.

Once a cat has a L:B of 12:1 or over you are in the performance range. These boats are designed to be built to a specific weight and carry a nominated load. Once these boats are built to heavily or are overloaded, they slow down. But if they are sailed at there designed displacement 12 knot averages are possible with peaks of 25 knots. A small amount of hull flair will help absorb small overloads but they are more sensitive to overloads.

But we will now talk about a real bridgedeck cat that was designed to displace 9500 lbs with 14:1 hulls. Its built weight came in at 12500 lbs. The 1000 square foot of designed sail area was upped to 1200 square foot after a few years. The additional sail area compensated for the extra wetted surface and this boat, although heavy, was capable of 22 knot averages over 50 miles and due to its heavy weight capable of ghosting well in small sloppy seas. In short overloads in narrow hulls are not be as bad as people think as long as the boat structure can take it and the underwing clearance is good.

The above is a very generalised, simplified discussion but the trends are real. If you want a high performance boat you have to limit your carrying capacity and build to weight. Very few designers can get true high performance heavy boats.

 

Sylvain Carignan is part of the design team of the TORO34 an open wing deck racer cruiser that has hulls that can fit berths (but not much else). The TORO34 is 34 x 20 foot weighs 2200 lbs and carries a 52 foot carbon wing mast with 527 square foot of main, 167 square foot of jib and 807 square foot of Code 0. These are characteristics of a fast boat. The goal was to produce a catamaran with large volume of buoyancy in the hulls to provide security and allow the pushing to the limits in downwind sailing, this volume also allows you to insert berths.

The original design was to be in foam fiberglass with aluminium beams and mast to provide a cheap fast boat. Before the first boat had been delivered it had morphed into a full carbon fibre boat with carbon cross beams and mast.

The Toro34 was first delivered in 2011. The Carbon front and rear carbon crossbeams will improve performance and overall stiffness of the boat while reducing the overall weight. The catamaran that can be put together by 2 persons without motorize assistance (crane or other) in 4 hours. Catamaran transportable by an extendable width trailer or by container (40') and has 4 berths.

The hulls are resin infused carbon epoxy corecell baked in an oven. The cross beams are 250 x 250 mm carbon epoxy infused and baked. The mast is a carbon epoxy wing autoclaved as are the rudders, boards, tillers, spinnaker poles. All components of the boat are painted to reduce weight. No gel coats.

The non foiling version of the boat had a light helm, precise and very responsive even in the gusts. It did 16 knots of boat speed going upwind in 18 knots of wind without code zero. The next day, in light air, 8 knots of wind it did 12 knots of boat speed with the code 0 and 7 people onboard. Later the boat reached 16-18 kn with the jib in 15/16kn of wind and wave around 1m. Then touched 20kn of speed in 15kn of wind with the code zero.

A foiling version has been developed and is faster. But they have now developed a “cruiser” version with a hard deck cover for some protection. A fast interesting design.

 

The following is a dream for all bar the top money earners. Nekker Belle (formerly Lady Barberetta) was brought for $5.3 million and upgraded by Richard Branson. It is 105 x 46 foot, displaces 224,000 and carries a 3200 square foot main and a 2100 square foot genoa. The length to beam of the hulls are 12:1. The boat can seriously sail with speeds up to 22 knots and it can really go to windward with its daggerboards and 10 foot plus draft. But I suspect the two 425 horsepower engines and 7000 litres of desil is the main motive power in a lot of short trips.

Now we get to the boat’s history original design intent and the reason for a 2 year upgrade/refit in Australia. Lady Barberetta was build in Carbon fibre and nomex sandwich with titanium metal components and intended to weigh 135,000 lbs with a rig designed for that weight. The original French owner ran out of money with a subsequent owner fitting out what was intended to be a sports cruiser but was turned into a charter yacht. The accommodation and equipment was increased which upped the weight to about 180,000 lbs but the rig was not upgraded and inadequate. When Branson purchased the boat he understood it would require an upgrade. He did not anticipate 2 years. The reason was the original build documentation was very poor and no one could remember details of the build. Result the boat had to be completely surveyed to understand the structure and equipment then virtually redesigned to suit the new weight displacement requirements. EG the new rig required a new carbon mast, rigging and sail plan. We are talking at least a $2 million upgrade. When the boat had been used for a while Branson sold the boat for $3 million. As one rich person said he enjoyed his boat for 2 days. The day he brought it and the day he sold it. The rest was an expensive toy.

But this boat is about internal and external luxury as the jpegs show. If you can afford the mooring fees, the 6 crew required to assist in driving and maintaining it and the regular upgrades of all the equipment on the boat it will be perfect and it will only cost $3 million and requires $1 million per year to run it. No wonder it charters for $60,000 per week. Also shown is one of the toys, the boat carries a submarine. Dreams.