Multihull Structure Thoughts

Hello Michael thanks for participating, I think it’s great this boat exists. But I am mystified as to why I have been told on repeated occasions by both you and Laurie McGowan that plans were never completed ?
Does 02 mean there is an 01 ?

 

This thread continues to surprise me with people who give their time to help us all understand what is being or has been built in multihulls. Thank you all. My aim of this thread is to gather information about interesting multihulls, so that all people can understand what makes a reasonable multihull, its structure and how to build a good boat.

Back to the 23 x 10.2 foot proa discussed yesterday. This part will discuss the rig(s), sails and voyaging done by this proa.

First, we will discuss the mast. The mast is bamboo, glassed, with cleats epoxied and lashed on with epoxied cotton string and has been reliable. There’s a bottom plug of wood and the masthead is a foot or so of pine with relieved holes for the attachment of stays and halyard block. 3000 miles later – zero mast trouble. The yard and boom are made of four carbon fiber windsurfing masts, joined in pairs at their fat ends with rods of hickory then cut to length and the ends finished with inset knobs of wood, with holes drilled in these to tie the corners of the sails out to. The mast sections are filled with foam to prevent sinkage and the carbon is painted to protect it from the sun. A string runs from one end to the other and back to make sure the sections don’t pop apart in use.

The builder has made all his own sails and has learnt a lot about cutting a shape, materials, assembling the sails etc. His most successful sails have been made from heavy duty polytarp. The largest sail is the latest one made with the best shape and has powered the proa over 700 miles to windward on various parts of the voyage. No. 1. Heavy-duty polytarp 160 square feet, No. 2. Dacron about 130 square feet, No. 3. Cotton canvas about 120 sq. feet, No. 4. Cotton canvas 65 square feet, No. 5. Heavy-duty polytarp 50 square feet. All the preceding sails are Oceanic lateens.

During the development cycle the builder also made a Gibbons battened sail made from light-duty polytarp of about 130 square foot. This sail was very powerful, more powerful than the 160 square foot Oceanic lateens sail, it moved the proa at 12.5 knots on flat water, and the builder believes that it could go even faster with this sail in the right conditions. The issue with the Gibbons rig was the difficulty in handling and reefing the sail in higher winds due to the battens.

To quote the builder “The Oceanic lateens is easier to stow in a hurry – just stick out your arm and drop it, and there it is all folded up in your arm. Even at sea you can get the Oceanic lateens down easily enough, if all else fails just drop it in the water, then grab the yard and pull it aboard bit by bit. This is harder to do with the Gibbons because of the battens. Unless you can point right into the wind you have trouble reaching them. After much experience I have come to the conclusion that rapid stowability is an extremely important attribute – there are so many times when getting that thing under control quickly is important eg. when being backwinded, when approaching a dock too fast, or a tree, during squalls or when the sail is stuck under a drifting boat.”

Remember, this man has sailed over 3000 miles with these rigs in coastal conditions and did not capsize his proa. Later when he made it to Panama, he capsized 12 times in the different sea and wind conditions. The proa’s asymmetric hull goes to windward at between 45 and 55 degrees depending on the rig and sea conditions. The 1.3 foot draft is an advantage in shallow waters. He would consider a lee board and a shallower hull if there was to be another boat. The proa’s average speed is about 7 to 9 knots with peaks of 14 to 15 knots. This proa is a cruiser not a racer even though the length to beam on the main hull is about 20 to 1. He has sailed the proa as a live aboard cruiser for over 18 months often single handed and occasionally with a crew. The major luxury that he purchased was a foldable beach type seat that allowed him to seat with a back rest while sailing.

His experiences living in Mexico and dealing with Mexican authorities makes me glad I live in Australia. Corruption and silly regulations can make life hard. EG It takes over a month to register a boat, and any size of boat is required to carry EG 40 lbs of sand to act as a “fire extinguisher”.

Again, this shows if you have little money and a sense of adventure you can cruise for a long time. The jpegs tell more of the story. Part of the blog describing the proa (near bottom of page) is at: July 2011 – Grillabongquixotic’s Blog https://grillabongquixotic.wordpress.com/2011/07/

 

Here’s the story: The builder contacted me back in the summer of 2019, inquiring about plans for Evergreen. I gave him the usual answer: that no plans exist and none are expected. However that didn’t put him off. He wanted to build it anyway, using only the lines drawing. Said he was an experienced builder and had done it before, sending along a pic of a lovely sloop on a trailer. So I figured, what the hell. I sent him a hi-res file and then every few months he’d send me some progress pics. He never asked for advice, and he worked out all the tricky parts himself, like the custom carbon masts, rudders, sliding beams and folding cockpit floor. He did not build it exactly like the drawing, you can see his custom deck hatches in orange, but since there ARE no plans, what could I say? Covid slowed him down since suppliers were slow to fill orders and the kids were home all day, but he kept plugging along and finally had the boat in the water and sailing the summer of 2021. She was built to a very high standard. I couldn’t have wished for a better builder.

So how did it all work out? She is fast (as you’d expect) and he says the acceleration out of stays is like a rocket. She tacks well despite the keels. The biplane rig is handy but has its quirks and one sail will backwind the other at times. The sliding beams are problematic at best.

If you’d like to know more PM me or use the contact form at Proafile.com, I don’t want to hijack this thread anymore than I already have.

 

Hi Oldmultiu!
Do you have any insight into the Crowther 220 design? I have found one that I am seriously circling and thinking to buy as a cruising boat. Seems like a very well-designed cat with OK living space and great performance. Wondering if you have ever sailed one or know anything about the 220?

 

Dan. I have not sailed on a Crowther 220 but have sailed on a Crowther 226 and other Crowther cats of similar bridge deck, full ended hull concept. They all have good to very good sailing characteristics especially upwind and in heavy weather. There is only one proviso. They should not be over built or carry an excessive payload. The original Design 220 was 43.8 x 24.1 foot weighing at launch about 10,000 lbs and displacing 14,000 lbs if well done. The 60 foot mast carries about 1030 square foot of sail area. The daggerboards draw about 6 foot with a 2.9 foot draft over the spade rudders.

The Design 220 structure was foam e-glass with polyester or vinylester in the hulls, decks and bulkheads. The accommodation layout was originally conceived for extended cruising or charter work. The layout is 4 double berth cabins in the hull and partially on the wing deck. The main cabin has the galley dinette and navigation. The cockpit is large and effective.

Now I will speak about the Crowther design 226. Design 226 was designed about a year before the above cat but is very similar and I have more knowledge about 226. Design 226 is 42 x 22.7 foot and weighs 9,300 lbs with a displacement of 13,200 lbs. The aluminium mast is 60 foot high with an 827 square foot mainsail and a 248 square foot jib. The hull length to beam is 12.2 to 1. The hull draft is 2 foot, over rudders 2.7 foot and with daggerboards down 6 foot. the underwing clearance varies from 2 to 2.7 foot. The hulls are full ended and splayed out at the bottoms.

The layout is 4 double berth cabins in the hull and partially on the wing deck. The main cabin has the galley dinette and navigation. The cockpit is large and effective.

The structure of the 220 design would be virtually the same as the 226 as Lock at that stage had a fairly standard structural approach for similar displacement cats. The hull skin and underwing had 1150 gsm triax outside 20 mm PVC foam and 760 gsm triax inside. The underwing has 4 deep underwing stringers and sides of the aft berth pods. Decks had 706 gsm triax outside 20 mm PVC foam 650 gsm biax inside. Secondary bulkheads had 650 gsm biax on either side of 20 mm PVC foam. Main and aft bulkheads had at least 2 layers of 1150 gsm triax on each face and a solid unidirectional e-glass flange top and bottom with timber inserts under the mast. There was a triax, Kevlar hull option but I am not aware of any being built that way.

Sailing characteristics of the 226 would be similar to the 220. In short it can sail very well across the wind range if you have a good wardrobe of sails. It will peak at over 20 knots and cruise at 10 to 12 knots under good conditions. If you have a large genoa it will do at least 75% of wind speed in lighter conditions. The cats real capability is going upwind in heavy airs as Lock had worked out good hull shapes by that stage for most wind speeds. Under reduced sail area in moderate winds the cat is much easier to handle and the cat will still maintain a good speed. If the rudders are correctly shaped the cat will be easy to steer and control under all conditions. Check the deck gear as these cats are powerful and need powerful winches etc to handle the loads. Electric winches or muscles are required to handle the sheet loads especially when tacking in close quarters. Understand who built the cat and get a surveyor to check out the structure. If it is built to plan it should be OK.

Hope this helps. Jpegs give some idea of the 220 and 226.

 

Oldmulti, thank you so much for your in-depth reply. I very much appreciate it. Would you happen to know who might currently have the designs/blueprints for Crowther 220? I contacted Brett Crowther but he wasn't able to help, unfortunately. At some point, the Crowther portfolio must have been sold on and I'm trying to find where I might be able to source / buy a set of 220 designs to help with a major refit of the boat.

 

Dan. Stuart Bloomfield has most of the Crowther portfolio and may be able to help. He is a little difficult to contact. But here is a start to his web address: Multihull Sailboats - Designs - Bloomfield Catamaran Designs http://www.bloomfieldinnovation.com/Multihull-Sailboats.html
Also another info page: About Us - Bloomfield Innovation http://www.bloomfieldinnovation.com/about.html

 

Thanks so much! Actually, I emailed him this morning after going down a deep research rabbit hole and finally finding his name and contact! Seems like he is the right guy to contact. Cheers!

 

How good is this thread !

 

Bloody Terrific!

 

Today, I am not going to be kind. The following 2 documents are from DET NORSKE VERITAS (DNV) which is an autonomous and independent foundation with the objectives of safeguarding life, property and the environment, at sea and onshore. DNV undertakes classification, certification, and other verification and consultancy services relating to quality of ships, offshore units and installations, and onshore industries worldwide, and carries out research in relation to these functions.

Translation: if you wish to have a vessel certified as a “safe” vessel in all aspects, you build the vessel to the standards listed in the following documents and you will have no problems getting the vessel registered, be able to do charter work or insured. But be aware as Rob Denny has said scantlings define by DNV and ABS certification groups are often conservative, highly theoretical with large safety margins and are tested by techniques that may not be real world loads.

Once standards in the old days were a set of rules like EG a rib will be placed every 500 mm if the skin thickness is 12 mm. Now standards are mainly mathematical calculations based on a materials characteristics and loadings that will be experienced by the vessel under specified conditions.

But please understand that the DNV standards can give a very good guide as to the loadings that can occur in a vessel just by surface reading. If you can do the mathematics, you can calculate the actual loads and EG the skin thicknesses that may be appropriate. Please understand the reason you pay a naval architect is they understand where the standards are appropriate and where they can be ignored. I can assure you, a lighter and just as seaworthy boat, can be built if you do not use the DNV structural standards. This is the reason a charter boat is often heavier than the same design built for please use.

So, hopefully you can at least speed read over the documents to get a greater understanding of some of the load points and thoughts required in designing. The first PDF is about general structural and equipment standards of a vessel. The second document is about Rig design for larger vessels. Sorry for not being kind but you may appreciate why you have to pay real money for next set of plans.

DNVG hull standard 2-21.pdf - https://rules.dnv.com/docs/pdf/DNVPM/stdcert/2010-04/standard2-21.pdf

Yacht Rigs DNVGL-ST-04121.pdf - https://rules.dnv.com/docs/pdf/DNV/ST/2016-12/DNVGL-ST-0412.pdf

 

The New Zealand designer, Ron Givens designed Tiger series of power cats. The power cats range from 27 foot to 60 foot, powered by outboards in the smaller sizes and diesel engines in the larger sizes. We will focus on the up to 40 foot outboard powered versions, one of which is an ocean crosser.

They range from 27 x 11.8 foot displacing 6,675 lbs powered by two 100 HP to 150 HP outboards that carries 590 litres of fuel and runs at 20 knots to a max of 26 knots. The accommodation is a double cabin, toilet and galley with dinette in the main cabin.

The next is a 31.8 x 13.5 foot displacing 9,400 lbs with two 150 HP with a maximum of 225 HP. Maximum speed is 25 to 30 knots depending upon the power and propellers. It has 700 litre fuel tanks and can do 300 mile range. The accommodation is a double cabin, toilet and galley with dinette in the main cabin.

The next is a 36 x 14.75 foot displacing 15,400 lbs with two 225 HP 4 stroke outboards. Maximum speed is 22 to 30 knots depending upon the power and propellers. It has 800 litre fuel tanks and can do 300 mile range. The accommodation has 3 double cabins, toilet and galley with dinette in the main cabin.

The next is a 39.2 x 14.8 foot displacing 16,100 lbs with two 250 HP 4 stroke outboards. Maximum speed is 17 to 20 knots depending upon the power and propellers. It has 1200 litre fuel tanks. The accommodation has 3 double cabins, toilet and galley with dinette in the main cabin. This Given-designed T11 powered from New Zealand to New Caledonia in 2010. It took two days and 18 hours (51 hours run time) to deliver the cat 936 miles to Noumea via Norfolk Island (fuel stop), at an average speed of 14.18 knots. Her twin 250hp outboards used around 2,500 litres of fuel – an average consumption of 1.335 litres per nautical mile per engine.

The 39.2 foot cat is an outboard powered version of the T11 design with accommodations and other custom modifications to suit client’s requirements. Accommodations are 2 double berth cabins forward, one compact double cabin aft to starboard with toilet. Port side hull has a generous ensuite aft under galley. Spacious saloon and galley area with fold down table when extra sleeping arrangements are required. Flybridge has a hardtop and zip-up plastic clears all round, and from the aft end steering station the skipper has good forwards and aft visibility. Cockpit has uncluttered open area with easy access to the large boarding platform on the extended bridgedeck between the motors.

The common point of the above cats is the construction which is composite epoxy, wood, plywood for the majority of the shell with foam inserts in some plywood sandwich panels and fibreglass taping etc. The whole system is designed to be totally unsinkable, even in the event of very serious damage at sea, as the floatability of the construction material is greater than total dead weight of the motors, engineering etc. Tomorrow we will feature the next cat designed and built for the New Caledonia owners of the outboard powered T11.

Some jpegs of the Given cats.

 

The next phase of the Given power cat development is aimed at those who didn’t want a planning boat, and didn’t want outboards, but wanted something cheap to run, big and comfortable and really good in a sea way – a good sea power boat. The Ecat series was born. The Ecat 10.6 was launched as the Econocat 10.8 that is 35.4 x 15.75 foot with a displacement 11,000 lbs. She is powered by a pair of 87hp Lombardini diesels with sail-drives. The displacement hulls are thin requiring less power to drive them with a fuel consumption of 0.55 litres per nautical mile per engine at cruising speed of 14.5 knots.

Another version “Margaritaville” has a few modifications. The hulls are from the same moulds but are 750mm longer for more storage space in the saloon and a larger galley. Overall, she’s also 250mm narrower, to fit into a marina berth. “Margaritaville” is 37.7 x 14.8 foot with a displacement 13,000 lbs. Again, she is powered by a pair of 87hp Lombardini diesels with sail-drives. At around 2,600 revs she has a comfortable cruise speed of 14-15 knots. At that speed Margaritaville is only using around 1.3 litres (total) of diesel per nautical mile.

The accommodation suits the owners’ requirements but in each case there are 2 double cabins forward an toilets in the hulls. There is work space and engine “rooms” available in each hull. The main cabin has a large, open central saloon and galley occupy the bridgedeck. The table can be lowered to produce another double berth in the main saloon, but this space is mostly for relaxing and entertaining. There is a navigation and steering position available.

The construction of the first Ecat was constructed of strip plank cedar and fibreglass. After testing, this boat was then used as a male mould to create female moulds for future production boats. The following cats’ hulls are moulded in foam and glass, with the wing deck and cabin top made of vacuum-bagged ply-foamply sandwich. Carbon fibre components are used in some areas for added strength without increased weight, such as the slimline mullions in the large wrap-around windscreen and lightweight carbon rudders and tillers. The hull and top of the main saloon are female-moulded foam and glass, with double-diagonal plywood topsides glassed both sides, and the cabin-top and wing deck vacuum bagged ply-foam-ply sandwich.

An owner reported: “A breeze kicked in for our cruise out into the bay, enjoying the smooth ride and easy handling. The hulls travel very cleanly through the water, with the knuckle just touching most of the time. We don’t get a chance to try much wave action, but Partridge is “absolutely confident that the boat is going to take the rough stuff. But we haven’t put any water over the foredeck yet – we don’t even get any spray.” Both the Ecats jpegs shown here have travelled many coastal miles to the satisfaction of the owners.

In both cases they have previous Given planning power cats and are much happier with the lower fuel consumption and easier motion at the cruise speed of about 14 knots. The jpegs tell part of the story.

 

This is a tale of 2 Russian designed cats. The first one is featured on Page 120 of this thread and is called “Timur”. That cat is 19.7 x 11.1 foot with a 1350 lbs weight and a 2700 lbs displacement. The wooden mast is 30.5 foot with a mainsail area is 129 square foot, a staysail of 86 square foot. The hull length to beam is 7.5 to 1. The draft with the board down is 3.25 foot. The underwing clearance is 1.7 foot.

The hull shape of Timur is flat bottomed. Now the following cat is similar but different. The second design has a V and has thinner hulls and a slightly bigger rig. The reason for the comparison is the design of many of the structures which appear to have a common designer. Less is known about the second design but there are more plan details.

The second cat is a Russian catamaran camp cruising design is 19.7 x 10.5 foot. The mast is 29.5 foot high with a 140 square foot mainsail and an 86 square foot jib. The hull length to beam is 10 to 1. Draft is 2.4 foot over centre boards and rudders.

The accommodation is simple with 4 berths, 2 foot wide at the head and 1.5 foot at the base, all 6.5 foot long. The cabin has 4 foot headroom with 3 foot over the bunks. The cockpit can sit 6 comfortably.

The construction is plywood and timber. Hull skin is 6 mm plywood. Frames are 20 x 40 mm, 20 x 45 mm, 20 x 50 mm. Chines, gunnel, stringers 20 x 20 mm. The 50 mm wide x 470 mm high and 9 foot long cross beam structure has 6 mm plywood faces over a 20 x 40 mm top and bottom flanges and cross support timber.

There are 2 sets of jpegs. Attached to this item is Timur the flat bottom design. The next set of plans is the V bottom design. Interesting camp cruisers.

 

The second Russian 19.7 x 10.5 foot plywood camp cruiser plans.