Multihull Structure Thoughts

The ISLAND SPIRIT 410 Catamaran is from Spiritdesigns in about 2017. The cat is a performance cruiser/charter cat that is 41.5 x 23.1 foot and weighs 18,000 lbs with a displacement of 22,400 lbs. The standard aluminium mast is 50 foot high and carries a 576 square foot mainsail and a 320 square foot jib. There is a performance rig option that has a 54 foot mast and a 730 square foot mainsail with a 342 square foot jib. The length to beam on the hulls is 9.5 to 1. The low aspect ratio keels draft is 4.1 foot. The hull prismatic coefficient is 0.624. The underwing clearance is 2.7 foot. The hull freeboard at midships is 6 foot.

The performance of the cat under the standard rig (aimed at the charter market) will be good with these numbers. The performance rig will help improve performance for faster cruising. The accommodation layout gives the clue as to the intention of this design with 4 separate double cabins and 2 full toilets in the hulls in a charter cat configuration. The main saloon cockpit combination will comfortable sit 8 and the galley area is large. I especially like the “roof” solar panel supports over the dual steering cockpit areas. There is a 3 cabin “owner” accommodation option available.

Spiritdesigns generally design in DuFlex glass, Balsa/Foam glass or Plain Foam glass. This cat is described as resin infused foam glass construction. If the cat follows the normal build procedure there will be a lot of flat foam glass or duflex glass panels with a separately built round bottoms attached to the rest of the shell. Spirtdesigns has been designing boats since about 2000 using these techniques and has a very good understanding of how to design fast build cats.

The jpegs give the idea. The PDF is the specifications, many drawings and jpegs of the 3 cabin version.

 

Old Multi - informative thread, as always.

As your currently perusing Thailand (Island Spirit Multihulls), care to comment on Alan Carwardine's designs over at Asia Catamarans? Their current Stealth Series seems very innovative. And the keen eye can discern several unique and evolutionary design features (even though they do not print them out on the internet). They seem to be 'oozing' the tropical lifestyle there in Phuket!

Home - Asia Catamarans Award Winning Sail & Power Catamarans

Cheers - Ogre

 

Coastal Orge, I will do an update on the STEALTH series soon.

The following is a small cruising cat that was probably inspired by “Miss Cindy” the ocean crossing 16 foot cat. The design came off a Russian web site Cyber Boats Co. | Многокорпусники https://smallboatsplans.com/plans/multihulls/

It is a small 17 x 8.2 foot cruising catamaran that is designed for small trips on bays, rivers and lakes by a crew of 1-2 people. On day trips, the crew can be increased to 4. The weight is 670 lbs with a displacement of 1250 lbs. The rig can be off a Hobie 16 etc with a mainsail 107 square foot and a jib of 52 square foot. The hull length to beam is 10 to1. The rudders are kickup and the centre boards draw 2.5 foot. The underwing clearance is 1.55 foot and if you put a foot well in the bridge deck the footwell clearance is 0.9 foot. The motor is a 2-3 HP outboard.

The hull lines with a relatively narrow flat bottom, large side camber and chine steps on the outer sides. The centre of buoyancy is slightly shifted to the stern and is located in the area of the aft bulkhead of the cabin. This prevents trimming aft when the crew is in the cockpit.

The interior of the catamaran is simple. Two full 2-meter bunks in the aft hulls are part of the structure. There are lockers under them. All other interior details - galley, etc. - the builder can place where he likes. The bridge deck is designed with a step pod, but you can skip it and make the bridge flat. Then you can place one more bunk on the bridge, and you can sit on the bridge with your legs lowered into the hulls.

The construction is plywood and timber with the following bill of materials.

1. Plywood Marine 2440x1220 9mm 6 sheets

2. Plywood Marine 2440x1220 6mm 18 sheets

3. Timber: o 20x30 mm 60 meter o 40x20 mm 20 meter o 20x80 mm 10 meter

4. Stainless steel fasteners

5. Glass fabric 40 meter.

6. Epoxy resin 20kg

7. Epoxy putty 5 kg

8. Polystyrene foam grade PS-1 with a density of 200-300 kg / m3

An interesting design for simple camp cruising. Jpegs give the idea.

 

This thread has been interesting and your efforts appreciated. Just a thought..

These boats fall naturally into categories, but there is nowhere that I know of where you can choose a "class" and peruse the variations. As an example if you were considering self build 30'ish bridgedeck cabin cats there are plenty of options (RW alone has at least 3!) but there is no way I know of of comparing the options to shorten your list. Much of that information is in this thread, but not in one place.

You know what they say, an idle mind is the devil's friend, and I'm still recovering from surgery last week. I need to think less...

 

A boat wiki with templates might be ideal for this. Still easy text editing, collaborative, and the template gives some structure but remains flexible and can be read by a script to generate categories etc.

 

Two items. There is a web site about the “Design of Sailing Yachts and Motor Boats: Scantling, Main Structures and Detailed Components” that is a multiple part tutorial about the design and scantlings of sailing yachts. Don’t do the registration just click on one of the blue text in EG Advanced Design: Bulkheads and Penetration Reinforcements and you will see a series of videos on an aspect of design. Some videos are 50 minutes long. This will take some serious time to view. More monohull based but informative. I do not know if there are any limits on viewing. The site is: Design of Sailing Yachts and Motor Boats: Scantling, Main Structures and Detailed Components ⛵ - CompoSIDE http://www.composide.com/webinar/design-marine-components/

Second item. The topic of capsizes was raised recently so I will just do a small piece on the best research I have found about multihull capsizes. Also, a comment made by Lock Crowther after he did some research in the 90’s.

The first piece of research came after the Fastnet ocean racing disaster with a short summary of: “The reasons for capsize of catamarans were: wind 56%, wind and wave 16%, pitchpole 12%, wave 8%, breaking wave 4%, flooded 4%.” And “84% of the catamaran casualties were the result of wind induced capsize or pitchpoling“.

This research has been shown before and is a 2.5 meg PDF download. http://www.wumtia.soton.ac.uk/sites/default/files/uploads/pages/STAB2000BD.pdf

The second paper is a follow up of the above paper refines the investigation of capsizes. A nice piece of information is:

Recommendation 8.4: “In severe breaking waves of height similar to or greater than the beam of the yacht…encounters on the beam or astern should be avoided”
This is interesting as it goes against the series drogue data (start here) which suggests that a drogue off the stern of a multihull is better than off the bow.

The paper is a 14.5 meg PDF at: http://www.wumtia.soton.ac.uk/sites/default/files/1441_merged.pdf

Lock Crowther's independent research found as follows:

“This work [tank testing multihulls] has indicated that the well designed catamaran is remarkably safe in breaking waves up to considerable height, even when beam on, we were unable to capsize a power catamaran yacht in the largest wave which could be generated. This corresponded to a 52′ wave for a catamaran of 40′ beam. Scaling this down to a typical 24′ beam cruising cat means she should be O.K. in a 31′ breaking beam sea. An equivalent size mono-hull power boat was easily capsized by a 25′ breaking sea, and in tests with conventional yachts after the Fastnet disaster, it was found that a 40′ mono-hull yacht could be capsized in a 12′ breaking sea.”

Interesting stuff that is worth a watch or read if you need greater detail on have to design a multihull.

 

It's possible I don't understand the above, but I believe deploying a drogue from the stern would point the bow into the waves ? Wouldn't that then mean both statements accept bow to swell as ok ?

 

No, stern drogues point stern into sea.
Wave comes, boat accelerates, drogue creates drag and holds sterns to windward.

 

Edit; The main concern with stern drogues is rudder damage.

 

The Roger Hill designed 10 meter plywood cruising cat is 33 x 18.6 foot weighing between 7300 lbs and 8800lbs depending on information source and the quality of materials sourced. The 44 foot aluminum mast carries a 440 square foot mainsail and a 185 square foot self tacking jib with the option of a genoa and gennaker. The single spreader mast has caps going to the gunnels meaning either the genoa size will have a limited overlap and a gennaker that cannot be closely sheeted in unless the tack can be moved along the forebeam. The hulls have a 10 to 1 length to beam. The dagger boards draw 5.9 foot when down. The underslung spade rudders are protected by a mini wedge shape keel. The hull shape is a single chine. The underwing clearance is 2.1 foot. The cat is powered by 2 outboards, one either side of the cockpit.

Roger Hill has designed many sailing cats and power cats. All his boats have good to very good performance and the numbers indicate this boat will be sail well. PS look up his 20m resin infused all carbon high performance cruising cat ‘Kotuku’ for an idea of what a “cruising” cat can be.

The accommodation is 2 double berths forward on the wing deck (not the best position in a seaway). The single berth aft is well positioned. The galley and toilets are in the hulls with full headroom. The bridge deck cabin has 5.2 foot headroom. The cockpit is a good size.

The 10 meter is a plywood timber construction with single chine hulls. The design has a simplified but effective structure to simplify building. Construction is almost totally out of sheet ply materials with foam cored panels for the cabin top, bridge deck. and 2 primary bulkheads, there is a minimal amount of ‘solid wood’ required in areas such as the stems and connecting some panels where access is prohibited for coving and taping. The structure has plywood secondary bulkheads and stringers throughout. The jpegs give the idea of the details. A ‘beaching’ skeg is detailed that will allow the cat to take the ground without damaging the bottom and simplifies the steering with fixed, under hull rudders (and for prop protection should a small sail drive unit be fitted instead of an outboard). The rig and sail plan is simple and the deck layout has all the sailing controls lead aft to the cockpit for ease of handling and safety.

The jpegs give some details. Roger Hill is a good designer and this cat would be a good cruiser.

 

Was anybody else bored enough,, or have spare time enough to check the clearance of this boat. I get 21 inches clearance on 33ft length, or 5.3 percent of length. And that is before your average so and so adds too many items and weighs it down. Also some of that 'low' clearance is close to the bow, if say the clearance was lower in some parts, higher than others it may be less of an issue. Say aft of amidships it was 21 inches, but forwards there was more clearance, then might be less an issue. Trouble is you start raising the boat higher to increase clearance, you raise the center of gravity

 

5.3% is okay, a bigger issue is what happens when a wave IS encountered...the bottom of the cabin can help break the wave with a vee, for example as Woods recommended

5-6% is the general goal from my reading

 

Here is an 8 page PDF on bridge-deck clearance. I dont know if this has been posted earlier. I started reading every post of this thread, but is harder when you reach page 117. The pdf in part says that bridge-deck should be kept to 60 percent of LOA maximum, e.g. the bridge-deck of a 40ft cat should only be 24ft from bow to stern. I measured the above cat and I calculated 62 percent of LOA if you start where the front of the bridge-deck becomes horizontal, and 71 percent if you start when bridge-deck is at deck level. Note there is a couple feet at the bow where the bridge-deck curves down. It is not my boat, so it does not affect me directly
https://catamaranguru.com/wp-content/uploads/2020/03/catamaran_bridge-deck_clearance.pdf

 

Peter's link above re bridge deck clearance did not work for me for some strange reason - this photo is about the closest I could get to the link :
https://catamaranguru.com/wp-content/uploads/2020/03/bridgedeck-clearance.jpg

Here is another article about clearance on this site :
https://catamaranguru.com/why-does-bridgedeck-clearance-matter-in-a-catamaran/

 

Interesting. I hadn't considered the bdeck length, obviously a longer deck requires more clearance. Ours is a bit more complicated, for comparison. We have a low section and then a higher section, but the bdeck length with the vee is about 12' long against wl of 32' or 38%. We gain another 8' or so of cockpit with the deck clearance at about 24" of clearance or 6.25% for the aft cockpit section. I have to measure it to be sure, but even at 21' we are at 66%.

I think the combination of large and low is a certain problem. I wish I had made the vee a hair deeper, but we also have a wave splitter forward which is what happens when you go from a vee to a scow shape. I expect it'll slam a bit in anything horrible.

Keep in mind, my design is not considered a bluewater.

oldmulti-if any of my contributions are a bother, let me know

also, these pictures are not showing completed work, all seams get tabbing, etc