Multihull Structure Thoughts

Thought I would post an update about my Skip Johnson designed Seawing 24 Trimaran that was featured in post #1575 on page 105 of this thread.

After a number of years of ownership, sorting out and updating, it is finally back in the water and so far I have to say I am impressed with its functionality and performance.

I knew the boat was light based on its foam glass construction and use of carbon in the floats and beam arms, and that was further supported when a friend who built and owned a F25C helped me disassemble/assemble it for trailering/launching and remarked how much lighter the floats were than his.

And then once it was on mooring, I immediately noticed how lively and reactive it was to wind and water conditions, and when viewing from the shore, it is always the first boat to turn with wind.

Sailing the boat has confirmed a stiff lightweight structure with no strange noises or behaviors when tacking thru and launching over swell. The boat is well balanced and requires minimal rudder effort even when sailing downwind in a following sea.

Performance wise the boat sails in the 7-8 knot range effortlessly in our late summer swelly trade wind sea states, and thanks to the extra freeboard occupants are kept dry.

On a beam reach she’ll hit 12 knots without much effort in open ocean trades, however I haven’t yet pushed the boat as the mast is in desperate need of a makeover. But given our sailing conditions this time of year, size of the boat, and age of the sails, I’m inpressed.

While not “high volume” by todays standards, the floats do have a good amount of volume by 1990 standards, and being tall and streamlined they tend to recover very quickly when pressed hard, resulting in minimal healing/slowing.

On the accommodation side, I have been spending about 3 nights a week on the boat, and for a 25’ Trimaran the interior is quite functional.

I much prefer the layout of sliding bunks in the wings, the settee with flip up/down table, and aft galley with unobstructed passageway to the bow over the more common layout you might see in Trimarans this size where you have two seats facing that convert into a bed.

Although the main hull has a rounded bilge that adds a little more to the boats draft compared to modern designs, the trade off is that it allows for headroom that’s more on par with a small mono.

If you put the Seawing 24 next to say a Corsair F24, you’d be amazed how much more volume and freeboard the Seawing has, almost as if someone over inflated it, and yet it still looks very streamlined which is no surprise given it was designed by an aeronautical engineer.

As I’ve said before, it’s unfortunate more Seawing 24’s weren’t built. It’s a great design.

Here are some updated photos. Also a link to video footage of my Dad at the helm on the first sail and a link to a video of me pushing the boat a bit harder:

 

Michel. The first of the information about SIB. Sorry about the quality of the jpegs but this is out of an old magazine. Tomorrow I will put up more info about the Cabsib 32 which show more structural detail but is very similar to the original sib. The first jpeg is of the original Sib 30 with lashed on beams.

On the third jpeg there is a number on the side of the constant camber mould frame 9 - 4. That gives the coordinates of the size of the mould frame.

 

I get Red's Piver/ Neel reference but for lots of elbow room it isn't as boxy as some. I'd like at least a flat walkway in front of the cabin and netting, you need someplace for the kayak. Deck utility is compromised for that interior room but for some that may be more important. In flat panel foam this might tick more of Guzzi's boxes than most. I'd probably stick with a Cross or if I wanted this much room Horstman as I like the rounder hulls smoother motion in waves. This is apt to be a bit bouncy but I'd like to see it sailing, could be fun.

 

I said I would do more on the Cabsib 32 but what I had was a repeat of the Page 39 entry of this thread, so we will look at the Marples CC 40 which I think is one of the best versions of the Constant Chamber series. The CC 40 tri is a good cruising tri for a couple capable of 200 mile day averages. As you will find this tri is stronger than most and very capable of world cruising. The hull shapes are partially controlled by the build technique and reflect the era it was designed. It may not have the latest full ended float shapes but the design sails well and according to owners is comfortable in a seaway.

The CC 40 is 40 x 26.5 foot with a weight of 8,000 lbs and a displacement of 10,600 lbs. The 50 foot fixed aluminium mast (other options are available) carries 755 square foot in the main and fore triangle with a maximum sail area of 870 square foot (excluding spinnaker). The length to beam on the main hull is 8.2 to 1 with the length to beam on the floats is 10.6 to 1. The draft is either a centreboard of 2.5 foot to 6.6 foot. The other option is a fixed keel of 3.9 foot. The engine is a 20 HP or 25 HP inboard diesel with direct drive. Speed under power is 6 to 8 knots depending on engine power.

Accommodation is a double berth aft cabin, then a spacious cockpit with all sail controls leading to it. The main salon has a U shaped galley aft, 2 settees (storage under), a fold down table with 2 wing berths above (ideal for sea use). Forward is a toilet/shower area with a V-berth double cabin in the main hull bow.

The construction is interesting. It is constant chamber which is strips of timber laid over a simple mold to forma a timber panel. The curved panels are then tortured into a hull shape. Now the fun part. I have read of CC 40 built with 4 layers of 3 mm plywood (12 mm thick panels for main hull and floats), another was 15 mm of 5 layer thick layers. The CC 40 in most of the jpegs was professionally built 4 layers of cedar core cold molded with West System epoxy to give a hull thickness of 19 mm with 10 oz. fiberglass sheathing. The decks are plywood with stringers and deck beam support. There is a jpeg of the web structure of the main and aft crossbeams. Have no illusions, John Marples the designer knows how to design a strong, light, relatively easy to build structure.

The performance of this tri is about good average speeds across a wind range. 10 to 15 knot speeds can be achieved with 200 mile days possible.

This is an excellent tri for cruising and if built to plan will serve you well. The jpegs give the idea.

 

Hi Peter, thats a big hunk of boat there. It will be interesting to see what you do with her. An aquaintance of mine had the original NZ Shilo 30+ ago. It already had 5 foot hull extentions when it came into his ownership. I can't find the photos I had of it but it is/was pretty much a dead ringer for the Samoan boat. So I would say thats probably a 30 thats been extended.

Malcolm did design a larger version, the Valencia but I don't know if any were built (see comparison). The Northern 11 looked a great boat and by all accounts it performed well. One of those with a sensibly designed light pod cabin would be a great cruiser IMO. Shame it didn't work out commercially. Malcolm did have Northern 11 plans for sale on his website so there may be some other versions out there.

 

Hi Jamez
Thanks for your info, she sure is a chunk of boat and it seems there are a few possibilities for her now that I know it is a Malcolm Tenant designed Shilo. (maybe even turn her into a power boat) I am intending twin approx 10kw electric motors..Mine seems to be the same size as the Valencia, however, it may be narrower by some 2 feet. What I know is that I have a lot of work to still do to get her in the water, but I do have great plans..
If you google Valencia charter Aukland, you will find an example of the Valencia, which as you say, is basically an extended Shilo. They rent it out on several charter sites. The original dimensions of the Shilo were pretty small being some 9,2mx5,6m. The Valencia is some 38ftx20ft.
I attached the story of Eric Knight and the Northern for you to read also.

 

Sorry, I only saw later you already included the dimensions...

 

Hi Jamez again..
What would you do with her? How did your friends Shilo perform and any photos, esp of the interior.
I think the payload will be some 3000 lbs with the extensions, I am hoping I may even be able to go bluewater with her..You can see the extended sides lend to more headroom especially in back cabins..lots of storage too.

 

Hi Jamez again
Its a 38ft Sienna, also very similar to the Shilo, not a Valencia..boat name is Cruczar..The Valencia and Sienna look identical to me.

 

Thomas Costa Lima of TCL Design Studio on the Iberian Peninsula, has a project to produce more of the following 50 foot “day sailing” power catamarans. The 50 SL Catamaran is designed to enjoy the experience of a day in the sea with all the comfort and plenty of space.

The 50 SL Catamaran is 49.2 x 17.4 foot with a weight of 24,600 lbs. The hull length to beam is 10 to 1. The draft is 2.65 foot. The underwing height is 2.5 foot. There are two 150 HP outboard engines driving the cat. The fuel tanks total 1400 liters (about 320 gallons). Inboard engines are an option.

The accommodation is focused on outside living with the Flybridge or deck with sufficient area for a generous solarium, a second command post, an area for the support boat with loading arm, a refrigerator and sound deck with two double solariums + one triple bow solarium, a table for 12 people, a cabin with one command post, a WC, kitchen, living area that converts to a double bed. The hulls have cabins, toilets and storage.

Construction is unknown but foam glass would probably be the method judging by some of the construction jpegs which had a fiberglass hull mold.

Performance is unknown but the outboards are relatively small compared to the weight so I suggest a max of 20 knots but reasonable fuel economy. Yet to be verified.

The jpegs give the idea.

 

Hard to say without looking at it in person. While the hulls look quite Shilo-like the cabins on the hulls and BD cabin (even allowing for what look like later additions) don't. I'd say that removing the worst of the bridgedeck cabin extensions and building more streamlined/aesthetically pleasing BD cabin would certainly be an improvment. No I don't have any photos, he still speaks fondly of the boat as a good sailing cruiser.

In addition to the racy stuff he is most known for (and the large custom sailing cats, trimarans, powercats, ferry cats etc.etc.) Malcolm designed designed a bunch of bridgedeck cats in that 10-12 metre range in a lot of different styles. Possibly some inspirations for a new cabin design for yours? All the best with your rebuild. It would be great if you could start a new thread and update it from time to time.

 

Thomas Costa Lima of TCL Design Studio has done an interesting charter cat design. Authors Yacht, commercializes this type of vessel and gives the possibility of it being customized and adapted to the type of operation for which it is intended. It is a product developed in conjunction with the customer and accompanied until delivery.

The Author Event 65 is 65 x 28 foot with an estimated displacement of 60,000 lbs (It can carry up to 150 passengers or about 25.000 lbs of payload). Mast is 85 foot high with a sail area of about 2,140 square foot in the main and 100 % foretriangle with a self-tacking roller furling jib. The draft is 5 foot over the fixed fin keels. No power details are available.

The accommodation is mainly based on a charter function. Much external deck space on decks and flying bridge. The main saloon is an open space with minimal internals beyond seating bar etc. Down in the hulls are cabins and toilets but are not fully specified. I suggest as this is a custom cat that can be configured to suit your wants.

The construction is foam glass but no further details are available. Thomas is a capable designer and has a creative flair. His designs should sail well as they appear to have good numbers but until we have a test or more design detail to confirm the numbers I can only suggest it can do 10 knot averages with peaks near 20 knots.

The jpegs give the idea.

 

Wow! No stone has been left unturned to create the ugliest possible cat...!
What was wrong with the "old" sleek style?

 

Not really a fair comparison.
An efficient seaworthy sailing vessel for 6-8 people to travel oceans.
Or:
A party venue for up to 150 people to booze-cruise across the bay on a calm day. Sails go up for atmosphere and "vibe", but the motors stay on.

The sailboat would cost about $1.5 million, the party venue about $25 million.
It's all a long way from a small Wharram......