Where do I start? Build a Trimaran from an Australian Surf Boat and Hobiecat

[Chris71]

Hi
Is there an easy book to tell you calculation , Arma, cross beam, breadth etc etc and things i need to turn our Surf Boat into a Multi Hull. We want to buy a Catamaran like a Hobie 18 or bigger if it exist. And blend it into the Surf Boat as much as we can. The Surf Boat usually has a crew of 4 on the oars and a sweep. Its just under 28ft long. about 250kgs about 500lbs Surfboat https://en.wikipedia.org/wiki/Surfboat . Beam is about 165cm.
We boat the boat for $300 bucks. It was used was a prop on the ceiling of a restaurant that didnt survive covid. It has holes through the foam sandwich from chains and sprinkles going through it.
We would love to make it into a basic multi hull, usually two or three crew. To go for extended camping trips, coastal cruising and go into the blue water and racing. Foils? Used to live in the Islands North of Australia and New Guinea so could sail back if its strong. What do you guys think? Any opinions, thoughts, points etc. A few days ago we didnt know it would end up on the front lawn and had no thought on a boat project at all. My mum is just shaking her head with another boat out the front. Ive added a few photos of the boat and some other pics we were just thinking. We dont have alot of cash so nothing too fancy, We bought some inflatable rollers to move it up the beach

 

[bajansailor]

Chris, I think you best bet is to simply ask on here for suggestions as to how to go about doing your proposed conversion.

Can you tell us a bit more about the surfboat please? I presume that it has positive buoyancy already built in, for the inevitable times when it is swamped in waves.
But are you planning on keeping the cockpit area open, or do you intend to deck it over?

Re sailing the finished boat north - I guess you could do it in day sails, and camp ashore at night? How long would the sea passage be to sail across to New Guinea?

Re buying a Hobie Cat (I think that the largest they built is a 21'), a rig like this is probably a bit too 'high tech' for what you have in mind - I am thinking that a rig like the simple lug sails as shown on the blue proa inyour attachments would be more practical.
And to save expense, you might be able to buy a single Hobie cat hull to use as an outrigger for a proa for less than what two hulls would costs.
There are some folk on here on who very knowledgeable about proas, and I am sure that they can give you good advice.

 

[Chris71]

Hi Brian, Yes is is positively buoyant , even when the two bulk heads at either end have been damaged. We were thinging on keeping the cockpit open towards the stern. And forward of the center enclosing this area for kit etc. We talked about making the gunwale higher mid ships and building wings around the gunwales, to stop getting splashed and to sit on with legs in the cock pit. Maybe a canvas folding dodger to go from Mid ships to the cockpit to sleep under. Just a few thoughts while were chatting .

 

[Chris71]

Hi Brian, Yes is is positively buoyant, even when the two bulk heads at either end have been damaged. We were thinging on keeping the cockpit open towards the stern. And forward of the center enclosing this area for kit etc. We talked about making the gunwale higher mid ships and building wings around the gunwales, to stop getting splashed and to sit on with legs in the cock pit. Maybe a canvas folding dodger to go from Mid ships to the cockpit to sleep under. Just a few thoughts while were chatting .

We were hoping to find camps over night but would like to stay at sea. My friend wants to Race it in the Darwin to Ambon Race, about 650nm . Papua is only a few miles from Australia

 

[bajansailor]

Re the Darwin - Ambon race, are you hoping to participate in the race in August this year?
Darwin to Ambon Yacht Race https://www.darwinambonrace.com.au/
The distance appears to be about 600 miles - you would definitely want to have some type of basic shelter built in to your craft, as you would probably be at sea for at least 4 days.

 

[Rumars]

Plenty of conversions exist, usually with Hobie 18 or Tornado hulls and rig. Fit reefing points to the sail, make sure the beams are good for the loads. Rudder and daggerboard and it's ready to go. Visit Small Trimarans http://smalltrimarans.com/ and use the forum search function for inspiration.

 

[Chris71]

Re the Darwin - Ambon race, are you hoping to participate in the race in August this year?
Darwin to Ambon Yacht Race https://www.darwinambonrace.com.au/
The distance appears to be about 600 miles - you would definitely want to have some type of basic shelter built in to your craft, as you would probably be at sea for at least 4 days.

My mate wants to Aug of next year.

 

[Mr Efficiency]

I guess it could work OK, but you best talk to trimaran sailors with enough experience to know what the essentials of a good sail trimaran are. Without that kind of input, it is too much guesswork. I'd be no help in that regard. Or you should get hold of a book(s) devoted to sail trimarans, to get a feel of what works, and why. The advice might be that it is a little wide (the main hull), but someone will know, you will also need guidance about any requirements for centreboards, which would complicate matters. In the end, is it a saving on buying a tri, or is it the novelty aspect of the re-cycled surfboat ?

 

[jamez]

I'm not sure I'd want 18-20' floats on a 28' center hull. Tribesman (built c. 1980s) used a pair of Buccaneer 24 floats with a surfboat based center hull approximately 26 feet long. Although the center hull was supposedly a bit wider on the waterline than ideal (by comparison with purpose designed tris) the boat still sailed very well.

 

[bajansailor]

The Tribesman in the photos above does look very impressive.

Chris, Richard Woods has a range of trimaran designs, but they are all under 20' however there might be some useful design ideas there, especially re the Strike 18 -
Sailing Catamarans - First Choose a Design http://sailingcatamarans.com/index.php/designs-2/27-trimarans-under-25

 

[Mr Efficiency]

Certainly looks a good job done on that conversion, something to emulate perhaps.

 

[peterAustralia]

please see below a conversion from an Australian surf boat to a multihull

 

[peterAustralia]

 

[trip the light fandango]

As you can see a proa is much less work, tribesman looks great what an accomplishment, those buccaneer floats are probably around 200% floatation, weight is the big issue for speed and engineering. Whereas the proa can get away with a hobie float, a tri will be around 100% buoyancy with Hobie floats so a nasty wave combo or freak wave could roll you especially if you're pushing it... it would help if it's 20ft wide but that makes the beams quite difficult to build light and strong enough.
A proa kept light has more chance of righting if you make provision[inflation bag , a long enough spar and mounting point for leverage when upside down] for it and there is a few of you.
My float percentages are approximate.

 

[guzzis3]

Quite a few have been done. Some with cabins some open. They have very little resale. I seem to recall a very nicely done cabined version sold some years back asking about $3800. (brisbane)

Cat hulls aren't ideal as tri floats want a lot of buoyancy forward but you can muddle through.

Hulls are not the expensive nor time consuming bit. If you want to cruise most of your money will go to equipping it, navigation, safety, cooking sleeping, head etc. A second hand cat rig is fine for pottering around a bay but if you want to sail in open water, well you'd better have a decent motor.

Do you want to trailer it ? If not have you looked into the cost of storing it on or near the water ? If you intend to trailer it have you looked into demounting and having enough space on land to park it ?

Plenty of people have gone down the surfboat route and it's not the worst idea, but it isn't the best idea either. If you just want to go cruising monohulls are virtually worthless these days. You can pick up a plastic boat and refit it and have a decent outcome. If you specifically want a tri have a look at Ray Kendrick's designes.

Plans for the Scarab 650 folding trimaran http://www.teamscarab.com.au/scarab650/design.html

is one of his most recent. It will give you basic accommodations, folds for trailering and will sail well.