Tacking Drua Camp Cruiser

[luomanen]

I’ve been drawing thin water camp cruisers. Something that can do multi day trips, with one other person, and sleep aboard. I’ve drawn sail and oar boats. I’ve drawn birdwatcher-cabin style boats with cozy enclosures (and scarry windage). I’ve drawn lots of trimarans with mirage drives in them. I’ve drawn SO many proas.

Catamarans with the rig in one hull intrigue me. Two Dierking Tamanu hulls, with a couple of lug rigs in one hull, is how this idea started out. It had two rudders and one big leeboard. My Dierking designed Wa’apa outrigger has made me a believer in leeboards for narrow boats. They are so simple and rugged. And they leave your hull intact. Also they are good enough for Meade Gougeon, so…

Anyway my asymmetric catamaran started to get more asymmetric as the rigless hull shrank down to a 100% displacement ama like you might find on a trimaran. Folding meant that the beam went from a trailerable 8’6” BOAto 9 feet center to center. One of the rudders disappeared and I was left with a tacking outrigger with a really big outrigger, and a lot of beam. Or maybe a tacking drua. Or is it a really wide trimaran, with one side chopped off?

Anyway the big hull is LOA is 20’, with 10:1 length to BWL, the ama is 18 feet long and almost 17:1 length to BWL. The hulls are 9 feet apart centerline to centerline. The whole thing is a bit over 13 feet wide, but folds down to 7 feet and change. As drawn it has 1100lbs displacement. I hope it can weigh less than 400 pounds. Bridgedeck clearance is 20”. I had enough room to put a centerboard under the cockpit sole, so I went ahead and did that.

This is a cruising boat, so I don’t want a super tall rig, with a ton of sail area. The 20’ mast is from a Hobie Wave, with a 110sq ft fat head main, (up from 95 on the Wave) and a 40 square foot jib. I like the lower center of effort, and reasonable area. Tridarka was definitely an influence here. The curved sprit boom works just like a straight one except there is a short line between it and the mast to keep the curved bit relatively horizontal--like a topping lift. The downward one could be a bungee. A screecher, or asymmetric spinnaker could fly from the stem which is 31” in front of the jib tack.

So in trimaran mode, with the ama to leeward, it’s a 20’ wide trimaran, without the windward hull bashing into anything, and 1100lbs of displacement with the ama not quite submerged (1100*9=9900 ft pounds of righting moment maximum). In pacific proa mode, it’s a hiking machine, with the crew scooching to windward to take advantage of the huge beam. I weigh 250lbs, so If I were scooched out 8’ from the centerline of the main hull that’s 2000ft/lbs of righting moment, just from me. A small amount of water ballast out on that long arm could do a lot of good, and not be too big of a liability when you are back in trimaran mode.

My outrigger has a safety ama, and it has been put to good use MANY times. Usually just being thoughtless, but it has saved me from some close calls too. I’m hoping that the more extreme beam and more substantial ama will make this boat feel more like a catamaran, and be slower to get to the magic heel where they just pop over, all sudden-like. That said, if the ama is kept reasonably light, this might be recoverable from being turtled--though I have not done calcs on that.

The cockpit is 7’ long and the sole is 26” wide. The side benches are also sized to be berths. There is a removable extension (shown bright finished) that bolts onto the ama side akas when it is unfolded. Those holes reduce windage and provide tie down points for kayaks or whatever gets carried out there. There will be some kind of tent, probably a store bought one modified for my bridgedeck.

Compared to a lot of things I’ve drawn, I’m attracted to the low parts count, and simple to build hulls and connective. This feels like building a trimaran, and leaving one whole side off. Or building 1.5 catamaran hulls, or like a proa, without all the complication that comes with going forwards and backwards.

What do you think?

Chris
tackingdrua.20.5.wavesloop.hero.jpgtackingdrua.20.5.wavesloop.F3qtr.jpgtackingdrua.20.5.wavesloop.under3qtr.jpgtackingdrua.20.5.wavesloop.folded.R3qtr.jpg

[luomanen]

More pics

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[skyl4rk]

It is a shame to clutter up the prime cabin space with a centerboard. You need to add a centerboard trunk to your drawings, it won't work as shown. Or switch to a leeboard.

[Yeadon]

Why couldn’t they install the trunk against the starboard wall of the big hull? Then do twin rudders on the big hull and the ama?

I like the concept, overall.

[luomanen]

tackingdrua.20.5.wavesloop.CBcutaway.jpg

It is a shame to clutter up the prime cabin space with a centerboard. You need to add a centerboard trunk to your drawings, it won't work as shown. Or switch to a leeboard.

I agree that it is a shame to clutter up the cockpit sole. I can move the centerboard trunk over to the edge of the flat bottom--which moves it about 7 inches off center. That also makes a more uncluttered storage space under the sole. A leeboard would totally work. But there's something tidy about not having a surface piercing foil too. I imagine the centerboard going in like a Tornado--slips in like a daggerboard, and then pivots back on an axle.

But I think I have room for a centerboard trunk.

[amish rob]

I like it!

Of course, being me, I’d either proa it, or full blown trimaran it.

[skyl4rk]

I missed that the centerboard was under the sole. A centerboard is a good solution, better than a daggerboard in any case. Perhaps a cable lifted centerboard instead of a horn to keep the cabin sole flat. Is the cabin sole 14" wide? Thats wide enough for feet and storage, but thats about it. Not sleepable under a tent.

[skyl4rk]

Thanks for the rudder pic. Are the gudgeons steel or aluminum? Does the outboard mount box twist at all under power, or when the rudder is under stress?

Also, just wanted to say I still like and prefer the Scampi design.

[Woxbox]

I like the concept. I do wonder how all the gear for a multi-day trip would get packed away. Lockers in the ama, I suppose, plus in the ends of the vaka. I've seen small dome tents set up on such boats, and I think I'd explore that option.

[luomanen]

I like it!

Of course, being me, I’d either proa it, or full blown trimaran it.

I get going in those directions. The potential upside is one less hull to make than a trimaran, and fewer connectives (but a bigger hull and bigger connectives). And it avoids all the duplication of rigging and steering that a proa has. My Wa'apa outrigger sails shockingly similarly on both tacks, but it's really a bodyweight boat. This one would be a bodyweight boat on the proa tack, and less so on the trimaran tack.

[luomanen]

Thanks for the rudder pic. Are the gudgeons steel or aluminum? Does the outboard mount box twist at all under power, or when the rudder is under stress?

Also, just wanted to say I still like and prefer the Scampi design.

I'm surprised you remember that! That was a long time ago.

[luomanen]

I like the concept. I do wonder how all the gear for a multi-day trip would get packed away. Lockers in the ama, I suppose, plus in the ends of the vaka. I've seen small dome tents set up on such boats, and I think I'd explore that option.

It looks to me like there's a lot of space in the vaka in the compartments ahead and behind the cockpit. Each of those is more than 4' long, and about 24" at the waterline. Pushing stuff into big compartments through tiny holes has some big downsides, though.

[luomanen]

I missed that the centerboard was under the sole. A centerboard is a good solution, better than a daggerboard in any case. Perhaps a cable lifted centerboard instead of a horn to keep the cabin sole flat. Is the cabin sole 14" wide? Thats wide enough for feet and storage, but thats about it. Not sleepable under a tent.

The cabin sole is 26" wide. The flare of the hull and length of the centerboard trunk are what are keeping it from being further out--unless I pierce the side of the hull, or cant the angle of the board. I might as well go leeboard at that point...

[NeilMB]

The cabin sole is 26" wide. The flare of the hull and length of the centerboard trunk are what are keeping it from being further out--unless I pierce the side of the hull, or cant the angle of the board. I might as well go leeboard at that point...

Honestly, I think that a Michalak pivoting leeboard like Gary has used on his outriggers could be a great choice for these small trimarans with sufficient straight runs on the main hull to make it easy to locate it at the right position for balance. I was just looking at Bernd Kohler's Little Tri https://duckworks.com/little-tri-plans/ which has an external daggerboard case and wondering why it wasn't an external centerboard aka pivoting leeboard.

[Gary Dierking]

Thanks for the rudder pic. Are the gudgeons steel or aluminum? Does the outboard mount box twist at all under power, or when the rudder is under stress?

Also, just wanted to say I still like and prefer the Scampi design.

They are stainless steel. The box has been okay with a 2hp motor. The only failure I've had is the quarter round piece of plywood that the rudder attached to. Better to make it a sandwich between metal plates.
My deleted post was actually for a different thread and I posted it by mistake.

[amish rob]

I get going in those directions. The potential upside is one less hull to make than a trimaran, and fewer connectives (but a bigger hull and bigger connectives). And it avoids all the duplication of rigging and steering that a proa has. My Wa'apa outrigger sails shockingly similarly on both tacks, but it's really a bodyweight boat. This one would be a bodyweight boat on the proa tack, and less so on the trimaran tack.

Oh, I’m intrigued by your idea. Completely.

It’s all compromises, choices, and sometimes just what we like.

I hope you get to realize this pretty canoe in real life!

[Todd Bradshaw]

Don't underestimate the value of net areas on multihulls. Wing nets and bow nets can add considerable "living space" with minimal weight gain and design complications.

[luomanen]

Don't underestimate the value of net areas on multihulls. Wing nets and bow nets can add considerable "living space" with minimal weight gain and design complications.

You make a great point. I wonder if the wooden bench to windward would rather be a net. Less nice to sit on, but maybe better for reducing windage.

[luomanen]

tackingdrua.20.6.sidetop.jpgtackingdrua.20.6.front.jpgtackingdrua.20.6.btm3qtr.jpgtackingdrua.20.6.R3qtr.jpg
I've been refining the design a bit. I wanted to try a leeboard. And to do that I figured it made sense to break up the sides of the main hull into two planks with a chine--that way the upper plank was more vertical--keeping the leeboard close to the hull. Even if I don't decide to use a leeboard, I like this arrangement for building, because I'll be able to get the bottom, bottom plank and bulkheads installed without having to reach into the full depth of the hull to do the fillets between the bottom and the bulkhead. It makes those long side planks a little more easy to handle and the chine stiffens up the hull as well.
The concerns about leeboard ventilation are real, but I haven't experienced them on my Wa'apa. The advantages are no cases to make. No penetrations in the main hull--easier to build and lower hull drag. Reduced part count. And if a board is the wrong size, or in the wrong place, drilling some holes is a lot easier than rebuilding a centerboard case. I got the lead wrong on my Wa'apa, and just drilled a hole 2" forward of the old one. Maybe I could try a setting where the board is canted just a little bit forward, if I have ventilation problems.
Gary's ingeniously minimal leeboard design uses a single threaded handle on the pivot bolt--oriented so that hitting something unscrews the handle. But the lever arm, countering the lateral deflection of the board, is between the threaded handle (in tension) and either the top or bottom edge of the circular board surface pushing on the hull (compression). So the lever arm is 1/2 the diameter of the circular surface touching the hull.
I'm hypothesizing a 3 bolt solution. The middle bolt is a pivot, and does not tightly hold the board against the side of the boat. It could even be a pin. The top and bottom bolts can act in tension. In my boat the bottom bolt is in tension on a starboard tack, and the top edge of the circular surface is in compresion--so the lever arm is almost the entire diameter of the circular surface. Twice as much leverage, half the load. The downside is that there are slots for the top and bottom bolts in the circular surface that weaken the board. And the downside of any leeboard is that the CLR already acts along a longer lever arm--because of the height of the pivot.
The board will have a bungee downhaul, wrapped over the circular top of the board, and a 2:1 uphual that touches the board in the same way, and emerges from the front edge of the bridge deck and is cleated there.
I also revised the ama design to have a bit more forefoot and a more narrow entry. I tweaked the rig a little--but it's still 150 sq ft.
And I really like the design of the outboard platform. I want it to be somewhat air permeable so it doesn't act like too much of a sail as the ama lifts. The holes also look cool--and hint at the structure under the deck. It also gives me lots of places to tie down stuff out there.
Time to get deeper into the calcs.
And I'll make a close up view of the leeboard bolts and slots the next time I'm in Solidworks.

Thanks for your thoughts!

[skyl4rk]

If you make the leeward deck out of foam, it will serve as an anti-capsize device. Maybe 4" of foam wrapped in glass and epoxy, with a few fore and aft stringers.

[AnalogKid]

Your design immediately reminded me of the boats sailed by the Rarotonga Sailing Club on Muri Lagoon. Unfortunately I don't have any photos of the board details.

https://rarotongasailingclub.org/

P1010982.jpg

[Gary Dierking]

[QUOTE=AnalogKid;6601309]Your design immediately reminded me of the boats sailed by the Rarotonga Sailing Club on Muri Lagoon. Unfortunately I don't have any photos of the board details.

https://rarotongasailingclub.org/

QUOTE]

I have some more shots of the Vaka class that sails in Rarotonga. See them here: http://outriggersailingcanoes.blogsp...ng-strong.html

[luomanen]

Your design immediately reminded me of the boats sailed by the Rarotonga Sailing Club on Muri Lagoon. Unfortunately I don't have any photos of the board details.

https://rarotongasailingclub.org/

P1010982.jpg

Those are so cute! I love the low aspect rigs and their general adorability!

They look fun to sail too.

Gary, what did you think of them when you crewed on one? They look a bit narrow in beam over all, but they also have that nice short rig.

[Gary Dierking]

Those are so cute! I love the low aspect rigs and their general adorability!

They look fun to sail too.

Gary, what did you think of them when you crewed on one? They look a bit narrow in beam over all, but they also have that nice short rig.

They had big centerboards in the hull and good sails. They went pretty fast I thought and handled like any monohull racing dinghy. I've never seen a plan for them.