Hobie 18 Tri

Greetings gang!
Been lurking and now and again asking questions regarding my other boats (1974 Laser, 77 H16, 81 US Yachts 21, 82 Sol Cat 18 w/10' beam). I'm hesitant to wear out my welcome with flurries of crazy questions so I'll try to be succinct. I've got the Gougeon Bros book, I'm a master timberframer (savvy with full scribe, center-line, reference face, square-rule, and s4s layouts, strings/lasers. Art teacher and I can lay a decent weld/fabrication. It's gong to take time (may a year or two) but I'm diving in.

I've had a couple of H18 hulls kicking about and decided pull the trigger. What better thing to do during the lock down than piddle about in the garage Picture shows starboard hull. Top sides are soft, dagger board has classic lower flange leak, patch on bottom; it won't be a catamaran again... but a new life is in store!...a center hull.

Design Brief for tri (open to discussion and modification):
--1 up plus camp gear (say 350-450 lbs dis??)--trapeze at upper limits but very tame-able, reef often
--2 up day wet day sailer (Hobie 16 fulfills this niche already...)
--Beam (center of float- float): 14-16'??
--Float length: 14'? (stitch and glue 3mil ply, cedar strip, solid foam??) I've considered using H14 hulls as floats but I want more bow volume, less banana
--Rig (for now): Hobie 14 or 16? Eventual diy wing mast...custom sail.
--Ply Box Beams, 3' long from hull center line to folding hinge then "x" more to float.
-folding trailer-able as in Waters W17 (I see it ending up with similar #'s to his tri)
--same transom hung single rudder, same dagger board

I realize I can spread the beam out (RM), add float rudders, go crazy with float volume and fly the main hull... I'm not interested. My aim is to require solo trap at the top end, but not often. I don't want to create another crazy wet trapeze hull flying ride like the H16...think slightly more docile.

Mostly solo is my aim. I love my Hobie 16, can solo trap it but I've been in over my head a few times I want this tri to be a degree or two below the wet thrill factor of the H16. Perhaps that makes it slower but I envision this tri should have better vmg related to windward angles.

Legitimate question 1: So to jig up the main hull...I can balance several aspects (bow plum line, transom gudgeons, various laser points port to starboard) and correct for roll side to side. How do I find water line fore and aft or a parallel reference related to it? Mast point load will be the same, over the front beam, weight placement fore and aft will be similar to the original cat, using body position where the trampoline would normally be. Shall I give it a guess looking at floating pictures of an H18?

I want a slight 6-8" foot well or cockpit with tube drains heading to transom. Dagger board case will stay but top elevation might lower. Side seating or basket style like Jim Gallant's "Best Guess". Slight curved coaming "cabin" bump for dry storage in front of forward beam socket.

Legitimate question 2: The picture shows my tentative thoughts on where to open her up. The H18 hulls are symmetrical save for the curved beam pockets. Thinking I might open the deck from the blue tape to blue tape where the curved pocket just flattens back to symmetry. It would make the cockpit (including dagger case) about 9' long. I'm 6'7" and want to be able to camp under the boom, bridging over the cockpit sole or on the seat. That said, a 9' chasm in the hull (in order to remove the asymetric pockets) seems unnecessarily huge. I would have to add longitudinal reinforcement through the cockpit in any case via the seating. I might even raise the gunwale 4" in the seating to give the feel of a deeper well. If you're aware of how the factory glues up the top-bottom, the flange would stay to help with longitudinal integrity.

Looking at the photo, is 9' too much? Should I just live with the asym beam pockets and make the cockpit long enough to sleep, say 7'? Regardless, the new forward box beam will be right through the existing fore asym pocket so perhaps leave the aft asym pocket.

I'm dying to open this thing up and start thinking about bulkheads, mast load point...beam location, etc.

Thanks generous forum. In the meanwhile, I'm going to brush up on the blue bible.
cheers.

 

With your skillset shouldn't you be considering just building in ply and epoxy. The main reason to stick with fibreglass [apart from not wasting a well made thing] is for resale value and it sounds like that isn't a factor in this type of build . They would make excellent raised garden beds for a deck. By the time you have worked out how to get the beams /amas/seating to work the centre hull skin isn't that much extra and you can get plans that really complement what you want to make cheap. There's still plenty of customising you can develop, for instance curved transverse beams made of mast section[or timber] that curve above your beams ,with the sail tracks pointing inwards. These can be the tracks for 2 awnings that slide across the decks making covers /tent at any size or shape you need while sailing or camping. That way you can help me develop the idea ...ha. However you choose to go ,good luck with your project.. Your hulls are even the same colour as the ones I've been extending for my trimaran.

 

You ought to worry about adding so much weight that the H18 hull will sink excessively.
A foot well in a 10" wide hull? You ought to think about raising the crossarms so that you will think you have a foot well, by raising your seat up about 10".
Sounds like a fun project.
Please keep this up to date with pictures.

 

Fandango,


You mention I shouldn't be just considering ply and epoxy. Would you suggesting infused foam for things like floats, box beams, primary/secondary bulkheads, etc? Most all my savvy is with wood. I've done a little fiberglass work, never vacuumed (although I've spent many hours looking at viddys of vacuum layups, flat panel or otherwise). Obviously, part of this project is the learning curve and doing what I can with the bits I have and on a shoestring. The distant fantasy is to take my SolCat 18 (10' beam) and put a stitch n glue center hull together, parting out the SolCat to build the larger, cutty cabin'ed tri. This H18 tri is going to be my "learning project" in the meantime.

Yes Upchurchmr,
weight will be the killer. A together Hobie 18 has a dis of 400lbs. My (yet to be designed) floats will combine to be the volume of a single hull, or close to it, perhaps less(?). However, the dihedral while floating motionless will have an effect on displacement of center hull. I envision a flatter dihedral that has floats just touching the water when still...nothing near the angles of a Weta or Multi 23 which seem to have excess dihedral (Multi 23 being an extreme). In 10 knots, I envision the windward float will be airborne while the leeward gets pressed. In 5 knots, I can see the windward float still touching or skimming.
I realize my center hull being an h18 will be lacking vaca volume/dis; I hope the less aggressive dihedral will allow the floats to take some of that work load while at rest.

(and I fully realize that I know just enough to get into trouble...I'm not afraid to start a project, realize I'm doing it all wrong, and scrap the whole thing. It passes the time

Low dihedral examples I can think of:
Trika 540
Trimore 560
Jim Gallanant's "Best Guess"
Sea Clipper 16
Warren's Ultra light 20 (super flat, looks like 3 identical hulls, all on the same waterline)

I've cut out the foot well. See pic. Deck opening is 35"long, averages 17" deep, and is 12" wide at surface. I would build the sol up so the well is only 8-10" deep using stringers on sides or thin half moon bulkheads spaced beneath (or a likely combination therein). If I raise my box beam so it's top surface is 4-6" above gunnel, I've created a 12-16" foot well... just enough to bend the knees a little, keep a dry bag secure, and keep my beer onboard.

Second pic is a doodle on top of a cut/paste H18 hull. I've doodled this sort of thing many times while not having the parts to play with. Now i have the parts! Drawing is close to scale. Red bulkheads are primaries, the rest are open for change relative to build method of floats, and the valid concern of added weight to vaca. Hammock style benches (keepin it light) look a little small right now. As I have a lot of experience in log work, the "mock-up" technique works in my mind. Sleuthing through problems as they arise is part of the fun...fluidity in design; nothing set in stone.

So...I'm chomping at the bit a little ...if you feel like doling advice:
1. what thickness of ply for primary bulkheads? 3mil glassed both sides w uni of medium weight?
2. what thickness of ply for box beam? 6mil top sides, 3mil verts, reinforced with curved stringers (something to fix to) and/or laminated curve top side. I don't know...maybe a swing-wing would be lighter than box-beams?

For floats, any thoughts on per-existing plans that I can modify to suit?

Suggestions on overall beam?
Suggestions on float length?

I'm having fun! Looking forward to mixing some poxy

Thank you! Say anything! I will cherry pic what I think is best to move slowly forward
cheers.

 

I was actually trying to say just build the main hull out of ply it wont take much longer,..ha, but that's your next project.
Two things I can think of at the moment are ,- your beams will need to be set higher so that you don't get a soggy bottom[got a laugh out of that film] , the forward bulkhead wont be necessary and would be very tricky to build unless you cut the deck out each side temporarily,
I'm glad you didn't cut the daggerboard case out.
If you have the other hull consider glassing on an extra 3or 4 ft to the stern. There may be a sweet spot length where the main hull will accept it .
That way your forward beam could mount just ahead or behind the case and the rear mount would become the donor.
longer amas, and mast base will need more support..ha, which you know. .it's a fun project.
There is waterproof board [polyurethane ?PVC] that's fairly cheap but it isn't flexible as far as I know, others could detail types of sheet , maybe start a new post in that category for it ,..

 

Fandango,
set beams higher, yes.
mast post support, yes (might glass a load bearing boot in the bottom)
The 'sweet spot' if mating hulls together is between the dagger case to just in front of original front beam pocket. The aft end of boat tapers quite quickly. I don't want to butcher the other hull. Save it for...

I've begun looking into other catamaran plans as an option to building floats from pure scratch.
I sort of like the Woods Quattro 14 or 16 to use as floats. I realize most designers would cringe at the notion of one of their boats being bastardized into something else.... structural integrity is always in question when things get modified beyond their intended purpose. The aforementioned catamarans seem to have a similar shear, bottom curve, bow line to the H18 hull. I don't want to put wave piercing or axe bows on the floats if the main hull has the classic curving banana rise bow profile.

I guess there may be factors that effect my notions such as:
does it matter if the float is wider than the main hull? (H18's reserve buoyancy is vertical and forward of midships) The hull profile is the classic "U". The Quattro 16 has a wider topsides, gunnel to gunnel than the h18 and is a softened "V" hull profile (which facilitates the two-piece tortured ply/stitch and glue hull shape)

I'd like to think my box beams would mate to any hull form. It just might take extra framework, bulkhead(s). But it seems like a legitimate way to build my floats, poaching from a known catamaran.

Any other stitch n glue cat plans you guys think of ? 14-16' range

lots of daydreaming I'm trying to remind myself of the goals... 1up, camp racer/cruiser. Keep it lite.

Thanks!

 

Hi Bigfork,
What a fun project! Look forward to following along.
I think you'll probably want to look at Bedard's Forro 550 which seems to have a similiar design criteria. I was thinking of building one (he'll be selling the amas from a mold laid up in glass or carbon) but I decided to go with the RoG. The kit showed up first week of Covid SiP so I'm busy in the shop.
FORRO550 Trimaran – Bedard Yacht Design https://www.bedardyachtdesign.com/designs/sail/forro550-trimaran/

 

Thanks Buckethead! The Forro 550 looks very similar to my idea. Is there a small coffin berth in main hull? I'm currently amassing a 3 ring binder with similar designs in order to contemplate beam, float length, etc... The Forro goes in the file.

I'm beginning to mockup bulkhead (bh) locations and profiles with scribed cardboard but I have some questions to throw out there:
If you look at the picture, you'll see my bhs are labeled 1-6. The Hobie 18 hull is just a glass, foam, glass layup with no internal structure other than thru-dagger box. Factory drain plug on transom.

How should I treat the internal space in regards to water that finds it's way inside (if any ?)... My Hobie 16 will take on a couple cups in one hull; never could find the source.
BHs 1-2 and 5-6 are integral to the box beam structure and need to be full (?)

What I have illustrated below shows 4 air tight boxes:
1: bow to bh #1
2: bh #2 (going under forepit) to aft of dagger box bh #4
3: bh #4 (going under aft pit) to bh #6
4: bh #6 to transom

The green curve on some bh's shows the relief to allow air (water...) to move between full bhs.

So already, I've negated the full length air/water flow from tip to tail (as it is from the factory).

As far as flotation and sealed "buoyancy" goes, I'm not sure what I really need...? Do I need to have inaccessible sealed "zones" for emergency buoyancy?

A forward hatch will access bow space just in front of the fore box beam and bh #1.
A round access port will access the aft space between bh #6 and the transom.

Was thinking of a cockpit drain that would go from pit sol via hose to new hole on transom in the traditional manner, in order to alleviate inevitable water in cockpit.
Fore pit is a little different beast...Can't figure how to drain water from there without linking it to the aft pit and then out as mention above. Or perhaps a bungee/neoprene kayak type cover over the forepit would suffice during heavy weather.

So...which bhs to leave full and which to relieve for water movement...? or am I inventing issues that don't exist As it is now, the factory transom drain will only drain between bh #6 and transom....

Thoughts?

 

Drain for the cockpit would normally go straight out , slightly lower, the side of the hull., my Trem has one each side, with a cup shape facing aft to stop water getting forced in,.. above the waterline. As you'd know H16 just have blocks of polystyrene ,one each end , it's a simple solution and they don't absorb water because the rectangle shape sits above the bilge. Bladders , plastic milk bottles securely tied in can do the same.

 

Thanks Trip.
Yea, my 1977 H16 still has it's foam blocks beating around inside...no worse for the wear. My US Yachts 21 came to me with old gallon vinegar jugs packed under the forward berth to add additional emergency buoyancy. I might do the same in aft box and bow area of my current dink around.

Looking at my convoluted doodles above (that likely only make sense to me...), I guess there's only one real question.

Should I leave interior air (water) flow along the length of the hull below my bulkheads so theoretically, water or air could travel from tip to tail (out factory transom drain)?
or
Is it OK to box off areas completely, breaking up the space similar to what I've drawn? In this case, any water egress would have to be pumped in the event of a holing or seepage...

thanks!

 

I'm no expert Bigfork but sealing off compartments completely doesn't sound right, I like the idea that the bilge can pass all the way to the stern plug.
The hulls with the deck left in place don''t need much re enforcement to take the cross beams, that top flange is very strong.
Any bulkhead only needs to be a ring . you don't want to create a rigid point that doesn''t flex at all, any re enforcement wants to be spread out with some extra matt internally,say 6 inches to avoid a sharp edge or folding point in the hull. That's how I look at the potential stresses any way, regards .

 

Thanks Trip.
I hadn't thought about the need to maintain some flex... And I can always use plastic jugs or whatever for additional buoyancy. Ring frames would also lighten things too. Every stick of wood and fiber layed contributes to end weight. That's going to be my killer. To keep things light, I was planning on secondary bulkheads being 1" blue foam, backed w 3mil door skin (all glassed/matt, sealed up).

The flange on the hull is very strong. That said, my box beams are going to be 2" above flange so I won't be counting on it's inherent strength; my bulkheads rise above. I've raised the box beams in order to raise the trampoline or other seating arrangement (less wet at the cost of more windage, slightly higher boom). From top of box beam to cockpit sol will be about 15". Enough to bend the legs a little.

I'm having fun.

 

I think I'd leave the deck in place and re enforce it below the beams, then build the beams to fit snugly into a glassed profile made on top of the original deck. You're right about weight.

 

Some cockpit sol progress. Decided to follow advice leaving air/water flow capacity from bow to stern in order to allow drying out and use of factory transom plug. The sloping bulkhead is made from door skin laminated to 1" pink foam board, glassed on two sides and sealed with epoxy. Filleted and tabbed to sides. This makes a surprisingly strong shape. I'm experimenting a bit here, I know. Not concerned with resale value while trying to DIY with a "keep it lite" thought towards every move. I know DIY and staying lite are strange bedfellows

The two bh's under the sol are just 1" foam skinned with fiberglass, sealed, and filleted to hull. I've made similar progress on the fore-pit further forward.

A factory naked Hobie 18 hull weighs anywhere from 110lbs (90's red-line batch) to 135lbs (late 70's, early 80's). I'm guessing mine is on the heavy end before I started as it's an early 80's and has undergone several "fix the soft deck" remedies (although those areas are now mostly removed). Now that I'm into this a little, I have a better idea of finish weight of vaca... I bet I'm adding around 15-20 lbs by the time i have all filleted, glassed...30lbs tops. So let's just guess 150 lbs overall as a target. The vaca will be able to receive a beam of any sort; either nested tubular or hinging box beams (what I really want so I can trailer).

tell me I'm a fool I'm learning alot about how to do layups. My processes are getting more efficient, less wasted glass, and evolving as I go.
Feel free to comment, tell me I'm all wet or otherwise going about things wrong.

thanks and cheers!

 

Bigfork,
Your decisions that affect things are long in the past.
Getting better at layups is only good.
Carry on and don't second guess yourself.
Things will finally become obvious just before you finish!

Good progress.