Power trimaran/stabilized monohull, 6M LOA, 60HP outboard

Apologies since this vessel already has a very long and at times gloriously contentious design/build thread as a monohull (Custom 19' all weather, minimalist, strip plank composite 'go fast' https://www.boatdesign.net/threads/custom-19-all-weather-minimalist-strip-plank-composite-go-fast.66022/page-17#post-970376), but I'm thinking of adding training wheels to make her into a tri hull, specifically to improve static stability. I have limited experience with multihulls--they aren't super popular in southern California, likely a result of 'tradition' and our sea conditions. Also, please understand I am a professional captain who runs a freediving business and she is purpose built to transport freedivers/swimmers/spearfishermen, and not intended to be idiot proof for drunks, infants, the infirm, and definitely not everyone's taste. She is specifically designed for southern California waters which feature on most running days long period large swells, mild to moderate wind chop, lots of following seas.

I only have three days of seatrialing so far, so the process is just beginning. She needs a few simple mechanical tweaks, but I am quite happy with her seakeeping when running. When designing her, I had envisioned a minimalist boat that would be really fast, highly fuel efficient, not super comfortable but at least tolerable, pretty wet, but very seaworthy and particularly good in long period swells and various following seas, as well as nice looking for a work taxi boat. Her use profile would be much like the little Milpro Zodiac RIBs that are very popular among divers out here. Once I got the vessel done and off the back porch and into the water I discovered:
(1) she's very eye-catching, more than I expected; my business has become more premium since I started the build four years ago and I see this now as possibly a great asset.
(2) not to say there isn't any spray mist but she's remarkably dry for such a low freeboard
(3) she's a good deal slower than expected in flat water with a max speed of about 32kts
(4) she's a good deal faster and more comfortable in all kinds of sloppy water--ride quality is really stellar when I compare her to other vessels of her size and class -- little Boston Whalers, RIBs, inflatables, center console fishing boats.
(5) Even moving at a crawl stability is fine and you can comfortably walk her deck, but static stability is MUCH worse than I expected. She was never designed for that--just for freedivers in wetsuits to shoot the channel, anchor in a sheltered cove, then hop right in the water... but her static properties make her problematic as a 'homebase' even for freedivers. In truth, when designing her, I felt like this would make boarding from the water easier--and it does! you just tip her towards you and slide in--but in practice this is cruel to anyone already in the boat, and for novices this behavior will be terrifying. It's just not 'boat' enough to suit the rest of her. Static stabilitywise she's good to about 350lbs but anything above that and she just gets cruel and incredibly fussy about weight distrubition. Again, running stability at 800lbs (what I had designed her for) is great.

In short, certain aspects of her turned out much more 'nice boat' in the traditional sense than I expected or intended--looks, build quality, ride quality, etc. I think little outriggers could address the static stability without affecting seakeeping or reducing max speed. A buoyancy collar would do the same, but I think little amas would look unique and sexy whereas any type of buoyancy collar would detract from her aesthetic.

I have scoured but not seen anything like I'm proposing done on this scale, except sometimes as a record setter experimental type craft. Is this because it doesn't 'work' or just that--for anyone in their right mind--the build complexity is so insane for such a small vessel, and/or because one could just mold tunnels into a fiberglass monohull to get the same effect?

From a 'SOR' perspective I'd like to modify the vessel to:
(1) Improve static transverse stability, particularly when weight is concentrated aft.
(2) lift her stern a few inches. Scope creep, hubris, and inexperience meant she turned out 33% heavier than I ever expected she would, and a good deal of that is aft.
(3) fix her transversal static balance. Her console is starboard offset to make it easier to longitudinally traverse her deck, and to offset the weight of a diver boarding her port aft side. But this means when docked she has a list and it does bother me now that I see it. She's like a pretty girl in a pretty dress who happens to only wear a single high heel. When designing her I figured she'd be a dirty and reliable girl in overalls whose static limp wouldn't matter but she just built out very different.
(4) keep it easy for a freediver/swimmer in the water to board her from the port side
(5) maintain (or IMPROVE!!) her max speed, seakeeping, and rough water comfort particularly in following and quartering seas
(6) keep her looking good

1, 2, 3: are very easily addressed by the position and size of the amas; the dock list is also naturally addressed by making the port (boarding side) ama more robust and heavier than the starboard ama since people will be clamoring around on it.

4: is addressable by keeping the amas in tight. maybe another molded step or hand hold, plus the aft sugar scoop + radar tower should make boarding still easy

5: this is trickier. So, so much of the information about multihulls I can find relates to sail boats. My gut tells me that if the amas are almost entirely out of the water when she's running fast, relatively fine of entry, very close to the main hull, and overall quite narrow, they shouldn't introduce much drag or vertical heave and corkscrew movements. Much is made in design literature of wet deck slamming but if this vessel is moving quick and on plane there is so little vertical heave or slam that I don't feel that is a huge concern. I have also spent some time on power cats that were designed to have an air cushion under the hull which in some speeds and sea states was great while in others not so much. The vessel was always intended for passengers in wetsuits but since she ended up so much drier than expected, I'm thinking it would be nice to keep that and she could even be a Sunday boat for people that love being close to the water. Would the proposed amas throw green water into the main hull? I don't think so but thoughts? More distance between hulls would, in our waters I am fairly certain, tend to introduce corkscrew and ugly heave. But for the amas, how close to main hull is too close?

We tend to always have large long period swells, plus occasional windchop that doesn't amount to much, HOWEVER around here swells and wind often run in a different directions which lead to broaching and misbehavior with a lot of hull forms. The narrow hulls have always felt great to me in those seas, and she really shined here in the brief seatrialing I've done so far. As is, she heels a lot during fast turns, but it's somewhat safe since centrifigul forces are pinning passengers into the boat. Less heel on fast turns probably wouldn't be a bad thing, but I also don't want to create a vessel with so much stern lift that she's prone to bow stuffing (her bow is very light; longitudinal COG feels about midway between the engine and seat). I don't think the amas I've roughed out would do this unless I'm very much missing something. I also understand that from a structural standpoint the amas will have challenges but I'm very comfortable with that aspect and the fabrication steps.

6: this is in the eye of the beholder but I think the right amas don't detract from her looks.

Here she is now:


Video footage from first sea trials:



Draft of modifications (this would address SOR 1,2,3,4):

 

I told you so, but you argued no slip space, iirc.

I think you are going to need to work awfully hard on ama connections versus adding sponsons or hull width.

Maybe speak to the slip question a bit. I think sponsons will be best, but designing them for 25mph is outta my league as to how water hits them on the bottom and sides.

The other thing about sponsons is they offer more displacement for the same width or less with for same displacement (as amas).

 

The other thing with sponson is you could wrap the transom and get some more buoyancy aft if needed. But like I said, outta my league to draw.

One thing you ought to do is see if you can develop the hydrostatics..

 

I found one picture of the basic idea. For your case, I’d probably wrap the transom/maybe incorporate some boarding ideas? These would build fast with some 6-9mm okume plywood..you can use aircraft techniques for the inside tabbing.

 

I’m confused by #3 above. I’m curious if you are hitting top rpms. At 60hp, that thing should be doing 40 scary easy. Prop wrong? I watched all the videos and it looks like the stern is just too buried and dragging, so all the more reason to sponson. Maybe add a picture of the stern out of the water off one side a bit. I think, like I said, doing the sponsons properly will make boarding easier than it is now, done well. But not my thing to coach you on it!

 

Once in winter we went down the river with my 18'x3'(5.5m x 0.9m) jonboat 3 people onboard with 40hp. Freeboard 16" (40cm) only . A lot of clothes ,fuel and equipment caused to spill over the transom when we reached a higher sandbank.

Whatever you add you will lose speed and agility which can be seen in the video.
Adding floats/amas , the boat will be wider at the stern , sharp floats tips will be prone to damage itself or something or somebody.
Adding collar is not allowed .

#1 increase displacement without widening hull and without adding much weight.
#2 as above
#3 center console
#4,5,6 modify transom .

My first sentence in this post , I wrote about my river boat which I once sailed across to the sea , which wasn't very smart idea.

 

If his stern is squatting; then wider hull might actually help. Lotsa if, but it doesn’t look like he is getting out of the water to me..

The aft sections of the hull are just not doing enough I’d say. And the original thread went around in circles about this design being a v hull too narrow and anticipated stability issues for narrow beam..

 

I've read original thread beginning. Kaboat. LxB ratio was the reason .

 

But he could add some chine flats; how big I dunno.

 

Thanks for the replies.

Yes, @fallguy you do get to give me some well deserved "I told you so's" for my stubborn commitment to the narrow beam but running stability is great; not everyone would appreciate a little gentle roll but from an operational standpoint it's fine--you can walk edge of the deck at 3kts and she barely heels... and if she'd not ended up heavier than expected static stability would be more workable. In the video some of the poor static stability is free surface effect from the gas tank--again, a truly custom baffled tank would fix this, so as mentioned, some regular mechanical stuff needs to happen. Some of this is that in four years what I want out of the vessel and my own tastes have changed...

My buddy who has a lot of custom fabrication experience and legit one-off boat building experience rode on her the first day and his first instinct was sponsons. The sponsons in your photo look a 'little tacked on' but obviously the owner wasn't overly concerned with aesthetics and I could do a more molded look for this vessel. High speed structural concerns are also always a thing but I really thought this vessel might touch 50kts so her hull was built for those kind of forces and that's something I can do; since it's a fully built wood boat inside a skin, I don't need to rely purely on tabbing and can attach sponsons or amas with thru hull dowels and proper joinery so anything that is added is truly structurally integrated into the hull. Again, when looking at what other people have done in the past it's hard to separate what they would have done if build complexity and 'structural attachment' weren't actually concerns of theirs.

I was working in isolation and this was my first boat build, so I didn't realize what I was doing as far as build quality. Once she was getting surveyed and on the water and out in the world at the boat yard getting the outboard finalled, etc, it became apparent that build quality is one of her strengths. She was built for longevity so that'll remain to be seen but I didn't cut corners there either.

As for max speed, I could likely get more than 32kts with a little bit of playing--since I was in engine break-in period I didn't run much wide open, and it was very obvious the zinc trim needs to be set more aggressively as she was heeeling quite hard to port when running fast. I did fit her with about the most aggressive prop I could for that engine, so I'll double check RPM's next time. One biggie is that on all these runs I always had a friend/passenger on the vessel, she's uninsured and still in an 'experimental stage' and so I was a little tentative with the throttle as the consequence of a cartwheel or big jump could be really, really bad. Nothing about her behavior or design makes me think that's likely, but easy does it in the beginning. Ride isn't perfect for passengers at all speeds but the intention is small bean bag cushions, so our assessment of ride comfort was based on, 'how would this thing feel with a little bean bag'? We use those a lot on small vessels around here and on a lot of the RIB's you're still getting slammed around pretty hard even with a bean bag--this vessel would be quite comfy at higher speeds in a rougher water than a RIB her size.

As for adding weight, it would be a concern but I can certainly fabricate ultralight amas or sponsons and relishing the challenge, especially given that the vessel doesn't 'need' them--meaning, if they fail catastrophically the vessel isn't suddenly sinking or unmanageable.

^^^ never a calm sea did a good sailor make!

Okay, the 'without adding weight' part I'm looking forward to as the vessel was built light but still very sturdy ('high speed workboat'), almost all WRC and a good bit of carbon fiber, but the ama's could be ULTRALIGHT and so probably still foam filled but stringer frame carbon fiber skinned under vacuum and I think fabricating those would be fun. She's perfectly safe ultimately stablewithout any additions, so the additions don't need to be indestructible/heavy.

Console position is not anything I want to change as it'll make almost impossible to get into the bow of the boat and that console is truly structural, not just tabbed in. Maybe next time? #3 is very fixable with just a little bit of offset weight, and I can put her in the water and stack weight on her until I get this exactly right. Starboard side ama/sponson should be lighter than port side because people won't be stepping on it so much. The amount of weight needed is also small; when I docked I of course spent a lot of time stepping and lifting on her stern to see what it would take

As far as lifting the stern, the amas or sponsons can function as trim tabs. The amas design in the first post actually has an AOA of about 1 degree (this is very mild), but that should add some stern lift with minimal drag. Also, the stern drag, while possibly an issue, isn't as severe as it looks--the rear airboxes are functioning much like trim tabs (although they're on the same plane as the bottom of the vessel), and because they're long they're a good deal more buried than her actual transom. Here is a what her waterline footprint actually looks like when she's running 20kts+. Because she's a narrow hull, her stern will probably never get as high as an equivalently powered broad planing hull of her displacement:



Again, not to say stern couldn't be higher but it isn't as egregious as it looks; center of pressure seems about right to me. I do plan to get more drone shots from the side running in rough water.

Sharp pointed amas and damage--yes, this immediately jumped out at me. It's hard in Fusion 360 for me to round the points and filet edges quickly while sketching the ama shape but in practice they'd have a much much blunter point; in practice my vessel has way softer edges than the computer model in all respects. As is, the ama tips are visible from the pilot's position; they way they're drafted here they can be seen at a glance, but they can't be further aft, otherwise the pilot has too look over his shoulder to see them. Unlike many waterways I've been in, one nice thing about southern California ocean is we don't have much flotsam in the water. We don't have real river mouth harbors and very few trees so driftwood, etc, isn't common. We'll get ghost lobster floats sometimes but amas and sponsons won't make special problems there. As for diver safety, it's easy for a small vessel like this to be boarded by freedivers while not under anchor, but still she's ALWAYS at least in neutral when anyone is entering/exiting, and part of the original SOR and my stubbornness on certain design points was about reducing windage so her low profile has that going for her; windage is a huge safety concern when it comes to boarding/exiting on the water. As for docking, since I have my own slip it's easy to add appropriately sized and shaped floats to my own slip; most of the guys in my marina have little floats and modifications on the fingers because besides the broken sailboats that never leave, most of the vessels see a lot of use.

Incidentally, this is a superyacht with lots of computer optimized strakes and steps, their actual waterline hull form is not unlike the amas I have been mulling:
Bladerunner 45 https://shuttleworthdesign.com/Bladerunner45.php

....

Sponsons could look better than a buoyancy collar, that's for sure. I'll sketch some out, but I still think the amas look the sexiest. From a weight/structure standpoint they'd be more challenging but again, due to the way the vessel was built and my own strengths as a fabricator I think I can venture much further afield than the average backyard guy. If anyone with deeper multihull experience could advise I'd appreciate it. I can accept that nearly anything I add can reduce her handling or manueverability but the question is largely 'how much?' She isn't commercially viable if she can take only one passenger (static stability is okay with one passenger); she can lose *some* of her running qualities and still be a very fine boat for my purposes... just want to land on the right compromise. Thanks.

 

Not sure how hard to take off the tower, but you ought to test the speed without it.

Look, I have no c.v. in naval architecture, but my gut says you are not getting up high enough to reduce planing area. A fairly cheap idea would be some 1/4” aluminum interceptors maybe 1-1.5” bite, but I’m not wanting to cut any divers or lines.

So, speed is not the primary concern. But the initial stability. Is the tower for freediving? That is adding to your problems.

What if you just made a hinged ultralight ama that flops down off the tower when you stop? Its connections are not designed for speed, but they make the platform super stable at rest. Basically some aluminum tubing and some marine foam block styled to your liking. Flop them down and pin the connections in a low or up position.

Or is the boat also too wet inside with 4 passengers? …thus requiring more displacement?

 

Yes, getting up higher could help with planing area, but the hull form isn't really designed for true planing, so I have to be cautious there. It's in between a planing hull and displacement hull. It has a more smooth feel and acceleration than a planning hull; it just lifts higher in a somewhat linear fashion as speed increases, unlike a planing hull that might kind of squat and dog down at low speed before getting powered 'over the hump' and then throttled back once on plane. This was the design intention and I love this linear behavior for her use. If you put enough power into this hull shape it'll lift just fine (look at the deep vee offshore racers of the 80's) but you lose efficiency... Ama's though could be easily angled to give more lift; I know with airfoils for instance an angle of attack of up to 3-5 degrees results in a big lift increase without a lot of drag increase. I don't know exactly how important this is ultimately, and deeper amas would likely result in following seas type behavior that I don't want.

Speed in flat water was never a primary concern because there's tons of little boats already out there that are optimized to go very fast in flat water--this is almost any of the light planing hulls that dominate this displacement class--but they tend to pound something awful in our swells unless you slow down a lot. Flat water for us is the exception, not the rule, and so for something that's for the business, the ability to maintain good speed and comfort in our 'normal/roughish' water is much, much more important that top speed in flat water since I run year round and it's valuable to be able to say with some certainty to a customer that, 'yes, we should be perfectly fine to run next weekend as there are no large storms in the forecast' as opposed to 'well, we can pencil in next weekend but if there's a bit of wind on the day you'll want to reschedule'. In the video, all the drone shots would be what we'd consider 'really flat water' around here... 1 to 2 foot swells and 5mph wind, inside the Santa Monica Bay. Those type conditions are more typical of our mornings maybe 80 days a year, which for most days would be when running to the islands, and then in the afternoon the wind comes up and maybe swell picks up and we get some flavor of following sea when returning home. If used commercially I can't pick my days and only go out when it's perfectly flat. For freediving operations, the seas have to be diveable, and for CYA I follow USCG weather advisories but at least 320 days out of the year, I can shoot the channel (which may be very sporty 30 mile run by small boat standards), get into the lee of the Catalina Island, and find a cove that is dead calm and glorious for diving. So, this vessel was more or less designed to perform with a pretty good average speed/comfort across all sea types we have here and I do think she does that better than anything else her size. As for as 'pure speed' even 32kts with a 60hp is still top of class when I compare her to RIBs and Whalers of similar displacement when powered with an equivalent engine. I think this is mostly due to her lightness; she isn't a 'real' planing hull like those vessels but even fully decked out with all her batteries and electronics she's about 1/2 - 2/3 the weight.

The tower is not removable without a hacksaw, and reattaching it would also be major as it's cold molded deep into her hull to withstand a lot of G's. Nothing you see on her is bolted and tabbed on--every piece of her is cold molded to every other piece of her for better or worse. I do think it creates a lot of air drag and while it is incredibly light (by volume about 80% wood/fairing compound and 20% carbon fiber, smidge of glass) the electronics in it add weight, about 30lbs, to the stern. The original design did not have the tower but I needed somewhere to put radar and antennas, and in service it will be a luggage rack for longfins and spearguns. All the electronics are a little bit of commercial use CYA.

The boat isn't too wet with four passengers, these are divers in wetsuits, and would already be drier if I changed the scuppers into drain socks, its simply logistics of commercial use and what people want to do... In the first three days of sea trialing, I didn't bark a lot of orders at my friends because I wanted to see what they'd do and how they'd respond to the boat.... She was never intended for standing operation because I knew that would make her tippy--you're supposed to move across her at crouch, using the handrails and maintaining easy 3 point contact. Everyone wants to stand. Weight distribution is always a thing with a little narrow hull, so if people are boarding they oughta get in and move to the bow, making room for the next person. Instead they want to cluster in the stern. Two people in the boat can do this sort of thing but once you add more people it's too many variables. One friend of mine, he is a very experienced diver/boater/sailor. The two of us working and diving off her found her perfectly nice but he understands how to work with this sort of hull. He remarked to me that 'for anyone who grew up sailing a dory' the vessel is perfectly intuitive. But that would be a very small percentage of my students/customers. I could bark orders at everyone who comes on board, and on day 3 I did start laying down the law to my friends as we were diving and she 'worked' better but I'm going to get tired of training a parade of people on how to use her. I already have to do that to a certain extent with my the kaboats, but a novice's instincts on those inflatable vessels are very different.

So, the only issue is initial stability. If the boat was intended purely for personal use this wouldn't even be an issue. It's just that, I designed the boat as an experiment that I thought would take 6 months to build and about $12k in hard costs, and maybe something that I could use for my business. But then over the course of the four year build I decided I really wanted and needed to use the vessel for business; I can't justify this thing (obviously went way over $12k, but still scads less than a new boat) as a purely personal use vessel as I don't even have time for 'personal' stuff on the water in California anymore, in the last four years the kind of student I have has also changed--much less of the die-hard, blue collar or ex-military type hunter angler, and more just 'LA' type water person. So the vessel needs to reflect this.

Folding amas were suggested by a few friends as well. They could even be designed to fold up and tuck against the radar tower and probably look pretty good. One problem is that I'd have to clamor over customers to raise and lower them, or create some kind of mechanical system that would probably need maintenance, two things which I know are going to drive me crazy and defeat the purpose of the original SOR. I'll mull it over some more though... I did the 3 days of sea trials then immediately had to leave the country for work and so I was just playing with my CAD model for the last two weeks. Now I'm back home and get to spend time walking around the vessel with coffee or beer in hand and study her which is part of my design spiral.

I do think I'm talking myself into adding the ama's I need to decide how deep they should be.. I think the fundamental design compromise lands right there. I mean, now that I study the vessel I don't think they're going to screw her up in any major way, and I really want to build more boats. As far as I can tell, an awful lot of folks have speculated on doing something like this with a fastish narrow hull but no one has explored it in real life, besides Jim Antrim's electric Duffy record setter. I've reached out to a couple local legend designers but they seem not interested in taking something like this on. One of my dive buddies is a naval engineer (offshore platforms more than boats though) so I'm going to rope him into the next sea trial run and pick his brain a bit. I can 100% understand from a manufacturing standpoint why no one has done it, but for higher end custom vessels done with modern materials and manufacture (ie plywood CNC) it's not cost prohibitive. As I understand more about design and market forces of boats I think part of the reason we don't see anything like this is market driven. Small boat buyers are all recreational users and they naturally want the fastest thing they can get for the least money, so a fiberglass planing hull does a fine job there; legitimately fast, cheap, and comfortable... but often in a very narrow segment of sea states!

I'm going to make some of the easy mechanical tweaks while she's out of the water, then do another sea trial in rougher waters with more targeted drone footage. Day one of the video was fairly rough, about as heavy as anything she'd run in commercially, but we were super tentative then because the outboard was brand new and at the same time we were trying to get around the Palos Verdes peninsula into the SM Bay as a gale was building, and it was too windy to drone out there.

 

I think by continually moving running surfaces aft as drawn ama corrections, you are making a mistake and always adding L when you only need B. This will continue to add drag and slow the boat and make it more difficult to get speed, imo.

It would be best if you could do some modeling at this point.

 

Modeling would be good... I need to get to work on other stuff but I am just obsessive (obviously). I think 'trim tabs' are as relevant a way to think about the proposed amas, and the current transom, as is 'hull'. Since this hull doesn't plane in the traditional sense, trim tabs which provide the right lift without a lot of drag would increase top end. The ama could potentially do this with more efficiency/less drag than a traditional trim tab. Running attitude we also noticed, in our very brief and informal assessment coming around the peninsula in the wind on day one, was largely indifferent to the loading situation. Once she's going she runs more or less the way you see her in the video no matter where you put people--more weight in the stern didn't cause her stern to sink, even at low speeds. This is the airboxes doing what people install trim tabs to do (albeit without the advantage of adjustability). They're on the same horizontal plane as her hull bottom (zero angle of attack), so they don't introduce much drag or provide much actual lift but the hydrodynamic forces against them are very strong so no practical amount of cargo will bury the stern which is of course really important for weather ability. Early in design I clumsily ran some calculations on air/water drag (ie, literally an airfoil shaped radar tower for more lift), but quickly realized how insignificant this would be compared to almost anything happening underwater.

I don't have the skills to do a lot of hydronamic modeling but maybe it'll be worth it to hire someone. I've been reading what I can digest in various naval texts and studies, which are helpful, but so much of it admits 'real world results may vary', largely I think because much is modeled in 2D. Modern software is likely better with this... I think one way I can actually look at this most accurately and cheaper is to simply build out some amas and test them at a few different angles actually on the vessel, in the water, before fabricating them to be final. Amas which can be dynamically angled would be the perfect way to go... I bet that's what a lot of the superyacht tri's probably tried. Allen with LEEN mentioned to me they messed around with foils to do this but ended up just optimizing weight distribution, which I can understand since those kind of moving parts under those kind of forces plus salt/ocean would likely be a maintenance nightmare.

 

At this point you are describing a 3 point hydro. In that case, the sponsons or amas should be further aft but not longer; maybe shorter.