26' power trimaran, trawler style family boat

I am far along enough on my other build (very small but somewhat sophisticated strip plank carbon fiber composite) that I am stupid enough to start thinking about the NEXT boat, which is very much on the opposite end of the spectrum.

I want a cheap and cheerful family boat for southern California, mostly to run from an LA harbor to Catalina Island (about 25kts) and hang out for a few days to a week, anchored in one of the coves or on a mooring, maybe head to a further island time and weather permitting. I have been looking around and seen some interesting designs but understably at that size, most boats have some compromises in order to be easily trailerable. I just want something that will fit my 26' slip, or that can be dry docked for even cheaper at one of our harbors, and I am not interested in an easily trailerable boat.

What I want out of the boat:
-Outboard powered
-Lots of living space, much of it indoors/covered
-Stable and comfy on anchor (good initial stability)
-Cheap/efficient to run, doesn't have to be super fast
-Space for 4-5 adults to sleep comfortably with some options for privacy
-Decent cooking area
-Head/shower
-Diver/watersport friendly
-Needs to fit in 12' wide slip but does not need to be easily trailered
-Seakeeping is always a concern but it would not be intended to run 'all-weather' or far offshore where weather changes suddenly from good to very bad with no warning
-maybe 300 mile range
-hook and line fishing is not a concern whatsoever me but it would be nice for possible resale if a more serious fisherman could look at it and see himself/herself fishing while the rest of the family is happily occupied

I do not have a ton of experience with multihulls but I am liking the idea of a semi-displacement trimaran--something that would have characteristics that fall between a mono and cat. I liked the floor plan of the Skoota 24'. This build I would do plywood stringer frame with a layer of glass, and generally try to keep everything above the waterline light and sturdy enough for a family but not a commercial/industrial strength.

I am away from my computers so this is not much more than a 'back of a napkin' sketch for now but this concept has:
-seperate/private aft queen berth
-midships area is covered outdoor galley, shower, head (composting toilet)
-all tanks/batteries, etc, in the center hull down low
-spot for tender and little lifting boom
-twin berths in the pilothouse/fore cabin
-self bailing

I'd to rough it out in Fusion 360 to work out basic weight/buoyancy/waterline, but if I drop the center hull deeper it would take on more monohull characteristics, right? If the hulls in general are also finer and deeper, that could also reduce tendency for vertical heaving also? Not sure if I am totally missing something, but why are seagoing trimarans of this size almost never done with this type hull in order to maximize deck space? Would it be a nightmare in certain seas for some reason I'm not understanding? Also--for our waters we have consistent long period swells 2-3 ft on many days, 4-6' is a 'rougher day' but outside of storm conditions not much more wind than 15-20kt gusts in the afternoon, with calm mornings, evenings, and nights. Our waters are also deep so draft is not a big issue.

 

Hi,
I've studied the hydrodynamics of this boat LEEN 56 - LEEN-TRIMARANS https://leen-trimarans.com/fr/gamme/leen-56/ . A little bigger than yours, but numbers can be extrapolated, i think


I drop the center hull deeper it would take on more monohull characteristics, right ? If the hulls in general are also finer and deeper, that could also reduce tendency for vertical heaving also? => That's partially true. However, it shall be balanced with the "semi-planning" navigation mode that you are seeking, taking into account, also, that in general, such configuration is not really made for planning. Their advantage is their low drag and their excellent stability. So you have to carefully balanced the hulls depths and the position of the centers of the hull. In the drawing down below, are represented the Center of Floatation (CF) and the Center of Buyoancy (CB) of the central hull and the two floats, put together, for the LEEN56. We explore designs with the center of buyoancy placed in front of the center of floatation, in order to help attaining the semi planning mode.



Down below are the final hulls parameters, where CC stands for Central Hull, and FL for Float. Dimensions are in m, areas in m2, Displacement in m3. location are given from the forward perpendicular.



To illustrate the trade-off that shall be made between beam, depth and drag, here is the result of drag calculations @ fixed speed, made by CFD computations, for the LEEN70, big brother of the LEEN56. I hope this will help in giving you a direction where to go when drawing your hulls.

 

Thank you! That is very helpful for understanding the multi-hulls. I did not consider Cb and Cf in the last design but for a well behaved multi-hull I can see how that would be critical.

The LEEN boats were obviously something I looked at for this design as they were the closest thing I could find to what I was imagining--3 similarly shaped and sized hulls to create a large platform as opposed to center hull with tiny amas. Having the hull detail and numbers will be very helpful for study.

 

By the way, it would seem like the foils on the LEEN hull could help immensely when it comes to finer trim. Because of our kelp forrests such a thing would be absurd out here unless on an enormous scale--but if it could be designed to 'clean' itself by folding the wings down flat against themselves then swiveling up to be longitudinally in line with hull it might work. That would be a lot of moving parts and so outside my scope but anyway a little adjustable foil seems like a way to improve multihull planing.

 

Yes that's true. We had issues with earlier hull design with a too excessive trim of the boat. That's why we choose to explore the foil option, together with a trim tab option, as trim is essential for a boat to achieve planning In the end, the simpliest solution has been retained, with a re-centered weight and a lowered maximum speed, keeping a desire distance range in accordance with the choosen engine while having a correct trim angle for the vision at helm. Sadly leaving the foil option aside...

Still, the conclusion of our study was :
- at speed below 12tks, a trim tab reduces the boat drag more efficiently than foils.
- above 12kts, a trim tab does not change the boat drag compared to a configuration without trim tab
- above 12kts, foils diminuishes the boat drag, and would have allowed a greater speed with the same propulsive force.
- the use of foils allows to control the planning speed of a multihull


Please note that the value of 12kts is specific to our earlier design. This value will differ from one configuration to another. In the figures given in the graphic downbelow, the surface of the trim tab and of the foil, as well as their trim variation with speed, have been specifically choosen to overcome the formation of a big bow wave at 12kts.

Theoretical power curves without foils (wofoil), with foils (wifoil), with trim tab (witrimtab). Horizontal : Boat speed in kts. Vertical : Propeller power in kW.


Theoretical trim curves without foils (wofoil), with foils (wifoil), with trim tab (witrimtab). Horizontal : Boat speed in kts. Vertical : Trim angle in degree.
The data points wifoil and witrim are superposed, as we deliberatly impose a choosen trim angle.

 

I had a little time to CAD the concept for this and start to think about the hull design more, settling on a 29.5' x 12' hull size. So long as the boat is under 30' LOA and with a max beam of 12' it would be fairly easy to slip in most of our Los Angeles marinas, and also I think possible to dry-dock. It would be intended for a modest outboard (60hp - 100hp), or by the time I actually get to it perhaps the price of electric would be low enough that I could do a direct drive electric inboard. I need to do some displacement and weight estimations but the bridge deck is intended ride at least 1' off the waterline.

@Alan Cattelliot Thank you for your detailed responses and sharing your data. I studied your hull bottom models and tried to digest as much as I could of the numbers. This shape I sketched out should put the center of floatation aft of the center of bouyancy. I'd be willing to lose a little bit of planing behavior for less vertical heave so the center hull grows somewhat fine and deep in the fore sections. The center hull would house all the mechanical and operational stuff, and I'd build with an eye towards being able to shift the water tank and batteries to get the balance right as I sea trial. This boat build would be a big commitment for me so before I pull the trigger on this thing, which is still at least a couple years out, I'm thinking to build a 15' catamaran barge, powered with a 15hp engine, to use for freediving competition to replace the tow barge I currently have. If I build the 15' cat hulls to this same exact shape of this tri's center hull, I should be able to get fairly accurate assessment of how the hull shape behaves in the real world, correct (adjusting of course for the fact that the cat is two hulls instead of one)?

 

Despite my thoughts on using this design at 12' beam constraint, can you explain how far lcb and lcf can be apart before things go wrong?

 

What is the advantage over a monohull or cat here? At 12' beam; it seems below the lower limits of any benefit to 3 hulls.

 

I share your concern about the limits of benefits of 3 hulls. Using 3 hulls increases the stability, if a stability "full over all" is considered to be the first requirement for the project. Trimaran crew ships are known to be good at minimizing platform movements, facilitating transfer in heavy conditions.
How far lcb and lcf can be apart before things go wrong ? The positive heave (trim) at rest could not fit the visibility requirements for the operating conditions. Together with a poorly trimmed and powered propulsion, the boat could not take off. The boat wake keep getting awfull and awfull, as you've reached the maximum couple available from the motors, while standing on your tip toes, struggling to see the sea waters.

 

Does the lcg have anything to do with all that reasoning?

 

I will pick apart the hulls I sketched to see where CF and CB actually lie in this iteration. They're probably somewhat far apart although I needed to digitally chop up the hulls and really check.

I've spent some time doing freedive operations off flat platforms in just average afternoons and the vertical heave quickly becomes absurd so I might be overthinking the fineness of the forward sections. These hull shapes can be squeezed and stretched to move CF and Cb around to some degree.

Stability at rest is a primary design concern, and it would mostly live in very sheltered to somewhat sheltered waters. It is not intended to be an all weather open ocean type cruiser or serious work boat, more a houseboat that makes channel crossings comfortably if you pay attention to weather. The crossing from Los Angeles to Catalina is mellow outside of storms and our afternoon wind, and all off our nearshore local islands have sheltered bays and coves once you get there. For the most part, and certainly while underway, I expect most of the passenger weight would be slightly fore of the middle, so the aft end stays light. It's intended to have a tender on the roof of the rear cabin, but something small and light, probably weighing about 250lbs--an inflatable or stripper catamaran powered by either 6hp or 15hp.

My buddy has a similarly sized monohull, and it's definitely a seaworthy beast with somebody who knows what they're doing is behind the wheel, but it's lack of stability at rest is off putting to all but the saltiest boaters. Our long period swells aren't kind to catamaran hulls from what I've gathered. I do know a couple guys with small cats who flog them relentlessly for offshore fishing but they seem to be the exception and the boats have limited lives before the bridge starts to split. I was thinking that a tri hull would put me somewhere in between those two worlds--great stability at rest and for our conditions reasonable seakeeping, but admittedly I don't have much experience with multi-hulls and can't seem to find anyone who's made a power cat of these dimensions. In research I did see a lot of stuff about 'cathedral hulls' in much smaller boats, which seems to be a tri hull that becomes a mono in the rear, and they seem to get very high ratings for stability and ride in calm water but fall apart in swell and chop for obvious reasons.

Here are some more polished renders:

 

Again, I may be very inexperienced here but the other advantage over monohull would seem to be efficiency. My buddy's big converted 30' fishing trawler has something like a 500hp inboard diesel to push it at 20 kts (this is also a weight thing for the boat) and other big monohull sleepers I have been on with clients are usually something I find absurd--triple banger 200hp outboards (fast though) and loads of belowdeck space but not much actual privacy. I am sure the LEENS are incredibly well designed, tested and engineered but they're 50' boats making 15kts on 200hp. The 50' monos I have been on need a couple big inboard diesels to do that.

 

My intuition suggests you have too much cabin mass too far forward for all the fine entries of all 3 hulls. (By a lot)

I am having my own issues with my boat being too heavy forward, so experience or bias, pick one or both!

I would think you need to carefully model the hydrostatics which is outside my paygrade.

A Hickman Sea Sled would be a good option, but those have proven tricky to propel. @baeckmo is a local expert on them

Keep in mind, a fine triangular hull behaves very differently with a stone in the back versus a stone in the front..

 

@fallguy Thanks for the info on the Hickman. That's very interesting to me--it reminds me of my father's little WorldCat in some ways (the airpocket concept) although I think the WorldCat hull didn't flatten out some much in the back. He had that in Florida out of Destin so my time on it was in very different seas than southern California.

Something like that could be exactly great as a little tender for this big boat, and I feel like I've seen very similarly shaped hulls in some smaller tender type vessels.

My issue with the Hickman hull is that it looks and sounds like it does great running at speed, but at rest it seems like it could have potential for loads of vertical heave in our swells. This is just my off the cuff, first impression though. This boat I'm designing, would, when in use, be on anchor at least 95% of the time in somewhat protected to very protected water so stability at rest is the main thing. It's intended usage is weekends at Catalina and the like so we're talking 2 hours each way and 48 - 72 hours on anchor, or perhaps just moored for 3-4 months out of year in a sheltered spot on the island, or maybe sitting in a harbor and rented out as on water airbnb.

As for the fineness of the forward sections, I will examine that. I certainly may have overdone it there. I do have a question for you though about your vessel--how is it at rest in the swells vs running? I'd be willing to trade off some of the running behavior for better at rest behavior. You're out of Florida, right? I know it depends on where you're at but from what I remember of my time in Florida waters is that you do get a lot of wind chop and some good waves but they're relatively short period. We have can have fairly large swells but they're very long period. As far as I can figure, that's why on the west coast monohulls are so much more popular than multihulls--market forces and boating conventions taken into account. While the long periods can be unkind to anyone prone to motion sickness, the boats feel steady. I have not spent very much time on multihulls here but based on speculation and what I've been told, the long periods can make cat hulls really uncomfortable from the standpoint of vertical heave. I don't know anyone with a trimaran but since it seems like it could be designed to have more of a monohull characteristic it might be good here.

EDIT: by the way, just to explain, 'sheltered' to me means more or less out of the wind. But our swells can and often do come from a different direction than our winds. So, we can be anchored in some cove that feels really safe and calm but there's still some swell finding its way in. It's hard to escape the swell unless you're inside a harbor.

 

I apologize, but my boat is in the Mississippi River and now on the frozen ground. She is headed for the Gulf of Mexico. I expect with the fine entry; the boat would be good in vertical matters, but we are adding bow extensions which will probably increase that some or add vertical movement perhaps with some width forward, that is..

Based on my limited knowledge and time fishing in California, my guess is the driver to the monohulls is related to costly slip fees.

My boat is 17' beam or so and needs a slip 19' wide by 40' deep. In many marinas, they make you pick up a double or in some, a 60-80' slip to get the beam.

My boat is a Skoota 32 demountable. About 10,000 pounds.

The Hickman would indeed probably ride atop more...but in long period, there is no getting around it, pick your boat, you are riding them. My personal opinion is if you are choosing that hull to gain 6" against 6' long period seas; you are going to be disappointed in the tradeoffs.

Building 3 hulls and marrying them is going to add a lot to the build time and material budgets. Same for a cat. For my boat, probably about a year per 36' oal hull, then a year for bdeck. Project was 5 years, 12,000 hours.

To get that cabin that far forward, gonna need more forefoot. Have you seen Red Sea Diver's posts on the cats with big feet?