Custom 19' all weather, minimalist, strip plank composite 'go fast'

I'm a licensed captain building a very specific kind of boat for my business (southern California freediving and spearfishing instructor). I'm extrapolating this design off an inflatable type--the Saturn Kaboat, AKA "world's best sh*tty boat"--that I've been using now for some years including back in the day in some stoopid weather... ...and am rapidly outgrowing, which has some incredible advantages for what I do as well as all the problems of a cheap inflatable.

In short, dimensions of the intended boat are 19' x 46", 20" transom and loaded heavy probably a 15" freeboard (bear in mind it is carrying freedivers in wetsuits). V-bottom is about 23 degrees at the fore waterline and 13 degrees at the transom. I'm doing 1/2" WRC planks and thinking two layers of CF on the exterior and one layer of 4 oz S-glass on the inside, wooden deck at least 8" from the keel and foam under that except where there are necessary compartments.

This is a very long post but I've been lurking on this site ever since I found it and there seems to be a great deal of good info and feedback...

What I am after: Low fuel cost, good to great speed, good seakeeping, kinda sexy looking, moderately comfortable ride for passengers. Fuel and weight are a big deal since I am using the boat for work multiple times a week so savings here go right into my pocket. Running coastal and island hopping in SoCal we get long period swells, some wind chop in the afternoon, almost always a good bit of following seas on the way home. I'm finishing much of the interior bright since clients will dig it even though this is gonna cost me some durability. I don't take out scuba divers so no tanks rolling around but I will have lead weight belts, and heavy anchors and bottom weights for training rigs are part of my gig. I'm trying to follow ABYC guidelines as much as possible although so long as it's Coast Guard six-pack legal I expect my insurance co will not have a problem since they're currently insuring my inflatable. At under 20' requirements are pretty simple.

What I'm willing to sacrifice: Dry ride (me and my passengers are in wetsuits the entire time), interior space, stability at rest, construction expense. This kind of a boat is premium labor so no point in trying to save pennies on materials and I use my current boat many hundreds of hours per year. It's emphatically not a fishing, family, or sit on the deck and drink beer boat. It's emphatically not designed for 'standing operation'. It's only going to have a 46" beam, and very low freeboard although very shallow draft; passengers must stay seated when boat is underway. I fell in love with long and skinny and plus to fit my slip space it needs to be very narrow. I use my current boat often as an enormous dive float; big boat with a high freeboard never works well for that since the hull becomes a sail even in slight wind.

If anyone with some experience cares to comment on my plans I'd really appreciate it, particularly in the composite realm and weight/strength/possible handling. I've built my strongback, cut out section forms, and laminated and blocked out the stem, keel, and transom. I've already bought some necessary components since a boat this small and minimal almost needs to be designed around the fuel tank, engine, life jack storage, etc. I'm almost ready to start planking. I haven't done composite work on a large scale but I do have a ton of experience with resins, urethanes, silicones, casting etc for building custom dive electronics, and I while I'm slow my woodworking is decent. Based on my research I *think* my intended boat should be ultralight but extremely durable although I haven't seen much info on strip-plank boats of this size--most of what I've read concerns canoes or 30'+ sail boats (ie Gougeon Bros). I don't intend to put a big engine on it but I think a light boat of this size and dimension would experience some pretty serious forces if running wide open with even a mid-size engine.



Materials for boat construction:
Wood: western red cedar, white oak, some 3/4" marine ply.
Glues and adhesives: West Sytems 105 and G-Flex (for the white oak).
Strip planks: 1/2" WRC, bead and cove.
Stem: steam bent/laminated white oak.
Keel: shaping it from 2" x 1.75" laminated white oak.
Transom: 3/4" marine ply laminated with 1/2" western red cedar plank (to be finished bright for interior). I may need to add a 1/2" white oak plate on top of the WRC if my transom isn't thick enough right there for the engine mounting. I'll also do some knees out of shaped 2" x 2" WRC I think.
Exterior fiber: Carbon fiber, two layers. First layer of 4.8 oz warp CF, second layer 5.8 oz warp/twill. This will be painted a few inches above the waterline but below that I'll finish it bright so on closer inspection it reads not as a black bottom but as CF, entirely for the cool/sexy factor.
Interior fiber: S-2 glass, one layer 3.7 oz. Finished bright. Long term durability concerns me but I suppose if it's getting beat up I can always sand this down and coat it with kevlar.
Below decks fiber: Misc S-2 tails from above the deck and biaxial tape plus 6.5 oz unidirectional S-2 strips.
Deck: 1/2" wood laminate, probably WRC to keep it light with a thin layer of white oak on top for look and impact resistance. Maybe a glass sandwich for strength. I wasn't planning to glass the top of the deck, just finish with epoxy (probably the G-Flex) and clear urethane. Finished bright. Again, not sure about durability but white oak is tough and I can always coat it with kevlar at some point.
Transverse deck supports: I probably won't need many of these since the boat is so narrow. I will more or less combine them with transverse walls of fuel tank compartment, battery compartment, and bilge. I'll probably use 3/4" marine ply even though it's heavy; some weight below decks will be a good thing for stability. For longitudinal compartment walls I'm thinking WRC.
Foam: lots of it; I intend for boat to be self bailing and I have a belly tank that will go as far forward as I can stuff it so max deck height will likely need to be 10" - 12" at the bow, slopping to maybe 8" high towards the transom. Low density; foam should not be needed for structure. Plan is to terminate the deck about 8" from the transom and have big scuppers down in that well, and then fore of that well under the deck a small, narrow closed bilge area for cable/wiring access. That closed bilge is designed to stay dry in all but bow stuffing weather which is never impossible but to be strongly avoided. Actual bow will have about 5' of cowling over it with a bulkhead to make two compartments for an anchor box (interior lined with kevlar) and second compartment for life jackets. First couple feet of bow (too far to reach and too small to be useful) will be sealed watertight, mostly foam filled except for space to repair/replace nav lights and bow eye.
Gunwales: laminate WRC strips clad in white oak and kevlar, probably about 3/4" outwale and 2" inwale. For floatation I intend to use a good bit of WRC here.

Trailer: Will live in the water at my slip.
Engine: Suzuki 60hp (best power to weight for 3 cylinders)
Fuel tank: 18 gallon belly tank, intended to go below decks about 1/3 the way back from the bow.
Steering: Dual cable rack and pinion
Batteries: 2 x AGM, in dry hold below decks, roughly midships
Bilge: Regular one in the actual bilge compartment (which will be relatively low volume anyway) and then 2000 GPH emergency bilge, deck height at the transom. Planning some very generous scuppers, 3 x 3" down in the transom well that should cover anything non-catastrophic.
In hull transducer: Probably in the battery compartment. From research I'll need to core out a circle of the plank and fill this with solid epoxy since the transducers won't shoot through wood (air pockets).
Console: Still figuring this out, probably need to get the hull pretty well put together, get my electronics and then design around that. Likely a midships low pedestal type thing, connected to the hull on the starboard side, and a very low saddle seat. Definitely not intended for standing operation. Nothing about this boat is intended for standing. Moving about it when not underway is done at a crouch while holding onto the gunwales and/safety rail.
Passenger seating: Probably very low kneeling bench or very low saddle type seat. Keeping within 2 miles of shore I feel like I can responsibly take me + 4 passengers, and running to the islands I would probably do me + 2. I did an Orca sim on the hull and it should easily carry quite a bit more weight plus the ridiculous amount of floatation but paying customers run the gamut and very small boats do not well suffer kooks.

 

Well presented
A couple of thoughts

Have you considered all plywood construction? I see mostly flat panels that might be easier in sheet material than strip planking.

The FG and carbon are nowhere near balanced.

That's a lot of carbon used in comparison when it shines in tension.

Was the carbon free?

Bright finished carbon underwater. What a nightmare to keep growth off of. I guess you can have the customers wipe it each dive.

Good luck

 

@socalspearit, a pair of considerations, in addition to praising the detailed description of your boat. Have you done any weight estimation, hydrostatic calculations or stability studies? The beam / length ratio seems to be very low and the boat is likely to have poor initial stability. On the other hand, in the view of the cross sections some lines seem indicate some irregularities in the side. Also, if you eliminated the double curvature of the bow surfaces it is very possible that you could use plywood instead of strip planking. But perhaps that does not interest you, it is only a proposal.

 

@socalspearit how long was / is your Saturn KA boat?
I just had a look at their website, and the longest boat appears to be 18' - is this the one that you are using for reference?
18' Saturn Inflatable Boat https://www.saturnrafts.com/18-saturn-boat-en.html

Note that their overall beam is 6'8", while your boat's width is 3'10" (46") - that is a huge difference!
(Or is the 46" beam a mistake?)
And for two boats of the same width (re comparing a wooden boat like yours with an inflatable) I would think that the inflatable will generally have better stability than the wooden boat.
Or are you planning on fitting an inflatable collar around your boat, to effectively make it a RHIB?
If you did do this, then I think you would be ok re stability, but it will be very tippy without.

Agreed - the sections above the chine between amidships and the bow do look rather strange, and I think they might need some more work?
Also, re the bow sections below the chine- they appear to be concave when looking from the outside. Should they not be slightly convex?

 

When I started this I found mostly plans for plywood construction but couldn't find any plans for boats that were remotely close to my very narrow, specific application, although military/SAR and race hulls looked close but of course much, much larger. I'm pretty well committed to the strip-plank thing at this point since I cut the forms and paid a very dear price for a few 20' x 2" x 8" western red cedar planks. I suppose I could use different types of plywood and I certainly see your point but once I started stripping out the WRC I found it eye-poppingly light especially compared to marine ply. Weight savings for this craft = fuel savings and as much as I use it--the high intial cost of very light materials should pay for itself in gas savings relatively quickly. I don't actually have a ton of experience with strip plank boats--I've been on countless fiberglass center consoles and RIBs, and many of my slip neighbors are small commercial fishermen with very solid work boats and even one of the original Radon's (something like 60 years old and likely to last another 60), BUT one guy across from me has a strip built center console fishing boat. He has a 70hp 2 stroke on it and says just him he's made 35 knots. I haven't seen it on the water but in the harbor it's so light it sits practically out of the water. The strip thing also looks pretty; I sell a premium service so I think if the boat looks a little different it can't hurt. After so many years in the rubber navy I guess I'm willing to pay and sweat more for a pretty boat. One day I might build a stitch and glue family boat (the Bluejacket boats look terrific for family weekends at Catalina).

The bottom cleaning isn't a big deal for me since I'm a diver and underneath my boat all the time. I also found some clear catalyzed teflon urethane bottom paint which I have on my BOM (although I'd like to get more info to know how it works): Smooth Sailing Boat Bottom Paint Products http://www.boat-bottom-paint.com/products.asp

No, carbon fiber isn't free I did a lot of research and reading but I am not so versed in the composite properties when used on a boat hull like this. I do have several buddies that do pretty good boat work but they're more traditional fiberglass, foam core, chop strand and poly. I'll try and research more on balancing the composite materials. To my understanding the S glass actually has comparable mechanical properties to CF it's just heavier. The other thing which I potentially have going for the design is a wood deck some distance from the bottom of the hull that should provide a good bit of stiffening? I have not yet bought any of the fiber.

re: plywood, see above... think the irregularities you see in the side are some kind digital artifacts left over from Orca (the plug-in I ended up using the make the digital model) but the sections themselves when cut out from MDF and stacked together are true. I did actually run some sims with Orca on this hull design. To be honest I did not understand all of the calculations the software spat out but I did play around with center of gravity and loading, etc. It floats very high on the water Based on the weight of the WRC, etc, the hull when completely finished with all deck and hardware will probably be less than 200lbs, then you figure 70lbs of batteries, 18 gallons of gas, 230lbs engine, some electronics and steering I guess total weight is still around 500lbs. Most fiberglass boats and RIBs that could handle a 60hp are coming in at three to four times that weight.

A fair bit of this design is my practical experience with many thousands hours and miles of running my inflatables over the years which have a somewhat similar hull design. Initial stability is misleading for these long skinny vessels. My current inflatable is crazy, it's 15' long and about 40" wide. It was designed as a soft flat bottom but I have a couple solid planks of white oak between the bottom and air floor which give it a shallow V (not unlike this hull design). We haul ourselves over the side all day and I can even stand on one of the side tubes (obviously not while underway) and it barely heels which most people don't expect. A lot of this is because it's quite boxy. The coefficient of the waterline plane is extremely high and the COG is very low. It wouldn't be good for heavy cargo or tons of fish but it's more than adequate for the weight of people moving around on it and clambering over the sides. People look at it and think it's like a kayak or canoe which is very tippy but the waterplane area on those vessels is tiny since they have such fine bows and sterns; this thing is more like a barge. A proper self bailing deck will raise the COG more than I love due to passengers but they're basically on the floor and I'm trying to keep any of the structural components, batteries, and gas tank down as low as possible.

 

My Saturn is an SK470 the exterior width is 42" and it's 15' long. They may have released a wider one but for years they've all been extraordinarily long and narrow by most standards: 15' inflatable Kayak & inflatable Boat crossover - KaBoat SK470. https://www.boatstogo.com/kayaks-sk470.asp?gclid=Cj0KCQjw7MGJBhD-ARIsAMZ0eeswDYZ-eNGx0QZDQFw9-Xzj0ZKP9rx4bMD-lKcwdvkz2-FlZ4oC_5QaAmJJEALw_wcB

Thanks, re: sections do you mean how they're sort of concave? Or how there are diagonal sections in the model? The diagonals are leftover from how Orca made the model. The concavity is minimal except right at the bow but when I actually cut the sections out of MDF I cut them with no concavity.

re: tippy. I'm not planning on tubes around it. I'm sick of repairing inflatables. Everyone who looks at this thinks 'tippy', but it's the same with the kaboat. It does concern me, maybe I'll build a scale model. One thing though is that most people assume you walk on a boat deck. My kaboat can be walked on pretty well if it's lashed to the slip but in open water NOBODY stands on it. Hauling oneself over the side of my intended build though the leverage is a little different but the kaboat is so stable in this regard, I dunno...? Maybe my standard for tippy is very different in this application also since most boaters are averse to falling into the water. To be fair, if I have 2 people in the kaboat and two are sitting on the same side tube (underway running wide open with 15hp we make about 16 knots so everybody sits down in it like a tobagan), I always go in on the other side to keep it balance. It's virtually impossible though to actually tip and roll the kaboat. Doing a sudden sharp turn at high speed might do it, and once landing on a beach when it got turned sideways in a breaking wave I was sitting on the side and felt it going. I just rolled off and it righted itself but that's by far the closest that vessel has ever felt to tipping.

 

Would such a vessel pass the requirements of a boat plying for hire, offshore, in your jurisdiction ? I very much doubt it would here, and what would be the number of people you would be allowed to take out ?

 

I don't think it's an Orca flaw but, if so, I would stop using that software.
Although the cross sections may seem correct to you, I would check the shape of the water lines and longitudinal sections.

 

Yes, my inflatable is legal (but was challenging to insure for hire) and my captain's license allows me to take up 6 people on what's called an 'uninspected vessel', which means if it's legal to be on the water period I can use it for hire provided I have a few additional basic safety devices (which I do). I would not put 6 people on this vessel except maybe buddies on a coastal run but why? I also don't take anyone who is not a competent swimmer/trained freediver and everyone is in a wetsuit before they even get on my boat. I have to double check the regs but for certain types of offshore use (it depends on how far offshore) there are additional safety requirements such as an InReach type device (intended on the new boat), and at a certain point radar, etc, but I would not intend to take the new boat that far out. It would be irresponsible to take people out for hire to the islands but I used to run my inflatable out there all the time solo or with a buddy (Catalina is 25 miles, Anacapa is a more realistic 15). More than 6 people and more than 20' long, and more than a certain distance our regs get much stricter.

 

Okay, thank you I will research this.

 

One look at that boat, and hearing it will have to run a significant following sea, the term "submarine" comes to mind. I would have thought a significant upsweep to the sheerline forward, would be strongly indicated.

 

If you only have to travel say three miles out, you can get away with low comfort levels, but a boat you can't stand up in, underway offshore, would get very wearing, over much longer.

 

I do get your thinking here. I do intend on having a curved cowling that extends 5' back from the bow. My application is very different though; I have been running around for years in what's basically a dive float with a gas engine for years and while I'm not currently running 40 knots in a big following sea with it I've definitely been on somebody else's 'traditional' boat in the same seas as I've taken my crappy inflatable in and thought to myself that I'd just as soon be on my unsinkable gas powered dive float, and granted this is usually on somebody else's boat with a not very experienced guy at the helm who may have never really run his boat in any weather (downside of the inflatable of course is that it's only unsinkable until severely punctured). A lot of the smaller Zodiac type boats would be similar--they have so much floatation they ride in pretty much anything and if lots of water is in the boat they more or less instantly drain. What I'm thinking to build is intended to be similar--there's enough foam in the plan that the boat would float pretty comfortably level if swamped with people (although a swamped boat doesn't exactly handle well) plus the entire hull being primarily wood is already positively bouyant. Upside to low bow and long skinny is the ultralight boat is not so much affected by wind. My inflatable is self bailing but the scupper is tiny and there isn't room for much of a bilge so I do not run it if conditions or the inexperience of passengers could cause it to swamp. I want to design this boat so it drains very, very fast (much faster than regs or ABYC standards). The inflatable also is wicked in a following sea. It surfs so well I can almost always run wide open in any following sea although weight distribution matters. I haven't stuffed its bow in years once I figured out the weight distribution. I'd like to keep this characteristic as much as possible. I'm obviously not a naval architect but I have a lot of sea time in a very nonstandard vessel that behaves in a very nonstandard way so I'm trying to understand why it does what it does and how I can build on that. Also while it's a point of discussion and much to chew on I'm certainly not intending to run in gales (taking my inflatable out back in the day in those conditions ranks among the dumber things I have done) but I do want a very very healthy margin for sea conditions. I don't do sessions with students and clients if we have any kind of small craft advisories. In socal we have storms but even a very stormy winter might be 6 days out of the month where we have small craft warnings. What I do with my boat on my own time of course is different.

Comfort is pretty low on my list, maybe 'moderately comfortable' should be downgraded to 'bearable' but again, my application is very different. Being stuck for hours on a boat for hours and unable to stand up would be lousy. But, I'm a freediver and I shuttle freedivers to spots then get in the water with them. We usually want to get to a spot as quick as possible then immediately get in the water. Once we're out of the water we want to immediately get home. It varies with time of year but say I go to one of the near islands on my inflatable to spearfish and I'm 8 hours on the water. Maybe 2 hours of that is running wide open to and from the island, 60 minutes is sitting on the boat at anchor throughout the day eating, warming up, farting around with fish/rigging/kill bag, and the other 5 hours we're in the water. This is how it goes on my inflatable as well as any small center consoles or RIBs or even jet ski (which can be VERY quick but running long distances this way is exhausting, too much for me and not realistic for taking other people out). About 30 minute runs at a time is the realistic limit on my kaboat for my students. This vessel I think could realistically be more like 60 minutes. A bigger boat that allows us to change in and out of wetsuits and be dry is of course a different experience, and of course some private clients have some very luxurious vessels but then the day tends to be centered more eating, cooking, socializing, probably some drinks, etc, with a little diving... I actually have to spend a lot of time maintaining the inflatable (it is also very customized) but fuel efficiency staggering--in calm seas with three people in the boat I can make about 16-17 knots and 12 miles to the gallon. That's hard to give up, fuel is barely even part of my overhead. No paying clients are paying for the pleasure of my boat, they're putting up with my boat to dive with me, although sometimes my students get a big kick out of zipping around in a camo wetsuit past the cliffs, getting smacked by the occasional wave, it's not a typical boat experience.

Anyway, thanks for the thoughts, lots for me to chew on from this thread.

 

I still think that your beam of 46" on a length of 19' is not going to be enough.

I think it is quite telling that the larger 18' sister to your 15' boat has a beam of 6'8".

I know that you are already fairly advanced in the build of this boat, but I am just wondering what you think about making it 2' wider (this would still only result in a beam of 5'10") by cutting it down the middle, separating the two halves by 2', and creating an assymmetric catamaran instead?
I have a feeling that your miles / gallon might even improve if you do this.

 

I did actually think about a small catamaran at one point from the standpoint of fuel economy but I have this dive platform thing in half my slip so can't fit anything that wide. I could actually go up from 46" but also based on weight distribution (that engine is very significant), just looking at it plus some of the sims I did in Orca, the actual waterline length/beam (and this of course depends on loading) isn't really 4.75/1, it's closer to 4/1. This is very long and skinny by today's standards but the old pangas, raceboats, etc, were much longer and skinnier than modern sport boats. Historically, the long skinny boats had good seakeeping albeit less initial stability, and this is my experience with my inflatable. It's kind of weird--it planes easily because it's so light but in reality it's more of a semi-displacement hull, but unlike a lot of planing hulls it never ever squats, it just sort of skims. Some of this is that it's so long and skinny weight has to be spread out. I like that characteristic. For this application I want to stay well away from deep V type squaty planing hulls. We don't really do a lot of uphill charging and I find a boat that surfs well in a following sea (longer, more shallow V) is a good compromise for what I do. I dunno, I could make this thing a little wider though. I've been second guessing it a lot especially when I dropped the forms onto the strongback to just see the shape at scale. I don't think the 'tippiness' is as a big a problem for me as it would be for other users though, and I don't know that it's as tippy as expected. It's a little counterintuitive but because it's so long it will have a lot of bouyancy from the edges that adds up if it starts to tip. Based on my inflatable it will be VERY LOUSY if sideways certain types of wind waves but in emergency this becomes a steerage fix and when we have those seas they are predicted and accompanied by small craft warnings, and my inflatable has made those seas alarming in no small part because it has a freeboard of like 11" with 3 people in it and drains too slow...

Hmm, that being said the waterline beam of this design is a lot less than 46"! They are built on a much larger scale and I know there are loads of design elements that I don't fully understand but many of military and SARs type boats currently in use (where again, seakeeping and speed trumps stability/comfort and cargo) are built with a very high length to beam ratio, much higher than typical 3:1 which is what most mass market sport type boats seem to max out at. It's tough because I can't find much literature on this, many of the longer vessels I am researching are actually full displacement hull sailboats so somewhat N/A, and real raceboat and military designers don't publish much on the internet, or if they do their modern designs are so far beyond this--multiple steps and pads and tunnels etc, which are meaningless for me. I feel like I'm making sort of a old school race hull with a more conservative engine; looking at race boats from close to a hundred years ago when 60 horses was a 'big' engine they are designed similar to this with speed and reasonable seakeeping as priorities.

I did realize that for this thing I should do at least a pair of longitudinal stringers reinforced with unidirectional fibers under the floor. There's the potential for a lot of sagging force for something like this. I ran into this on my inflatable--it's been through several iterations of longitudinal reinforcements under the airfloor... I snapped and cracked a lot of materials before just putting some heavy white oak timber under there with the 15hp engine.. first time running it with that size engine it was kinda alarming--transom swaying and buckling under the torque and the front half of the boat wanting to taco up. It's a crappy inflatable but the forces on the long skinny design get extreme in the center, and this boat has the potential for so much more speed and much much greater forces.