pontoon that masquerades as a powercat -overview

I apologize ahead of time because this post will run a little long. I don't see any way around it. As a new member, at the start of a build, it seems necessary to be thorough so that you know my skill level and what I am trying to realistically accomplish. Also this is probably posted to the wrong section of the forum. I wasn't sure to go here, or multi hulls, or what.
So, I've always been comfortable with a canoe or kayak and a tent but my kids not so much. I have decent woodworking and caprentry experience so i figured they would be happier (and i'd get alot of of brownie points) if I build something they can have more fun on. My woodworking skills are enough that I am confident building a basic hull shape that will float and stay together at slow river cruising speeds. But I don't know anything about designing stability into hulls; a pontoon boat seems to take care of that. However, I can't stand the whole "I mounted my front porch on some 55 gallon drums" look, so I figured I would make it look like a sailing cat minus the mast & rigging.
Now, I'm not expecting to have any resale value, or to be efficient moving through the water, or to really do anything other than push against the current at maybe 12-15 knots. Just enough to be able to move around a bit without wasting half the day.
I'm not using the best of materials and I have read many of your warnings against it in other threads. The obvious dangers of its durability, the increased maintenance and initial sealing costs, etc. It is a trade off I've decided to accept. I also do not know traditional boat building methods. I can make a square, racked, level, plumb, fair, etc. 3-dimensional structure with my methods though. So some pics will probably look like crap from a boat building stand point but the structure is square in 3d to within 1/4" tolerance before sheeting. After sheeting it should be within 1/16 to 1/8". I also like to build full scale and deal with the extra time/expense of reworking things due to the incomplete design. I just seem to work better when I can see full size pieces in front of me.
The main reasons I am sharing this is for safety and education. If I begin to move in a direction that is fundamentally or structurally unsafe it would be nice to have someone experienced tell me to stop. I am not trying to get any of you professionals to design this for me bit by bit either. I am only looking for a "you cant do that dumb***" but if someone wants to link reference material or even explain it, I would certainly not complain. I am reading everything I can find and have already made several modifications based on things I have read in old posts here. But reading the literature isnt always enough. For example, the hull I shaped out seems to me like it is a semi-planing hull. I think the design will be able to get to something of a plane with something more than dual 10-15 hp outboards. But I've only got what I've read to go against and I'm not sure if I have calculated properly. So getting a yay or nay on things would be great, and again if you want to link reference or go into detail- awesome but not expected. The secondary reason for sharing is because I would hate to build and run it for a number of years and then run into someone who knows better and find out that some slight variation would have made an exponentially large performance difference. Whether it is fuel economy or range or safe operating zones or whatever. This part of it does seem to me like it is trying to get free design bits though and if I get no input on that front i would get it. Basically, I just want any advice that more experienced people are willingly to give.

So if you've made it this far, thank you and this is where I get into the fun stuff.

Starting goals:
Sleep me, my wife, and two kids (not necessarily spaciously)
Trailerable
minimal draft
fit smaller rivers
10-15 knots
16-20 ft LOA
no greater than 11-12 ft BOA

What would be cool:
Gulf of Mexico coastal area capable
faster speeds without huge fuel consumption or ridiculously large motors


Concept:
roughly 30" wide hulls with a single berth and another compartment for either a generator or a small galley (one per side), a center cuddy cabin big enough for a small double berth, head (composting not trad. marine), & helm. Under the cuddy would be a rail system like under a kitchen table that would allow the cuddy to open in the middle and panels placed in the opening. There would be 2 support beams, the normal one close to the bow and another one behind the cuddy. They would be bolt in and would have two sets of receiver brackets. One on the freeboard for when opened up and the other on the deck for when closed. This should allow for the same beam to be used for support in both open and closed positions. Opening and closing would be limited to calm waters or docked situations.


Current Progress:

First hull

I went with a 6" flat bottom that goes into a shallow V that rises 1" for every 3" and runs to a 24" width where it is 3" up from the flat. I then connected that point to a point 24" up from the bottom and 30 inches wide. In the first 5 ft, I brought the bottom up from 24" to 9" and the width from 30" to 3" and I plan on capping and shaping the nose after sheeting. The pics will show where ive gotten to.
The longitudinal stringers are missing a few because I am still kicking around whether I should use a 10 ft piece of 1x4 right in the middle. This would then be a stiff, weight distributed spot to tie the beam supports together at the lowest level. They also were not necessary to keep the frame square and it gave me a little more room to reach around.
I didn't know exactly what the dimensions would look like after bending the stringers so the nose piece is missing a few pieces yet. The bent stringers are pulling the front 2 ft or so out of position so I was going to get the side pieces of ply laid out and cut and then temporarily tack them in place to hold everything racked while I fill in the rest of the nose support. Once those pieces are in place, when I remove the ply everything should sit like it should.
Then I will finish the stern/transom. I am trying to figure out if this will be worth keeping a flatish bottom all the way to the transom in an attempt at planing or whether I should forget about achieving plane and pull the bottom up over a couple feet. Upside to that is more water flow around the prop at a shallower depth.

So that's where I am at. Worse case scenario, I should be Ok as far as a slow cow that wont kill anybody. If I have inadvertently done something right, we can celebrate after the thing floats without killing anyone.

Pics to follow.

 

Seeing you are well along the path of making this thing, it is a little late to be inviting suggestions, possibly. Some better pictures might help you get an assessment of it, but the idea you will plane with small outboards can safely be discounted, imo.

 

I'll see what I can do about some better pics. The thought on planing was more of a "maybe i stumbled on something" thing. Im not gonna try and put anything large power wise on unless someone knowledgeable were to look at things and be able to easily say, "yeah that can handle more" without taking measurements or doing calculations.
I have trouble sitting down and designing something on paper. I could design furniture or single to two story houses and know they would hold up right, but i would still have to start building and design each stage after the prior one was built. I have been lurking here for a few weeks now soaking up info and i rushed to join today and get the post up because i am still at a point where some modifications can be done. Another hour or two of work and and i will have lost that window. I am looking at this as a learning thing so even if you just want to tell me why I have been an idiot, that's okay. If i won the lottery tomorrow I'd still finish it, but i'd be on the phone with Woods or Hughes to get one done right at the same time. Most likely this thing will be a cow, I'll learn a bunch of stuff and run her till she falls apart. Hopefully I will be able to get some real plans and have something decent built before then.

 

You have started with a good attitude Pirate Twig. A lot of newbies have their minds made up before they even make their first post.

From what I see in the pictures, you have framed one hull. It would not be unthinkable for you to stop now and start over with a better game plan. You can get some good advice from our members. This is the first bit of advice, not the one you want to hear. Stop....start over with a more practical build.

What thickness ply skin do you intend to use?
Have you carefully calculated the all up weight of the boat, motor, occupants, supplies and all the other stuff that you will have on the boat? That is the first order of business because that will largely dictate the size and design of the hulls.

You say that you will make it 11 feet wide.....Why more than half as wide as it is long?
Will you be leaving all that framing in the finished hulls?
Will the boat be trailered or will it be permanently moored in the water?
Do you intend to use fiberglass sheathing on the outer skin?
Why a pontoon or catamaran?

You are located in Florida. Where? There may be some knowledgeable builder/designers near by who would be willing to offer helpful suggestions. you.

The generality is that we like boat guys and boat builders and we would like your project to finish well and satisfactorily.

 

Pirate Twig, I read somewhere that the beam of a catamaran should be 1/2 the length. The longer you make those beams, the stronger they will need to be. Finding information on building beams is an almost impossible task. For some reason no one wants to say "This is how you build them" I think I have seen every picture of a catamaran beam on the WWW, and never once did anyone say why they did it the way they did.
I'm sure you can build a long, strong beam, but will you need a crane to lift it? That's where the tricky stuff comes in. Light, Long, and strong just don't seem to go together so well.

 

Is it the hull shape in relation to how it will interact in the water, or is it the overall concept for the stated use? Yes, starting over isnt a really great thought but I can if the overall shape really needs it. If it is in regard to the frame & stringer method i'm using, I choose it because it plays most closely to my existing skill set, even though it will be heavier and may limit what can be done in other areas due to weight concerns.

What thickness ply skin do you intend to use?

I initially had been thinking some stuff that I now know is probably better suited for battleships , so I was thinking 3/8 for the bottom and v and 1/4" for the sides.

Have you carefully calculated the all up weight of the boat, motor, occupants, supplies and all the other stuff that you will have on the boat?

This was a problem area for me. Since i don't know proper boat building techniques and Im designing as I go, I couldnt really figure a way to estimate empty build weight. Ive got things planned to be able to be completed in phases. I laid out the general phases assuming I would have no outside advice and would need to test as i went. So, phase 1 (current) involved sealing the skeleton (cheap ply) with as many coats of CPES/git rot/ or thinned epoxy as necessary for the end grains to stop soaking. Skin and seal with glass cloth/epoxy/varnish. Get some buddies to help spot it so I don't drop it and then get it up on my shoulders like I was portaging the canoe. This would give me a point close to cb. then float test it; ballast the rear for motor weight and set ballast where I guesstimate beam location to be. Then adjust as necessary. I have figured what I believe to be conservative buoyancy calculations. I didn't take all of the angled areas volumes and didn't count the front 5 ft where the height changes are. I am coming out with around 1500 pounds at 10" draft. I have been reading alot of the cat stuff but most of it applies to the sailing cats. So, I see with them that your max load should be equal to the buoyancy of one hull. This appears to be because of waves and/or one side lifting due to the force of the wind on the sails. So minus the sails and considering this subject to small craft advisories, it seems like I would be okay to figure a max overall load of 1.5x one hull. 2,250 pounds at 10" draft with about 12" of freeboard. Again, no exp here, so i planned to modify the topside design to accomadte for the weights and drafts I was seeing at the point. So if the draft started getting to deep to early in the build, the cabins (lol, theyre small, more like coffins) would go from hardtop to ragtop, etc.


You say that you will make it 11 feet wide.....Why more than half as wide as it is long?

I'm not sure of the final width yet. i don't want it to be wider than 11 or 12 ft because there are a few bridges on the local river that I want to be able to fit through (take my tape measure for the next canoe trip). I want to get it wider than 8 ft for a couple reasons. It seems like it would have a little more stability, I like the squat wide look of it and i would like to have some space between the hulls and the center cabin so there is space to walk between them to get to the front tramp/net area. Final width was planed to be done around stage 4 or 5. the possibility does exist that I wont go that far with it though. Initially it would be locked at an 8 ft width. I may decide I like it there and not go any further.

Will you be leaving all that framing in the finished hulls?
Yes, it is how I know how to build and will give me something to tie interior sheeting to. Going to fill the area between the outer hull and inner wall skin with packing peanuts and then pour the 2 part foam to fill the voids and create a sandwich. There will be 3 total solid bulkheads to tie of wtc's. Of the two visible in the pics, the one further from the bow was an early design screw up and will get the center cut out. Also the ones that are currently cut out will have the center piece at the top cut out; it was left for initial build stability and a common flat spot for squaring.

Will the boat be trailered or will it be permanently moored in the water?
def trailered. I dont have any land and can't afford a mooring.

Do you intend to use fiberglass sheathing on the outer skin?
Yes

Why a pontoon or catamaran?
The inherent stability of a spaced double hull offers a more forgiving platform if my hull design ended up being absolute crap. I am confident in making a hull that will float and hold some weight, but if it was a monohull, can I use it as a boat or will it be suited for log rolling sports? I dunno.

You are located in Florida. Where? There may be some knowledgeable builder/designers near by who would be willing to offer helpful suggestions. you.
Near Ocala/Crystal River

 

ElGringo,

Yeah, I couldn't find anything like that either. The closest I came was a couple cross section pics that really surprised me. I think one of the pics was from a build page of a Wood's Wizard or Merlin. I was expecting solid beam but they appeared to be hollow. With solid wood top and bottom and ply front and backs. When I got to that point I was going to look and see if I could find any data related to the stresses put on the beam. I'm thinking I would need figures for up/down, left/right, and rotational. I was then going to apply what I know about the resistances the wood has and come up with a solid wood beam. If i detect any unwanted flex during testing, I can rout some places to insert some box tubing or square stock aluminum to reinforce.

 

PirateTwig, watch this video, it will give you some things to think about. I wish I could help you but I'm still trying to design my own. The hulls don't worry me, almost anything will work to some degree. https://www.youtube.com/watch?v=UspM9qOdAmI

 

It wasn't a typical I beam setup. I've done red iron construction and have hand built wooden i's but it looked different than what I am used to. Rather than a continous top and bottom chord with a central web, it looked like two pieces of wood box tubing had been face joined. That was the new part on me.

 

I saw one that had a top and bottom plate with four web pieces. I couldn't tell from the picture the size of the plates or webs but it did look like it would get the job done.

 

That sounds like what I saw it just looked to me like there was two tops and two bottoms. I wasn't going to try to use an I even though it is lighter because I only know their strengths for static spaces. The connection beam from my understanding will be a dynamic stress with loading and unloading of pressure. Those calculations would be pushing my experience a bit too much I think.

 

Yep, I spent my life as a Soldier, Toolmaker, and Musician. Now I dream about building a catamaran and don't have one clue as to where I should start. You are fortunate in understanding what you need to be asking for help on. I'm not smart enough to know what to ask.

 

I'm guessing your trying to do a real boat. I'm more or less just trying to learn by polishing a turd.

 

Thinking my attempt to use proper nautical terms may have been a little misleading when I described the concept. Could be affecting the weight image you guys are getting. The "head" would just be a self contained composting toilet in a closet, no water in or holding tank out needed. The "galley" would have a rinse sink (fed from river and drained back to river), cooler or small 12v fridge and enough counter space to make simple meals with my camping gear. Again no holding tanks or built in cooktops,microwave, etc. The berth area in the hulls wouldn't even have an actual berth. The kids love the idea of just hanging hammocks. So they would just be empty space.