pontoon that masquerades as a powercat -overview

Pirate Twig; Your skill set would appear to be entirely adequate to build a serviceable boat. The pictures show that you can do some decent joiner work. You are doing it the old fashioned way as if building a house. There are other ways such as stitch and glue that is the most recent way of doing as small boat that is both simple, lighter and presents a cleaner less cluttered interior. What you are doing is perfectly alright but there are better, easier ways.

What we need to do is arrive at an initial design that can be simple, safe, and serviceable at the same time. We just have to know at least the probable weights of the family members, the likely engine or engines that you will use, and the extent of the food, water, supplies, and fixtures like Porta Potties and such that you will anticipate. After that we can come up with a general idea of the dimensions of the hulls that you will build. You really need to do this before you take a wild guess at the hull dimensions that you guess might work.

Well OK Crystal River A really nifty place to play in the water. I know it well. There is a group of old fart builders in Crystal River who might help you face to face. I cannot remember the name or the exact location but they are there. The last thing I remember them doing was an old time Florida river sailing boat that was built with primitive tools and materials. Not because they had no power tools but because they wanted to do it the old time way. It was a big ugly boat that worked pretty well. If you can locate that bunch of guys it may be an advantage for you.

Hang in there. We will try to keep you from building a disappointment.

The distance between hulls is a matter of calculating the beam strengths and the anticipated static and dynamic loads that they will endure. The longer the beams the more rigid (spell that big and heavy) that they must be. If you are planning a cuddy over the center then that structure can be designed to take some of the load. A big Garvey would be every bit as serviceable and more spacious too. There is one of those under construction at Lucas boat works in Bradenton. That is another group of amateur builders who get off on making all kinds of boats. You might check out the group. Try Skipjack@TampaBayrr.com Dave Lucas is the proprietor and main offender there. He is a pretty good guy too.

 

Messabout: for the methods, i thought about the stitch and glue and if it was a canoe or kayak I think i could have figured out the cuts needed to get the shape to pull together. I ran into problems with this when I tried to figure out how to tie in a walkable top deck and the tie ins for the beams. To try and tie in that kind of structure outside my stick frame knowledge seemed to be pushing the envelope of where i should really pay someone to do. That is a big sticking point for me. Designing with building methods i don't know means the design needs to be out of my hands and should be payed for. That is outside current budget. So if the current design needs to be scrapped, well crap but ok; but I would like to stick with the building method I know. I do still have some play with what is here as far as the deadrise angle, width of the flat, existence of the flat, and the taper of the nose. Some of the existing frame was meant to be cut back after sheeting stiffened everything up. i could also shave some of weight from the frame work without impacting the strength i was building for. The hull also is currently about 14 ft long; the back end and transom don't exist yet so the length is up in the air as well, but i don't want to go so long that it can't turn around on a small river.
I was going to stitch the seams where the panels joined together though cause a wood to wood joint beats filler any day in my book. The framework gives me a base to build an A-frame type beam support that can distribute the load at connection throughout the hull. I was pretty much looking at Garveys (though i didnt know that was what they were), jon boats and the noses of the aluminum pontoon logs when I shaped the hull. The only other thing I considered was how much space i would need to crash in without feeling too cramped. So the 30" width allowed for stick building, some foam, the inner and outer sheeting and width for me inside.
Weight estimates for the family would be around 5-600 pounds. Maybe 200 pounds for a long weekend in camping gear and food. For motors I was only thinking about 2 2stroke 15's (but who wouldnt want more power if it was feasible?) Based on what I have so far Im guessing at each hull weighing under 300 pounds if hard-topped. The center area was planed to be something that would be built up as much as weight would allow after the hulls were completed. It may just be flat deck with a console for helm and room to pitch a tent. I was looking at alot of freeboard so that I had room to squat if we wanted to take several people out on the river. Is this enough to get an idea of weight or am i still missing some stuff?


Thank you for your interest and help. even though its not meant to be the best boat from the git, I certainly appreciate any help to make it as good as possible despite the limiatations.

 

Okay, I goofed slightly somewhere in the first calculations. Ive redone them and attached a pic of the calculations in case Im still messing it up. Im coming out to 22.9025 cu ft displacement at 10" equaling 1429.116 pounds. I figured total max weight at 1.5 x one hull equaling to 2143.674 pounds at 10". I didn't count the front 5 ft where the dimensions change.

The hand drawn one shows where they are meant to be cut later. The top section was left for stability during building and the bottom section will be cut to slope the floor so any water will run to one location.

 

Your calculations look a little off on the sloped portions for instance the 3" by 9" with the slope going from 0" to 3" in 9" would be calculated as 1.5" X 9" and the 1" X 7" sloped portion with the sloped portion going from 0" to 1" in 7" would be calculated as .5" X 7" and I don't understand why you are figuring the total displacement at 1.5 times the displacement figured for one hull. If the two hulls are the same dimensions then the total should be twice the displacement of one hull or 2858# at 10". You mention that you want the boat to be trailerable but you are also saying 11 or 12 ft. wide I would definitely not go any more than 8.5' as this is the maximum width allowed in most states without a special permit and probably a lead vehicle. Good luck with your project it's better to be heavy and strong than light and weak and remember the golden rule of engineering "If there's any doubt build hell for stout". Yours and others lives may depend on it.

 

Dave: I had read that the usual method for figure total max load on a cat was to look at the load of one hull. This was for a sailing cat though and it appeared the reasoning was that at some times you would only have one hull in contact with the water. I don't expect that to ever happen with no sail but for a good safety margin I figured max at 1.5 instead of 2. To figure the sloped areas I counted both sides together as one box. so the 1.5x9 added to the 1.5x9 on the other side boxes to 3x9. Not saying I'm right, thats just how I remeber how to calculate volume from HS almost 20 yrs ago. The concept is to have a slide system under the deck like an extendable kitchen table. So on the trailer it would pull into somewhere between 8 & 8'6" but on the water it would get opened to the larger width. Im not sure at this point if it will be feasible to do with proper support while staying at a reasonable weight.

 

I guess I didn't look at your calculations close enough you are correct on the sloped areas as you aren't times 2 as I thought. I don't believe you will need to figure the displacement at 1.5 without a sail if you have enough power to make the boat plane then the displacement will be reduced and if you only have enough power to move the boat at displacement speed then there should only be a slight change when turning or in rough water. Making the boat so you can adjust it's width for towing is going to be real complicated and add a lot of weight, I would suggest just staying with the 8.5' width. A boat is always a compromise between what you want, what you can afford and what is practical. I've looked at what you have built so far and I would be concerned about the strength of the frames and I would suggest covering them with a sheet of plywood to make water tight compartments between them and for strength. Anyway good luck with your project and I hope you will actually finish it and we'll see it on the water post a lot of pictures as you proceed.

Dave T.

 

Dave: I wasn't meaning to have a planing hull. I just goofed that way. So I was wondering if the larger motors were feasible for the structure and/or if the concept I am looking at is even worth attempting to make it plane.

I may be using the calculations in an unorthodox way? I am not meaning these numbers to be a listing of actual stats. I am using them as target weights. If i can stay within those numbers, I should be pleasantly surprised during float tests. I stated mininal draft as a build goal and this is true. The main reason for that is I go down some shallow rivers and don't want to be stuck in the channel. I have seen some 18-22 ft bayliners on the river and I believe their drafts are well over 2'. So I should have a lot of wiggle room. The lowest open point of the finished hull was intended to be about 3.5-4" higher than the hull is now. I don't know how to figure what the min freeboard should be so I cant calculate true max weight and draft should be, but if i run the same 15' section as before at 2' draft each hull comes out a little over 4300 pounds displaced.

The beam doesn't seem too complicated other than figuring out how to calculate the pressure values that it will need to resist. The essential idea is this: https://tse1.mm.bing.net/th?id=OIP.M75f65e7baaa6561f73961acb546f8baeo0&pid=15.1&P=0&w=300&h=300
It would need to have some box tubing sections to wrap the beam and act as deck mount points. The inner dimension of the tubing would need to be small enough to keep the rails from jumping alignment but big enough to allow them to flex, probably around 1/4-1/2". The play would allow the system to absorb some of the energy away from the main beams like Messabout refered to a couple posts ago. The main problem is weight. I'm trying to spend as little as possible so material other than wood is likely out of the question. Until I get data on the forces in play I don't know if the weight and physical size required will be feasible. I actually have a couple different ideas on how to accomplish the fold but most of them would need to have a real engineer calculate the fine details to make them work. I think Wood's and Surtee have the best K.I.S.S. method in play for theirs but I don't want to go ripping off someone elses design. If i can make it work with something I haven't seen before, that'll be sweet. Ive been trying not to think about this too much because it is still down the road a bit and I am tryng not to get too distracted.

I am a little confused about the framework not being strong enough? i think it is the first time I've been accused of under building something. It's usually way way.... way on the other side of the scale. The plan had been that the front 5ft and back 3-4 ft would be wtc and most likely foam filled. The area between would be 2 wt compartments one about 8 ft and the other whatever was leftover. 8 ft section being bunk area and the other would be storage or a generator (weight allowing) for charging 12v system and emergency trolling motor power on one side and area for the "galley" in the other hull.

 

I was initially confused by this but i just reread my op. It was meant to say "something more than dual 10-15 hp" rather than "planing with 10-15 hp". The correction has been made. Sorry

 

So to get into specifics.... I added a v to what was originally a flat thinking it would grab some of the river's current and help keep the boat from spinning when floating with the current. Seemed Ok on paper but when i had it all shaped out I realized that the angle itself was way too shallow to grab enough to make a difference. I also wondered if i should increase that angle dramatically as I approached the nose (like in the kohler design from another post) so that it would have a sharper edge at the start of water contact. After reading & thinking about replies my questions are:
1. Would it be worth considering a larger motor and attempting plane?
2. Does it really have to be scrapped or can I modify the existing to some extent with the understanding that I am not trying to optimize the hulls in regards to wsl drag etc. Just a basic shape suitable for a beginer to hull theory to start with. I listed a few different ways I could modify the existing in an earlier post but I could also round it off.
3. This seems dependent on the answers to 1&2. Would the flatter v angle currently used cause to much drag? Snow plow?
4. This also seems dependent on 1&2. Rocker or no?

 

More pics

I tried to get the camera in spots where you can see some of the angles better. Sorry it took a few days, it was the first chance I had to get some in the daylight. You'll probably notice the high spots on the webs, uh yeah, i dun wanna talk about it. They'll get cut back to where they should be. The 2x4 on edge that is notched for the two full ply pieces is not attached. It was a spacer during initial setup, i just haven't pulled it out yet.

 

Pirate, how much rocker do you have in the bottom? Isn't this going to be a power cat? Manfred Pech usually comments on catamarans and gives pictures in his posts to explain what he is saying. I think he's mad at me because I don't research enough before I ask.

 

No, no, no, I am absolutely not mad at you. Don't worry. Here are so many good comments. Life will go on without or with rocker -- and without me. M.

20ft Cat



https://www.ebay-kleinanzeigen.de/s-anzeige/katamaran-sofort-startklar-top/525004110-211-4481

 

Hi Manfred, I thought it had too much rocker for a power cat.

 

I haven't done the back 7 ft yet. So i haven't done any. In case I have the terms mixed up, I am referring to tapering the bottom up towards the transom. But if I need to scrap and completely start over as messabout suggested or modify the lines drastically, the answer will change, the way I am currently understanding things. And I dont think it will actually count as a powercat. The thinking was more pontoon boat made to have the appearance of a sailing cat like Surtee's or Wood's Gypsy but minus the sail.

 

Pirate, many of the shorter power catamarans are being built with very little, or no rocker or tapering to the transom. Just flat bottom and straight sides. On Bernd Kohler's designs there is even a slight down turn at the aft end. Then on the other hand Richard Woods Skoota 28 looks to have about 3 or maybe 4 inches of rocker at half the length of the hulls, and it works well for him and moves right along for a 5,000 pound cat with two 20 hp outboards.