Small, light weight, take apart fishing pontoon

[msaxton]

So, I wanted a small (12') fishing pontoon that would be fairly lightweight, able to take apart quickly(separate deck from pontoons), able to load into a full size pickup. But, I could not find anything that suited my needs, so decided to design one. I did it in sketchup but could not attach a sketchup file, so did a 2D screenshot. All comments good or bad appreciated.
Some of the criteria for my design:

1) Easy to build with some carpentry and fiberglass skill

2) Deck sized 4' wide in order to slide into full size pickup truck between wheel wells. Pontoons could be loaded on top of a ladder rack or tied down on top of deck in back of truck.

3) Pontoons to be 2" CC styrofoam stacked, glued, carved/shaped to form desired and fiberglassed over with raised runners for channel in bottom of deck to ride on.

4) Bolt holes spaced 1 or 2 feet apart on deck for mounting the deck to the pontoons, toons will have a raised runner with threaded inserts every 6-12" and the bottom of the deck would have an inset slot that rides on the runners so the deck could be slid back and forth to achieve Center of gravity to adjust for different sized motor and weight distribution of occupants.

5) The pontoons could be adjusted to size in order to get proper displacement but at the size they are designed, I roughly figure it out to have a displacement of 1915 pounds

Even though I am fairly new at sketchup, it turned out ok, if any one knows how to upload a sketchup file let me know, the 2D screenshot is not great

 

[aussiebushman]

Might be quicker and cheaper to use the hulls from an old Hobie or similar beachcat - you will often find them being wrecked and sold for next to nothing. That way all the hard work has been done and you only have to make the deck

 

[rob denney]

So, I wanted a small (12') fishing pontoon that would be fairly lightweight, able to take apart quickly(separate deck from pontoons), able to load into a full size pickup. But, I could not find anything that suited my needs, so decided to design one. I did it in sketchup but could not attach a sketchup file, so did a 2D screenshot. All comments good or bad appreciated.
Some of the criteria for my design:

1) Easy to build with some carpentry and fiberglass skill

2) Deck sized 4' wide in order to slide into full size pickup truck between wheel wells. Pontoons could be loaded on top of a ladder rackor tied down on top of deck in back of truck.

3) Pontoons to be 2" CC styrofoam stacked, glued, carved/shaped to form desired and fiberglassed over with raised runners for channel in bottom of deck to ride on.

4) Bolt holes spaced 1 or 2 feet apart on deck for mounting the deck to the pontoons, toons will have a raised runner with threaded inserts every 6-12" and the bottom of the deck would have an inset slot that rides on the runners so the deck could be slid back and forth to achieve Center of gravity to adjust for different sized motor and weight distribution of occupants.

5) The pontoons cold be adjusted to size in order to get proper displacement but at the size they are designed, I roughly figure it out to have a weight capacity of 1080 pounds

Even though I am fairly new at sketchup, it turned out ok, if any one knows how to upload a sketchup file let me know, the 2D screenshot is not great

Pretty cool. Use plywood inserts in the foam wherever you want to bolt onto the foam and cast the bolt threads in epoxy (see Gougeons on Boat Building). Big diameter bolts are easier.
You won't need 12 bolts as only the corner ones do any work.
If shape and looks are important, it is easiest to split the hulls along along the centreline and shape the foam half a hull at a time. Actual hull shape for speed, maneuverability, etc is not critical. You will need at least 600 gsm/18 ounce glass to prevent ding damage. Maybe double on the bottom if rugged beaching is required.

 

[msaxton]

Pretty cool. Use plywood inserts in the foam wherever you want to bolt onto the foam and cast the bolt threads in epoxy (see Gougeons on Boat Building). Big diameter bolts are easier.
You won't need 12 bolts as only the corner ones do any work.
If shape and looks are important, it is easiest to split the hulls along along the centreline and shape the foam half a hull at a time. Actual hull shape for speed, maneuverability, etc is not critical. You will need at least 600 gsm/18 ounce glass to prevent ding damage. Maybe double on the bottom if rugged beaching is required.

I was actually going to use 2"x4"x10' inserts on top of the foam set into the foam 3/4" so that there is 3/4" sticking out for the channel on the underside of the deck to ride on for sideways stability and for sliding the deck fore and aft as needed. I do think you are correct about just needing bolts in the corners of the deck especially considering that the channels on the underside of the deck will be locked into the runners on top of the hulls.

 

[msaxton]

I worked up the approximate weight of the finished craft less any seat, hardware other than the deck fasteners, any accessories, or motor and it came to 206#:

Specs for 12’ x 4’ take apart fishing pontoon

Weights:
Pontoons:
Foam in pontoons: 31 cubic feet approximately 36 #
2x4x10’ runners approximately 26 #
Fiberglass cloth and epoxy approximately 15 #
Total pontoon weight 77 #
Deck:
¼” plywood 22 # per 4x8 sheet x2 44#
1 sheet 1.5” foam 6#
2x4 framework 5-8 footers 11# each 50#
Fiberglass cloth and epoxy 10 Oz cloth.140# per sf 14#
Transom lumber 2x6 10#
Hardware 5#
Total deck weight 129#
Total bare finished weight: 206#

I have an older long shaft 20hp johnson motor that weighs approximately 100#

 

[msaxton]

Might be quicker and cheaper to use the hulls from an old Hobie or similar beachcat - you will often find them being wrecked and sold for next to nothing. That way all the hard work has been done and you only have to make the deck

Not a bad idea there, but that would be defeating my purpose of wanting to build the hulls out of foam and get the exact length and style of attachment of the deck that I want, as well as the amount of displacement desired. Displacement is important in this design as I may want to make a small removable light weight cabin for it in the future for some overnights on the water(or just use a tent). If my calculations on displacement are correct it should be about 1915 pounds. That number was derived by info I found that said the foam has 60 pounds of displacement per cubic foot.

 

[msaxton]

Thanks to whoever moved this thread, I wasn't paying attention when I started it

 

[Boat Design Net Moderator]

You're welcome. No problem at all.

 

[upchurchmr]

60# per cubic foot is heavier than solid Oak (40# per cubic foot).
I assume that was just a mis-typing.

You seem to have a very narrow space between the hulls.
This is going to have a very high drag due to water piling up (inteference between bow waves) under power.
I did the same thing on a 100# catamaran row boat. 4' width overall with 1' beam for each hull giving a 2' gap between the hulls. 11' length, to fit in the bed of a full sized pickup, same as you. Rowing means there is very little power, but I could watch the water rise between the hulls until it hit the "deck" structure. Very noticeable drag. Finally I widened the crossbeams to 6' and it was much faster. This means the crossbeams sit on top of the bed rails with the hulls on the outside of the bed.

If you are doing shaped foam, there is no reason to make flat angles on bow and stern. Fill it out to a nice curve and have more displacement and less drag. Now you can make the width of the hulls a little narrower.

Have you ever looked at the displacement of a 4' wide by 11' long flat bottomed boat? I don't think it is anything as high as 1900#.

 

[msaxton]

60# per cubic foot is heavier than solid Oak (40# per cubic foot).
I assume that was just a mis-typing.

You seem to have a very narrow space between the hulls.
This is going to have a very high drag due to water piling up (inteference between bow waves) under power.
I did the same thing on a 100# catamaran row boat. 4' width overall with 1' beam for each hull giving a 2' gap between the hulls. 11' length, to fit in the bed of a full sized pickup, same as you. Rowing means there is very little power, but I could watch the water rise between the hulls until it hit the "deck" structure. Very noticeable drag. Finally I widened the crossbeams to 6' and it was much faster. This means the crossbeams sit on top of the bed rails with the hulls on the outside of the bed.

If you are doing shaped foam, there is no reason to make flat angles on bow and stern. Fill it out to a nice curve and have more displacement and less drag. Now you can make the width of the hulls a little narrower.

Have you ever looked at the displacement of a 4' wide by 11' long flat bottomed boat? I don't think it is anything as high as 1900#.

Thanks upchurchmr,
The 60# per cubic foot isn't the weight of it, it's the displacement for the foam, not sure if the 60# pcfd is correct, but I saw several places where it said the displacement for the foam was 55-65# pcf so I went with the average of 60. Not sure if I am calculating total displacement right, did seem rather high to me. I just calculated the total cubic feet of the pontoons x the 60cu ft of displacement. Please do correct me if that is the wrong way to calculate it.

I could go with narrower hulls, I was shooting for high displacement, but, I guess I don't need to be too worried about achieving high displacement, would just ever be me or one other person, a 20hp outboard, and some fishing and lightweight camping gear, so not too much weight. 800#s max?

I did think about the drag with it being so narrow, but this is not being built for speed and will mainly be used in small lakes and rivers on relatively calm waters for some backwater fishing.

The shaped foam will be gently curved using several different shaped hot wire cutters, the flat angles in the pic are just due to my inexperience with sketchup, and was having a hard time drawing the curves right, so just went with the sharp angle for ease of drawing.

 

[upchurchmr]

The density of fresh water is 62.4# per cu ft.
The density of styrofoam you get at Home depot for insulation is about 2# per cu ft.
Of course to make the hull you need the wood and the glass, so your 55# flotation might be a good number.

Hot wires work well, except if you are bonding the layers of foam together. The adhesive doesn't cut well and will mess up your cut. If you just dry stack them then cut then bond together that could work OK.
Another way is to use sandpaper on a "high speed" disk. it will cut nice and smooth, but your ability will decide if you get a nice shape.
You also might just try sanding with a fairly coarse sandpaper by hand.

The row boat was never doing more than 5mph and showed a very distinct increase in drag for a narrow boat. The effect will happen even at slow speeds.

You might calculate how much volume you actually need for 800# displacement to get a better idea of how big the hulls need to be. Be careful with solid foam, the weight adds up surprisingly.

It might be easier to just make two plywood hulls, 1/4 ply using stitch and glue construction. It would be plenty strong with a little thought. Epoxy / glass would make it last. Or you could just glue it together with epoxy and paint it. Multiple coats of paint would last for a couple of years at least - could be longer with care.
A 1' wide X6" deep X 11' (the volume in the water) would have 5.5 x 62.4# = 343# flotation for each side. So 6.4" deep gives 400#. This wouldn't be very good since it would be rectangular front and rear with huge drag.

 

[msaxton]

I suppose I could just go back to my original plans of just building a simple small 10' or 12' jon boat, punt, or Garvey, just kind of like the idea of a pontoon.

 

[upchurchmr]

Catamarans (pontoons) have some really good benefits, with some very specific limits.
Something so small will be limited in the load carrying and if overloaded will be very slow.
More like a raft.

Most of the time you just can't have everything you want without actually doing some engineering (or design/ testing).

Here is a 11', 50# rowing catamaran I built for my wife. It fits in the back of a Tundra pickup, with a special built rack.
Really limited carrying capacity (135#), no actual deck - to limit the weight.
It works great, but probably slower than a traditional rowing shell which can be lighter, but is usually 20+' long and requires balancing (somewhat like an acrobat)

11' just doesn't give you many practical options.

Here is another design which has more carrying capacity. Could this do anything for you? http://duckworksbbs.com/plans/gumprecht/cat12/index.htm

 

[msaxton]

Yep, thought about it for a few days, and am just going to go back to my original plan to build a jon boat. But, it was fun to work on the design any way. Does anyone have a preference for good 12' jon boat plans, I will most likely not follow any plans completely as I will be building it with 1/4" ply/foam/1/4" ply sandwich construction w/ fiberglass & epoxy, but do want to get some plans to get measurements and angles right.

 

[msaxton]

Aha! I believe I found exactly the idea to build, a 12 foot folding jon boat that the hinged front flips over onto the back for hauling. wood fit in the back of a pickup or on a small trailer. https://www.youtube.com/watch?v=ZFA8OTjuoQ8