Pontoon/houseboat conversion

I have a 24ft Harris pontoon boat with 24ft length pontoons that are 23inches in diameter along with a factory aluminum deck. All furniture is removed other than the Captain's chair. I contacted the manufacturer and they sent me an original spec sheet indicating that the "weight" is 1410lbs and the capacity is 17 people or 2420lbs. I want to build an enclosure on it and am very conscious about weight. I am opting for a 12ft enclosure, using 2x2's, 16 inches on center with 2x4's for the roof, 2ft on center with perlins (1x3's) on the 2x4's every 16inches to screw the metal roof to. Between the perlins and the metal roof I am going to install 1/2 inch rigid foam panels for insulation and sound proofing from the rain. I have been weighing most everything I can think of. For exterior walls I was going to use 3/8 plywood covered with quality paint. I have a donor travel trailer for windows and doors. Motor is a late 80's 110hp Johnson. Gas tank is 13 gallons.
Questions I have are these; any glaring flaws with this design? Where should I located the 12ft enclosure on the deck? With the above mentioned weight and capacities, how much can weight can I safely add? The boat will be used primarily on the Mississippi River and small inland lakes. Any thought or help is appreciated. Thanks in advance

 

Welcome to the Forum dhk.

Can you perhaps post on here your tally of your weights so far?
If you don't know the weight of something, then you have to try to calculate it - eg for plywood, measure the total surface area and multiply this by the thickness and the density of the material. Calculating weights is one of the more tedious aspects of boat design, but it is something that is very important.

You could start off by locating it amidships, with 6' forward and 6' aft of the centre, and see how that works out.

One rule of thumb with a pontoon boat is that you do not want to let the draft be any deeper than half the diameter of the pontoon.
Calculate (roughly) the volume of the hulls at this draft, and see how it compares with the quoted weight and load capacity.

A suggestion re calculating your weights - use a spreadsheet, as this will make it much easier.
Apart from the weight of each item that you have on board, you also want to include it's longitudinal position in the boat (typically measured as a distance from the transom) and it's height (measured above the keel). Use your spreadsheet to calculate moments for all of these items, and you will then be able to calculate where the longitudinal and vertical centres of gravity will be, approximately.
For a pontoon boat you basically want the LCG to be pretty much just aft of amidships - not too much, otherwise she will be trimming down at the stern, possibly excessively if your LCG is too far aft.
Re the VCG - draw a sketch of the midship section to scale, including the house - if you can include a human figure in there to scale as well, this will make it look more realistic. And plot on this midship section where your calculated VCG is.
Once you have done all this, please post your results and sketches on here.

 

Great project and, yes, welcome to the Forum.
Bajansailor is spot on with his advice but generous in his weight limit.
I would encourage 1/3 loading limits as round pontoons have diminishing buoyancy.
Boats always end up heavier than planned and the perfect storm of submergence with pontoons can be tragic, rollover.
2x4's on 16" centres is pretty heavy for a roof but let's have a look at your sketches/drawings/design.

 

Given the manufacturers hull weight of 1410 lbs and a capacity of 2420 lbs (total displacement of 3830 lbs), the manufacturer feels that having the pontoons submerged half way is safe.

If you plan on running faster than 5 mph I would want more weight in the stern. When crossing a boat wake pontoon boats will stuff easily and it happens a lot because everybody likes to sit in the front. For what its worth, my pontoon boat sits 3" lower in the stern compared to the bow.

 

Okay, I did some calculations for weight. The completed 4 walls of a 12x8x7 ft tall enclosure, minus the weight of the two doors and fasteners would be 502lbs. The weight of the roof, minus fasteners, would be 217.44lbs, bringing a total weight of 719lbs for the structure. Now I would have to figure in all other components/supplies but I think I'm comfortable with the weight. Thoughts please?

 

Ok, you are up to 719 lbs so far - you still have a long way to go.
Let's add on 10% to this as a margin - and this is not a lot. So we are up to 800 lbs.
I calculated that the displacement would be a bit over 4,000 lbs at 11.5' draft (ie half the depth of the pontoon).

The manufacturer notes that the Lightship weight (I presume they mean lightship) is 1,410 lbs, and the loading capacity is 2,420 lbs, giving a total displacement of 3,830 lbs. So they are recommending that you stay less than half immersion of the pontoons at maximum load, which is good.

You have 2,420 - 800 = 1,620 lbs remaining now to play with.
It sounds like a lot, but you will find that this allowable weight adds up very quickly - remember to include the weight of the crew (at say 180 lbs each), coolers, provisions, water, beer, safety equipment (including anchors), fuel, that 110 hp Johnson O/B motor, steering and controls, two doors, furniture, a toilet compartment, a basic galley with a stove and sink, personal effects, fishing equipment....... and I am sure that you can think of more things to add.

 

Just some thoughts and checks
I didn't see any interior sheathing
3/8 ply seems thick unless it is actually the only skin
Do the weight calculation again, but "wet" -if any materials or spaces hold water, consider this state. Vinyl floor good, thick carpet bad. No futons!
Consider weight placement and then consider worst cases for crew position -"hey watch me" everyone goes to a corner. Or you have some water toy full of water that takes everyone on one edge lifting to dump it out.
That 'halfway' advice means everywhere. If pontoons sit tail-heavy past that halfway, righting moment couples with trim. When wind pushes the boat over, the stern is tilting into declining buoyancy while the bow is tilting to an increase -weight shifts further aft. You only had 12" of vanishing buoyancy to lose.

 

Thanks for the replies. Couple of questions; I'm not sure what the manufacturer means by "weight"? I'm assuming it means the weight of the boat when it was purchased from them, at which time it had hundreds of pounds of furniture. Does that sound correct and if so, I should then have that "extra" capacity to play with?? Also, I did not take into account interior skin, which would add another 157.5 lbs to the walls (I would leave the ceiling alone). Should I reconsider the exterior walls and go with 1/4 inch on the exterior as well as the interior and glue and screw it on for strength and stability? By doing both interior and exterior walls in 1/4 inch, it would add 45lbs to my original 719 lbs, bringing the new total to 764 lbs. I'm open to using any lightweight material that is reasonable in cost, if there is such a thing in our present economy. Heck, at one time I was thinking of wrapping the two exterior side walls and back wall with old vinyl billboard material.

 

My non-expert answers
The weight on the tag is the registered weight when the boat left the manufacturer. It would include all the furniture and parts included on the standard model.
It wouldn't be too hard to weigh the boat on the trailer and also weigh the trailer. It also wouldn't be hard to calculate displacement in fresh water.
1/4" ply screwed and glued inside and out seems plenty strong to me -avoid seam match. If you want even lighter I would consider 1/4 ply scarfed together to make single continuous sheets for each interior wall, screwed and glued to those 2 by with 2 2x4s in the building corners. Paint all wood, even inside the wall. For outer siding use aluminum corrugated like camper siding. Aluminum dents, but plywood on exteriors gets ratty edges fast. Vinyl and aluminum siding are flimsy but the plus side is they shed water and retain none. There are professionals building what you describe that use these materials. There are sidings made for the camper industry.
I don't know this vinyl billboard material, but it would be a very entertaining look.

 

Not much to chime in....but let's all appreciate 17 people equating to 2420 lbds.... must been skinnier southerners back when it was built.

 

dhk,

I would suggest you follow-up on post #2.
Post #10 raises a good point. Today that equates to over 3000 pounds not 2420.
I'll add that the manufacturer weight is not necessarily correct nor does it account for differing heights of weight which hugely affects roll stability.
You may find this dead thread of interest:
I know nothing about boats but am building a Pontoon, where to post trivial question? https://www.boatdesign.net/threads/i-know-nothing-about-boats-but-am-building-a-pontoon-where-to-post-trivial-question.66810/

BB

 

Beware the “ pontoon effect”. If you load the pontoon to the point where it is at or more than half submerged, you can have a nasty result. In this condition, should the load shift to one side, the vessel may exhibit the effect of flipping over.

 

What's with all the flaky pontoon modifiers lately?
Full of questions, then they just disappear never to be seen again.
(It's a rhetorical question.)
To me it's the "Pontoon Effect"!

 

Get some 22 gauge aluminum sheet, 2X2 X 14 gauge aluminum square tubing, a Milwaukee unishear and a screw gun. Fab the armature, then insulate with 2" expanded polystyrene. You may surprise yourself with how skilled you will become in an hour or so and have a lightweight and sturdy result. Prime the exterior with zinc chromate then finish paint with an automotive urethane.
This opinion is worth what you paid for it.

Ryland

 

we have the poontoon effect,

Nice, well made toon, how old is it?

As mentioned, we have the pontoon effect, the original poster is missing, where are you on your build?

I think you were talking about coroplast material being billboard material. I like the idea of using this stuff, it's tuff, very light weight, water/weather proof and if not shot with #7 buckshot will last a long time.

If you don't mind a textured look, painting coroplast with bedliner makes it really tuff stuff, I think you can get that in various colors. That over 1/2 inch foam board and 1/4 inch interior ply walls should be more than sufficient for a cabin or camper wall.

I'd second the motion to use aluminum box for your framing, I'd use rivets.

I'm considering doing a Landau 23.5' with 24" tubes with a 60 HP Bigfoot cabin for the same purpose as yours, just another Mississippi Shanty.

However, I'm thinking more about a removable tear drop style that can go on a trailer or the boat or, perhaps even smaller to load in the back of the F-150. Not trying to get off topic, just mentioning alternative camper ideas.