Advice - can this houseboat be made to sit level with extra buoyancy?

Personally, I’d start cutting things off.

That massive top deck looks ridiculously too big.

 

Also, if the roof is designed to support a live load of 48 persons or even 12; it is massively overbuilt. You can probably see the added sections from below.

 

Possibly you could bridge the gap between the catamarans and then plate it, turning the cat hull into a monohull.

 

From your statement it appears that you do not understand that buoyancy is a function of the displaced volume NOT the horizontal cross sectional area . Cross sectional area x depth of immersion

Vertical sides or not, the deeper the hull is immersed, the more buoyancy the hull provides.

SamSam probably has it right. You don't want to extend the length, so move the sullage tank forward. Not sure exactly from the 3 pictures what you meant by a floatation tank. Is this
a tank that exists but is not immersed, a provision in case the other hulls are ruptured.
In any case, you might be able to move the sullage tank ahead and incorporate it into the third half pontoon and if the floatation tank is not immersed now, make it immersed and contained within the
third pontoon.

 

I had not used the upload feature here before so not sure - I just uploaded mine to imgur.

I had been thinking something similar - any rear extension could be made deeper than the rest of the hull, but rather than floodable hull up front, I would add a streamlined frame between the hulls up the front to act as 'legs' for it to sit on when on the sand. The downside being having extra drag, structure for marine life to grow on, and still needing a reasonable surface area to avoid it sinking in when sitting on soft mud (which we do frequently on our existing houseboat).
I like your idea of adding extra bouyancy below the existing hull though, to avoid extending out the rear.

I did some experimenting with people on the front - 300kg or so standing on the front deck moved the waterline down around 9mm. There is a lot of bouyancy up front, and a lot of weight aft, so I think the amount of weight needed to trim it down at the front is excessive. Plus the freeboard is not massive so I would prefer to add buoyancy to balance it, rather than ballast.

I am not very graphically inclined so I am still working on a diagram, but I think I included everything about the hulls and draft in the first post?

So mostly it is cat hulls - rectangle in shape, 1.2m wide (each), 12.1m long (waterline), 0.58m high until the bridgedeck. 1.8m between the hulls.
Then the front 1.8m between the hulls is sealed, and the front is like a punt. The rear of the sealed section is vertical and the cat hulls just extend back from there.
At the rear between the hulls is a 4.8m long, 1.18m wide, 0.5m high tank with a streamlined bow on the front (0.6m, not included in the 4.8m). The front 2.4m of the tank is the sullage tank, and the rear is just sealed off and used for extra buoyancy.

The current draft is 7.5cm at the front, and 19.5cm at the rear. The bottom of the hulls are flat, and it is very rigid, so it sits 1cm deeper into the water for every 1m you move towards the rear.

Based on adding up the immersed hull area, or weighs around 4900 KG.

Compared to some houseboats i have been on, I actually felt the top deck was small! It.s 2.2m wide and 10m long. It's a bit under 1/3rd of the downstairs floor space, and around half of the actual roof.
I still need to measure everything used on it and see how much it weighs, but it's not massively heavy duty aluminium, and the panelling is not too heavy.

Otherwise to the rear most of the weight seems to be in the extra structure for the stairs and stairwell, bathroom on the other side and various sections for storage. While this could all be chopped out and moved, i don't think it would be economical for me to do so, even if the boat was significantly cheaper.

Yep, the roof has some chunky extra support beams in the middle of the main downstairs section. Since as a houseboat we don't need to be rated for loads of passengers on the room, the plan is to remove them and add some supports across to the side, so the area is then totally open. The actual roof itself is marine ply, with a fibreglass cover, sitting on aluminium crossbeams.

Good thought - but correct me if I am wrong, is the entire gap between the hulls was plated in, then the buoyancy would be increased equally down the length, which would result in it sitting higher in the water, but still with the rear deeper than the front? (it should even out a little of the tail heavy trim though, since the punt bow is already sealed between the hulls, so if already adding buoyancy to the front?)

Sealing just the section behind the balance point is an option, but since the tanks take up 2/3 or so of that space, the increase is not enough by itself from my calcs. It could be combined with a rear extension too, which could also be a mono hull, which reduces the length needed. But I am not sure how that will effect how it goes through the water - it will be big and slow anyway, so I imagine it won't handle too much different.

Yep, I worded that poorly - apologies for the confusion, the precise terminology is new to me. All I mean is that (for example) if it was sitting level, the weight to immerse the hull 1cm (379kg) is the same for every additional cm of immersion, because of the vertical sides. Unlike my current houseboat, which has rounded hulls that flair out up to the bridge deck, and take an increasing amount of weight to immerse them each additional centimetre.

What's the most accurate way to express that? That the buoyancy increases in a linear fashion?

I added another pic to the album from the rear that shows it. - Imgur https://imgur.com/qxOYS7D

So the tank starts at the rear of the boat - about 1m behind where the actual cat hull rear waterline starts (under the square sections at the rear are outboard mounts). The tank is a 4.8m long, 1.18m wide, 0.5m high tank with a streamlined bow on the front (0.6m, not included in the 4.8m). The front 2.4m of the tank is the sullage tank, and the rear is just sealed off and used for extra buoyancy. The bottom of the tank is level with the bottom of the hulls.

So currently that tank sits about 20cm into the water at the rear and around 15cm at it's front. The sullage tank is currently empty (and never used).

The bathroom is in the rear structure, and piped directly into the tank. It is a standard (Australian) low water use house style toilet, with a full size pipe into the tank. There is a long aluminium pipe from the sullage tank right through to the rear of the boat for connecting up to a pump out station.

I don't think the tanks can be easily separated. But yes ideally the entire length would be moved forward until the right point so the sullage portions weight when full is spread equally front and rear. And then another tank added behind to extend back out to the rear, adding buoyancy.

I don't have enough experience to know how big a job that would be, so that will be something I will ask when I can get a marine surveyor out to go over it.


I had been mostly focused on the idea of building an extension and connecting it to the rear, so I am loving all these different suggestions of ways to correct the trim!

 

So that roof...

Is it 3/4 plywood all the way with glass on top?

Let’s look at that briefly with some estimates.

A 3/8 foam core roof would weigh about 1/2 pound per sqft, whereas a 3/4 ply roof with some glass would weigh about 2.5 pounds per sqft or more.

Lets say the roof is 10x30 or 300 sqft. That would be 600 pounds of unneeded weight to support that live loading, plus the supports. I’d bet you have nearly a half ton extra weight there. And it is high in the air. I could be wrong, but let’s say they used lighter ply, it’d still be 3x heavier than the foam and be 300 unneeded pounds. If you removed juat that last hundred feet of roof and made an open cockpit; at the 2.5 number; you are shedding 300 pounds or so at the stern in JUST the roof.

I doubt you need to make a flat bottom boat.

Then what gauge is the AL? If they used 1/8” thick, it weighs about 1.5 pounds per sqft or roughly 3 times the 3/8 foam panel. Of course a very thin AL would get closer, but I have doubts.

Rather than adding buoyancy to an overweight, likely top heavy boat; I’d put it on a massive weight modification.

Sure; it sounds daunting, but making a nice aft open cockpit and building a lighter structure is really going to be the best route here, imo.

The back of the boat pic shows walls everywhere. Walls for two heads maybe, walls for the stairwell. All aluminum or just paneling or a mix?

I bet if you make a nice open cockpit at the back; you’ll need no hull mod and end up with a nicer; more attractive and safer boat.

 

Also, AL on the sole would be very uncomfortable in the heat. I’d tear it off as well aft and put in a lighter foam floor. Less weight; more comfort.

 

Finding this thread convoluted but wanted to comment on roof construction.
I built my houseboat 18 years ago.
The roof is 1/4" good-one-side ply (3/16") on 2" polystyrene foam (solid, no stringers or beams) , on 1/4" ply, on 2 1/4" laminated spruce beams.
The spruce beams are exposed inside and were made on a jig using three layers of 2x1 (3/4" thick).
They curve 6" over 10 feet (1 1/2" spring back coming off the jig).
I glassed the top with polyester and gelcoated it black.
I've re-gelcoated it every five years and after 18 years it's been fine.
Oh, and it's all held together with 4" #10 plated wood screws (pre-drilled) from the outside.
Simple, light and effective.
I also ran an over-width strip of mahogany on the bottom of the beam for esthetics and added strength.
I (200 lbs) can walk on the roof no problem.

 

I can't see any edges of the roof to measure, as the ply is all covered. It's actually a thinner layer of the beams, the a second flat layer in the middle that the top deck got built on.
I will try and find same Ali edges too and measure them. But likely heavier than you suspect.

The sections at the back are aluminium frame with the aluminium + plastic covering. It has the stairwell, two outboard covers, two storage areas, the head, and a section enclosed on three sides. They all appear fairly overbuilt in terms of the amount of framing involved.

I agree though that removing all the overbuilt sections to lighten it and redistribute weight is a good idea. That sort of work is beyond me to do personally though, and I doubt the boat could be bought cheap enough to make it economical for me to pay someone else to do it.


At this stage I am still waiting for paid feedback from the marine surveyors I contacted, but I am not confident it can be re-balanced without excessive time or money spent.

 

The boat is not worth much if it has been modified well beyond its original design.

Someone suggested the thread was convoluted; I think that is more the story of the boat.

I’d walk if you can’t perform the work.

 

I am prepared to pay someone else to do the work rather than walk away, since I have no experience welding aluminium. In fact, I would prefer that, and even if I could weld aluminium I would not want to take on even more of a project than it already is.

But you are right that it is looking like the boat is not worth much - but pending the report from the surveyor, I think there is still potential for the right price. And there has been some great feedback from people here, and some good ideas in terms of options.

 

You don’t need to do much welding of aluminum.

You need to cut the aluminum overbuild away and build it out with some lighterweight products.

If the sole is all aluminum; you won’t like that for a houseboat floor anyhow.

You would cut away the unneeded areas and pay a welder to install aluminum angle and then build either plywood or foam onto the angles.

You don’t want AL plate walls in a houseboat; you’ll roast inside.

There would be very little aluminum welding.

 

So, when does the paid help arrive?

 

It was surprisingly hard to find someone who was willing to help with bouyancy calcs, but I made contact with a Naval Architect and went through everything with him last Friday.

He is currently modelling the hull in software and will find out out how much weight needs to be removed for it to float level, or how much extra bouyancy is needed etc. I will post up those finding as soon as I have them of course, and go from there.

 

Sywofp so what about the 500kg (approx) of glass in the front half and then the furniture and the galley/kitchen plus accommodation . A simple look at any Murray river house boat will tell you there is a lot missing, that needs to be taken into account before you break out the angle grinder!