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

Discussion in 'Stability' started by Sywofp, Apr 23, 2018.

  1. Sywofp
    Joined: Apr 2018
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    Sywofp Junior Member

    Long time lurker here, who finally is looking at a boat that needs some designing.

    I am looking at a purchase of a houseboat shell - however it sits very tail heavy. I want to know if it is feasible to make additions to the rear so it sits level. I have made a bunch of my own calculations, and reached out to some local marine surveyors for paid feedback, but wanted some feedback here to make sure I am on the right track.

    Some pics of the boat can be found here - Houseboat

    It was built as a river cruiser that could also be used to operate pedal boats from, and was designed for commercial use with 48 passengers. It's (mostly) made from 5083 aluminium, and appears to be very well put together. Underneath it has twin cat hulls (dimensions below) with a punt style bow. At the rear it has outboard mounts. At the rear between the hulls it has a sullage tank (with streamlined front end) and an extra buoyancy tank. The hull is divided into sealed and pressure tested sections 2.4m long.

    Weight: 4900 Kilograms (based on my measurements of displacement)
    Length: 13.7m
    Width: 4.7m
    Waterline Length: ~12.1m
    Individual cat hull width: 1.2m
    Hull Depth: 0.58m
    Punt bow length: 2.4m

    So the punt bow is 4.2m wide, and extends back 1.8m, after which it transitions to the cat hulls. The front 60cm or so of the punt bow is out of the water. The sullage tank is empty, and holds around 1200 litres. The sullage tank had a mild curve in the underside so the drain can get everything, but otherwise the hulls and tanks have vertical sides, so buoyancy does not increase with depth.

    It was moved around out of the water using wheels under each hull, and a towbar on the front end. The balance point is 8.4m from the from, or 7.8m from the waterline start, and around 4.3 from the waterline end. So half the weight is in approximately the rear third of the waterline.

    The front currently floats much higher than the rear.
    From immersion depth: 0.075m (7.5cm)
    Rear immersion depth: 0.195m (19.5cm)

    Based on my measurements the hull has 37.9 m2 of underside area, and if the current weight was evenly distributed it would sit 0.129m (12.9cm) into the water.

    In front of the balance point, it weighs 314 KG per M, and to the rar of the balance point, 570 KG per M. So the rear is about 1100 KGs too heavy, compared to the front.

    My problem comes from how I should be calculating the extra buoyancy needed. The area it would be used in is a tidal river system, so it needs to be able to sit on the sandy or muddy bottom at low tide. So I don’t want to have any extra additions extend below the depth of the current hull. Since it is quite light overall, extending the hulls does not add a lot of extra bouancy unless they are very large.

    My outboards weigh 120KG each, plus fuel and hydraulic steering etc. The effect of the sullage tank being full also needs to taken into account - I have used 600KG (half) as a starting point, since it likely will be emptied long before it is every totally full, and a portion will be below the water level. There is currently no fresh water tank, which in theory I can add wherever and whatever size - I had been thinking either inside the punt bow, or in each hull, or underneath between the hulls. I do want to avoid balancing the boat by adding too much extra weight forward, as the bow has little freeboard (and is designed for calm river use).

    I will be adding furniture etc, and have a separate tally of that weight, which could be used to help the balance. Windows will also add weight forward, but I want to start with understanding how to make the empty boat float level.

    My first thought experiment was that if I had a mirror image of everything forward of the balance point, and stuck it onto the rear of the boat, then the heavy tail end would be equally supported from both ends and the boat would float level, albeit a few cm deeper. The sullage tank would then be somewhat in the middle too, so would not change the forward / aft weight distribution when filling up. Is that correct?

    The problem of course if that I don’t want to add 7.8m to the rear of the boat. Of course if the structure of the extension was built lighter, this could be reduced. My calculations come in around a 5m waterline extension if it was just a big party deck to the rear with the cat hulls extended.

    My other option is to extend out the sides and just have the pontoons that would have extended to the rear bolt on each side, with walkways on top. The added advantage here is that I could move them front to rear to fine tune the balance. The downside is that I need the same 5m of so of (each) 1.2m wide hulls, which makes the boat a lot wider. It also means the sullage tank is no longer in the middle.

    So I would love some feedback if I am on the right track with my thinking. I don’t have much experience with this sort of thing, so I don’t know if I am missing something in terms of how the extra buoyancy will make it sit in the water.

    I am also open to any other ideas (wacky or not!) on how to make sure it sits level.

    Of course the ultimate solution is to buy a different boat, but if this one can be fixed without the extras costing too much, it could potentially be a well priced, and unique riverboat.
  2. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell . . . _ _ _ . . . _ _ _


    No offence, but you need to hire a professional here or at least talk with the designer.
    The designer is likely the best person to speak with.
    You'll have to pay him/her.
    There should be no need to modify the hull and if you do, simply extend the pontoons with pods to mount your outboards on.
    You'll need to do a full weight and balance including people loads, provisions, ground tackle, beer, 2/3 holding tanks, everything must be accounted for.
    Including wind loading, icing, etc, etc, etc.
    You need a professional, it's not guess work.
  3. Sywofp
    Joined: Apr 2018
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    Location: Australia

    Sywofp Junior Member

    No offence taken - as I mentioned I've already organised professional paid advice.

    But that's going to take a little while, so in the meantime I'm hoping someone here can give me some feedback on my understanding of the maths involved.
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Ok..firstly the best way to approach this is by you understanding what is wrong, then you can self correct later. So, what you need to do is create a list that has the following:

    Item, weight (kg), Lever (m), Moment (tm).

    So, 4 columns.
    Item = outboard, table, desk, chair, window, genset, deckhouse...whatever, an item of 'something'.
    Weight, W, this is then the weight of THAT item in kgs. So either weigh the item, or calculate ir, or, if the sales brochure gives you an approx weight.
    Lever. You need to select a datum, so most likely best for you is to use the back end, the transom or stern. What is the distance from the transom to THAT item in m. It would be the centroid, or CoG of THAT item is the distance. Not to the back of the chair, but to the centroid, CoG, of THAT item.
    Moment. This is the the weight, W, times the lever, in tm.

    So, you then add you all the items in the weight column. This gives you a total weight of everything. So this weight must equal the weight as if you picked up the houseboat with a crane that has a set of measure scales on it.

    Then you add up the column of Moment.

    Then you divide the Moment total by the weight total.

    This will then give you the distance of the centre of gravity from the transom.

    This is your first step.

    A full list is shown here, as an example for you, with the LCG, VCG and TCG shown:

    fallguy likes this.
  5. messabout
    Joined: Jan 2006
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    messabout Senior Member

    You have provided a good set of dimensions that affirms your dimensions and weights because the moment arms are equal. That means that the balance center needs to be moved forward by whatever means are possible. Failing that, the buoyancy of the after part needs to be increased so that the hull will trim approximately level. That is what you have posited. The balance point should be half of the pontoon length if we agree to ignore the forward punt section. That stuff is calculable.

    The pictures show a rather attractive looking purpose built vessel that appears to be more bulky aft than forward. That is only picture impression however. That you have 240 kilograms of outboard attached to the stern is one of the most influential of the factors. Remove the outboards....yes I know that is not feasible.....and the pivot point will move forward a meaningful amount. Thus the vessel will trim more nearly level. The arithmetic is not difficult. According to your dimensions the trim is a little less than 6 degrees from level. That is a bit more than would be comfortable for landlubber passengers.

    Keep in mind that the passengers will be mobile and possibly unruly. "Oh look at that huge croc over there". Now the passengers all run to one side or to one end of the boat which wrecks the trim in both X, Y, and Z directions. That is also a factor that deserves exploration. I expect that it is best to let your paid consultant do the math and advise what to do.

    I like the boat and probably the environs in which it is to operate. Maybe another member will jump in here with a more encouraging post.
  6. Sywofp
    Joined: Apr 2018
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    Sywofp Junior Member

    Thanks Ad Hoc and messabout for the feedback, that's given me some more things to Rickt about and calculate from.

    One thing I didn't mention is the outboards are not yet installed. Basically it's an empty shell, with only the structure and a bathroom and plumbing. So no weight can easily be redistributed.

    But it sounds like I'm not missing anything obvious with my understanding, so I need to start compiling weights of everything that will be added and their location to calculate moment. So then I can have it calculated exactly much bouancy needs to be added.
  7. latestarter
    Joined: Jul 2010
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    latestarter Senior Member

    Probably a silly question, does the floor slope by the same amount as the difference in the immersed depth.
    There seems from the photos to be more mass towards the stern so the draft would tend to be deeper at the stern.
    I would have thought having a level floor on a houseboat is the aim.
    fallguy likes this.
  8. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    I am a casual observer and boat lover. Ad Hoc’s post is a great way to go about addressing the issues. The boat looks arse heavy to me and my gut hunch is the design got away from the designer...either by demand or during the build.

    Once you do the spreadsheet; you’ll know in a hurry what abovedeck issues caused the stern issues.

    We see this issue to a smaller scale in putting four stroke engines on fishing boats designed for two strokes. Add a hundred pounds to the transom and now the fuel tank designed at the center is now adding stern weight, too. Suddenly; you need to find ways to remove weight from the aft sections. You move a seat forward; lighten an anchor; remove an auxiliary tank, etc.

    My honest view is that you need to modify the abovedeck structures; not the hulls. Someone went too far with it all.
  9. TANSL
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    TANSL Senior Member

    It may not be necessary to change the entire structure and adding a weight on the bow steps is sufficient to correct the trim. That's as long as the resulting freeboard is not a problem.
  10. fallguy
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    fallguy Senior Member

    How very true. The possibility the forward area of the boat was designed for more weight (paddleboats?)exists. Knowing the design waterline would be helpful, but your point is very valid.

    That massive tank on the back is what makes me suspicious about the design getting tweaked, but I could be wrong.
  11. SamSam
    Joined: Feb 2005
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    SamSam Senior Member

    Add a third catamaroon at the back. Un-enclose the stairwell and move it forward. Move the awning forward or lose it altogether, it looks overbuilt, too.
    It looks like with 48 people and all the gear, seating and supplies the bottom step on the front porch stoop will be underwater. It looks like waves will go over the front and smack into the steps making for a wet ride.
    Was this designed by someone that does it for a living i.e. certified or licensed or degreed? Are you going to use it for what it was built for?
    fallguy likes this.
  12. Sywofp
    Joined: Apr 2018
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    Sywofp Junior Member

    Yep, the floor and the bottom of the hull is parallel. Level is the aim of course.

    Yeah, the design was modified at least once along the way, but unfortunately I don't have the full story.
    I would love to modify the above deck structures and re-balance it that way, but that is a huge amount of work and not something I really want to take on. But certainly an option if the price was cheap enough.

    Because of the stepped bow, I don't have a huge amount of free board to play with at the front. And I still have to add weight with the interior fitout, so I very much keen to add bouyancy, rather than ballast.

    You are correct, and I actually found out some more today. The original design was actually 9.6m waterline length and had no provisions for engines, but then was extended 2.4m (waterline) and had outboard mounts added so it could be moved by itself. As far as I am aware the stairs, bathroom, sullage tank and extra pontoon were added at the same time, as well as the top deck.

    So it looks like the design went awry at that point - it has been miscalculated how much extra buoyancy was needed to account for the extra added weight.

    The question is how easily it can be corrected...

    Between the cat hulls at the rear there is a sullage tank and then a flotation tank, so there is no more room to add buoyancy easily without extending to the rear, or out to the sides at the rear.

    I need to calculate the weight of the awning up top - it's aluminium frame with sheets of this plastic panel with aluminium faces for the actual covering, so I don't think it is terribly heavy. Ideally I want to be able to use the space up on top so a sunshade of some variety is must have.

    The calculations I did of the original build show enough flotation with the 48 passengers and gear, for it to float with the water just below the bottom step. But it would not be moving around then so less of an issue with waves coming up the steps. I will just be using it as a houseboat, with few passengers so not much of an issue as it should sit much higher if level.

    It was designed and built by a qualified builder, and was signed off by the Maritime in it's original form before being modified. But modified since, and I am not able to talk to all the people involved, so can't query that way.
  13. latestarter
    Joined: Jul 2010
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    latestarter Senior Member

    My concept would be to add below the hulls but not all of the addition would provide buoyancy.
    It would be divided into sections some sealed others floodable.

    To get a rough idea of what is needed, earlier you said you needed 1100 kgs.
    A sealed length of the box below each hull behind the centre of gravity say .12m deep 4m long 1.2m wide x 2 hulls produces 1152 kgs of buoyancy.
    The part of the addition in front of the centre of gravity would be designed to flood so not changing the buoyancy there.

    With adding the same depth along the length of the hull it will sit on the sand.

    If you were wanting more freeboard at the bow you could seal a section near the bow and make the addition deeper through out its length to increase the rear buoyancy to keep the boat level.

    Beyond a plywood canoe and a part built SOF rowing boat I have no knowledge of construction rules so will leave that aspect to others.

    I have drawn a sketch however do not seem to be able to upload it, do I need to put it on a photo hosting site first?
  14. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I think the simple thing is to splash the boat and start moving things around and/or adding some weight. Sand bags work, though you do have to shovel a bit. A concrete block weighs about 35 pounds and this is easy enough to load and move around. I'd try to concentrate weight forward, but not too much forward. It will require more weight to level up her trim, but will not affect her handling as much. In the end, you may find you don't need much weight, if far enough forward. If this weight isn't much more than a few well fed guests, consider it.

    Conversely, you can play the guessing and center of masses game, which is a fair bit more tedious, then a half a pickup truck load of concrete blocks. You can always return the undamaged blocks to the big box store when you've found her trim weight. Yeah a half an afternoon moving blocks or . . .

  15. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    We could give you more accurate and practical advice if we knew the shapes of the hulls as well as current draft and trim of the boat.
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