Question About how to calculate Floatation of my design

Discussion in 'Boat Design' started by AdamOMahony, Sep 11, 2017.

  1. Nick.K
    Joined: May 2011
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    Location: Ireland

    Nick.K Senior Member

    Maybe it'd be a good idea at this point to put down some basic expectations for the project:

    What is the expected / desired life span of the boat?

    Is it to be used purely as a canal boat or do you maybe intend to explore some of the more open Shannon lakes where wind and chop could be an issue?

    Is this a significant investment?​

    If you are planning a kind of boat experiment that you expect to last a few years and maybe lead to better things, it isn't going to be a major investment and so long as it floats you'll enjoy it...then I don't see any problems. On the other hand if you are planning on it lasting a decade or more and have ambitions to explore the wider Shannon water ways, I think it would be a good moment to take advantage of the forum and reassess the design and construction.

    As I said in a previous post, the ply you are using appears to be a low grade (as far as I can see from the photos. It is white inside and the surface layer is very thin and a pinkish colour?) As an experiment, try boiling a piece for an hour or so.
    It is very difficult to seal moisture out of wood effectively. In 'sealed' timber moisture can be absorbed through tiny pinholes, cracks and around fastenings but the rate of evaporation through these is negligible. The sealer coating prevents or slows the evaporation through the wider surfaces and the moisture level rises.

    I've no experience of the Shannon (beyond crossing it in a ferry:)) but I've often heard that the open parts can be fairly choppy. I'm not an expert on design but chop could be quite uncomfortable in a light weight, short, flat bottom boat. I would expect the boat to start pitching and slapping the water.

    If I was building a boat for the Shannon waterways, I'd do it in steel and I'd chose a design that could combine shallow draft with an ability to handle chop.
    Steel has some advantages: Its fast to build with and a forgiving material to build with too. Its extremely strong in the context of small boats. Steel is not expensive, actually it may be the most economic material to build with, though admittedly it is costly to coat and needs maintenance. On the water, steel is a hardy material. It would take a major collision to cause damage that would need a repair. The weight of steel gives a steady motion and if the weight is kept low, it gives great stability.
     
  2. AdamOMahony
    Joined: Sep 2017
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    Location: Ireland

    AdamOMahony Junior Member

    Thanks for taking the time to write out all that Nick and I have the answers to these questions :)

    The intended use is to travel as much of Ireland's inland waterways as possible and providing a nice comfortable space for me and my wife to enjoy the canals, we're a young couple but the budget for the project is solely from building websites online which is actually a fairly easy way to fund the project. Thus far I have the entire budget for the wood and fiber so we would at least get the hull completed without much strain on finances or investment.

    We would like it to last 5-10 years if possible but are quite aware that it may not be able to last much longer.

    The main use would be canals although some adventuring into the Shannon was something that would be great.

    Although it would never be at significant speed more a snails pace along the way.

    I'll pop a piece to boil tomorrow and let you know what happens with it, probably it will peel apart before the hour is up.

    What's on my mind now bouncing around is what scenario would my design be better for as in if the shape of the hull was not intended to be for a canal but more open or chopped water would it be better than the flat bottom?

    Or if cutting it with a shallow V as in Tansl's suggestion of modifying the current hull shape be better for the canal and handle the Shannon a little better?

    FYI I'd love to give steel a shot some day but really wouldn't have a notion of shaping it or any of the tooling required where as I have most of the equipment to shape timber and intermediate knowledge of joinery.
     
  3. Nick.K
    Joined: May 2011
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    Nick.K Senior Member

    In the current version of your boat, how do you intend to make the hull surface and what is your plan for the finish of that surface...will it be faired and smooth or will it show the texture of the glass reinforcement? Do you have a method statement for the application of the glass and resin, are you familiar with the process or is it going to be a learn-as-go procedure?

    To build with steel, basically you need only four tools. A big grinder (€90), a little grinder (€60), an inverter welder (€250) and a big hammer (€20). You'd get along better with a decent face shield than a cheap one (€120) and you'd also need gloves, ear and eye protection and respirator etc.
    Some clamps, crowbars and maybe a small chain hoist would help too.
    You would chose a design where the surfaces have only 2d curves (like your current design). There are various ways of building small boats but all involve pulling the plate in to curves and tacking it at the edges. Surprisingly little force is needed, in fact steel plate has a natural curve from the mill which can be used to advantage.
     
  4. Heimfried
    Joined: Apr 2015
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    Heimfried Senior Member

    If you click top left pic on page "choose / alter frames" you will get the frame, Tansl suggeted. But be careful with the results because it depends very much on VCB. You should try to calculate weight and VCB it first, which is not difficult, but very labourious.

    Hints to play with the choosen frame: input only in the right column (use "i" buttons for information)
    e. g.: input box "mass to load / unload" input 310, then button "load" : displacement and draft increase a bit (4310 kg disp, were causing 500 mm draft on your first frame).

    input box "mass to relocate" input 160, input box "transversal" input 2000, then button "apply relocation" : shift of the CoG (marked "G"); then button "find equilibrium" : computer heels the boat to achieve equilibrium.
     

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    Last edited: Sep 16, 2017
  5. AdamOMahony
    Joined: Sep 2017
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    Location: Ireland

    AdamOMahony Junior Member

    Really it will be a learn as you go procedure I will look into best method's for in although I have been given good instruction of how to apply it and have a pdf on application and process that I will be going through before applying the epoxy. I made the suggestion to my wife and the plain answer was "we have wood already for the boat, your not going out to get steel and equipment for it at this stage"

    The plan is 6mm ply outer skin which will have two coats of epoxy, 1 layer of 270g/m glass fabric and then a final layer of epoxy which will be to protect the glass fabric from light sanding of defects.

    Thanks again Hiem although I admit I am lost with that site, I clicked all the information button's placed in rough value's or estimations and I've no idea what im looking at with the results from it..... I'm sure there is crucial information there that would be invaluable but im not sure how to interpret it.

    So far it seem's I've two options to go with before continuing any construction, the original design and the more flat bottomed or shallow V from sam's and tansl's suggestions. Is there a way to find the advantages or disadvantages of each?
     
  6. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    A monocoque is easier to build than a catamaran. This one has more stability than a monocoque but I do not think stability is a problem for you. The monocoque will probably need less depth than the catamaran. And so we could continue to note the advantages and disadvantages of each of these types of hull.
    In general it does not seem necessary to build a catamaran but the problem, for those who observe your project from outside, is that we do not know most of your needs. That is why, if you define your SOR, we could give a more accurate opinion on what the correct hull is.
    As it has been said before, it is very important to have well defined the boat and all its equipment to be able to make an estimate of the weights and their centers of gravity. Full-load displacement is a primary, but not the only, factor in deciding the most suitable type of hull.
     
  7. AdamOMahony
    Joined: Sep 2017
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    Location: Ireland

    AdamOMahony Junior Member

    Thanks Tansl,

    My main requirements are that the boat is very stable when sitting in the water (moor on the canal for a few days and then go again), can take some amount of wave if it came to it without issues. (Ireland is far too windy to expect to never meet some amount of chop in the lakes)
    The heaviest Items I can see that will be in the boat is a bed area, waste water tank, fresh water tank, outboard engine, small cooker, small wood stove, I'm guessing the overall weight of the boat to end up between 4.5 and 5 tonne's on the higher end and possibly as low as 3.5 tonnes.

    Originally we had the thought of crossing the English channel at some point but realistically it's never gonna happen with this particular boat.

    The main requirement is that it is a space on the canal or lakes/rivers that we can hold up in at night before moving along slowly around the country.
     
  8. TANSL
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    TANSL Senior Member

    There are many things that you must define. For example: do you need a generator, what fuel capacity do you need? Do you need a food store, cold store, anchor windlass, warehouse for and with spare parts? capacity of the gray water tank ?, .... washing machine and dishwasher ?, .... autonomy ?, .... air conditioning?, heating system ?, fire pump?, bilge pump?,... comfort isolation?. If you are going to sail, even if only once, by open sea, the depth of the boat has to be higher, so the total weight increases and also navigation equipment is needed. Lifeboats or rafts?
    Do you need to hire an insurance policy for the boat and what do you need, technical documentation, to obtain it?
    In the end, many things that need to be taken into account and that only you can define.
     
  9. AdamOMahony
    Joined: Sep 2017
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    Location: Ireland

    AdamOMahony Junior Member

    Okay so the maxium would be lakes in shannon, never ocean with the vessel

    No washing machine or dishwasher, possibly 100L tank one for grey one for fresh, fire canister is needed according to the by-laws for the waterways Ireland only needs to be 2-3kg though, + anchor , small wood burning stove 70 kg don't really need AC in ireland just open a window to cool down, Compost toilet (very light weight), will go for solar for inner power on the boat so the batteries for that are kinda heavy, lifeboat or raft probably not. so yeah I'll avoid open sea.....
     
  10. TANSL
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    TANSL Senior Member

    You should make a General Arrangement drawing (sketch) to indicate where each of the things that the ship needs is located and to check that there is enough room for each of those things.
    Apart from the technical aspects, imagine that you are already living in the boat and check that you do not have to go over the bed to access the kitchen, or that there is space on the outside to put the clothes to dry, try not to spoil the solar panels when you have to work outdoors. Is the height of the house sufficient for a person to stand inside?, Allows to pass under all the bridges that are usually in the canals?
     
  11. AdamOMahony
    Joined: Sep 2017
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    Location: Ireland

    AdamOMahony Junior Member

    We have a floor plan of the general area within the cabin.

    as for air draft there is a pdf document I received from irish waterways which indicates the draft's and air drafts of the different canal systems.

    It seems the minimal draft is at some points 1.2m and air draft of 3m for the majority of the canal system, 1.2 being the lowest safe draft for some of the system where as 3 M being some of the lowest bridges the canal's go through.

    there's enough room to stand up although it is just a few cm above head height....

    Here is the floor plan from our drawings. our plan is that the maxium of the boat above the water line (at .5m draft) would be 1.75m

    Boat inisde.PNG
     
    Last edited: Sep 16, 2017
  12. SamSam
    Joined: Feb 2005
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    SamSam Senior Member

    I just looked at Ireland on google maps, I'm amazed at all the small farms and how it is all actually Emerald Green. Too much.

    I went and re-read the op and a boat roughly 8' x 22' probably won't cost anywhere near the 10s of thousands of moneys I mentioned, and it won't be a crippling expense if it doesn't work out as expected. Of course, I didn't know then it is being publicly funded. That's probably the most interesting thing here...how does that work?

    There are much better designs, but yours will work if you don't expect too much. Since you started building with no clear, thought out plan, you're going to run into all kinds of problems such as "This won't work, what now?"

    Since Nick.K asked what the expectations were, a lot of questions have been cleared up, especially the water conditions to be operating in, such as lakes rivers etc.

    Since power will be outboard, a fine choice, you'll need to tilt the transom 15 degrees or so.

    The roof only needs a crown of an inch or so (I was thinking, a lot of these guys are Irish and it's supposed to be hard to understand what they're saying, but I understand them perfectly. A lame attempt at accent humor. But then I realized that when you start talking metrics, I don't understand what you're saying, it's like a heavy mathematical accent.) Anyway, a crown of 24.5 mm or so would be plenty to shed rain and would allow you to use the roof as a deck, a wonderful thing. I'm not sure if the stability calculations on the hull would allow it though, it might be too top heavy with any people up there.

    Seriously consider starting over, you haven't invested very much time or materials into it yet and what ply you have cut up can possibly be used elsewhere as bulkheads, shelving etc. Using ply as frames is questionable to begin with, if the ply falls apart with getting wet, it's not a good thing. Solid wood frames glued with gussets or bolted are better.

    Have you considered how you will attach purlins or stringers or strakes or chine logs or even the skin to the plywood frames? Glue won't be enough and screwing or nailing into the edge of ply wood is very weak structurally and generally a very bad idea.

    Look through various designs here, not to find a design shape to use, although there are some that would work fine, but to see how they actually construct the various parts of boats. They are old plans and some methods are outdated, some new, better methods have been developed since, but they worked then and will still work now.
    www.svensons.com - Free Boat Plans From "Science and Mechanics" Magazines http://www.svensons.com/boat/

    .
     
  13. BlueBell
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    BlueBell Ahhhhh...

  14. TANSL
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    TANSL Senior Member

    You could get something similar to the figure. It will probably be necessary to increase the freeboard. The space in the hull to accommodate the various elements does not seem sufficient
    Snap20.jpg
     

  15. Nick.K
    Joined: May 2011
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    Nick.K Senior Member

    Ahhh wives are sooo sensible (apologies to wife readers, I'm sure there are a few that are not sensible too..)

    Are you going to use the same ply for the outer skin? I think you should use a quality marine ply. Don't assume because it says marine, 1088, Lloyds etc etc that it will be OK because often it will be expensive rubbish. Look for ply from a marine supplier, it is worth the extra cost.
    The schedule may be technically strong enough, others can comment better than I can; but it looks too light to me. In your first post you were talking about a displacement of between 2.7 and 4.2 tons. Think of a docking accident for example which could involve you bumping in to a corner of another boat or a protrusion from a wall. Think also of a windy night on a strange dock, your fenders come off and the boat is rubbing on the dock while you are asleep. Three tons of boat will be behind a small impact area which could be underwater. Your laminate outer layer will finish at under 1mm thick, it may add some stiffness to the ply but it won't give it much impact or abrasion resistance. If your hull ply begins to go soft there will be very little margin of safety.
     
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