Small Power Cat Design

Discussion in 'Boat Design' started by small cat, Nov 22, 2002.

  1. small cat
    Joined: Nov 2002
    Posts: 11
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: barbados

    small cat Junior Member

    I’m looking for is input on the specs for a powered cat design, I want to build a small “shallowish” draft power cat that can handle moderate chop and keep me dry.

    (I’ve changed the name of the thread cause it seems that the R/C model I was considering might be a waste of time)

    My thinking goes like this:

    small - easy to build low power requirements
    low power - wide hulls thick chine
    smooth ride with wide hulls - 20 degrees deadrise
    low draft with lots of deadrise - wide hulls again
    engineering concerns - keep space between hulls small (18")
    uncluttered cockpit - external motor bracket - adds effective length to boat

    Length: 12'
    Beam: 6'
    Individual hull width: 2.25'
    Max Gunnel height: 2.5'
    Deadrise: 20 degrees at transom
    Power: Max 30hp

    It’s been suggested that I go flat at the transom (how flat?) and move to a vee section toward the bow. It was also suggested that chines won’t be effective at the speeds I’ll be encountering.

    Take a look at what I’ve got so far .........

    All comments welcome ........
    Small Cat

    Attached Files:

  2. JPigott
    Joined: Oct 2002
    Posts: 27
    Likes: 0, Points: 0, Legacy Rep: 10

    JPigott Junior Member

    My two cents. Remember or consider Archimedes Law, a boats displacement (the volume of water it displaces) is equal to its mass. This applies on a sectional basis as well. Consider where weight is going to be placed, engine, fuel, passengers, the hull itself. Those sections should have a sufficient hull volume to support intended payload, power, equipment. If you don't the boat will sink too low in the water to perform properly, it can also be dangerous depending on water conditions.

    A flat bottom provides more volume under the water per inch of immersion, supporting more weight than a v shape. It also will get on plane quicker because a flat surface provides more effective hydrodynamic lift. A poor example perhaps but a flat rock will skip on the water better than diamond shaped one. If you think about it this very simple example covers three important subjects. Lifting shape, weight or mass and power applied (the throw). Along the same lines the design should sit in the water in such a way that the power applied creates the most effective forward movement, trim or in the case of the rock perhaps angle of attack.

    So in the case of a smaller boat or RC model you may need to find ways to compensate for the disporportionate weights of motor and crew to a small displacement boat.

    For a parallel discussion check this out.

    Scroll down to this section

    Pocket drive, catamaran tunnel and some theory:

    Your first discussion made me think of what I believe is called a slot hull, similar to a mono hull but with a slot cut through the middle.

    Finally, If you think of the boat longitudinally the further out you place the motor with limited displacement the more it might effect the trim angle and create porpoising. In a monohull this would be less impactful.

    Looks like you have made quite a bit of progress. Devlin uses pennies as scale weights to determine displacement characteristics of hull design. Also if I'm telling you things you already know my apologies.

    Best Results.

  3. Bill the Cat
    Joined: Oct 2002
    Posts: 27
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: West Florida

    Bill the Cat Junior Member

    What worried me in my Cat plans was - How am I going to hold these two hulls together? In any water and especially rough water the hulls want to go their seperate ways. With the construction techniques and materials availalbe to me I considered it one of my greatest obstacles.

    I like your forms and model - please remember we can fill up books with what I don't know about boat design - but they seem a bit short for the shape. A larger hull could take advangae of both your deadrise and the flattening toward the stern.

    With your construction method - you'll have great compartments for in-hull storage, but the entire working surface will be deck (no cockpit). Now, I like that for my application - is that what your trying to achieve with your design?

    For help in stitch & glue / hard chine design - down load Carene - it can achieve what you want in deadrise and shape with four flat sheets rather than the complex (but real nifty looking) method you're using

  4. Jimboat
    Joined: Feb 2002
    Posts: 248
    Likes: 15, Points: 18, Legacy Rep: 130
    Location: Canada

    Jimboat Senior Member

  5. JPigott
    Joined: Oct 2002
    Posts: 27
    Likes: 0, Points: 0, Legacy Rep: 10

    JPigott Junior Member

    This might give you some ideas on integrating hull shapes. You might be able to simplify the hull shape to something more asymmetric as shown. If it works for these extremes it will work for you given the scantling thickness is sufficient.

    My thought on cats is you get better stability for effective (high) aspect ratio of wetted hull (length width).
  6. yipster
    Joined: Oct 2002
    Posts: 3,486
    Likes: 95, Points: 58, Legacy Rep: 1148
    Location: netherlands

    yipster designer

    :) i like (low) power cats (+ arrowed tri's) what was the R/C model?

    did not see the deck structure to keep you dry in your design.
    i think that can hold the hulls together.

    i saw a similar in size and power split V tunnel cat (not symetric hull design) made by jeff, do think it was on also...

    iff interested in displacement multihulls read also:

  7. Guest

    Guest Guest

    ivb hull

    the IVB Hull has far better characteristics than any cat and requires less hp and only one motor to drive them.
  8. yipster
    Joined: Oct 2002
    Posts: 3,486
    Likes: 95, Points: 58, Legacy Rep: 1148
    Location: netherlands

    yipster designer


    IVB ??? a "TRI" you mean?


    alta vista on IVB: FN-DD-IVb Destroyer
    Displacement: 3000 tons Hull Armor: 84
    Length: 69.52 meters Volume: 42000 m3
    Mass (Loaded/Unloaded): 32019.6 / 30156.5
    Power Plant: 10252 MW Fusion Power Plant, 1 month duration (85.4 m3 fuel) Jump Performance: 4 Jump Fuel: 13125 m3
    G-Rating: 5G HEPlaR (1601.04 MW/G), High-Efficiency CG Lifter (300 MW) G-Turns: 37 (J4: 50.1; J3: 63.2; J2: 76.3; J1: 89.5; 102.6 using all jump fuel), 200.1 m3 fuel each
    Maint: 1143 etc etc.

    IVB.. IVB.. so many abriviations! but IVB..? :confused: please help!

  9. JPigott
    Joined: Oct 2002
    Posts: 27
    Likes: 0, Points: 0, Legacy Rep: 10

    JPigott Junior Member

  10. yipster
    Joined: Oct 2002
    Posts: 3,486
    Likes: 95, Points: 58, Legacy Rep: 1148
    Location: netherlands

    yipster designer


    i should have guessed it!
  11. Guest

    Guest Guest

  12. small cat
    Joined: Nov 2002
    Posts: 11
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: barbados

    small cat Junior Member

    I’ve got to say thanks for all the useful input I’ve been getting and I am trying to incorporate it into my design. Gonzo and JPigott have got me heading towards a more flat bottomed approach.

    The deadrise is now 5 degrees at the transom and gets significantly vee’d towards the bow. I've dispensed with the chines ..... I understand they make for a noisier boat especially if I want to pole on the flats.The other dimensions have stayed the same and I’ve build another card mock up ....... and it sure looks a lot sexier

    The new mockup was sealed with tape and I’ve started some float testing but I’d like some help with the physics and math.

    The model is 1/12 scale so I assume it’s comparable buoyancy is 1/1728 or (1/ 12 x 12 x12) of the real thing. So if I want to hang a maximum of say 300 lb off the back of the real boat then my single hull model should be able to support 300 / 1728 / 2 = 0.87 lb or 1.4 oz and still maintain a decent trim angle .... Is that right?

    Well here are the latest pics, the hull seems to have a good attitude in the water but I will wait on your input before trying to hang 1.4 oz off the back.

    With respect to full size construction I won’t be doing this frame up stuff. I hope to build the two hulls separately ( stitch and glue) and then use ply bulkheads to hold the thing together. Bill you’re quite right I’ve already begun to worry about my ability to pull off that aspect of the project ......

    Well thanks again guys, I’ll keep you posted

    PS: I think I will build the R/C version ...... it will give me a chance to check out the stitch and glue method and as an electric boat the model should be way heavier than the real thing.

    Attached Files:

  13. JPigott
    Joined: Oct 2002
    Posts: 27
    Likes: 0, Points: 0, Legacy Rep: 10

    JPigott Junior Member

    That sounds correct to me. The relationship is the cube of the linear ratio.

    Of course I'm not an engineer by any measure. Have studied a lot of boat design theory without putting it to actual practice. Right now my suggestions are more intuitive than mathematical. However since you have engineering experience you're one up on me.

    A lot of your choices will be trade offs based on your needs.

    Planing shapes versus displacement. Will it climb on top of the waves or ride between it's bow and stern waves. Based on the 30hp I assumed you were looking for a planing model. A displacement version would have limited speed but good efficiency can be achieved with less horsepower.

    However with 30hp I would guess you would need sufficient planing surface. If your boat doesn't plane properly then you're overpowered for a displacement model. A displacement is more efficient if it designed to ride between those waves and a rounder shape has a lot of advantages both logitudinally and transversely.

    Pinning down the practical objectives your trying to accomplish and modifying the features of the boat to accomplish those goals is important.

    From what I have seen cat's have a tendency to squat in the rear. That's why I was little concerned about rear volume and a drop back motor mount. That also requires some engineering to ensure that the power or weight doesn't create high stress concentrations and fatigue in a small area. As was mentioned to me and was inferred in Option one, it requires a bit of engineering.

    Along the same lines I'd imagine in Cat design your torsional box is deck and bridging structure based.

    It would seem to me that your looking at an upright center console type design. Of course this creates a higher verticle center of gravity. Perhaps requiring more beam. More length might be required depending on the draft requirements.

    I would think the Cat Book would be a good idea. Although I have no idea of the breadth of coverage. Working off a successful design is a good approach.

    Similarly, Designing Power and Sail by Arthur Edmunds is a good primer with a lot of useful tables and formulas. However, I don't remember it covering cats.

    You might want to build a bigger prototype at some point so you your materials and model power will be more comparable to a full size model.

    I wouldn't rely entirely on anything I have told you. I think there is a lot to evaluate and to figure out. However, looks like you can work through that relative efficiently. Your modeling capability looks good!
  14. Guest

    Guest Guest

    IVB HUll

    with that cat design hull you will not get shallow water operation as the hulls will not support the weight without being forced deeper into water..
    the IVB hull is the same length to beam ratio as a cat but will support a greater weight with less depth into water ..
    the other main factor is there is NO BOW WAVE at all all the water is forced into and under the hull.
    the other factor is that the centre of gravity is much lower in an IVB that a cat again making it a much more stable platform ..

  15. Mike D
    Joined: Sep 2002
    Posts: 104
    Likes: 18, Points: 0, Legacy Rep: 465
    Location: Canada

    Mike D Senior Member

    You certainly need help with the math :)

    Try 0.087 lb = 1.4 oz:!:

    The 1728 for a volume (or mass) is quite correct and 144 would be the ratio for any area and obviously 12 for any linear dimension.

    If you intend to do any testing then check out the following notes.
    • The model should be in the same type of water as the boat will operate and at the same draft (scaled down, of course). If you perform the tests in FW but will operate in SW you will need to make corrections as the displacement volume is correct but the mass is incorrect. FW weighs 62.4 lb/cu.ft and SW is 64.
    • The model vertical centre of gravity should be close to the the boat's because it affects the magnitude of the trim. If the model VCG is too low it will trim less than the boat, if higher than more.
    • Have the load on the model suspended in the correct way or in some Rube Goldberg way that emulates it. The load and it's VCG affect the model and the VCG of the load is at it's suspension point not[/] in the middle of the load itself. This is one of the reasons why fishing boats like trawlers get into difficulties so often. If it is has been a good trip with the fish hold nearly full the boat is fairly deep in the water and its stability is not too good but sufficient. But if the next catch is very good then as it is being hauled aboard the net clears the deck and is swinging freely. The centre of gravity of the fish is now acting at the block way up there in the a-frame.
      [*]If you have the model correctly set up for the test then you'll get the correct trim but don't try and model any heeling with only a single hull. You can have a good, stable catamaran but each hull on it's own could be completely unstable and even roll over.
      [*]If you don't believe this here's a little test. You'll need two pieces of wood about 12" or 18" long and very rectangular say 2" or 3" by 1/4" or 1/2" and a thin, narrow light cross piece that will hold them together, the "deck" if you like.

      I'm going to describe two 12" x 3" x 1/4" hulls with a cross piece of a broken hacksaw blade. Each hull has a large staple at the mid-length on the 1/4" side (edge). The blade will slide thro' the gap but is fairly tight so everything stays in place. We want the hulls to float vertically but if we let them both float freely without connecting them they will only float horizontally, in other words they are unstable. But if we connect them with the blade the "cat" will float in the correct attitude. If we slide the hulls closer together there comes a position where it will also become unstable. This position depends on the the proportions of the parts and their weights as well as the weight of the cross-bar or blade.

      So even a spacing of only one or two hull widths is probably enough for stability purposes but insufficient for a deck working width.

    Good luck with your tests and make sure you can measure properly the effect of what you're doing.

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.