How should I start (hull design for catamaran)

Discussion in 'Boat Design' started by justinDesign, Oct 3, 2007.

  1. justinDesign
    Joined: Sep 2006
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    justinDesign Junior Member

    I am new to boat design and have a few questions. First let me inform you on my design. I am designing a small fishing vessel that is meant to be used in hazardous conditions like low water and debris laden water. The vessel is meant to be a catamaran and be approximately 12’ by 5’. The vessel is designed for two occupants that will be seated in the individual hulls while underway; this means that the hulls must be at least 20” in depth. This vessel is meant to be powered by an outboard surface drive so there is the possibility of speeds up to 30mph.


    Hull material:
    Due to debris I wish to make the hull of aluminum or stainless but am open to the possibility of using fiberglass or roto-molded providing they can meet my criteria.

    Hull design:
    I have never designed a hull before and can only assume that making a cat hull is even harder. The good news is that I am very familiar with Rhino and if any advice can be given in this area I can most likely understand. I would really appreciate any tutorials or some 3dm files of cats that I may pull up in Rhino and reference.

    Other questions:
    How important are chin lines/splash guards (not sure if right terms) on a hull and if I do need them do I have to have them on the inside and outside of the hulls or just the outside edge.

    Any recommendations/files/websites are appreciated.
  2. messabout
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    messabout Senior Member

    Twelve by five is not the best proportion for a cat. The hulls will need to be 16 inches wide more or less if a person is to sit in the hull. That leaves only 28 inches between the hulls. Not nearly enough. You seem to be thinking of going fast. If so, you will need to use a planing planform. Twelve feet is not long enough for a boat to be constructed of heavy material. You mentioned stainless, aluminum and FRP. A bulletproof boat will necessarily be heavy and the short length will not work well. Stretch the boat out to 15 feet or more. Widen it to seven feet more or less and widen the hulls at the chines if it is to be a planing cat.

    Seems to me you'd be better advised to use a monohull design for what you are after. Question: If you are to run in hazardous water, why are you thinking 30 MPH ? Would that not be foolhardy?

    Apparently you intend to build your boat. I promise you that the build will cost more than a suitable used boat that you can buy and use without all the work, time, skinned knuckles, profanity and possible disappointment.
  3. justinDesign
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    justinDesign Junior Member

    The 30mph is while underway in ideal conditions

    I do not really plan to build this boat, however I want it to be buildable. I am designing this boat for my Industrial Design thesis. I am primarily a bass fisherman who likes to stand up and fish thus the catamaran for a stable platform. This boat is meant as a stalker to find and peruse the fish. The speed issue has to deal with the fact that modern bass boats have massive engines for speed but these engines create limitations and are very costly. I want a low cost boat that has certain advantages over an 18ft Skeeter or something like that. Something like a glorified Pelican roto-molded boat. These boats can have a 5hp motor and has a plastic deck that bounces when one walks on it. I want something between these two styles. Any suggestions?
  4. Trevlyns
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    Trevlyns Senior Citizen/Member

    Here's a good FREE start on the basics of yacht design.

  5. messabout
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    messabout Senior Member

    For purposes of the thesis, I suspect that something out of the ordinary might get better reviews. But that is only if you have an outstanding concept. I am sorry to tell you that there are great hordes of people who have tried to develope an improved boat design. This has been going on for a very long time. Most of the boats that you see out there are merely variations on a basic design theme. Sad to say some of them have been developed and/or built by people who had no credentials for doing so.

    You are a bass fisherman who wants a stable and probably silent running boat. There are degrees of stable and degrees of silent. You also want exceptional durability while you also want good speed potential. You can not have all that in one boat. So let us pursue the practical and achievable.

    A small skiff of 14 or 15 feet length can have entirely adequate stability. Put in enough beam at the chines to make it so. But not excessive beam. Too wide and it is harder to drive at stalking speeds. Hard chines or only slightly eased chines will help the initial (walking around or playing a fish) stability. Practical chine width for such a boat might be 48 inches. Now if you have a flat bottom that is 48" wide you will have some structural issues that must be accounted for. If the bottom is to be thick enough to contain itself without a lot of interior bracing it will be heavy. It will also be quite durable in the process. The weight is in the right place too. On the other hand increases in weight will require progressively more power to drive at speed. A slight transverse arc in the bottom will help the situation but it is more difficult to build. Too much arc and the initial stability will diminish.

    Think in terms of building efficacy, economy of construction, and reasonable, even if compromised, performance, add to the list, stability and above all safety. A 9.9 HP outboard will drive a decent 15 footer at more than 20 MPH if the total weight is not excessive. Not more than, say, 600 pounds all up weight. A boat of this sort is likely to be trailered and so facility in loading and unloading is worth consideration. That means light weight. A flat bottom helps too. The flattie will also draw only 4 or 5 inches of water so you can stalk 'em in thin water if need be.

    Sounds like I'm trying to talk you into a simple flat bottomed skiff doesn't it. Of course there are countless boats of that type already out there. The reason is that the design fulfills most of the the needs and many of the wants of the fisherman. They can also be attractive if well executed.

    Your catamaran idea has some merit if properly proportioned. The down side is that it is more involved to build, uses more material, and is more difficult to maneuver in tight places. It will also be heavier, require more elaborate trailer supports and be less comfortable to fish from. On the plus side you will be walking around on the center deck which will be elevated somewhat. Better to see the fish I suspect. But also better for the fish to see you. I don't know about the bass in the Grand Rapids area but the Florida bass are very wily and they will haul *** when they see a fishing boat. Especially if some dude is holding a fly rod or casting rig. For pure stealth there is not many better boats than a Barnegat Bay Sneak Box. You might check out that design just for fun. The sneak box is primarily a sailing and or sculling boat contrived to sneak up on ducks. It is a very quiet runner. It could be made into a power boat easily enough. It is unique and not often seen these days. Fairly simple to build but not as easy as a flattie. Then there a thousand other well proven designs that are worth study too.
  6. justinDesign
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    justinDesign Junior Member

    I would like to go with the cat design for shock factor if nothing else. The main premise behind the design of this project for me is to have an uninhibited platform for two to stand and fish from. I even plan to have the seats recess into the hull like Chryslers fold and go seating. I would like the vessel to be 12ft in length, how large would this make my beam/hulls. The selling points are the features added in via my product design background. Just want a functional hull design with low draft, stability, can go fast (20mph) and is a head turner, to lure people in. This vessel is intended to use electric propulsion 90 percent of the time for stalking along reeds stumps etc. and is not intended for tournament use (no money) just want to get the best fishing experience possible for a user that goes out say twice weekly to try to catch something to hang on the wall. Think armature/dummy anglers Cadillac If you have any ideas on my proportions to achieve this with a catamaran please let me know. How about a tunnel v, is that any better and how do they work.
  7. Chris Ostlind

    Chris Ostlind Previous Member

    This is not going to be a 12' design unless you want to do endless "wheelies" when you punch it with the necessary power to hit 30 mph... planing vessel or not.

    Personally, I wouldn't be wanting to go that fast in anything shorter than 18'. That's especially true if you have a load of expensive fishing gear on board, a $10,000 engine and two priceless dudes sitting down in the hulls.

    Please rethink this design motif. There's a reason why other boats of this general type have a lot more length and a lot more longitudinal stability.

    Hefty aluminum is the only way to go while still getting something like a light enough hull to make this all work. Unfortunately, it does not pencil-out at puncture resistant thicknesses until you start to get up above 20' in length.

    If you want help with the Rhino thinking for a boat along these lines I'll be glad to help you.

  8. Navy Dave
    Joined: Mar 2004
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    Navy Dave Junior Member

    Good idea, with several similar examples in the market


    Sounds like an interesting idea. Here are a few links to source info.

    This software is written by "Jimboat" as he's known on this forum. It will help you determine what dimensions are optimal for your application. The software is named 'Tunnel Boat Design Program'. Jim's customer service is outstanding, and I am sure he can help you through the process.

    Here is a link to a 12' catamaran that was built as a home project (similar performance expectations, but a higher top speed than you were looking for).

    Here is link to a product with a similar seating arangement as you propose.
    Last edited: Nov 13, 2007
  9. kengrome
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    kengrome Senior Member


    Chris is right. You seem to have great ideas that won't happen in the real world, at least not all in one boat the size you're talking about. 12 feet is way too short to achieve everything you've said you want. In fact, I doubt you'll end up with half the features you've mentioned so far, especially in a boat this small.

    Why don't you list your features in order of their priority so you can start crossing some of the less important ones off the bottom of the list until only the most important features remain? Then we will have a practical list to work with, and maybe we can focus on a useful effort for you.

    I think your fancy design motives are getting in the way of reality here. At the risk of insulting you (which I'm not trying to do here) I might suggest that you forget about the head turner stuff until you settle on a hull design that gives you a few of the most important features on your list -- whatever they may be. You can always make the boat pretty later.
  10. kerosene
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    kerosene Senior Member

    agree with the previous posts - I recommend reading some books on the topic + lots of this forum. You'll learn that its always about a compromise - gain in this, lose in that...

    The fact is that good design is not likely to be very revolutionary. Having graduated from industrial design myself I know that it is important to have something sexy in your portfolio. Also it is far easier to impress the crowd with "Urban techno nomad style wearable microwave, mp3, gps, shoulder strap thongs" (made of recycled fluerescent tubes of course) than with a somewhat conservative yet good quality design.

    Boat design is so strictly dictated by the hull design that it doesn't lend itself very well for wild visions - especially if you plan on making a working prototype for yourself. If you would settle for a scale model you could probably take more freedoms and trust that the jury is clueless and read more Wallpaper(tm) than professional boat builder.
  11. Gerald
    Joined: Oct 2003
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    Gerald Junior Member

    Here is a cat that looks like it is going fairly fast in the video. Cat AMI Boats.html
    They sell one under your desired dimension and one over your stated dimension. The design is straight forward enough to build in fiberglass. There is no need to have 1,300 kilos of floatation so some of that should be designed out it you want to be seated inside the hull.
    Good luck.
    Gerald Niffenegger
    Florianopolis, SC Brasil

  12. Jimboat
    Joined: Feb 2002
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    Jimboat Senior Member

    justinDesign - there are certainly design methods to help you achieve your performance goals. Check out the "Secrets of Tunnel Boat Design" book and the "Tunnel Boat Design" software ...both will help analyse the performance and stabilty characteristics of your proposed design, and also to help make key modifications to your designs that are best suited to help you achieve your design.

    Give me a call or PM me if you'd like to discuss your ideas. I am happy to help if i can.
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