suggestions for a small narrow hull for rough seas

Discussion in 'Boat Design' started by kurtjfred, Apr 13, 2009.

  1. kurtjfred
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    kurtjfred Junior Member

    I have been wanting to build a small one or two person boat that can cut rough ocean waters while still maintaining at least 20 to 25 knots. I seen an earlier thread on some very long narrow and small boats in Bermuda. I also like that they were powered with small outboards. I dive from a 12 foot inflatable and the ride in windy conditions is brutal trying to maintain any speed. I have been trying to think of ways to accomplish this such as hydrofoils to keep it up and over the waves or some type of very skinny and possibly heavy hull that would penetrate and go thru the waves. I had a very interesting conversation with an old navy guy in Point Arena California one day about adding water ballast to smooth out the ride and than offload it to add bouyancy while stopped or in calm conditions. He was very partial to slow displacement type hulls. When he understood that I was wanting at least 20 knots and a smoother ride even in rough conditions, he suggested making a covered/watertight cockpit on a long boat that could be ballasted to to push thru the waves.
    Do any of you have any suggestions to offer?
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    You have said whether you're looking at a mono or cat hull form? A catamaran would be preferable, and then you can utilise the longer length to good effect. The lower the length displacement ratio the lower the vertical accelerations as well as better powering too.
     
  3. kurtjfred
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    kurtjfred Junior Member

    open to anything that works

    Thanks for the reply Ad Hoc. Not dead set on anything yet. Multi or mono doesn't really matter. I have been 26 miles offshore in my 12 foot inflatable and several times we have gone offshore while down in baja, I simply desire what most would consider a "small boat" that can still maintain a decent distance covering speed in rough wind chop. Big swell never is the problem in having to slow down or get beat to hell, it's always the wind chop, 1 1/2' up to 3 or 4 foot waves. I wasn't against the idea of making a hatch so to speak over a cockpit and peirce thru the waves. Although if it was a hydrofoil, the speed would diminishing thru a wave might be a problem. I understand very little about the hydrofoils. Thanks for the referal to the other threads also, I really appreciate your help.
     
  4. peter radclyffe
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    peter radclyffe Senior Member

    ray hunt designs, the guy who pretty much re-invented the seaworthy speedboat
     
  5. kurtjfred
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    kurtjfred Junior Member

    I was researching historical torpedo info and was quite shocked by the speeds and distance they were acomplishing during WWII and awed by what they are doing today. I am confident they were not using power equivalents of small outboards but HOLY ****! What if you had two small cylinders skegged to a very sharp v hull of about the equivalent size of a kayak and hydrofoiled them to lift the hull clear around 5 knots +- ? Would the two cylinders facilitate (the word escapes me right now) left and right balance? Yaw is not the word I am thinking but balance out the heeling of the craft. Would you need torpedo/cylinder shapes (I was thinking of keeping the fuel and power source below water) or would simple hydrofoils suffice? I have yet to look at any fluid dynamics of the drag on a cylinder below water but it can't be any worse than some of the large deep V hulls of the mid size boats. I am not attempting any break neck speeds though.
    On that note, does anyone know off the top of their head if the drag coefficient is the same in liquid as air? I am familiar with aerodynamics but at times you are considering the compressibility of air which I believe is not a consideration in water.
     
  6. tspeer
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    tspeer Senior Member

    Check out Gold Coast Yachts. They make wave-piercing power cats. A smaller version of this one might be what you need.
    [​IMG]

    I suggest getting a used beachcat sailboat and put a platform on it. They'll do 20 kt under sail, so doing it with an outboard should be possible, too.
     
  7. erik818
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    erik818 Senior Member

    A long and narrow boat with low deadrise, very little rocker and a long sharp wavepiercing fore would do the job. I've tried such a boat, and it gave a smooth fast ride with little horsepowers even in a chop because it cut through the waves instead of crushing them from above. The price, which I should have realised beforehand, was that the ride got very wet. If you add a cockpit to handle the spray the concept should work.

    A long narrow sharp hull is nothing new by the way. It was the way to achieve high speed before powerful motors were available.

    Personnally I dont think ballast is a good idea in a planing boat. The power needed to drive it is approximtely proportional to the weight, so the boat should be as light as possible. If you need a lower CoG for stability it's better to reinforce the bottom.

    I don't have a proper ocean nearby so I don't have an opinion on whether a boat is oceanworthy or not. Sometimes common sence is the key, not the boat.

    Erik
     
  8. marshmat
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    marshmat Senior Member

    Hi kurtjfred,

    I think something like Erik describes could work pretty well for your situation. It will be a wet boat, unless you include an enclosed cockpit, but since you're already going offshore in a 12' inflatable I suppose you're used to wet boats.

    Hydrofoils work very well and can maintain a constant high speed, until you reach the point where the wave height is comparable to the flying height. Then, you have to slow right down to displacement speeds, at which point they just plain suck. I don't think it would be feasible to do such a small boat, designed for such rough conditions, as a hydrofoil.

    The submerged torpedo hull idea- like a SWATH ship- has its applications, but I don't think this is one of them. It would be very hard to achieve reasonable stability in this size with such a design, unless you make the waterplane of the struts so large as to negate any advantages the submerged hulls might offer.

    You are correct that compressibility is not an issue in wate. The usual equations work the same way for a submerged body as they do for an aircraft travelling well below Mach 1, just substitute the appropriate density. (Indeed, it's common to test models of new submarine designs in aircraft wind tunnels.) When operating a surface craft, though, there are of course a heap of free-surface effects (wake, etc.) to consider that tend to be far bigger contributors.
     
  9. kurtjfred
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    kurtjfred Junior Member

    Thanks to all of you for the replies. Erik and Matt - A couple of friends from UC Davis read your responses and think you two are dead on. We are trying to decide if we build from scratch or start looking for something to modify. Thanks again.
     
  10. Commuter Boats
    Joined: Oct 2006
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    Commuter Boats Commuter Boats

    I was curious as to handling of a long powerboat that I felt would be optimum at a speed to length ratio of 2.0 - 2.5, and built this prototype / test model to play with and further my understanding.
    I first drew a double ended 34 footer with a small outboard in a well, then I started cutting cost of the build and maximize the use of 4 x 8 sheets of particle board for the mold and 38 inch fiberglass for the construction and ended up with a 28'x4' that weighs about 1400 pounds and was meant to carry a 15 horse outboard on the transom.
    In order to meet the electrical requirements of a spotlight, windshield wipers, bottom machine, and a couple fans it was necessary to purchase a 25 horse for its 15 amp charge system and I was pleasantly surprised when the boat handled well at speeds up to a speed to length ratio of 4 (21 knots) although the stern was settling.
    Ballasted with water, rocks, gear, and people to somewhere around 2800 pounds it would still run well over 16 knots but really seemed to like 12 knots and burned about 1.2 gallons per hour ( when lighter around 1900 pounds it burned three quarters of a gallon per hour at 12 knots).
    Design and construction consumed one month and with power about $10,000 , it was well worth the education.
    When time and money allows I look forward to the next evolution, it will have a 7 foot beam and I will again try to optimize it for speed length ratio of 2-2.5
    As to rough water handling, this one is handicapped by the CG of the passengers being too high and the engine weight being too far aft and to high, but even so I've seen the spotlight underwater while pushing into them and ridden in the trough of a 35 knot inshore blow.
    I think you can accomplish your goals in a semi-displacement design but because 4 feet is not wide enough you're going to end up with a larger,higher volume boat than you're talking about in order to satisfy stability requirements.
    Both of the running photos are at near 20 knots.
     

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  11. Daniel Noyes
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    Daniel Noyes Junior Member

  12. kurtjfred
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    kurtjfred Junior Member

    Thanks commuter, have a couple of sketches from the last few months that look similar. I am happy to hear of your results, especially the fuel mileage. We have several power plants to work with from small 8 horse up to 30 horse diesel and gas from 5 up to 28, that can be modified and used as inboards. Even have a 20 hp and a 25 hp outboard to work with. We will probably start with the inboards, keeping them very low and some where amidship. An oldtimer that lives a few minutes away towards the bay area said he would modify a few props for us once we decide on a couple of engines. We've been looking at aluminum the last few days, all three of us are experienced welders with no past work with composites so it was our first consideration. Quite a few people are encouraging us to try fiberglass though. Thanks again and if we get something floating that looks similar to yours, I'll send you some pics.
     
  13. Commuter Boats
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    Commuter Boats Commuter Boats

    Daniel Noyes, Thank you. I have a small pile of cartoons ( napkin sketches ) of variations of this design, most of which include a inboard engine.
    One of the most exciting incorporates a 10 hp Sabb with a variable pitch propeller which is presently sitting under a tarp outside. It is drawn at 37' x 6' and my math would suggest 7.6 knots on two quarts per hour.
    I'll try to shrink my drawings so that I can scan and post them.

    kurtjfred, You're welcome, this is a design in progress and I'm open to all ideas. I built the prototype 10 years ago and have only played with drawings on this design since then, I stay busy in repair and occasionally build a somewhat more conventional boat on commission. My other boats are relatively narrow planing designs, the last one was 28'x7' and powered by a 200 hp outboard, my largest is 40'x9' and runs a surfacing propeller with a yammar diesel.
    I've spent a few years lurking around this site without posting much and during that time I've been very intrigued with Fast Fred's pursuit of improving on and modifying for cruise purposes, Atkin's rescue minor. The 37' x 6' that I mentioned above would incorporate Atkin's tunnel stern but I must admit that I've gotten cold feet due to the lack of widespread acceptance of that design.
    A couple years ago I had the pleasure of meeting Steve Dashew and got a complete tour of his Wind Horse ( there's a couple threads here on boat design.net).I need to be careful here in that I made no attempt to put myself in his class but just the same, there are some similarities between the shape of Wind Horse and my prototype commuter canoe,Coho.
    It was while studying the similarities that I was once again struck with how much easier it is to control the relationship between the center of gravity and the center buoyancy as the vessel gets larger.
    I look forward to continuing this conversation and I'm sure that my prototype would have benefited from the scrutiny of the experts on this site.
    I'd suggest you be brave and post your preliminary drawings, the most knowledgeable are gentle and you can ignore the rest, your project will benefit.
     
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  14. garydierking
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    garydierking Senior Member


  15. erik818
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    erik818 Senior Member

    Aluminium is not a bad choice for the boat. The advantage of GRP is that it lends itself better to mass production, but as I understand it that is not a consideration. Unless you go advanced and use a foam core, carbon fibre etc. aluminium will give you a lighter, stronger boat. Why give up the advantage of using a production method you're comfortable with?

    Erik
     
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