Modular Cruising Catamaran

Discussion in 'Multihulls' started by ImaginaryNumber, Jul 19, 2009.

  1. yellowcat
    Joined: Feb 2007
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    yellowcat Junior Member

    Hi Frenette
    That's a french name ... if i can give an advise from my grand father who was near engines all his life, it will make you deaf.
    If you are doing long distance, diesel is still the way to go. New stuff will come out but it is too early. I am planning to house 2 x 90 hp mercury outboards into the hulls. They weight 420 pds each. I would like to use electric propulsion boosters to get the cat planning or on foils assisting the outboards . The idea is to get the combustion engines running at their best rpm ratio. I intend also to recup as much as possible the heat generated by this type of engine. My goal is to also be able to use those engines 2 or 1 at a time for wakesurfing on an intergrated design into the large cat design. Sounds crazy, but doable. We are now experiencing on various boat shapes and power.
    I see a potential for the propane etc gas outboards if they get large units out , those schiste gas wells can lower the cost of fuel and better weight to btus. It may seem out of context for this thread, but outboards are a big part of modular designs.
    My sailing areas will be more like the maritimes (shippagan NB for kiting) and Bahamas (i wont tell). Also on the Richelieu river and St-Laurence river for some must marinas stay for work ... we will see about that ... i have to get out to fix the docks, water flooded and wood logs are sitting and banging with the waves. One is so fr... heavy, no one wants to hit that at 30 knots ... floods bring all sorts of things, we got ourselves a new propane tank and few bumpers ...
    cheers
     
  2. yellowcat
    Joined: Feb 2007
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    yellowcat Junior Member

    by the way has anyone test the app boat sentry ?
     
  3. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    I've seriously considered a wide, heavy displacement trimaran, such as the one designed by Len Surtees for Tristan Jones, except I would want the floats to fold back against the hull, for ease in docking or haul-outs. More complex, more costly, but still an idea...
     
  4. Deering
    Joined: Feb 2005
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    Location: Juneau, Alaska

    Deering Senior Member

    Yeah, you have a lot of competing demands for this boat - liveaboard in cold climate, tropical living, big blue-water passages, and now possible Northwest Passage...you forgot to mention the ice-hardened hull you'll need for that.

    Responses:

    Head - outfit the boat with a marine head. It will meet your needs in most places. If you travel to a location where a composting toilet is the preferred solution, buy one there and install it. Go with the solution that is most universal. I assure you they have marine heads in Hawaii and Mexico too.

    Cruising by sailing - I live here. I can tell you that you'll miss an enormous amount of cruising opportunities waiting for the just-right winds. Many of the best locations will be completely off limits with that strategy. And cats are notoriously poor light-wind performers (too much wetted surface) so that will limit you even more. Did your sailing proteges' use multihulls?

    Insulation - 4 inches of foam might get you an R-value of around 20. That's pretty good. But you'll still need an energy source to heat it.

    Drying out - easier to do with a monohull. With a multihull you need two (or three) level spots for your hull to rest on. Good luck finding that here. Frankly, I don't see the point. Bring a good tender with you and let that dry out while you're ashore. BTW - you didn't mention your tender, and it'll be one of your most important features. The rocky beaches here are murder on tenders, so you need to think this one through carefully. I had an aluminum-hulled RIB on my last boat and it was terrific.

    Trimaran - one with folding amas would maybe meet your transport requirements. But bear in mind that it'll be a dog sailing in light winds in SE Alaska. Equip it with a good motor...

    I suggest you come up here and charter with someone to get a better sense of the lay of the land before you make any final decisions.
     
  5. groper
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    groper Senior Member

    That is a very inaccurate statement.

    Most well designed modern multihulls will sail very well in light winds, sailing faster than any comparable use monohull.

    Your powercat, has too much wetted area for efficient cruising at displacement speeds, compared to a monohull- because it was designed as planing powercat and its displacement to length ratio is poor. Its has fat hulls to provide sufficient dynamic lift area and aspect ratio, so it will plane once enough power is applied.

    Efficient displacement catamarans such as sailing cats, should have a good displacement length ratio, and if they need to carry a heavy payload, then they really should be made longer to preserve the favourable ratio. The same applies to monos or any hull, but stability is a problem for a single very narrow slender hull. With the narrow hulls, comes the ability to exceed the hull speed without excessive power to overcome the hump. Narrow slender hulls don't generate a hump to climb over and they don't have a "hull speed" like froudes law predicts for fat hulls < 6:1 length beam ratio.

    Efficient displacement cats also need to have considerable separation between hulls to avoid the wave interference drag - another reason planing powercats exhibit poor efficiency at slow speeds.
     
  6. Deering
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    Deering Senior Member

    True enough. But if you look at IM's design criteria, he is proposing a heavily-built liveaboard sailing cat with the ability to carry 6 months of provisions on it and flat-bottomed hulls. If I recall his target length was in the 10M range. This boat will be a poor performer in light winds.

    At the risk of opening up the whole debate of a few pages ago, isn't it true that a catamaran of equal displacement to a monohull will have more wetted surface? And at low speed conditions, the dominant form of resistance is skin friction rather than wave resistance?

    Your points regarding power catamaran design are all true, though typically light-wind conditions do not adversely affect power boats - quite the contrary. Regarding hull spacing, what would be the optimal hull spacing for a displacement powercat? Leo Lazauskas did a study of catamaran hull spacings at various speeds and the results weren't what I expected: http://www.cyberiad.net/library/multihulls/solar1/solar.htm

    And finally, I am unaware of any sailing catamarans that live in Southeast Alaska, though there may be one or two outliers. I am aware of a few 'former' sailing cats which have had their masts removed...
     
  7. groper
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    groper Senior Member

    I have no idea on the suitability/practicality of a sailing catamaran in Alaska - i accept your reasoning on that as you live there. Light wind performance can be good tho.

    My comments as to performance, were equally for the OP as much as you or anyone else reading this. I use Leos program to model catamaran configurations so i can quantitatively see what effects the design parameters have. I remember your thread about your cat as you wanted to modify it so it performed more efficiently at slower speeds. But im sorry to say, theres not alot you can do as you cannot alter your hull spacing, nor the hull width... so simply extending the transoms and adding aft run wont do much and hardly worth it. You really need a completely new design if efficient displacement cruising is your goal.

    You will notice in the solar race link you provided, the mono had the least resistance... but do you understand why? The length of the mono was 20ft, the waterline beam was 15inches... do you think you could make a workable cruising boat with such a ratio??? The mono in the solar race analysis, was a hull identical in form, to a single catamaran hull we see in performance cruising catamaran reality - that of efficient displacement catamarans that is :) The stability of such a slender hull makes them impractical for anything but an olympic rowing shell... If you increase the beam of the mono hull to eliminate the stability problem, then a catamaran or stabilized mono (tri) becomes the most efficient configuration.

    I admit i havnt read IM requirements, all i can say is that if he fails to understand what makes a good multihull efficient, then he too will have to live with poorly designed boat for his intended usage. At the end of the day, the fuel and energy bills determine how often your able to use the boat and how long you can stay at sea.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Adding buoyancy and length helps to increase the L/D ratio and so long as the stern isn’t dragging i.e. trimmed, it’s a no brainer, it’ll be an improvement. This method has been done so many time on so many different vessels. I’ve employed many times when asked for advice on a poor performing hull. Last one I did was on a 65m RoPax vessel, worked a treat. Its simple naval architecture.

    This is just a L/B ratio of 16….slender but nothing out of the ordinary. Many fast ferries are in the 12-20 range depending upon their application. There is nothing “wow” about this at all.
     
    Last edited: Sep 5, 2013
  9. Deering
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    Deering Senior Member

    Groper, since you are very familiar with Leo's program perhaps you can tell me what the optimal spacing would be for a 15M catamaran in the 12-15 knot speed range. I seem to recall attempting to model my plans on his program to determine the effect of hull spacing, but was unable to obtain usable results. Leo stated that my hull configuration was outside the parameters of his program, but I confess that I don't recall the specific reasons why, though I think it was due to the fact that the L/B ratios were outside Michlet's ranges. He was talking about releasing a new program but I haven't seen it yet.

    I've built an exact half-sized model of one of the current hulls and tested it with various bow and stern extensions, and the results were exactly as the math predicted, with a very significant improvement in displacement performance. The extensions yield a L/B ratio of over 10, and a DL of 59 compared to the original 232. But I realize that a real-world model is probably inferior to a computer model in many ways...

    What I did not test was a model with both hulls, so the impacts of hull spacing remain unclear. Since Michlet is not suited to provide a definitive answer, can you please provide me with the mathematical methodology you used to determine that my efforts would be fruitless? I'd like to fill in the gaps in my knowledge, which I acknowledge are many. Thanks.
     
  10. groper
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    groper Senior Member

    Of course it's an improvement, but is it worth it..? In commercial vessels, the fuel savings pay for the modifications in short order, but for private vessels one has to burn a hell of a lot of fuel before the break even point is reached in order to pay for the modification.

    Finally we agree on something...
    Tho I'd like to see a personal/recreational cruising monohull in context with this thread, with a 16:1 length beam ratio... Do you know of any under 100ft? That would be "wow"...

    Deering, I can run the program for you but I'd need to know more info, such as hull length, hull waterline beam, draft, block coefficient, prismatic coefficient, displacement, etc. if you don't know all this, we could assume a generic shape and simply compare hull separation alone. Displacement and length would be enough to get a reasonable idea...
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    A monohull no. It would suffer from extremely poor stability...very limited use, such as rowing shells...but cat's, plenty.
     
  12. Deering
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    Deering Senior Member

    Groper, don't have the coefficients that you outlined, but the hull has very straight, flat runs, flat, nearly vertical sides...it's a block, thought the aft 14 ft have the bottom of the hull sloping up to the WL. For the hull with proposed modifications:

    Displacement = 37,000 lbs
    Length at WL = 52 ft
    WL beam = 5 ft, possibly a bit less depending on where the draft ends up at
    Draft = ~2 ft

    Fuel economy is but one objective. Increased speed is another - running at 8 knots is tiresome. If I can increase my cruise speed by 50% or more, that's worth a lot of money in itself. I also get a larger working deck and a more comfortable ride out of the deal. My current cost estimate for doing this work is about $50,000, with me building the aft extension myself. That aspect of the project is quite simple.

    Thank you for your assistance.
     
  13. groper
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    groper Senior Member

    Godzilla predicts the optimal spacing for your hulls @ 35ft, waterline edge to edge, so 30ft centerline spacing @ 13.6kts nominal speed. This is hardly a practical size for you id imagine... The above results in no negative wave interference at the design speed.

    So i think you might be more interested in what the difference is in terms of non optimal hull spacing ie how much difference will it make in terms of horsepower or fuel consumption to what you already have? If so, what is your hull spacing?
     
  14. groper
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    groper Senior Member

    I assumed a demihull centerline spacing of 4 meters until you get back to me...best guess based on the images ive seen of your boat...

    Here is the file so you can view the results via michlet - you will notice that the wave resistance is a significantly higher proportion of the drag around hull speed compared to viscous drag derived from wetted area. You can also see that the added resistance from negative wave interference alone is between 0.7-1.9kN @ ~5-7m/s (9-13.6kts) respectively. To equate to added engine kW, multiply by speed in m/s and then add prop loss (probably around 30%). So at 6m/s or around 11kts, you need an extra 1.9*6*1.3= 14.8 engine kilowatts just to account for the negative interference wave drag alone. Theres more interesting data in there aswell...

    Michlet file, model based on your parameters, ended up a touch over 16,000kgs displacement --> View attachment in.mlt
     

  15. Deering
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    Deering Senior Member

    Centerline spacing is more like 2.8 meters, I'd have to take actual measurements from the boat.

    The added drag due to wave interference is interesting. But what I'm most interested in is how that would differ from the current hull. The current hull, unextended, has a WL length of 9.5 meters and a displacement of 14,060 kg.

    14.8 kW is not an unacceptable penalty.
     
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