Monohull verses Multihull powersailers / motorsailers

Discussion in 'Motorsailers' started by brian eiland, Aug 8, 2004.

  1. icetreader
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    icetreader Senior Member

  2. I need a stronger engine in my boat. Or, more hot air in me.
  3. sharpii2
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    sharpii2 Senior Member

    Multi more efficient than Mono?

    Hi edaydesign

    The answer to your question depends on what you intend to use the boat for.
    Judging by the way you worded your question, I'll assume you are talking about speed. There are three criterias of speed and all are based on the length of the boat at its waterline. These speeds are multiples of the square root of the waterline in feet expressed in knots.
    The first is called displacement speed and that is approx. 1 to 1.34 * sqrt WL. For a boat that has a 9ft WL that would be 3 to about 4kts.
    The second speed is what I call sub-displacement speed and that is usually significantly less than sqrt WL. With our 9ft WL dinghy that would be, say 1.0 to 2.25 kts.
    The third speed is what I call super displacement speed and that is from say
    1.5 * sqrt WL on up (usually to 2.0 * sqrt WL). So at this speed our 9ft WL dinghy would be doing 4.5 to 6.0 kts or better.

    All these three speeds are based on wave making. With sub displacement speed, very little energy is spent on wave making. That is why rowing dinghys and container ships almost always travel at this speed. It requires the least hp (hourse power) per ton. At this speed the vessel with the least wetted surface area (WSA) wins. And because a monohull has less WSA per ton it is usually the best option for this speed. If you are rowing a few hundred pounds of provisions out to your yacht with this 9ft WL dinghy. This dinghy had best be a Monohull.
    At Displacement speed a lot of energy is spent making waves but not so much as to make it prohibitively expensive in energy use. at this speed our rower of provisions would be working his guts out but would be able to do it if the natives were wading in after him. This is the most contentious speed for multihuls vs monohulls. If the boat is heavy for its length, the monohull is better (our provision laden dinghy). But if the boat is modrate weight for its length and is powered by sail or carries a cargo that makes it top heavy (like a multi deck pasenger ferry or our 9ft dinghy with just the sailor and his sweet heart on board day sailing) The multihull is the better choice. Our 9ft WL dinghy would be able to carry more sail and/or carry it longer as a multihull than it would be able to as a monohull even if, as a monohull, it had the same width (Beam). For this reason, it would be able to reach and maintain displacement speed over a wider range of wind conditions without its crew having to go to uncomfortable contortions such as hiking over the side.
    At super-displacement speed there is no contest. The multihull, because its hulls are narrow and sharp, make smaller waves than a monhull with the same Beam. Here, light weight is essential. If the multi is light enough, it should be able to easily reach these speeds under sail while being rather easy to mannage. A mono can reach these speeds (only if its light for its length) under sail if it has either a very deep ballast keel or a long hiking board. If its very narrow, it can power through its bow wave just like the multi. If its reasonably wide (to carry the huge rig it would need in even light airs without hiking or a ridiculously deep keel) it would have to plane. That means it would have to climb over its own bow wave and that requires a lot of energy, so it needs either a large rig or a strong wind or both.
    When I was comming of age, light Mono sail boats were designed to the IOR rule and had pinched ends. Because of their light weights, they often planed. But when they did, they were squirrely and hard to control. In those days. a cheap multi could easily beat an expensive mono of the same length. Now light monos are designed so they can plane much more safely and they have much bigger rigs and deeper (and sometimes shifting) ballast. Now it takes an expensive multi to beat an expensive mono of the same length under sail.
    The reason I spent so much of my explaination with sailboats is because they are always designed to use minimal energy per given performance. In an age of cheap, portable, energy (an age not likely to occure again in our lifetime and is now fading fast) it was economical to build heavy planing power boats (at about 40hp per ton). But now, the times, they are a' changing.
    In the near future, I expect to see more and more multis in the top two speed ranges and monos relegated to the slowest speed range and/or the heaviest displacement for length (such as rowboats, tug boats, large cargo ships, and boats needing ultimate stability). (Alas, you multi lovers. Monos, like the poor, will always be with us.)
    There is yet another difference between multis and monos which is worthy of considering. Ant that is that a mono has accumulative rolling where a multi dosen't. For this reason, some are considering multis for even slower speeds as motor sailers (see 'The Concept Catamaran Project' thread).

    I hope I have answered some of your questions.

  4. edaydesign
    Joined: Dec 2004
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    edaydesign Yacht Designer

    future trends

    Thanks Bob for the reply!

    Ill tell you a little more of my project.

    So I am trying to bring the marine industry into a more sustainable world. Speed is a major factor in my projected market. A modular interior will be designed to make the vessel more friendly to be refitted down the line instead of wasting alot of materials in this "refitting" process. The overall size of the vessel will be around 55'. Now for the tricky part, detemining the power source. I too believe that the cost of energy is only going up and up. what I am percieving is generating clean alternative responsible fuel sources. What I read from your reply, if the vessel i am creating is lighter and if you want speed, then go with the multi hull. Was this correct???? One possibble solution to this "lightness" could be sail power (as you mentioned) but what if there is no wind? You cant go back to your alternative propulsion, powered by diesel.

    *****what if this vessel was powered with the use of electricity. Sortof like a diesel-electric motor to turn your props. Instead of a diesel motor to supply an electric current you used a fuel cell powered by hydrogen. These fuel cells are tremendously smaller(and LIGHTER) than diesel motors and only emmits h20!! the storage tanks for the hydrogen are actually smaller-to the same size as diesel tanks.

    Am I still looking at a multihull or a mono??

    Getting enough HP should not be a problem, just look a locomotives. These trains pump out HP and torque to pull tons and tons over a mile long.

    One might say that the infrastructure is not in place so it may never work. But this is not like the automotive industry and infrastructure, because the marine industry does have to worry about roads. refueling stations can be very easily installed at marinas all over. Hydrogen can also be manufactured at these facilities very cost effectively too.

    Sorry for getting off track....

    Also among my research I have all the power specs needed to run a 68 Searay. It uses a pair of 1540HP Catapillar diesels a total of 3008HP for propulsion, 27.5KW Generactor for AC and a 500 amp intermittent DC load. I know that i will not need all of this power to run my 55' but hopefully it will get me started.

  5. sharpii2
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    sharpii2 Senior Member

    Hi edaydesign.

    Thanks for the kind words!

    Your right in your last question. A multihull is the superior choice to meet your criteria (light weight + speed).

    Your propulsion sytem, using H2, Is, I believe, unrealistic. The problem with H2 is that it comes as a gas, not a liquid. It must be compressed by several hundred pounds to get any range at all. This would mean heavy, expensive, high pressure cylinders for your fuel tanks.

    The alternative is biodeisel which is made from recycled frying oil. This way you can use conventional deisels into the forseable future.

    The only snake in this paradise is that the fuel will be very expensive, say, 5 times the present price adjusted for future inflation. In view of this likely reality, I think it best to plan on having all the super displacement speed happen under sail, and all the motoring be at sub-displacement speed, say, 5kts in your case. In this case, you would need 1hp per ton. And your vessel would displace about 12 tons. You would need about 12hp per hour/16hp per hour per 1gal. of deisel, or about 0.75 gal. per hour. And you may be able to do much better than that.

    By going to a hybrid (deisel electric) system, you may be able to wring out more effeciency. The deisel can be sized for the average power requirement. whereas your electric motors can be sized for your peak requirements. This might get you down to 0.5 gal per hour.

    So lets say you want a power range of 1,000 nm. We take 1,000 nm and divide it by 5nm hour (5kts) and that gives you 200 hours. Now multiply that by 0.5 gal. per hour and that gives you a tankage requirment of about 100 gallons. If you wanted enough power to do your sailing speed of, say, 20kts, you would need 20hp per ton or about 10 gallons per hour divded by 20kts (your new speed) times 5kts (your old speed) times 100 gal.(your old tankage) to get 500 gal. (Your new fuel tankage). In short, you would need five times as much fuel. This is , of course, asumming a dead calm for the entire trip (which, I think, is as it should be).

    As I hope I have demonstrated, There is a high penalty in fuel usage for trying to go fast under power, even with multihulls.

  6. DiverDown
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    DiverDown Junior Member


    If you want to look at an interesting powercat,goto,advanced search for power cats.There is a 65' aluminum Prometa,Santorini.

    From memory: 7000 litres, 15 knots top speed,cruise 12 knots(range 4300 nm).I read somewhere,using one engine at 8 knots gave a range of around 7000 nm.

    For drives, I've often thought a surface drive would be good.It would have to be propped for lower speeds of course,and would never act as a true surface drive,always driving submerged.There are two types from what I've seen: the steerable type,and the non steering type with a rudder behind, like this:

    The steerable type may seem a little complicated,but yachts have hydraulics anyways,and trawlers go in long straight lines.With easy access,carry a couple spare cylinders,lines, and oil.

    The main advantage would be,with a dual engined vessel or cat,the ability to lift one out of the water so as not to drag when running economically on one engine.Another:not having rudder skegs,and prop shafts and all that stuff interrupting flow and possibly leaking, several feet under water.

    I'm sure there are other qualities.

  7. brian eiland
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    brian eiland Senior Member

    Diesel-Electric Systems

    Forget the hydrogen fuel cell for the moment and concentrate on the diesel-electric possibilities....but not in the 1000hp plus units for pleasure boats. Have a look at the real good article in Dec/Jan issue of Professional BoatBuilder on the subject. And pay particular attention to the DC systems coming on line in leu of the AC systems.

    If you don't get this magazine apply for a trade's well worth it if you deal with boats
  8. brian eiland
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    brian eiland Senior Member

    I looked quickly at this vessel and noted that supposely it is powered by a single diesel engine. It doesn't appear to be so from the looks of the wake and the central hull nacelle. Do you know where there is more detail on this vessel?

    On a multihull trawler type I think you would have an EXTREMELY difficult time making a surface acting prop work. It would, as a result of the props location and pitching of the vessel, be constantly immersed too much and then not enough which would subject the prop and its shaft to huge short peak loadings.
  9. DiverDown
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    DiverDown Junior Member


    It definitely is twin engined,with two 6 litre 135 hp Perkins Sabres.I have a magazine article from Multihulls World.Not other info online other than yachtworld selling info.
    Personally, I wish it was a little wider with an interior helm.
  10. brian eiland
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    brian eiland Senior Member

    Santorini power cat

    What issue of that mag was it in? I will be next to them at the Miami show and will have a closer look.

    Don't know why you would want a space eating interior helm in this modern day of hand held controls, and considering her covered helm already, unless its because of your cold Canadian air.

    And if you get her too wide you limit your berthing choices. No need for really wide unless you want stability for a sailing rig. Looks to me as they have a slender enough hulls so as to not get a peaking of bowwash in the bridgedeck area, as well as a central nacelle to try to limit wave slap in this area.

    Santorini power cat engines:
    And six cyl diesels, very good choice. Do you realize that the six cyl configuration is the most inherently balanced configuration for an internal combustion engine
  11. brian eiland
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    brian eiland Senior Member

    Trawler Stability verses Motorsailers

    I've always appreciated the seaworthy look of trawlers, but often I must remind myself that they are subject to rolling at sea....sometimes precariously. With this in mind I happened on to another thread on this forum that I added a photo and a few comments. Have a look at this photo

    And maybe this comment if your interested, This reported incident was a serious situation which could have resulted in abandoning the ship for many owners....just another reason I like the concept of motorsailers over trawlers for REAL ocean going boats.
  12. DiverDown
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    DiverDown Junior Member


    After digging for 15 minutes...I found the article,which I had taken out of the magazine,no date or number.Would be from spring or summer 2002,they offer reprints of articles-they'll know.

    Some quick info for you: Length 20m.,Beam 7.4 m,Light 20 tons,Loaded 28 tons,11knots at 1500 rpm with 4200 n.m range,7000 l diesel,consumption at cruising speed 1.65 l per n.m(both engines)

    So,18.15 l per hour at 11 knots,385 hours to empty is 4250 nm. If one engine is off,would burn 9 l an hour,run 777 hours and go how fast/far? 7 knots=5450 nm, 7.5 knots=5800 nm, 8 knots= 6200 nm. If it will do 8 knots, thats enough to get direct to Australia from L.A with a little reserve. You'd carry extra.And top up in Hawaii anyways.Beats the heck out of sailing,and cheaper too.

    Being indoors and climate controlled while enroute is so much easier and less tiring than being buffetted by wind (hot or cold)rain or sweating all day.An indoor helm needn't take up much space. I like the option of choice.And yes, those outflow winds straight off the glaciers,even in August, can put a real willy up your back.
  13. Guillermo
    Joined: Mar 2005
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    Guillermo Ingeniero Naval

    Inboards converted to outboards

    I'd like to know how you convert inboards to outboards. Can anybody help here?


  14. mattotoole
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    mattotoole Senior Member

    I'm intrigued with diesel-electric too -- especially for catamarans. There are a few things I wonder about though. What is the duty cycle of the genset, and how does it compare to its equivalents? Also, what about high voltage (144V) DC systems in a marine environment? I'm sure it can be handled with good engineering, but does the insurance industry and Coast Guard agree (esp. if you plan to carry passengers and hired help).

    Perhaps this ought to be in a new thread...

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    FAST FRED Senior Member

    "but does the insurance industry and Coast Guard agree (esp. if you plan to carry passengers and hired help)."

    The Cruise Ship industry and the Staten Island ferry use electric drives with no CG or insurance co hassles.

    Its ALL on the shelf , just the price is a shocker compared to a simple diesel and std shaft setup.

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