Only one steerable engine on small power multihull?

Discussion in 'Boat Design' started by massandspace, Nov 16, 2018.

  1. fallguy
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    fallguy Senior Member

    Well done Sam Sam.
     
  2. Ilan Voyager
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    Ilan Voyager Senior Member

    There are plenty of steering systems that can work. Each with this pros and cons.
    Think about another system , rather than dismounting in pieces, if you do frequent trailering. If the work is too long, tiring and/or difficult, the boat will stay useless on the trailer. Nobody wants the pain of totally dismantling a catamaran after a Sunday on the water.
    Often the motor multis have a width of 50% or slightly less of their length. That makes the structure far easier to design, as the stresses are kept moderate. There no need of a wider beam as the stability is largely enough for a motor boat. The sail multis are in a very different league.
    It's essential to keep "displacement" multis light because to be efficient ie fast with minimal power, the hulls must be slim, the mandatory feature. This fact limits their underwater volume.
    A good ratio length/beam is 10. The optimal as we have calculated and tried being 12. Over the gain is marginal, except in some spacial cases where you want a minimal hump and/or you're designing a trimaran. Under 10 the performances degrade vastly, 8 being the extreme limit with very poor performances compared to the 12. (that doesn't mean it's bad but that you have made an architectural choice of ratio length displacement and speed). Under a ration of 8, better to go to a "planning" monohull, 24X8 simpler to design, build. and trailerable with no thrills.
    In multis length is speed, there is no doubt. If you do not get enough displacement at a given length, the simpler is to design a slightly longer hull than to begin to torture the shape. And keeping it light. It's less expensive at short and long term than bigger engines, if you want a "multi" speed..
    Good displacement motor multi hull shape are pretty simple: prismatic coefficient as big as possible (very easy on slim hulls) somewhere around 0.60 to 0.70, center of displacement at 65% of the length, rocker very moderate, that looks at first sight like a sailing multi hull but noticeable differences for the trained eye: the hulls are fuller, the sterns larger and flatter, less rocker, the consequence is that the center of displacement is at around 60-65% of the waterlength and not 52% like on a sailing multi which has the defect to sink their sterns in the water when motoring, dragging tons of water. In fact that looks a bit like a Tornado hull with fuller sterns.
    For the spray rails I never went to the complication on multis to make the hulls with spray rails included in the design like on a planing monohull, I simply used molded plastic spray rails (there several in the market) bolted and glued with 3M 5200 to the hulls. These spray rails are not mandatory but are very convenient to impede the water of climbing along the topsides and by directing the water outside and down keeping the boat dryer.
    On a 28 feet around 1.3 metric ton loaded (very feasible if it's a day boat) 10/1 ratio beam streamlined hulls you can expect around 20 knots with 2X60HP engines and carefully tuned propellers. I insist on the tuning of the propellers, displacement multis are not monos in that chapter.
     
  3. Richard Woods
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    Richard Woods Woods Designs

    a 28ft 1.3T powercat that can only do 20 knots with twin 60's is a very poor design. Our live aboard 28ft Skoota can do over 16 knots empty (1.8T) and 13knots loaded to 2.4T. And that is with twin 20hp

    Written on board Skoota
    RW
     
  4. Ilan Voyager
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    Ilan Voyager Senior Member

    I've always been extremely conservative about speed claims.
    Speeds made on a mirror sea, no wind, no current on short distances are of little use in real world navigation. In such conditions a 28 feet 2X60HP (largely overpowered but you'll understand why infra) 1.3 tons is able of more than 25 knots on GPS over 1 good hour.
    Small displacement boats even very slim have limits, physics are stubborn and power requirement exponential. They are more limited by the length of hulls than by the weight within some limits.

    If you want more speed with the same power and similar weight make longer hulls and keep them slim. A bit of care about wet surfaces and you get interesting results.
    A 40 feet catamaran of 5 metric tons 240 HP twin gas outboards gets around 23 knots, a 70 feet trimaran of same weight and power with a Yanmar LH4 will get 28 knots.
    Interestingly the same 70 feet trimaran at 6.5 tons will lose only 2 knots, and will have a cruising speed of 22-23 knots very close to the max speed of the 40 feet catamaran.
    Part of the gains come from a better propeller (HB propellers are notoriously too small in diameter, and are not the best for efficiency), and a steady diesel engine with big torque able to stay permanently at 90% of the max power. The rest comes for the longer and slimier hull with a very small drag hump.
    The tri is more seaworthy and can cope with hard conditions. But it has a less livable surface and thus less nice amenities than the 40 feet catamaran. In fact it's a long corridor.

    Coming back to the 28 feet 1.3 ton, I note that in ideal conditions a planning flat bottom monohull of same length and weight will go faster than the displacement catamaran, with a 1X120 HP outboard, but if the sea has a few small waves the boat will slam so badly that the skipper has to slow down. maybe faster but a slower cruising speed. It has also the inconvenient of a hard bump before planing.

    Normal conditions of navigation are almost never ideal. And bad conditions happen often and suddenly. At least in Brittany.
    So I prefer to claim the following assertion: in a 2+ feet choppy sea, a 15-20 knots breeze, maybe a little tide current, a common sea state condition in my reference of navigation which is Brittany, the usable top speed that you can get every morning without stressing the structure, being fully loaded and without overusing the engines is around a big 20 knots. It's maybe underrated but it's obtainable in a wide range of conditions of sea and wind.
    With 60 HP engines, apparently a bit too big indeed, you'll get a better mean speed cruising speed. There is no gain of max speed searched over the 50 HP outboards but more to get a rating of 45 HP usable over long periods, so the cruising speed will be optimized on this rating. That gets a sustainable cruising speed well above the cruising speed of a 50 HP. You'll get also fuller torque and power curves. The difference of weight between a 50 and a 60 HP is negligible. The same between the 100 and 120 HP outboards, which have similar weights but the the 120 has 20% more HP for the cruising speed with generally a better specific consumption.
    The close examination of the curves of power and torque are primordial, some engines may be unsuitable.
    Better cruising speed in normal condition is more important than top speed in optimal conditions in my opinion.
    So it seems more interesting for me to use the working boat logic; to choose the engines and to tune the propellers for an optimized cruise speed with the best consumption for that speed in the most prevalent conditions. This cruise speed has to be well above the hump, thus staying in the flattest part of the drag curve and not being affected by small variations of speed. I mean not falling down in the hump of drag when slowing a bit.
    Thus the boat will be able to return safely to the harbor with a 30 knots wind and 6 knots tidal current in the nose as there is more low end power and a flatter torque curve. These conditions happen too often in the Channel and more in the treacherous Sea of Iroise. The boat will be able also to have eventual overloads and the boat won't become too sluggish. You can use the boat rather safely in difficult conditions.
    Forcefully that working boat logic sacrifices the top speed, it's not its goal.
    In different sea conditions or with diesel engines the calculations and obtained speeds will be different.
     
    Last edited: Dec 1, 2018
  5. Richard Woods
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    Richard Woods Woods Designs

    I understand what you are saying. But my comment is based on personal experience with my Skoota 28 powercat which I have had for 6 years and travelled over 6000 miles. At first in British Columbia and then later from Texas to Florida and on to the Bahamas. We have crossed the Gulf Stream to/from the Bahamas four times now at an average of 11 knots, taking just under 5 hours anchor to anchor. Going to the Bahamas we are always fully loaded with food staples for three months and fuel for 250 miles. We also have a rigid sailing dinghy, a kayak, a bicycle, a sewing machine etc on board so we are not light.
    see here


    and here for empty boat but before the engines were properly trimmed, hence the bow up trim


    RW
     
  6. SamSam
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    SamSam Senior Member

    Thank you for nothing, meaning in that I didn't actually hardly do nothin'. Thank you though. I just re-clarified the push-pull cable system that Mr Wood mentioned, vs the pulley and cable system that was being imagined. I had a question framed up for him on how do you rig it up and go about connecting two cables to a single helm, as I couldn't recall ever seeing that before, and as I was googling to help make myself appear knowledgeable, lo and behold somebody had already solved the problem perfectly, or as Dad would say, slicker 'n' snot on a broomhandle.
     
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  7. fallguy
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    fallguy Senior Member

    I rather enjoy the back n forth between Richard and Ilan. I think they are agreeing with each other.

    Of course, I am building a Wood's designed cat, so there is bias.

    The cat I am building is a 32'.
    L/B is 11.307 or so. If anything it is less beam, I never did check to see if this was before we moved the moulds in. If not; it would he higher ratio. This, of course, was my primary concern with the design and Richard did not need me to tell him as he was more the teacher.

    The power recommendation (from Richard) for the 32' boat is 2x70 (yammie 4 stroke), which is also closer to what Ilan is suggesting for the smaller boat, relatively speaking. Of course, there are some serious tradeoffs for this massive bump in speed and efficiency. The hull cabin size shrinks to a single bed rather quickly.

    We did briefly discuss going to say a Suzu 90 2 stroke iirc, but the weight tradeoff was awful and I suspect they'd be louder by much.

    The available loading for the Skoota 28 is not as big as the 32, of course, but the 20hp engines are fine because I am sure the 28 has very narrow hulls.

    And the ultimate point is for the OP. If your hulls are too wide a/o heavy; you'll have trouble attaining efficiency and speeds with the 2x20.
     
  8. JamesG123
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    JamesG123 Senior Member

    I doubt the OP is still around.

    Its amusing that guys will come on here asking for opinions and when they don't get the ones they were looking for, will argue defensively and then disappear in a huff when they can't convince everyone of their brilliance...
     
  9. Richard Woods
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    Richard Woods Woods Designs

    I forgot to say that you can use twin outboard steering onto one push/pull wheel (double cable wheel unit of course). However you need to put the cables in from opposite ends, and that means moving the gearing about 1/8th turn after putting in the first one and before fitting the second. Bit of a fiddle but it does work even though the suppliers say you cannot do it and it voids any warranty

    There is a good pdf here

    http://www.jfaughn.com/logs_other/pictures/2011/oct/Installing Steering Cables.pdf

    and some youtube videos as well

    RW
     
  10. Ilan Voyager
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    Ilan Voyager Senior Member

    Richard, I love your Skootas. Simple, roomy and efficient.
    Simply the goals are different. Yours is the most economical cruiser at a very good speed.
    Mine is the fattest cruising speed possible without disintegrating the structure, plus the most reliable and the safest possible in that you can name dangerous conditions. Sun, force 3-4 winds, 10 feet waves and tidal currents up to 6 knots are the marks of a day of nice weather, everybody is on the water. With that conditions, in Mexico the boats less than 40 feet are forbidden to go in sea...
    You take the same way as the liner fishing boats of the Ile de Sein, which have 200 HP on 27-29 feet hulls built like tanks to cope with the local conditions. The necessities and means are totally different.
    fallguy. ordinary 2 strokes on yachts are useless. generally noisy and too much thirsty. For the same power 35 % more consumption than 4S and direct injection 2S. no good for cruising.
    The outboards Yamaha Enduro 2S have a TBO (time before overhaul) of about 600 hours while a 4S is more than 1600 hours (the TBO are nominal, a well cared engine can get more). That gives you an idea.
    For cruising boat needing reliability simplicity of maintenance and quietness a 4S is preferable. There are plenty in the market.
     
    Last edited: Dec 1, 2018
  11. fallguy
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    fallguy Senior Member

    That guy is still here. He might even be checking out his hydrostatics now.
    Can I find yours on the internet? I think the boat I am building is well capable of the 'nice days'. Where are you having the sunny day 10' seas.
     
  12. JamesG123
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    JamesG123 Senior Member

    Hasn't been on the site since day he posted this thread and got told it was a bad idea.
     
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  13. Ilan Voyager
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    Ilan Voyager Senior Member

    I've been extremely busy. No I have nothing in Internet as I have retired before the wide spread of Internet, also I was engineer working in teams where ego has no place. Yes your boat is able of the nice days but if the Mer d'Iroise (the Iroise Sea see the map) was your normal place of navigation you'll put beefier engines. You'll understand with the following documentation as I do not want to be accused of exaggeration.
    It's a small sea enclosed between 2 lines of islands and lots and lots and lots of reefs. There are 76 days in the year with winds above force 7 and 49% of the time the wind is force 4 and above. Good for sailing, but the problems are the tides ranging from 6m 20 feet to 10 meters 30 feet giving currents until 8 knots in high coefficients. As the sea is rather shallow the conjunction the wind and tides gives enormous seas disproportionate to the wind. As the fetch is small the sea becomes fierce in hours. There are more than 400 documented shipwrecks from 1780 until nowadays.
    Iroise Sea - Wikipedia https://en.wikipedia.org/wiki/Iroise_Sea
    Île de Sein - Wikipedia https://en.wikipedia.org/wiki/%C3%8Ele_de_Sein
    Read it...
    Some illustrating videos:
    The tide current at the Raz de Sein (in the south right of the map) by nice weather so the kayaks are having fun. Look at 25s. All the pass between the land and the island is like that. so you have to pick carefully while to cross the Raz in function of the tide's hours.
    https://www.dailymotion.com/video/x2kfgmc
    --Abeille Bourbon is a high seas emergency tow vessel (salvage tug), 80 meters (260 ft) with 22000 HP. The video is in a wind of 35 knots and look at size of the waves while the big tug more than 100 feet high is getting out to high sea Abeille Bourbon stays until 8 days out in the storm to assist the ships entering the Channel in direction of the north

    --The frigate Latouche Fréville is 139m long 456 feet, and 39 meters 129 feet high above the water. The video was taken in 2007 in the passage de Fromveur (high left corner of the map) during an ordinary winter storm. Admire...
    https://www.dailymotion.com/video/xpw93t
    --last, short but nice the 60 feet trimaran Fujifilm passing the Nividic light house during the 2002 Race of the Light Houses beginning of July if I remember well. very fresh wind but note that there is only 1 reef. We are far from a storm. I love how Fujifilm goes swiftly into that sea, Peyron and his crew are true pros.
    https://www.dailymotion.com/video/x2p7a8

    A crazy fisherman of the Ile de Sein going to fish sea bass by force 6. These guys fish alone on 28 feet boats, as a local tradition says that never 2 members of the same family will be in the same boat One dead in the family is enough in case of accident. Even by nice is choppy because of the tides.


    Have fun with the vids. Happily there are nice days with good navigation only you have to be very cautious. After all 51% of the time the wind is under force 4.

    Carte_mer_d'Iroise.png
     
    Last edited: Dec 7, 2018
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