Mount Seagulls to Cat

Discussion in 'Multihulls' started by rosbullterrier, Jan 10, 2010.

  1. rosbullterrier
    Joined: Feb 2005
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    Location: Cornwall, UK

    rosbullterrier Junior Member

    I'm building a twin hulled power for our next Seagull Race. There are some very helpful design ideas for much larger engined boats but mine needs to be a lot more like a light weight sailing catamaran I think with such small engines.
    A main difference for the regs is it needs to be a displacement boat. Although for another race the possibility of planing.
    One question is placement of the engines. What would be the difference of one mounted directly behind each hull; two nearer the transom middle in clean water; one on the inside of each hull further up towards the middle?
     
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    How did they define planing speed?
     
  3. garydierking
    Joined: Sep 2004
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    Location: New Zealand

    garydierking Senior Member

    If you mount the engine anywhere other than behind the hulls, you need to either build a fairing in front of the engine or a fairing on the engine leg itself like this one: http://homepages.paradise.net.nz/garyd/outboard.html
    Modifying the engine like that may not be legal for your race though.

    Gary
     
  4. Guest625101138

    Guest625101138 Previous Member

    Andy
    Is there a length constraint.

    I did the attached drawing for a 4HP racer capable of almost 20kts.

    Rick
     

    Attached Files:

  5. gonzo
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    gonzo Senior Member

    What was the top speed?
     
  6. rosbullterrier
    Joined: Feb 2005
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    rosbullterrier Junior Member

    Rick, that hull looks amazing. There is no length constraint, but bearing in mind this not as technically serious as the Bermuda/New Zealand races, I thought a two sheet length of 16ft would be practicable.
    Gonzo, the rules state: 'Open to any NON-PLANING hulled craft with a crew of at least two, using only a TRADITIONAL (twin legged) Seagull engine(s) for propulsion'
    Now this river speed limit is officially 6kn, last race I was doing about 11mph without being disqualified - so they accept that. The rules were changed a few years ago to ban planing boats with 5R engines because of an accident. Its more of a carnival atmosphere.
    So, if I make a hull which is semi displacement and keep the speed down, the same rig could be used in other more open races which are starting up.
    Gary the info I am looking for is whether the prop is best behind the narrow hull or clean water. I appreciate the leg fairing but only the gearbox is immersed with an anti ventilation plate fitted directly above the prop.
    The engine placement question was possibly to determine the configuration of the hull(s)
     
  7. rosbullterrier
    Joined: Feb 2005
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    Location: Cornwall, UK

    rosbullterrier Junior Member

    There are three classes:
    A, Canoes and Catamarans
    B, Boats 12ft and over
    C,Boats under 12ft
     
  8. Guest625101138

    Guest625101138 Previous Member

    Andy
    If you use a long slender hull then a single motor in the middle would be OK. It allows you to get the heavy bits closer to the middle of the boat so lower pitching moment.

    If temperature is not excessive you can bog up around the leg with chockfast so it fits neatly. I have attached some photos of a pedal drive leg that mounts into the bottom of a boat just forward of centre. There is a keel post in front of it and it is faired into the keel.

    This 8m long boat has got to 6kts under pedal power. Imagine what 3 to 4HP could do on a skinny stabilised hull. I have got to 10kts in a 7.2m hull with 630W.

    Rick W
     

    Attached Files:

  9. Guest625101138

    Guest625101138 Previous Member

    I would go one main hull with stabilisers and use 4 sheets on one hull for 32ft rather than 2 sheets on 2 hulls.

    For a given displacement a long monohull will do better than a cat if you are not length constrained.

    Rick W
     
  10. rosbullterrier
    Joined: Feb 2005
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    Location: Cornwall, UK

    rosbullterrier Junior Member

    Ok I'll find another bigger power single Seagull engine. They come in either 1.8hp or 5.8hp (genuine power) Shame, I was looking forward to multi engines!
    Is the pedal power motor fitted into a unit which is then fitted/removable from just forward of the boat centre? It slots through a watertight hole in the hull? I presume this allows you sit behind and your combined weight is central.
    I could fabricate a similar arrangment, the motor could be just behind me. I suppose this would be neccessary in a long lightweight hull, and would allow a streamlined stern. All that would be below the keel would be the gearbox to skeg. An entirely unique setup!
    I might chicken out at 32ft. How about 24ft?
     
  11. Guest625101138

    Guest625101138 Previous Member

    I would need to do some calculations to determine the relative merits of 24 to 32 but with 6HP and the likely weight of 2 people plus boat it will be better at 32ft.

    A boat that is long and slender like this does not look that big. If you do not put a lot of buoyancy in the ends then structural loads are not high.

    The attached picture shows the engine room in the pedal boat. You can see the feet on the cranks. The drive leg fits into a well formed in what I refer to as the torque tube. Without a cabin the motor could be mounted in a well between the bottom and the deck. Something with tapered sides and ends that it wedges into with nice fairing on the bottom. From memory the parts do not get to boiling point so a cast block will handle the temperature.

    If you make an all-up weight estimate of the boat, gear and people I can compare what you will get with 6HP in different lengths. Lets say the hull and stabilisers is 40kg.

    I occasionally see trailers with a number of sculls loaded for transport to rowing competitions. The eights break in two but I think the fours and smaller are transported in one piece. There are probably legal limits. Have seen boats to 8m on top of cars. I have carried 7.2m on a medium sedan.

    Rick W
     

    Attached Files:


  12. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    I understand that they state "non-planing". How do they define it? Unless there is a definition of "non-planing" anything goes. Racing to a rule means that you design the fastest possible boat allowable.
     
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