12ft Cat for Solar Splash Competition - first time tips needed

Discussion in 'Boat Design' started by JacobMUSI, Oct 13, 2021.

  1. kerosene
    Joined: Jul 2006
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    Location: finland

    kerosene Senior Member

    50lbs is not light. A rowing boat capable of carrying 600lb lbs of cargo weighs that much.

    I take it this one is optimized for the fast speed event as it will be quite bad for the distance event.

    Now it isn't my project but here is what I would do.
    (read some basic yacht design book, I have done this but you should hit the library. Principles of yacht design is famous. Nature of boats is easier but not technical at all. Skene's book is simpler than the principles)
    Download a copy of freeship. I have not vetted the link but it can be found in many places for example here:
    Download Free!Ship 3.43+ https://www.softpedia.com/get/Science-CAD/Freeship-Plus.shtml

    Create a design as long as feasible (within reason). If your power output is about 200w then you can go up to 20' but bit less will do. Round bottom, reasonably flat ion the middle with some beam to give stability. 5" draft at the deepest point. Create the design from XPS. Good vacuum is essential. DIY hot wire (with good ventilation) might be a good idea. Use bondo to smooth it out, cheaper than epoxy. Once straight roughen out a bit as a prep for glass.
    Cover the bottom in single layer of glass and epoxy resin.
    Make the battery compartment as deep a hole as possible ie. egainst the bottom. Add a layer of glass inside here.
    Off you go.

    Can't recall the solar array size - if it is reasonably large then consider outriggers.
  2. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Jacob; Kerosene is a knowledgeable guy. He is telling you that the design you have posted might be appropriate for the speed event but it is very bad for the endurance event. I am also suggesting the same bad news. In addition, the design is not the best even for the speed event. That is because you are shackled with a low horsepower and high weight situation. A vee bottom is not the best go fast design for low horsepower and too much weight. A flat bottom is much better in that regard.

    If you want to compete in the endurance event your straight keel design is a sure loser. You will need to get the transom up, or at least on or above the waterline in order to reduce drag from the immersed, gurgling, transom. That means that you will need some rocker in the bottom. How you design the rocker curve is a large determinant of whether the boat will plane or not. Several of our members have suggested that you do some reading in Dave Gerr's book: The Nature Of boats. He explains the critical way that aft half beam buttocks influence planeing performance. There is more...............>>>.You are drawing boats that have a forward profile that raise the bow in a rather sexy curve. Sure enough there are hot rod boats out there with that sort of profile. Those boats have huge horsepower to burn. That is not what you need for this contest. Put as much boat length in the water as is practical. That will get you some lesser dynamic drag when at low speed in the endurance part of the event. Try to design such that the plus and minus acceleration of the displaced water is as low as you can make it. That'll get you in the ballpark for good performance with very low power.

    When sufficient power is applied, the bow will raise such that there is an angle of incidence at the front end of the planing surface. (called the stagnation point) Swoopy forward profiles are not useful for this particular purpose. A good start is to have the forefoot ( the bottom of the stem) just barely touching the water surface or perhaps a little bit below the Waterline. That will help the boat go in an economical straight line when you are going slow. Plotting a curve of areas will give you the general idea of minimizing the acceleration of the displaced water. Presenting a wide and flat surface to the water at planeing speeds is advantageous if maximum speed is the aim. Yes you can kinda sorta compromise between going slow and far, and going fast and not so far.

    Concluding: If the
    contest points favor one or the other of the tests then go for that part of the test. If on the other hand, the points award are equal, then you have to do a design with the best compromise.
    portacruise likes this.
  3. kerosene
    Joined: Jul 2006
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    Location: finland

    kerosene Senior Member

    Uffa Fox did Ankle Deep as his (at least more or less) last boat. The idea was that at slow speed the transom is pretty much at water level diving decent slow speed resistance. As speed pics up and bow rises the rear flat area becomes a planing surface. Something like that could be a decent compromise for both events.
    Maybe shifting the battery between events to change the angle of attack.

    here is a thread with some illustrations that I have copied here:
    Ankle Deep launch by Uffa Fox http://forum.woodenboat.com/showthread.php?243000-Ankle-Deep-launch-by-Uffa-Fox

    ankldeep.jpg Ankle deep chine1.jpg
    Ankle deep chine2.jpg
    portacruise likes this.
  4. Jimboat
    Joined: Feb 2002
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    Location: Canada

    Jimboat Senior Member

    Looks like a fun project, and design challenge.
  5. portacruise
    Joined: Jun 2009
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    Location: USA

    portacruise Senior Member

    If you have pictures and specs on last year's winners, maybe use that as a guide for your design? Then you might tweak it a little by making it lighter, assuming that's possible, to see if that improves things on your practice tests. If not, you can always add back some weight or move things around in the right places, and have something competitive. The torque output can be tweaked using a Quick Change pulley gearbox with idler for the different events (so you don't have to carry a heavy spare motor on board) and prop size and pitch can also be tweaked to fine-tune it, RC model props are usually the cheapest way.
    Last edited: Nov 1, 2021
  6. Waterwitch
    Joined: Oct 2012
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    Location: North East USA

    Waterwitch Senior Member

    Some teams seem to be refining their previous years entry.
    kerosene and portacruise like this.
  7. portacruise
    Joined: Jun 2009
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    Location: USA

    portacruise Senior Member

    That video looks a lot like kerosene's sketched boat...

    What is the minimum speed for the endurance event? If it is under 3 knots or so, a shorter Hull could be used? Or is winner based on the distance covered within a (12 hr?), Or other fixed time period?

    Oops, just saw rules- it is distance covered in 2 hours, so no short hulls....
    Last edited: Nov 2, 2021

  8. S V
    Joined: Jan 2019
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    Location: Lithuania

    S V Junior Member

    I think the good way to go could be removable hydrofoils. On foils when speed event and no foils when endurance or hullvane type for endurance. I searched the rules with Ctrl+f there is no mention abaout foils, so they are allowed? If you go catamaran - single foil at centre of gravity or hysucat/hysuwac style could work. For monohull some 1960s style US navy wings could work also. Btw, you dont need to raise all the ship out of the water, just a fraction of it. In that way you will go away with simple propulsion system.
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