Project Solar Boat

Discussion in 'Boat Design' started by drbobbob@aol.co, Nov 6, 2009.

  1. drbobbob@aol.co
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    drbobbob@aol.co Junior Member

    Hello to all,
    I am a newbie, not only to this website, but to the world of boating.
    I am working with a great group kids from Santa Monica HS (called Team Marine) who are building a solar powered boat as part of the Solar Cup competition put on by the MWD (Metropolitan Water District) in So Cal.
    We built a boat last year flying by the seat of our pants and this year we hope to put a lot more thoughtful engineering into our project.

    Your input, experience and advice (which may include referrals to a good psychologist) are appreciated.

    Here is what we are working with. All teams start with the same hull. (pictured below) I would describe it as a 15' flat bottom canoe. The electric motor (Agni Motors 95 series) powered by 2 12v Optima Yellow Cap batteries in series. From the performance graphs, at 24v, it puts out about 10HP at 800 RPM. Weight of the boat with skipper is about 300 to 350 lbs.

    Project 1 is the Drive Train.

    Given the HP / Weight / Hull type we are trying to determine the best propeller Diameter, and Pitch to use as well as what kind of gearing ratio we might be able to use. I have been to some sites with "select the right propeller" calculators, but our motor and hull type are never an option.

    We can go with two propellers as there are two very different races.
    One propeller would be to provide the most power for a 200meter sprint.
    The other would be to provide the best efficiency as we try to see how far we can go in 90 minutes balancing the power drawn by the motor with the the solar panels ability to recharge the batteries.


    I would think that the smaller the angle of the drive shaft, the more forward force produced. (The prop cannot extend beyond the stern, so the max shaft length would be about 6') I realize also that as shaft angle decreases, the diameter of the prop you can use will decrease.
    Presently we have a 12° angle. (Would a flatter angle and the use of twin (smaller diameter propellers be an idea worth pursuing?) I am not sure if we have the means of dropping the shaft at 90° to the bottom of the boat and gearing another 90° shaft so the propeller angle is Hz (or // ) to the boat's bottom. Other thoughts include Duo propellers.


    With respect to gearing.
    Previously we ran a direct shaft to the prop. This year we are considering increasing to a 1.5 or even 2.0 to 1 drive. Would this be too much for the motor? ( I am guessing this would also depend on or change the parameters of the propeller)

    I know this is an odd collection of parameters and applications to work with, (at least it feels that way to me) but we greatly appreciate and help you can give us.

    Thanks
    Bob
     

    Attached Files:

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  2. Guest625101138

    Guest625101138 Previous Member

    Bob
    There are a few basics that are worth considering.

    For the high power run I expect the batteries will limit the performance you can get. It is unlikely to be anywhere near 10HP unless they are very large. What Ah rating are they?

    The best prop configuration will be a large diameter propeller, turning slowly on a curved shaft.

    A pushing propeller is self-aligning to the flow so does not need much support (really none) if it is aligned with flow. Without any support it dangles when at rest and wanders to the side when turning so it is best to have a shaft strut that provides some constraint. The strut can be made very small.

    I use spring steel for curved shafts. They can be designed for infinite fatigue life.

    Marine propellers have evolved the way they are for a variety of reasons and efficiency has not been a primary driver. Hence the typical marine propeller is not very efficient and really unsuited to what you are trying to achieve.

    You are likely to find a large model plane propeller that will be most suited to what you want to do.

    I can do design for you but it is a school project so it is a matter of how much learning you want to get from the exercise.

    The first thing to determine is the maximum power that the batteries can deliver for the 20 to 30 seconds it will take to do 200m. You may be able to ditch the solar panels if that is permitted. The weight reduction will reduce drag. If the batteries have been sitting around then cycling them to about 50% capacity a few times will probably lift their power output.

    The second step is to determine the drag curve for the hull. There are ways to do this that give reasonably accurate results in displacement mode and not quite so accurate at planing speed.

    Third is to estimate the combined power you can get from the solar panels and batteries for 90 minutes.

    Let me know what input you would like and we will see where it goes from there.

    This is one of my boats fitted with a 59gram model aircraft motor geared to swing a model aircraft prop:
    http://www.youtube.com/watch?v=GJedBprmSkk
    It did 12kph in this configuration.

    This shows the drive set up:
    http://www.boatdesign.net/gallery/data/500/V11JE_Drive.wmv
    The motor is a small black outrunner that you can just see turning over driving the two stage belt reduction. I used 12:1 reduction to get the required prop speed using this high revving motor. The prop shown will get efficiency around 85% in applications like this.

    Rick W
     
  3. gonzo
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    gonzo Senior Member

    Weight is the most important parameter that you can control. If you have all that equipment that is the same for all, your skipper should be as light as possible. That alone can make you win.
     
  4. marshmat
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    marshmat Senior Member

    Within a tightly controlled fleet like this, resistance is pretty closely linked to weight.

    In solar car races, weight is so critical that they had to set a minimum weight limit for the driver- 80 kg fully dressed, and if you weigh less, you have to add ballast. If your class doesn't have a similar restriction, training your lightest jockeys to drive is a wise idea.

    Rick makes some good points- he's done quite a few ultra-high-efficiency, low-speed designs and you'd be well advised to consider the suggestions he made above.
     
  5. drbobbob@aol.co
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    drbobbob@aol.co Junior Member

    Great feedback! Thanks! You are correct Matt, the weight of the skipper is set at a minimum, with ballast added to make up the difference. Weight is one of the big issues and a tricky one when it comes to batteries. They are a set to a maximum of ~55lbs.
    Rick, your offer to run some numbers is really appreciated. The set up in your video is simple, but so innovative. I have to check the rules to see what, if any limits there are on propeller location. I am surprised to hear that airplane style propeller would be more efficient than boat propeller.
    I will get back to you with more info. In the meanwhile here is the power curve for our motor.
    Thanks
    Bob
    /Users/drbob/Desktop/Agni Motors :95.png
     

    Attached Files:

  6. Guest625101138

    Guest625101138 Previous Member

    Bob
    Are you permitted to use lithium batteries or is the type set? Can you afford lithium if permitted?

    What is the time scale for this project?

    Unless you are permitted to use lithium batteries the batteries will be the limit rather than the motor on top speed. You need to do power test on the batteries.

    If you provide the dimensions of the boat and the loaded weight for the races then I can produce a drag v speed curve for it.

    Rick W
     
  7. drbobbob@aol.co
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    drbobbob@aol.co Junior Member

    The short answer is no. We are presently using Optima Yellow top batteries.
    12v Cold Cranking Amps @ 0 F: 750 Cranking Amps @ 32 F: 870
    Reserve Capacity: 120 Capacity (C/20 Rate): 55
    We use two in series.

    The long answer as written in DA RULES is:
    "Only secondary (electrically rechargeable) batteries are permitted. Fuel cells, primary batteries, or mechanically rechargeable batteries will not be approved. Each team is responsible for supplying their own batteries. The batteries must be commercially available, lead-acid, unmodified with their weight consistent with the Rules. Craft are allowed to carry a total battery
    weight of not more than 25 kg (55 lb). Batteries must be absolutely stock (as manufactured) in every sense. The battery modules may not be modified in any manner, including the addition of electrolyte additives, case modification; or plate addition, removal, or modification."

    IMO the highest CCA batteries that weigh in under 25kg would be the ideal for the 200m Sprint.
    I am also researching if there are batteries that have a higher reserve capacity for the endurance competition.
     
  8. drbobbob@aol.co
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    drbobbob@aol.co Junior Member

    Rick, The boat is 4.6 meters (15 ft, 1 in) and .81 meters 32 inches wide. pictured above. It is also referred to as the 6 hour canoe. The bottom is flat, the engine/drive train and batteries must reside behind the bulk head which divides the boat into 6 1/2ft and 8 1/2 ft sections. We work to keep the batteries and motor (the bulk of the weight) as close to the bulk head as possible. The skipper must be a min of 40" ~1 meter from the bow.

    Hope this gives you what you need.

    Once again, Da Rules state:
    7.2 Technical Specifications
    7.2.0 If a boat does not meet any of the technical specifications penalties will be assessed by the event officials.

    These are the two rules that might dictate what type of prop design we can use.
    7.2.1 Length - The length of the craft will be approximately 4.6 meters (15 ft, 1 in). Nothing may extend forward beyond the bow of the boat. Only the rudder may extend beyond the stern of the craft.
    7.2.2 Width - Nothing may extend more than 36 cm (14.2 in) to the boat center line beyond the deck edge of the craft at any point.

    7.2.3 Height - The maximum allowable height above the waterline is 1.5 meters (4 ft, 11 in). This height can never be exceeded during the events.
    7.2.4 Depth - No restriction. An excessive depth may make the craft awkward to handle near shore and may increase the likelihood of encountering underwater obstacles.

    Thanks again

    Bob
     
  9. Guest625101138

    Guest625101138 Previous Member

    Bob
    The motor controller will likely have a low voltage cut out around 18V. Aiming for a battery voltage around 19V at full power should give the best overall result for the sprint.

    Are you able to do a load test on the battery or find data on voltage droop with current?

    Do you know if the motor controller has a low voltage cut out?

    Can you advise your project time scale? Is there time for testing?

    Marine propellers are defined by draft constraints and cavitation. They have evolved to be quite inefficient in small craft because power is abundant and fuel was cheap. In lightly loaded application like you have, with no serious draft restriction, an aircraft style propeller will be more efficient.

    This is an example of a model plane type prop in operation with a boat doing around 15kts with less than 1HP:
    http://www.youtube.com/watch?v=jh_RhkejWLw
    The prop used is identical to the black one shown in my video linked above.

    The people who have made curved shaft systems have had difficulty getting suitable shaft material. Spring steel is tremendously strong and tough. It is possible to use other materials but spring steel is the best without going to really expensive alloys. You are likely to need about 3m long that is say 10mm diameter for a shaft and I suggest you get enough for at least two shafts. I do not know size for certain until the prop is determined. If you can locate a suitable spring steel supplier that will supply small quantity it could save time.

    Rick W
     
  10. gonzo
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    gonzo Senior Member

    Is the flat bottom a requirement?
     
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  11. Ad Hoc
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    Ad Hoc Naval Architect

    drbobbob

    If you wnat to understand more about the prop, i would suggest you contact baeckmo or daiquiri. As can be seen here:
    http://www.boatdesign.net/forums/props/prop-doing-bad-29864-3.html
    rick doesn't know how to design a prop nor what cavitation is...he only has a program with numbers, and doesn't design for real. As such doesn't understand what happens on real boats. It is your call, but i would go with beackmo...Rick is just a PlayStation designer.

    You have said:
    "..We can go with two propellers as there are two very different races.
    One propeller would be to provide the most power for a 200meter sprint.
    The other would be to provide the best efficiency as we try to see how far we can go in 90 minutes balancing the power drawn by the motor with the the solar panels ability to recharge the batteries..."


    In either condition, as already noted by Matt, weight and weight and weight. Weight shall be the key to your success. this reduces the length displacement ratio. Reducing this parameter reduces the residuary resistance and trim effects considerably. It also smooths out any humps in the total resistance curve.

    As for prop's:
    Only 3 things to note, the speed of water in way of the prop, the power given, at the prop, and the rpm of the shaft. These will drive the design of the prop. The hardest, shall be the speed of the water, ie what wake factor to use which is related to your hull shape and location of the prop and any appendages. The rest, are details...but are driven by the basics above.

    From the small pic, it looks like your prop is pretty much "open" water, the only other issue is to try and keep the angle of the shaft less than around 7~8degrees. Above this losses are incurred with increasing angle.
     
  12. Guest625101138

    Guest625101138 Previous Member

    Bob
    I have modelled the hull for purposes of determining the drag. The curve attached is based on total displacement of 150kg. The dark line is a best second order fit of the calculated drag shown by the thin pink line.

    There is an inconsistency in information you provided as the battery you nominated has a weight of 20kg alone while you are only allowed a total of 25kg. I have attached some information on the batteries.

    Working on the smaller 38Ah batteries I figure you will operate at around 20A for the 90 minute distance run depending on the level of sunlight. Under these conditions you could expect around 7kts using the APC prop pictured. If you are able to use the larger batteries then you can get a bit more. The APC will still work well.

    The figures in the battery table for internal resistance are very low in my experience. These indicate you can pull a lot of power from the batteries without losing much voltage. You may get up around 4kW. This is much higher than I have been able to get from similar size batteries. You need to determine what they can produce.

    At the high power the APC prop is not the best choice. However before going into this I would like the battery details to be verified.

    You have still not given the time frame that you have for the project.

    Rick W
     

    Attached Files:

  13. gonzo
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    gonzo Senior Member

    What crew weight are you calculating the speeds with?
     
  14. drbobbob@aol.co
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    drbobbob@aol.co Junior Member

    Gonzo
    The total weight of the boat with skipper is about 350 lbs or 160 kilos.

    AH,
    With respect to the parameters you talked about, "As for prop's:
    Only 3 things to note, the speed of water in way of the prop, (we're in a reservoir so it's flat) power given at the prop , and the rpm of the shaft. " (per the motor chart posted about 10Hp or 80 newton meters at 1600 rpm)
    What is your opinion with respect to use of a more airplane style propeller?

    Like on the 190SL forums, another project of mine, there are some strongly differing opinions. Now I'm just the guy that walks in a bar to ask for directions. So please, if I am getting differing information, nobody be offended if I don't take sides.http://www.boatdesign.net/forums/images/smilies/smile.gif

    Rick
    My bad, I originally listed this HP at 800 rpm. I am not sure if this changes your calculations. Also, from the pict of the APC prop, I could not tell what model, size, pitch etc you were suggesting.

    Sure I may sound like a broken records, but we really appreciate this help The kids are getting more and more pumped with the prospect of coming in with some designs and application never used before.
     

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  15. gonzo
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    gonzo Senior Member

    That would be a 10 kilo skipper
     
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