Electric Row Boat conversion update...

Discussion in 'Hybrid' started by sparky_wap, Jun 1, 2008.

  1. sparky_wap
    Joined: Mar 2008
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    Location: Suffolk, VA USA

    sparky_wap Junior Member

    Today I had the chance to open-up the electric row boat with the surface prop to see what she could do. The fixed prop shaft comes out near the bottom of the transom. With standard prop, I experienced ventilation at almost all power levels.

    So...

    I had an 8 X 12 surface prop handy, ballasted all the weight I could forward and gave it a try.

    I collected all the data I could and it looks like the surface prop does not have enough 'bite'. Running through the numbers, the surface prop had about 60% slip at low speeds and up to 77% as I approached (s/l) of 1.5.

    I think the problem was as the speed goes up, the stern drops and the prop drops down too far. Since I can't adjust the trim, the surface prop might be out of the question. If she got up on plane, the prop should have been about the right height.

    Next I will try a standard 3-blade 10-11 prop and try to ballast the stern and make an anti-cavitation plate. Any suggestions? With the weight shifted aft, the top of the prop sits around 5" below the surface. If she planes, I'd expect the stern to rise at least 3" from static.


    Joe
     

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  2. Guest625101138

    Guest625101138 Previous Member

    Joe
    Can you give us some basic data on what you have and what you want to achieve?

    Surface props will only get efficiency around 70% maximum and will give the best overall performance above 40 knots. The reason being that you can eliminate the drag from underwater appendages.

    What motor power have you got?

    What is the total displacement?

    The efficiency of a prop goes up with the apsect ratio of the blades providing the loading is in design range. If you have no constraints on diameter you can easily achieve prop efficiency above 85% and get close to 90% if loads are not too high.

    The shaft may be the cause of the prop ventillating. It will create a rut in the water that may be bleeding air to the prop. Fairing down the leading edge of the shaft will help streamline flow to avoid a deep rut.

    An old outboard leg may do the best job for you. I have used an off-the-shelf right angle drive. This allows me to swing a big diameter prop without angling the shaft:
    http://boatdesign.net/forums/attachment.php?attachmentid=20726&d=1209288392
    The test boat shown does 6 knots before the two 12Ah batteries get into their power limit. The little batteries give a useful range of 7.5km at 7kph.

    Rick
     
  3. sparky_wap
    Joined: Mar 2008
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    Location: Suffolk, VA USA

    sparky_wap Junior Member

    Some Specifics...

    Rick,

    The displacement is around 750 lbs and about 1/2 of that can be shifted fore/aft (myself and batteries) easily. The two 6.5HP PMDC motors power a single shaft via chain drive. The shaft runs 80% of the motor RPM. Max motor rpm is 4000 at 48 volts. I will never achieve 48 volts due to losses. Assume 44 volts and 3700 rpm.

    The chart from today's data is shown. I actually ran her up to 7 mph briefly but didn't collect data. I'd like to see her hit 20 mph and cruise around 12 mph on plane.

    The length is about 12.5' at the w/l. The bottom is about flat with very little deadrise. There is quite a bit of drag from the Keel(s) but 13 hp should plane her with the right prop. I am limited to a prop diameter by the depth of the water leading to the main lake. I have to use oars to row out the first 1000 feet or risk damage to the prop. Eleven inches is pushing it.

    At 7mph, she felt like she was close to planing but the prop was slipping way too much.

    I never thought of fairing the shaft as it is entirely under water from the transom back. It could help...

    I'm covering the outboard idea with a separate project.

    Look at the picture of the DBD set-up. I though about doing this with an automotive half shaft and a heavy duty cv boot. I bet it could be done for $250 and some parts lying around the basement. Would be crude but might solve the ventelation problems.

    Joe


    Joe

    drive
     

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  4. Guest625101138

    Guest625101138 Previous Member

    Joe
    You certainly have the power to get it to plane. You now need to deliver it efficiently.

    I have only done enough playing with surface props to know they do not act as foils. They are effectively pumps that jet water backwards. I gave up on developing my knowledge on them when I realised how poor the best efficiency was and the fact that I do not see myself doing 40 knots with low power applications. The blades are different shape to submerged props.

    I think you will have difficult planing unless your prop sits below the transom. You might be able to get a bit higher using a "cavitation" plate from the transom over the prop - a partial ducted tube.

    An outboard would give you the ability to tilt it up to inch along in shallow water but sit down for deep water operation. Having two motors makes it complicated though.

    Rick W.
     
  5. sparky_wap
    Joined: Mar 2008
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    Location: Suffolk, VA USA

    sparky_wap Junior Member

    Prop height

    Rick,

    I can adjust the angle of the shaft slightly up and down to allow more or less exposure under the transom. Right now, the center of the shaft is a couple of inches below the transom when looking aft from the bow under the boat. It is about 9 degrees down form the water line. I was hesitant to lower the shaft much more because the keel provides some protection now. The pictures below should give an idea of the set-up but it has been refined since.

    If the larger 3 bladed 10 x 11 submerged prop provides no better bite, I might abandon the inboard set-up and switch to the 1970 Johnson 20 with a single 14 HP forklift motor. It's a tiller so I would need to rig the steering to match my current set-up. I will also need a really good prop to match the monster torque and lower rpm of the series motor. Gearing the motor up might not be enough.

    I must be doing something right because one of the neighbors saw me open it up and reminded me that there were no gas engines allowed on the lake. I told him it was electric powered and he looked pissed off and went back inside. I offered him a ride and he wouldn't even look at me. Guess he figures I was full of it.

    For my calculations, I am using a lift to drag number of 5 and assuming a prop eff of 50%. At 12-14 mph, I expect to be fully on plane with about 7 HP at the shaft. Do you think this is a good estimate for planing speed, drag and prop eff?

    At one point, I had a nice 11 x 10 3 bladed submerged prop installed. This prop had a good seat-of-the-pants feel and the motor loading numbers looked good. Too bad for me the prop ended up on the bottom of the lake due to poor Engineering Judgement on my part...

    Joe
     

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  6. Guest625101138

    Guest625101138 Previous Member

    Joe
    Your performance estimates are conservative. You have plenty of power if you can deliver it and the boat has reasonable trim.

    This is a good site to determine planing power:
    http://illustrations.marin.ntnu.no/hydrodynamics/resistance/planing/index.html
    You do not need to get too concerned about the last three parameters.

    Javaprop is a good way to determine the best prop but it takes a bit of learning.

    I think the outboard offers the best option. Find the biggest diameter prop that will fit the unit. The best result should come from using the reduction in the bottom of the leg with the biggest diameter prop that will fit.

    Rick W
     
  7. sparky_wap
    Joined: Mar 2008
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    Location: Suffolk, VA USA

    sparky_wap Junior Member

    Outboard progress

    Rick,

    Since the lake is too low to get the boat out now, I am working on the outboard conversion. The Johnson 20 HP donor O/B looks like it is in great shape even though it is almost 40 years old. One question, the original power head rotated CW looking from the top of the motor while the electric motor I have, rotates CCW looking from the top of the motor. I know the gears in the lower unit will work in either direction but will using the 'back' surface of the gear teeth cause damage? I can switch the rotation of the (Series hyd pump) motor but it is not easy. (Yes, I know enough to be dangerous) The series motor will have at least double the torque of the 20 hp powerhead so maybe I just answered the question myself.


    The Johnson 20 hp OB now has a 9" x 9" prop with a 1.75:1 drive ratio. There is no way to fit a larger diameter prop on the lower unit without some serious modifications that I don't want to do. My options seem to be from a 9" x 7" to a 9" x 11" prop. Based on the power curve of the motor and the lower unit gears, I should use the highest pitch prop available. The motor is rated 10.4 HP @ 1670 rpm @ 36V. I have the ability to go to 72V with my controller and battery pack if necessary. A series motor will run much higher rpm when lightly loaded so I am building an electronic rpm limiter. The total boat weight should be about 750 lbs with just me in the boat.

    The guy trying to use the s28 magmotor in the other post will run into some serious thermal issues if he keeps the power on for more than a few minutes. Those motors are not vented and are only rated for a few hundred watts continuous use. I looked at those motors a few years ago when I first started this project.


    Joe
     

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  8. Guest625101138

    Guest625101138 Previous Member

    Joe
    Good to see you are pushing on. I will be surprised if you do not get a nice result with this set up.

    The prop is reasonable. Definitely go for the 11" pitch.

    My calculations are suggesting 15kts should be achievable with the motor doing 4200rpm. The power about 9HP. The power estimate is very rough. I am taking drag as 1/8th of the weight plus a bit. Is OK for a reasonable planing hull. I expect the motor will handle this speed but brushes might not like it.

    Under these conditions the 9 X 11 prop will get efficiency around 75%. That is quite good.

    Running the gears in reverse should not hurt the gears but I am not sure about the change mechanism. It may have a better dedent for going ahead although I have driven small outboards hard in reverse without them jumping out of gear. If you have to get into the motor windings to reverse it then it would be reasonable to do some initial testing with it without rewiring. If it jumps out of gear then you can surmise it does not like the vertical shaft spinning the wrong way.

    Using these small props on a reasonably light boat will limit the gear loading. The motor will just wind out until the power demand reaches the applied power. Providing the motor does not throw its windings. It is rated at 1670rpm but do you know its safe maximum speed.

    I would be wary of pushing the voltage too high. Sure the current is the main constraint but if you are spinning fast at high current then the brushes willl take a hammering. By doubling the voltage you are making it a dragster rather than road car. The former is not intended to last. 10.4HP should be ample to give you a thrill.

    Keep us posted.

    Rick W.
     
  9. tinhorn
    Joined: Jan 2008
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    Location: Massachusetts South Shore.

    tinhorn Senior Member

    At what point might it make sense to consider smaller dual props?
     

  10. sparky_wap
    Joined: Mar 2008
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    Location: Suffolk, VA USA

    sparky_wap Junior Member

    Dual Props

    My original plan had dual props from trolling motors and two submerged dc motors. It was working great but I overheated the motors due to over current.

    I have considered using two 8" props with one on each of the 6.5 HP dc motors. I have the props (LH and RH 8 X 8) and might try on a different boat.

    I'm working on the outboard project now to use on the row boat as I can tilt it out of the water until I row to deeper waters.

    I'm looking to get the boat on plane and to max out around 20 mph. Should run all day at 5 mph but will only run on plane for 20 minutes before draining batteries. I'm on an electric only lake so I decided to see how far I could push it.

    A couple of weeks ago, I alarmed a neighbor with the inboard set-up because he was sure I was using a gas motor. The gps read about 8 mph and the prop was ventilating so bad I couldn't go any faster. It's amazing how much wake a row boat can make at that speed.
     

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