Project Log - New Keel, Who Dis? (Pearson 26 to Electric Cruiser)

Discussion in 'Boat Design' started by Jeff in Boston, Apr 23, 2021.

  1. Jeff in Boston
    Joined: Sep 2020
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    Location: Boston

    Jeff in Boston Junior Member

    The Requirements:
    • Electric power for quiet and serene propulsion.
    • Capable of 6mph with electric power. This is the maximum speed in the harbor I am usually in.
    • Capable of carrying 8 adults for a cocktail party.
    • Can be helmed from the rear of the cockpit so the captain can converse with the passengers while underway. This is possible due to the quiet motor.
    • Use solar power to avoid having to plug the boat in somewhere.
    • Has a head (toilet)
    • Has room for a 6 foot tall person to lie down comfortably, in relative darkness.
    • Trailerable to avoid marina fees.
    • Shallow draft to be able to get in close to shore.
    • A canopy to stay out of the sun.
    • Less than $6000.
    The Solution:

    The desire for a good berth, head, and rear helm rules out any trailerable powerboat I have ever heard of or seen.

    This led me to a used sailboat. They are cheap, helmed from the stern, efficient at low speed, have a head, and have places to lie down. I could take off the mast and put on a canopy of solar panels!

    The hard part was room for 8 adults. Most sailboats in the trailerable range handle 6 in the cockpit. And then I found the Pearson 26 One Design. It has a beautiful and comfortable 8′ 11″ long cockpit. Just one problem: A three foot tall 2200 lb cast iron keel.

    A related problem was the fact that they didn’t usually come with a trailer and if they did it was designed for the full keel.

    No problem! Hold my beer and I will unbolt the old keel and put on a new shorter one!

    This is my project log. I expect it will be more effort and money than I had hoped, but I still expect it to work.

    Hint: If you want to do something similar to this, and are okay with a standard sailboat cockpit stick ONLY to shoal draft sailboats that come WITH a trailer. Your wallet and sanity will thank you!
     
  2. Jeff in Boston
    Joined: Sep 2020
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    Location: Boston

    Jeff in Boston Junior Member

    Status:

    Keel removed and waiting for new trailer to arrive for transportation.

    I'd add pics, but waiting for gallery upload review.
     
  3. Jeff in Boston
    Joined: Sep 2020
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    Location: Boston

    Jeff in Boston Junior Member

    The Question of Boat Stability:

    I'm aware of the relationship between metacentric height and roll period.

    I know that when most sailboats lose their mast they tend to have a very uncomfortable snap roll. By removing the mast and the keel I'm completely upsetting the current balance. And reducing displacement.

    To correct this I will be adding a 4" x 4" x 120" keel. I will then put the boat in the water and test the roll period with a stopwatch. I expect I will have to add ballast to increase the displacement, and I will need to use the roll period to tell me to add ballast high or low in the boat.
     
  4. Jeff in Boston
    Joined: Sep 2020
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    Location: Boston

    Jeff in Boston Junior Member

    Pictures:
     

    Attached Files:

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  5. Dejay
    Joined: Mar 2018
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    Location: Europe

    Dejay Senior Newbie

    Interesting project. Do you know what she weights without the keel?

    I wonder if you could stabilize a sailing monohull by adding some amas. Or even just one ama. Maybe could even be something very lightweight like two kayaks or canoes that can be detached easily. Could add some space for solar panels too. The connection could be part of the canopy maybe.
     
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  6. Jeff in Boston
    Joined: Sep 2020
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    Location: Boston

    Jeff in Boston Junior Member

    3000 lbs without the keel.

    I was thinking of adding amas, but I think it would complicate the trailering process too much.

    I think I will be able to have enough solar on a canopy over the cockpit.
     
  7. SolGato
    Joined: May 2019
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    Location: Kauai

    SolGato Junior Member

    How are you planning on powering it?

    Given your budget and speed goals, I think you are going to find that the propulsion, battery and solar system needed to achieve your goal of 6MPH for a hull with that much displacement loaded with that many people will put a big dent in your budget if you want it to be safe and reliable.

    If you could live with 4MPH, it could be done with off the shelf components that, but it’s still going to cost a good chunk of change.
     
  8. Rumars
    Joined: Mar 2013
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    Location: Germany

    Rumars Senior Member

    I would wait with a new keel design until I would have the exact weight and distribution of the new propulsion. You will need a lot of batteries, and with your budget it's likley they will be lead, wich are not only heavy but also bulky, and it will affect trim, your ballast might not end up where you think now.
    It's also possible that you don't like the boats behaviour in waves, and the best cure would be bilge runners, and maybe a only a short skeg for directional stability, instead of the long keel you plan now.
     
  9. Dejay
    Joined: Mar 2018
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    Location: Europe

    Dejay Senior Newbie

    According to hullcalc.xls you need 2.5kW for 6 knots, 1.17kW for 5 knots and 0.5kW for 4 knots (for 2000kg and 7.9m length and 2m beam). I believe these numbers are a bit optimistic. You could try prelimina.com.

    If you want to stay below $6000 you'd probably need to find some repurposed cheap electric motor because the ~5kW electric outboarders are already pretty expensive. You could retrofit one on a broken outboarder.

    Or is this a boat with an inboard?

    If you're fine with 4 knots for the most part you could do maybe with 2kWh battery? LiFePO4 costs you something like $450/kWh. Not sure about current prices. You probably need more, depending on how far you want to go, and on how much solar you can fit and want to rely on.
     
  10. Jeff in Boston
    Joined: Sep 2020
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    Location: Boston

    Jeff in Boston Junior Member

    Sadly, I've already blown past $6000 due to the lack of trailer and needing to buy a new trailer as used ones are rare at this size. See note above in bold about this being a bad idea on anything other than an *actual* trailer sailer. :)

    The guy building the trailer thinks I need a keel to center the boat on the trailer and the boat was designed to rest on a stiff keel, so I'm thinking I should have one, but will keep it on the light side. Maybe 200-300 lbs. I've seen similar keel / trailer setups on lobster boats.

    No inboard. I have a 6 HP gas outboard to start with for the first year. Will look at moving to electric after the family uses this for a year and the approval of the admiral.

    My calculations at 5000 lbs (which I hope to stay well under depending upon stability) is 4 mph at 1200 watts, 5 mph at 2500 watts, 6mph at 6400 watts. I think 6 MPH is unrealistic, but 5 MPH is certainly doable on a brushless trolling motor.

    I'm leaning towards the Haswing brushless trolling motors. It seems likely that I can do 4 mph to 5 mph on one, and the videos I've seen back that up. - Haswing protuar 5 hp electric - YouTube
    The Caroute brushless motors have a well known speed regulation problem. The biggest Haswing Protruar is $800 at 2500 watts.

    I know that the speed is limited by the prop, so I may have to play with that. Also, two motors would be nice for redundancy and ability to spin in place.

    For batteries, my options are:

    * lithium (light weight, powerful, light enough to remove for charging in the house )
    * lead carbon (very tolerant of partial charging)
    * locally sourced used AGM (fairly plentiful on craiglist)

    As I don't need to go too far (initial trips will be about 5 miles at most) I figure I can get away with 2 to 4 KWH. For lead carbon, I can do that for under $1600.
     
  11. Jeff in Boston
    Joined: Sep 2020
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    Location: Boston

    Jeff in Boston Junior Member

    Huh. I'm thinking that the bottom of the hull is unusually V shaped for a sailboat. Maybe that is enough to keep it centered on the trailer? That would lower the draft, cost less, and move faster.
     
  12. SolGato
    Joined: May 2019
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    Location: Kauai

    SolGato Junior Member

    One problem with going by estimations based purely on displacement, weigh, thrust, and power with regard to speed is that it doesn’t take into account prop slip and cavitation.

    Just like a Hot Rod, you can have a big motor that makes a bunch of power, but if you can’t get that power to the ground you won’t be fast. Small tires will spin out while big tires bite and launch you forward.

    The props on most trolling motors are very small and often when used on large boats like yours, especially when also used to steer, you will find they will start to suck air and slip in the water lowering your overall top speed. Adding a cavitation plate over the top of the prop can help a bit with this, but only so much.

    If this were my build I would seriously consider an inboard electric motor with a propshaft like a sailboat once everything else is proven.

    With the right prop, you could see the speeds you had originally hoped to achieve, and the system would be safe and reliable and up to the task of moving a boat load of people around in a variety of conditions.

    Check out Thunderstruck’s brushless motor kits for reference:

    Sailboat Kits & Accessories https://www.thunderstruck-ev.com/electric-sailboat-kits-and-accessories-inboard-motor-ev/

    And you mention a well known issue with Caroute motors, are you referring to a hiccup?
     
  13. Jeff in Boston
    Joined: Sep 2020
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    Location: Boston

    Jeff in Boston Junior Member

    I would probably use an E propulsion Navy 6 before I did an inboard:

    Navy Series Electric Outboard Motor | ePropulsion

    The props on the trolling motors I'm looking at run 10 to 12 inches in diameter. These are pretty hefty motors, much larger than you would put on a bass boat.

    The thunderstruck stuff looks great and I would totally look at that option if the boat was set up to use an inboard.
     
  14. kerosene
    Joined: Jul 2006
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    Location: finland

    kerosene Senior Member

    Are these "net" watts, as in propulsive power required? Of the shelf trolling motors - even the bigger 24V ones - are generally optimized for very slow speeds with their propeller. Propeller efficiency is quite bad at higher speeds and on the other hand making a higher pitch custom propeller easily over burdens the motor and can damage it.
    Using 50% as a ratio for motor input watts (usually the power they post) to propulsive power is on the optimistic side. 1st of all I don't know of 2500W trolling motors. Torqueedo maybe but the more typical ones are less than 2,000W - input power.

    But even going from that 2000w to propulsive is bit sad unfortunately. 1st you should use multiplier of 0.85 to 0.9 (good brushless) for motor and controller losses. You are now at 1700-1800W. Then 50% is likely optimistic prop efficiency. With the higher RPM it is a challenge even with a custom prop to get to 70% efficiency.
    So now we are at 850-900W propulsive power with std prop to 1100-1200 with a custom prop.

    Electric is compelling but being realistic about it's capability is important. You can get roughly 1.5kWh battery capacity for 500-600$ if you build the pack yourself from LFP lithium cells. This would include BMS but not controllers or chargers. I wouldn't use lead even if the weight is not so detrimental in the style of boat you have.

    edit:
    MinnKota 112lb Riptide has ~3 grand price tag. 52 amps and 36V so that comes at 2160Watts of current draw. It is not a cheap engine and it still has limited output.
    Haswing Proturar 5.0 is radically cheaper, ~750 euros or hair over 800USD here including our pretty high VAT. Note that it is advertised as a 5hp replacement but actual draw power is 2560watts (24V btw). This is not exactly honest as, yes they have similar thrust at slow speed but in the end power is what matters. If you want to go 6mph the hp count not slow speed thrust - and the prop needs to be a match to the speed.
    Also gas motors report their power from shaft output. In the electrics there is a fair bit of difference between draw and output.

    Conclusion: All electric gets pricey very quickly with range and/or with speed. In casual use (not every day) the main benefit is silence and no smell. So why not both? <500 USD trolling motor with a <500USD battery. You will be able to move about at slow speed (3mph) for quite a while as you socialize and sip drinks. But if weather turns worse or if bit more serious distance needs to be made pull the rope. You can call it a hybrid :p
     
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  15. kerosene
    Joined: Jul 2006
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    Location: finland

    kerosene Senior Member

    about the navy series marketing. See the number below. I seriously question how realistic it is that the range goes UP with speed. With a tiny planning boat this could be possible but not very indicative for a displacement boat.

    upload_2021-4-30_11-50-4.png
     
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