Looking for basic power formulas for electric drive...

Discussion in 'Propulsion' started by JunkRat, Jan 27, 2019.

  1. Dejay
    Joined: Mar 2018
    Posts: 721
    Likes: 138, Points: 43
    Location: Europe

    Dejay Senior Newbie

    Good empirical data would be ideal of course. But only if it's for your exact use case that also solves your problem.

    If you consider converting a boat to electric propulsion you will have to change something compared to the previous performance of the boat (go slow, make lighter, make boat longer, short range or expensive batteries). So lets say you throw the diesel overboard and chop the mast off and think 6 knots is totally fine for your case. How far will the boat go then with $1000 worth of batteries?

    Like JunkRat says the question has too many variables. Both length and displacement have exponential effects, unknown motor or propeller efficiency for your use case (speed) or you might only get anecdotal evidence about performance for a boat you want to convert.

    Of course we want to know the secret formula you are all trying to hide form us! Just kidding.

    Lets just agree there are different ways for skimming a cat :)
  2. HereandNow
    Joined: Feb 2020
    Posts: 1
    Likes: 0, Points: 1
    Location: Knoxville,TN

    HereandNow New Member

    I am looking for the same thing.
    I found this simple basic powerboat formula. from The Lowdown on Boat Horsepower (And How to Make the Most of It) | BetterBoat Boating Blog https://betterboat.com/boat-horsepower/

    You need 1 hp for every 40 pounds of boat to go 20 mph.
    so you need 100 hp for a 4000 lb boat to go 20mph

    to go 30mph you need more.
    you need 160hp for that 4000 lb boat to go 30mph.
    so 1.6 per 4000lb to go 30 mph.

    very rough estimate but that is what you need right?
    for a sailboat they dont give speed but just have a rule of thumb of "An old rule of thumb says that for every 2,200 pounds of weight (AKA every Metric ton), you need four horsepower."

    now just convert electric to hp and you get about
    1kilowatt (1000watts) = 1.3 hp
    or one gas hp = approx .745 kw (745 watts)

    lets try that on the silent 55
    which is 1.8 tons and goes 5 knots at 2 kw of electric motor power.

    first lets use the motorboat formula

    2 kw is 2.6820441777 gas hp
    5 knots is 5.7 mph
    1.8 tons = 3800 pounds

    so lets rough our boat to 4000 pounds and say to go 20 mph it would need 100 hp like the formula said

    but its going roughly 1/4 that which wont scale linearly but says it should need 1/4 the hp or 25 hp to do that but the silent 55 is only using about 2.68 so clearly that doest work at that high speed.

    lets try the sailboat rule of thumb.

    for every ton you need 4 hp. well they are almost 2 tons and using about 2.6 hp.
    so clearly those rules of thumb really fail for the silent 55 which does much better.

    I think the big factor is the MUCH slower speed. if you idle along at a slow speed a boat can use barely any hp at all and 5 kt or 5.7 mph is pretty slow.

    now the electric boat xzells mentioned has this infor translated from german on their youtube video

    In the meantime we have covered over 6000 km and have been totally supported by our new propeller. It runs at almost 15% more effectively. Today's video shows us on the day trip from Hiddensee to Wiek. Here we become aware of a drone whose pilot we discover in a bay. When asked about it, we got the great video. Thanks a lot for this. It shows how we continue from the very shallow bay towards Hiddensee. The transition from shallow to deep water on the stern wave is very nice to see. First bubbling (flat water) then the transition to a smooth longer wave (deep water). From here we are traveling at around 4.5 KW and 14 km / h. Electric boat, solar drive, solar boat, solar yacht, electric drive, 48 volt, electric drive, solar houseboat, the quiet boat, electric boat, solar boat, electric drive, solar yacht, 48 volt, electric drive, Bellmann, Bellmarine, Mastervolt, Kräutler, Kraeutler, Elektroyacht, Torqeedo. The video shows the conversion of our 'boat into a solar boat, assembly of the solar panels, installation of the electric motor and our successful test drive"

    so they say they do 14 km / hr at 4.5 kw approx 6 hp at about 8.7 mph

    they also say they do
    " 4,5 kw beziehen sich auf den Verbrauch bei 14km/h. Meistens fahren wir mit 2 kw Verbrauch bei 10 km/h. Eingebaut ist ein 10kw Motor und die max. Geschwindigkeit liegt bei 20 km/h."

    which i guess is 4.5 kw or 6 hp at 14km/hr or 8.7mph
    2 kw or 2.68204 hp at 10km/hr or 6.2mph
    10kw or 13 hp gives 20 km/hr or 12mph

    or 6 hp
    Last edited: Feb 23, 2020
  3. Gravio
    Joined: Feb 2020
    Posts: 4
    Likes: 2, Points: 3
    Location: USSR

    Gravio New Member

    This sail will perfectly provide your boat with electricity!

  4. Yobarnacle
    Joined: Nov 2011
    Posts: 1,747
    Likes: 130, Points: 63, Legacy Rep: 851
    Location: Mexico, Florida

    Yobarnacle Senior Member holding true course

    Diesel electric. Rather than batteries, use a generator to provide the electricity to your electric motors. Diesel fuel in concentrated energy.
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