Small electric powered catamaran for safety boat use?

Discussion in 'Boat Design' started by Pete Smith, Oct 21, 2023.

  1. Pete Smith
    Joined: Oct 2021
    Posts: 4
    Likes: 0, Points: 1
    Location: Hampshire

    Pete Smith New Member


    I'm thinking about changing one of our safety ribs to electric powered.

    From our testing of electric motors on Rigiflex 360's and so on, the available motors are not really up to it so I'm wondering about a small catamaran as the basis for the safety boat instead of small rib and plastic Rigiflex boats as they should be more easily driven.

    We need to be able to have two people on it, space for a third following a rescue and preferably console control on jockey style seat perhaps so there is some space either side of the driver for the crew to get around as required.

    Looking at using 6kw motors and we can build in lithium batteries etc.

    Easy entry from the water would be useful as currently dragging people in over the side is pretty tough in crappy weather esepcially if the crew is a smaller female and picking up some lump of a bloke.

    I'm thinking a small landing craft ramp up front could be used - drop the front of the deck so they can slither/pull/be pulled up it.

    Whaly Boat 355 have a dropping bow section for landing purposes for example.

    There are potentially something like this:
    SEAHULL: The Best Commercial Boats, Barges and Small Landing Craft
    Zego 300 - Zego Sportsboats
    Zego 400 - Zego Sportsboats

    Does anyone know of a potentially useful starting points for cat plans as I can knock up a prototype quickly and cheaply to get an steer on whether this might have any legs as an idea?

    Approx dimensions in the order of:
    Length 3.5-4m
    Width 1.8-2m

    Some weights to think about:
    Boat 150kg
    2 x crew 160kg
    Outboard 35kg
    Possibly rescued person 80kg
    Various gear 30kg
    Couple of hundred amp hours LiFePo4 batteries 90kg

    So normally around 465kg all in and 550kg perhaps with a rescued person.

    With two people we must be able to get a a resuce pretty smartly - hence looking at this as the current ribs can't plane. Need to be able to get some reasonable speeds of 12 knots+ but only short bursts as our sailing courses are not huge.

    Once loaded with a rescue we will be dragging a boat (or several) so we will be chugging along and no need for great speed.

    Battery and motor testing on ribs shows range with the batteries is not a problem for our use including some full throttle. We can potentially get some higher power motors but looking at the easily available 6kw initially.

    To be used in the Solent on the UK south Coast so some significant chop and swell and conditions can be reasonably rough.

    A reasonably tall cat hull would be comfy through the water and more efficient so it seems like a good plan.

    If anyone has any thoughts would be great.
  2. SolGato
    Joined: May 2019
    Posts: 370
    Likes: 231, Points: 43
    Location: Kauai

    SolGato Senior Member

    Your best bet is to look at electric outboards which are built on/around a traditional outboard motor leg/housing, but the problem with these motors is the gear reduction in the lower unit which will limit top speed as most electric motors put out half the Max RPM as an ICE, and the lower unit then cut that in half again.

    Up to its Max RPM however, an electric outboard should perform as well if not better thanks to the thrust advantage.

    It’s this inability to reach the same Max RPM as an ICE that will make getting a boat to plane a challenge. If you can’t get it to plane, you are just pushing it through the water which is in efficient and you’ll be limited by hull speed.

    Typically, bigger the electric motor the higher the max RPM, however it often comes at a cost of higher voltages and a bigger more complicated battery and charging system.

    That said, I don’t think 6kW is going to do it, especially when that’s input not output power, and based on your passenger requirements and the need for good maneuverability in challenging conditions, you’ll also need to swing an appropriately sized prop. Most smaller electric outboards can fall short in this department. Their props lose bite, and again with the lack of RPM, can make navigating current and headwind challenging.

    Hull-wise I think your on the right track with a catamaran which offers good stability and reduced wetted surface allowing better efficiency at slower speeds.

    There are manufactures that make HDPE pontoons that might work well for building a small catamaran with a flat deck, center console, and rear swim platform to allow easy access. The big volume plastic hulls would provide good buoyancy and stability and be nearly indestructible.

    I’m imaging something like a coach boat but bigger and more capable.
  3. Pete Smith
    Joined: Oct 2021
    Posts: 4
    Likes: 0, Points: 1
    Location: Hampshire

    Pete Smith New Member

    Thanks for the resposnde Solgatop!

    Yes I am looking at getting some more powerful motors in the 10kw range which will therefore be pulling around 225amps flat out.

    Flat out is VERY unusual though so 200Ah of batteries should easily be enough as will be pulling around 20-30 amps probably most of the time so around 6-8 hours.

    I've seen various hdpe pontoon kits available and it my be a useful option to think about but I think I will look further into a rough and ready test bed to see how things might work.

  4. Algie Bennett
    Joined: Oct 2023
    Posts: 1
    Likes: 0, Points: 1
    Location: Lymington

    Algie Bennett New Member


    This is Alg from Lymington and I will contact you directly as we are working on this exact scenario currently and I think we are probably fairly close by.

    We have the motors and batteries you need and have started work on a small test catamaran that will do what you want. 48v 7.2, 10.5, 13 and 22kw motors available.

    A 96v 30kw motor is also available but this is clearly massive overkill for what you are looking for. We will have some even larger 96v motors aavilable towards the start of the new year which will be quite amusing!
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