How to select motors for catamaran?

Discussion in 'Propulsion' started by Afilimon, Nov 4, 2018.

  1. Mark Morwood
    Joined: Sep 2018
    Posts: 5
    Likes: 0, Points: 1
    Location: Brisbane

    Mark Morwood Junior Member

    I think we must be talking at cross-purposes because that makes no sense to me. Are you suggesting changing from two engines to one engine will give twice the run time with the same boat speed? Or are you suggesting twice as long at something approaching half the speed? In which case no - that is not what I believe the OP is trying to do. My understanding is that he has a self drive tourist boat on a canal, that he wants to operate at a moderate speed. He's trying to work out what configuration/design changes might make a difference to his run time given a particular operating speed. With that set of assumptions, then I would argue that it is not going to make a big difference whether he uses two motors or one to his run time at a particular speed. One motor should offer slightly longer run times because of reduced drag, but otherwise you still need to use the same amount of power to propel the boat at a given speed, whether it all comes from one motor, or half each from two. I believe electric, unlike ICE engines have relatively flat efficiency curves. Two motors does have the separate advantages of providing a simple effective steering system without adding a rudder system, and is an easy mounting and installation option on a catamaran. If he was approaching hull speed, then increasing the length of the hulls would help, but that doesn't seem to be the case. And as always reducing weight, which will reduce wetted area and hence drag, will also help, but that might be an expensive option with diminishing returns as a significant proportion of the weight of the boat is in the passengers. I will admit I've ignored the impact of getting the right pitched and sized propellor, but I think in his case it is probably fixed by the trolling motors he is using. It would certainly be an interesting place to try optimising later.

  2. gonzo
    Joined: Aug 2002
    Posts: 15,512
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    Location: Milwaukee, WI

    gonzo Senior Member

    You are getting confused, because thrust and power used by the motor are not a linear relationship. Further, the advertised thrust is only a marketing tool and not a true specification. The only way to relate input electric power to thrust is to have a table or an equation that gives values at different RPM and Amperes.
    A motor that generates 250 lb of thrust will make the boat accelerate if the hull resistance is 172 lb, until it reaches an equilibrium between thrust and hull resistance.
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