# Help with calculation for electric catamaran hull: speed, length, with, depth

Discussion in 'Boat Design' started by Jon E, Jun 25, 2016.

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The video shows a little Mercury engine which is identical to the one that I have. Mine is a 1999 model claimed to be 2.3 HP. Actually I think that it is a re branded Tohatsu. My 15 .5 foot little sharpie style boat happily exceeds the speed claimed in the vid with the little gas engine.

I have argued about minute details but my pet sharpie/flattie has none of those attributes, except modest weight, It even has hard chines and lots of wet surface but it does achieve a sloppy plane with that little noise maker two stroker engine. ......But you are going to use the three quarter horse Torqueedo which is probably not equal in real world output to the wee little Merc. The electric one may not deliver the same total thrust but it will deliver what it can in relative silence.

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### keroseneSenior Member

I would consider xps foam with glass and epoxy. You would need to rig a hot vire cutter but that is not hard if you have the space.

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### Jon EJunior Member

Torqeedo Cruise 2.0 R:

Input power in watts 2000
Propulsive power in watts 1120
Comparable petrol outboards (propulsive power) 5 HP
Comparable petrol outboards (thrust) 6 HP
Maximum overall efficiency in % 56
Static thrust in lbs* 115
Integrated battery -
Nominal voltage 24
Final charging voltage -
Total weight in kg 15.3 (S) / 16.2 (L)
Standard propeller
v = speed in km/h at p = power in watts v19/p4000
Alternative propeller options v30/p4000, v8/p350
Maximum propeller speed in rpm 1300

* Torqeedo static thrust measurement is based on internationally accepted ISO standards. Static thrust figures for conventional trolling motors are typically measured differently, which results in higher values. To compare Torqeedo static thrust data with conventional trolling motors, add approximately 50% to the Torqeedo static thrust values.

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### sigurdPompuous Pangolin

Jon E, you are most welcome.
I have run several different rocker and waterline shapes on 7m and 125 kg per hull, around 8-12 knots. From elliptical to parabolic and in between. It makes very little difference. So it would seem that all you need to know is the beam, and then an estimate of what draft that will give.

The beam is 24-26 cm, and the draft is about 12 cm.
If you can then make the waterline fair and nice, and the rockerline so that the ends are close to the surface, you should be golden.

If you use more than 90' chine angle, like Rick's yellow 6m and your drawings, then draft or beam at waterline (BWL) must increase or the shape be a bit more fuller in the ends. I believe you have a good amount of wiggling room on all these, before anything too bad happens.

Let me know if you want me to generate station offsets for parabolas or ellipses. Good luck!

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### Jim CaldwellSenior Member

Jon, for electric power 748 watts = 1 hp.
Their "Propulsive power in watts" (shaft power ?) is 1120 watts so it is a 1 1/2 hp motor period.

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### keroseneSenior Member

Actually shaft power is more, I believe the 1120 is actual propulsive power that has propeller inefficiency taken into account. Shaft power is probably .85-.9 * input power.
And the shaft power to propulsive power is probably better than outboard due to prop design and direct drive. Still its purposefully made so that apples to apples comparisons are hard to make.

Last edited: Jul 13, 2016
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### HeimfriedSenior Member

Jim, the different values are not generated by a false conversion factor between the units Watt and Horsepower.

Torqeedo claims to include in its stated values of propulsion power all losses from the electric motor over bearings, gear and propellor.
The output power of gas engines is normally given as the motor shaft power, therefore it includes no power losses in bearings after motor, gear and propellor.

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### Jon EJunior Member

Today i replaced the Mercury 3.3 hp with a Mariner 5 hp. Much more powerful, but still not that big change in speed.

The 6 meter hull is optimized to 12 km/h (design weight 170 kg), and both engines reach that speed with almost no power.

Full power Mercury 3.3 hp (average) = 8.9 knots (16.5 km/h)
Full power Mariner 5 hp (average) = 10.4 knots (19.3 km/h)

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### Skagit gadgetNew Member

Plan A:

Take 17 ft aluminum catamaran, called "pontoon boat" in the US, and affix a pair of trolling motors to transom. Cat is a beat up well-used boat that served a houseboat cleaning service on Lake Mead, NV for several years and was bought for cheap. Two trolling motors for redundancy in case one fails, also because then you could run each at half throttle for less strain and perhaps max efficiency. I chose new "46 lb thrust" salt water rated motors. These are light enough to physically handle easily. For batteries I have 35ah deep cycle "wheelchair" batteries which are a very handy standard size cost: ~\$80-US. No need for expensive Lithium batteries as the boat needs weight for stability since at present there is only the perimeter railing on the deck. Later a cabin will be built after initial testing is completed. To support the solar panels for the time being a framework of pvc tubing will be erected sized for eight panels. Likely only 4 panels to be purchased initially for testing under max sunshine in June 2017. A simple battery charging controller should keep the half dozen batteries from overcharging. I have 6 more batteries to be installed at a later date.

Boat to be anchored in a cove under full sun. First test will see speed of battery recovery after x miles of mid-throttle operation at resultant y speed. Trolling speed y should satisfy intended goal of the project, but speed runs are not out of the question. GPS will have to be purchased to record speed and will come in handy for navigation.

17 ft cylindrical flotation should qualify as "torpedo" hull for max efficiency through the water but perhaps this is not really enough hull length to qualify (?). In any event we will try to chart as many graphs as possible. The most interesting will of course be the speed available under steady sun with zero net flow in/out of the battery pack: the "driving on sun" speed value.
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