Have you looked at the posts by
gianlucacarusots?
While his extensive experiments with motors applies to Tandem Islands, most would also be relevant for your craft.
https://www.hobie.com/forums/viewtopic. ... nd#p291107gianlucacarusotsForum topic :-
Torqeedo 403 Ultralight Motor / Hobie TI Application GuidePage 8 ->
My experience with a rather humble 34 lb Watersnake & 50Ahr lithium Ion battery on a Tandem Island is it pushes the boat along for the best part of a morning at 5kmh. I estimate my range in calm conditions to be between 25 & 35 km. (about 15 to 20 miles in Imperial).
I'm told that an over-sized motor does not increase speed very much but greatly reduces range.
In my case, 80% power even with a 18kmh head wind and 500mm chop towing an occupied kayak towed behind, is about the maximum use of the motor. Above that the propeller undergoes aeration (sucks air down - Speed boat creatures call this cavitation). A horizontal foil above the prop & a second on the shaft may well stop this & increase power and efficiency.
Also some motors slow speed by increasing resistance within the throttle. This wastes energy & decreases range. I suspect a Torquedo does not do this but you could check.
To get around this you wire in a
Pulse Width Modulation & leave the motor at full throttle but control speed via a small rotating knob. Check out Youtube videos.
As you may well know, a PWM turns the electricity on & off rapidly with bigger gaps at slower speeds. This doesn't hurt the motor or battery but extends range. They cost about $30 AUD and are very easy to install
As for prop pitch, most electric motors are built to move big heavy plaining hulls below plaining speed when they have huge turbulence at their blunt stern.
I understand the Hobie & Torquedo kayak motors have props and power suited for Kayaks. Of course they would be aimed at average Hobie & other kayaks. If your boat is heavier or less efficient than this average, you may have to deal with it. I suspect the Hobie adapted Torquedo would be a little underpowered on a tandem island. My 34lb Watersnake is designed for plaining dingys and may well be more efficient with a larger and slower prop?
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Below is a quote on prop pitch & hp on Outbacks from somewhere in the Forum that I picked up last year - (my apologies to the author for not remembering him/her) Note: As the author points out, some of the calculations below may be wrong as kayaks often have wave piecing hulls and they can almost double their
Hull speed. Under sail my TI often reaches 15kmh so I suspect it is wave piecing. Again apologies to the author - I am not an engineering/phsics expert.
2 hp gas motor with a 4 1/2 inch pitch prop it propels TI up to 4mph (3.5 knots/6.5kmh).
This is a factor of the prop pitch more than horsepower, hull speed, or weight (basically the max hp of the motor is not exceeded) regardless of boat length since we are below the max hull speed on all 3 boats.
TI running 2hp and 7 inch pitch prop, 4.4 knots max with the 7 inch prop (max hull speed is 5.8 knots). ~ maxxed-out the hp of motor at 4.4 knots.
The 2 hp motor just doesn't reach max rpm when on the TI with the 7 inch pitch prop as it did with the 4.5 pitch prop (HP limitation of the motor).
As long as you are not exceeding the max hull speed of the boat or the max horsepower of the motor, its the propellers pitch and efficiency that determines your speed on boats with displacement hulls (kayaks, and most sailboats).
How a hull behaves at that speed depends not only on hull length, as is commonly used to calculate hull speed, but also, displacement, length/width ratio, bow and stern shape and several other lesser factors. Most significantly, the traditional 1.34 x sq. root of the water line length formula was derived experimentally by William Froude for heavy displacement conventionally shaped hulls such as military and cargo ships, not planing hulls or kayaks which are usually classified as semi-displacement hulls, among others.
The Hobie Outback has a comparatively "hard" hull speed where the amplitude of the wave trap is greater than many kayaks such as the Adventure. Regardless of prop pitch, the thrust requirement increases rapidly approaching and exceeding this hull speed. For instance, the Hobie eVolve electric motor uses 90 watts to obtain 4.0 MPH, 200 W for 5.0 MPH and tops out at 345 W and 5.4 MPH. I can briefly double the thrust with Turbofins to gain another .1 MPH for a top speed of 5.5 MPH as the boat remains effectively trapped between its bow and stern waves, unable to climb out under
Drag in lb vs speed (knots & kmh) short wide kayak)
ordinary circumstances. We usually acknowledge that Turbos are not speed limited by prop pitch anywhere near that speed. The idea here is that you don't get a lot of bang for your buck beyond about 4.5 MPH with this boat.
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