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By the lee

Torqeedo leg comparative drag

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I don't know. What I do know is that I like that mastervolt design a lot more than the torqeedo design. it's a flaw in all torqeedos except deep blue that the motors are immersed to work. my 1003 ate itself up because despite taking what I thought was really great care of it, the seals failed. the design of these two units basically ensure that the motor on the torqeedo is always immersed in a corrosive environment. the mastervolt should at least be serviceable in comparison.

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Or find an old saildrive and fit an electric motor to it yourself. The torqueedo looks like a 12 year old designed it.

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Torqeedo claims that the resistance will reduce speed about 0.56% according to their study. We have Cruise FP 2.0 under 23 footer. In reality the speed drop tends to be much higher. Of course, our boat has much less wet surface than the 30 footer in their study but there are other aspects. Because you are not supposed to cover it with anti-fouling paint it will be covered with marine growth pretty quickly. The other issue is that the propeller tends to unfold in speeds over 5.3 knots. But in the other hands it is several times cheaper than the OV. Maybe even 10x compared to this version?

 

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So you are comparing the 2/4 kW Torqeedo model to the newest controlled pitch Oceanvolt model (Servoprop). The lowest power available from Oceanvolt is 6 kW. Servoprop must be much more expensive than the standard Oceanvolt drive.

Torqeedo recommends 2/4 kW models up to 4 tonne boats. I think 4 kW is too small for 4 tonne.

Are the Figures 1 and 2 of the Torqeedo claim available somewhere? The claimed "average speed loss" of 0.56% or 0.04 means that the speed was about 7 knots. Yes the loss will be low at such a high speed for a 30' boat. I don't know any Dehler 30', but looking at Dehler 29 it reaches 7 knots only between 75 and 135 TWA at 14 knots. At those points the speed difference between standard drive (VP or Yanmar) and no drive is less than 0.2% according to ORC VPP. So even if the Torqeedos claim is true, the drag is about 3 times more than the drag predicted by ORC VPP for normal drives.

It is expected that the speed loss will be low when the boat has a high drag due to moving over its hull speed (about 6.9 knots for Dehler 29). Beat VMG at 14 knots wind is predicted to drop 0.6% and at 6 knots 1.4% (VP/Yanmar ORC VPP). These points Torqeedo will be addiotional 1-3% slower, if the Torqeedo claim is true.

Torqeedo drive hub doesn't look at all well designed for low drag. The frontal area is huge (due to motor inside the hub), the tip looks too blunt and the diameter reduction towards the propeller hub way too fast causing flow separation.

You can find the drag formula used in ORC from the VPP documentation (6.22). Note that the hub diameter (ST4) would be taken to be the end diameter, which is equal to propeller hub diameter. More realistic for Torqeedo would be to use the maximum diameter of the hub. The latter part of the drag area formula is dominent (0.4*(0.8*ST4)^2).  For normal drives ST4 is from 85 to 112 mm. What is the maximum diameter of the Torqeedo hub? 200 mm would give 4 times more drag than 100 mm with that formula.

Oceanvolt drive looks like to be as low drag as the normal drives.

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