<instructions>Your goal is to analyze the following comment and estimate how highly it will be upvoted by the Hacker News community.</instructions><comment><author>Animats</author><text>That&amp;#x27;s a nice application for brushless motors with variable-frequency AC drive. The motor is so simple - it&amp;#x27;s just windings and the propeller ring. No seals, no gears, and no bearings that carry heavy loads. All of those things give trouble underwater. The drive electronics is more complex, but drives for variable-frequency AC motors are well-debugged now. Every drone has several of them, and electric cars have big ones.&lt;p&gt;Some pictures of existing units: [1]  They&amp;#x27;re mostly used as steerable thrusters for boats that need to be highly maneuverable, such as ferries. They&amp;#x27;re becoming very popular for ROVs, because they&amp;#x27;re much lower maintenance than existing thrusters used at depth.&lt;p&gt;[1] &lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;www.youtube.com&amp;#x2F;watch?v=Te_xejpriFM&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;www.youtube.com&amp;#x2F;watch?v=Te_xejpriFM&lt;/a&gt;</text><parent_chain></parent_chain></comment><story><title>China&apos;s new submarine engine</title><url>http://www.popsci.com/china-new-submarine-engine-revolutionize-underwater-warfare</url></story>
<instructions>Your goal is to analyze the following comment and estimate how highly it will be upvoted by the Hacker News community.</instructions><comment><author>mannykannot</author><text>I wanted to know more about the rim-driven thruster technology, and a quick search revealed that it is being deployed in a number of areas where hub-driven electrical thrusters would otherwise have been used. One manufacturer&amp;#x27;s blurb[1] states that they use no seals, so I am curious as to what sort of bearing is used - could the rotor be magnetically suspended within the stator, at least while turning?&lt;p&gt;It also states that their device is symmetrical and so is equally efficient in either direction. My understanding is that, for optimal efficiency, a ducted propulsor should be narrower at the exit, but I guess there is no particular difficulty in making a rim-driven thruster of this form if its primary use is one-directional.&lt;p&gt;[1] &lt;a href=&quot;http:&amp;#x2F;&amp;#x2F;www.tsltechnology.com&amp;#x2F;marine&amp;#x2F;thrusters.htm&quot; rel=&quot;nofollow&quot;&gt;http:&amp;#x2F;&amp;#x2F;www.tsltechnology.com&amp;#x2F;marine&amp;#x2F;thrusters.htm&lt;/a&gt;</text><parent_chain></parent_chain></comment><story><title>China&apos;s new submarine engine</title><url>http://www.popsci.com/china-new-submarine-engine-revolutionize-underwater-warfare</url></story>