Motor hp, tube diameter, and batteries size are common things people discuss about thrusters. *I guess because they have numerical values that we can grasp and readily compare.
However if you look at a thruster as a system, instead of a component, then it becomes apparent that they are really only two attributes in selecting a thruster. *Installing a thruster is really about optimization and efficiency.* A lot of it comes down to placement of the tube.
The first thing is to determine the interior location for fitting a thruster, that is, if you are installing a thruster in a boat that is already finished.* Keep in mind that these are big motors that will throw carbon off of the brushes.* The space will get dirty and it should be no other flammables or explosives (hydrogen from a charging wet cell battery).*
You want the tube to be as far forward as you can get it to achieve the most leverage. *But you do have to take the depth of the tube placement in consideration. Optimally you want the tube at least one diameter below the water line to ensure adequate head pressure.* I’ve also seen it recommended that you want 1/3 diameter distance from the bow and* the keel to prevent circulation.* Tube length is recommended to be 2-4 diameters.* Too short it will cavitates as the water won’t be laminar flow prior to prop.* Too long, and you have additional frictional forces in play.
Keep in mind that the tube to be physically able to be installed as well.* I say this from experience.* It is hard to tab the bottom of the tube.* The more clearance, possibly the better quality layup at the end.
Large radiusing the inlet of the tube at the hull penetration makes a huge difference in efficiency.* This allows the thruster to make its rated torque, and also quiets the thruster down significantly. **I look at some “professional” installations and cringe.* They do everything right electrically, structurally, and cosmetically, but then then leave the tubes with a hard edge at the hull.* Why?* Because they either don’t know, don’t believe, or quite possibly because it adds a lot of time to the job.* You have to create a big fillet on the inside in order to obtain the radius on the outside.* I would ask to see photos of previous installations of other boats if I was paying someone to install for me.* Or I would learn to live with a noisy cavitating (rocks in a blender) thruster.*
I was fortunate that I had my boat on the hard and I could measure all dimensions and fit up directly.* I had drawings of my boat that I could use to measure to scale with.* I was able to call Nordic Tugs and ask their advice.* In fact a good thing to do is to look at sisterships that have thrusters fitted from the builder.* Ideally they’ve spent some time and effort optimizing the location. *
That’s why sometimes, but not always, bigger is not better.* A smaller tube diameter with lower thrust may be able to give more leverage to a boat than a larger tube that is further aft.* With the blunt bows profiles of many of our boats this may not be an issue.* I actually ended up installed a bigger bow thruster than is typical for my boat because I could easily make it fit with all of the above design attributes.* I have a Sidepower SP55 with all of the upgrades (Q prop and electronics) .* I’m pretty pleased with my install.* It took a longer time than I had anticipated, but it was done right.* It provides fantastic thrust with very little noise and cavitation.
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