Good points.
For most of us, we're going fast if we get 7knots. And from what I have read, at speeds below 15knots, Kort nozzles do not present significant friction and drag and present a net advantage.
From: electricboats@yahoogroups..com [mailto:electricboats@yahoogroups.com]
Sent: Friday, March 8, 2019 7:27 PM
To: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] Re: Sailboat Props - Data Please
Two points to consider; Kort nozzles are good for tugs and trawlers because they increase the thrust at low speeds when there is a lot of propeller slippage. But they also increase drag and friction at higher speeds. So not worth considering unless you want high bollard pull. Propellers are most efficient at 50% slippage so you can calculate the rpm at cruising speed. The pitch is the amount the propeller moves forward with no slip per revolution.. Use the biggest diameter prop you can fit in you arpeture with 5 -10% clearance. The other thing to consider is shaft revolutions should be around 800 to 1000 at cruising to full speed.
On Sat, 9 Mar. 2019 at 12:07 pm, GNHBus@aol.com [electricboats]
<electricboats@yahoogroups.com> wrote:
http://gentleseas.blogspot.com/2016/05/secret-german-high-tech-submarine.html
-----Original Message-----
From: tvinypsi@gmail.com [electricboats] <electricboats@yahoogroups..com>
To: electricboats <electricboats@yahoogroups.com>
Sent: Fri, Mar 8, 2019 10:25 am
Subject: [Electric Boats] Re: Sailboat Props - Data Please
Thank you for the thoughts, Kevin. Much appreciated! I've seen at least one other boater's suggestion that the prop calculators are intended for ICE driven boats and not electric, although the Vic Prop calculator seems to be designed the goal of attaining as close to hull speed as the engine/motor seems able. The torque it assumes is the maximum as calculated for the user-entered horsepower. I don't think any by highest RPM attainable by the user-described system is considered in its calculation.
With this in mind, the problem with using the Vic Prop calculator is knowing what figure for HP should be entered. Thunderstruck states their 10Kw motor will generate up to 10HP continuously, but offers a peak of 28HP. What would be the correct figure to use? I haven't been able to figure it out.
But then there's also the question of what ratio of reduction to use. A big wheel is most efficient, but if constrained in diameter, I've read, increase the RPM or increase the pitch at the expense of efficiency.
Not being an engineer of any kind, it's all quite a lot to get my head around!
On the prop size/kort nozzle concepts ..... My prop diameter is constrained by being in the aperture of a full-keel boat. In my readings I have indeed run across kort nozzles and thought about the possibility of such a retrofit. We're in the Great Lakes, so beyond the material and structural design questions that I'm not entirely competent to answer, the potential for fouling is a serious consideration. A couple of different types of mussels are particularly pesky. So, I doubt I'll attempt this. Great suggestion though .. sort of like an aft-facing bow thruster!
Posted by: "Myles Twete" <matwete@comcast.net>
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