T-Pain Is On A Boat: Re: [Electric Boats] Re: Sailboat Props

Friday, March 8, 2019

Re: [Electric Boats] Re: Sailboat Props - Data Please

A comment for " tvinypsi". I needed to have max thrust from a given (max allowed) prop diameter. So I contacted Korts in the UK. A year later, and countless emails I got my final estimate of cost vs thrust, when all else was constant. In my application, They(and I) settled on a type 37 airfoil for the nozzle. (I needed good reversing thrust also, or would have went with the 19a.) Their research went like this: my 3-blade, 14/11 prop in open water would produce 500# thrust. When using one of their 4 blade, 14/11 nozzle-type props specifically designed to work in a nozzle, they claimed the thrust would be 1500#. Added to that, they wanted $5,000 EACH for props(X2), and $15,000 EACH for nozzles welded to my converted lowers. That would be a cool $40,000 just for their name and engineering.

Now there are several factors that most either don't know about or seem to over look. This message would turn into a wall-o-text to go into them all, to explain the significant differences. And I know, there are all the "armchair engineers", and "designers" out there that say that a max of 30% increase is all that can be expected.

At this point I thought I might see what I could do it for(or even if I could.) I had designed, an ABS "plug nozzle" to match my lowers. Then I had marine-grade castings made from them, and hand fitted to my lowers. I have a whopping 1/8" clearance around my stock props.

NOTE: One engineer said this was to much, another said it was too little.

NOTE: neither "engineer" could intelligently discuss my hull shape and contour, even after I sent them partials of the hull portion of the plans.

Bottom line I have discovered is, that I am in the ball park. I studied all the books I could get my hands on, picked the brains of all the prop guys that would talk to me, and then made a difficult decision! I am willing to try SOMETHIN, and adjust accordingly...

My suggestion to you about the duct is, if, after you have done your due-diligence, and think you need the added thrust, Go for it. I found the engineering cost me a hundred bucks, printing cost me $300+/-, ... twice. The castings cost me around $300 each, and I got 3 (2 for the project, and one to spare or sell). The final machining took 2 months of a couple hours each day, blueprinting and hand finishing. All this took about a year of phone calling, late-night emailing to the UK, all-day conferences with EVERYBODY INVOLVED, ... from start to finish.

I am hoping to get it all wet this summer. Good luck, and if you need files for printing, pics, or information on castings PM me. My work can be scaled, my printer will print it, and my foundry will cast it. It would be up to you to finish and coat it.

I found a REAL prop guy in Oklahoma that says that he can convert the stock elliptical tips to nozzle tips, to further improve the efficiency. There are "plastic" ducts out there sold as "prop guards", but don't be fooled. They have even more that the "max allowable type" tolerances, engineers will allow. Also, please note that ducts/nozzles are a serious drag on sailing boats (any boats), especially at about 10 MPH and above. According to most published information sites and journals, they are for higher HP, work boats that need a lot of slow but high thrust output.

I guess I got carried away again. You got the wall-o-text anyway... I'll shut up now and go back outto my shop.

Go for it, it is only money, and cheaper that some boilin LiFePo4s.

Cheers,

Dan

----- Original Message -----
From: tvinypsi@gmail.com [electricboats] <electricboats@yahoogroups.com>
To: electricboats@yahoogroups.com
Sent: Fri, 08 Mar 2019 09:39:23 -0500 (EST)
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!

Dan Hennis

CTR Services

P.O. Box 254

14237 FR 1155

Cassville, MO 65625-0254

417-396-0228

__._,_.___

Posted by: DAN HENNIS <dhennis@centurytel.net>

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