Glen-L.com

I am doing the research to build a steel (or aluminum) "Goliath." It will be an electric powered product. In my research, it apears that I am going to use the 36-72 Volt D&D ES15-6 6.7" Motor (approx. 10 H.P.Cont., 40 H.P.peak). And, in an effort to keep things as slow turning as possible and still reach maximum torque & thrust, the plan calls for something in the range of a 21" dia. screw. I am currently leading towards a 4 blade model from GETAPROP.COM, (HUNG SHEN KCA4). However my research is incomplete due to needing to know what pitch for the screw, and what reduction rate to get. The D&D company needs an output RPM to reduce their motor (normally running at 4,000RPM), to the desired best screw RPM (presumably somewhere around 1,000-1,200). Each vendor of a component, seems to want information the other vendors won't seem to release, or can't give till I give them the other vendor's information that they won't give..... Confused yet???!! I am wishing at this point that SOMEONE can make a fairly close "shot in the dark", and let me close this visious circle here. I am needing a Pitch and Rpm of screw-shaft, to complete the powerplant planning. I have tried two different prop companies that offer"free sizing analisys", but none seem to be able to help. In stead, they ask for info I can't get wo/the other. What can I do, or where can I cakk to resolve this??

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Better, faster, cheaper. Only ever found 2 of the 3! (But still lookin.)

Built 4, repaired 30+, ...So many boats, ...so little time.

Yes I am confused. Its all a trial and error in what you are describing.

aero_dan,

I wish I could give you the specs you need, but I can't. You do have my attention. The Goliath is a neat boat and adding an electric drive makes it even better. I'm wanting to use the motor and controls out of a golf cart to drive a 16' pontoon boat.
D&D has worked with several electric boat companies and you can bet your bottom dollar those companies have legal papers restricting what information D&D can release. The others venders don't want to commit, because they would have to stand behind what they suggest. It's a shame they won't work with a costumer and experiment with him. They all have used or demo parts that a person could try out.
One last thing, you say the plan calls for a 21" screw. Is that Glen-L's plan, yours or someone elses? I'm just a dumb old country boy, but a 21" screw sounds too big for your electric setup.

Why metal? Wood Wood Wood!!!

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A secret to a good marriage is to have a quick mind and a slow mouth!!!

Until the Tugboat Guys chime in, I'm going to really stick my neck out and offer up the following.

The information you need to define your propeller specifications can be found in Dave Gerr's Propeller Handbook. Wasn't too long after perusing the book we developed a spreadsheet to check out our prop selection. Looks like our combination was in the ball park !

We modified our DIAMETER-HP-RPM spreadsheet and input some numbers for a 10 HP motor with 2:1 reduction gearing. We're using BHP for these calculations. Electric motors may be rated differently. I'm sure the engineers and NA's out there will be quick to tell us if our numbers are out to lunch ...

http://www.chelseacoachworks.com/boats/ ... _calcs.htm

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Paul Kane Chelsea, PQ

Building the Glen-L Hot Rod : http://www.boats.chelseacoachworks.com

OK guys, I am so glad you all have more time in front of the computer than me. I have a few more answers from some folks that think they know. I still have some questions though, but first YOUR answers. The plan from G/L shows room for a total of 23" with no rake. My best information is that you need 1" for clearance, any more and you develop increased loss of thrust, etc. The goal is to keep the screw speed way down, like the "big boys" do. The hull is most efficient with a large slow turning screw, ... and that is what we need to tow large, multi-slip docks around the lake(s). If I follow most of the prop sizing programs, it is as though they are gaging for an outboard or stern drive. They suggest a 14" pitch and a 10-12" prop! I have priced props for the "trial & error" method. At $899.00-1,700. per "trial, I don't think I want to do as much "trial" as some would suggest. I did find a site that will rent a prop for a given short period (30 days or less fron out-the-door till returned). Most of the calc programs want a reduction rate and an "engine" RPM. I think that is where I make the program puke. Generally, these motors are wired for 1750 or 3400 for AC, and 1200, 3600, 8,000 (and even 10K) for DC. Granted they are using a variable controller, but if you are wanting to get the most out of the battery, you should consider the "sweet spot" designed into each motor. Most do not even know about this electrical phenomenon. I discovered it when I had wind turbines at my old home. DC is more forgiving, but this is where I am at. I would be willing to settle for an 18", 4 blade screw, that would turn a max of 1125 RPM, push the hull of 4800-5,000 lbs, at the advertised calculated speed of 6+ kts. Reduction rate is variable, and proper use of the battery bank will determine the duration. The 18'-6" could get pushed to 19' if I get a wild hair on the jigging table, and the beam will not exceed the designed 8'-2".

I also read in one of the other posts where somone had a bow thruster in their Titan. It would be great to find out more on that, so I did not have to reinvent it. This would greatly enhance the dock management mission I am building this for. Again, thank you for the responses, and don't be afraid to tell me I am beached. Better to be beached here than on the lake. (and far cheaper)

I have contacted 3 of the larger prop makers with on repies in weeks. I think they thought I was pulling their legs. Ine place said if I sent them $100, they would "engineer" a screw option, and guarantee the report, for my application. Is that normal?

DMH

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Better, faster, cheaper. Only ever found 2 of the 3! (But still lookin.)

Built 4, repaired 30+, ...So many boats, ...so little time.

I think you need to provide them with more information. You havent mentioned hull displacement that I read so far. Displacement is the very 1st number needed in a prop selection.
As Paul stated, get Dave Gerrs book on props, it is useful for your application.

!st you need to know weight, hull shape, & HP. This yields a estimate of speed.
From speed, HP, & prop dia allowed, you can get estimate for prop RPM.
Prop RPM divided by engine RPM yields reduction gear ratio.

Some things are accepted as rule of thumb. One of those is 100 rpm per knot of speed.
If Goliath gets 6-10 knots then a shaft rpm 600-1000 is real close. If your motor runs 4000rpm, then a gear ratio of 3:1 or 4:1 should work. Since 3:1 is a common gear, you couls likely use that.
Can you get the motor with a standard SAE bellhousing bolt patter?

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God Bless our Troops; especially our Snipers

Sorry I am not a fast respondent... I am working out side too much. For the author of this one, "One last thing, you say the plan calls for a 21" screw. Is that Glen-L's plan, yours or someone elses? I'm just a dumb old country boy, but a 21" screw sounds too big for your electric setup."

On Ken's plans, it appears to allow for this size. I am guessing he had the intent to make it as scale as possible, Ah-La "Theme boat". Also, I am told there is a significant efficiency savings in running a large, slow screw on a full displacement design as this. To be honest, I haven't a clue, but liked both answers I got.

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Better, faster, cheaper. Only ever found 2 of the 3! (But still lookin.)

Built 4, repaired 30+, ...So many boats, ...so little time.