T-Pain Is On A Boat: Re: [Electric Boats] General direction in propeller efficiency

Thursday, November 10, 2016

Re: [Electric Boats] General direction in propeller efficiency

I went to the Landing School for yacht design. This was given to us as a rule of thumb, mostly for small yachts. And this is what I remembered off the top of my head.

I think this rule is for designing new boats. If you have the prop to close to the hull, you can get a slapping noise on the hull, and it could possibly cause a turbulence around the prop, which would effect the performance of the prop.

You wouldn't want to build boat mold($$$$) for a production boat, to find out you can not put the size prop you designed it for. So the rule might error on the side of caution. Although, you are right, when the prop gets real big, 2 inches might not be enough clearance.

Alright, I looked it up in Gerr's book, so this is the professional way to calculate prop clearance for displacement hulls.
"

Genereally, there should be a tip clearance of 15%..

Ideally, it should be 20%...

But, here is Gerr's chart for slower shaft RPM

RPM 200-500 SL Ratio under 1.2 Min Tip Clearance 8%

RPM 300-1800 SL Ratio 1.2-2.5 Min Tip Clearance 10%

RPM 1000 and above SL Ratio over 2.5 Min Tip Clearance 15%

Note in the above table SL Ratio is speed to length ratio.

SL Ratio = Kts / square root of Waterline length in feet

So for a boat to do 6 knots, and it's waterline length is 30 feet, you would get a SL Ratio of 1.095

What king of neworleand said is also true, there are other factors in specifying a propeller besides prop diameter, pitch and blade count.

Here is a list of things you need to specify a prop: (from "Propeller Handbook" by Dave Gerr)
"

1. Diameter

2. Pitch

3. Number of Blades

4. Hand (Left or Right turning)

5. Propeller shaft diameter and keyway

6. Blade Area

7. Cupped or uncupped blades

8. Supercavitating or standard noncacitating blades

9. Blade section shape (airfoil, ogival or compbined)

10. Skew

11. Rake

12. Blade thickness

13. Hub Diameter

1-6 must be specified for every propeller and every installation. 7-13 are of greater importance for different types of craft and in solving specific problems."

On Tue, Nov 8, 2016 at 2:59 PM, sabre281@yahoo.com [electricboats] <electricboats@yahoogroups.com> wrote:

Kev, where did you come up with 2" of clearance from the hull, just wondering.
If you size a wheel to that becomes a pretty small wheel.
The rule of thumb has been 1" if memory serves me well.
My Sabre 28 turned a 14" wheel which gave me 1" clearance. Some of the folding prop vendors claim it can be within 1/2" clearance.

Personally I'd want to run the largest wheel I could get away with going the EP route.

Bob

__._,_.___

Posted by: Kev <captainyoung@gmail.com>

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