prop size?

Rick, please don't get angry on me for this comment, but I really doubt that a 18" prop will ever get to have 84% efficiency in the real world, under the hull.
84% is a big goal even for carefully optimized tankers or bulk-carriers, with their slow-reving engines.
In the Wageningen basin test report, there is not a single prop ever going over 80% in ideal, free-flow conditions.

 

daiquiri
I acknowledge this by adding the qualifications for practical consideration like shaft angle and appendages noted in earlier post #12. Our conclusions are much the same by the way. On the other hand ships have a serious problem with flow characteristics off the hull that can be avoided on a small craft.

I am achieving prop efficiencies as high as 87% for low power applications but there is a lot of effort and careful design to get this result. This link shows what can be done with as little as 120W:
http://www.adventuresofgreg.com/HPB/HPBmain.html
I have attached a photo of the prop I designed for the boat. This particular one has an efficiency of 86% at design condition. I have my own software for optimum prop design but it is not a lot different to JavaProp, which I used for the above analysis.

My method and JavaProp have the advantage that they are based on analytical methods and do not depend on empirical data. It is much more flexible and shows what is possible. There are no inefficiencies already wound into them. Helps focus the mind on eliminating losses.

So efficiencies in the high eighties and maybe even low nineties is possible but you have to know what you are doing to achieve that result.

Rick W

 

Fantastic adventure and great result! I loved reading it.

And that prop is a true example of what can be achieved by avoiding the standard schemes.

What airfoil sections did you use for the prop, if it is not a secret? I'm an aerospace engineer, you know, so I'm really affascinated by these things...

 

The sections close to the hub were a little bit thicker than the majority of the blades. They start with a 20% thickness right at the hub and quickly reduce to 8%. The section is a modified NACA 4 series of my design. They have similar performance to an MA409 section but simpler shape. This is a highly regarded low Re# foil.

At the time the prop was designed I was hand fabricating them from stainless flat bar and the design was suboptimal to simplify hand fabrication. Greg was able to find a friendly machine shop able to produce blades from my CAD file. I did a bit of rework on the design for CNC production but did not take it as far as I could have. With complete freedom on section shape I could get 87% with a CNC prop at these power levels and even better efficiency with slightly higher power on an easily driven hull.

My own hand fabricated prop was also tested on the boat and was between 1 to 2% less efficient than the CNC prop. I calculated it would be 1% worse. It is a matter of how much time you spend to grind the profile. I fabricate a complete prop in about 4 hours including cutting, welding and profiling.

My objective is for high efficiency at relatively low power for electric drive and solar/wind energy collection so I have done a lot of work aimed at reducing losses. Understanding the basic physics and using good analytical tools gives reliable results. Performance gaps can be eventually nailed down and reduced. On Greg's boat, the lowest loss component we considered was 0.23W lost in offsetting the rudder drag due to the offset thrust. So we looked at what would normally be regarded as minute components. The shaft is 1/4" spring steel unsupported at the outboard end. I am yet to find anyone who can appreciate this without seeing it work. But it means the 2 to 3W strut loss is completely eliminated.

Rick W

 

Daiquiri/Rick:
Thanks for the sound technical advice.
I work in shipbuilding and repair and would return the favour should the need arise.
I will repost real life performance figures when they are available.
john

 

John
I will look forward to some test results. Always good to get feedback and it will provide you with a good point of reference for next prop if you choose to go that way. Don't forget to weigh the boat if the opportunity presents.

Rick W

 

it might not apply but the bigger the prop the slower it needs to spin compaired to a smaller prop to provide the same amount of thrust. there is a very big difference and the bigger prop the more efficent

 

You're welcome. If you have any other need/question ecc. you know where to find us.

 

prop calculator problem

Ok , soooo,,,, where is the option for single, twins or trips?
Do we use total HP for the 2 engines, & divide the prop pitch in half ???not

 

prop repitch - results

Albin 27 - Westerbeke 70hp - 3800 rpm max power

original owner prop - 18x15 - only 3000 rpm acheived mello cruise @1800 for hull speed - 7knts - begin to overheat

re-pitch prop 18x10 - spools to 3600 maybe 3800 but scary - about 13knts - I think it will spin more - might be a bit under prop. Cruise now at 2600. Not as mellow.

see chart for power curve - is 2600 where I want to be? Im not trying to get the best fuel burn - Id rather get have what the engines really like to spin.

 

You're right, it has none of these options. It's a very simple propeller calculator and performs a preliminary calculation for a single prop-engine combination. It doesn't take into account the increase in effective power for multi-engine boats, and I suspect it doesn't take into account the wake factor either. Yet it appeared to be suficiently usable for what the initial request was.

 

According to Propcalc, the correct prop for your application, "Planing Heavy Cruiser" w. 300HP @4500 rpm and 4800lbs is a 13 x 10, 55% D.A.R min. - providing max of 32.5 Knots w. 12% slip.
With 200 HP @ 3500 rpm, correct prop would be a 14 x 11 providing 26.5 Knots w. 14% slip. You will note with higher shaft speeds, i.e. 1:1 ratio, recommended Prop diameters are smaller.
I would suggest a 14 x 10 since 3500 rpm efficiency is preferred.

 

thanks jango:

I purchased a used 14x12 for cheap and will be launching in Feb some time.
I will post some performance figures when they are available
john

 

hello, and thank you Rick
I: density = 1000; kinematic visco = 0.0000013, and the result is the attached image.
What's wrong?

 

I expect that your result is based on the default foil section, which is a flat plate. This would be the most conservative.

If you want something more realistic then you should select an appropriate foil. JavaProp has a limited range included. For this sort of application I usually select the MH 9.8% at Re#500,000. You need to then adjust the angle of attack until you get the blade area near something that matches the EAR you are interested in. This could mean the AofA is negative because the cambered foil will still provide lift down to about half the camber.

With any propeller you have to be mindful of the resulting blade shape. It has to have enough metal to take the imposed loads.

One minor aspect is the density. If is for salt water then use 1025. The viscosity is temperature dependent as well but it will not make much difference in the normal range.

Rick W