propeller comparison

[Floyd S.]

I currently have a boat with 32in hung shen nibral 4 blade props the pitch is 41.5. I am considering switching to Mi wheel X series 4 blade 32in nibral with the same pitch. Can you advise me of these in comparison?

 

[jehardiman]

Welcome to the forums.
Assuming you are not concerned about the material, the most important thing when comparing two well designed props of the same diameter and pitch is Blade Area Ratio (BAR) and/or the Expanded Area Ratio (EAR). Then come the nuances of area distribution and section shape...
Can I ask what type of vessel and why you are changing wheels? FWIW, a P/D of 1.3 is towards the edge of good practice

 

[Floyd S.]

The current props have apparently been repaired several times. The shop that just repaired one said they told the previous owner that they needed to be replaced and would not tolerate any further repairs.

 

[jehardiman]

Hummmm...
"Repairing" a 32" nickel-aluminium bronze prop requires a specialized furnace big enough to hold the wheel. If it has been "repaired" several times...perhaps I maybe would cast doubt on the integrity of the material at this point...lots to sort through there. Do you know what "repairs" were made?...and why?

But back to the original question, talk to Michigan Wheel, they can best comment on how the wheel they offer compares to the wheel you have. It might require you taking a few measurements, but at $4K+ a pop, they will spend a few minutes talking to you.

 

[Floyd S.]

I don’t know the previous repair history of the props. I only know the repair facility told the previous owner that they would not be able to properly repair them another time. I purchased the boat a year ago. It is a 580 PH Ocean Alexander. The DAR on the current prop is 0.75. The Michigan prop would be DAR 0.81.
Can you tell me what effect this would have?

 

[jehardiman]

So internet data says Monk designed, 69,500 lbs, twin direct drive 700 hp CAT diesels with the wheels in a partial tunnel...cruise 13knt @ ~1850 rpm....max 20.5knt @ ~2300 . Slip with a 32x41.5(?) inch wheel is 486% and 383%, which seems way high unless it is a fully-cavitating propeller because it is way off the standard prop charts even with a partial tunnel. Are these the original wheels? It's not that props seem too small, but the pitch seems too large. Perhaps this is why they have been "repaired" so much.
Anyway...
If the area distribution is approximately the same, then a larger DAR will mean increased torque for a given rpm, which means to get to the same rpm, you need more shp. On the other hand, a larger DAR means that there is, generally, more thrust at a given rpm; which for all other things being equal (i.e. same cavitation number), means less slip so less rpm is needed. So a larger DAR hurts torque, but helps thrust; assuming props of similar development. Whether you will make the the same speed at the same shp would require a detailed analysis of the powering curve, engine output curves, and propeller curves. Basically, what was done to select the props to begin with.

 

[baeckmo]

No way you could have direct drive here; the numbers don't match! Floyd S: please give us correct engine type, power/rpm and speed at max throttle setting, plus gear ratio (from plate on transmission, not from hearsay...) before speculations get wild.

 

[Floyd S.]

Cat C-12 705 hp 2.41 gear ratio 2360 rpm 20.5kn these were the original props supplied by Hung Shen

 

[baeckmo]

That makes more sense, but it also shows that the diameter is too small for the power and rpm. To compensate, a high pitch has been applied. The result is low efficiency and serious cavitation, which may be the reason for previous repairs. In this case, the blade profile must be carefully selected to cope with the high loading and cavitating operation; blades should have some camber (or cup), which allows a reduction of pitch. Question is: could a larger prop be accomodated?

Be careful to present all the relevant info to the Michigan staff (not only the dimensions of the old screw); that will give them a better chance to find the right unit.

 

[jehardiman]

No way you could have direct drive here; the numbers don't match! Floyd S: please give us correct ....

Cat C-12 705 hp 2.41 gear ratio

That makes more sense,.

Yeah, pretty much what I was thinking. Still haven't seen a good photo/description of the "tunnel". Could be anything from a prop arch to reduce draft to something that actually plays with Va...<shrug>
Anyway, a direct replacement 32" 4.75 to 32" 4.81 will likely depend on things other than just the DAR...(i.e. engine torque curve, blade area distribution, cupping if any, etc.) As baeckmo says, the Michigan Staff will help.

 

[comfisherman]

I've moved between hung Sheng, Michigan and some other brands. The hung Sheng was bought for a boat that's use case is hard on props to try. It did alright, it was a bit thicker in the hub, fluke base and flukes, can't remember the exact weight difference but it was non trivial for what appeared to be identical skew and spec dimensions. Wouldn't be the first pac rim product that increases volume to compensate for slightly lower qc in metallurgy.

At the end of the day it boils down to dimensions, material and the qc and final finish of your local shop that seem to really make the difference. Currently have a Michigan on at the moment, as the hung Sheng is rotated out being re finished (normal for my work to rotate props out from gravel damage). Can't say as there is a big perceptable difference.

Over the last now 20 years, I've bought in no particular order hung Sheng, Michigan wheel in bronze stainless and nibral, a rice, one teiginbridge and a veem. Volume wise most have been Michigan, mainly due to the local place stocking them. Add another decade to changing props as a crewman and it feels like I've touched them all.

Every new prop or reconditioned wheel feels good to put on. Only one that truly made me step back and go "wow" was the veem.