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prop guru question

T

tbrodes

Regular Contributor
Gents,

Looking for advice from the guru's on this board (Jeff Mark, Dave, Chuck, Al). I blew out my starboard prop running home last fall (hit a log coming home in the dark from tuna fishing the last day of the season). Anyway, the boat currently spins 4400-4500 RPM fully loaded (which isnt that often) and cruises at 23-24 knots at 3300 RPM. Boat has 1.5 inch shafts, 2.57:1 trannys. And currently the boat has 22x22 3 blade props.

Been looking around for used props, and have stumbled upon 2 pairs, one of them being 22x25 3 blades, and the other 21x21 4 blades. I am hesitant on both, and am looking for advice. I am nervous the 22x25's will overload the engines, and the 21x21s 4 blades wont harness all of my torque.

Any advice guys?
 
A good prop shop could enter these numbers into a calculator and get a readout with some certainty. But, here is my take, without the calculator!
One thing missing is the cupping description. Is there cupping on any of the props in question? A medium cup may be good for a full inch of pitch.
Overall, I see no reason to go to 4 blade props. And, this is from a guy who just went to 4 blades on my own boat. First, you have plenty of blade area for those engines with 22" diameter. You have no cavitation or other blade loading issues, I'm sure. It appears your optimum pitch is 23" with the 3 blade version. I'd call the 25" too much and your current 22" very close. The 21x21 4 BL will likely run the "correct" WOT and be smooth, but I fear you will give up some speed; and, not that this is super important, but the 4bl props may prove more challenging to remove with some prop pullers due to blade to blade clearance.
 
Ok not a gurw and props are a big subject you may wish to call a prop shop but if I had to pick do to money or what ever id go with the 22×25 s 4 blades it will hold back your Rpms on that engine and you can back off on the other and just be slower / less gas to the other will probley put your engine to over rev not good and you would realy have to wach that to keep from blowing it if you don't have rev limiters. If your not changing both that is
 
If the price is right, I'd get the 22*25's as I suspect you are needing another set of wheels.

I'd then take them to a "prop scan" equipped shop and have them reworked to whatever you need. As the old ads used to state, even "new, out-of-the-box props will be off"...I've seen it first hand and on more than one occasion. Getting them 'dialed in' is worth it, to me.

Like DD said, a good propshop will be able to give you real numbers based on experience, with a high degree of confidence.
 
Dave, actually VERY good point on getting 4 blades on and off. As it is 3 blades are a tight fit!! The 22x25's are 190 bucks, but I'll need to have them bored out, currently 1-3/8 inch mine need to be 1-1/2 inch. I'll also need to ship them up from vero beach to NH. The were tuned 3 years ago, never used again, and have been sitting in the guys closet. Have a buddy going to go look at them hopefully tomorrow night just to make sure they look half legitimate. Any idea what it costs to rebore and usually take an inch or 2 of pitch out of them?
 
Flounder Pounder has a bunch of props around your size range. Most are 4BL. They are at Port Canaveral, FL. fpmarine.com
 
Memories..... We beat the heck out of this situation between my brother and my self a few seasons back. Our 34C with 1 1/4 shafts ran 18x20 3 blade, jumped to 21x23 4 blade. The dropped our WOT WAY TOO much but the experiment was highly informative. Boiling it all down...I would agree with Dave and Mark. Prop scan guys can plug in your numbers to a modeling program a come very close to your requirements. We did have some massive bite in the water though. Kinda like a car .... 427 with 4.11 gears, off the lline was hot but near top end was gone
 
Al,

exactly what I am afraid of, albeit I am not going to be making that big of a switch! (i'd need to be running 25x25's to make a similar move)! I think I will certainly be staying with 3 blades, as I think I have enough surface area to begin with (my wheels look HUGE compared to friends boats that have 454s) and I have a big tranny ratio. A guy off another forum is running some #'s for me, showing the prelim results that I am not using all of my available HP with the 22x22's, while the 22x25's will use just about all of it, with less slip, less cavitation....but more tweaking needs to be done.

Anything else that pops into your heads appreciated!
 
I can assure you from personal experience that those 21x21 four bladers will perform virtually the same as your present 21x22 three bladers. The boat will move a bit faster at idle, plane off a bit easier, and run abut the same rpm at WOT. If they are reasonably priced, I'd go for them.

Jeff
 
Don't go with the 21X21 4 blade, they do not equate closely to your 22X22 3 blade props. 4 blade vs 3 blade is aprox equal to adding 1 inch of pitch, however the loss of diameter equates to reducing the pitch by 2 inches and will result in being underpropped. The 22X25 can be repitched and rebored at aprox. $350 each. Buying the the 22X25 and incurring the cost of having them altered will bring them into the cost range typically seen for good set of used props and they can be tweaked for your boat. Is your existing prop damaged beyond repair? A good prop shop can repair almost any degree of damage. Try consulting with Bossler & Sweazey.
 
The old rule of "1 inch diameter equals (etc, etc.)" is less valid when a prop gets large in diameter. (For example, the loss or gain of an inch is meaningless for a prop 5 feet in diameter!) In this case that loss of 1 inch on a 21/22 inch prop is not significant COMPARED to the gain in blades: 3 to a 4 blade. The end performance should be fairly identical to the present props with the advantages I posted above.

Hey, I DID this on mine and know what I'm talking about.

Jeff
 
No matter which you choose it will be SWAG without an actual in water test. Unfortunately it is pretty much out of the question to do one. It involves testing several different props over a timed measured course. On a bigger boat that means pulling it out of the water and changing wheels several times until hitting on the ideal set. I don't know how much it would cost to tie up a Travelift for half a day....
 
Guys,

Thanks for the comments, as always learning a lot from you all. Jeff, current props are actually 22x22, so downsizing to a 21x21 4 blade I think would prove less efficient. What do you think the most efficient top end RPM is? Should I be shooting for 44-4500 RPM or 3900-4000?

Cheers Guys! Had a buddy take a look at the 22x25's, said they are in great shape, and look new. My damaged prop is missing an entire fluke...so its beyond repair unfortunately :-/
 
I concur that 1 inch difference (smaller) on a 5 ft dia prop may not be significant. However, on a 22 inch dia prop it is significant, the area of the thrust face is reduced by nearly 10%. Increasing from 3 blades to 4 blades will provide an increase in efficiency of the propeller, however, the efficiency of each blade decreases due to turbulence created by the blade ahead of it. The efficiency gain by going from 3 to 4 blades will not offset the thrust loss from reducing the diameter. Additionally, the increase in drag of the 4 blade prop over the 3 blade will result in loss of top end performance. Mid range and initial acceleration will improve (until increased drag exceeds the efficiency gain). I have studied this extensively (Fluid Dynamics), thoroughly read the "Propeller Handbook" (by David Gerr) consulted with both prop shops and Michigan propeller before buying new props. Hit the numbers right on during the first sea trial following the repowering project. I also know what I am doing.

Regarding wide open throttle, What does your engine manufacturer recommend? 4200 - 4400 RPM at WOT sounds about right. Something lower than that will not allow the engine to develop full rated HP.

As they say "You pay your money and You take your chance"
 
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Viking76 said:
Increasing from 3 blades to 4 blades will provide an increase in efficiency of the propeller , however, the efficiency of each blade decreases due to turbulence created by the blade ahead of it.
Click to expand...

I would substitute the word "slip" for efficiency in the first part of that sentence, and sub decrease for increase. Per Gerr, the efficiency of a prop is a complex relationship between slip and friction, and the optimum solution will even depend on the rpm and speed at that moment. Thus, the birth of in-use variable pitch props.
Actually, did he not say the most "efficient" prop is a very long one-bladed prop?
 
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I do not concur with your recommended word substitution. Slip and efficiency are two different things. Also slip and drag are not the same thing and should not be confused as such. As efficiency goes up, theoretically slip goes down. However, this is not necessarily true, as the number of blades increase, the hydrodynamic drag is increased on each blade due to wake turbulence created by the blade ahead of the subject blade. The net gain is positive from by increasing the number of blades (evidenced by larger pleasure boats using 5, 6 & 7 blade props) Your concept of the optimum solution being dependent on RPM and speed is only partially correct since in H2O the drag increases at a rate of 4X the speed. The one very long blade prop might, theoretically the most efficient, it would be very difficult to balance, therefore being impractical.
 
Viking; Your missing the point about when it is best to increase the number of blades. Miss Budwiser runs an extreemely engineered and trialed prop system and guess what, it's two blades. Saying generally that more blades gives you more efficiency is dead wrong. Look up the section in Gerr's book about "diameter limited" props and how more blades helps in that regard. He goes into great length into explaning that DIAMETER is best for increasing efficiency and more blades is second, due to the reasons you explained (more friction due to more blade area and leading blade turbulence). You have a lot of the points correct, but somehow are missing the blades/efficiency relationship. My point stands on going from 3 to 4 reduces slip. Only if the added friction/turbulence component is a lessor effect than the reduction of slip, does the efficiency increase. My def. of efficiency in this context is distance vs fuel, given a target speed.
 
Diver, I'm not missing the point about anything. Miss Bud is in a different category than most of us can play in. The Miss Bud Organization has nearly unlimited $ and big HP. Most of us are "diameter limited" (also funding limited) and therefore we must look to other means of reducing slip or improving the overall efficiency of a propeller. Methods such as increasing the number of blades, increasing the blade area, optimizing the blade shape and profile, and employing cupping are available options. I clearly understand the "blade/efficiency" relationship in that as you increase the number of blades, the efficiency of each blade is diminished, however, the net gain (propeller total) is increased. In your context, I can concur with your def. of efficiency, however, it appears to be more adaptable to a complete system(boat, engines, etc.), not propellers which is/was the subject of primary focus.

Putting all this aside, it appears that the question of "what props should he buy" appears to be unresolved. There may be no precise answer. We and the pro's can only offer up suggestions and recommendations.

As stated before, "You pay your money and You take your chance!"
 
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As stated before, "You pay your money and You take your chance!"

AND...To prove a point, we had the option when we changed props to experiment A LITTLE....Our vessel was re-powered from 350's to 454's BUT, they used the old props to run...They would spin over 5k and we never took the chance of finding out the limit because she really screamed.... DESPITE all of the the models and wisdom to be found here and locally, we grabbed a great deal on those big 4 blade props so for the price of a short haul, we used them and discovered ..: GREAT idle forward and reverse movement ( 30% or so faster), superior turning in tight quarters, improved fuel economy under 2000 rpm BUT WOT was severly reduced as was cruise speed.

We then believed we had discovered a few things about our WOT and engine limits. After finding a set of 4 blade props , and I think I remember these as 20x22 ?, we seem to have struck a very happy medium from our original 18x20 #3 cup 3 blade wheels.

WE GOT LUCKY, we sold the big wheels, paid for the hauls and new props and MAYBE pocketed a few bucks in the long run. The educational value for us would have been enough. Getting behind the wheel and feeling the changes to OUR boat were more illustrative than all of the reading material available DESPITE its validity. Like reading about an experiment and knowing the possible outcomes OR doing the experiment and seeing it happen in front of you.
 
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