Gas Powered boats 3 blade vs 4 blade

[trace elements]

I am looking at putting a pair of Acme props on my boat to improve the slip numbers. I currently am powered by 454s with 1.5 gears and 18x19 michigan propellers. I calculated a slip percentage of 25% at cruise.

I know all of the old adages of 3 vs 4 blade, what are everyones thoughts on 3 vs 4 blade on gas boats?

I am looking at Acme 18x18 3 blades or Acme 17x18 4 blades, which would you choose?

[Tony Meola]

I am not a prop guy, Carl is better at that, but if you run 3 or 4 blade why change the size of the prop. I can understand pitch and cup, but I would think the diameter remain the same.

If you are not running Nibral props, I would include that in your calculations also. Props flex less with Nibral.

3 Blade props will give you a higher top end, while 4 blade should give you a little better cruise speed with a lower top end speed.

[trace elements]

Thanks, Tony.

As I understand it, the 4 blades have more blade area and therefore more drag requiring a reduction in prop size to maintain correct RPMs. Same thing with the larger DAR props like ACME. Increased drag requires a reduction in pitch or diameter to keep up RPMs.

[Carl]

First I would not consider going down in diameter to offset the extra surface area of a 4th blade. A larger diameter provides less slip, think of wider tires for more traction on a car. If going to 4 blade, reducing the pitch or cup would be my instinct.

Next, I'd ask what are you trying to accomplish. Yes, I read reducing the slippage as your intent, but to gain what in it's place? Economy? Speed? If speed top end or at cruise?

Tony is right, nibral can add some performance, at the cost of cost.

You offered info on the wheel, diameter, and pitch but did not mention cup. If you do not have a cup in the wheel you may see some benefit of reduced slippage from adding or increasing the cup. You may have to reduce the pitch though, depending on where your WOT rpm is.


Acme makes a great wheel with many seeing performance improvements. But I'd suggest finding a really good prop shop, sitting down with them and all your specs...motor, boat configuration and weight, trans ratio, speeds and associated rpms, WOT and let them go over to explain the difference between the word improvement and expectation. Lots of people expect removing 15% slippage will see a speed increase of 15%...not usually the case. And sometimes as you get closer to optimal, a change...whether it be blade count, pitch, cup, wheel style and brand can have you moving away from your target. Or put another way...playing with wheels can be trial and error making it tiresome and expensive.

Whatever you do, I say beware of the pitfall of hitting your performance hopes...but not quite attaining your recommended WOT...that is a surefire way to motor problems.
Many will toss on a set of wheel, go wow, I'm really moving now...but miss WOT by 200rpm. Eh..I'll go easy, it will be fine. Then comes spring, then summer with all the family gear n supplies coming aboard, the bottom no longer 100% and that 200 now becomes 4-5-600 off the rated WOT.

[trace elements]

Thanks, Carl.

My goal is just to improve fuel economy. I get 1.1 at 22 knots. Any improvement on that would be nice. Like I said, after calculating, I have a very high prop slip at cruise (25%). Getting this number below 20% or even 15% would be significant.

I currently have Nibral 18x19 with some degree of cup. I had them computer-measured with propscan this past year. I am looking to go with 18x18 Acme 3 blades. I am hoping the modern geometry and improved DAR (disk area ratio) will help me attain my goal of lower slip and higher efficiency.

[Geebert]

If you want guaranteed improvement and have money to spend, call sharrow marine and order a set.
Id love to know how they'll do on a bertram.

[Tony Meola]

Todd

I don't think Sharrow has experimented with an inboard prop. Not sure they actually have the clearance to do it without increasing the length of the shaft.

[trace elements]

I think a set of sharrow props for outboards are in the range of 5k. It would probably be cheaper to do a diesel repower then buy a set of sharrow props for inboards.

[Carl]

Have you spoken with ACME, I've found them pretty helpful.

I'd hate to spend that price on perfectly machined ACME wheels only to have to tweak them...no longer perfect, will they be. They'll get them close but they get heated, banged n twisted by hand ground to balance then onto a prop scan to check.

[Geebert]

I think a set of sharrow props for outboards are in the range of 5k. It would probably be cheaper to do a diesel repower then buy a set of sharrow props for inboards.

What about a diesel repower AND sharrow props. With all the fuel you'll save, it'll pay for itself by the year 2089

Btw, i bought an acme for a shamrock. I had no idea what to get but they knew exactly what it needed because they have a data base of info they've accumulated over the years. I'll bet a phone call to them will get you real close, if not spot on.

[Carl]

I don’t know of anyone with First hand info on Sparrow props, I like innovation and have looked at the ads and articles with peaked interest.

What is presented looks compelling, but maybe it’s just the graphs I have seen, they compare the Sparrow prop to another showing better speed, economy but the Sparrow prop in the graphs I have seen are a good 500rpm lower at top end (wot). So is advanced performance due to overpropping? Adding 2”+ or pitch can increase your cruise, run at lower Rpm for better fuel economy…but motor needs to hit Rated WOT. At the least, I’d like to see comparisons with both props dialed in 100%.

Still a very cool design innovation, but is the lack of traction because of price, availability or maybe not quite what they claim. They have been out a good long while…Remember when the dol-fin came out…every outdrive, every outboard seemed to have them.
Ok, the fins were cheap… how about pods, those are pricey to buy and fix… when parts are available.

[Dug]

Trace,
Personally, I have a strong preference for four blade propellers. I have run four blade propellers on my boat when it was both gas powered as well as diesel. There was a noticeable improvement in close quarters handling and docking. I also feel the boat ran more smoothly with a little less vibration. You should have a more efficient cruise however you will lose top end. But who runs about it top and for any prolonged period of time anyway. I will take more efficient cruise any time. Be sure no matter what propeller you purchase that you buy nibral instead of just plain bronze. As it was explained to me years ago are hulls may provide comfort but in order to do that they not particularly efficient. They take a lot of power to move through the water as does any deep vee. That means there's a lot of torque being applied to the blades and the Nybral are stiffer which means less flex which means more efficient. I hope that helps!

[scot]

The chase for higher MPG is non stop with 454s. You are currently seeing 1.1 MPG with twins at 22 knots.. correct? That works out to 10 GPH on each engine. For 454s pushing a 31ft deep Vee, I think that is outstanding. 25% slip is also a decent number on that setup. Imho you may be close to the top on that hull, power, prop, gear, engine setup.
I’ve alway been a fan of 2:1 gears in smaller boats which affords you a larger diameter wheel, which equals higher cruise efficiency.. but potentially at the cost of top speed.
To make things better you will need to get down to a 9 GPH burn, on a 454, or increase speed but 3-4 KTs.
If you get there (and I hope you do) let us know, good luck.

[Geebert]

Another factor to consider that is often, if not entirely overlooked, is tip speed.
There is a limit to what rpm you can spin a given size propeller before it starts to eat more power to turn, than it generates in thrust.
With gas engines(high cruise rpm) , 1.5 trans, and 18" prop, you're near if not over the max tip speed that size wheel should be at, at your cruise rpm. You're well above it at max rpm.
Max tip speed is entirely subjective, but there is a general "do not exceed" number for efficiency. Just one other part of the "props are magic" phenomenon.

Personally id go 4 blade 16" and up the pitch until your max rpm is in correct range. I may be the only one with that opinion, but i think mine smells no worse than anyone else's.

[Carl]

Rule of thumb I am most familiar with- go with the largest diameter while keeping the wheels square. 16 x 16, 17 x 17, 18 x 18...when you cannot turn larger add a blade, pitch, cup, rake etc. Rule of thumb with many exceptions which I tend to leave to a good prop shop.

[trace elements]

I've read Dave Gerr (Naval Architect) book on propeller sizing and deisgn. He recommends maximizing diameter and using cup if you cant fit enough diameter. Still pondering this issue, but 25% slip seems way to high. When I rebuilt my onboard mako, I was able to get the slip down to 9% at cruise. Gained 5 knots just by switching to acme.

[bob lico]

if it is any consolation i went thru 18 prop changes over the years to get were i am now. i have slowed down the wot. speed down 4 knots to increase cruise speed and dramatic increase in fuel economy. so many factors to consider but Dug is on the right track, that is get as much Nibral as you can in the water but remember the enemy of slip is agitated water. look what is in front of your props like transducers, raw water pick ups, thru hulls etc. i have reduced my slip to a bullshit number no one would believe so i will leave it at that.

[Tony Meola]

Give Atlantis propeller in NO a call. I know someone who had them make an Acme prop using an oversized blank and them machining it down in size. This gave them more blade area, picking up additional cruising speed.

[Geebert]

I've read Dave Gerr (Naval Architect) book on propeller sizing and deisgn. He recommends maximizing diameter and using cup if you cant fit enough diameter. Still pondering this issue, but 25% slip seems way to high. When I rebuilt my onboard mako, I was able to get the slip down to 9% at cruise. Gained 5 knots just by switching to acme.

Second what bob said. Weight, clean bottom, and up stream obstacles of the prop are important conciderations when scrutinizing slip.
Just out of curiosity, what were the rpm, gear ratio, prop and re-prop size of that mako?

[bob lico]

nice ----9% Slip very impressive, this takes quite a bit of planning and prop knowledge . to gain 5 knots is a tremendous fuel economy gain by just backing off the throttle and obtain the same cruise speed .Geebert your on the ball!

[trace elements]

Second what bob said. Weight, clean bottom, and up stream obstacles of the prop are important conciderations when scrutinizing slip.
Just out of curiosity, what were the rpm, gear ratio, prop and re-prop size of that mako?

It was a 236 inboard. Had a 260 hp mercruiser 350 mated to a 1:1 trans. The original prop was a 14x9 Michigan. Repropped to a acme 13x10.5.

Cruise with the Michigan was 20 knots at 3000
Cruise with the acme was 25 knots at 3000

[Geebert]

This is a tip speed graph. I dont remember where i got it from. Seems like it was an article written by a Naval Architect or engineer, making the point of the importance of proper gear ratio/prop diameter selection.
It is not universal for every application. It was a general guideline for planing boats, open propeller etc.
The idea is the line is the max, so stay under the line, not get as close as you can to it. The closer to the line, decreases efficiency and increases cavitation tendency.
The graph is in meters per second and kts.
Your 14" prop at 3000rpm with a1:1 trans would be spinning at approximately 183 feet/sec or 55.7 meters/sec.
The graph limit at your speed of 20kts is about 147ft/sec. Or 45meters/sec. So you were a good amount over.
You new prop 13" at 3000rpm is spinning about 170 feet/sec or 51.8 m/sec. Almost right on the line at your new speed 25kts. Heading in the right direction of lower tip speed, per the graph.
This is why there is a limit to how big a prop you should stuff under the boat.
There may be more efficiency dropping even further but it takes a lot of dollars to experiment, and your targeted performance may not be the same as maximum achievable efficiency. (Good enough economy for desired speed/performance)
There's plenty of combos that people blame the engines for poor efficiency when they should be blaming the trans and prop.
Gas engines running to 4000rpm or more, should not have a prop larger than 13" with a 1:1 trans. If a larger prop is needed, you would need to drop the gear ratio to keep the tip speed in check.
This is why i would stay with 16" or 17" max with your 1.5:1 transmission and gas engines.
For more anecdotal evidence, search this board for the members stated performance.
The gas guzzling original power plants were 1:1 trans and 16" props.
Repower with new gas, or lower rpm diesel engines and 1.5:1 or 2:1 trans, and the engines get the credit for the new found fuel efficiency, when a large portion of the original problem was incorrect gear selection.
This turned into quite a bit more than i set out to write, hope it helps, or at least causes an argument and name calling, id hate for it to go to waste

[Carl]

I cannot argue about the Acme wheel being the perfect end choice for your boat, the performance is great with very little slip on an incredible rebuild/restoration, guys you should check it out.
https://forum.classicmako.com/forum/cla ... oard/page5


What I can debate is the performance comparison of the Michigan Wheel to the Acme Wheel.

From what I read Michigan wheel was "reworked many times".
-Each time a wheel is "reconditioned, reworked or tweaked, the blades are forced into a new position, AKA- Bent. Bend a wire hanger a bunch of times and what happens? But even more so, if the blade placement is changed, the balance is thrown off. This requires the prop to be "rebalanced". Bring a car wheel to be balanced, they add weights where the wheel is light to bring it into balance. On a propellor, they balance by grinding material off where heavy, thinning the blade in the process. So if really "reworked many times" the tired, thinner blades can flex. Flex will be read as poor performance with high Slippage.


The boat ran 4500 RPM WOT with the Michigan wheel. The rated WOT is 4200, the boat was underpropped.
The boat ran 4050 RPM WOT with the Acme wheel initially, then the motor was tuned up to get 4200 ( I hope). Did motor need tune-up when hitting 4500 rpm with Michigan too?

The Michigan wheel was underpropped by 300-450rpm. Using the minimum 300rpm to play with, an inch of additional pitch plus some cup could have been added. That would likely add another 2-3knts.

It's just tough to say what would have happened if you bought a new Michigan wheel with additional pitch n-cup, dial that in as well as possible. At that time try the Acme Wheel for a true apples-to-apples comparison.



Now the tip speed is a whole other area that has me thinking...interesting for sure. But wonder if your plugging his numbers into your chart is more of a hope to see Tip Speed as proof, nullifying the fact the boat was originally underpropped. So in addition to changing Tip Speed, there was a change in pitch with cup added...new Nibral compared to tired bronze, old style Michigan to new style large blade Acme wheels.

I think with so many variables being changed at once, trying to pinpoint any one item as being the reason for performance gain is nothing more than a crap shoot.

How was that for stirring the pot on the 1st day of Spring??

[trace elements]

Impressive, Carl. Thanks for the kind words on the Mako. The only boat that could have ever made me give it up was the bertram 31.

Agree on all of your points. Simply chalking it up to new wheel isn't completely fair.

I never put much stock in the tip-speed theorey. I was a student of Dave Gerr at Suny Maritime, (hence why I bought $$ and read his book). He had mixed feelings on the thought process. It is a guideline, certainly not a rule.

[Geebert]

Alright Carl, i see you've come out swinging(a big prop)

I think with so many variables being changed at once, trying to pinpoint any one item as being the reason for performance gain is nothing more than a crap shoot.

How was that for stirring the pot on the 1st day of Spring??

We agree on that Carl.
I wouldn't try to hang innumerable variables on one observation, but i wouldn't ignore the availble data point either.

And not hoping it fit the chart, pointing out that it fit the chart.

The only hoping is to meet and armchair race in person one day, other than that, i let the facts stand on their own, its up to the reader what, if anything is gained from it.

Also agree on that project mako.
That's a fine job on another classic boat that always catches my eye.
Student of Dave Gerr?
Shouldn't you be the one answering the question of which prop?
Whatever you decide on, please post the before and after data.

Mine is also 454s 1.5 trans currently with 17x19 3 blade props, but i have no performance numbers as i haven't launched her yet. Between hurricane Ian and the construction craze in florida right now, I've been too busy to get back to it. Hopefully i'll be in the keys with it this summer.

In the meantime i read about everyone else's boats and argue with Carl.

[trace elements]

Let me know how your boat runs when you get it in. Look forward to hearing about it.

[Carl]

Well not much to debate if everyone agrees…. Lol.



Geebert, it fits the chart, but is it because of tip speed, pitch change, less flex, increase blade area, added cup or…

I was once told larger wheels have a downfall and that is increased drag and or friction. When going over a certain speed a smaller prop is better. I wonder if true…then look at drag boats and say…I guess so as they turn tiny props

[bob lico]

carl i disagree to a certain extend with diesel power everything changes a gas power plant spinning a shaft at 4500 rpm with 1 to 1 yes slippage is critical however if we are using tip speed chard with 1.5 to one at 2600 rpm (to obtain maximin cruise speed for fuel economy ) tremendous difference i say go with the biggest prop you can fit under the boat to utilize the 700 plus pounds of torque that a cummins/ caterpillar puts out ) and spin it slower, thats the final analysis after sooooo much experimenting with a unlimited amount of other peoples props.on my boat in a real comparison situation , changing props in 15 minutes with giant forklift lift boat out of water back up and air impact to remove and reinstall put back in water and test.

[Carl]

Bob, I believe we are on the same page with us going with reduction and the largest wheel for our 31's. My comment about drag on larger wheels is when going much faster than we do in 31's...

Where the wheel size starts to factor in I do not know, that's more your area of expertise.

https://www.youtube.com/watch?v=i3fCVHT9kYM

[Geebert]

Im sure there's an argument here, we just haven't found it yet.

The chart does not apply universally to all craft. It is limited to normal planing hulls, like bertrams, makos, shamrocks, with submerged props. The article was targeted toward inboard power so outboards would certainly have a different curve.
It dose not apply to either extreme of drag boats on one end and tug boats on the other.
There was even an exclusion for multi bladed propellers like the 5, 6, 7 blades you see on some boats, and ducted propellers.

They all have different efficiency curves, BUT they all do have an efficiency curve.
With so many variables its impossible to have a catch all graph.

The intent of the author was to illustrate the most common vessel type, with the most common propeller type, and offer a rough guide line for the difficult task of choosing the correct size prop and to point out that gear selection should be paramount to prop selection, not an after thought.

You can use any engine in the world to power a boat, by selecting the proper gear and prop.
But if you only have one gear ratio available, than you are limited on engine selection, to what is compatible with that shaft rpm and power curve.

All of this for the sake of optimum performance.
Not to say you can't use whatever you want or that this is all that will work, we know that's not the case.

Its just one of the many indicators you observe and evaluate, if you're pursuing the optimal set up for your boat.

I've been plugging every combination ive come across into that chart for fun(my version of fun) since i found it a couple years ago, and the combos that people change out are always closer to the line, or past the line, than the ones they finally settle on as the best compromise of speed and efficiency.

Even yours, Carl.

Pete Fallon claims some of the best efficiency numbers ive seen, for 454's. I know some of it was attributed to shallow shaft angle, which I'm sure contributed, but he was also running 17" props, where others are trying to run 19-21s (with gas engines)and with less efficiency.

Again, not saying it's a stand alone fact, but it dose fit well, where good combos are identified.

I'll see if i can find the article it came from. I screen shot the chart but not the article.
If i can dig it back up i'll post the link.

[Carl]

Im sure there's an argument here, we just haven't found it yet.

Todd,

I prefer "debate" over argument. LOL

I really do like hearing every aspect and view from everyone, especially from those that know why things work or at least should work or shouldn't for that matter. I like it more than "This is what works, go with it". Yes, the outright answer is great when doing a build and needing to get it right the first time.

Hell, I used the tried and true "This works" method when seeing if I could swing a budget to convert to diesel. I had motor cost, I had a pair of props that I knew should work with shafts, I knew what Struts should look like to figure a price to make. Yup, when it's "need to know" I go simple tried, and true, but...

But beyond that...well I find myself like our old math teachers " If I don't see your work, it's wrong, even if it's the right answer". I find the Tip Speed interesting and while one hand looks and goes, cool, yeah it works the other hand tries to debunk. In the end, I'll keep looking, and someday maybe when I have time for pure personal fun...I'll put together a test platform (AKA- Cool boat) to test each individual variable to compare hoping one day, maybe I'll know why it works instead of that it just works.

The drag boats was put in post as I love watching and hearing them.

[Geebert]

Found it.
Let me know if the link doesn't work.


https://www.hydrocompinc.com/wp-content ... ertalk.pdf

someday maybe when I have time for pure personal fun...I'll put together a test platform (AKA- Cool boat) to test each individual variable to compare hoping one day, maybe I'll know why it works instead of that it just works.

That's my dream for retirement as well.
There's tons of things I'd like to tinker with, just lack the time
and$$

The drag boats was put in post as I love watching and hearing them.

#metoo

It's hard to argue with someone you agree with so much, i guess we'll have to settle for "debate"

[Carl]

It's hard to argue with someone you agree with so much, i guess we'll have to settle for "debate"

Come on, you have to try harder! Lol


Interesting article…. I have to read it a few more times to really grasp. But is it possible at some RPM,s the water just scatters.