props finally on, repower nearing completion.

[Carl]

I'm not calling anyone stupid.

Many people do however hear and see what they want to hear when it comes to propellers and gaining performance.

Case and point- my buddy wanted new props, would like to go a little faster. Can I have pitch increased. I know his boat and motors are supposed to crank up to 4800 and they just barely do. He's telling me no, they hit 5K and the company who built the motors said they only need to go to 4600. Long story short, motors just barely hit 4800...unless he's running down the back side of a swell and standing off to the side when reading the tach. As to the 4600 part, it was on a tag that said between 4600 and 4800, he just eliminated that "between" part.
I made him happy for a little while by changing knots to MPH on his chart plotter.





Why did 3" change in pitch only reduce RPM by 40...Governor? Slippage and big changes do not always stay in line with the rule of thumb.



When you had the guys do the prop analysis, I am sure they had the motor info, torque curve, transmission, weight of boat...but did they account for the deep Vee? Our boats eat alot of power over say a Hatteras with a flat bottom.

[mike ohlstein]

The engines will turn to 3450 under no load. You can run them continuously at 3100. You can run them 10% of the time at 3300.

[bob lico]

mike you and i went over the exact same boat and hit on the first try. back off on pitch and go for cup pitch IS not your friend . mike tell him what we did.

[saburke17]

3000 Rpm's 27 knots, WOT 3200 rpms 28.4 knots. Will take 2 inch's of pitch and call it day 21x22 light to medium cup

[Yannis]

Saburke,

I think Dug has the same engines but his numbers are better, give him a call.

I have the same engines and I do 24 knots at 3000 and 30+ at wot 3300.

[Carl]

Appropriate wheels are 20X20, 20X21, 21X20 or somewhere in there. If the boat is light, maybe 21X21

Cruise is about 22, top end is about 25, give or take a knot. This is 20 year old science.....

Can you tweak another knot out of it? Sure. But you have an 18 in your equation, you're on the wrong track.

You took the long way around...but you got there.

Saburke,

I have the same engines and I do 24 knots at 3000 and 30+ at wot 3300.

Yes Yannis...but Saburke has a real boat.










Sorry Yannis...you know I just couldn't let that slide.
I tried and tried, it wouldn't slide.





Glad you are there Saburke, now go out and enjoy her!!

[Yannis]

Carl,

You knocked my socks off!!...I’m watching the evening news while playing with my iPad, nothing to watch really, it’s all coronavirus...so in a semi lethargic mood I read your comment and laugh! Lol...

[mike ohlstein]

3000 Rpm's 27 knots, WOT 3200 rpms 28.4 knots. Will take 2 inch's of pitch and call it day 21x22 light to medium cup

I would still advise 21X20 or 20X21 three blade with a more severe cup, especially at the tip.

If you want to out-troll everyone in the canyon (except me...) go with a 20X20 four blade. You won't be a speed demon, but you'll be super smooth and the wake pattern at 1100 rpm will raise fish like no other boat.

ISYN

[saburke17]

sounds like a challenge Mike, where are you out of?

[mike ohlstein]

Long Island, center for uncontrolled disease.

[Carl]

Carl,

You knocked my socks off!!...I’m watching the evening news while playing with my iPad, nothing to watch really, it’s all coronavirus...so in a semi lethargic mood I read your comment and laugh! Lol...

Before this "pandemic" the news in general wasn't overly uplifting...today its beyond depressing. That semi-lethargic mood you spoke of is going around strong for sure and needs a relief valve. Glad you took the ribbing in the spirit it was meant.

[Carl]

If you want to out-troll everyone in the canyon (except me...) go with a 20X20 four blade. You won't be a speed demon, but you'll be super smooth and the wake pattern at 1100 rpm will raise fish like no other boat.

ISYN

well I wasn't planning on keeping the 20 x 20 4 blade wheels on too long, but maybe I will. I was hoping they put me on the under proped side so I could go 3 blade, increase pitch a bit, then add more cup to hit correct WOT. Guess when boats in I won't need to be guessing.

[mike ohlstein]

100% on the right track.

[Tony Meola]

Ok so what am I missing. The 4 blade smooths out the running, gives you a higher cruising speed and a lower top end. Since we don't run on the top end why not go 4, unless you want to troll for stripers. Then go 3.

[Carl]

Tony--

To keep it in super simple terms that even my mind kinda understands...the extra blade

-gives extra traction (each blade takes a bite, think slippage, hole shot...going up the back of a wave, think big fat tires on a car)

-divides the blade load (3 blade wheel with 3 hundred hp is 100hp per blade, 4 blade wheel its 75hp...think blade flex under load )

-better balance (smoother)


The extra blade also tends to lift the stern. Beyond my brain power...but "I think" of it as a paddle wheel effect. "I could be 100% off"...but the reality is a 4 blade will add lift to the stern. Nice if your stern heavy and boat tends to squat. Not so nice at higher speeds when boat comes up higher out of the water then starts pushing the bow down.


So that is sort of the up side of 4 blade.
The down side

- the extra blade is extra friction in the water. That takes power, so figure a loss of 200 rpm for the extra blade. That is compensated for by reducing the pitch...AKA go slower.



The compromise The Trade Off

So then its how much more efficient is the 4 blade wheel due to less slippage, less blade flex. Does the extra blade lift the boats transom higher allowing less boat in the water for a given speed making it easier to push...making it more efficient or faster. OR does the extra blade job rob you of extra pitch so you go slower and not as efficient.

Do you want to give up a little on your cruise speed for extra smoothness.



As for trolling...stripers where you want to go slower, a 4 blade with less pitch would allow you to go slower. I think what Mike was talking about is offshore trolling where the boat noise has a big part in raising fish. I never heard about 4 blade being better...but lots of things I never heard about. My dad and his bud swore when his buddy swapped out generators they started catching at the canyon. For a couple years they could be in the middle of the hot spot and not produce. Then after, they could do not wrong.



I was starting with 20 x 20, 4 blade wheels as I had a pair in stock.

I think they will put me in the ballpark...but I am pretty sure I can loose a blade for more cruise speed with a 3 blade down the road. Smoothness should not be a factor anymore...shafts turning dead slow with Cummins and 1-1/2 :1 compared to my 440 gas days with 1:1 egg beaters.

[mike ohlstein]

I think what Mike was talking about is offshore trolling where the boat noise has a big part in raising fish. I never heard about 4 blade being better...but lots of things I never heard about.

I was actually referring to the way that the boat makes it's wake, but the sound factor can't be discounted either. Anyone who has been to the canyon with me will tell you that I catch a lot of fish less than 10 feet off the stern, or at least I did when I was using a 4 blade 20X20 wheel. It's that first wave. There's something about the shape of the bottom, the 4 blade wheel, and the sound of the boat that creates the magic. 1100 rpm. 6.7 knots. Engines synced by EAR (not by rpm). Green baits on bright days, purple baits on overcast days.

And sure, I catch them out in the spread too, but the biggest fish I've raised simply come right up to the boat and hit the flatline.

[Carl]

I was actually referring to the way that the boat makes it's wake, but the sound factor can't be discounted either.

I thought it was all pieces to the puzzle starting with noise that travels out bringing fish in closer to take a look, then if they like what they see...

At anchor what makes one boat hot and another not? Wake, wash and props out of the equation...generator? noise sound some they may have an attraction to, curious...do fish get curious??? Teasers...is it the splashing or the noise, maybe both.

I guess if we had a sailboat we could see if they caught more with motor on or off when moving along

[Tony Meola]

Well I can attest that two 31 Bertrams will not raise fish the same way. New ours was a 75 FBC with 454 Mercruisers. Our friend was running a 68 FBC with big block Chryslers. Offshore in the Canyon, our boat would easily raise 3 fish to his one. And when I say side by side I mean 50 to 100 yards apart.

I always attributed to the engine noise and the bottom color. He painted his bottom black ours was blue. Plus he docked on Forked River and we were up on a creek. His bottom was always stained brown from the Cedar Water in the river. Ours was always fairly clean. I kept telling it was because the fish thought they were looking up at the sky when they approached our boat, and his looked like a big shadow to them.

[Tony Meola]

Carl

One other thing in what you said, I think the 4 blade gives you a higher cruise and the 3 blade gives you a higher top end. To me top end is not as important unless it is to somehow save my tail, but at 30 knots I should be able to get out of almost any trouble when a short spurt is needed.

[Carl]

Tony-

All things being equal a 4 blade prop has to have the pitch reduced by at least an inch to attain the rated WOT. An inch of pitch can be a couple mph at top end.

The math-

3 blade with 12' pitch goes 12" forward every revelution
4 blade with 11" pitch goes 11" forward every revelution

In theory the 3 blade "should" be faster at every RPM from Just In Gear to WOT...right?


But One benefit the extra blade brings to the table is more "traction" or in prop terms, less slippage. When does a prop slip most....when it's pushing the most and that is when getting on plane, low speed cruise when much of the hull is in the water.

Benefit Two is the Extra blade add lift to the stern. That can put the boat on plane sooner, more efficient and faster at lower rpm. Come up on the throttle a bit more to cruise that extra lift in the stern allows a faster cruise...like a little trim tab without
the braking effect of tabs.



Bring it together,
At cruise a 3 Blade at same RPM "should" be faster but has more slippage, boat may be sitting deeper in the water so it's less efficient falling far behind its theoretical speed. A 4 blade with a better bite and sitting higher Runs closer to its theoretical speed. And that is where the performance curves overlap.

I stuck a lot of shoulds, could and mays in there as well "in theory" and all things being equal as often there are lots of variables that toss the rules of thumb out the window.

[Tony Meola]

Carl

Got it. Interesting break down. Now if we want to get real technical and go Bob Lilco's route, find out what size blank our props are cut from. Go up a size on the blank, then have it cut to the size we need. Gives you more blade area, decreasing slip, picking up speed and reducing fuel consumption. In theory.

I know someone who did that with his 38 Henriques and he told me it made a big difference. Not sure it works on every boat.

[Carl]

Tony-

I am in awe of people who can see a propellor for more then a simple screw going through the water...like I do.

I can grasp diameter, pitch, blades and cup...some of the obvious blade shape and size. I can be handheld to follow along with blade design, rake, variable pitch and whistle edges. Putting anything more then the basics together and coming up with an expectation and actual ability to hit those expectations with actual performance numbers...kudos to them as it mind boggling to me.

[Yannis]

....as much as I cannot understand WHAT these doctors see in that microscope and can come up with a vaccine for a given virus!

....or how mathematicians out of a double blackboard FULL of numbers, letters and bizarre shapes, can tell you if that fuse will meet that comet and even land on it, and when!

Damn it , I thought I was smart...well, ok forget it!

[Carl]

Yannis your smarter then you think, knowing what you don't know. Others think they know, when they really don't know.

You know?

[Yannis]

ΕΝ ΟΙΔΑ ΟΤΙ ΟΥΔΕΝ ΟΙΔΑ

(one thing I know, that I know nothing)

Socrates, before his execution by the state of Athens for “ introducing new ideas to the public”.

[Carl]

well then I guess you and me know that we don't know and thats more then enough to know now.

I like that Socrates...had a way with words.

[Tony Meola]

Leave it to those Greek Philosophers. They make us all believe we do not know what we are talking about.