4-Blade Props on Sailfish with OX66's

Tucker

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#1
Don't really care about the hyped quicker hole shot the 4-blades give; or worried about loosing a few MPH on the top end. Looking for better fuel economy at the same cruising speed. Is this possible with 225 OX66's on a sailfish? Thinking about doing the research but before I invest the time, wondering if someone has lived this. Thanks...
 

seasick

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#2
Tucker said:
Don't really care about the hyped quicker hole shot the 4-blades give; or worried about loosing a few MPH on the top end. Looking for better fuel economy at the same cruising speed. Is this possible with 225 OX66's on a sailfish? Thinking about doing the research but before I invest the time, wondering if someone has lived this. Thanks...
Although prop selection is not an exact science, in general the more blades, the more drag. More drag means less fuel efficiency. There ate two easy ways to increase fuel economy: clean the hull and reduce weight. One way to reduce weight is to not carry more fuel than you need for your outing plus of course a saftey margin.
Changing pitch can help but when you factor in the cost of new props, the return on investment may not be attractive especially if your annual fuel consumption is relatively small.

I often note that captains will identify the sweet spot where fuel eficiency is maximised but will run at higher speeds because the sweet spot seems a tad slow. I am as guilty as the next person. I like running a bit faster to get where I am going sooner:) On my smallish single engine vessel, going faster means that my gph almost doubles but my speed doesn't double.
 

SmokyMtnGrady

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#3
Seasick, I will challenge the theory of more blades equals more drag thus less fuel effecient. While this may be true both in theory and the real world, there are other factors impacting performance. Since you cited Bernoules, I can assume you might be a scientist or engineer of some sorts. While I only took a year of physics in college for my biology degree and I am no engineer, the reality of it adding the 4th blade means less slippage and reduced cavitation and so forth.

I have found going from a 3 blade SSII Yammy prop to a 4 blade prop of the same pitch but an inch smaller in diameter has resulted in my fuel economy going from 3.0 to 3.2 mpg depending on seas and so forth. The 4th blade does add drag, but that drag loss is more than made up by moving more water at lower RPMs, for me and my F250 I can cruise at 100 RPM lower for similar speeds as the 3 blade. So, my motor is working less hard to do the same amount work, thus it is more effecient and it adds up for us. My top end has been reduced by about .8 knots per hour, but my cruising performance has improved and that is where I do most of my boating, not at WOT.
 

seasick

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#4
SmokyMtnGrady said:
Seasick, I will challenge the theory of more blades equals more drag thus less fuel effecient. While this may be true both in theory and the real world, there are other factors impacting performance. Since you cited Bernoules, I can assume you might be a scientist or engineer of some sorts. While I only took a year of physics in college for my biology degree and I am no engineer, the reality of it adding the 4th blade means less slippage and reduced cavitation and so forth.

I have found going from a 3 blade SSII Yammy prop to a 4 blade prop of the same pitch but an inch smaller in diameter has resulted in my fuel economy going from 3.0 to 3.2 mpg depending on seas and so forth. The 4th blade does add drag, but that drag loss is more than made up by moving more water at lower RPMs, for me and my F250 I can cruise at 100 RPM lower for similar speeds as the 3 blade. So, my motor is working less hard to do the same amount work, thus it is more effecient and it adds up for us. My top end has been reduced by about .8 knots per hour, but my cruising performance has improved and that is where I do most of my boating, not at WOT.
In the real world, it is not practical to make a one to one comparison of a 4 blade to a 3 blade based on blade count alone. The other factors are never the same for both props. I believe your numbers, an increase in fuel economy of about 6% but that could be due to other factors such as the quality of the new prop, the blade shape, cupping or even reduction in drag due to the outer edge of the rotating prop being farther away from the cavitation plate where turbulence adds drag.
All that mumbo jumbo said, as I mentioned, cleaning the hull and reducing carried weight will always help.
For the math end of the argument, that 6% gain in fuel economy in my case would save about 30 cents a gallon based on my last purchase price for regular on the water. In a typical summer, I only burn about 200 to 300 gallons of fuel. The new prop would save about 90 bucks a year. A decent stainless prop would cost a few hundred to several hundred, meaning that there would be a several year return on investment. Twins make the payback look even less attractive.
My advice is that boaters in general may want to address performance issues by changing props but it often doen't pay to do so purely for fuel economy.
Now if you use a lot more fule per year, it may offer a faster return on investment.
 

Tucker

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#5
Good stuff here gents. Couple comments:

1) The price of gas, which is suppose to increase Monday, is the obvious variable. A small increase in price means huge gains in recouping the cost of the props.
2) I believe the prop guys have figured out how to negate the drag of the 4th blade by reducing the diameter. I'm not smart enough on the subjet to challange that but the rotating mass changes things a bit. Believe smokymtn may have more to add on that.
3) As far as increasing fuel economy I believe that is well documented. Now I don't savy math too good, but a small increase nets huge savings in fuel because we consume so much.
4) The biggest factor, I believe, is boat design. Yea seasick's right about the bottom and dinged props are also a huge factor. BUT, these Sailfish are dogs.
A) Somebody screwed up with the stern design of the older 272 Sailfish.The eurotransoms with big block 2-strokes have buried cav plates. It's no getting around it. Raising the motor puts the cav plates above the bottom of the boat
B) The wide beam and putting the waterheater in the front makes things worse.

The problem is even worse with older Marlins with 250 OX66's. I'd love to have one but they burn much more fuel than the Sailfish. Now, the new ones with those new 4-strokes are much more economical.

Cheez, I was really hoping a Sailfish guy would chime in about trying this. I have a lot more research to do but the guys at Propgods said they would lend me the props. I have to pay shipping back & forth, so worse case is about $60-$70 out of pocket.
 

Grog

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#6
Seasick summed it up pretty good with the props. There's a lot of different factors to make one prop better then the others and shortcuts to overcome problems which in turn have drawbacks (thinner props have less drag but will bend easier). Just trying to understand the cup of a prop will make most people's eyes roll back. When the gained efficiency overcomes the extra drag, then the prop is better for you.

When you have to dump $600 for a new set of props, the payback is going to take a while.

I have an older Sailfish with a bracket that is almost as deep as the hull. You can't get the cav plates up?
 

SmokyMtnGrady

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#7
If one is looking at this soley from a cost benefit perspective, then changing the prop to gain fuel savings and hoping to recover the money and then save a bit more, then perhaps it is not a good investment.

Now if you need a new prop or props and you have to spend the money regardless, then 6% savings is a no brainer, it is like a tiny rebate every rotation of the wheel. In my case I needed a new prop, well a spare to run to the Bahamas last year, so I bought the 4 blade prop. Overall performance for my 228 improved beyond fuel economy. Our engines have a finite duty cycle or life. It seems to me if my motor is working less to do the same job, it should have some marginal benefit toward increasing the life of the motor. Just speculating on this though.

I can tell you my boat runs better in sloppy seas and following seas with the 4 blade and unless you only plan to use that sailfish in a lake or something, you will appreciate it if you notice the improvements in handling in following seas, better hole shot ( while this may not make a big difference, the quicker the boat gets on plane the more fuel you save due to that hole drag thing) sure it is pennies, but pennies add up. Anyway, I am very pleased with the results of changing props. Granted your boat may not respond like mine did or does. Something to consider.