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I have a 1984 Sport 15 Center Console with a 1984 Mercury 50 HP (4 cylinder). It currently has a 13 inch pitch prop that allows the engine speed to go above the suggested operating range (5000-5500 RPM). The 13 inch pitch will allow RPM around 5800-6000 at WOT. I get 30 MPH with the 13 inch pitch, but thought I should get more. Seems like it would be great for a heavy load, but feels like it cuts/slips the water too much in my case, only increasing RPM and not MPH.

What would I get with either a 15 or 17 pitch prop?

Here are some shots of her. "Three Days Grace"

Thanks,
Greg


http://www.youtube.com/watch?v=qWJjJK...WJjJKOHw_o

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The 1984 Mercury 50 has a 1.83:1 gear ratio. If you are only able to hit 30 MPH at 5800-6000 RPM then that propeller is slipping badly, about 25 percent, so your instincts are correct.

13" pitch would normally be about right, but, of course, it depends on what model of propeller you are talking about.

I recommend the 10-3/8" x 14" Mercury Vengeance (aka Quicksilver Silverado) or the 10-1/2" x 13" Stiletto Advantage (aka Turbo Hot Shot)

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Thanks a bunch.

I know several guys that have 17's on their 15-footer. They get high top end, but lack the hole-shot and control at lower RPM's.

I was actually considering the newer Mercury Spitfire 4-blade in 10.1 x 15. I do putz around alot from fishing holes and often troll. I'm also looking for performance/fuel economy as well. I am just looking for around 35MPH. Are you familiar with this prop?

.

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A 15 Whaler with a 50 Merc should do about 35-36 MPH I would think. That little classic four is great engine.

The new 4 blade Spitfire aluminum may be worth a try. Mercury has been promoting it widely. I have no idea how it would run in comparion to a good SS model.

If it were mine, I'd be looking at the Mercury Trophy Sport, maybe in 14" pitch. For $250 delivered, not a bad deal. It is recommend for flats boats, and that is basically what you have. You might be able to locate one used.

http://www.mercurymarine.com/propelle...phy-sport/

http://jacosmarine.com/new_mercury_pr...r_sale.cfm

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Thanks Finn.

That is a nice prop too. I will have to see what my finance officer (wife) says. :-) as I have already spent a bunch on the boat this spring.

The Spitfire does come with that hub system that has been widely publicized. Do you have any experience with that?

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The Spitfire is an aluminum prop and you are going to get aluminum-prop performance with it.

Why bother?

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I've never had any issues with newer aluminum props on my Yamahas (15, 90). They both seemed to be fine and fast.

I've never had a stainless prop before. I wouldn't know the performance difference. I know it is twice as much, what makes it twice as good? Obviously, it's stronger, especially if it you hit something. What else?

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Actually, stainless props are nowhere near twice as much, unless you insist on getting a manufacturer's own model or buying from local prop shop. From the Jaco's ad, the aluminum Spitfire is about $149. The Stilettos go for a bit more than $200, including the hubkit - Dan's Discount Props is one vendor.

To me, it's 50 or 60 bucks well spent - better grip, better mileage, better durability. But of course finance officers have been known to make acquisition cost the only factor:-).

Tim

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My 1984 and 1985 Mercury catalogs show that the 1984 Mercury L4 50 has 2.0 gearing, not 1.83.

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If the gear ratio is 2:1 then the calculated slip is still about 19 percent.

My propeller recommendations remain the same though you *might* be able to use the 10-1/2" x 14" Triad instead or the 10-1/2" x 13" Triad with a 2:1 gear ratio.

My data base is showing 1.83:1 but the manufacturer's specifications are a better source.

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Thanks guys.

I will consider the SS. What are your opinions on 3 and 4 blade variations? As I said, I putz around alot at slower speeds. I do like to go fast on occasion, and I often travel with an extra soul on board. Sometimes the kids like to goof off with either a tube or skis.

What is the formula for slip calculation?

Greg

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I can't think of any reason a 15 would need a 4-blade prop - those are best for big, heavy, high-speed boats, pretty much exactly the opposite of the BW 15.

Follow Tom's advice - sometimes he's a little brusque but he sure knows props.

Tim

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The formula for slip calculation is simply the difference between actual boat speed and the theoretical boat speed (based on the propeller's pitch) divided by the theoretical boat speed.

Theoretical Boat Speed: T

Actual Boat Speed: A

Slip = (T - A)/T

The more complicated arithmetic comes from calculating the Theoretical Boat Speed. To do this we need to know three things:

Propeller Pitch (measured in inches)
Engine RPM (this is why you need a tachometer)
Gear Ratio (which converts engine RPM to propeller RPM)

*If* boat speed is measured in Miles Per Hour (MPH) and it does not necessarily need to be, then Theoretical Boat Speed is...

(Propeller Pitch x Propeller RPM)/1056 = Theoretical boat speed

There are 12 inches in a foot, 5280 feet in a mile and 60 minutes in an hour, so that's where the constant 1056 comes form:

(5280 x 12)/60 = 1056

For other units of boat speed (or propeller pitch), the constant, 1056, changes.

But there is really no need to go through all this tedious arithmetic when you can just use any of a number of handy online propeller calculators. Just Google: "propeller calculator"

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Tim,

I've always had new OEM props that were fitted to my outboards when rigged. I never had to worry about it.

I did this project and fixed this outboard myself. I'm still learning and take in all the info people have to offer. I simply want to know the "why" and "how", and try to come to a personal consensus based on all of that info and opinion. No feelings hurt here.

I went through the prop selector on Mercury's website and I have contacted Mercury myself in regards to their offerings. The gentleman I spoke with was "big" on their four-blade performance and prop designs. That is one major reason why I even considered that option. When I always thought of four-blade props, I always thought of the little "high thrusters". This newer generation of four-blades offer different blade configurations and geometries that offer better performance than their three-blade counterparts in the same pitch family.

I have decided to go with SS, and I am leaning towards athe Trophy Sport in 14 pitch. As Tom mentioned earlier...why bother with an aluminum prop when you have the option of SS and its better technology. I feel the same way in regards to the four-blade variety. Why go with three, when the newer four-blade varieties offer better perfomance with no sacrifices?

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What does it mean to be "big" on a four blade propeller?

The 1984 Mercury 50 uses a 3.5" (small) gearcase. Mercury offers exactly one four blade stainless steel propeller for this motor, the Trophy Sport. It should come as no surprise that if you ask Mercury what four blade propeller they recommend, it would be the Trophy Sport.

The trouble with Mercury is that they offer dozens of different propellers for large outboards but damn little for small and intermediate outboards. If you want a stainless steel three blade prop from Mercury you also have exactly one choice: The old Vengeance. That's it.

So take propeller recommendations for Mercury with a grain of salt.

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Tom,

Aside from the aluminum prop offerings from Mercury, yes, the Trophy Sport and Vengeance are the only applicable ones for my outboard. What I meant by "big" is that the man at Mercury I spoke with seemed to prefer what I mentioned in the prior post...that why not get the 4-blade instead of the 3-blade, as speed is not compromised and performance is increased.

It also echoes what Finnegan stated earlier.

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I doubt that is what your gentleman at Mercury told you and I do not see where Larry (Finnegan) said anything like that either.

If that were true, why would there even be a three blade propeller offered? In fact there are far more three blade propeller models than four blade propellers offered. Why is that, do you suppose?

Well, here's my theory: There are dozens and dozens of different propeller models offered for outboard motors, in both three and four (and even five) blade designs because every boat and motor combination is different and what one boater needs in terms of performance is different that what another boater needs. One guy might just want to see how fast he can go. Another might want to go water skiing. Yet another may want to simply cover as much ground with as little fuel burned as possible.

We don't need to discuss every model of propeller offered here because your application is specific: a 1984 Sport 15 Center Console with a 1984 Mercury 50 HP outboard. The stainless steel propeller choices are few, only two from Mercury, two from Stiletto/Turbo/Yamaha and likewise from Solas.

[ Interesting aside: PowerTech! offers no less than 17 different models of propeller to fit your Mercury 50! I've never understood how, or why, PowerTech offers so many models, but they do. ]

I would not assume that because the Trophy Sport has four blades, it is better. It might be on your boat. I don't know because my experience with the Trophy Sport is exactly the same as Larry's: I've never tried one.

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I think it would be resonable to say that Mercury knows propellers better than any outboard manufacturer -period. This is the only brand of propellers that we see widely used on all brands of outboards. When people are looking for the best performance from their engine, they often end up with a Mercucy prop (if Mercury makes one that fits their engine - sometimes they don't).

For the 40-60HP engines they make with the small gearcase, you don't need many offerings to completely cover the field of need. These engines don't go that fast, rarely over 40-45 MPH, and for that range, they offer 4 prop lines: the basic Black max 3 blade aluminum, the 4 blade Spitfire aluminum, the 3 blade Vengeance SS and the high performance Trophy Sport 4-blade.
What else could possibly be needed for boats that don't go all that fast, and for $250 or less?

Same for the mid-range, for which they make the above 4 lines, plus also the Laser II and High five, for a total of six lines. Once, agin, how many prop lines do you need for a mid-range Mercury engine, 75-125HP, with Mercury gear ratios? Those six cover the field pretty well. The only reason not to go with some line of Mercury prop for your Mercury engine is lower pricing.

On these older Mercury engines, sometimes the transom bracket design, with thumb clamps and limited mounting holes, prevents elevated engine mounting. In those situations, a Spitfire aluminum or Vengeance SS is probably all the prop that is needed.

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I can't find the article here anymore, but Joe had an Evinrude prop guide attached here somewhere. It had a discussion about 3 blade vs. 4 blade, and as I recall, it recommended 3 blades over 4 blades unless there was a vibration issue in the boat. Maybe Joe can re-link it or direct us back to it?

I think I remember that the 4 blade would work better than a 3 blade in a situation where the prop was likely to experience more air in the water (cavitation), like mounting a motor high and trying to make high speed turns.

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Is this the one?
http://www.whalercentral.com/articles...cle_id=108

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I'm done discussing anything on here since Tom calls me a [ xxxxxxx ] liar and knows everything there is to know about props.

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:-)
You were right Tedious.

Correction...I'll be back on when he chimes in. Does Moderator mean you are the authority? I'll stick to what the Mercury man tells me. Swing by Annapolis and say hi Tom. I would love to meet you.

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Greg -- We don't use that sort of language here. Review the Code of Conduct you agreed to when you signed up.

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YUP! That's the one, I found it very educational as I am currently studying props for the repower of the Outrage 18 in refit. I found the section I was looking at :

Blade Count—Theoretically, the fewer number of blades a propeller has, the more efficient it will be. But as the number of blades is increased, vibration is decreased. For most applications, three blade propel- lers provide the best balance between efficiency and smoothness.
However, BRP/Evinrude engineers have created a series of four blade propellers that provide increased efficiency and a superior grip on the water. The result is improved acceleration and better cornering in all water conditions.
In addition, when operated in a surfacing application, a four blade propeller keeps more blades in the water for maximum thrust and efficiency.