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Would a 4 blade work well on a 1973 Boston Whaler Bass Boat with a 115 Mercury?

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That would depend on what four blade propeller you are talking about. There is far more to propellers than the number of blades.

It will also depend on what you mean by "work well".

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It would be a Diamond 4 from West Marine. The size is 14-1/2" x 18" and it will replace a 21" pitch stainless prop that I had reworked to a 19" pitch. The 21 at WOT turns only 4600 RPM and the reworked prop turns 5600 at WOT.

Will the 4 blade improve the hole shot performance with less vibration and steering torque at high speed? Will it plane at lower RPM? It planes at 3000 RPM with the 3 blade. Will it boost fuel economy? West Marine says the four blade is recommended for bass boats and boats with high performance hulls driven by high-horsepower motors. Is a 1973 Boston Whaler Bass Boat 16 with a Mercury 115 HP a rig that will benefit from the Quicksilver Diamond 4?

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OK, now we have some good information to work with. Let's start with a few basics:

The 14-1/2" x 18" Quicksilver Diamond 4 four blade aluminum prop that West Marine sells is the same as the Mercury Alpha 4. This is a propeller that only fits large gearcase motors (V-6 and sterndrives), not intermediate gearcase motors like your Mercury 115.

By the way, what Mercury 115 do you have? Model? Year? Four cylinder? Six cylinder? EFI? DFI? Two stroke? Four stroke? Gear ratio?

I do not believe Mercury ever offered a 115 HP with an V-6 gearcase. You will have to select a different model of propeller for your motor. But before we get to that, let me digress and talk a bit about the Mercury Alpha 4/Quicksilver Diamond 4 propellers:

Mercury's description of them is as inexpensive and easy to repair aluminum prop for runabouts and daycruisers. My personal experience with them is that they suck as far as performance is concerned. Very little grip or top speed. They are just a cheap aluminum prop, that is all. Best kept as a spare.

I am having a hard time reconciling your data for the prop you have now. You say you had its pitch reduced by 2" from 21" to 19" yet you report an WOT RPM gain of 1000 RPM. This is two or even three times the engine speed gain we would expect. Typically one inch of pitch reduction will result in a about 150-200 RPM gain. Something else is going on here.

A sound recommendation for a good propeller will depend on what motor you have. If you can tell us the gear ratio and recommended WOT range of that motor, that will give us a good satrt on what we need to figure it out.

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thanks so much for your time. this is good stuff.
it is a 1992 mercury outboard and written in the owners manual is "engine tach in 5200"
the full throttle rpm range from the specs are4750-5250
i do not have in the manual a gear ratio.
the prop was a 21 i don't know what it is now but first i put a 17 pitch on the motor and it ran like a wildman could not wot she would have blown. she shot up to 5800 and i backed off but boy did the boat jump so the 19 is the play or my tach may be done for. it is old.

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I believe a 1992 Mercury 115 uses a 2.07:1 gear ratio. If the redline is 5250 and the boat is capable for 45 MPH (which I know it is with that power) then it ought to have no trouble turning a 21" pitch propeller.

What is the top speed you are seeing now?

There are several excellent three blade propeller you could try including the Mercury Laser II in a 20" or 21" pitch. The Stiletto Advantage I in the 13-1/4" x 19" would be very fast as well.

Mercury only makes one four blade for your motor, the Trophy Plus. You would want to try the 13-3/4" x 19" or 13-3/4" x 21" size using a Flo-Torq III hub kit. This is a big prop for that motor. In 1992 the lower units were not as robust as they are now. The Flo-Torq II hub will offer some "cushioning" when shifting.

The 12-3/4" x 20" Mercury High Five would be a good choice for acceleration, but again that is a lot of stainless steel for that motor. Experimentation is the only good way to find out if a given propeller will work well or not.

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I believe i can give an answer to this post.
I have a 1980 montauk with 115 mariner 2 stroke 1994 2+2.
The gear ratio is 2.07:1 as is yours too. The WOT range is from
4750 to 5250 Rpms.
I have a High Five 20'' and i can get 4800 Rpms and top speed to 37 Mph.
I have amazing acceleration, handling and a planning time of 3 sec. with 1800 Rpms.
I wanted to move to the trophy plus and after a long consideration and testing i figured out that i must use 19'' Trophy plus (4900-5000Rpms).
But i preffer to loose 5 mph from top speed than loosing this amazing cruise with ANY weather that 5 blade offers me.B)
Don't drop below 19'' with 4 blade and this setup. Your engine will work like a blender!!!:o

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tom, could you tell me about lower unit not being robust and the flo torq hub giving it cushioning when shifting?
when i shift it she really grinds.
lets say the tach is right. i'm turning 5600 with the cut down 21. i'm ok? but i need to be carful and not run wfo! or should i keep trying.
tasosk, do you get better fuel milage with that wheel?

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5600 RPM is not good. Way too much as your redline is 5250. I would dispose of the modified prop you have and get something better.

Mercury's Propeller Guide suggests that the three and four cylinder 75-125 HP models made before 1998 use a rubber hub (Flo-Torq I) instead of the Delrin Flo-Torq II hubs which are now standard. The Flo-Torq III was designed to help with "prop rattle" on the four strokes, but it has a cushioning effect when shifting as well.

I have a Trophy Plus in the 19" pitch I would like to sell if anybody is interested.

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Tom, thanks again but I'm still not clear on the flow hub thing. This will help with the shifting using the flow Torq I, II or III?

Tom, your profile photo of the boat jump is a place I have been many, many times. We used that road to get to a duck camp on Oak River.

What's the story on the 19" Trophy Plus prop?

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Mercury's Flo-Torq hub kits use a field replaceable insert in a square casting of the hub of the propeller instead of a pressed-in rubber hub.

The Flo-Torq I *is* a rubber hub that slips into the square Flo-Torq bore.

The standard Flo-Torq II hub (supplied with all Mercury propellers) uses a hard plastic hub insert made of DuPont's Delrin. Because the plastic is hard, the shifting of your outboard is hard.

The Flo-Torq II hub uses a two part Delrin hub that is held together with titanium rods that act like a spring to cushion the torsional inertia of the shifting.

My Trophy Plus was part of a lot of propellers I picked up from a dealer going out of business. The prop is used but "like new". I would sell it for $275, less than half of retail list.

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tom, that is a 4 blade? the question is what if it turns too fast or too slow? i would like to try it but what if? can we work this out? thanks, anthony

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ok, i got it. thanks to connie and her shop, jefferson propeller. she is not only smoking hot but knows her stuff about propellers. thank you connie. the right pitch for this set up is a 20 pitch! with this pitch, and balance it is a dream! runs so well that i love you connie! a perfect 5300 at wot and will plan at 2600 and 4200 is so fast but easy. the right pitch and balance is so important.
the four blade is over kill too much to repair, buy, and does'nt offer much more than the three blade.
:D