posted 12-05-2013 10:52 PM ET (US)     

My "last forever" Mercury 90 2 stroke saltwater series has been a lot of trouble over the years between warranty claims and now greater repair bills from electrical issues. I am at point where I have zero confidence in the engine and its ability to bring me (more importantly my family) back to the dock.

I am sure Mercury makes a lot of great products and people are very happy with them but for my personal experience it didn't work for me and it is time to move on.

What type and size aluminum or stainless steel propeller is going to be the best fit for the 170 with a F90? We primarily use the boat for running the family andfriends to the beach and tubing for the kids, so the boat is typically loaded up. Do I really need to go SS with this setup?

posted 12-06-2013 10:30 AM ET (US)            

Regarding the choice of aluminum or stainless steel for the propeller, at the 90-HP level I don't think you will see a huge gain in performance from a stainless steel propeller, but a stainless steel propeller will be much more durable.

For selecting a propeller, we need an idea of the speed potential of the boat. Using the Boston Whaler performance reports, we see that a 170 MONTAUK with 90-HP can reach a top speed of 42.8-MPH at a test weight of 2,291-lbs.

Cf.: http://www.bostonwhaler.com/boat_graphics/eprowebsitemedia/2904/ EnginePerformance/2014/323825_EnginePerf.pdf

We enter the 42.8-MPH and 2,291-lbs into my implementation of George Crouch's speed calculator, and solve for the hull factor: 215.9--a very impressive hull factor.

http://continuouswave.com/cgi-bin/crouchcalc.pl

Now we assume all the variables will be the same, except the weight. The F90 (372-lbs) is lighter than the Brunswick outboard (399-lbs) in the test by 27-lbs. We recompute the speed with the lighter weight (2,264-lbs): 43-MPH. This will be our target speed. Now we turn to my implementation of a propeller calculator:

http://continuouswave.com/cgi-bin/propcalc.pl

We enter the target speed (43-MPH), the gear ratio of the F90 (2.27:1), and the maximum engine crankshaft speed (6,000-RPM), and set the SLIP for 10-percent. The calculator predicts the pitch: 19.09-pitch.

Based on these estimates and calculations using the estimates, it looks like a propeller pitch of 19 would be a good starting point.

A three-blade is probably the best all-round choice. Now for a particular propeller: it is a huge field.

The first place to look is to Yamaha. You might find that when you buy a Yamaha engine the propeller can be included in the purchase for a nominal additional price. Yamaha propellers seem to work well.

If Yamaha is not your cup of tea, there are many other choices. One propeller maker that I recently took note of is SOLAS.

SOLAS propellers have two interesting features: their aluminum and their hubs.

The aluminum propellers are manufactured with a method called squeeze casting. This method is said to produce finer grain size in the metal, reduce porosity in the metal, increase strength of the metal, and permit easier repair. In addition, the propellers are painted with a four-layer coating to provide excellent corrosion protection. For more information see

http://www.solas.com/newweb/propeller/products/al.asp

The RUBEX brand of SOLAS propellers utilize a replaceable hub insert, a common feature, but the RUBEX hub employs an interchangeable molded rubber over brass design. The rubber hub is said to offer several advantages over the usual plastic insert hub:

--reduced hub chatter

--less deterioration than plastic

--one piece construction

--elasticity to deflect up to ten-degrees when shifting to reduce banging or to be able to absorb impacts if striking a submerged object

SOLAS has an on-line propeller selection guide at

http://www.solas.com/newweb/propeller/products/brandSearch_rubex.asp

Again, I don't have any experience with SOLAS's RUBEX hubs or their Aluminum propellers, but they do seem to have a few features that distinguish them from the rest of the choices on the market.

The SOLAS company was formed about 25 years ago by Dr. Solas Y. J. Lin, and is located in Taiwan, Republic of China. You can learn more about SOLAS propellers and the technology behind them from

posted 12-09-2013 08:59 AM ET (US)            

quote:

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To select the correct propeller for your boating application,
your boat and outboard MUST be water tested.

IMPORTANT: The correct propeller for your boat, under
normal load conditions, will allow the engine to run near the
midpoint of the RPM operating range at full throttle.

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