posted 03-07-2008 07:42 PM ET (US)        

I have an identical temptation that I have just re-powered with twin 2000 Yamaha 250 OX66 motors, and I am looking for maximum performance out of this rig. My previous set-up was a set of 1994 Yamaha V-X 76 225's that served the boat very well. When in tune, it pushed the Boston Whaler boat to 57-MPH in calm water, light fuel, just me aboard. The trouble of keeping these multi-carburetor engines in tune became too much, so I [purchased] some EFI motors. I need to outfit the new 250's with some propellers and need some suggestions on what to consider. Also, if there are other things to consider that would improve the overall performance and top end of the boat, I'm all ears.

In addition, I am doing a progressive restoration on the boat replacing other functional and cosmetic elements on the boat in order to keep it as original as possible. If there are [unrecognized acronym] part suppliers or others who can provide good, solid replacement boat parts, I would love to hear about them.

posted 03-07-2008 09:18 PM ET (US)            

Although I am boat and engine literate, I would consider myself prop illiterate, so don't take offense to my questions-just trying to learn.

First, why a four-blade prop? I was considering a Mercury MIRAGE 14 7/8 x 21. Will the surface area of the four-blade drop my [engine speed] at all?

As for mounting the engines, can you suggest at what relationship of the anti-ventilation plate to bottom of hull I should be considering?

posted 03-07-2008 11:36 PM ET (US)            

First of all, there is no such thing as a "stock prop" at least in regards to an outboard from a manufacturer. Except for a few small kicker motors, no outboard is shipped with a propeller at all. It is up to the owner, dealer or perhaps boat manufacturer (if they rig their boats at the factory with outboards) to install a propeller that meets the needs of the boat in question. All large outboards are shipped sans prop.

Manufacturers, like Boston Whaler, who now install the outboards for each model of boat at the factory usually have a propeller they use that they have chosen because they have tested and determined the prop works well. This does not mean it is optimal for everybody and every boat.

In the case of dealer installed propellers, all to often it was just a matter of what they had on hand or what cost the least. This is why so many aluminum props are used today when they really shouldn't be.

Now you say your 225s were equipped with the Yamaha painted stainless steel props? Those are very common and good propellers. It would not surprise me if that is what you had. It surprises me VERY MUCH that they you could ever reach 57 MPH with only 20 inches of pitch. Are you sure about the pitch? The speed?

I am also unclear if in fact you were referring to the old motors limiting at 5300 RPM or the new motors limiting at 5300 RPM. Are you now running the old props on the new motors?

The old Yamaha 225 had a 1.86:1 gear ratio. Does the 250 OX66 use a 1.86:1 gear ratio or a 1.75:1 gear ratio? This information will be in your owner's manual.

What we need to know for your new motors is the following:

- WOT range, especially the redline (5600 RPM you say?)

- Gear ratio.

- Current pitch of props and model of prop or part number.

- Top speed measured with a GPS, preferably multiple runs measured in two opposing directions and averaged.

posted 03-08-2008 08:58 AM ET (US)            

I do not yet have the new 250's yet, so all data is for old 225 set up. As far as gear ratios, the 225's have a 1.81:1 ratio, not 1.86 as you suggest. In 1994, 150 HP through 200 HP had 1.86:1, regular 225 had 1.8:1, and V76X 225 and 250 had 1.81:1. Not sure how much difference that makes but thought I would stipulate the correct data.

As for the current pitch and speed, I am sure of only the speed, however, I should clarify. The speed has been measured 57 on fuel management speedometer and 55.6 on GPS. The boat is run in brackish water, calm winds, no gear, less than 25-gallons of fuel, ebb tide, and only my hulking 6'0" 180-lb frame in the boat.

As for the props, I am not sure of the pitch at all. Diameter is 14-1/2. Last year I told my mechanic I wanted more performance out of the prop and had the pitch adjusted. I am not sure where it ended up. Unless there is an easy way to measure pitch, I probably won't go there; they will not be put on the new engines. They have been re-conditioned several times and after 14 years, it's time to retire them.

Moving to [engine speed], I am referring to old 225's at 5,300-RPM max. RPM range is to 5,500 RPM so I am not far off. As I climb my speed to mid 50's, I find getting to 5,300-RPM requires excessive trim and the bow begins to porpoise at about 54 MPH.

Since my motor knowledge runs out at mid-1990's era carburetor Yamaha motors, I won't know for sure the WOT RPM and gear ratio for the new motors until I get them, unless there is an on-line source for that information.

Perhaps I have over complicated things. At this point, since the old propellers are not going on the new engines, a best-guess recommendation based on the known and make some assumptions on the unknown would be best. We'll assume a WOT range to 5,600-RPM and similar gear ratio as previous model year 250's of 1.81:1 Without a viable set of propellers, there is no way I can get in the water to record any test data. Any suggestions on mounting the engines would be helpful as well.

posted 03-08-2008 10:22 AM ET (US)            

The EFI 250 has a 5500 RPM redline and 1.81 gears. With a 10 percent slip factor, you'll need a 22 inch pitch propeller to reach 58 MPH.

posted 03-08-2008 11:25 AM ET (US)            

In your situation, I would recommend you mount the motors with the lowest set of bolt holes. In other words, as high as they will go with the mounting holes already in the Whaler Drive.

posted 03-08-2008 12:05 PM ET (US)            

As for mounting height, P 38 photo looks like the OB's are about 3" off the top of the mounting bracket; my 225's are 3/4" up in mounting hole #2. I think I will raise to hole # 4 (last hole) pushing them up another 1 1/2".

posted 03-08-2008 12:16 PM ET (US)            

The Revolution 4 is known a a small diameter prop which also (and contrary to common theory) provides good stern lift and is also fast. In spite of its small diameter it has a lot of blade area and may help explain why it does well at handling a lot of horsepower on larger boats. It also grips very well and tends not to ventilate.