prop test results...advice please

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Well-known member
Feb 25, 2006
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Rowayton, CT
So I did some testing today with two diffrent props. Light breeze, small chop about 70 gallons of fuel and two adults.

Boat is a 1991 25' Parker MVSC. The engine is a 2001 225 Opti. WOT should be around 3600, according to the manual.

Here's what I found:

15X17 three blade aliminum Black Max

Speed is via the GPS

4500 - 21mph
4700 - 22.1mph
4900 - 23mph
5100 - 24.2mph
5300 - 25mph
5500 - 25.9mph
5600 - 26mph this was wot

15 1/4X15 three blade aliminum Black Max

Speed is via the GPS

4200 - 19.7mph
4400 - 20mph
4600 - 21.9mph
4800 - 22.1mph
5000 - 23mph
5200 - 23.9mph
5400 - 25mph
5600 - 26.2mph this was wot

In both tests the engine was trimmed slightly (1.8 on the Smart Craft)

To me, none of this seems right. Judging from what some of you are getting with similar boats and 225 Yammies, I've seen wot speeds above 30.

Any suggestions would be extremely welcome!

Holy smokes! I'll launch this weekend and get some newer numbers, but my mod-V '92 with an OMC 225hp will usually cruise 22+ knots (25 mph) @ 4000 RPMs [for ~2.3 mpg] and I've topped 34 knots (39 mph) @ WOT.
I meant 5600 wot, not 3600 (why do I keep making that typo...).

Dale, that's what I thought I should be getting. The engine was brand new in 2003 and only has 70 hours on it. The bottom is in great shape and has new bottom paint. The trim tabs were up, the boat was well balanced and, as I mentioned, not loaded down with a full tank of gas.

What could be the issue here?

Tomorrow I plan to check the GPS. It's a brand new Raymarine E80. All the other data is fine, so It would be very odd if that were the problem, but still...

Should I try going down in prop diameter to, say a 14 1/4 and a 16 or 17 pitch? Four blade, maybe? Steel instead of aluminum?

Help... :shock:
Call Andy at SIM on THT. You are blowing out. The merc black maxes stink. I would blow out too.

By using a prop calculator and assuming your 2001 Opti has the same 1.75:1 gearbox as the current models, it puts you at 50% slip across the board :shock: . Dunk believe you should be around 14-15% with a premium 3-blade prop and around 10% with a premium 4-blade prop.

You need a SS wheel on that motor. A 4th blade will usually let the prop hookup better but you need to watch the pitch, as you cannot let the pitch drop below the diameter size or performance and speed will really suffer.

What is your MAX RPMs? You want to be a few hundred under that with that load and proppoing to be near max allows the boat to be overall more responsive. Mercury makes some of the best props in the business ... do you have a good dealer you can go to? You should consider a Merc Mirage, but also check with your dealer.

Do you also know where your OB is mounted? Mine was mounted too deep, as the vent plate on the OB was under water (mine is on a bracket). I raised the OB 2 bolt holes.

You can print out this prop slip chart or I send you the excel file that it runs on ... same for anyone who wants it, just PM me with your email address.


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Excellent info, Dale. Thanks. As you casn imagine, this is a little perplexing :shock:

I'm interested in your bolt hole issue. I'll have to check mine. Did your change have a significant impact?
Well, the mystery is solved :D

Turns out that my GPS was getting its speed data from the transducer paddle wheel. Once I reconfigured the speed data box to show SOG the numbers jumped right up to where they should be. My top speed at 5600 was 39mph, and I found a comfortable 30mph cruise at 4500, burning 11 gallons per hour. All this with the 15 1/4 X 15 three blade aluminum Mercury wheel.

As if that wasn't good enough, I headed out to a favorite mid-sound fishing spot and picked up a dozen stripers. The smallest was 24" and the batch included my first two keepers of the season.

Thanks for all your input and suggestions!