2000 VENTURA 18 OptiMax 135 Propeller Change

Optimizing the performance of Boston Whaler boats
keithp66
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2000 VENTURA 18 OptiMax 135 Propeller Change

Postby keithp66 » Sun Mar 27, 2022 7:18 pm

I have a 2000 Ventura 18 with a 135 Optimax that we bought in Fall 2021. The propeller is a MIRAGEplus 15-pitch.

We have only run the boat a few times. In testing with two adults aboard and a half-tank of fuel, the engine accelerates to 6,000-RPM and boat speed is 37-MPH. The boat feels over-propped to me.

I want to change to a propeller with 17-pitch, and perhaps an aluminum propeller for testing, before buying a more expensive stainless steel propeller. But most aluminum propellers with 17-pitch have a diameter of 14.25 or 14.5-inches. I want an aluminum propeller with the same diameter as the MIRAGEplis, 15.5-inches.

Q1: If the propeller diameter is limited to 14.25 to 14.5-inches, should I get an 18-pitch propeller--if I can find one?

Q2: If the propeller diameter is limited to 14.25 to 14.5-inches, should I get an 19-pitch propeller?

It seems to me a 19-pitch propeller would be too much for the Mercury 135-HP engine.

Advice will be appreciated--Keith

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Re: 2000 VENTURA 18 OptiMax 135 Propeller Change

Postby jimh » Mon Mar 28, 2022 7:19 am

keithp66 wrote:....the engine accelerates to 6,000-RPM and boat speed is 37-MPH. The boat feels over-propped to me.
The condition of being "over-propped" means the propeller pitch is too great. But you want to increase pitch, which is the opposite of what would be done if the engine were "over-propped."

From the limited data provided, determining if the current propeller pitch is too high or too low is difficult. You must give more information about the engine in order to assess the boat performance with the engine and propeller you have now. In particular, tell us:
  • engine gear ratio
  • maximum allowed engine speed
  • recommended range of engine speed for full-throttle operation
You should find all that information in the engine owner's manual. To look up this information on-line now is difficult. Mercury stopped all production of the OptiMax engines in May 2018, but they had dropped the 135-HP model before then.

Right now, the only advice that can be offered is that increasing pitch to 18 or 19 should result in a lower full-throttle engine speed. A rough estimate is that a two-inch increase in propeller pitch will typically result in a reduction in engine speed of about 400-RPM.

Most engines have a mechanism to limit engine speed to protect the engine from harm, called a "rev-limiter." When the speed-limiting function is initiated, the usual method is to suppress engine ignition. The result is the engine begins to misfire and slow down.

Q3: Because 6,000-RPM is very likely near the maximum engine speed permitted, did you notice any indication that the engine was beginning to self-limit its speed?

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Phil T
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Re: 2000 VENTURA 18 OptiMax 135 Propeller Change

Postby Phil T » Mon Mar 28, 2022 1:46 pm

You do not mention the mfg year of the Optimax 135. If original to the boat, the following applies:

The 1996-2007 Optimax 135 has a WOT range of 5000 - 5600 and a gear ratio of 2.00:1

Aluminum props do not perform as well as painted or polished stainless. They also damage easily.

The membership of Ventura 18 owners who still have an Optimax 135 is very low so direct prop comparison is not available.

In reviewing the threads and running a prop calculator I would recommend you read this thread discussing the same engine on a 2004 Dauntless 180. While the hull is a bit different, it is very close to the hull in question.

Tom Clark is a respected prop person so his advice has serious weight.

https://continuouswave.com/ubb/Forum4/HTML/005554.html

Let us know your thoughts/questions.
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keithp66
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Re: 2000 VENTURA 18 OptiMax 135 Propeller Change

Postby keithp66 » Mon Mar 28, 2022 8:52 pm

The engine came with the boat new so it is also a 2000. The ratio is 2:1. The WOT range is 5000 to 5600. We only took the boat a few times after we got it in the fall and the one time I ran it wide open it did not seem like it like being at 6000. It felt like it backed down the rpm's quickly. It did not back fire though. Honestly I didn't want to experiment after that. It just feels like the 15p is the wrong prop for us. Still wondering if I should move to a 17 p same diameter or an 18 or 19 with a smaller diameter? I know it makes a difference but I read that you shouldn't get too hung up on diameter within a certain range. It was saying focus on the pitch and let the manufacturer worry about the diameter. The more I read the more I find that all propellers are not created equal and estimates about the effects of changes in pitch and diameter are just that, estimates. Our typical day is cruising around Buzzards Bay or Vineyard sound with 2 to 5 passengers.

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Phil T
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Re: 2000 VENTURA 18 OptiMax 135 Propeller Change

Postby Phil T » Mon Mar 28, 2022 11:03 pm

Keith--Moving up in pitch in the same propeller brand and model results in a reduction of 150-200 RPM at WOT for each two-inches of pitch difference.

To get the engine RPM down to 5,600, you'll need to go up to a 1Mirageplus 19 pitch of the same diameter. This is not unexpected as the V6 gearcase of the Optimax 135 takes large propellers.

Also, the engine should be mounted 2-holesup or 3-holes-up with the top bolt using the third or fourth hole, counting down from the top.

Don't move the engine mounting position until you have the propeller sorted. The engine gains 150-RPM for each hole raised.
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jimh
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Re: 2000 VENTURA 18 OptiMax 135 Propeller Change

Postby jimh » Tue Mar 29, 2022 7:22 am

With the new data of a gear ratio of 2.0 and recommended full-throttle speed range of 5,000 to 5,600-RPM, we can evaluate the performance of the 15-pitch propeller at an engine speed of 6,000-RPM and boat speed of 37-MPH using a propeller calculator.

With those inputs, the SLIP calculated for the propeller is 13.2

At a 37-MPH boat speed, a value of SLIP of 13.2 is a bit high, and particularly unusual for a Mercury MIRAGEplus propeller. For almost all propellers, the SLIP value will be decreasing as the propeller rotation speed increases toward the maximum boat speed, and the slop value would typically fall below 10.

Using the propeller calculator to predict engine speed and entering SLIP = 10 results in an engine speed calculation of 5790-RPM for a speed of 37-MPH

There are four possible causes of the SLIP to calculated quite that high:
  • the engine speed measurement is off
  • the boat speed measurement is off
  • there is a significant problem in the propeller hub causing the propeller to rotate more slowly than the propeller shaft is rotating
  • the engine is trimmed out too far, causing the propeller to operate in aerated water and reduce its thrust

You should probably redo the performance test and see if you get data that is similar to your initial test.

UPDATE: I reviewed some data of my own involving a MIRAGEplus propeller, a 17-pitch model, but it's on a much larger (24-foot) and heaver boat, and powered by a 225-HP engine. Looking specifically at the SLIP values for the MIRAGEplus propeller, among several others tested, I see that as the boat speed tops out around 44-MPH the SLIP is still quite high, around 12. I think that is due to the relatively high-engine mount, and from the Whaler Drive tendency to create aerated water into the propeller.

In the application on your boat, a small and lighter hull, and with a 15-pitch propeller, I would expect the SLIP to be lower than 12.

Regarding what might cause an engine speed to vary 150 to 200-RPM between tests: almost ANYTHING could cause that. Influences in the weather like temperature, humidity, atmospheric pressure will affect engine power output. Water temperature affects the density of the water and thus the propeller performance. Variations in fuel being used, such as fuel octane and fuel purity affect engine power output. Changes in weight on the boat, changes in sea state, and the presence of any current will all affect the data.

In my testing, I see variations of 150-RPM for a certain boat speed on the same day of testing, in data taken just minutes apart. Also, unless you have a digital tachometer getting data directly from the engine's control module, a change of 150 to 200-RPM is going to come down to how you read the dial pointer on most tachometers. In order to reliably observe a small engine speed change like 150-RPM will require the test conditions to be kept exactly the same between tests. Any other variable between the two tests could affect engine speed, too, besides the variable you think you are testing.

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Re: 2000 VENTURA 18 OptiMax 135 Propeller Change

Postby jimh » Wed Mar 30, 2022 1:30 pm

As a general rule, most two-stroke-power-cycle engines will make their peak horsepower at an engine speed below the rated maximum engine speed allowed, and that is probably why the OptiMax is mentioned as having a recommended operating speed range (according to PHIL) with 5,000 to 5,600-RPM being recommended.

Since the 15-pitch MIRAGEplus propeller is letting the engine accelerate to 6,000-RPM (according to the tachometer you are using), a propeller with more pitch is probably going to cause the full-throttle engine speed to decrease, and, again using a rule of thumb, a 2-inch pitch increase will probably drop 400-RPM. That would put the OptiMax engine speed at 5,600-RPM, right in the high end of the optimum range.

We can predict the boat speed with the propeller calculator using inputs of

RPM=5600
RATIO=2
PITCH=17
SLIP=10

and that results in a boat speed of 40.6-MPH. If the SLIP is still around 12, the boat speed would drop about 1-MPH.

Propeller testing is most effective at producing useful data if only one variable is changed between test runs.

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Re: 2000 VENTURA 18 OptiMax 135 Propeller Change

Postby jimh » Wed Mar 30, 2022 1:59 pm

I found a Mercury Service Manual for an OptiMax 135. It shows the Rev-Limiter is set for 5750-RPM. I would be VERY surprised that you could get the engine to run at 6,000-RPM fo any length of time and not encounter very certain evidence of the rev-limiter kicking in to limit engine speed. This is another problem with your performance data.

Cf: Page marked 1A-1 in this publication:
http://www.hedges-uk.com/boat/Merc/outboards/2-stroke/optimax/Opti_135-150/135-150%20OPTIMAX-DIRECT%20FUEL%20INJECTION-With%20Serial%20Numbers%20OG590000%20and%20Above.pdf

The value of 5750-RPM engine speed limit is also mentioned in some other non-authoritative posts by users of the engine.

Again, re-run the test and carefully check the tachometer reading.

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Re: 2000 VENTURA 18 OptiMax 135 Propeller Change

Postby keithp66 » Wed May 25, 2022 10:11 pm

Update: I was able to test a new propeller, [an aluminum] Turning Point Hustler 14.25 x 17-pitch. The 2000 18 VENTURA fuel tank was full and only I was aboard.

With the Turning Point Hustler 14.25 x 17-pitch [aluminum] propeller, the Mercury OptiMax 135-HP engine was able to accelerate to 5,800-RPM at full throttle (as indicated on a tachometer of unknown accuracy.] The 2000 VENTURA 18 boat accelerated to 41.5 MPH speed-over-ground by GNSS receiver.

The acceleration onto plane was rapid and the engine seemed at ease. In full turns [something] grabbed strongly.

A nice on-plane cruising boat speed was 24 MPH, requiring an engine speed of 4,000 RPM.

Overall I was very pleased [with the new aluminum propeller and the boat performance that it gave]

[In considering what propeller to buy] I vacillated back and forth between an aluminum and a steel propeller. I was going to pass on buying the aluminum propeller as it was only $87. However, Turning Point's representative said [with the aluminum propeller] there would be no flex, [and the difference in boat speed with a steel propeller would only be] 1-MPH to 2 MPH more [than with the aluminum propeller]. [I decided that spending] more than $250 [to get the steel propeller] would not be worth [the extra money].

ASIDE: I soon will be selling the MIRAGEplus 15.5 x 15 propeller.

ASIDE: on another boat, a DAUNTLESS 16, I am also using a Turning Point Hustler aluminum propeller but with a smaller diameter.

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Re: 2000 VENTURA 18 OptiMax 135 Propeller Change

Postby jimh » Fri May 27, 2022 3:31 pm

Thanks for the report of the test of the aluminum propeller.

Steel propellers tend to be more durable, and a blade strike with a steel propeller usually does not cause much damage--if any. A blade strike with an aluminum propeller usually results in a bent blade.

What is the fuel tank volume when the tank is at FULL on a 2000 18 VENTURA?

I don’t know that specification.

keithp66
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Re: 2000 VENTURA 18 OptiMax 135 Propeller Change

Postby keithp66 » Mon Jun 06, 2022 6:49 pm

45 gallons

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Phil T
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Re: 2000 VENTURA 18 OptiMax 135 Propeller Change

Postby Phil T » Mon Jun 06, 2022 7:38 pm

The problem with the Mirageplus was the diameter is much too large for that engine/boat combination.

A 13 x 15 of the Mirageplus would be appropriate to test.

I am glad you found a prop that works for you.
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