CONTINUOUSWAVE

[Moderator's note. This thread is an new and update report on performance of a what is believed to be a 1986 MONTAUK 17 with an E-TEC 90-HP. This is a new report on the performance of the boat, which follows up on a prior, very long, 45-post thread (which is now closed) in which all sorts of odd sidebar discussions ensued on tangental topics (such as how to compute SLIP and how the Crouch Speed Prediction formula might be improved) and in which there was generally significant ambiguity about many of the measured data values (particularly engine speed) which caused the information from the performance testing to be skewed from normal outcomes.]

Boat Speed at Full-throttle
Four runs were made with the engine at wide-open throttle. In three instances the engine speed was 5,200-RPM and the boat speed was 35-MPH. In a fourth run the engine speed and boat speed were close to those figures.

The boat speed is down by 2-MPH from an earlier test which showed 37-MPH, but in the recent testing the crew weight was increased by one additional adult and by additional fuel aboard.

I believe the E-TEC 90-HP engine produces maximum power at 5,200-RPM, so the maximum engine speed during these tests was where it should be.

During these freshwater tests the propeller was an aluminum [BRP] 13.25 x 17-pitch and the engine mounting height was two-holes up. The hull had old bottom paint that was mostly worn off.

Q1: would changing to a Stiletto 13.25 x 15-pitch propeller make an improvement?

I have read good comments about the Stiletto propeller. The existing propeller works, but if I can improve on its performance, I would like to.

[Moderator's note: the original version of this article as written by the OP included many other topics not related to the performance of the boat. All of those topics have been moved to a separate thread in THE GAM where the author reports how he obtained a remedy for several problems encountered with the boat and its gear during testing.]

Details about the boat and engine [with much repetition]:

The boat is a [unspecified model year] Montauk 17, with apparently very dry [hull]. The bottom paint was worn.

The testing was in fresh water.

The engine is an [unidentified model year or production epoch] E-TEC 90-HP and is mounted two-holes up

During the testing the boat had:

• 15-gallons of fuel [in a 22-gallon fuel tank located] forward of console;
• a Group-34 battery under the center console;
• two adults, one large one small;
• a 13.25 x 17-pitch [BRP] aluminum propeller [of unspecified model and unspecified number of blades].

My expectation were for a boat speed exceeding 40-MPH, which was from [many unspecified comments read on continuousWave].

In three test runs on August 17, 2025 [the maxum boat speed was] 35-MPH [at an engine speed of] 5200-RPM.

[Boat speed was measured by a] new Lowrance Eagle 7 [with the unit of speed set to statute miles per hour] but I will, however, check the speed setting next outing [by comparison with a reading from] a smartphone app.

Also, I have a [hey you continually refer to BRP as BRG so please stop it] aluminum [unknown number of blades, unknown model] 13.25 x 19-pitch propeller still in the box which the retailer will allow me to exchange.

Q2: what propeller should I exchange the 19-pitch for?

Q3: should I increase engine mounting height to three-holes up?

ASIDE: I have read a thread about a Montauk 17 E-TEC 90 by mikepte dated 25 January 2021 relating to propeller use. [Moderator's note: please give readers the courtesy of including the URL to any other information for which you wish readers to be cognizant.]

[Moderator's note: please include the model year of the boat and the engine. Please mention the model designator and the number of blades for the propellers. ]

glen1941 wrote:My expectation were for a boat speed exceeding 40-MPH, which was from [many unspecified comments read on continuousWave]...I have read a thread about a Montauk 17 E-TEC 90 by mikepte dated 25 January 2021 relating to propeller use.

The unclear reference to a prior discussion perhaps refers to this thread:

Montauk 17 E-TEC 90
https://continuouswave.com/forum/viewtopic.php?t=6132

In that thread there is no mention of a boat speed exceeding 40-MPH, but there is a reference to another thread, pointing to a 19-year-old discussion at

Montauk: E-TEC 90 Propeller comparison.
https://continuouswave.com/ubb/Forum4/HTML/004278.html

In that discussion a performance report is given which mentions a top speed obtained as 42.5-MPH.

Unfortunately, the boat, engine, installation, and testing conditions are significantly different than the situation that is under discussion here. The variations are:

• engine model year of the E-TEC; in the cited reference the engine is c.2004, which is known to be producing close to the rated 90-HP;
• engine mounting: in the cited reference the engine is mounted on a 4-inch set-back bracket, and not on the transom;
• the environmental conditions were
• air temperature 60-degrees; in the test under discussion the air temperature is not mentioned but is probably warmer as the test occurred in August;
• water effects in the cited test include saltwater (based on my personal knowledge of the boat and its owner, and where it is located (San Francisco Bay), and in the presence of some possible currents, where the colder, higher-density saltwater improves performance and the presence of currents affect boat speed;
• crew weight, one very fit and lightweight adult in the cited test compare to two adults
• stainless steel propellers were used.

Environmental conditions such as air temperature, humidity, water temperature, and water salinity will all affect the performance, with low air temperature, low humidity, and cold saltwater all being enhancements. The presence of any current in the water will also affect measured boat speed if done with speed-over-ground via a GNSS receiver.

Crew weight will also influence any performance test outcome.

Also influencing performance will be operator skill in trimming the engine for best performance.

And use of a high-performance stainless steel propellers enhanced performance.

Also, as previously mentioned (in the cited thread) the power output of the E-TEC 90-HP engine has been known to vary depending on the model year and the EPA certification. Apparently Evinrude had to alter the engine tuning to maintain certain exhaust gas emission characteristics for the three-cylinder E-TEC 90 over the years.

Another influence on engine performance is the quality of the fuel being used. This clearly affects performance. In actual EPA engine testing for emissions, a particular certified gasoline fuel is used to eliminate variation in engine performance due to the gasoline quality.

That 19-years-ago another owner with a similar boat and engine achieved a certain boat speed in their particular testing and conditions of that test does NOT mean that in 2025 it will be possible to achieve the same results with a different boat and engine without duplicating all the parameters of the original test.

ASIDE: as it happens, I have been to San Francisco Bay, have met Rob (ratherwhalering) and have been out in the actual boat, the MONTAUK 17 with the 2004 E-TEC 90-HP engine mentioned in the 19-year-old thread, and at the helm. The boat ran extremely well and the (then new) E-TEC 90 seemed like the perfect match to the hull. And, no, I don't remember how fast it went.

Something is wrong with your boat. The slowest I would expect your top speed to be is 40+ MPH under your load conditions, prop, and engine. I know you weighed the boat, but my guess is that it is heavy. 5200 RPM and that prop has to produce results of 40 MPH. I guess there could be slippage in the prop hub allowing the engine to hit those RPMs without the prop itself turning a corresponding number of turns-

Blackduck wrote:The slowest I would expect your top speed to be is 40+ MPH...

In order for your comment to be correct, with an engine turning 5200-RPM, a 2:1 gear ratio, and a 17-pitch propeller, to hit, say 43-MPH the SLIP would have to be a negative number, which is not possible, unless the actual propeller pitch were much higher than its marked pitch.

Blackduck wrote:5200 RPM and that prop has to produce results of 40 MPH.

In order for your comment to be correct, with an engine turning 5200-RPM, a 2:1 gear ratio, and a 17-pitch propeller, the SLIP would have to be a very, very low 4.4 to achieve 40-MPH. Obtaining a SLIP of 4.4 is not likely, unless the propeller pitch is inaccurate, that is, unless the propeller marked as "17" pitch is actually a higher pitch. This sometimes occurs with certain propeller models and certain manufacturers.

Q4: what is your calculation of the current propeller test data for the SLIP factor where the top speed was only 35-MPH, the engine speed was 5,200-RPM?

Using RATIO = 2, PITCH = 17, my Propeller Calculator shows a SLIP of 16.4, which is a bit higher than expected, but that could be due to the engine being trimmed out too far during the test. A SLIP of around 10 is more common. My calculations are from the PROPELLER CALCULATOR I have created at

https://continuouswave.com/calculators/propCalc.php

Blackduck wrote:I guess there could be slipping in the propeller to hub allowing the engine to hit those [engine speeds] without the prop itself turning a corresponding number of turns...

As for loss of propeller shaft speed due to the coupling between the propeller shaft and the propeller actually slipping, that sort of problem can be easily detected by looking for circumferential scratches or marks between the thrust bearing on the propeller shaft and the metal hub of the propeller. I doubt there is such a problem, as with 90-HP and a 17-pitch propeller and a light boat there generally is not a problem like that. A loose hub is more likely on a much more powerful engine trying to turn a much higher pitch propeller on a much heavier boat. In that situation the propeller load is very high, the engine torque available is very high, and the non-metal-to-metal coupling element will be first to yield.

In any event, the OP has never given any information about the coupling mechanism in the propeller, as he just describes in as a "BRG" and never gives a model designation or mentions the coupling mechanism or the age of the propeller. He does mention the propeller is aluminum, in which case the blades are more likely to flex than the coupling is likely to break down. Actually, the rather high SLIP numbers (16) are likely due to the aluminum propeller blades deforming at maximum load. That is just one of the reasons that people who want to get best boat speed always use steel propellers.

Blackduck wrote:I know you weighed the boat, but my guess is that it is heavy.

I don't see any mention of the exact weight of the boat during the testing reported in this thread. If the OP has a good figure for the total weight of the boat during this test he is keeping it to himself.

Particularly on small boats with generally light total weight, the influence of any extra weight is immediate and noticeable on performance. On that basis, the speculation that the boat is "heavy" is a very reasonable explanation for why the performance is not meeting the owner's expectations of greater-than-40-MPH top boat speed as rigged.