1990 Montauk 17 Re-power Yamaha F90LB

Optimizing the performance of Boston Whaler boats
A2_Mark
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1990 Montauk 17 Re-power Yamaha F90LB

Postby A2_Mark » Thu Nov 24, 2022 7:59 am

I have a 1990 Montauk 17 with the original Yamaha 90 two-stroke-power-cycle engine.

I am repowering with a new Yamaha F90LB.

Q1: what propeller should be used on a Yamaha F90LB for a MONTAUK 17?

An initial suggestion was to simply use the propeller from the earlier engine. After a little reading, I have seen it asserted that when moving to four-stroke-power-cycles engines from a two-stroke-power-cycle engine of the same rated horsepower, a slightly larger prop may be called for because power typically occurs at somewhat lower RPM.

A specific suggestion about a Yamaha prop would be very welcome.

Q2: is a stainless steel propeller preferred over an aluminum propeller?

I am looking for good general performance. Obtaining the maximum boat speed is not critical, but I do not want want to compromise [maximum boat speed by] too much.

On occasion the boat will be used to pull water toys.

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Phil T
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Re: 1990 170 Montauk Repower

Postby Phil T » Thu Nov 24, 2022 1:55 pm

A 1990 Montauk is referred to as a Montauk 17.

The Montauk 170 is a completely different boat and was introduced in 2003.

As for your repower.

You need a different propeller because the gear ratio of the new engine is not the same as the old one. The information you received is not accurate.

Hundreds of owners have repowered the Montauk 17 and the other models based on that hull. A few hundred have installed an F90.

A dozen or so have done extensive propeller testing.

The list of recommended props for the Montauk 17 and the F90 include:

13-1/3 x 18 Yamaha Talon (Performance)
13 x 19 Yamaha Painted SS
13 x 19 Powertech SCD
13.25 x 17” Stiletto Advantage

*** NOTES:
The propeller size listed only applies to the specific model and is not transferable.

The propellers also require the engine be mounted [two holes up, that is, ]so that the top bolts are in the third hole down from the top.

If the blind holes were used for the existing engine, you must fill those holes with plugs and re-drill them using the BIA pattern.

Information on the pattern and hole position is on this site. Do a search.
1992 Outrage 17
2019 E-TEC 90
2018 LoadRite 18280096VT
Member since 2003

jimh
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Re: 1990 Montauk 17 Re-power Yamaha F90LB

Postby jimh » Fri Nov 25, 2022 9:32 am


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Re: 1990 Montauk 17 Re-power Yamaha F90LB

Postby jimh » Fri Nov 25, 2022 9:38 am

A2_Mark wrote:After a little reading, I have seen it asserted that when moving from two to four stroke at the same power, a slightly larger prop may be called for because power typically occurs at somewhat lower RPM.
Wrong.

Typically a naturally-aspirated four-stroke-power-cycle outboard engine only develops its rated power at the very highest engine speeds permitted, say at the 90 to 100-percent of allowed engine speed range, while a two-stroke-power-cycle engine will produce its rated power output at a much wider range of engine speeds, typically in a range of 70 to 95-percent of the maximum allowed engine speed.

Daleshank
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Re: 1990 170 Montauk Repower

Postby Daleshank » Wed Jun 28, 2023 1:08 pm

Phil T wrote:The list of recommended props for the Montauk 17 and the F90 include….
I just repowered my Montauk 17 with a 2023 Yamaha F90. I want to make sure I’m getting the right propeller.

PHIL T suggested the 13-1/3 x 18 Yamaha TALON Performance. I can only find the a 13-1/8 x 18 TALON

Q3: will a 13-1/8 x 18 TALON suffice?

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Phil T
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Re: 1990 Montauk 17 Re-power Yamaha F90LB

Postby Phil T » Wed Jun 28, 2023 3:41 pm

Daleshank wrote:Q3: will a 13-1/8 x 18 TALON suffice?
Yes.

Also the TURBO 1 is similar in the same size as the Performance Series TALON.

Shop the web. Prices vary.
1992 Outrage 17
2019 E-TEC 90
2018 LoadRite 18280096VT
Member since 2003

pcrussell50
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Re: 1990 Montauk 17 Re-power Yamaha F90LB

Postby pcrussell50 » Sun Jul 02, 2023 4:45 pm

jimh wrote:
A2_Mark wrote:After a little reading, I have seen it asserted that when moving from two to four stroke at the same power, a slightly larger prop may be called for because power typically occurs at somewhat lower RPM.
Wrong.

Typically a naturally-aspirated four-stroke-power-cycle outboard engine only develops its rated power at the very highest engine speeds permitted, say at the 90 to 100-percent of allowed engine speed range, while a two-stroke-power-cycle engine will produce its rated power output at a much wider range of engine speeds, typically in a range of 70 to 95-percent of the maximum allowed engine speed.


Correct.

There has been a bit of a misconception about torquiness, (made up word), two-stroke versus four-stroke, with the urban legend being that four-stroke technology is inherently torquier. I suspect this misconception was borne out of the original application of four-strokes in powersports, which was pro motocross, where the AMA (American Motorcycle Association) created a "conversion formula", whereby four-stroke motocross engines could be up to 450cc's displacement in order to be competitive in the 250cc two-stroke class. The formula was unfairly biased against two-strokes as it was just too permissive. With that much more allowable displacement (1.8 times), it was just too much for the inherent superiority of two-strokes to overcome, and racers wishing to be competitive were forced to switch over to four-strokes. The new 450cc four-strokes were said to have "more torque", and torquieness became their hallmark. But that is in fact a meatless, apples to oranges comparison due to the displacement difference. But it persisted and people came to think that "four-stroke means torque superiority over two-stroke". Which is diametrically opposite of the truth.

-Peter

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Re: 1990 Montauk 17 Re-power Yamaha F90LB

Postby jimh » Tue Aug 29, 2023 10:50 am

Let's get back to the original assertion made: There are two parts to the assertion.

PART 1 is the claim that when changing to a four-stroke outboard from a two-stroke outboard, the propeller will need to be "larger." This may actually be true because in many outboard engines using a four-stroke design the GEAR RATIO will be numerically higher than in a similar two-stroke outboard. The result of this is the propeller shaft turns less for each engine crankshaft revolution.

For example, if a boat speed was obtained with, say, a 17-inch-pitch propeller on the two-stroke engine, when changing to a four-stroke the pitch of the propeller may need to be increased because at identical engine rotation speeds, the propeller shaft on the four-stroke engine will be turning slower.

But typically the propeller pitch for a particular boat speed is constant, and if the gear ratio is higher, the engine speed must be higher to produce that same propeller shaft rotation speed.

The higher numerical gear ratio used on the four-stroke is also able to improve the torque output from the engine as delivered to the propeller shaft, because the slower propeller shaft speed, the greater the torque for the same horsepower being delivered. Torque is defined as

Torque = ( 5252 × HP ) / RPM

Another interpretation of "larger" propeller is one with larger diameter. This is also seen in modern four-stroke outboard engines in the propeller aperture size. Typically older two-stroke engines are limited to a propeller diameter of 15-3/4-inch or perhaps 15-7/8-inch by the location of the anti-ventilation plate above the propeller shaft. Newer four-stroke engines can turn propellers with diameters of 16-inches or more.

Another factor entering the discussion is the ratio of propeller diameter to propeller pitch, and how if affects propeller efficiency. There is some evidence for propeller diameter equal to propeller pitch is optimum. While this may only be a rough rule of thumb, there is no doubt that with four-stroke outboard engines, the design of the gear case and lower unit have been altered from the traditional outboard to permit propellers with larger diameter to be used. A reasonable inference about this must be that the manufacturers did for a reason: to be able to use larger diameter propellers that may be more efficient than propellers limited to less than 16-inch diameter.


PART 2 of the claim is rather vague; it claims the larger propeller is needed "because power [from what I read as meaning power from a four-stroke engine] typically occurs at somewhat lower RPM." This is generally not true. The power curve of most four-stroke-outboard engines requires the engine speed to be very close to the maximum allowed engine speed in order to reach the rated power.