1990 Montauk 17 Re-power Yamaha F90LB

[A2_Mark]

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

I am repowering with a new Yamaha 90 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 two stroke. 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.

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.
Thank you,
Mark

[Phil T]

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.

[jimh]

Read about engine mounting height at

https://continuouswave.com/forum/viewto ... ?f=7&t=739

Read about engine mounting hole layout on the transom at

https://continuouswave.com/whaler/reference/FAQ/#Q8

https://continuouswave.com/whaler/reference/FAQ/#Q9

[jimh]

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.