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Hi, All -

I contacted Marine Max in Jacksonville, Florida about getting a new prop for my 2008 Montauk 170 equipped with a Mercury 90hp 4-stroke. I bent my present prop on a rock, and have been looking into buying a new prop. The current propeller is a stainless steel Laser II 13.25 w/ 20P. With an almost empty boat, I can get the motor near 6400 rpm, and get just barely over 40 mph according to gps. However, I almost never run the boat near empty. Most of the time I am loaded to near the maximum weight capacity of the boat.

Two different folks at Marine Max in Jax recommended the stainless Mercury Trophy Plus four-blade prop, size 13.75 w/ 23P as the best replacement for my bent prop. That seems like a massive propeller for a 90hp 4-stroke, but they were sure my boat usage justified that sized prop.

Any thoughts about a Mercury Trophy Plus 13.75 23P on my Montauk 170?

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It sounds like too much pitch, but before I comment tell us what the red line RPM is on your engine.

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Here is the Whaler performance sheet for the Montauk 170. The boat/motor/prop is the same. Note the test date is 2007.

http://www.bostonwhaler.com/boat_grap...4732AM.pdf

What I find interesting is you report a higher rpm but lower speed.

I think the WOT is 6000.

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As Phil points out, something is wrong with your speed vs RPM report. It seems like you have excessive propeller slippage. is the boat bottom painted?

Based on Whaler's report, and if you need a prop for carrying heavier loading, a four blade could help out, with more blade surface in the water. But before you spend $479 for a new Trophy Plus prop (see jacosmarine.com), which I would recommend in 19" pitch, try the new four blade aluminum Quicksilver Nemesis in 19" pitch for only about $130 http://www.ebay.com/itm/New-Mercury-M..._476wt_847

The 4 blade aluminum Nemesis (also called Spitfire) will not be quite as fast top end as the hot Trophy Plus, but should help you determine if four blades is what you need for your application.

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Hmmm. I'm going to check the setting of the gps and see if it is showing knots instead of mph. If it is showing knots, that would seem to make it more in line with the Boston Whaler test data.