MONTAUK 17 with Mercury 115 FOURSTROKE

Optimizing the performance of Boston Whaler boats
MikeF1
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MONTAUK 17 with Mercury 115 FOURSTROKE

Postby MikeF1 » Mon Nov 30, 2020 5:38 pm

[The engine on a 1982 MONTAUK (probably a 17) boat] is a 1998 Mercury 115-HP with a recommended maximum 5,250 RPM.

[The 1982 MONTAUK boat] is going to be re-powered with a new Mercury 115 [probably a FOURSTROKE] with WOT range of 5000 to 6000-RPM.

[The old engine used] what is believed to be a 13-1/4 x 19-pitch propeller [of unspecified material and unspecified number of blades].

Will a new and different propeller be needed for the new Mercury 115-HP FOURSTROKE engine?

ASIDE: I can't find the [model] numbers on the old propeller.

ASIDE: I know the 115-HP is over the rated power for a 1982 MONTAUK 17. I think this boat may be carrying some added weight [from water ingress into the Unibond hull]. The old engine isn't producing the boat speed it should. The boat will weighed soon.

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Phil T
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Re: MONTAUK 17 with Mercury 115 FOURSTROKE

Postby Phil T » Mon Nov 30, 2020 6:19 pm

While the two-stroke had a WOT of 5250 and a gear ratio of 2.07, the new Mercury 115 FourStroke is not the same.

To determine the appropriate propeller pitch, look at the engine power, gear ratio, WOT engine speed, propeller SLIP, and target boatspeed.

The current Mercury 115-HP FourStroke model has a gear ratio of 2.07:1 and a WOT limit of 6000-RPM. With a target speed of 48 to 49-MPH and a SLIP of of 5=percent, a propeller calculator returns a pitch of 18-inches.

Propeller size is not universal across brands due to unique blade geometry. Stainless steel is better than aluminum.

As for brand and model, other owners with this engine on other Boston Whalers recommend the Mercury Enertia.

For you application, a 14.5 x 19 Enertia would be where I would start.

The engine should be mounted two-holes-up. Put another way, the top engine bolt would be in the third hole down from the top with two empty holes above the bolt.

It is worth noting propellers are wicked expensive. Going to an inexpensive propeller will not lead to a good result. It would be like buying a $90,000 Range Rover and then installing the tires from a Big Wheel.
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MikeF1
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Re: Cost of ENERTIA Propeller

Postby MikeF1 » Mon Nov 30, 2020 7:59 pm

Holy smokes! $686 on Amazon for that Enertia propeller.

I expected to pay about half that for a decent propeller. I think the current propeller's cost was in that range, or not much more.

Also, the Enertia [literature] says [that propeller is] recommended for [engines of] 135-HP and above.

jimh
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Re: MONTAUK 17 with Mercury 115 FOURSTROKE

Postby jimh » Mon Nov 30, 2020 11:14 pm

The ENERTIA propeller is high-cost because of the special material it is made from. It might improve boat speed by 1 to 2-MPH. If that increment in speed is worth $700, then the ENERTIA may be the best propeller.

If the old propeller can fit the propeller shaft on the new Mercury 115 FOURSTROKE, I recommend you try it. Report the performance results when you have conducted careful sea trials with the new engine (after break-in period). If the top boat speed is not to your satisfaction, you can try other propellers.

The suitability of the existing propeller with a different engine is entirely speculative at this point. Readers have no information about the boat's performance with the old propeller and old engine, and little certainty about the existing propeller's pitch and dimension, number of blades, material, and general style. With so little known, any advice offered would be speculative.

Please restart your inquiry after you have purchased a new engine, installed the new engine on the boat, broken in the new engine, and have collected accurate performance data for the new engine with the old propeller—assuming it will fit. Use a new thread for that topic.