Montauk 17 with 115-HP 25-inch-shaft

Optimizing the performance of Boston Whaler boats
ewalsh
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Joined: Tue Apr 13, 2021 9:13 pm

Montauk 17 with 115-HP 25-inch-shaft

Postby ewalsh » Tue Apr 13, 2021 10:18 pm

Hi all. I own a 1999 Montauk 17. In 2020 I re-powered a Yamaha 115-HP 25-inch-shaft two-stroke-power-cycle engine in near-mint condition that is older than the the 22-year-old MONTAUK. The engine is mounted on a no-set-back T-H Marine 50012 CmC 5-inch Vertical Extension. The propeller is a standard, original, aluminum Yamaha propeller, marked 17K ( I believe).

The boat pops up on plane very quickly and will do you about 45 at wide open-throttle. I cannot run it at open throttle though as the engine will quickly exceed the 5,500-RPM redline. I'm assuming lots of SLIP and deformation of the soft aluminum propeller. I know there's a lot more speed in it, but that's not necessarily my question.

The question is related to the problem that hat the propeller blows out [ventilates] when taking even a slightly aggressive turn. Currently, when on plane, the anti-ventilation plate skims right at water level, which I believe is perfect. I'm assuming the old generic aluminum propeller [is causing the blow-out], which leads me to my question:

How in the world do I figure out what the best-all-round propeller is for my set up?

I'm sure with the proper propeller, the ventilation in corners will end and I'll pick up efficiency and perhaps a few more miles an hour.

I hear and read all the time about people testing different propellers, but for the life of me, I can't find somebody that's going to share stainless steel props with me to play with.

So I'm hoping the pros out there can lend a hand.

Give me a place to start.

I'd like to stick with a three blade stainless steel propeller, but I'm not against a four-blade stainless steel propeller. The ideal propeller would yield 5,500 PRM with a 600-lbs load. That might mean 5,800 RPMs with a light load, but I can modulate.

Please ask me anything that will help you (and help me) to mate the right propeller to the engine. I promise to capture and share the adventure.

Thanks for reading, Ed


BACK STORY: For 21-years great years the 1999 MONTAUK 27 was powered by a three-cylinder Yamaha 90. That engine was so good for so long, but it finally suffered corrosion in its water passages. There was no repair without disassembly, cleaning and, of course, extracting and tapping all of the bolts that snapped.

I really like the old-schoolYamaha 115 engine. It has no automatic fuel-oil mixing so you must pre-mix the fuel and oil in the fuel tank at 50:1 ratio. Mixing the fuel with oil in the fuel tank (in lieu of "Precision blend") is simple, slightly lighter weight, and--perhaps oddly--very satisfying as it helps to condense the evaporation of time. I love this engine, and it runs like a gem.

Subjectively, the boat looks admirable by eye with the extension. On top of that, the extra space between the motor and the engine splash well renders the splash well much more useful. But enough about that.

ASIDE: I can't believe this is the first time I've posted on the new forum. I participated on the old site for many many moons (with username ewalsh). I want to say I joined in 2001. How time evaporates right in front of you.

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Phil T
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Location: Was Maine. Temporarily Kentucky

Re: Montauk 17 with 115-HP 25-inch-shaft

Postby Phil T » Wed Apr 14, 2021 10:52 am

Ed--welcome back. Here is a simple answer for the propeller:

For the Montauk 17 with the classic Yamaha 115 2-stroke the ideal prop is the 13x19 Yamaha Painted Stainless Steel.

For mounting, you want to make sure the anti-ventilation plate-- just above the propeller--is 1-1/2 to 2-inches above the bottom of the keel

Based on past owners with your model engine, expect a WOT of about 47 to 49-MPH running solo and light.

Post some photos when you have a chance.
1992 Outrage 17
2019 E-TEC 90
2018 LoadRite 18280096VT
Member since 2003

jimh
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Re: Montauk 17 with 115-HP 25-inch-shaft

Postby jimh » Wed Apr 14, 2021 11:31 am

If the current Yamaha propeller is marked "K17" I believe the K refers to the gear case diameter, and K propellers fit the Yamaha 70 to 130-HP engines. The K gear case engines are 4.25-inch hub diameter. The "17" is the pitch rating.

The propeller may be a stainless steel or an aluminum propeller and is just painted black. You can deduce the propeller material from the weight. If the propeller is painted white, then it is more likely to be aluminum.

Since the engine can turn the 17-pitch propeller above the engine maximum rated speed, you should be able to increase pitch to 19-pitch. This will give you more top speed but acceleration may not be as good as with the 17-pitch.

Any replacement propeller from Yamaha in the K series should fit. If you get a non-Yamaha propeller, you should check that it fits a 4.25-inch hub diameter, has the proper number of propeller shaft splines, and is recommended for fit the on Yamaha 115-HP engine.

Whether or not changing to a steel propeller from what you believe is now an aluminum propeller will reduce ventilation in turns is difficult to predict. But on a general basis, at 115-HP power level a steel propeller should work better than an aluminum.

The propeller blade design, blade rake, and blade cup will also affect how well the propeller maintains grip.

Another influence in propeller ventilation in turns is how much the hull leans into the turn. Any propeller will ventilate in a turn if the boat has a significant list to one side in the turn. On my own boat the only time I have ever experienced propeller ventilation was while trying to make a tight, 180-degree, hairpin turn at about 30-MPH. The boat was leaning very much into that turn, as you can imagine. Other than a situation like that, the propeller never ventilates. If not having ventilation in high-speed and aggressive turns is essential, you may have to lower the engine mounting height to permit that to happen.

ewalsh
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Joined: Tue Apr 13, 2021 9:13 pm

Re: Montauk 17 with 115-HP 25-inch-shaft

Postby ewalsh » Wed Apr 14, 2021 7:03 pm

Dear Phil and Jim,

Great to hear from you and I thank you for your help.

I'm having technical difficulties trying to upload a couple of pictures. Apparently they are too large. I'm not really that good at this kind of stuff but just want to report that I can confirm, based on the propeller stamp, that there is a white Yamaha aluminum 17 K prop on the leg.

I could not Measure the anti-ventilation plate to the keel because the motor is tilted and still in storage. However, I've known about this measurement for ages and I'm sure that I followed the recommendations. I am going to try out a 13 x 19.

I thank you for the recommendations and insight. Regards, Ed

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Phil T
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Re: Montauk 17 with 115-HP 25-inch-shaft

Postby Phil T » Wed Apr 14, 2021 8:49 pm

Ed -

Note the size suggested above ONLY applies to the brand/model listed. Not any 19" pitch prop will necessarily work as desired.
1992 Outrage 17
2019 E-TEC 90
2018 LoadRite 18280096VT
Member since 2003