1979 MONTAUK with Tohatsu 90-HP

Optimizing the performance of Boston Whaler boats
Mattwarner
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Joined: Sun Sep 25, 2016 1:57 pm

1979 MONTAUK with Tohatsu 90-HP

Postby Mattwarner » Wed Jan 02, 2019 10:13 am

Hi. I have a 1979 Montauk and I need a propeller recommendation, please. The MONTAUK is fitted with a Tohatsu 90-HP two-stroke model M90a, and is currently running a 13 x 19 [aluminum] propeller. I do not waterski--just general boating with wife and two young kids.

The engine is mounted two-holes-up. I am not sure of RPM as I have not rigged my tachometer yet.

I have found two propellers:

--Mercury Vengeance 14.25 x 15

--Michigan Wheel Rapture 14.25 x 17

Both these propellers are very well priced, so I wanted to check whether they would be suitable or whether I am best to hold off for maybe something in the 13-inch-diameter range with a 17-inch [pitch]. In the past I have always dropped 2-inches in pitch when swapping from [aluminum] to stainless.

jimh
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Re: 1979 MONTAUK with Tohatsu 90-HP

Postby jimh » Wed Jan 02, 2019 12:11 pm

The link below will lead to prior articles about a MONTAUK 17 with 90-HP and a Vengeance propeller:

LINK to about 60 SEARCH RESULTS

Please give more details about the engine:
--recommended full-throttle engine speed range for optimum performance
--gear ratio
--maximum propeller diameter permitted

Knowing the gear ratio and the recommended engine speed range will be very important in selecting the propeller pitch. You really must have an engine tachometer to assess performance. Also, it would be useful to know the present performance of the boat with its present propeller, and to know the pitch of that propeller and the maximum crankshaft speed the engine can accelerate to. (I think you have a 19-pitch on the engine now.)

You can get an idea of what the target boat speed should be from this listing in my article in REFERENCE on the 16/17-foot hull:

http://continuouswave.com/whaler/reference/16-17/repower.html

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Phil T
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Re: 1979 MONTAUK with Tohatsu 90-HP

Postby Phil T » Wed Jan 02, 2019 1:15 pm

Matt -

I would wait till you have your tach rigged and run the boat with its current configuration.

Ratherwhalering has a good post here: http://continuouswave.com/ubb/Forum4/HTML/004594.html

I don't think those props are the right size. You would be looking for something in the 13'xx by YY inches.

Using the basic prop calculator with a WOT of 5840, Gear ratio of 2.00, slip of 5% and expected WOT of 44 generates a pitch of 16.

cf: http://www.csgnetwork.com/marinepropcalc.html
Member since 2003
1992 Outrage 17, 1992 Evinrude 115

stubaird
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Joined: Tue Sep 04, 2018 1:24 pm

Re: 1979 MONTAUK with Tohatsu 90-HP

Postby stubaird » Fri Jan 04, 2019 2:25 pm

The [WOT speed] range for a Nissan M90A engine is 5,000 to 5,500-RPM, and the gear ratio is 2:1.

I have a 1972 19 Outrage with the same Nissan M90A engine. [For the 19 OUTRAGE] I have a Michigan Wheel APOLLO 13.5 x 15 stainless steel propeller. Top boat speed with a light load is in the low-30-MPH range at about 5,200-RPM. Acceleration is good, and the 19 OUTRAGE will plane with a heavy load. Obviously my 19 OUTRAGE is under-powered by any normal standard, but it was converted from a sterndrive. I am a bit wary of putting too much weight on the transom.

I previously had a 17 Newport, and the 19 Outrage is a much bigger boat.

The TOHATSU M90A (which is a surprisingly decent engine) [and if used with a classic MONTAUK 17] should be able to easily run a 13.5 x 17 stainless steel propeller, maybe even an 18- or 19-pitch propeller. I don't think the gearcase will handle a 14-inch-diameter propeller.

Stu

jimh
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Re: 1979 MONTAUK with Tohatsu 90-HP

Postby jimh » Mon Jan 07, 2019 7:00 am

STU--thank you for the information on the gear ratio and recommended full-throttle engine speed range for the TOHATSU 90-HP engine. Based on that information, and on the general observation of performance for the MONTAUK 17 to be able to reach about 42 to 44-MPH with 90-HP, we can calculate the propeller pitch:

RPM = 5500
RATIO = 2
SLIP = 10
MPH = 43

Solving for PITCH gives 18-inch.

Mattwarner
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Re: 1979 MONTAUK with Tohatsu 90-HP

Postby Mattwarner » Mon Jan 07, 2019 10:47 am

Thanks for the info guys, will keep an eye out for a 13.5 x 17 or 18 pitch stainless.

Currently it has a 13 x19 but without knowing the current RPM's I cant tell whether I am hitting max RPM on this ALU prop. I agree that actually this is a great little motor, I was planning on repowering but for an old 1995 motor it performs very well on the 17 hull.