1979 MONTAUK with Tohatsu 90-HP

Optimizing the performance of Boston Whaler boats
Mattwarner
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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 [but much later in the discussion this is corrected to be the engine is mounted only on-hole-up--jimh] 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--wait till you have your tachometer 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 propellers 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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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.

Mattwarner
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Re: 1979 MONTAUK with Tohatsu 90-HP

Postby Mattwarner » Mon Apr 08, 2019 7:02 am

Just to update, I ran for the first time this season with the tachometer hooked up and WOT engine speed was a disappointing 4250-RPM. Boat speed was 37-MPH with two young kids on board (8 and 6) and about [8-gallons] of fuel.

[The 1979 MONTAUK 17] hull has not been [bottom painted with anti-fouling paint]. [At static trim the hull] sits perfectly. I am not worried about waterlogging more so than might occur in any other old Boston Whaler boat.

[The maximum engine speed] is 1000-RPM down [from the manufacturer's suggested speed range]. And I don't think the 13x19 aluminum propeller is too much pitch for this engine; the boat felt slower [in this test] than last season.

I therefore have stripped the carburetors and soaked for three days, following by a thorough cleaning, and reassembled them. I performed a link-n-sync procedure. The carburetors were were very clean to start with.

I have purchased a BALLISTIC brand 13.5 x 17-pitch stainless steel propeller.

Now that the carburetors had been cleaned I will test again with the aluminum propeller to see if this has improved the performance.

I would be expecting a boat speed of around 40-MPH (with the aluminum propeller) and hoping for 42-MPH with the stainless steel propeller.

stubaird
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Re: 1979 MONTAUK with Tohatsu 90-HP

Postby stubaird » Mon Apr 08, 2019 8:16 am

What is the engine mounting height?

In my experience with [a MONTAUK 17 hull and a Johnson 115] raising the [engine mounting height] [improved] boat speed and [reduced a tendency for the bow to oscillate up and down while on plane].

Stu

Mattwarner
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Re: 1979 MONTAUK with Tohatsu 90-HP

Postby Mattwarner » Mon Apr 08, 2019 8:24 am

Thanks Stu.

[The TOHATSU 90-HP on the 1979 MONTAUK 17] is mounted on- hole "up". I figured [this mounting height] would be ok.

To raise [the engine mounting] another one- hole or two-holes higher would not be difficult.

What is the general consensus [on engine mounting height]?

Mattwarner
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Re: 1979 MONTAUK with Tohatsu 90-HP

Postby Mattwarner » Mon Apr 08, 2019 8:30 am

Just to note, my initial article mentioned motor mounted two holes up but this was a mistake; the engine is only one hole up.

[I emended your initial article in this thread to make note of that--jimh]

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Phil T
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Re: 1979 MONTAUK with Tohatsu 90-HP

Postby Phil T » Mon Apr 08, 2019 8:35 am

MATT--for the sake of testing, only change one variable at a time. Test the current propeller with the same engine height. Change propellers and retest. Try to run with and against any current (if on a river or tidal body of water like the Solent).

Once you are in the ballpark, raising or lowering the engine will be the fine tuning.

Remind us which prop was tested first.

Cheers.
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1992 Outrage 17, 1992 Evinrude 115

jimh
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Re: 1979 MONTAUK with Tohatsu 90-HP

Postby jimh » Mon Apr 08, 2019 10:20 am

I don't know what the consensus of all participants here is going to be regarding the engine mounting height. I am not sure anyone has obtained that information on a statistically accurate basis.

I can tell you that the engine mounting height is usually raised until there is no further improvement in performance or until the propeller begins to ventilate or until the boat owner is satisfied and tired of fiddling.

That the Tohatsu 90-HP on your MONTAUK 17 is now mounted at one-hole-up is a reasonably good starting point for testing propellers.

jimh
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Re: 1979 MONTAUK with Tohatsu 90-HP

Postby jimh » Mon Apr 08, 2019 10:31 am

It is somewhat tedious and confusing to have to refer to earlier posts to figure out exactly what results have been obtained. If I am correctly reading the narrative, the most recent test was conducted as follows:

BOAT: 1979 MONTAUK 17
ENGINE : Tohatsu 90 mounted one-hole up, but before a thorough cleaning of the carburetors was done; gear ratio is believed to be 2:1 and recommended engine speed range at full throttle is believed to be 5,000 to 5,500-RPM based on information about another engine;
PROPELLER: we can infer that the propeller was an aluminum 13 x 19-pitch based on a careful reading of the narrative
CREW: One adult, two children, ages six and eight
FUEL: about 8-gallons.

Results:
ENGINE SPEED in RPM and BOAT SPEED miles per hour
At maximum throttle
4,250-RPM and 37-MPH

Unless the two children are going to always be available for all future testing, I recommend retesting without them aboard. Changing crew weight will have an influence on boat performance on small boats where the total boat weight is not great. Eliminating variation in crew weight from test to test will produce better test data.

Please clarify if your boat speed measurements are in statute miles per hour (based on 1-mile is 5,280-feet) or in nautical miles per hour (based on 1-nautical mile is 6,076-feet). For some reason I think you are in the U.K. and perhaps are using nautical miles per hour.

It is also useful to note how the speed was measured. If speed is from a global satellite navigation system (GNSS) receiver, the speed will be speed over ground (SOG). If speed is from a Pitot tube water pickup, the speed will be speed through the water. The influence of current effects must be compensated. If speed is SOG, you have to subtract a favoring current or add a opposing current to get the true speed through the water. Knowing the speed of the current with any precision is difficult, so the usual method is to test in two opposing directions and take an average. If speed is from a Pitot tube pick-up you don't have to compensate for current, but the absolute accuracy of those speedometers may not be good, particularly if using just an inexpensive device.

Mattwarner
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Re: 1979 MONTAUK with Tohatsu 90-HP

Postby Mattwarner » Mon Apr 08, 2019 11:17 am

Yep, I am not going to try the new stainless [that is, a BALLISTIC brand 13.5 x 17-pitch propeller] until I have re-tested [the aluminum propeller] with the [engine having clean] carburetors. I will also test with just me in the boat so I can do some more testing.

Unfortunately the speed limit in Poole Harbour where I boat gets enforced between 1st April - 31st October. That means traveling a fair distance out of the natural harbour before the boat can be run over 10-nautical-miles-per hour, and conditions are rarely suitable for sustained high speeds.

Once I can [get a test result with the engine speed] around 5,000-RPM or higher, I will be happy. I will use that test as a benchmark, and then swap to the Ballistic propeller and play around with engine mounting height.

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Re: 1979 MONTAUK with Tohatsu 90-HP

Postby jimh » Mon Apr 08, 2019 11:36 am

Your most recent reply did not clarify the speed measurement method or units. Please clarify how you are measuring the boat speed and in what unit you report those speeds.

As a general rule the speeds mentioned in the forum and throughout the other contents on the website are based on statute-miles-per-hour. Small boats seldom make long ocean voyages where they will be out of sight of land for days or weeks. Therefore small boats don't need to measure their progress in nautical miles when navigating, and small boats usually use a land-based unit of distance, such as statute miles or kilometers because they are closed to land. In order to make your test data comparable to all the other test data, the boat speed will have to be provided in units of statue miles per hour.

Also, please confirm what propeller you are reporting as the propeller under test. You have mentioned four propellers, but I think the only propeller tested is an aluminum propeller described only with dimensions 13 (diameter) x 19 (pitch). The make, model, and number of blades has not been provided.

Sea state will have a significant influence on propeller testing results, as will other environmental factors. The best testing procedure is to test several propellers on the same day under the same weather and sea state, as much as possible. Cold and dry air and cold water both tend to improve performance compared to the same engine, boat, and propeller tested in hot and humid air in warm water. Salinity also has an effect. If testing in saltwater the results will be different than testing in freshwater due to the difference in water density.

Mattwarner
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Re: 1979 MONTAUK with Tohatsu 90-HP

Postby Mattwarner » Tue Apr 09, 2019 12:15 pm

I confirm the speed was measured by GPS so the speed was speed-over-ground or SOG. The speed was measured at 37-MPH in one direction and 36-MPH in the other direction.

I will retest with just myself onboard and post new baseline results.

Mattwarner
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Re: 1979 MONTAUK with Tohatsu 90-HP

Postby Mattwarner » Tue Apr 23, 2019 12:12 pm

Just to update got some good weather over the weekend and headed out to take some more readings with just myself on board and 60l of fuel results as follows:

These were taken SOG by Raymarine GPS which records max speed and reset after each run.

average of 34.5kts = 39.7mph @ 5000rpm

Just to recap motor is mounted one hole up with a 13 x 19 aluminium prop. I have yet to try the 13.5x17 stainless as i am told i need a specific thrust washer for it.