1989 15-footer; old Tohatsu 50

Optimizing the performance of Boston Whaler boats
Tailwind
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1989 15-footer; old Tohatsu 50

Postby Tailwind » Sun Feb 05, 2017 7:20 pm

I have a 1989 15-foot Boston Whaler boat (with a side console) with an old Tohatsu 50-HP two-stoke-power-cycle outboard engine. I only recently bought the boat and the whole thing needs a lot of love. I'm now at the point where the engine is running well, and it's time to replace the beat up, under-pitched aluminum propeller.

Currently, the boat will only run about23-MPH (as measured by a GPS receiver). The main reason for this is that the prop is only 9-pitch and the engine stops accelerating at 5,700-RPM--which all lines up pretty well, mathematically. How much more pitch should I run? The choice appears to be either 10-pitch or 11-pitch. Either way, the thing will still be slow--that is, seemingly much slower than others report, even with 40-HP engines.

The Tohatsu 50 is really just a re-jetted 40 from what I can see, and, as a 1992 that was not properly cared for (until now), it's at best a strong 40-HP by today's standards.

I don't want to overload the old thing, but also don't like running at 5000-RPM, so I would like to get the cruising engine speed down a bit and more top end speed --not for speed's sake, just to have it right.

Absent any advice to the contrary, I'm currently leaning toward a stainless steel propeller of 11-3/4-diameter and 11-pitch.

Thank you, in advance, for any tips or advice.

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Phil T
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Re: '89 Side Console 15 with (old) Tohatsu 50

Postby Phil T » Sun Feb 05, 2017 7:27 pm

Blade design and size are not universal.

What make and model of prop are you looking at? Is the beat up prop a 11.25 x 9? Is it stainless or aluminum? Are increasing to 11p to reduce WOT rpms?

What is the lower gear ratio and WOT rpm range for that motor?
1992 Outrage 17
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Tailwind
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Re: '89 Side Console 15 with (old) Tohatsu 50

Postby Tailwind » Sun Feb 05, 2017 7:34 pm

The prop is aluminum and has "Japan" stamped on it. I cannot discern the maker or model. I measured the radius at 5.25", so it is close to a 10.5" diameter. I know that the previous (dad) owner intentionally under-propped (pitched) the motor to keep his son from going too fast. The gear ratio is 24/13 (~1.84) and the operating range is 5000-5700. The current prop, despite its wavy, blunt, damaged edge allows the motor to run right up to 5700 with just me or with all 4 of my small family aboard.

I want to get to a max of 5400-5500 and lower RPMs at cruise.

Thank you!

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Phil T
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Re: '89 Side Console 15 with (old) Tohatsu 50

Postby Phil T » Sun Feb 05, 2017 7:42 pm

Here is a thread from the archive with comments by prop guru's regarding the same engine. I would follow their advice.

http://continuouswave.com/ubb/Forum4/HTML/005004.html
1992 Outrage 17
2019 E-TEC 90
2018 LoadRite 18280096VT
Member since 2003

Tailwind
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Re: '89 Side Console 15 with (old) Tohatsu 50

Postby Tailwind » Sun Feb 05, 2017 8:02 pm

Thanks, Phil, that is an interesting thread and a good read on that motor. I'm also trying to find information for the 15', so the pitches they discuss there would all be too high.

jimh
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Re: 1989 15-footer; old Tohatsu 50

Postby jimh » Mon Feb 06, 2017 1:56 am

I used to have a 15-footer. The engine was an old two-stroke-power-cycle Mercury 50-HP. The top speed was around 34-MPH. The engine was a 1976, which perhaps was a power-head-rating of 50-HP, so you might consider that the power at the propeller was less, maybe 45-HP.

Here is an appropriate discussion on 15-footers, horsepower, and boat speed:

http://continuouswave.com/ubb/Forum4/HTML/007439.html

The REFERENCE section also gives some information on the expected boat speed as a function of horsepower as provided by Boston Whaler themselves:

http://continuouswave.com/whaler/reference/15/

Again, 50-HP is predicted to hit 34-MPH. And 40-HP should get the boat going 28-MPH.

If you want to hit a target speed of 28-MPH with an engine at 5,700-RPM turning 1.85:1 gears on the propeller shaft, and if you estimate propeller SLIP at about 10-percent, then the pitch should be about 11-pitch. If you want to hit 34-MPH with that same engine speed and gear ratio, the pitch will have to be around 13. (Calculated with a propeller calculator.)

I think that bracket, a pitch of 11 to 13-inch, will be about right. What will work best for your boat and engine will depend on how much actual power the engine can muster, and how much weight is aboard. Usually you can find those smaller propellers in one-inch pitch increments, so perhaps you can start at a 12-pitch and then only have to go up or down one-inch in pitch to get it right if the first propeller is not on the money.

Also, going 30-MPH in a SPORT 15 feels like you are flying because the boat is so low to the water.

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Re: 1989 15-footer; old Tohatsu 50

Postby jimh » Mon Feb 06, 2017 2:07 am

What model is your 1989 15-footer? I know you remarked it has a side console. See the REFERENCE section with model information for help in identification. I think the choices are SPORT, SUPER SPORT, SUPER SPORT LIMITED, and CENTER CONSOLE. Which one is it?

Tailwind
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Re: 1989 15-footer; old Tohatsu 50

Postby Tailwind » Mon Feb 06, 2017 9:37 am

Hi Jim--I think mine was a side console that was ripped out and replaced with a side center console with a flip-back seat/cooler combo. It has a portable tank and there are old holes in the sides where the bench may have been.

As to propeller. I think we are in complete agreement. I wasn't happy with the speed because it was max 23-MPH and probably pinging the rev limiter. I also take no joy in running a boat around all the time at 5000 rpm and 20 mph.

I chose to prop the boat like a 40 hp and went with an 11 5/8 by 11p. I couldn't justify SS on a motor that could grenade at any moment. The extra 2 inches of pitch should get me closer to 5400/5500 and the extra diameter (vs. 10.5 x 9p) should make the boat more responsive in the mid range.

We shall see.

Thanks so much for the advice!

Tailwind
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Re: 1989 15-footer; old Tohatsu 50

Postby Tailwind » Sat Feb 11, 2017 11:17 pm

FOLLOW-UP: I replaced the existing, beat up, 10.5 x 9-pitch aluminum propeller with a brand new Solas Amita aluminum 11.6 x 11-pitch. The engine speed dropped to 5100-RPM from 5700-RPM. I'd like to be turning more than 5100-RPM (and the boat loading was light), but something did not add up right.

At 5100, I was running 27 mph. That results in a slip of 6%, which cannot be. Thus, either the prop is more than 11-inch pitch or my tachometer is reading 200-RPM low. Both the propeller and the tachometer are brand new.

In any case, I then lifted the motor one hole and can run 28.2 at 5300. The numbers are still off.

Anyone dealt with something like this? If the tach is off by 200, then I'm getting 5500 rpm and right where I want to be. Not sure, though, and may need to find a way to test the tachometer calibration.

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Re: 1989 15-footer; old Tohatsu 50

Postby jimh » Sun Feb 12, 2017 10:11 am

There is a notion that a 2-inch change in pitch ought to reduce engine speed by about 400 to 500-RPM. You have observed a drop of 600-RPM. That is slightly more than expected, but later you moved the engine mounting height upward and saw only a reduction of 400-RPM. That is just about what would be expected.

For the propeller SLIP to calculate to 6-percent does not seem completely beyond the range of expectations. I would not infer that the pitch was incorrectly stated unless the SLIP value approached zero or negative numbers. Also, the statement of the pitch is not precisely uniform among all propeller manufacturers, so a propeller of a particular pitch from one manufacturer in one propeller style may not produce the identical outcome as a propeller from a different manufacturer of a different style with the same stated pitch. In the case of your original propeller with damage, the notion that it must be considered the standard of comparison for pitch is not realistic. Perhaps it once was a 9-pitch, but now with damage to the blades no longer functions as a 9-pitch.

As for the accuracy of the tachometer, an error of 200-RPM in a reading of 5,500-RPM is an error of only 3.6-percent. As a general rule, a calibration standard ought to be ten-times more accurate than the amount of error you want to detect. Since you are dealing with only a 3.6-percent error, this suggests the calibration standard tachometer would have to be accurate to 0.3-percent. I don't know where you'd find such a tachometer.

If you have access to a good frequency counter, you could attach the input of the frequency counter to the tachometer signal and read the frequency of the tachometer pulses. You could deduce the ratio of pulse frequency to engine RPM as a whole integer, and then use that to divide the frequency reading into an RPM value. The range of tachometer signal frequency should be the related to the engine speed and the number of alternator poles. For example, if the engine runs at 5,500-RPM and has a five-pole alternator, then the pulse frequency on the tachometer signal would be

(5,500-Revolution/1-minute) x (1-minute/60-seconds) x (5-pulses/1-revolution) = 458.33-Hz

If the engine had a six-pole alternator, the frequency would be 550-Hz. This is enough difference in frequency to deduce the number of alternator poles. Once you have deduced the alternator poles, then you use the measured frequency to work back to engine speed.

For example, if. when your tachometer said the engine was running a 5,500-RPM, you measure a frequency of 464-Hz, then you would first deduce the alternator must have five poles, and then compute the engine speed as:

(464-pulse/1-second) x (60-seconds/1-minute) x (1-revolution/5-pulses) = 5,568-RPM

To have a frequency meter these days that can read low frequencies like these is not especially unusual. Many of the better digital multimeters (DMM) products incorporate a frequency meter. As long as the frequency meter is sufficiently accurate, you could use it as a means of tachometer calibration. The only caution is to assess how well the frequency meter reads the pulsed DC waveform, and how absolutely accurate its measurements are. Even without knowing the absolute accuracy, you could use the frequency meter to judge the tachometer by comparing readings at two engine speeds, say at 2,00-RPM and at 4,000-RPM. The frequency meter should read those engine speeds as two frequencies related by a 2-to-1 ratio. Compare that to what the tachometer says. This can give you a sense of tachometer error, as well. A readily available frequency standard for low frequencies is the 120-VAC 60-Hz electrical power. The electrical power grid in the USA maintains 60-Hz power quite accurately, so using a frequency meter to measure the 60-Hz power will give you a notion of the meter's accuracy. If the meter reads within 0.18-Hz of 60-Hz, then it has a 0.3-percent accuracy.