1985 Super Sport 15 60-HP Re-power

Optimizing the performance of Boston Whaler boats
davidB01
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1985 Super Sport 15 60-HP Re-power

Postby davidB01 » Sun Sep 22, 2024 5:39 pm

Q1: what is the expected maximum boat speed for a 1985 SUPER SPORT 15 with a TOHATSU 60-HP engine?

Q2: will a reduction in total boat weight due to reduced engine weight help improve performance?

Q3: are Tohatsu 60-HP engine made in Japan by Tohatsu and not by Honda?

Q4: is there any reliable metric that allows the comparison of the reliability of outboard engines of various manufacturers so I could come to some conclusion about the reliability of the Tohatsu engine?


VERY LONG BACK STORY

I spent the summer of 2024 fighting with a 1986 Johnson 70-HP two-stroke-power-cycle engine on my 1975 SUPER SPORT 15, until I finally got the engine to run well. But I need an ultra-reliable engine for my wife--not an engine that needs me to tinker with it (which I enjoy)--but this is about her. I want an engine that is "bullet-proof."

I have considered a Yamaha 70-HP [of unknown model year, unknown type] but it is a very expensive engine. I have also considered a Tohatsu 60-HP engine--available from a dealer who reputation is "great." The cost of the Tohatsu 60-HP will be about $4,000 less than the cost of the Yamaha 70-HP engine.

The Tohatsu 60-HP will be to 50 to 60-lbs lighter than the Yamaha 70-HP two-stroke-power-cycle engine and the Yamaha 70-HP four-stroke-power-cycle engine (as well as the engine on the boat now). The reduced engine weight will in my opinion increase the performance of the boat by some degree which will compensate for the 10-HP less engine power.

Maximum boat speed is not a consideration. I want overall good performance, with the occasional ability to pull up a skier and to take kids out tubing.

ASIDE: Thanks for letting me join this forum. I grew up with a 13-footer.

I spent hours in last two days reading many very helpful threads as I start a restoration project of a 1985 SUPER SPORT 15, which is a project boat for me to both have fun in, keeping me busy in my retirement, as well as wanting an easily controllable boat for my spouse.
David
On Lake Winnipesaukee in NH. :D
Current project: 1985 BW 15 SuperSport

jimh
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Re: 1985 Super Sport 15 60-HP Re-power

Postby jimh » Sun Sep 22, 2024 8:33 pm

davidB01 wrote:Q1: what is the expected maximum boat speed for a 1985 SUPER SPORT 15 with a TOHATSU 60-HP engine?
To answer this would be much simpler if you could tell readers what the existing maximum boat speed of your boat with its current engine and horsepower. I predict 36.5-MPH.

How I Got the Predicted Speed

Back in the 1990's I owned a 1976 SPORT 15 with a 1976 50-HP engine. At that time the hull was very light; and in later manufacturing years the hull weight increased. The 1976 50-HP was very light. With two adults aboard the top boat speed was about 34-MPH. I would estimate the actual engine power was about 45-HP because in that epoch the power was measured at the crankshaft output, not at the propeller shaft. The speed measured was accurate as if was from a GPS receiver. Now I put those two data, 45-HP and 34-MPH into the Crouch Speed Predicting Formula (using 190 for the HULL COEFFICIENT) for moderate planing hull boats, and find the approximate boat weight, which is 1,405-lbs.

To predict your boat, I will add 100-lbs for the heavier hull lay-up that would be appropriate for a 1985 boat, and 50-lbs for the added wood seating of a SUPER SPORT, and another 75-lbs for a heavier engine to to get 1,630-lbs. Then I will assume the engine produces its rated 60-HP. The predicted boat speed is then 36.5-MPH.

jimh
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Re: 1985 Super Sport 15 60-HP Re-power

Postby jimh » Sun Sep 22, 2024 8:33 pm

davidB01 wrote:Q2: will a reduction in total boat weight due to reduced engine weight help improve performance?
Yes. Boat speed is always a function of power-to-weight ratio. Reduce weight and speed increases.

jimh
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Re: 1985 Super Sport 15 60-HP Re-power

Postby jimh » Sun Sep 22, 2024 8:35 pm

davidB01 wrote:Q3: are Tohatsu 60-HP engine made in Japan by Tohatsu and not by Honda?
As far as I know, yes. TOHATSU only partnered with HONDA to sell higher horsepower outboard engines under the TOHATSU brand rather recently. TOHATSU-made engines are sold under the brand names of Mercury and Nissan, and possible other brand names in the world we don't hear much about.

jimh
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Re: 1985 Super Sport 15 60-HP Re-power

Postby jimh » Sun Sep 22, 2024 8:37 pm

davidB01 wrote:Q4: is there any reliable metric that allows the comparison of the reliability of outboard engines of various manufacturers so I could come to some conclusion about the reliability of the Tohatsu engine?
There is absolutely NO publicly available information that allows boaters to get information about comparative reliability of engines by brand. All information that boaters pass around among themselves is based on anecdotal data they heard about from someone who heard about it from someone else who read it on a website written by someone else.

The best indication for a particular brand about the reliability of their outboard engine products will be the length of the manufacturer's warranty offered to your when you buy the engine of that brand.

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Re: 1985 Super Sport 15 60-HP Re-power

Postby jimh » Sun Sep 22, 2024 8:41 pm

Regarding engines that are described as being "bullet-proof", that adjective has often been used by fans of a particular brand who have an amazing ability to forget how often the engine they owned of that brand needed repairs.

davidB01
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Re: 1985 Super Sport 15 60-HP Re-power

Postby davidB01 » Sun Sep 22, 2024 10:46 pm

jimh--thank you for the very detailed reply. To answer your question, [the present top speed of my 1985 SUPER SPORT 15 with the 1986 Johnson 70-HP engine] with just me (200-lbs) in the boat as indicated on the the 1985 pitot speedometer is about bout 32-MPH, a bit under your predicted value. The pitot tube speedometer is a a 40-year-old gauge. I don't know how to use my iPhone as a speedometer.

Q5: what IOS application will allow an iPhone to show speed?

I cannot comment on your calculation of total weight at this time. I have the original spec sheet from the original purchase (the owner saved everything down to the packing list from factory to the dealer--even the sales person’s notes and comments from 1984.

Based on the Johnson manual for the weight of a 70-HP with tilt-trim, I should be saving about 50-lbs going to the Tohatsu. Using your calculator, and keeping all else constant, it looks like dropping the 10-HP would cost me about 4-MPH, then the decrease in weight will make up about half of that.

[When most of the operation of the the boat] by me or my wife will be at less than wide-open-throttle and for less than three miles , [a reduced stop speed] will not make much time different, and the Tohatsu 60-HP will save $4,000.

My greater concern is for time to accelerate from a standing start while pulling a water skier. I plan to talk to the dealer about the best propeller for [lower engine speed] power for skiing and getting up on a plane, even if I have to sacrifice (what I hope is only) a few top end MPH. Frankly, in the medium chop on this lake, safely operating at full throttle may not be a frequent occurrence.

[A SUPER SPORT 15 with 60-HP] may not be the best boat for that use, but it beats the alternative I have, which is [a boat that weighs 7,000-lbs with a 400-HP engine]. [A SUPER SPORT 15 with 60-HP] should be faster.

ASIDE
I learned to ski on a 13-foot classic with 25-HP, and that makes me hopeful that a [A SUPER SPORT 15 with 60-HP]--while not ideal--may still do well enought.



jimh--thank you for your thoughtful responses, not only to this thread, but in all of the old threads that I have read in the last 72 hours and learned extensively from

@ others. If you either have a) experience in taking a 15 classic or SS from 70 to 60 hp or b) putting a Tohatsu 60 or 70 (70 not in current lineup) on a 15, please let me know your experience.

Gratefully yours,
David
On Lake Winnipesaukee in NH. :D
Current project: 1985 BW 15 SuperSport

jimh
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Re: 1985 Super Sport 15 60-HP Re-power

Postby jimh » Mon Sep 23, 2024 7:25 am

davidB01 wrote:Q5: what IOS application will allow an iPhone to show speed?
Try this search:

https://www.google.com/search?q=IOS+app+to+show+speed

jimh
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Re: 1985 Super Sport 15 60-HP Re-power

Postby jimh » Mon Sep 23, 2024 7:35 am

davidB01 wrote:I plan to talk to the dealer about the best propeller for [lower engine speed] power for skiing and getting up on a plane...
The propeller does not increase the engine power, but by using a propeller with a lower pitch the effect will be to reduce the load on the engine from turning the propeller. Because the load will be less, the engine will be able to more easily accelerate under load.

The power output of the engine is proportional to the engine speed, more or less, so allowing the engine to accelerate to a higher engine speed will allow the engine to produce more power output under the load of the the propeller which is trying to push the boat forward while towing a water skier.

You do not mention if this TOHATSU 60-HP engine you are planning to buy is a four-stroke-power-cycle engine or a two-stroke-power-cycle engine. Typically a two-stroke-power-cycle engine will produce useful power output over a wider range of engine speeds than would occur with a four-stroke-power-cycle engine. Often a four-stroke-power-cycle engine power output only reaches the rated power output at the very maximum permitted engine speed.

In general when you have a relatively small and light boat, any change in load, either from weight added to the boat or from drag added by attaching a person in the water with water skis, will have a very pronounced effect on performance.

To get the best acceleration under heavy load you may need to change the propeller to a lower pitch than you would typically use when the boat was lightly loaded. The drawback to changing to a lower pitch propeller is that when the boat is very light, the engine may be able to accelerate to an engine speed that is greater than the manufacturer recommends, and the operator must be careful not to allow that to occur.

In order to make any realistic propeller evaluation you must be able to accurately measure engine speed and boat speed. You need an accurate tachometer for engine speed, so be certain to include an OEM tachometer in your purchase of the new TOHATSU 60-HP engine--and possibly a digital readout model.

Using an IOS app to measure speed is likely to be sufficiently accurate, as long as the display of the speed has a resolution to 0.1-MPH. If the displayed speed just jumps by increments of 1-MPH, there might be some ambiguity.

jimh
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Re: 1985 Super Sport 15 60-HP Re-power

Postby jimh » Mon Sep 23, 2024 7:51 am

Using your data of a top speed of only 32-MPH with the 70-HP engine, and using the hull coefficient of 190 (which is a good value for a semi-V rounded bottom hull like the classic 15-foot hull), it looks like the total weight would be about 2,468-lbs.

Reducing weight by 50-lbs to 2418 and reducing engine power by 10-HP to 60-HP then predicts a new boat speed of 29.2-MPH. That is a reduction in boat speed of 2.8-MPH.

However, I don't trust your data, based on my experience of being able to get 34-MPH (measured by GPS) with only 50-HP on a slightly lighter 1976 SPORT 15.

By the way, those three-cylinder OMC 70-HP engines are really very good engines, probably about the best-ever three-cylinder 70-HP two-stroke-power-cycle engine design. They were closely copied by Yamaha for their 70-HP engine.

If you have the OMC 70-HP engine in proper tune now and it runs quite well, I would not be in such a rush to discard that engine. It might burn a bit more gasoline and consume a bit of oil, but it probably can run forever if well maintained.

However, I know the allure of a more modern engine, which will start very reliably, run without smoke in the exhaust, won't require continual mathematics to figure out the correct mixing of gasoline and oil, and consume less gasoline. But the cost is quite steep for a new engine these days, and any savings in gasoline used will never pay back the fantastic expense of a new engine. Also a four-stroke-power-cycle engine will need lubricating oil changes probably once per season, which can be an ugly chore.

Jefecinco
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Re: 1985 Super Sport 15 60-HP Re-power

Postby Jefecinco » Mon Sep 23, 2024 9:48 am

Waze is a free app for road navigation that provides current speed. It may also work for determining boat speed.
Butch

davidB01
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Re: 1985 Super Sport 15 60-HP Re-power

Postby davidB01 » Mon Sep 23, 2024 10:00 pm

jimh wrote:However, I don't trust your data, based on my experience of being able to get 34-MPH (measured by GPS) with only 50-HP on a slightly lighter 1976 SPORT 15.
I don’t trust the data either. I feel that both the resolution and accuracy of the 1985 OMC [pitot tube speedometer] is suspect.

ASIDE ON HOW TO FIND AN IOS APP THAT CAN SHOW VELOCITY
[To find an IOS app that can show velocity from GPS reception I will] try a Google search. I have [the IOS app] WAZE, [and I will try it].
David
On Lake Winnipesaukee in NH. :D
Current project: 1985 BW 15 SuperSport

Kapharms
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Re: 1985 Super Sport 15 60-HP Re-power

Postby Kapharms » Tue Sep 24, 2024 8:06 am

I have a 1977 SPORT 15 with an added after-market center console and a 2022 Suzuki 50-HP engine. With two adults aboard the maximum boat speed is 34 to 35-MPH, as indicated on a speedometer and on the WAZE iOS app.

ASIDE: the original engine was an OMC 49-HP.

jimh
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Re: 1985 Super Sport 15 60-HP Re-power

Postby jimh » Tue Sep 24, 2024 10:23 am

Kapharms wrote:...a 1977 SPORT 15 ...with a 2022 Suzuki 50-HP...maximum boat speed is 34 to 35-MPH....
That is the same top boat speed I found on a 1976 SPORT 15 with a 1976 Mercury 50-HP four-cylinder two-stroke-power-cycle engine with just one aboard, no canvas up. That old outboard did not have power,was mounted at the lowest possible position, and had an aluminum propeller.

Image
Fig. 1. With 50-HP, top speed is about 35-MPH, bimini down, one person, calm water, small fuel load.
(From the REFERENCE article on the 15-foot hull.

davidB01
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Re: 1985 Super Sport 15 60-HP Re-power

Postby davidB01 » Fri Sep 27, 2024 7:14 pm

Update.

Based on Jim’s recommended google search and one nice user here with a PM, was able to find what looks like a good basic GPS IOS app called, simply enough, “speedometer”. Tested with the big boat GPS (which cruising in the same range) and our car, and it seems to be fairly accurate.

So, with that, did some speed runs this morning before pulling the boat (the new engine has arrived). 6 MPH wind and very light chop. Tried various engine trims to see how what I could get out of it, and was averaging 38.2 mph (several upwind and downwind runs combined/averaged). The notes from the dealer in 1985 on the specification to the original owner indicate it should have been able to do 42 mph, but there is no written history on this particular boat if that was ever achieved (surprising actually, as he kept every other manual, note, and record)

Using Jim’s online Crouch’s calculator, this would, with a 70 HP 2S Johnson on it with a factor of 190 put the boat at 1732 lbs. Seems heavy, but better than the above calculations. Most of what I have read here is with that engine it should be 40+ mph, but a) the boat may indeed be heavy, b) not sure the 39 y.o. Engine is putting out full power, c) the engine is in full down position, which may be inducing more drag, d) it currently has a big dolphin fin, and I am not sure, especially give the lower mounting of the engine, if that is adding a lot more drag, c) unknown prop size/pitch at this point. There is no Tacometer on board at this time.

With the starting numbers now revised, the loss of 10 hp and 40 lbs, per the calculator, will drop me about 2.4 mph. I can live with that in a boat that I really want set up as a family boat for the spouse/kids. Mid 30s is really fast enough. And, if b) above is true, then it might not even be that much of a loss. 2 weeks and we will know.

The Tohatsu 60HP I selected, while nominally the 20in size, measures actually 21.4 bracket to AV plate, so we have some work to do in measuring how high to put this once we can get accurate measurements with the old engine off. But, that is a different thread! And, of course, selecting a prop.

Will post results, and pictures with the new engine, when done.

Thanks to all, but especially to Jim H for all of the detailed advice and calculations
David
On Lake Winnipesaukee in NH. :D
Current project: 1985 BW 15 SuperSport

davidB01
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Re: 1985 Super Sport 15 60-HP Re-power

Postby davidB01 » Sat Oct 19, 2024 9:13 am

I got the new 60-HP Tohatsu 60-HP four-stroke-power-cycle engine installed on my 1985 SUPER SPORT 15. Dave Rossetti and his apprentice Doug of Lighthouse Marine in Tuftonboro, New Hampshire, did a great job in the engine installation, in cleaning up the space behind the main bench seat with moving the battery into the storage box, in deleting the oil pump, rerouting the cables and wires, and cleaning up the gel coat and old mounting holes. My spouse--for whom the boat is being configured--is very happy.

IMG_4300.jpeg
Fig. 2. The new Tohatsu 60-HP engine on the SUPER SPORT 15.
IMG_4300.jpeg (58.97 KiB) Viewed 321 times


engine.jpg
Fig. 3. Clean aft cockpit deck and transom with swim platform on Port.
engine.jpg (39.49 KiB) Viewed 283 times


PERFORMANCE DATA

From sea trials on an a flat lake, the boat was able accelerate to 43-MPH with the 60-HP Tohatsu engine, mounted three-holes-up, with a 15-pitch propeller, and trimmed up as high as I dared (at this point). The engine speed indicated on the new tachometer was about 6,000-RPM, which occurred just before the first warning buzzer from the engine.

Using Jim’s Crouch’s calculator with inputs
    HP=60
    MPH = 43
    HULL COEFFICENT =190
the calculated boat weight is 1,070-lbs.

I estimate the actual boat weight as follows (all in lbs):
    Base hull = 525
    Tohatsu engine = 216
    Crew weight = 195
    Battery = 35
    Six gallons fuel and tank = 40
    Swim platform = 20
    Stuff = 30
    TOTAL = 1061-lbs
The two estimates (1,070-lbs and 1,061-lbs) are close, so I think the weight is just about correct as deduced from the performance data.

My mechanic, Dave, noted that the propeller pitch should be increased to 16-pitch to stay within the Tohatsu engine speed guidelines, and the larger pitch might produced a small increase in boat speed, but that would come at the expense of acceleration. [With the 15-pitch propeller] the boat does nicely get up and go from a standing start.

The installers and I also noted that with the actual length of the shaft of this engine at 21.4-inches the engine mounting height could be raised one-hole higher--if I wanted. I have not had a chance yet to see where we the Anti-Ventilaton plate is sitting at full speed.


The use of this boat will:
  • occasionally be to pull tubes and skiers; and
  • mainly be by my spouse (who will be unlikely to run at full throttle) and 19-year-old daughter (who may all too frequently be at full-throttle.
We will see if we like [the engine mounting height and the propeller pitch] as they are now. We really don’t need to be going any faster on this lake, and in anything over light chop the boat will likely never be running at full-throttle.

Thanks to Jim H and others whom contributed to this thread with their ideas and data.
David
On Lake Winnipesaukee in NH. :D
Current project: 1985 BW 15 SuperSport

jimh
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Re: 1985 Super Sport 15 60-HP Re-power

Postby jimh » Mon Oct 21, 2024 10:03 am

Getting 43-MPH from the 60-HP engine on the SUPER SPORT 15 sounds very good. I think you are well configured.

Adding an inch of pitch should pull down the engine speed by at least 200-RPM (using an old rule of thumb). But boats always seem to get heavier as they get used, as more people and more stuff comes aboard.