1986 SUPER SPORT 15, 70-HP

Optimizing the performance of Boston Whaler boats
Rocky
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1986 SUPER SPORT 15, 70-HP

Postby Rocky » Wed Jul 13, 2022 9:03 pm

My 1986 SUPER SPORT 15 has an Evinrude 70-HP engine with a stainless steel propeller [of unspecified number of blades] of 13.5-inch diameter and 18-pitch. [The engine can accelerate to a speed of] 6,000-RPM, but I seldom operate at that throttle because "the book" recommends (I think) to run at 5,800-RPM.

WIth only me aboard the maximum speed over ground by GPS is 34-MPH.

My expectation is the boat should be able to accelerate to 40-MPH.

Give your analysis of the performance data given above.

Thanks

jimh
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Re: 1986 SUPER SPORT 15, 70-HP

Postby jimh » Thu Jul 14, 2022 8:46 am

To assess the performance, the engine GEAR RATIO must be known. Please give the engine gear ratio as indicated in the owner's manual.

If the gear ratio were known, then a PROPELLER CALCULATOR (such as the excellent propeller calculator available on continuousWave) could be used to assess the propeller.

For example, assuming the GEAR RATIO was 2:1, then your perfromance data would be entered into the propeller calculator in order to compute a value for SLIP:

RPM=5800 (as mentioned as the usual engine speed employed)
RATIO=2 (assumed)
PITCH=18 (stated pitch)
MPH=34 (stated speed)

This produces a SLIP value of 31.

A value for SLIP of 31 at a boat speed of 34-MPH is very unusual. The propeller SLIP should be about 10 when the boat is on plane and running well. There several explanations for this:
  • the engine trim is much too high and the propeller thrust is being spoiled by airy water
  • the propeller is badly damaged, which would be clearly visible by bent blades or other obvious damage
  • the propeller hub is slipping
  • the engine speed is incorrect, probably due to an error in the tachometer or in its calibration.

Among those possible explanations, a slipping propeller hub or a bad tachometer are perhaps the most likely to be the actual cause.

Regarding your expectation that the boat should be able to accelerate to 40-MPH: generally to have a reasonable expectation of top speed for a moderate V-hull planing boat, the boat weight and horsepower are needed, along with a coefficient for the hull design. With those data a method developed by naval architect George Crouch can be used to predict boat speed.

The method is embodied in another calculator, which I have created and called Crouch's Calculator.

Entering the following data into the Crouch's Calculator:

POWER = 70 (stated)
COEFFICIENT = 190 (based on experience with Boston Whaler hulls)
SPEED = 40 (expected)

The expected hull weight is calculated: 1,579-lbs.

For a SUPER SPORT 15 hull with a relatively light 70-HP engine and one person, the total weight should not be more than about 1,600-lbs. On that basis a boat speed of 40-MPH is expected.

ASIDE: I used to have a SPORT 15 with a 50-HP engine. It would reach 34-MPH. Using Crouch's Calculator to find the weight, the computed weight is 1,560-lbs. On that basis, I would say that 40-MPH with a 70-HP is in the ballpark of expected speed.

dtmackey
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Re: 1986 SUPER SPORT 15, 70-HP

Postby dtmackey » Thu Jul 14, 2022 9:19 am

Rocky wrote:My 1986 SUPER SPORT 15 has an Evinrude 70-HP engine with a stainless steel propeller [of unspecified number of blades] of 13.5-inch diameter and 18-pitch. [The engine can accelerate to a speed of] 6,000-RPM, but I seldom operate at that throttle because "the book" recommends (I think) to run at 5,800-RPM.

WIth only me aboard the maximum speed over ground by GPS is 34-MPH.

My expectation is the boat should be able to accelerate to 40-MPH.

Give your analysis of the performance data given above.

Thanks


Q1: what prop do you have on the boat? If you are running a three-blade those numbers do not make sense at all.

Q2: what we you using for speed? GPS or speedometer? [See above where GPS speed over ground is mentioned]

Q3: is your tach functioning properly? Some have an adjustment on the back depending if the motor is 2-, 3-. 4, or 6-cylinder. If the calibration setting is wrong, the tachometer RPM indication will be way off..

Q4: what is your engine mounting height? You can run the engine very high on the 15.

The boat should easily hit 40-MPH with a good running 70-MPH engine that is setup properly on the boat.

My SUPER SPORT 15 can continually hit 40-MPH with a Yamaha 70-HP two-stroke and now 43-MPH with a Yamaha F70, a jackplate, and a 17-pitch propeller.

I know [the F70] has more [power] it, and if I were to run an 18 or 19-pitch I'll see better top end boat speed; my estimate is 45 to 46-MPH.

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Phil T
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Re: 1986 SUPER SPORT 15, 70-HP

Postby Phil T » Thu Jul 14, 2022 12:22 pm

When discussing props, you MUST include make, model and size.

Saying you have a 13.8 by whatever means nothing and is not helpful.

Look at the hub or inside the hub for the serial number and then google it.
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Rocky
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Re: 1986 SUPER SPORT 15, 70-HP

Postby Rocky » Thu Jul 14, 2022 8:23 pm

dtmackey wrote:Q1: what prop do you have on the boat?
The propeller [previously described as having unspecified number of blades and diameter 13.5-inch and pitch 18] is an Evinrude Raker 394758.

dtmackey wrote:Q3: is your tach functioning properly?
By the sound of the motor I can't imagine it running at any higher RPM than indicated at full throttle, with motor trimmed rather high, about 6000-RPM.

dtmackey wrote:Q4: what is your engine mounting height?
The [engine mounting bolts are] in the third hole; that is, the engine mounting can only be further raised by one-hole or about one-inc or about 1.5-inches.

jimh wrote:...a slipping propeller hub or a bad tachometer are perhaps the most likely to be the actual cause.
With the engine not running, I put [the shift] in gear; i cannot cause the prop to slip. Also, the [boat] speed and [engine speed in] RPM correlate exactly every time I'm out. If the propeller hub were slipping it would be slipping exactly the same every time I accelerate.

The engine does not accelerate to 5,800-RPM unless I trim the engine rather high.

Q5: I know this sounds crazy but could [the engine] be operating on two cylinders without me knowing it?

jimh
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Re: 1986 SUPER SPORT 15, 70-HP

Postby jimh » Fri Jul 15, 2022 8:33 am

Rocky wrote:Q5: I know this sounds crazy but could [the engine] be operating on two cylinders without me knowing it?
It is possible for a three-cylinder engine to run on only two-cylinders and seem to be running normally, but if this occurs the power output from the engine will be reduced significantly.

In the case of a three-cylinder 70-HP engine, while running on only two cylinders the power output would decrease to perhaps 45 to 50-HP.

If the reported engine speed of 5,800 to 6,000-RPM at full throttle while turning a propeller with 18-pitch is accurate data, I doubt that a 45-HP engine could actually do that.

If you are completely confident in the accuracy of the engine speed as indicated on the tachometer, this is further indications that the problem must be in the propeller and the propeller hub.

Rocky wrote:With the engine not running, I put [the shift] in gear; i cannot cause the prop to slip.
Your test is not equivalent to the engine apply 70-HP of power to the propeller shaft while the propeller is trying to accelerate an enormous weight of water.

Try this inspection:
  • remove the propeller from the engine propeller shaft
  • inspect the thrust washer to propeller hub joint
  • look for circumferential scratches on the propeller hub face that bears against the thrust washer.

jimh
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Re: 1986 SUPER SPORT 15, 70-HP

Postby jimh » Fri Jul 15, 2022 8:53 am

Phil T wrote:Look at the hub or inside the hub for the [part] number and then google it.


Rocky wrote:]The propeller is an Evinrude Raker 394758.


You were supposed to search for more data with the part number. A search with arguments "Evinrude Raker 394758" reveals that the propeller is a BRP three-bladed 13-1/2-inch diameter, 18-pitch stainless steel propeller with significant blade rake.

Also, that propeller is obsolete. I suspect that is because of a change in the hub design. Based on some photographs of a part number 394758 propeller, it appears that the hub coupling between propeller shaft and propeller is a pressed-in rubber insert.

A propeller hub with a pressed in rubber insert could be slipping, although with only 70-HP that seems less likely that with more powerful engines.

But checking more thoroughly for a slipping hub seems prudent to me.

Rocky
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Re: 1986 SUPER SPORT 15, 70-HP

Postby Rocky » Sat Jul 16, 2022 8:34 pm

Well I was wrong. Full throttle and trimed it is only turning 4800 rpm not 5800 as i reported earlier
I was doing 35 mph

Any more help is appreciated

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Phil T
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Re: 1986 SUPER SPORT 15, 70-HP

Postby Phil T » Sat Jul 16, 2022 8:51 pm

What year and model of Evinrude 70 do you have? 2-stroke or DFI?

This will help determine the gear ratio/RPM range to determine the proper pitch.
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Phil T
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Re: 1986 SUPER SPORT 15, 70-HP

Postby Phil T » Sat Jul 16, 2022 9:02 pm

After using the advanced search feature and reading a few threads I think there is a different problem in this instance.

In this thread

https://continuouswave.com/ubb/Forum4/HTML/005216.html

there is a note to checking the TACH to ensure it is operating correctly, and ensure the carburetors are working correctly. It reads as your engine may not be operating correctly.

Once you verify these items, we can go back to the prop conversation.
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jimh
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Re: 1986 SUPER SPORT 15, 70-HP

Postby jimh » Sun Jul 17, 2022 9:10 am

WHAT IS THE GEAR RATIO OF THE ENGINE?

Until we find out what the gear ratio of the engine is, there is no way to assess the performance data.

SECOND REQUEST: give us the gear ratio, please.

I am AGAIN going to make a GUESS: the gear ratio is 2.42:1.

Next I am going to assume the propeller is working properly and has a SLIP of about 10.

Next, back to the Propeller Calculator with these inputs:
    RATIO = 2.42
    MPH = 34
    PITCH = 18
    SLIP =10

The calculator computes engine speed to be 5,365-RPM. The Tachometer is reading 5800-RPM. Now we see tachometer error as the problem.

NEW QUESTION: by some quirk are you stating the boat speed in Nautical-Miles-per-Hour but calling it "MPG" as in statute miles per hour?
If so, then 34-Nautical-miles-per-hour is a speed of 39.1-MPH, and the boat performance is right as expected.

Rocky
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Re: 1986 SUPER SPORT 15, 70-HP

Postby Rocky » Mon Jul 18, 2022 8:23 am

I appreciate your help.
Gear ration is .48 it also says (12:25)

Im guessing that means 2 to 1 but Im not familiar the numbers.

Would compression have any thing to do with it?
What about it running on 2 cyl without me knowing? The engine does not sound rough

thanks again

dtmackey
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Re: 1986 SUPER SPORT 15, 70-HP

Postby dtmackey » Mon Jul 18, 2022 8:53 am

Trying to resolve your concern is getting more confusing with each new detail. I think you first need to know if your motor is running properly. Second, you need to validate each piece of information you've provided:

Prop pitch
Engine RPM
Gear ratio - Evinrude used 2.42 on the 70 for many years, which is 12:29.
Speed - I don't believe you answered my Question #2 above and would like to know how you are measuring speed? Are you using GPS or a pilot tube speedo (which can be very inaccurate).

So far, any of the numbers you've provided do not make sense when run through a simple prop slip calculator.

D-
Last edited by dtmackey on Tue Jul 19, 2022 10:49 am, edited 1 time in total.

Rocky
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Re: 1986 SUPER SPORT 15, 70-HP

Postby Rocky » Mon Jul 18, 2022 4:23 pm

Measuring speed by gps