1989 Super Sport Limited Honda 50-HP

Optimizing the performance of Boston Whaler boats
Tim
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Joined: Thu Aug 24, 2017 5:54 pm

1989 Super Sport Limited Honda 50-HP

Postby Tim » Thu Aug 24, 2017 6:06 pm

I have a [1989] Super Sport Limited. I just replaced the Mercury 60-HP engine with a 2006 Honda BF50. The propeller is 11 3/4 x 10.

The Mercury [60-HP engine] had no tach; its best boat speed was 36-MPH at wide open throttle.

The Honda [50-HP engine accelerates] to the limiter at 6,500-RPM before [the throttle handle is] reaching full, achieving a top speed of just 22-MPH.

All data at sea level.

Which propeller do I need?

Thanks for your help.
Sapienti sat.

jimh
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Location: Michigan, Lower Peninsula
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Re: 1989 Super Sport Limited Honda 50-HP

Postby jimh » Sun Aug 27, 2017 10:24 am

Increase propeller pitch by 4-inches and retest; report outcome.

Work with a vendor that will let you test and return for credit until you find the optimum propeller.

How long is your boat?

Tim
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Joined: Thu Aug 24, 2017 5:54 pm

Re: 1989 Super Sport Limited Honda 50-HP

Postby Tim » Sun Sep 03, 2017 6:23 pm

15'. No luck finding a prop shop with a trial-and-error prop lending program.
The Outboard Motor Shop in Oakland, CA, calculated a 17 inch pitch. That's the same scientific-wild-a$$-guess I came up with. I ordered one and am waiting to try it out.
Sapienti sat.

Tim
Posts: 6
Joined: Thu Aug 24, 2017 5:54 pm

Re: 1989 Super Sport Limited Honda 50-HP

Postby Tim » Tue Nov 28, 2017 9:31 am

Here is the test data from my 1989 15 Super Sport Limited with a 2006 Honda BF50 four-stroke and aluminum 11 x 17 propeller. Aboard were two people and 18-gallons of gassolin on board.

Whaler engine performance data
MPH   RPM
3 950
5 1600
10 3000
15 3500
20 3800
25 4250
26 4400 (WOT)


I'm looking to achieve 5750 RPM at WOT.
Thank you in advance for suggestions on propellor size and pitch.
Last edited by Tim on Tue Nov 28, 2017 3:37 pm, edited 1 time in total.
Sapienti sat.

jimh
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Location: Michigan, Lower Peninsula
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Re: 1989 Super Sport Limited Honda 50-HP

Postby jimh » Tue Nov 28, 2017 3:37 pm

An old rule of thumb with propeller pitch and engine speed: you get 450-RPM change with 2-inch pitch change.

The rule would suggest that if you reduced pitch to 15-pitch from 17-pitch the engine speed should increase about 450 RPM. If that obtains, you would then hit WOT engine speed of about 4900-RPM.

Your boat speed already seems a bit poky for a modern 50-HP engine on a 15-foot hull. The SSL must add a lot of weight. My old SPORT 15 with an ancient 1976 50-HP engine (rated at the power head, not at the propeller) ran about 34-MPH top speed.

With your initial data, and adding the GEAR RATIO (2.09:1), the propeller slip calculates to 23, which is too high. SLIP ought to be around 10 at best speed performance.

If you changed to a 15-pitch, if the engine accelerates to 4900, if the SLIP remains 23, then boat speed would (still) be 26-MPH. If the propeller SLIP could be worked down to about 12, then the boat could hit 30-MPH with the 15-pitch.

Drop to 13-pitch; if the engine accelerates to 5700 RPM, if SLIP is down to 10, then boat speed will be 30-MPH. That sounds like a reasonable goal.

The present propeller is not letting the engine accelerate into its power band. The typical four-stroke-power-cycle outboard engine needs to be able to accelerate to the very upper end of its recommended full-throttle speed range in order to produce its rated horsepower output. The 17-pitch is too much load for the 50-HP. Also, the 17-pitch is not turning at the propeller shaft speed it was designed for--it is turning slower--and that makes the SLIP increase. Your "wild...guess" did not produce good results.

All this means that going down to 13-pitch should:

--let the engine accelerate to the upper RPM range where it can make 50-HP
--let the propeller shaft speed turn where the propeller is going to get more efficient (less SLIP)
--and the two factors, more horsepower and more propeller efficiency, combine to produce more boat speed.

Also, in the smaller pitch ranges the pitch changes are usually in 1-inch pitch increments. I'd line up three test propellers in pitches of 15, 14, and 13 for sea trials. If not available, try a 14-pitch. That was the pitch I recommended to you in my first reply.

dtmackey
Posts: 273
Joined: Thu Sep 28, 2017 9:29 pm

Re: 1989 Super Sport Limited Honda 50-HP

Postby dtmackey » Thu Nov 30, 2017 10:38 pm

Before you start testing props, make sure the motor is mounted properly. Boat shops tend to mount motors lower than they need to be. Test the boat and while on plane, can you see the anti-ventilation plate? If not, raise up the motor one hole and retest. Ideally you want to see the anti-ventilation plate top and not have lots of spray. Only after than should you start testing props.

When you test propellers get RPM readings at each 500-RPM or at least 1000-RPM increments, and use a prop slip calculator to help dial in the exact prop that gives the best performance. I'd recommend reaching out to Ken on his Prop Gods forum. He's considered one the best in country at helping dial in on the best propeller for the boat. For the record, I have no affiliation with Ken, and he was spot on after I provided him my information. His prop recommendation was spot on and not what I'd have thought. Saved me time and money.

D-

Tim
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Joined: Thu Aug 24, 2017 5:54 pm

Re: 1989 Super Sport Limited Honda 50-HP

Postby Tim » Fri Dec 01, 2017 12:19 am

"Who would think it could be so complicated?"
Thank you for the useful, detailed information.
Sapienti sat.

dtmackey
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Joined: Thu Sep 28, 2017 9:29 pm

Re: 1989 Super Sport Limited Honda 50-HP

Postby dtmackey » Fri Dec 01, 2017 9:39 am

Here is a simple prop slip calculator from Merc. Just plug in your details (your gear ratio is 2.09) and hit calculate in the field you are solving for (propeller slip).

http://www.mercuryracing.com/prop-slip-calculator/

D-

jimh
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Location: Michigan, Lower Peninsula
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Re: 1989 Super Sport Limited Honda 50-HP

Postby jimh » Fri Dec 01, 2017 10:21 am

A propeller calculator is not the way to approach the problem of propeller selection.

Before you can use a propeller calculator, you must have some idea of a reasonable target for top speed. The estimate of a reasonable top speed can come from the experience of others, or from another calculator, like one using the Crouch Method.

In the case of a Boston Whaler 15-foot hull with 50-HP, there is a lot of experience on which you can base an estimate of top speed. A reasonable estimate of top speed is going to be at least 30-MPH and perhaps as much as 35-MPH.

Let's say we will use 32-MPH as the estimated top speed possible with the SSL 15 and 50-HP. Now we can use a propeller calculator.

The Propeller Calculator is used to calculate an estimate of propeller pitch, based on entering the following data:

--the engine RPM, taken from the outboard maker's specification for recommended full-throttle speed range and chosen to be about three-quarters of the upper range. For example, if the range is 5,000 to 6,000, use 5700-RPM as the engine speed

--the engine gear ratio, taken from the outboard maker's specification; in this case, 2.09:1

--the SLIP, entered as 10 because that is what we expect from a properly working propeller with an accurately marked pitch

--the MPH, entered from our estimate of speed potential, in this case 32-MPH.

When you enter that in a propeller calculator, you will get an answer for PITCH. That is where to start for propeller pitch.

If you enter those data into the calculator you get an answer for PITCH of 13.77 (or 14-pitch). You do not need to be a "God" to figure this out.

The reason to work with someone who sells propellers is to take advantage of their experience and their willingness to allow you to test propellers on a BUY--TRY--RETURN--TEST ANOTHER basis, until you find the right propeller. Also, such a seller will be familiar with how the pitch markings work in a particular brand and style. One brand's particular model might have a pitch marking that is different compared to other brands. For a small boat with a speed potential of 32-MPH there is not a great deal of propeller voodoo involved.

Clearly you don't want to work with the guy who told you to use a 17-pitch propeller. If you put on a 17-pitch propeller on your engine and it was able to accelerate to 5,700-RPM, the boat would be going 40-MPH. That is just not a reasonable speed for your 15-footer with 50-HP. But I would go back to them and ask for a refund, based on poor advice.