Classic 15-footer with F70

Optimizing the performance of Boston Whaler boats
Alaskaclassic15
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Classic 15-footer with F70

Postby Alaskaclassic15 » Sat May 05, 2018 11:54 pm

Recently I was able to upgrade the engine on my [1987] 15-footer to a new Yamaha F70. The dealer gave me a 13-1/2 x 14 pitch propeller to try out, and, as I suspected. [the engine] was very under-propped. [The engine accelerated to] 4500-RPM [and the boat speed was] 21-nautical-miles-per-hour]. [The engine accelerated to] 6400-RPM [and the boat speed increased to ] 31-nautical-miles-per-hour

I went back to the dealer, and he then gave me a 13-5/8 x15 pitch propeller to try out. I will have the outcome tomorrow.

I suspect that [the engine] will also be under-propped [with this new propeller].

From what I've been reading on forums it sounds like a 17- to 19-pitch is the sweet spot.

My 15-footer is a very light hull. [It has] a side console and a 15-gallon fuel tank. The engine is mounted three-holes-up on the transom.

I live in Alaska and often times we use our boats to haul ATVs to outlying island to ride, so I do haul a bit of weight at times. But ultimately [highest] speed is my goal.

Any input or general performance talk would be appreciated.

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Phil T
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Re: F70 performance 15 foot classic

Postby Phil T » Sun May 06, 2018 12:16 pm

There are dozens of owners of 15 Sport's with an F70.

The highly rated prop is PowerTech SCD 13x16. Tim (Tedious) is a veteran member and 15 Sport owner who did alot of testing.

The Stilleto Advantage many member use is no longer made.

Review this thread - http://continuouswave.com/ubb/Forum4/HTML/008226.html
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1992 Outrage 17, 1992 Evinrude 115

jimh
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Re: Classic 15-footer with F70

Postby jimh » Sun May 06, 2018 1:48 pm

A light classic 15-foot hull with 70-HP engine should be able to reach a boat speed of 42-MPH according to Boston Whaler literature. See

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

In addition, see many descriptions of top speed potential for the classic SPORT 15 boat in

http://continuouswave.com/whaler/refere ... nials.html

A boat speed of 42-MPH would be expressed in nautical-miles-per-hour as 36.5 nautical-miles-per-hour. Please confirm that your earlier mentions of boat speed are actually expressed in nautical-miles-per-hour.

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Re: Classic 15-footer with F70

Postby jimh » Sun May 06, 2018 1:59 pm

In order to solicit propeller advice you need to give more information. What is the recommended maximum engine speed range for the Yamaha F70 engine?

What is the gear ratio of the Yamaha F70 engine?

It is unlikely that using a propeller that maximizes the top speed of the boat when lightly loaded will provide good results when the boat is heavily loaded. Perhaps the propellers already recommended to you were chosen to give good performance under heavy load conditions, and by testing them with very light loads you did not really put the propellers to the test of a heavy load.

Going on the assumptions that the engine under discussion will produce its rated horsepower at 5,800-RPM and has a gear ratio of 2.33:1, in order to obtain a boat speed of 42-MPH as predicted by Boston Whaler, and figuring propeller with SLIP = 10, the propeller pitch needed would be 19.8-inch.

Alaskaclassic15
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Re: Classic 15-footer with F70

Postby Alaskaclassic15 » Sun May 06, 2018 11:41 pm

Thanks Phil T, I have seen that prop name come up a lot amongst the forms.
Jimh The speeds I gave are in naughts. The gear ratio is 2.33 as you stated, The RPM full throttle range is 5300-6300 with 5800 making the full 70HP
I was able to test the new 13 5/8 x15 prop today and noticed a decent increase in speed for the midrange (4500 @23kts) my top speed was (33kts @ 6300RPM) and was able to hit rev limiter still. So after doing some prop calculators I came to the conclusion that a 17 pitch or a 19 would get me to where I want to be. Im leaning towards the 17 more.

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Phil T
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Re: Classic 15-footer with F70

Postby Phil T » Mon May 07, 2018 7:12 am

In is important to note that it is prop manufacturer and model, not just size are critical since prop sizing is not universal. It is not like a vehicle's tire size and you have a choice of 20 manufacturers and 100 models.

The same size in a different model will produce different performance results due to blade design.

When we are suggesting a Powertech prop, or Mercury or Yamaha or Solas, it means that exact one in the specified model and size. Dealers don't often carry every model in every size.
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1992 Outrage 17, 1992 Evinrude 115

jimh
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Re: Classic 15-footer with F70

Postby jimh » Mon May 07, 2018 10:18 am

Alaskaclassic15 wrote:...The speeds I gave are in naughts.


I am afraid you have me more confused instead of more clear. A "naught" is a noun that means zero or nothing.

Again, are the speeds you mention given in nautical-miles-per-hour?

We need to get on the same speed measurement scale so we can know the boat speeds clearly.

If the boat has already reached 33-nautical-miles-per-hour, that speed in MPH is 38-MPH.

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Re: Classic 15-footer with F70

Postby jimh » Mon May 07, 2018 11:26 am

Calculating propeller SLIP from your report of the latest propeller test:

PITCH = 15
RPM = 6300
RATIO = 2.33
MPH = 38
SLIP = 1 (calculated)

That is a very low slip value. Most likely the pitch is understated. Typically SLIP would be around 5 to 10.

Calculating PITCH for SLIP =10 gives PITCH = 16.5 for RPM=6300 and MPH=38. Note: the calculator only works in MPH.

I never heard of anyone navigating a 15-foot open skiff out of sight of all land references, which is when nautical-mies as a distance measurement are generally used, e.g., long ocean voyages.

Alaskaclassic15
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Re: Classic 15-footer with F70

Postby Alaskaclassic15 » Tue May 08, 2018 2:12 am

Sorry for the confusion on the speed! I do mean (nautical miles per hour)

I went back to the dealer today and exchanged the props for a 13 1/4 x 17 pitch Aluminum Yamaha prop and tested it out. [The boat was] able to reach 35-nautical-miles-per-hour [with the engine at] 6,300-RPM, and was able to cruise 24-nautical-miles-per-hour at 4,500-RPM.

I do realize not all props are created equal across the board, that has got me thinking to try a 19-pitch--just for heck of it. The boat and motor engine obviously can take a little more and I really do like trying to maximize performance. Thank you all for the info thus far. The area I boat in is Sitka, Alaska.

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Re: Classic 15-footer with F70

Postby jimh » Tue May 08, 2018 7:45 am

Your latest data:

RPM = 6300
RATIO = 2.33
PITCH = 17
MPH = 40.3
SLIP = 7 (calculated)

The calculated SLIP value of 7 is a reasonable value and generally indicates good propeller performance.

Here is a presentation of your performance testing data, taken from your narratives. Organization of the performance data for presentation makes the information content more useful, and avoids future readers having to dig all the details from multiple postings and narratives.

BOAT = 1987 Boston Whaler 15-footer; unspecified model
ENGINE = Yamaha F70, 2.33:1 gear ratio, recommended full-throttle range 5300 to 6300-RPM
ENGINE MOUNTING HEIGHT = three-holes-up
BOAT SPEED MEASUREMENT METHOD = unknown
(Note: speeds were reported in units of whole "knots" or nautical-miles-per-hour and have been converted to MPH based on a factor of 1.151:1.)
ENVIRONMENT = assumed to be cold ocean saltwater near Sitka, Alaska
AIR TEMPERATURE = unknown; assumed to be uniform among all tests
CREW = one
GEAR = 15-gallon fuel tank

TEST-1:
PROPELLER: unknown brand; unknown model; unknown number of blades; unknown material; diameter 13-1/2
PITCH = 14
RPM = 6400
MPH = 35.7 ("31" nautical-miles-per-hour)

TEST-2:
PROPELLER: unknown brand; unknown model; unknown number of blades; material unknown; diameter 13-5/8
PITCH = 15
RPM = 6300
MPH = 38 ("33" nautical-miles-per-hour)

TEST-3:
PROPELLER: Yamaha; unknown model; unknown number of blades; aluminum; diameter 13-1/4
PITCH = 17
RPM = 6300
MPH = 40.3 ("35" nautical-miles-per-hour)

El Rollo
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Re: Classic 15-footer with F70

Postby El Rollo » Thu May 10, 2018 12:16 pm

If you're really looking to get maximum performance, you may want to consider a manual jackplate.
A 4 or 5 inch setback is all you need, and you will be able to adjust the motor height further, and more precisely.
I have a 1988 15 Boston Whaler, and when I originally purchased the boat it had a 'doel-fin' type plate installed by the previous owner.
I installed a Bobs Machine shop 5' Manual jackplate, and noticed great overall performance improvements.
As Jim will probably point out . . . there are downsides with the addition of a jackplate. There is some added weight, (approx. 15 lbs.). Backing down the boat is not as efficient, as the water that is typically 'thrust' towards the bow & under the boat, will now be pushed against the transom. Also if you really hammer the throttle in reverse, you can expect some water to be 'funneled' , through, up and over the jackplate and into your splashwell.

That being said, I really like the benefits of the jackplate on my boat, and you may too. On other thing to consider is hydraulic steering. While it may sound 'excessive' for a 15 foot Boston Whaler with a Yamaha F70, you would be amazed at how nice the boating experience becomes. I used a Baystar unit on mine and couldn't be happier!

Good Luck !