Repower for 2002 21 Sea Ranger

[hoganrj1]

Hi everyone,
I am repowering my Arima with a Yamaha F150 and need to choose a prop. Any advice on pitch and diameter?
Thanks in advance.

[Tom C]

What is the top speed of this boat with its former power? This is always the first question to answer when selecting a propeller.

[hoganrj1]

I was toping out around 28 MPH with a honda 130

[Tom C]

OK, then I expect top speed to be ~32 - 33 MPH with the new motor.

To get there with the F150's 2:1 gear ratio, and it's 6000 RPM redline, I recommend the 14-1/2" x 13" Yamaha Reliance SDS, part # 68F-45932-20-00

[Tom C]

Be sure the installer mounts the new motor one, or even two holes up, on the transom.

[hoganrj1]

Thank you very much for the advice. I will make sure he does. Ad thanks for the prop recommendation too!

[beancounter]

I would suggest waiting for some others with that same set-up to weigh in on this. I am fairly certain you can turn more than a 13 pitch prop.

[Tom C]

Yes, the recommendation depends on the top speed potential of the boat. If the boat goes faster, then you'll want more pitch.

Here is a cheat sheet. If the boat goes this fast at WOT, then you want the Reliance SDS in the size listed:

32-34 MPH - 14-1/2" x 13" (Yamaha part # 68F-45932-20-00)
35-37 MPH - 14-1/2" x 14" (Yamaha part # 68F-45930-20-00)
38-40 MPH - 14-1/2" x 15" (Yamaha part # 68F-45970-20-00)

[Tom C]

hoganrj1 — Exactly what F150 are you repowering with? I'm behind the curve here; just noticed Yamaha has  some new versions abailable.

[hoganrj1]

Its the F150XB

[Tom C]

What year? When was it manufactured?

[hoganrj1]

Its brand new, either a late 21 or early 22.

[Tom C]

OK, my advice stands.

It would be good to get some confirmation about the potential top speed, from others with similar rigs.

[hoganrj1]

will do Thanks again for your help.

[Chief of the Boat]

Here is a sea trial report on a 21 with a F115 and F200.

[Tom C]

OK, those reports are very helpful. Thank you for posting those.

Does anybody see the obvious red flag here in this thread regarding the top speed of this model?

[Danno]

OK, those reports are very helpful. Thank you for posting those.

Does anybody see the obvious red flag here in this thread regarding the top speed of this model?

A 21' SR is just a 19' SR with 2 more feet of deck space. It should run the mid 30's like any other Arima with the right outboard (140 to 150 hp) and the right prop.

[Tom C]

Yes, I agree, this boat should run mid 30's

What I was referring to is the odd data points. hoganrj1 says his boat goes 28 MPH with 130 HP, yet the factory performance report says the boat goes 34.9 MPH with only 115 HP. Something is obviously off, so be careful with the data. The more data points you can gather, the easier it is to spot the outliers to discard.

I think there may be several things going on here. Old worn out motors are not necessarily as strong as new motors. Old boats may have rough bottoms which can really kill speed. And factory performance reports sometimes seem to be a little "optimistic."

The two factory performance reports compared to one another are telling too.

If a given boat can go 34.9 MPH with 115 HP, then it will go 43.8 MPH with 200 HP

Conversely, if a given boat can go 39.6 MPH with 200 HP, then it will go only 31.5 MPH with 115 HP, even adjusting for weight. Something is off here too.

At any rate, if mid 30s is the top speed, the 14" pitch Reliance SDS is going to be the best fit.

[martin3]

Why would anyone consider putting a 115HP on 21' Arima?
The performance numbers show it struggles until the higher RPM range.

Testing the boat with 150HP (4 cylinder) and 200hp (6 cylinder) would make more sense.

[Tom C]

Manufacturers offer lower HP motors as options to help keep costs down, and capture more customers. It's just business.

It is also why manufactures who rig motors at the factory install aluminum propellers, even though they are almost aways inferior; they are just trying to keep costs down, knowing a customer can spend his own money later on to get a good stainless steel propeller.

[davidsea]

  The only figures I really trust are these:   My boat - on the water - I'm driving.

Otherwise, it's actual total weight, bottom condition, best trim settings, drag from add-on stuff (tabs, transducers, water pickups, skip tops, radar, launchers, canvas, etc.), actual RPM and torque at the propeller, prop efficiency (% of slip)... the list goes on and on........

[Tom C]

Yes, ultimately, experimentation is the best way to figure out anything. In the case of propping a brand new outboard, you have to start somewhere, and that will reply on some estimation.

[Tom C]

An important note on measuring speed: GPS is today undoubtedly the most accurate and easiest way to determine speed, but one must remember that it measures speed over ground, not necessarily speed over water.

The only test data I trust is the average of two top speed runs run on reciprocal headings, then averaged. This will account for wind and current.

Also note that "Maximum Speed" recorded on any GPS is just the highest data point recorded, even if it is wrong. I have seen huge errors over the years. This is because GPS really just measures positions, and from those it then it calculates speed. As with any set of data, there will always be outliers, but GPS simply records the highest.

As I have mentioned in another thread, I think the easiest way to get verifiable sped data is to video a GPS as the boat is running its speed tests, or use a GPS overlay. Here is an example from a few years ago when I was speed a testing a small Whaler I had rebuilt.

https://youtu.be/8ZFFnfTtFb4

[Rck18]

The speed box app on your phone also works well. Verified with car speedos, bike computers, etc.

[Tom C]

The speed box app on your phone also works well. Verified with car speedos, bike computers, etc.

That is because it *is* GPS. All cell phones are GPS receivers, and can tell you speed through countless apps.

I have several speed apps on my phone, and in addition to Videometer, my other favorite is "Speedometer" by Tim O's Studio. It is nothing but large numbers, good for my old eyes.

https://apps.apple.com/us/app/speedometer/id546560363