Choosing first prop to run .

[sphelps]

Guys , quick maybe dumb question.
So I repowered my 16.5 Wagemaker wooden boat with a nice 1958 35 hp Big Twin Evinrude . It came with 3 props .
3 blade 10-3/8 x 11-1/2 aluminum
3 blade 10-3/8 x 14. Aluminum
2 blade not sure what pitch but has numbers BSP19320. And AJC46J on it . This prop was being used on a 14’ Aristocraft little speedboat .
I’m gunna lake test this weekend on wondering which prop makes the most since to start with . I do not have a tach on it so I know it’s gunna be tough to really know for sure on the right prop . Hopefully one of the 3 will work well . I’ll have access to a hand held tach in my next AOMCI trip to the Suwannee in a couple months .
Just wanting opinions on which to try this weekend for a test run ..

 

[Scott06]

My swag would be the 11.5” just based off all the the older OBs like that I ran had lower pitch props like 9,10, or 11”

I would definitely take the 14” with you and swap out if you are way off. Would be worth getting a tach to see
Where you land. I have a wireless tach & kv meter that is great for situations like this.


That engine reminds me of the 1956 Evinrude 5.5 I have. My grandfather bought a 12 ft tinny with it on for me, my brother and cousins 40 years ago. Still have the engine but someone tossed the pressures fuel tank so I haven’t run it in years…

 

[airshot]

Take your time and try alk three, then go back and try them again !! I would spend as much time as possible on the water with a beauty like that !!! Cool boat !!

 

[sphelps]

Thanks guys . I did run the 11.5 today . Me with my son up front .. He’s a big guy .. Did ok but only got 22 mph with it . Didn’t seem like it was really high rpm from just the sound . I will give the 14 a try next .
View attachment IMG_4710.MOV

 

[jimmbo]

If the Rpm is low with a 11 1/2, it will be a lot lower with the 14
While our Ears are very good at detecting a Frequency Change/Shift, they are not very good at actually measuring the Frequency. A Tach is needed when evaluating/choosing a Prop
WOT Range for that engine is 4000-5000 rpm
In 72 my Dad had a similar boat, but 2 ft shorter, with a 57 35hp. We ran a 12" on it

What's with all the Rope tying the Motor to the Boat, and the Steering Bracket to the Motor? The Transom, appears to have 'Cups' for the Engine's Transom Clamps, which would require them to loosen quite a bit before the Outboard could attempt a Twisting Dive

 

[brodmann]

What are you using to check speed? It looks a lot faster than than 22 MPH to me. But if you're using a GPS, that would be pretty accurate. With a little digging it seems that the third prop has a pitch of 9. If so, that would get your RPM's up, but top end speed might suffer. Obviously, you want to get your RPM's within the "Happy Place" for that motor, and then look for the best top end speed you can get or want that keeps the motor within that range.
I don't want to insult anyone's intelligence here, but if you can get a tach and record the RPM's and speed on one of the props, the math is pretty straight forward for choosing the pitch that you want to look for. The pitch of a prop is the number of inches it will travel with a full revolution. So, RPM's X pitch / 63360 (inches in a mile) X 60 (minutes in an hour)/2 for the 2/1 gear ratio on my '99 Yamaha 115 gives me the theoretical speed for a 19 pitch propeller. I then compare that to the actual speed and I know how much "slippage" I can expect.
I developed this math when my boat was struggling to get on plane, but would reach 43 MPH, but it took forever to get there. I had a 19 pitch prop and was only running 4700 RPM's. I've since replaced it with an aluminum 19 pitch prop that gets me to 5300 RPM's and still 42 MPH. While an aluminum prop is much lighter and can spin faster, it also "flexes" at hard acceleration which increases the "slippage" factor. Again, I'm sure most of you know all of this, but just in case I'm sharing what I've learned over the years.

 

[sphelps]

If the Rpm is low with a 11 1/2, it will be a lot lower with the 14
While our Ears are very good at detecting a Frequency Change/Shift, they are not very good at actually measuring the Frequency. A Tach is needed when evaluating/choosing a Prop
WOT Range for that engine is 4000-5000 rpm
In 72 my Dad had a similar boat, but 2 ft shorter, with a 57 35hp. We ran a 12" on it

What's with all the Rope tying the Motor to the Boat, and the Steering Bracket to the Motor? The Transom, appears to have 'Cups' for the Engine's Transom Clamps, which would require them to loosen quite a bit before the Outboard could attempt a Twisting Dive

Yeah, I’m gunna need to put a tach on it no doubt .. On those ropes I shimmed the motor up another 1” from where it was already shimmed an inch . The clamps were getting real close to the top of the original transom . So being the worry wort that I am I decided to tie things down a little just in case .. The boat is rated for 50 hp .
The rope on the steering bracket is there because the little pin that goes through the knob that holds it to the motor is a bit short or more the back of the bracket on the motor was a bit worn so the pin wasn’t making me feel extremely safe . So the rope tied on at the last minute at the ramp gave me a little peace of mind for the test run . I installed a new to me steering wheel and cable hub . I think I will just take the knob off and put a nylock nut and bolt on it instead ..

 

[sphelps]

Brodmann, yes I was using a gps app on my phone . My son did the same with his and we both had the same number so probably fairly close .
Don’t worry about insulting anyones intelligence.. Ya got to have some of that before it can be insulted ..Lol ! So it would take a LOT to insult me ! Lol! Thanks for the detailed formula and post !

 

[jimmbo]

While an aluminum prop is much lighter and can spin faster, it also "flexes" at hard acceleration which increases the "slippage" factor.

Flexes. That bit of Misinformation has sold many SS Props. In reality, the Blade is very rigid, and the Water will yield far sooner than the Blade.
Lock a set of ViseGrips on a Blade and try flexing it. It is more likely to Crack than flex.

 

[jimmbo]

Here the Prop Options for that line of Engines from a 1980 Accessory Catalog


and here is the OMC Prop chart for the 1960s 40hp Speeds shown at a given RPM would be similar for your motor, as they all have the same Gear Ratio and Rpm Range. However the Load shown are for a bigger engine

 

[jimmbo]

The rope on the steering bracket is there because the little pin that goes through the knob that holds it to the motor is a bit short or more the back of the bracket on the motor was a bit worn so the pin wasn’t making me feel extremely safe . So the rope tied on at the last minute at the ramp gave me a little peace of mind for the test run . I installed a new to me steering wheel and cable hub . I think I will just take the knob off and put a nylock nut and bolt on it instead ..

Some Years, that Bracket was mounted like you have it, then the Next year the Part with the Knob was Flipped around so it was installed from behind, closer to the Engine. Our 57 35hp was that way. If the Face of the Part on the Motor is Flat, then you have it correct. If where the Pin goes through had Spiral Ramps, then the Bracket needs to be on the Other Side, which could explain why it seems too short

 

[flashback]

That's sweet! Can't add anything but just wanted to say how nice your boat is and the motor sounds great...

 

[sphelps]

Some Years, that Bracket was mounted like you have it, then the Next year the Part with the Knob was Flipped around so it was installed from behind, closer to the Engine. Our 57 35hp was that way. If the Face of the Part on the Motor is Flat, then you have it correct. If where the Pin goes through had Spiral Ramps, then the Bracket needs to be on the Other Side, which could explain why it seems too short

Hmm , that is a possibility that I will check out ..