Prop selection for 1963 Mercury 350 35HP (high altitude/low Altitude Application)

[radiocontrolhead]

[FONT=arial, sans-serif]I have an old 14' fiberglass boat (early 1960s) with a 1963 350 35HP mercury outboard. It will be running in an elevation of 7000' above sea level. and perhaps occasionally at a lower altitude (1000 feet above sea level). I am able to find these props for this boat (ebay) Used and new in various pitches so that isn't a problem.[/FONT]

[FONT=arial, sans-serif]I have been doing lower unit work and will need a prop to start off with so that I can later refine my prop selection, the problem is, what is a good starting point? Ultimately i would like to have two props, one for high altitude and one for low altitude application.[/FONT]

[FONT=arial, sans-serif]The recommended WOT RPM range [/FONT]is 4800-5200 RPM according to Mercury.

 

[steelespike]

Welcome I'll see if I can get you started. Prop size is written diameter X pitch Pitch is the main number
and diameter will tend to change with the pitch. As pitch goes up diameter will go down and vice versa.
I'm pretty sure your prop is similar to mine with an 11 spline and a basic diameter about 10 3/4".
Lower pitch props give the motor more power and will allow it to rev easier.Higher pitch are harder to turn but may increase top speed.
Prop size is determined by the size and type boat,the typical load and activities and of course making the wot rpm close to the max.
More to come.

 

[radiocontrolhead]

maximum number of passengers for this boat is 4. According to the part numbers i was able to pull for my outboards/lower unit, the prop is a standard 10-3/4" diameter, 11 spline, with pitches varying from 10 to 15 (straight from mercury)

I guess my next question would be, what did mercury equip these engines with from the factory? I need a starting point to work with because I have never taken the boat out before.

 

[steelespike]

Is this a 2 cylinder or a 4 cylinder,4 cylinder 350 would be a 61.
I doubt that it came with a prop and the prop would be selected based on the the needs I mentioned earlier.
If your going to be at about 7000 ft you will be down about 20% on power and thats if you reduce the high speed jet.
Looking at the numbers and not having any performance to go by.I would start with a 10" pitch.
If it turns out too light you can still use it by watching the rpm. If we go higher and its too high you will have poor performance
and the motor will lug below its rpm range which can lead to premature repair problems.
Two important things for correct prop selection;You'll need a gps and a tach, an induction tach will work.
We also need to know the gear ratio. You can figure it by pulling the plugs and rotate the motor by hand.
Count the revolutions for one revolution of the prop measure carefully I'm guessing some where around 1.6.
If you change the jets for elevation you really can't run it at sea level, you'll cook it.

 

[radiocontrolhead]

It's a 2 cylinder, 1963. I don't intend to change the jets as I would like to ride to have the flexibility of bringing the boat down into a lower altitude and only have the change the prop, which appears to be a lot easier than worrying about carb jets.

I wont mind having less power up at altitude. Would a 12" pitch prop work at say a lower altitude? I guess i'm starting to realize that it's hard to determine without knowing enough information.

according to a mercury service manual I have, the gear ratio for this engine is: pinion-13 / Propeller-24 , so 1.83?

 

[Sea Rider]

A 35 HP engine probably must have come with a medium size prop pitch, try to check which, Will need a induction tach and a wot spin with you alone and lightly loaded boat to check full wot rpm, then can go from there selecting best less pitch prop to pull revs up for engine to work much better at that given altitude. Stock props delivered with engine usually performs very well powering light boats at sea level.

Happy Boating

 

[steelespike]

It's a 2 cylinder, 1963. I don't intend to change the jets as I would like to ride to have the flexibility of bringing the boat down into a lower altitude and only have the change the prop, which appears to be a lot easier than worrying about carb jets.

I wont mind having less power up at altitude. Would a 12" pitch prop work at say a lower altitude? I guess i'm starting to realize that it's hard to determine without knowing enough information.

according to a mercury service manual I have, the gear ratio for this engine is: pinion-13 / Propeller-24 , so 1.83?

Excellent, the older motors sometimes finding gear ratios is a challenge.
I can manage basic prop application. Others may be more expert in more sophisticated props.
Is your boat a basic tiller steer or a more deluxe with remote steering,windshield,floor,Electric start deluxe seats etc.
If you know the brand/model we may be able to find the weight.
Disregarding the weight and style of the boat a 10" prop at 5200 = about 23 mph guessing 12% slip. slope for a 35 but a 7000 ft may be the number.
13" ay 5200 sea level is a more reasonable speed of about 30.
Do you know what sizes are available in the 10 to 15" list?
I wouldn't select another prop until you see all your results with a 10".

 

[radiocontrolhead]

I may have read the wrong gear ratio. I forgot the lower unit i'm receiving was reported to be pulled from a Merc 500 which was also shared with a merc 350(4 cylinder)? See attached screenshot from my service manual. I won't know until I actually measure it physically but I can safely assume the ratio is either, 1.64 or 1.83.

From the research I was able to find (and there isn't much on the boat except for an emblem that says "sportster") (see attached screenshot) I think it is a sportline by sportsman mfg in paramount, ca. ( i really don't know) It has remote steering, seats, fiberglass hull, windshield, etc. looks like a classic car on the water.

I was only limiting my prop selection to the oem numbers. I can find them on ebay and that was what I was going to go with. I believe propco makes props for my boat but they are identical to the oem props and the only change is the pitch (diameter stays constant at 10-3/4)

 

[steelespike]

Wow look at the gear ratio chart! I'm always trying to find various gear ratios.Do you think you
could blow it up as big as the page so I can print?
A nice looking boat. In the day the motor was probably fairly typical but a 40 or 50 would have been better.
A 1.64 ratio will further challenge your motor. My boat is a 18 ft bare utility (375lbs).
1,000 lbs all up weight. My Merc 500 (1.64)does well with a 11" prop. but a 12" was a little too much.
Is the Sportsman a model or a brand name?

 

[radiocontrolhead]

I think sportsman is the model name. Next time i'm with the boat i'll try to grab more information from it, although i don't remember finding anything else. i would agree that a 50 HP would be really nice for this boat. I will see how an 11" performs with this bad boy and go from there. I just need to find one available.

let me know if this works for you (enlarged gear ratio chart)

 

[steelespike]

I am getting a blank page when I push print in my file tab.Hoping when I post this it will come up.
Worked perfectly.Thanks for the page it looks great!
Also may help others with these old Mercs.
As far as trying to id the boat there may be a tag on the transom with the company name and hp
and capacity rating.

 

[radiocontrolhead]

Update, I purchased a 12 pitched prop (thanks steelespike) and a 10 pitch (brand new) prop. I will now have at least two props to help dial in the boat. I will update this thread in a few months when i am able to take her out. I understand I still need a tachometer and my smartphone for speed.

From calculators i used online, at 15% slip (hoping it's a conservative assumption), @ 5000RPM (between 4800-5200), with a gear ratio of 1.64, and a 10" pitched prop (the smallest prop i can get my hands on) i should achieve 25 MPH. That would be pretty awesome.

 

[radiocontrolhead]

From my research, these props contain the [h=3]Flo-Torq? Shock-Absorbing Rubber Hub[/h]
Can these be rehubbed relatively easy? where can i find replacement hubs?

 

[steelespike]

Usually a prop shop can rehub a prop.
I'm pretty sure the 12" was only used occasionally for maybe 2 years.
I think the pressed in safety hub dates back to the late 40s.

 

[radiocontrolhead]

Update,

12" prop was way too big for this 35 horse on a 14foot fiberglass hull in 7000' altitude.

Went with a 10" and still coudlnt' get her to run within RPM range. I think i'll leave it until a nice 70HP or 85HP comes around seeing as the max recommeded HP for the hull is 90. Hull is in excellent shape.

Numbers are:

Pitch: 10"
Speed: 20 MPH (pretty solid it will barely plane)
RPM: 4400 (should be in the 4800-5200)
Gear ratio: 1.84 (ended up rebuilding the original lower unit so the ratio was preserved)
Carb: rejetted one size smaller per Mercury Manual (didn't seem to make much of a difference)

Since a 10" is the smallest i can find for this motor/lower unit, i have come to conclusion that i simply need more motor, MOAR power. but for the size of the lake it's in and the size of the boat, i'm ok with keeping this engine on it until i find an old merc700 to restore as a project.

Thanks Steelspike for all the help!