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Where to start with prop size?

T

thebubba

Regular Contributor
I have a 1975 Crestliner Muskie 550 with a 1978 Mercury 70hp 3 cylinder outboard. I just bought the boat and it did not come with a prop. I am not sure what size to try to start with on this boat. The specs say the boat is 825 pounds dry and empty. 15.33 ft long with a 72" beam. The maximum HP rating is 75 hp. It is an open bow tri-hull fiberglass. Looking to take it to the lake next weekend to try once I get the impeller replaced this week and find the correct prop. I do most of my boating at 5000-8500 ft elevation. But may occasionally go to the lower lakes to pull the kids around in a tube or something warmer. Any help is appreciated.
 
Forgive me if this gets a little long.

Props are probably the most misunderstood and mismatched component in the rigging of any outboard powered boat. You can take two identical boats with identical motors and each will need a different prop.

You have some of the info already which can help you figure out what you need. The size of the boat, the motor, overall weight are three of the "main factors". But how "you" will use the boat under "normal" conditions will also have a significant effect on which prop is "right for you".

So let's look at your numbers -

Boat Dry Weight - 825 pounds
Motor Dry - 190 pounds
Rigging (battery, cables, safety gear, anchor, etc) maybe 150 pounds (better to be a little liberal with the weight)
Passengers @ 150 lbs each (coast guard average) - say 3, for 450 pounds
Gas - maybe 15 gallons @ 6lb/gallon = 90 pounds

So tipping 1700 lbs plus whatever other "stuff" you throw in for the day out.

Now a 1978, 70 horse is putting out about 65 Shaft Horsepower and since you are running at altitude unless the motor has been modified you are probably going to lose a couple more. On a positive note, this model has 2.3:1 gears meaning it won't be gasping in the thinner air compared to newer models with a higher gear ratio.

So I ran your numbers through "Prop Calc" (software to give you a starting pitch) and it spit out a 3 Blade, 17" pitch, 13 1/4" diameter calculating it should push your boat along in the 29 to 33 MPH range. If you plan on running heavier than the numbers I have used you may want to drop to a 15 pitch (13'ish diameter, 3 blade)

If you can find a shop that will "loan" you a prop, then you can test it out.

The only sure way to match a prop to your rig is under "real life" conditions.

You need to shoot for a max rpm of about 5000 on your particular motor. If your number comes up high or low with a particular pitch/diameter you can adjust from there.

The rules of thumb for props are:

a 1" increase in pitch will lower the rpms by about 200 (and vice - 1" decrease = 200 rpms increase)

a 1" increase in diameter will lower the rpms 200-250 (and vice)

going from 3 blade to 4 blade will decrease your rpms by about 50 (and vice)

going from straight blades to cupped blades will decrease your rpms by about 50 (and vice)

A 3 blade (straight) is considered an "all round" prop

A 4 blade will give you a better hole shot if you are pulling a skier or tube etc

A cupped prop will also improve the hole shot whether running 3 or 4 blades.

Stainless steel and Aluminum Alloy (eg. Solas modular props) flexes less (bites harder) than OEM aluminum or composite so will run 50 rpms or so less, all else being equal.

(Stainless runs about 3X the cost of OEM, Aluminum Composite is 20-50% more expensive and Composite (plastic) is about on par with OEM in terms of $$$)

There are a couple of companies out there that produce variable pitch props. I run a 4 blade "ProPulse" on my 140 Evinrude that can be adjusted between 16 and 21 pitch in 1" increments (not while underway - must remove the prop to adjust).

This allows one prop to be utilized for a variety of conditions (if you don't mind a composite prop as opposed to aluminum or stainless). You can run "more" pitch in the spring/fall when the air is cooler and less dense or when you are running very light and then adjust to less pitch in the middle of summer or when running heavy. (Piranha is another company that produces a composite prop that allows you to switch blades of different pitch on the "common" hub - either the Piranah or Propulse system price is competitive to an OEM aluminum prop - $150 range)

(Motors ratings tests are normalized to sea level, 30% humidity with an air temp of 70 degrees - we rarely run at that. If hotter and more humid you can lose up to 15% of your max rated horsepower, so in order to prevent the motor from bogging, which causes at minimum carbon fouling and in extreme cases, piston damage, just about every motor should run a lower pitch prop in mid-summer as compared to other times in the year)

So, in case my "guess" got lost :) I would try a 3 blade 13D17P and tinker from there..
 
Last edited:
Graham,

Thank you for that very clear tutorial. I really appreciate it. I will see if I can locate that prop size and try it out. I will post once I get the results.
 
So i did some measuring of the lower unit. there is only 7 inches from the center of the shaft to the bottom of the anticavitation plate I think it is called. So that would mean a 13 inch diameter prop would be pretty tight. Is that okay to have it that close or should I drop down to a 12 inch? I found an old prop to try that is a 10 inch diameter 9 pitch. That will probably be a little too small, but at least I can start with something I already have.
 
That is all very helpful. I want to get it out on the water to make sure it floats (no leaks) and the engine runs well. I will be careful with this too small prop and over reving until I can get to the jets and bigger prop. My minimum altitude to run at will be about 3500 feet and maximum is around 8800 for my favorite lake so I will work on the proper jetting to make that work best. Last question for now, Isn't there supposed to be a thrust washer or something behind the prop to keep it from sliding into the gear housing? I have the prop, locking washer with tabs and the nut. Got the new impeller installed today and changed the lower unit oil out. Just seems like there should be another part here I am missing.
 
Thank you for the help on the thrust washer. I ordered the thrust washer today. As soon as I get it I will take it out to test and report back. I am going to see if I can borrow another prop closer to what you all recommend and go from there.
 
["You measured the wrong thing; you want the cavitation plate about 1/2 inch below the keel."]

I wasn't trying to measure the motor positioning, but rather to see if a 13" prop would fit on my motor and spin without hitting the the foot of the motor above the propeller which I think is called the anti-cavitation plate. There is only 7 inches from the center of the prop shaft up to the bottom of the anticavitation plate. That would only leave 1/2" clearance of the propeller rotating and missing the lower unit. Is that okay or should I go with a 12" diameter prop is my question?
 
So I have been looking Online for a prop and put in my engine specs 1978 Mercury 70 hp. and the only props that fit the motor are 10", 11" and 12" props according to their charts. The best I can do with a 10" prop is a 17 pitch, the 11" 14 pitch, and the 12" in 9 pitch. These are all way out of range of the suggested size recommended above. My test prop is a 10" x 9 pitch which is way too small according to earlier posts. What should I do from here. It is a 13 spline thru exhaust.
 
You are correct -----The 78 - 70 HP has a small lower unit and it is common in size with the 2 cylinder 35 / 40 HP and the 4 cylinder 50 HP -----13-1/2 diameter props were never available for them.--------I would start with a 12 or 13 " pitch for those high elevations.
 
So an 11" prop with a 12 or 13 Pitch? I usually fish the higher elevation lakes here in the Rocky Mountains. My favorite lake sits at 8800 feet. It is much more difficult to plane a boat at that altitude. I don't usually travel long distances, just use the 70 hp to get on and off the lake. Maybe up to 5 or 6 miles at the most and then troll or use the electric motor to manuever. When a thunderstorm comes rolling in, I like to get off the lake quickly. Every now and then I may take it to a bigger lake, like Flaming Gorge where you can travel 15-20 miles, but that is the exception. Planing at high altitude with be the biggest concern.
 
That is what I would try ( 12 " pitch ) for 8800' elevation. -------A tachometer is a good item to have in the boat.-----------Also make sure boat is in good condition with no surprises ( water logged foam ) under the floor.
 
So i finally got my thrust washer and installed a 12" 9 pitch prop I found cheap. Took it to the lake this weekend to see how the engine ran and if the prop was good, also tested my trolling motor (7.5 Merc). Both ran great, except my 70 hp did not have reverse. It kicked into forward only when I put the throttle down a little. When I was changing the water pump I must have put the shift spline together incorrectly. Will fix this and try it again. Any tricks to making sure it aligns correctly? (it being a 1978 70HP Mercury) Is there a way to test it before I take it to the lake to make sure I got it right? Is it possible my reverse is just gone and how would I test for that? Second issue I found was a little gear oil leaking out behind the prop. I just changed it, so it was fresh and easily identified. I will pressure test it when I remove it to reinstall correctly. As far as the prop sizing goes, this prop performed pretty well. The boat planed nicely, and WOT sounded like the motor was not over-reving, but I need to install a tachometer so I can see what the engine is really doing and not just guess. Any ideas on an inexpensive tach for this motor, the boat did not have one.
 
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