1977 Evinrude 75 hp on 16 fiberglass runabout prop question

[Wilsonsk1966]

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I hit a rock yesterday and need a new prop. Currently I get about 28 mph at 5100 rpm on my 16’ fiberglass runabout with 1977 Evinrude 75 hp.


here is what I have:

1977 crestliner apollo 660 16’ fiberglass runabout
boat weighs 1600 lbs with people and stuff
current prop is 13 1/2 x 15
motor is 1977 Evinrude 75 hp model 75743s
current top speed is 28 mph
current rpm at top speed is 5100 rpm

as far as I can see my options for aluminum props are:
13 3/4 x 15. 1/4” larger diameter than I have now, same pitch
13 1/4 x 17. 1/4” smaller diameter, 2” higher pitch

max Rpm for my motor is 5500, which prop would get me closer to that? Seems like same pitch and even 1/4” larger diameter would slow the motor down with the 13 3/4 x 15. Not sure how the 13 1/4 x 17 would affect it, same rpm and a little faster?

Thoughts?

 

[Texasmark]

You shouldn't notice the difference in diameter....if you change manufacturers or models the new prop could run different due to different blade geometry, variable pitch and other things not related to the 1/4" increase in diameter.

The engine WILL notice the 2" change in pitch! Forget that.

 

[racerone]

The 17" would bring it down to about 4700 / 4800 RPM.-------I would think that boat if in good condition would run with a 17" pitch prop.-----That is if the engine is putting out the proper power.

 

[flyingscott]

Top RPM for a 75 hp is 5800 rpm and you need to be propped to hit that. They are not great low end motors.

 

[Wilsonsk1966]

The 17" would bring it down to about 4700 / 4800 RPM.-------I would think that boat if in good condition would run with a 17" pitch prop.-----That is if the engine is putting out the proper power.

That is what I was thinking, the motor has ~120 psi in all 3 cylinders, new plugs (Champions), carbs have been gone through last year, sounds good, just not a real power house, not sure what else to check there, link and sync is good too. if I figure correctly, if I were to go to a 13 pitch prop, I’d rev my motor to around 5700 and probably not go any faster. I have to be honest, I’m fine with the top speed and would like it to get there quicker, it feels like the motor would be happier revving a little more. I can get a 14 x 13.

 

[racerone]

I say there is something wrong with the boat or the motor , or both.------You should not need a 13" pitch prop on a 16' boat !

 

[Wilsonsk1966]

I say there is something wrong with the boat or the motor , or both.------You should not need a 13" pitch prop on a 16' boat !

Thoughts on what I should take a look at on the motor?

 

[racerone]

Sometimes folks are a little reluctant to do some simple trouble shooting.-----Is there a local shop that has a proper test propeller to determine motor power output ?---Any chance the boat has foam flotation that is water logged ?

 

[Wilsonsk1966]

Sometimes folks are a little reluctant to do some simple trouble shooting.-----Is there a local shop that has a proper test propeller to determine motor power output ?---Any chance the boat has foam flotation that is water logged ?

I haven’t run across any foam flotation, I installed seat pedestals and it was open under the deck. I ran out and checked compression and spark after my last post, ~130 on all 3, sparks all jump 1/2” gap and the throttles are vertical and timer base up against the bumper at WOT. Here are pics of the plugs, I put them in yesterday morning so they have an hour or so on them.

 

[Wilsonsk1966]

Anything else I should be looking at? Motor starts good, idles great, sounds good, smooth. In this heat I don’t need the choke, just fast idle for a cold start.