Prop size question.

[shawn.54]

I got a boat last September it is a 1977 starcraft capri 15 with 1978 75hp Evinrude has a machigan super prop on it. Was wondering a about what size it was on past boats size was stamped on them ie 13x17 this one is stamped spo 217. any have a clue? thanks

 

[steelespike]

Re: Prop size question.

If you go to the Michigan wheel site they can check archives and can usually tell what size/year motors it fits.
May be able to tell if it belongs on boat like your Starcraft.

 

[shawn.54]

Re: Prop size question.

Called Michigan Wheel they told me it is a 13x17p. i think that might be a little fast. When spring gets here i have to get a tach and check wot.

 

[steelespike]

Re: Prop size question.

"a little fast" Do yopu mean too much pitch or do you mean not enough causing too much rpm.
The 17 is one size up from mid way on the chart.

 

[shawn.54]

Re: Prop size question.

I was thinking to much pitch. only run boat 2 trips before winter was getting 30mph on gps with 12 gallon fuel and and two guys one 250 the other 300 (me). i need the tachometer to see.

 

[steelespike]

Re: Prop size question.

Calculating backwards, using a 2.42 gear ratio and guessing 10% slip a 17" prop at 30 mph indicates about 5000 rpm.
Not very accurate because the slip could range from about 5% to to 12 or 14% and still be considered treasonable.
Unfortunately we do need an actual tach reading to be sure where you are at.
I'm thinking the 17 is close to what you need and should produce more speed and rpm.
Could be a setup issue. Things to check. The anti vent plate(just above the prop) should be about even with or slightly above the bottom of the transom.Do you trim up for best rpm and speed? Is there a Whale tail or hydrofoil on the anti vent plate?
Check that the throttle opens all the way.Is the motor in excellent tune?Is the prop in excellent condition.Is it aluminum or stainless.
Stainless is magnetic and heavy ,aluminum is lighter and non magnetic.
By the way your prop size may be under the prop nut.

 

[steelespike]

Re: Prop size question.

Calculating backwards, using a 2.42 gear ratio and guessing 10% slip a 17" prop at 30 mph indicates about 5000 rpm.
Not very accurate because the slip could range from about 5% to to 12 or 14% and still be considered treasonable.
Unfortunately we do need an actual tach reading to be sure where you are at.
I'm thinking the 17 is close to what you need and should produce more speed and rpm.
Could be a setup issue. Things to check. The anti vent plate(just above the prop) should be about even with or slightly above the bottom of the transom.Do you trim up for best rpm and speed? Is there a Whale tail or hydrofoil on the anti vent plate?
Check that the throttle opens all the way.Is the motor in excellent tune?Is the prop in excellent condition.Is it aluminum or stainless.
Stainless is magnetic and heavy ,aluminum is lighter and non magnetic.
By the way your prop size may be under the prop nut.
I do believe a healthy 75 on a 15 ft boat with 500 lbs of passengers should run 35- 40 mph.

 

[steelespike]

Re: Prop size question.

Calculating backwards, using a 2.42 gear ratio and guessing 10% slip a 17" prop at 30 mph indicates about 5000 rpm.
Not very accurate because the slip could range from about 5% to to 12 or 14% and still be considered reasonable.
Unfortunately we do need an actual tach reading to be sure where you are at.
I'm thinking the 17 is close to what you need and should produce more speed and rpm.
Could be a setup issue. Things to check. The anti vent plate(just above the prop) should be about even with or slightly above the bottom of the transom.Do you trim up for best rpm and speed? Is there a Whale tail or hydrofoil on the anti vent plate?
Check that the throttle opens all the way.Is the motor in excellent tune?Is the prop in excellent condition.Is it aluminum or stainless.
Stainless is magnetic and heavy ,aluminum is lighter and non magnetic.
By the way your prop size may be under the prop nut.
I do believe a healthy 75 on a 15 ft boat with 500 lbs of passengers should run 35- 40 mph.
We do need to know if it is a long or short shaft.Short shaft is about 15-17 inches from anti vent plate to the bracket that rests on top of the transom. A long shaft is about 20-22 inches.The model number may tell us the shaft size.
Sorry about this double post I obviously have hit the wrong button again.My GOD! twice??

 

[shawn.54]

Re: Prop size question.

working on getting a tach have some time todays high was 8 but i can see why every boat needs one. Thanks for info

 

[jestor68]

Re: Prop size question.

Since your motor's WOT rpm range is 5000-6000 rpm, I'd be picking up a 15 pitch and keep the 17 for a spare.

 

[shawn.54]

Re: Prop size question.

I have 2 of these one on boat 1 that needs rebuilt looking at place that said they could drop pitch to 15. I never thought you could do that.

 

[steelespike]

Re: Prop size question.

Present day 75s with a 2.00 ratio run 15 and 17" props on boats boats weighing 1100 to 1300 lbs dry.
with speeds 34- 39.2 mph. rpm 5000-5500.
your Capri is probably about 800 lbs. dry.
Your motor being a 78 and likely a little tired your speed will probably be closer to the bottom figure.
Long shaft motor 2.42 ratio indicates 35 @ 6000 w a 17" prop.
Short shaft 1.85 ratio indicates mid 40s at 6000. w 17"

 

[shawn.54]

Re: Prop size question.

The motor is a long shaft and has probably not been run in the last 12-14 yrs. Compresion in all three cylinders was 125 but had bad o- ring on tester. She could be tired but i do not belive that she has alot of hours on it. Setting around my have done more damage than running.

 

[jestor68]

Re: Prop size question.

I have 2 of these one on boat 1 that needs rebuilt looking at place that said they could drop pitch to 15. I never thought you could do that.

According to the guys at Mercury Racing, they can adjust the pitch by one inch. A prop shop that claims to be able to do 2 inches will likely mess your prop up.

 

[steelespike]

Re: Prop size question.

The motor is a long shaft and has probably not been run in the last 12-14 yrs. Compresion in all three cylinders was 125 but had bad o- ring on tester. She could be tired but i do not belive that she has alot of hours on it. Setting around my have done more damage than running.

Sound reasonably good but will probably take some tinkering to get it up to par.
We do have to keep in mind it is rated at the crank where as todays motors are rated at the prop.
You would probably need a gps to tell the difference between a healthy 78 motor and todays motors.

 

[shawn.54]

Re: Prop size question.

have a tach ordered will not make any changes untill i can get wot and some more hours on her. The more i think about this and read on forum I need all the info or I could be throwing money away. Winter just to long.

 

[82rude]

Re: Prop size question.

i disagree with merc.2 p is not out of the question but is as far as i would take it.ive had my buddy do it for me a few times with excellent results.