Buying a new prop 75 hp Evinrude

[shawn.54]

looking to get new prop for my 15' trihull is any one prop better than others and why?

 

[shawn.54]

Should have said was looking at a 15" pitch has 17" now 5200 rpm max

 

[steelespike]

What year 75?prop brand.
We need your lightly loaded wide open throttle rpm and gps speed.
I'd say entry level props are similar and certain styles do things better than other styles.
Solas props are reasonable and fairly predictable.
A 15" should improve the performance but you may need to avoid over revving.
Thats why we need your present performance.

 

[shawn.54]

Lite load 2 adults to teenage boys 12 gallons fuel and wet foam and 1978 motor can get 5200 to 5300 rpm can get 28.5 on a good day I can ski behind it at 350 pounds but know that should be closer to 5800 rpm I have a tac and gps so numbers are close. I know wet foam needs replaced just a little hesitant to take a saw and start cutting.

 

[steelespike]

Is this a small or large gear case what is the diameter of the 17" prop.

 

[shawn.54]

Large gear case will have to get back on diameter. Diameter is 13 1/4 x 17 Michigan super prop

 

[steelespike]

According to my info the 78-85 75 hp are rated 75 @ 5200. I suppose people suggest 5700.max.
Rule of thumb 1"pitch generates 150 to 200 rpm.

 

[shawn.54]

Just looked at service manual it suggests 5200-5800 and 2 inches would put me very close to top may have to back it down when running light but would give me more power to do water sports tubing and skiing. Thanks for info I have to research as much possible don't want to rush out buy a new prop to find out it doesn't help. Boat $ are hard to come by these days. I have found that the folks here at Iboats forum are very helpful and knowledgeable and I have the I have the upmost respect for them and their willingness to walk fellow boaters thru their troubles thanks again Shawn

 

[shawn.54]

1 more question what would mph difference be between 15 and 17 pitch?

 

[Sea Rider]

Not trying to rain on your boat, the rule of thumb is tricky. Usually states + 150-200 revs going one less pitch , but that's relative, depends on engine HP, boat type and weight. The issue is that no one really knows which load is considered light, medium light or heavy one as you could achieve between 200 to 600 + revs with just one pitch less if boating inside any of the 3 categories.

Post whichever wot rpm you achieved when going 2 less pitch, just for the record. A 15 pitch should pull revs up + 200 to ? achieve better hole shot than before, but with punishment of slight less top end speed. When going for a wot spin and tach will know for sure as practice most of the time opposes plain theory.

Happy Boating

 

[steelespike]

Slip with your present 17" prop is 31% perhaps the wet foam is at fault
Assuming the 15 can make 5800 rpm guessing slightly better slip 28% we get about 29.7 mph.
But the there is a lot of guessing going on.

 

[shawn.54]

I have been looking into tear down and rebuild to eliminate wet foam I am a little intimidated by the glass work also keeping an eye out for a tinny. Thank you for your help. Even if I get prop this week two boys in baseball will delay testing at least 2 weeks but I will let you know results as soon as I get them thanks again.