New Prop = Bad

[dalton565]

Hey

Installed my new prop on my 75hp merc outboard 2 stroke, the prop came recommended for my engine.

However had it out today and realised its no good, doesnt have enough power to get me up on the skis, tube etc are still fine. Also top speed has dropped from 35-40mph to now 20-25

I dont have the measurements from my old prop, but it was alot smaller than the new one. Approx 1/2 the size. The pitch was similar

The new prop has the following spec 14 x 11p .. S/N 48-7738A33.

Looks like im going to have to revert back to the old. The only reason i changed was cause this one was going cheap and the old one is a bit bent

Any suggestions

 

[steelespike]

Re: New Prop = Bad

First we need motor year, boat type and size,your old prop size and max rpm and speed.Diameter of props is generally a result of prop design.and usually has little affect on performance except in the extreme.14" is within specs for your motor.11" pitch is usually seen on something hard to push like a pontoon.
It would make a typical runabout leap out of the water but produce little speed.You could get your old prop repaired.And if you get back to us with
your max rpm and speed we could figure a prop if necessary.

 

[dalton565]

Re: New Prop = Bad

Thanks for the reply

Motor year = 1999
Max RPM = 5250

Boat time = Fletcher Speedboat, 16.5ftlong

i have attached 2 pictures, one of the old prop and one of the new, dont have diameters or pitch angles, though i feel putting the old prop back on will make it run better.

Perhaps you can tell from a bit better with the images attached.

Any help appreciated

Richard

 

[steelespike]

Re: New Prop = Bad

It appears the old prop has more pitch.A prop shop can measure the props for you.Look carefully on the barrels or under where the prop nut goes for numbers.Which prop produced the 5,250 rpm.You can measure diameter by measuring from the center to the tip of a blade and double it.
Generally diameter is simply the result of the design and not considered in prop selection. For what ever reason it appears the new prop is not right.
Put on the old prop and give us the specs.prop size/pitch,aluminum or stainless(stainless is usually unpainted),max rpm and speed.With these specs we can tell if the set up is close or if some part of it is out of the norm.

 

[Silvertip]

Re: New Prop = Bad

Recommended by who and for what purpose???? 14 x 11 is way wrong for that engine. That engine has a 2.3:1 gear set and you should be running a 13-1/4 x 19P or 13-14 x 17 for skiing. The 11P is a pontoon prop and should have had no trouble getting you up on skis but as you found out, top speed suffered and I hope you have a tach as the engine would have easily over-reved. I run a 13-1/4 x 19 on my 75 HP Merc. When RPM at wide open throttle is asked for we mean what RPM do you actually see on the tach at wide open throttle.