72 85hp johnson help choosing new prop size and pitch.

[INDYCAVER1]

I am real new at this whole prop thing. I have finally got my motor running strong and now need to address the problem that is my current prop. Its a little beat up but not horrible. I cannot find any markings anywhere on inside or out, so i dont know the pitch of it. is there any way to calculate the pitch by measurement? I have a 72 crestliner appollo 16 ft open bow real deep v boat. I would say the total weight motor and all is about 1500 lbs. The motor is a 1972 85 hp johnson w/ elelctric shift. Right now the boat can rev well over 6500 rpm and the motor calls for 5500 wot. I can do 28 mph at 5500 and can do 30-32 if i over rev it a little. I dont run it like that though, I just did it to see how much over the recomended wot it would go. The hole shot is good, but i would like to get the wot down to the 5500 and get as much top end speed in the process. I was thinking about maybe a 13" 19 pitch, but i really dont know. Any help or suggestions would be great.

Thanks, John.

[steelespike]

Standard in 72 was a 13 1/4 X 17. The final size would depend on the application.
A 16ft fiberglass boat seems reasonable to have a 17" prop.
A p[rop shop could measure the prop for you.
I would also suggest you verify the tach is accurate. In my mind it seems unlikely that it would overrev by 1000 rpm with a 17" prop.Has this motor been on the boat since 72?

[INDYCAVER1]

i never thought about the tach being off, it seems to be working fine at idle and low speeds though. what would be the easiest way to verify the accuarcy of the tach? Im not sure if it is the original motor, but since both the motor and the boat are 72's it probably is. I guess i should start with the stock prop, but i dont have a lot of money to invest in props. I would like to get one and be happy with the performance and not have to upragde several times to get it right. I wish i knew someone that had a few they would let me try out and see what works best, but i dont know anybody local that has prop that fit my boat.

[INDYCAVER1]

Ok I got a new prop. It's a 14" x 19 pitch prop. I took it out yesterday and it was much better rpm range. With a really light load could only go 5800 rpm. The weird thing is I dent see any increase in top end speed. I tried to adjust my manual trim out two notches to lift the bow and get a little more speed and ended up overheating it cause the cav plate prolly wasn't level with the water and let some air in. I now know the 85 is not powerful enough to push my big deep v over 30 mph. I thought a bigger pitch would help, but it didn't. I came to the conclusion that my old prop was a 13 3/4 x 15 pitch prop stainless. Does anyone have any input why 2 degrees more pitch would not help top end speed?

[steelespike]

I think we still need to verify the tach.Perhaps you could borrow a shop tach or have a tech go with you for a quick test.Not real accurate way to check but what is the idle rpm.Should be about 700. When changing from ss to al if even the same size you could lose performance or get unexpected results.Changing brands mixes in more variables.
Rule of thumb is "about" 200 rpm per inch so a 2" pitch change could net 300 to 400 rpm.
Looks like you got about 700. Without knowing the tach there's no way to be sure exactly how much real rpm. A prop shop could clean up the old prop and tell you the size.If it seems needed they could repitch it.
As far as the overheat it isn't likely from trim.The water intake is just on the high side of the gearcase bulge. Is the antivent plate (just above the prop) about even with the bottom of the boat?

[INDYCAVER1]

I guess I need to find a way to verify the tach, but the idle and the rpm seems reasonable. My buddy said he found an illustration that showed if ur trim angle is too much and ur motor doesn't have enough power to keep the bow raised, the motor is not parallel to the water and creates an air pocket in the motor that won't allow it to stay cool. That is what I think I have happening. Even if I went from ss to al I would think the pitch difference would give me a small gain.