Correct prop for Lake Powell

[bunker108]

I am taking my 1973 18' Starcraft SS with a 1972 Mercury 1150 to Lake Powell next month. I would like more top end speed than I currently have. At 4500' above sea level with a 15p prop, I am not sure of the diameter, I am at 31mph at 6000 RPM. Can I spin a 19 or would a 17 be better... is a 21 out of the question? Thanks everyone.

 

[steelespike]

Re: Correct prop for Lake Powell

Your slip is less that 1% I have a feeling either your tach is low or your speed is high
Thats a pretty hard calculation.Assuming powell is roughly at sea level your gaining about 12% power.
My info also indicates your present rpm is about 500 high.
A 12% increase in pitch is about 2" if you want to be near 5500 + 2" would put you at 19.
I lean toward a 17 till you can get a base line with it to evaluate your results.

 

[fishrdan]

Re: Correct prop for Lake Powell

SS, is 5500 the recommended WOT RPM for that Mercury OB? (I don't know)

At 4500' above sea level with a 15p prop, I am not sure of the diameter, I am at 31mph at 6000 RPM.

Powell at 3700' isn't much less than your current elevation, so I don't think you have much to play with there. This is more of a propping for the correct RPM question, instead of propping for an altitude change.

When you go to Powell are you going to be loading down the boat for camping or hauling around extra supplies,,, extra weight in the boat? You might need to keep your current prop, if loading down the boat.

 

[bunker108]

Re: Correct prop for Lake Powell

Thanks for the replies. Steel, my tach is correct and the speed was gps. My gear ratio is 1.7:1 I believe, could that throw off the calculation?

Great point Dan we will probably have a full load at Powell. I think a 17 will probably be the best compromise.

 

[steelespike]

Re: Correct prop for Lake Powell

I don't know how I came up with that very low slip presently with a 2.00 ratio the slip would be 27% really high,with a 1.7 ratio slip would be 38% unreasonably high.High slip can be from a number of causes,poor prop,inaccurate tach would be most common when dealing with gps speed.typical slip numbers are 10% or less on a boat like yours.That would make your speed about 38.My info indicates 1.7 appeared in 76 that is not confirmed.throwing out the ratio there is still a lot of slip going on with your present setup.
It's all pretty much academic; though the results with a 17 could be unexpected if it produces 10% slip
it would load the motor even more over just the pitch change.

Thanks for the replies. Steel, my tach is correct and the speed was gps. My gear ratio is 1.7:1 I believe, could that throw off the calculation?More to come.

Great point Dan we will probably have a full load at Powell. I think a 17 will probably be the best compromise.

 

[bunker108]

Re: Correct prop for Lake Powell

I am guessing it is time for a new prop. Mine looks like it has been "worked" on. I think it has been ground down from its original diameter and it has always looked small. So I think a 17p 13" diameter would fit the bill?

 

[bunker108]

Re: Correct prop for Lake Powell

I know the pics are not great but maybe you can get an idea of what I am referring to about the prop diameter.

and a quick video last year. This was only at 5400 but she will spin over 6K.
http://www.youtube.com/watch?v=BhIdO5grOHY

 

[Texasmark]

Re: Correct prop for Lake Powell

Whatever you decide, and I agree that you will probably be sporting a higher than usual load, I'd make it alum and carry your current as a spare. I run SS all the time but don't boat in rocky/potential rocky water. If I had to sacrifice some performance to protect my drive train I would consider it a pleasant loss.

On your gear ratio, 2:1 is more like it. I ran the same engine a few years later and kept track of same and there were no 1.7's on that engine. Only other ratio they ran was 2.333 and that was on the 90 tower and 3 banger of later years which I currently have. Hight ratios like that were used on the mid range 50ish hp 3 bangers turning 10" props.

Mark

 

[steelespike]

Re: Correct prop for Lake Powell

Hard to tell but it does seem odd shaped.
Diameter is easy to measure.Measure from center to tip of a blade and double the number.
15" prop diameter runs about 13 1/2 to 3 3/4"

 

[ezmobee]

Re: Correct prop for Lake Powell

bunker, I'm running a 17" Turning Point Hustler aluminum prop on my SS with 90HP and can hit 40MPH. I'm a big fan of that prop because it totally eliminated the blowout issues I was having when nailing the the throttle from a stop and in some turns without losing any top end. And it's cheap!

 

[Texasmark]

Re: Correct prop for Lake Powell

bunker, I'm running a 17" Turning Point Hustler aluminum prop on my SS with 90HP and can hit 40MPH. I'm a big fan of that prop because it totally eliminated the blowout issues I was having when nailing the the throttle from a stop and in some turns without losing any top end. And it's cheap!

Mr. Moderator Sir: I have a 90 hp Merc on my little tin boat that I am sure you have seen pics of on numerous occasions. Having run a Merc 115 at a gearbox of 2:1 and the 90's at 2.333:1, I am interested in how you get the numbers you said you get with what you said your rig was. I have a TP Hustler of 23P on my 90 Merc with a 2.333 box, I also have a Ballistic 17P that came with my boat that I purchased used and it would snap your neck outta-da-hole and max rpm's at 35mph. Running the numbers on my 90, you didn't bother to say what yours is, if I got 40 with my 17P, my engine would be up around 7-8k just guessing, but could run the numbers.

Respectfully?

Thank you sir,

Mark

 

[ezmobee]

Re: Correct prop for Lake Powell

I don't know Mark, I think I'm being legit. I looked up my iboats order history to double check what prop pitch I ordered: 13-1/4 X 17 Hustler Aluminum. Motor is a 1981 Evinrude 90 HP that some dope put 140HP carbs on that my mechanic was able to rejet to run properly. Tach is a new Teleflex that reads somewhere in the 5k range at WOT. Speed is GPS and generally runs in the 38-41MPH range depending on load and conditions.


Benchmark: 20k posts.

 

[bunker108]

Re: Correct prop for Lake Powell

So it is time to make a move on a prop...Powell is right around the corner. I decided on a 17p, but I am a bit confused on what else needs to be ordered with the new prop. Hub kit, thrust washer...etc. Any advice is appreciated!

 

[fishrdan]

Re: Correct prop for Lake Powell

What prop are you getting, same need a hub kit, some don't.

 

[bunker108]

Re: Correct prop for Lake Powell

Thanks Dan, I ended up ordering the Solas that does not need a hub kit. I went ahead and ordered a new thrust washer and nut too. Got it all right here on site...hope it works!