Selecting First Prop

[sparkyz450]

Hi guys, I'm new to the forum and a new owner of a project boat. I've been reading all I can trying to get stuff figured out and am looking for any recommendations. The boat we purchased needs a new lower unit. The motor is a 1974 Mercury 115 inline 6. While looking for lower units we have decided to probably buy one from an 80 horse, which from my understanding has a 2.3:1 ratio.

The boat is also going to be operating mostly around 6000 ft elevation. The hull is an 88 Thunder Craft Capri 152. From my research its weight is 750 lbs and the motor weight is 273 (info from this website). I'm guessing the total loaded weight will be about 1700 lbs.

We don't have any props we will be able to try first so I'm trying to get as close as possible with the first try. From my calculations, based initially off of a recommended prop for the stock 115 motor, I came up with needing a 24 pitch prop to be approximately equivalent to the 19 that was recommended by the calculator, factoring in gearing change to the new lower unit. So then accounting for altitude I'm thinking a 21 would be a good starting point...

Does this sound in the right ballpark? Thanks for any help and I will be happy to provide any more info needed.

 

[etracer68]

Re: Selecting First Prop

Welcome to Iboats, I first would stay with the 115 lower unit, second, my Yar-Craft is about 200 lbs more, and I run a 90HP and I can run a 17 or 19 pitch and be with in my rpm range, although the boat reponds better with the 17 pitch. I also run a 3 blade aluminum prop.

 

[steelespike]

Re: Selecting First Prop

From my reading you lose about 3% for each 1,000 ft.That would seem to put you at 18% power loss also you should rejet for best output.The Merc forum or Johns Old Mercury Web Site may be able to help with the jetting
and prop selection. Your much better off going lower pitch if its too low you can still use it by staying within your rated rpm.A missmatched ratio can be very problimatic in selecting a prop.I believe a 115 is 2.07:1 a 80 is 2.30:1
Very roughly a 10% difference Should be easier to turn a prop.

 

[sparkyz450]

Re: Selecting First Prop

From what I found on a few sites the 115 in that year is a 1.73:1 ratio stock. That's why I was factoring a large change for the prop. I don't have a lower now so I'm just going off the info I'm finding online. Thanks for the tip on the jetting, I'm hoping that in its time in the Denver area that someone has taken the time to properly jet it, but I'm sure I will be doing some fine tuning.