Repower props and stern ratios

[Bruce P]

Wow. I just found this forum after many days of searching for info about my repower this winter.

I just bought a 1988 32 Fairbanks (Fairline) Sedan that needs to be repowered. Wet displacement is 14,000 lbs and it is a heavy cruiser planing hull. I will probably go with Merc 5.0 MPI 260 hp twins as these will drop in. After that I can use Alpha or Bravo II or Bravo III drives and need to pick a reduction ratio.

So, based on my assumption that for every boat displacement / hull / hp / goal there is a "perfect" theoretical prop diameter / rpm / pitch, I am trying to define this prop and then work backwards to choose the drive and ratio.

My goal is best full consumption while on the step, which the previous ower tells me is about 20 mph. Any suggestions or comments are very appreciated.

thanks - bruce

[baeckmo]

Since you are going to use an IO drive leg, the maximum prop dia is fixed, you only have a choice between ratio's and pitches. For this boat I would suggest that you use a Bravo 2 with ratio 2.0:1. Pitch will be something like 15" with standard props, and if everything is ok (including your weight estimate), you should certainly get moving.

[Bruce P]

Thanks baeckmo for the help. If I read between the lines, I assume you are targeting a larger rather than smaller diameter. The Alpha drive is max 15-16" while the Bravo II is 20" max diameter. So, are you also saying that the theoretical most efficient prop is larger than 20"?

[Bglad]

Originally Posted by Bruce P

Wow. I just found this forum after many days of searching for info about my repower this winter.

I just bought a 1988 32 Fairbanks (Fairline) Sedan that needs to be repowered. Wet displacement is 14,000 lbs and it is a heavy cruiser planing hull. I will probably go with Merc 5.0 MPI 260 hp twins as these will drop in. After that I can use Alpha or Bravo II or Bravo III drives and need to pick a reduction ratio.

So, based on my assumption that for every boat displacement / hull / hp / goal there is a "perfect" theoretical prop diameter / rpm / pitch, I am trying to define this prop and then work backwards to choose the drive and ratio.

My goal is best full consumption while on the step, which the previous ower tells me is about 20 mph. Any suggestions or comments are very appreciated.

thanks - bruce

The Bravo II is also a good choice if the boat is going to live in the water most of the time. You can use aluminum props and cut down on the amount of corrosion that is possible between big stainless propellers and the aluminum drive leg.

[baeckmo]

Originally Posted by Bruce P

............ So, are you also saying that the theoretical most efficient prop is larger than 20"?

With a touch of oversimplification, you might say that there is an optimum value of the ratio "Thrust/(prop disc area)", i.e. the working pressure. Without going into detailed calculation of your case, from experience I consider it to be closer to 20" than 15".

In a real world, the above note on corrosion is highly relevant; neglect in these quarters comes with a high cost!

[Bruce P]

I just got the "Propeller Handbook" by Gerr, available and bn.com. This is a really good book for novices like me.

Thanks for the stainless steel tip; I did not realize this could cause additional corrosion potential (pun intended) over using aluminum props.