Prop questions...

Discussion in 'Boats and Motors' started by SwollenGoat, May 9, 2008.

  1. SwollenGoat

    SwollenGoat Scourge of Hoover

    OK, I have a fairly good grasp on how pitch, diameter and rpms affect speed/performance. (I think, anyway.)

    My Fisher 17' aluminum deep V is powered by a Mercury 90hp 2-stroke. (My motors max rpms should be right at 5500 based on the book.) The boat came with a 21 pitch 13" aluminum prop and the previous owner threw in a new 13.25" prop with 19 pitch.

    I noticed with the 21 pitch prop, the boat was sluggish on the low-end and I could never get the max rpm's above 5200. Top speed with that setup was right around 37-38 mph with 1/2 tank gas and a couple people and gear.

    Last fall I switched out the 21p for the 19p. Bottom end was greatly improved and top end was 35-36 mph. However, according to my tac I can hit nearly 5700 rpm at WOT. *Probably not good, so I always keep the rpms right at 5500 on the tac.

    So I'm thinking there is room for improvement, just not sure which way to go...

    If I stick with aluminum, I'm guessing a 20 pitch prop sounds like the ticket. However, it seems that they are rare birds and a little pricey.

    The other alternative is to switch to a stainless prop. What I'm not sure about is how much difference does a stainless vs. an aluminum prop make? I've heard that there is less resistance in the water with thinner stainless props. How does that affect overall pitch and rpm's? If I were to buy a stainless prop do I want to stick with something in the 19 pitch category, or possibly jump back up to a 21 pitch prop? Also, how big a difference top-end vs. holeshot on 3 blade vs. 4 blade on the stainless props?

    Questions...questions? :confused:
     
  2. SG,

    Another option for better performance would be to go with the higher pitched prop and raise the motor a hole or two. Try a SS prop with vent plugs which will help with spinning the prop on the hole shot. Raising the motor should gain top end RPMs/speed as long as you don't raise it so high you start to get problems with porposing. Lots of combinations to "tweek" some more MPH and still get the boat out of the hole. Just a thought.

    Tim