14" vs 17" inboard props

Discussion in 'Gas Engines/Drives/Transmissions/Props' started by scrumper, Mar 8, 2021.

  1. scrumper

    scrumper New Member

    29
    Jul 10, 2020
    1981 310 Vanguard Express
    Twin Mercruiser 6.2 MPI Horizon FWC inboards
    I had 3 blade, 14x21 pitch (yes really) props last season; they came with the boat. The engine topped out just under 4,000rpm WOT, this is below what I'd expect (engines want 4,500-4,800rpm.)

    I found a spare pair of 4 blade 17x14 props on the boat, in tired condition. I've cleaned them up and polished them and can get them balanced easily. They look great now.

    Question is, am I going to load the engine less with 4 blade 17" diameter 14" pitch props or with the 3 blade, 14" diameter, 21" pitch props that were on it? It's not just an academic question: there's a nick in the edge of one of the three blade props, so I'd have to get that fixed anyway. If I swap them out, I'll save myself a propellor refurb bill.

    Seems very hard to find calculations for this stuff. I tried one of those online prop analysis things but nobody got back to me. And the prior owner has no memory of what he did with props (though he is a lovely old guy who is still in touch with me and very helpful every now and again.)
     
  2. dwna1a

    dwna1a Well-Known Member PLATINUM Sponsor

    Apr 23, 2012
    James River
    88 Weekender 300 "Seahorse"
    Twins 350
    Try BlackDog or Michigan Wheel.
     
    scrumper likes this.
  3. dtfeld

    dtfeld Water Contrails GOLD Sponsor

    Jun 5, 2016
    Milton, GA
    410 Sundancer
    2001
    12" Axiom and 9" Axiom+ MFD
    Cat 3126 V-Drives
    14" diameter to 17" diameter is a big jump. They may not fit (hit the hull) or produce undesired noise/vibration if too close to the hull. And who knows what performance you'll get with those props.

    I'd suggest selling both prop sets and get a reputable prop shop to help get you the right props. A good shop will get you dialed in on how you run/load the boat. Also, you can get a custom set worked up and dialed in https://acmemarine.com/pages/sizing.

    Keep in mind overloading an engine is detrimental long term (maybe even short term), and plays a big role in maximizing speed and minimizing fuel burn.
     
    scrumper likes this.
  4. scrumper

    scrumper New Member

    29
    Jul 10, 2020
    1981 310 Vanguard Express
    Twin Mercruiser 6.2 MPI Horizon FWC inboards
    Had not thought about selling both to fund new ones, that's smart David thanks.

    They do definitely fit, not least because the boat originally came with 18" props before it was repowered (and PO had those 17" four blades on the boat prior to my purchase, just didn't have anything useful to say about them. He says he put the three blade props back on because he thought they'd make the boat faster, which he just assumed I'd want. They're clearly too much prop for the motors.)

    I'll see what Acme has to say.
     
  5. scrumper

    scrumper New Member

    29
    Jul 10, 2020
    1981 310 Vanguard Express
    Twin Mercruiser 6.2 MPI Horizon FWC inboards
    Actually, I guess what I need to know is, how can I find out if the 17x14 four blade props I've got would be suitable? Whatever technique all these prop dealers have must be available surely? They all take the same inputs (waterline length, weight, engine power and so on).
     
  6. dwna1a

    dwna1a Well-Known Member PLATINUM Sponsor

    Apr 23, 2012
    James River
    88 Weekender 300 "Seahorse"
    Twins 350
    I know when I switched to four blade Michigan Wheel asked many questions about my weight, WOT, horsepower, and age of my hull. I can go back and find the ones they recommend if it will help. All of that was on the site. Both they and BlackDog were great to work with.

    IF I remember right mine are 14x14. I wanted to get on plane faster and if everything is right my 1 ton girl will do just that.
     
    scrumper likes this.
  7. scrumper

    scrumper New Member

    29
    Jul 10, 2020
    1981 310 Vanguard Express
    Twin Mercruiser 6.2 MPI Horizon FWC inboards
    I called MW - the guy on the phone was at first sure I had an I/O because 14x21 is so weird on an inboard. He said that swapping my 4 blade 17x14s in would absolutely be an improvement. Perfect, perhaps not, but it'd get me closer and I could go from there. Thanks all.
     
  8. techmitch

    techmitch Well-Known Member GOLD Sponsor

    May 1, 2008
    Chesapeake Bay, Middle River/Frog Mortar Creek MD.
    1999 270 DA
    Twin 4.3s W/Alpa I Gen II's
    I'd have to bet she is a bit more than 1 ton.
     
  9. dwna1a

    dwna1a Well-Known Member PLATINUM Sponsor

    Apr 23, 2012
    James River
    88 Weekender 300 "Seahorse"
    Twins 350
    Now your going to make me go look
     

Share This Page

Show Sidebar