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Thread: Right prop,wrong prop?

  1. #1

    Right prop,wrong prop?

    Hello, how do I know I have the right prop on my 251. It's got a 350 Mercruiser 260 hp motor in it. I'm happy the way it will idle in the canal, I can run as low as 2.5 mph just in gear at about 700rpm. The problem is I don't like the performance above 3500rpm. It seem like it doesn't have any more above that. It will rpm a lot higher but not going too much faster. I know I can't have the best of both worlds in a prop ie more mph and a hole shot. What should the prop be on this boat with the engine I have. Thanks-Russ

  2. #2
    These inboard cruisers are not gonna perform anywhere similar to a boat with a sterndrive of similar horsepower.. The 251's with the stock 225 HP 305 would "cruise" at 18-20 MPH at 3000 RPM and top out at 25 MPH at 4400 RPM.. Repowering one with a 350 will probably only gain you about 1 or 2 MPH at cruise or WOT if it's propped to reach it's rated RPM at WOT (4000-4500 RPM)..

    Anyone that expects these little boats to perform like a similarly sized & powered sterndrive powered cruiser will be seriously disappointed.. Their hull design coupled with their straight inboard driveline pretty much limits them to speeds of less than 30 MPH..

    If you have a need for speed, get a different boat.. Otherwise, just sit back and enjoy the ride..
    CC-John

    1978 251 Catalina Express
    Powered by: PCM 383 CID GM (Bored & Stroked Chevy 350, 330 HP)

    1973 Century Resorter 16
    Powered by: 1972 Chrysler LM318 (225 HP)

  3. #3
    Wide open a 350 should reach around 4500 rpm or so.
    Everything I've read goes by the WOT engine RPM.
    If the engine reaches that the prop is correct.

    I think a 251 prop is 13 X 10 ???? but that seems small.
    I run a 16 X 15 on my 280. I cruise around 1800-2000 rpm or what ever the motor seems happy with and never go full speed.
    1983 280 Catalina

    "When in danger, or in doubt.
    Run in circles, scream and shout."

  4. #4
    No CC-John, I don't need speed and am very happy just cruising about 2500-2800 rpm which is about 17 mph. I love the constant hum of the inboard and it sounds like it would run for hours without a hiccup. 50lbs. oil pressure,never gets over 160 degrees .

  5. #5
    Solly, You have a reduction gear on a 280, 251 is 1:1. Thats why the props so much smaller. 13x10 RH w/cup is the right prop for a stock motor.
    1982 Catalina 251 "OLD SCHOOL"

  6. #6
    Quote Originally Posted by a1251 View Post
    Solly, You have a reduction gear on a 280, 251 is 1:1. Thats why the props so much smaller. 13x10 RH w/cup is the right prop for a stock motor.
    Ditto...

    And a 251 with a "stock" 250-270 HP 350 would probably be running a 13x11 or 13x12 to achieve the proper WOT RPM's..

    My 251 with a heavily modified 350 (bored .030 over and stroked to 383 cu. in.) that's making a solid 330 HP and over 400 ft. lbs. of torque and swinging a 13x11 FOUR-blade, cupped ski boat prop has a top speed of 33MPH @ 4100 RPM and will cruise at 26MPH @ 3000 RPM..
    CC-John

    1978 251 Catalina Express
    Powered by: PCM 383 CID GM (Bored & Stroked Chevy 350, 330 HP)

    1973 Century Resorter 16
    Powered by: 1972 Chrysler LM318 (225 HP)

  7. #7
    Gents, what is the correct prop for my '78 280 with single 305? It's in the water now so not sure what's on it but it wont go past 3400 rpm and 13mph. I don't have a need for speed either just want to cruise at 17-20 mph..

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