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  1. #1

    Good cruise speed for an 85 45c?

    Vessel is still new to me, only have a few hours with it so far.

    Feels like it's running a little wet and struggling at 16-17 knots ~1900 RPM, 3/4 tank (500gal cap) without any trim tab usage. Idle speed is about 4 knots.

    550 HP / 6v92's and no idea what props. Engines run super clean, no smoke and start without any loping.

    It will reach full RPM under way easily, but have only done that on the sea trial.

    Bottom is clean, although ready for a repaint.

    Just fixed the trim tab pump and next trip will see if they help the wet and struggling feel any.

    In the mean time figured I would ask and see where my expectations should be.

  2. #2

    Re: Good cruise speed for an 85 45c?

    Try bringing the throttles up to 2100 rpm or so and then bring it down to your desired cruise rpm. 1950 rpm might be about right. Have tabs up, then play with tab settings to settle in.

  3. #3

    Re: Good cruise speed for an 85 45c?

    Does the boat have a tower? What gears are u running? Get the boat in deap water and run it wide open for a minute or two. It won't hurt it. With a clean bottom I would think it would cruise at 20knts pretty easily.

  4. #4

    Re: Good cruise speed for an 85 45c?

    Quote Originally Posted by dsharp View Post
    Does the boat have a tower? What gears are u running? Get the boat in deap water and run it wide open for a minute or two. It won't hurt it. With a clean bottom I would think it would cruise at 20knts pretty easily.
    No tower, Allison M15's, 2:1 Gear I believe.

  5. #5

    Re: Good cruise speed for an 85 45c?

    Allison M15 = 1.5 gear ratioAllison M20 = 2.0 gear ration

  6. #6

    Re: Good cruise speed for an 85 45c?

    Quote Originally Posted by Pacblue View Post
    Allison M15 = 1.5 gear ratioAllison M20 = 2.0 gear ration
    That makes sense.

    Waiting for weather now.

  7. #7

    Re: Good cruise speed for an 85 45c?

    I'll assume you have the original 26x28 three blade props. In my case WOT made 27K when all things were right (that means almost never). And easily cruse at 22/23K. Once the engines are at operating temp. push to WOT for 3-4 min. then back off 400rpm that should be your cruise speed.

  8. #8

    Re: Good cruise speed for an 85 45c?

    Quote Originally Posted by Captcoop View Post
    I'll assume you have the original 26x28 three blade props. In my case WOT made 27K when all things were right (that means almost never). And easily cruse at 22/23K. Once the engines are at operating temp. push to WOT for 3-4 min. then back off 400rpm that should be your cruise speed.
    What was your idle speed? At idle, I'm about 4-5 knots. WOT 2300 RPM is about 22 knots.

    I have two pairs of 3 blade props, the ones in my garage are stamped 26 28 FJ and some other markings 96 M 17? I can't quite make out. Stamped 5Cup on the other side.
    EDIT: Scraped off some paint. Looks like a Frank & Jimmies Prop set.
    Left: L961 M172 26 x 28
    Right: M173 26 x 28

    The previous owner said they were two different sizes but didn't know which ones were on the boat.

    My diver put a tape measure on the props on the boat and the blades measured out the same.

    I had good weather for a run on Saturday and brought it up to speed, ~20 knots @ 2100 RPM and settled in at about 16-18 @1950 RPM. Temps came up fast, upwards of 195ish. No steam out of the exhaust, but seemed a little hot, however the gauges are original, so who knows how accurate they are. DD manual says 170-195 as normal operating temps... Plenty of water flow and new impeller pumps. Engine water levels were full just before the trip and overflow tanks full to the hot mark while running. No funny smells or leaks in the engine room...

    It stayed at 195ish and 17 knots or so. Only ran for a few miles that way then slowed for an 8 knot ride so I could play with the auto-pilot.

    I'd love to get the 20 knot+ cruise, it's just a comfortable speed. 17 feels sluggish especially in a following sea.
    Last edited by CaptainRoberts; 10-29-2017 at 08:24 PM.

  9. #9

    Re: Good cruise speed for an 85 45c?

    At 2100rpm with no slippage the speed is 35.29K and you are getting 20K that's 44% slippage you have a bottom or weight load issue. That number should be around 20%. Doesn't take much on a prop to kill your speed.
    Good Luck

  10. #10

    Re: Good cruise speed for an 85 45c?

    Sorry I ran the numbers at 2300 not 2100 so your slip is 38%. Still way too high.
    Sorry for the mix up.

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