Prop Help: F27 Mercruiser 305

[zedtwentyate]

Hello firends,

Now that i've replaced the carb, it's time to move on to a new project. I think it's time for a new prop. I'm concerned that I don't have the correct prop on the boat and its in rough shape (i think). Can anyone offer advice? From what the old search function has told me, I need a 14 x10 on this boat and as you can see in the pictures, this prop is a little ambiguous. Looks like someone stamped over the original size. Maybe it was reworked?

WOT - 3900 RPM @ 20-21 MPH

[prowlersfish]

Looks like it was a 13x10 reworked to 13x12 . Your just slightly low on rpm 14x10 would be a good choice but you could always redo the prop to 13x11 and that would give close to the same results .

[The Dog House]

I believe the WOT range for that engine is 3800 - 4200, so it seems you are propped correctly. What is the horsepower rating for the engine? If the engine is only making 200 hp like many 305s of that era, I think the problem may be that the engine is undersized for the boat. My Chrysler 318 making 225 hp is a bit underpowered in my opinion. I'm cruising at 18 mph at 3000 rpm.

[zedtwentyate]

The 305 is a 230 hp motor...

I believe the WOT range for that engine is 3800 - 4200, so it seems you are propped correctly. What is the horsepower rating for the engine? If the engine is only making 200 hp like many 305s of that era, I think the problem may be that the engine is undersized for the boat. My Chrysler 318 making 225 hp is a bit underpowered in my opinion. I'm cruising at 18 mph at 3000 rpm.

[zedtwentyate]

To achieve a nice cruise of 18mph, I would be in the 3500-3600 RPM range...

[Paul]

I run a 14 x 9 NiBrAl Cupped on mine with a 350 and a 1 : 1 Velvet drive. Boat's a little heavier that most F-26s however WOT is 4600, cruise at 3600 and 19-20 mph.

[The Dog House]

To achieve a nice cruise of 18mph, I would be in the 3500-3600 RPM range...

I believe the common prop for a newer F26/F27 is a 14 x 10 cupped prop. The cupping basically makes the prop equivalent to a 14 x 11 uncupped prop. I would think that a 14 x 10 cupped prop should give you ~18 mph at ~3000 rpm, but I'm just guessing. Hopefully someone with a newer (straight inboard) F26/F27 and a 14 x 10 cupped prop can post their performance numbers.

[prowlersfish]

To achieve a nice cruise of 18mph, I would be in the 3500-3600 RPM range...

I believe the common prop for a newer F26/F27 is a 14 x 10 cupped prop. The cupping basically makes the prop equivalent to a 14 x 11 uncupped prop. I would think that a 14 x 10 cupped prop should give you ~18 mph at ~3000 rpm, but I'm just guessing. Hopefully someone with a newer (straight inboard) F26/F27 and a 14 x 10 cupped prop can post their performance numbers.

It lists a small cup so that maybe equal to 1/2"" of pitch

[zedtwentyate]

Thank you for all the info. Looks like i need to read a little about props...i have no clue what I'm talking about.

From the sounds of it, I have two options:

1. re work the prop I have
2. Buy a new prop...or a used one.

I saw "Prop Shop" in a few other threads. Is there a reliable one that I should contact? I'm not looking to pay an arm and a leg for a prop for an old boat, but I do understand they aren't cheap!

[prowlersfish]

What area are you in ? I can recc some shops but best to deal local ask your marina or on the dock

[gitchisum]

My F26 with a FB ran about 16-18 mph depending on load @ 3100 rpm's This was a 233 mercruiser ( 351 block) 1977 model year. I know I had one cyl that was about 5% behind the the other 7, and I had 4500 hrs on the boat when I sold it. This hull is REALLY sensitive to loading and I ended up with Tab extensions ( basically whale tails!!!) to compensate for having 5-6 people in the cockpit area. ( didnt help that I had 3 downriggers with 15lb weights hanging off the backend)

I was running a cupped 14X10.

[zedtwentyate]

My F26 with a FB ran about 16-18 mph depending on load @ 3100 rpm's This was a 233 mercruiser ( 351 block) 1977 model year. I know I had one cyl that was about 5% behind the the other 7, and I had 4500 hrs on the boat when I sold it. This hull is REALLY sensitive to loading and I ended up with Tab extensions ( basically whale tails!!!) to compensate for having 5-6 people in the cockpit area. ( didnt help that I had 3 downriggers with 15lb weights hanging off the backend)

I was running a cupped 14X10.

I'm in Syracuse, NY. Maybe there's a forum member who wants to part with a 14x10 pro?

[MattSC]

What rotation is your current prop? I switched rotation when I re-powered years ago. I still might have my old RH 14x10 nibral cupped dyna-jet prop laying around Prior to the repower my old 318 cruised around 17-18 knots at 3100 rpm and around 25 at 4000 rpm. That is the Diameter and pitch prop that my boat was shipped from the factory with.

[zedtwentyate]

Well thanks to MattSC, the boat is now running a 14x10 cupped prop. She runs waaaaaaay better. At 3100 she is doing just about 20MPH. Max seems to be between 24-26 but I can still only get to 3900 RPM. I'm beginning to think there may be an issue with my tach.

Thanks again for all the help. I couldn't have accomplished anything on this boat without you guys!

[The Dog House]

Well thanks to MattSC, the boat is now running a 14x10 cupped prop. She runs waaaaaaay better. At 3100 she is doing just about 20MPH. Max seems to be between 24-26 but I can still only get to 3900 RPM. I'm beginning to think there may be an issue with my tach.

Thanks again for all the help. I couldn't have accomplished anything on this boat without you guys!

Congratulations! It sounds like your performance is right where it should be. These boats are definitely NOT speed boats.