Prop size and pitch

[Mdave1956]

Hi I have a 1988 Astro bass boat 17.8. 115 mercury tower of power. It was my wife's daddy's, we are trying to restore. The motor is a 1978. When we got it had a 21p. Liked to never got out of hole. Found a 19p better but no go. Bought a 17p. RPM only 3200 at 30mph. A local prop shop told me to try a 15p. To get RPM to spec. Oh all props are 3.25 di. Any help will be helpful. And yes motor has been checked by and older tec. that only does I-6 merc outboards. Thanx

 

[JimS123]

3.25 di. ??

17p at 3200??

Check your tach and maybe try a younger tec.

Somting wong here. Not adding up.

 

[jimmbo]

If you are only getting 3200rpm at WOT, I don't think there a Prop with a low enough Pitch to to even 5000rpm. 1800rpm shy, means you would need an 8" or 9" Pitch,
As mentioned, you need to verify the Tach
My 78 90hp with a 17, turning 4800 was getting 38mph. WOT Range 4500-5000
My 84 115hp with a 19, turning 5600 was getting 48mph. WOT Range 5000-5500

 

[Texasmark]

I had a 1988 115 tower on the same length, pad hull, Ranger BB and a 19P original Laser would get that boat up on the pad and at 6000 RPM ran 50 MPH at max trim. Something is definitely wrong with the engine.

 

[airshot]

Definitely an engine issue, but how much water is in the hull ? Doesn't take much waterlogged foam to slow you down a bunch !

 

[Mdave1956]

Thanx to all. I can't find a younger tech around here for motor that old but Compression is good on all cylinders. Had carbs rebuilt all that. The techs I have talked to told me water logged was an Issue with that boat. I'll have it checked. Was trying to restore as it was her daddy's, he passed and she was wanting to keep it. TIME FOR A NEW BOAT LOL.

 

[airshot]

She can be rebuilt if you have the time and money.

 

[JimS123]

Thanx to all. I can't find a younger tech around here for motor that old but Compression is good on all cylinders. Had carbs rebuilt all that. The techs I have talked to told me water logged was an Issue with that boat. I'll have it checked. Was trying to restore as it was her daddy's, he passed and she was wanting to keep it. TIME FOR A NEW BOAT LOL.

Sorry for my snide remark about your older tec. Since your data didn't add up it sounded to me that your guy is giving you bad info.

 

[Texasmark]

Thanx to all. I can't find a younger tech around here for motor that old but Compression is good on all cylinders. Had carbs rebuilt all that. The techs I have talked to told me water logged was an Issue with that boat. I'll have it checked. Was trying to restore as it was her daddy's, he passed and she was wanting to keep it. TIME FOR A NEW BOAT LOL.

Hull to water contact is a big player in boat speed.....if you have power trim and tilt, just get up on plane, tuck the trim in to the point that steering gets to be heavy. Record your speed/RPMs. Trim out till the boat wants to porpoise and make the check again. Proof's in the pudding as the saying goes.

That's the reason Bass Boats have a pad.....a narrow flat surface extending from about mid boat toward the transom, which is extended about 6" or so from the end of the pad. They have enough HP to get up to a speed where the only thing in contact with the water is that thin flat portion...almost zero friction.