Another Prop/RPM question

[redmen62]

I picked up an Regal 277 Commodore with twin 305's for a steal at the end of the year last year. Because of mechanical issues with the Stbd engine I wasn't able to do a full sea trial, and once fixed and in the water because of weather, life, and outdrive problems on the port I wasn't able to get her out to let her stretch her legs.

Put on a new counter rotating outdrive this spring so I needed a LH prop.. The boat came with 3 blade 15p props which didn't seem quite right to me, and since I couldn't find a matching LH prop I decided to get new 4 blade props. Went through 4 or 5 prop selectors online, talked to several people (2 with the same boat), and read all sorts of forums, and at the end of the day went with 14.5x17 aluminum 4 blade. However, now that my break in on the new drive is done it seems I'm only running at about 3300 and 32ish MPH at WOT... do I have the wrong props?

Prop math always confuses me... but it seems that 200RPM per pitch change seems to be the going approximation, so does that mean I should be running 21-23p props?

 

[ondarvr]

You would typically drop an inch of pitch when switching from three to four blades, you went up two inches (15"-17"). The place to start would have been with a 14"

And what do you mean by " the boat came with 3 blade 15p props which didn't seem quite right to me"?

 

[redmen62]

What I meant was it seemed off from every boat I've ever had, the props I used to sell and recommend to people when I worked at west marine, and all the prop selectors I used... The people with the same boat with 350's are running 21p 3 blades. They don't have problems getting up on plane, so I figured a 19p would be right with the lower hp then dropped again for a 4 blade... Did I go the wrong way?

 

[ondarvr]

I can only make assumptions. I would think the first props were probably close, but you didn't say what the RPMs were with them, from the results of the 4 blade props it sounds like they were at least close. The reduction in RPMs with the 4 blade is in the ball park with would you would expect if the 15" were the right ones.

 

[Maclin]

"does that mean I should be running 21-23p props?"

In a word, Nope. Totally wrong direction in pitch.

 

[Maclin]

Do both engines act limited in RPM that way?

 

[redmen62]

I have no way of knowing what they previously ran... the boat hadn't had 2 usable engines in a few years. I bought it from someone who used it as a floating condo and jetski dock because the stbd engine didn't run and didn't want to even try to fix it for 2 years; it ended up needing the carb rebuilt. Got that done and running great, then lost the port outdrive. I tried to track the boat back the who he bout it from to try to get somemore history on it, but no luck.

I followed the prop selectors and what others are using... Seeing what I'm getting now, does that mean I need a 13p to get back up to where WOT should be?

This might be a stupid question, but when I run the selectors it gives you different results for SS than AL... is there really that much of a difference and flex of the blades?

 

[Maclin]

Yes on the flex.

Just going by your statement that others with exact same boat but with 350's are using 21p's, There must be something else going on with the engines methinks, down on power for some reasons(s). The 15p props oughta be way enough of an adjustment for the 5.7 to 5.0 difference.

Other things to consider...
Boat too heavy due to water logging (or maybe they left a jetski in it somewhere),
hull fouled with crud,
fuel delivery or quality issues
wrong ratio drives,
can be a pretty long list.

 

[redmen62]

Yes on the flex.

Other things to consider...
Boat too heavy due to water logging (or maybe they left a jetski in it somewhere),
hull fouled with crud,
fuel delivery or quality issues
wrong ratio drives,
can be a pretty long list.

I vote for the jetski!!! lol

but really, the hull is clean (fresh power wash, brushing, and paint this spring), fresh gas and 1 rebuilt carb (both engines are operating the same, so I don't think it's the carb), and I know the port is the right ration (1.5:1, so if both engines are behaving the same then I think that scratches that one too)


I'm wondering if it's not a fuel delivery or weight issue. I'll start with fuel since that's about the only thing I can fix... :bi_polo: