Could I get more top end?

[Cobalt226]

I recently purchased a 1999 Cobalt 226 with 7.4L Alpha One. Currently I run about 54mph at WOT @4000 rpm and trimmed. The WOT range for my boat/engine is 4200 - 4600 and I have a 21P SS single prop. Could I increase my top end with a 19P and push my WOT rpm more towards 4600? Are there pro/cons? It runs sweet at WOT at 4000, but I was thinking I could push 60 with less pitch? Appreciate your comments opinions. Thanks!

 

[jimmbo]

Re: Could I get more top end?

Well as far as theory goes, you should gain 400rpm which puts you in the middle of the rpm range. The hole shot will improve. You might gain up to 1, maybe 2 mph only because you are currently below the recommended rpm range. What kind of SS are you currently running?

 

[Cobalt226]

Re: Could I get more top end?

Well as far as theory goes, you should gain 400rpm which puts you in the middle of the rpm range. The hole shot will improve. You might gain up to 1, maybe 2 mph only because you are currently below the recommended rpm range. What kind of SS are you currently running?

jimmbo, a 19P would improve my hole shot? I thought going with less pitch would hurt the hole shot?

 

[jimmbo]

Re: Could I get more top end?

less pitch equals less load on engine equals quicker accelleration

 

[jestor68]

Re: Could I get more top end?

Going lower in pitch improves acceleration.

Depending upon how far out of wack you are , dropping pitch can improve BOTH acceleration and top speed.

Regardless, only turning 4000 rpm WOT is not good for your motor. It must turn up to at least it's HP rated rpm to perform correctly across the entire rpm band. Right now it's "lugging" and never getting to develop it's full power.

 

[Silvertip]

Re: Could I get more top end?

Here's the skinny on pitch effect: a 19P prop theoretically moves the boat forward 19 inches with each revolution. A 21P prop moves it 21 inches. If you compare engine rpm at the same SPEED (choose 30 MPH for example), RPM with the 19P prop will be higher than with the 21P. So dropping pitch raises RPM which is like downshifting in a car/truck. It increases hole shot but may or may not increase top speed. Keep in mind RPM increases only because the prop is taking less bite, therefore less forward movement. However in your case the engine is apparently already "overpropped" (too much pitch) so decreasing pitch may increase speed AND holeshot. Before making prop changes, always make sure the engine is properly tuned, is not "tired" (in other words not worn out) and that the hull is clean and not full of water soaked foam. A prop is not a fix for a poor performing engine or dirty hull.

 

[Cobalt226]

Re: Could I get more top end?

Thanks for the explanations! Silvertip, my boat engine is not tired at all and even though is a 1999, it only has 160 hours on it and it runs great. Bottom is very clean and nothing is water logged as inside is very dry.

Bottom line is it sounds like a 19P prop would be the right move since it seems that I am slightly over pitched at 21P.

Thanks!

 

[Scott Danforth]

Re: Could I get more top end?

my first question, is the motor healthy? good compression, etc?

assuming it is, then the lower pitch will help.

 

[syuhas1985]

Re: Could I get more top end?

This is a good thread for me but my problem is the opposite I have a searay srv 195 with 302 v8 I have a 14 x 19p vengeance and I'm at 40 mph at 5000rpm so in order to lower rpm I need to goto a 21 pitch and will this help top speed?

 

[Oshkosh1]

Re: Could I get more top end?

This is a good thread for me but my problem is the opposite I have a searay srv 195 with 302 v8 I have a 14 x 19p vengeance and I'm at 40 mph at 5000rpm so in order to lower rpm I need to goto a 21 pitch and will this help top speed?

5k sounds high for that motor...I'd try both 21 AND 23P props. People here will crunch the numbers, but the bottom line is you won't know until you put it on YOUR boat and give it a shot.

I run a 14/23P for "light" load cruising...40MPH WOT 4k
I run a 14/21P for a full boat "cruising"...36MPH WOT 4K
I use a 14/19P for pulling tubes and get about 36MPH @ 4kRPM...better hole shot...

All on a 160Hp I6...Tuned up to spec and a light chop.

 

[syuhas1985]

Re: Could I get more top end?

5k sounds high for that motor...I'd try both 21 AND 23P props. People here will crunch the numbers, but the bottom line is you won't know until you put it on YOUR boat and give it a shot.

I run a 14/23P for "light" load cruising...40MPH WOT 4k
I run a 14/21P for a full boat "cruising"...36MPH WOT 4K
I use a 14/19P for pulling tubes and get about 36MPH @ 4kRPM...better hole shot...

All on a 160Hp I6...Tuned up to spec and a light chop.

Thanks for the info ill try a 21 see if that helps does diameter make a huge difference like a 14 vs a 14 1/2? I can buy a 21p either or.

 

[jimmbo]

Re: Could I get more top end?

5k sounds high for that motor...I'd try both 21 AND 23P props. People here will crunch the numbers, but the bottom line is you won't know until you put it on YOUR boat and give it a shot.

I run a 14/23P for "light" load cruising...40MPH WOT 4k
I run a 14/21P for a full boat "cruising"...36MPH WOT 4K
I use a 14/19P for pulling tubes and get about 36MPH @ 4kRPM...better hole shot...

All on a 160Hp I6...Tuned up to spec and a light chop.

I am curious as to why you do not gain or lose rpm ever though the is a 4 inch difference in the pitch. You don't even lose any speed dropping from a 21 to a 19 even though your rpm stay the same

 

[Cobalt226]

Re: Could I get more top end?

Just thought I would provide an update on my situation. I installed a 15 1/4 19P and got improved hole shot, faster on plane and maybe 2 mph top end. I now run WOT at 4400 - 4500 and now hit 56mph. I feel this was a good move! Thanks!