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trawfish
04-17-2007, 09:59 AM
I'm sure there have been plenty of prop threads, but I didn't see a specific prop forum. Maybe this belongs in tech forum??
I have a 26' Commander Signature with a 7.4 M.P.I. (I know it's a slug) with a bravo 1 (1.5 ratio). I am turning a four blade 24p stainless bravo prop. My top speed is 55 mph @ 4600 rpm. I am to beleive this is where the rev limiter kicks in on this particular engine. According to the Prop Slip Calculator, my slip is 21%. The prop is not labbed, no dammage, but I swear it looks like it has a little wobble in it.
I am first looking for a better cruising speed, and would always enjoy a higher top speed. I have no problem getting out of the hole (on plane for all you smart A$$'s).
What are your expert opinions on prop selection for this application????
Thanks in advance.

Jetaholic
04-17-2007, 10:03 AM
No, there's no "Prop" forum, but this one belongs in the "Outdrive Tech" section.

trawfish
04-17-2007, 10:19 AM
No, there's no "Prop" forum, but this one belongs in the "Outdrive Tech" section.
Can I move it over there? or do I have to start a new one?
Sorry, not very forum savvy!

Havasu Hangin'
04-17-2007, 10:54 AM
Is your outdrive ratio, speed, and RPMs verified?

ANXIETY ATTACK
04-17-2007, 11:08 AM
First, pull prop and fix a dial indicator on the prop shaft and turn, should be with in .001 anything more than that and it is bent, have you ever tilled rocks?

burtandnancy2
04-17-2007, 11:26 AM
rpm x pitch x ratio x .00094697 over slip = speed. Now you just need to know slip for your particular boat. Best way to find that is radar or GPS...

trawfish
04-17-2007, 11:28 AM
Is your outdrive ratio, speed, and RPMs verified?
Drive says 1.5 on it, speed is GPS and RPM is from tach on dash...

Havasu Hangin'
04-17-2007, 11:34 AM
Drive says 1.5 on it, speed is GPS and RPM is from tach on dash...
Assuming the outdrive ratio is what the case says it is, that is ALOT of slip. How high are you trimming it?
Either way, I'd take the prop to a shop to see if it's bent, or try a different one.

trawfish
04-17-2007, 11:39 AM
Assuming the outdrive ratio is what the case says it is, that is ALOT of slip. How high are you trimming it?
Either way, I'd take the prop to a shop to see if it's bent, or try a different one.
I have a 23p that runs about 51-52 at same rpm, so I am assuming the revlimiter is hitting at that rpm. Sounds stupid, but I can't hear it studdering. I am considering a 26p.... do you thing that will be to much??

Havasu Hangin'
04-17-2007, 11:48 AM
I have a 23p that runs about 51-52 at same rpm, so I am assuming the revlimiter is hitting at that rpm. Sounds stupid, but I can't hear it studdering. I am considering a 26p.... do you thing that will be to much??
That's still not good. 20-22% means there is something rotten in Denmark (like the prop is surfacing from over-trimming, etc.). Since both props are in that slippage range, I'd check the outdrive ratio and tach for accuracy.
If that outdrive had 1.65-1, you'd be around 12%...which is pretty normal.
Hitting the limiter won't cause slippage...just slow the boat's top speed down.

trawfish
04-17-2007, 12:01 PM
That's still not good. 20-22% means there is something rotten in Denmark (like the prop is surfacing from over-trimming, etc.). Since both props are in that slippage range, I'd check the outdrive ratio and tach for accuracy.
If that outdrive had 1.65-1, you'd be around 12%...which is pretty normal.
Hitting the limiter won't cause slippage...just slow the boat's top speed down.
I have played with the trim alot, sure I can lift it enough to blow out and raise the rpm, but I think I have found the optimal trim position... for the highest speed anyway. It seems I can get to that speed/rpm in almost any position. I am seriously considering a larger prop to see if I can load the engine enough to reduce that rpm range and gain more cruising speed. I am thinking of trying a 26p.... Do think that is to big of a jump??

burtandnancy2
04-17-2007, 12:33 PM
Maybe you're right, your boat is a slug...

trawfish
04-17-2007, 12:38 PM
Maybe you're right, your boat is a slug...
I'm slowly coming to that realization...

Havasu Hangin'
04-17-2007, 12:47 PM
I have played with the trim alot, sure I can lift it enough to blow out and raise the rpm, but I think I have found the optimal trim position... for the highest speed anyway. It seems I can get to that speed/rpm in almost any position. I am seriously considering a larger prop to see if I can load the engine enough to reduce that rpm range and gain more cruising speed. I am thinking of trying a 26p.... Do think that is to big of a jump??
I'd borrow a 26" prop and see what happens. It could be something as simple as someone replaced the Bravo with 1.65 internals, and you are on the limiter.
Either way, if you have ruled out all the other variables, a different prop would be the next place I'd look.

trawfish
04-17-2007, 12:49 PM
I think thats where I will start. Thanks for the advise, much appreciated.

BajaMike
04-17-2007, 04:00 PM
I think thats where I will start. Thanks for the advise, much appreciated.
That engine at wide open throttle should get 4600 to 5000 RPMs. The rev limiter shouldn't kick in until over 5000 RPMs, and the newer engines (the last 3 or 4 years) start a warning buzzer if you hit the rev limiter.
I don't think you can prop it higher if you're not even getting 5000 RPMs now.
:confused:

bordsmnj
04-17-2007, 04:15 PM
try a 22" 4 blade. sounds like your prop is too big.

Pro Max
04-17-2007, 05:50 PM
I bet the tach is wrong, I know a few people with that boat and they run great. I think even BIGLOU has one and I think he runs a solid 65