Prop and Porpoising
07-22-2002, 04:46 PM
#1
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Prop and Porpoising
I have a 1997 Baja 240 with a 454 mag. I just puchased the boat. I got 2 props with the boat a 3 blade 21 p 14 3/4 mirage plus and a 4 blade 24 p 14 3/4 bravo I. Top speed with the 4 blade is abot 62 mph at about 5000. This prop makes the boat porpoise is this normal?? The 21p is almost useless at about 45 it is running about 5500. I am not sure if the tack is reading properly. I think the motor should be coming apart running at 5500. I am not sure what is the best pitch to run on this boat. Any suggestions I was thinking a 23p 14 3/4. Anything I can do as far as the porpoising? The trim tabs do nothing to stop this.
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07-22-2002, 06:31 PM
#3
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Regarding the porpoising, it's really tough to battle that condition, but one of the reasons might be the 4-blader...Traditionally, the bravo1's create quite a bit of stern lift which might be plowing the bow, this may lead to an oscillation of the nose plowing(lifting) and then dropping repetitiously.
A 3-blade(mirage) will be your best top-end performer. As far as your 21 pitch, get rid of it. It's way underpropped for your set-up. 5500 rpms is very excessive for your 454 mag, I hope you didn't leave it there long. A 23 or labbed 25 Mirage will be your ticket.
I would lean towards the 25, though, due to the fact that you're turning a 24 Bravo1 to 5000 rpms. General rule of thumb is to drop one inch in pitch from a 3-blade to a 4-blade and vice versa.
A 3-blade(mirage) will be your best top-end performer. As far as your 21 pitch, get rid of it. It's way underpropped for your set-up. 5500 rpms is very excessive for your 454 mag, I hope you didn't leave it there long. A 23 or labbed 25 Mirage will be your ticket.
I would lean towards the 25, though, due to the fact that you're turning a 24 Bravo1 to 5000 rpms. General rule of thumb is to drop one inch in pitch from a 3-blade to a 4-blade and vice versa.
Last edited by Baja Daze; 07-22-2002 at 06:35 PM.
07-23-2002, 08:16 PM
#6
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Double check your numbers, because I'm not sure how you are turning a 24 4 blade to 5K when I am making about 135 more hp and probably 150 more tq than you and can only spin a 23 Rev 4 (4 blade ) to about 5200 RPM (24 Outlaw, same haul). I know there is a variance between the two makes, maybe there is that much difference, but I doubt it. By the way, you should have a rev limiter factory set at 5200 on that motor I beleive. So something is up. The porpoise is a pain in the ass, hard to get rid of. Best bet is to play with props and see where on the trim and tab indicators the boat runs best. Regardless of the tach reading correctly, just by the numbers a 25 Mirage Plus is probably your best bet. Later. Never know until you try, they are all different.
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07-26-2002, 09:55 AM
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Bob,
Check your tach, on the back there should be a multi position switch. It may just be switched wrong. Where does the boat idle at? should be somewhere around 700 RPM's at idle. If it looks like it's idling at 1000-1100 or something like that the tach is probably set wrong.
Ted
Check your tach, on the back there should be a multi position switch. It may just be switched wrong. Where does the boat idle at? should be somewhere around 700 RPM's at idle. If it looks like it's idling at 1000-1100 or something like that the tach is probably set wrong.
Ted
07-26-2002, 10:04 AM
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Quick question to everyone reading this post. On my 252 I can usually run the trim only to about 1/4 on the indicator before the prop starts to cavitate. I am running a 23P Mirage Plus, which seems to be the best prop for my boat. Where does everyone else's trim max out on the guage before cavitation?
Ted
Ted
