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| Prop Question. https://www.smwebhead.com/phpBB3/viewtopic.php?f=11&t=4435 |
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| Author: | 07H200SS [ Sat Aug 29, 2009 1:39 pm ] |
| Post subject: | Prop Question. |
OK I have a 7.4L MPI with a Bravo 1. I have a 21P 4-blade stainless on now and seem to top out around 52 MPH, but this according to speedo. I also got a 21P 3-blade stainless with the boat. ANy guesses on the performance difference between the two? I was told the 4-blade gets the boat out of the water quicker, but the top end would be better with the 3. As I know nothing about the difference in props can someone educate me? |
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| Author: | M3eater [ Sat Aug 29, 2009 7:52 pm ] |
| Post subject: | Re: Prop Question. |
In theory .... you are correct. |
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| Author: | ht32bsx115 [ Sat Aug 29, 2009 10:13 pm ] |
| Post subject: | Re: Prop Question. |
Why don't you run both and report your results? Take a look at your engine owners manual. What is your WOT range? (I think for a 454 MPI it's something like 4400-5000?) Your drive should be a 1.5:1 ratio. It should be on the serial number sticker as long as someone didn't change it. You should "prop" for max RPM (5000) to get best all around performance. If those parameters are correct for your boat, 21p might be a little "over-propped" for your size boat. depending on what RPM gets you 52mph (is that GPS speed or speedometer?) |
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| Author: | 07H200SS [ Sun Aug 30, 2009 7:29 am ] |
| Post subject: | Re: Prop Question. |
I might try both next summer. Just not out enough yet this year to screw around with changing it. Boat has 4-blade on now and at WOT I am running about 4800-5000 RPM and 52 MPH on speedo. |
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| Author: | Walt [ Sun Aug 30, 2009 7:40 am ] |
| Post subject: | Re: Prop Question. |
Well traitor (kidding), the prop slip calc I use is really good for going from one pitch to another. Somewhat helpful for changing blade count as well. The problem I run into is the difference in prop diameter with different no. of blades. The more blades you have, the narrower they make it. eg. With my 3 blade 19p 15.75" dia, I run 52 mph by GPS at 5200 WOT RPM. That sounds pretty close to what you are showing with the 4. Remember, with each blade added, you pick up a little drag as well as lose diameter. My high 5 is a 21p with a 13.25" dia. I can hit 50 mph by GPS at 5200 WOT RPM, but she's a whole new animal throughout the whole range. I can easily get two boarders up with a full boat, the throttle is much more responsive, and much easier to hold plane speed for boarding. I hope that helps |
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