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Old 03-04-2002, 10:08 PM
  #31
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26 Scarab, Very close will be +4" from your current prop which at the same rpm will be about 12mph. Might need +6" if youre at the rev limiter now. When going from 3 blade to a 4 blade you need to -1" pitch. So for you 23" + 4" -1"= 26" bravo prop and ?maybe a 28" for more speed but less mid range.
Remember one prop will never do it all. I carry 2 different pitch props and own 3 different pitches. Depending on the load, fuel, water conditions, temp etc I pick a prop.
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Old 03-04-2002, 10:11 PM
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26Sonic - I used your actual test data, and a 650 HP engine that I built by adding a centrifugal blower to a 502 MPI on Desktop Dyno. Because of the small cam in the MPI engine, I had to use 8 psi, not 5 psi boost. More cam and a bit of head work would help this engine a lot.

Your speed would be about 85 mph @ 5000 RPM with 28" pitch. Going down to a 27 " pitch would still be 85 mph at about 5200 RPM. You would likely be happier with the bottom end with the centrifugal blower by using less pitch. This is assuming a 3 blade prop doesn't blow out. If it does, then a Bravo 28 might be your best bet.
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Old 03-04-2002, 10:14 PM
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Marty,
I think you meant to reply to 26Sonic, I'm all set with props.
I also suggested those props it will fall somewhere in between a 26 28 B1.

Here's a question though I'm spinning a labbed 26 B1 5800 , would it be better using a 28 B1 and spin it around 5500 rpm ?
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Old 03-05-2002, 08:34 AM
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26scarab

what does your hp curve look like. if you are still making more power past 5500 on to 5800, then that will probably be the fastest the way you have it propped. if the power curve is flat or falling off between 5500 and 5800, then prop for 5500, reducing prop parasitic losses by slowing a prop down and adding pitch, also it seems there's a reason why the Merc factory racing power (500hp's and such) only cam for about 5200 for reliabilty.

two bottom lines.

1) pitch for your peak power point
2) trial and error to optimize
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Old 03-05-2002, 10:25 AM
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Tomcat

just tell me your curve for hull power required vs speed fits through the one data point and is a function of velocity
[B]cubed[B], not squared.

also, can you plot the power curve for a 330 carb

I am getting my props labbed by john at York and want to specify the work. I am at 4600 right now at 64.9999 with 25's. when he labs them I want to know if I should raise the pitch slightly to taked advantage of the "reduced drag" or leave the claimed "200 rpm gain" because the motor makes more power at 4800. supposedly the 330 are pretty flat between 4200-4600.

Must...........have............data ............
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Old 03-05-2002, 11:24 AM
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Sorry Rambunctious, the equation is:

Hull HP requ'd = weight x (Speed/Constant)Squared

If this was a displacement hull being pushed through a fluid, I believe the cubed rule would apply, but this is a planing hull operating at the interface of two fluids, one of which is 600x denser than the other, so friction at the wetted surface takes over. I believe the drive dragging through the water operates on the cubed rule, that's why raising the drive a little bit can have a big effect. The boat moving through the air is also operating on the cubed rule, but that component must not be anywhere near as important as the friction at the wetted surface.

I know that this equation holds up to 200 mph, because entering the data for a 36 Skater running at 110, 150 and 190, with three different engine packages, produces three curves that overlap almost perfectly. It would be interesting to know at what speed aerodynamic drag becomes important. I know in cars aerodynamic drag begins to exceed rolling friction at 55-60 mph. Whatever its effect it seems to be included adequately in the empirical equation up to 200 mph.

I doubt if you will have any more power at 4800 RPM, but the lab prop will be faster because it will be more efficient. With thinner, identical, vibration free blades a higher percentage of propshaft power in will be converted to thrust HP out. More speed means more RPM. I had a 7.4L - 310 HP, which is a very similar engine, and it went faster spinning a 23 @ 4600 than a 21 @ 5000, so asking them to increase pitch to maintain 4600 might work, I don't know.

The real answer is to port the heads, bump up the cam, change intake and open up the exhaust. See below for more data...
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Old 03-05-2002, 11:57 AM
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thanks for the reply tomcat.

as long as you are saying that the drag (force) due to hull/water friction is a linear function of speed, then I am in total agreement with you on the sumation of forces. but (i thought) the friction drag is also a function of velocity squared and therfore the power (force x velocity) is a function of velocity cubed.

lets agree, (either way, I reserve the right to get smarter) so I'm not tootin my horn and it's full of poop. i want to understand the details. This is our (my and your) only discrepancy. upon resolution, we will be in "violent agreement"

can you resend the graph of my 330's to fill the whole screen like a previous post of yours, I can't quite see the numbers.

As far as engine mods, my buddy waterfoul has bought his boat and engines three times over now. Keeps breakin stuff. I like stock for now. ( i am faster than him. he hates that)
Heck, I'm still savin pennies to lab the props! Next, drill out the silent thunder some, maybe new intakes, and upgraded ignition. after that. sell the formula for a newer one with 502's or 500hp's. that's the 5 year plan.

thanks
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Old 03-05-2002, 01:59 PM
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Rambunctious - I tried to post your graph the same large size, but the board would't accept it. Funny thing is both the small and large size show as less than 640 x 640 pixels under properties. What gives?
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Old 03-05-2002, 02:25 PM
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640X480 Max.
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Old 03-05-2002, 02:44 PM
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OK, got it figured out. For some reason if I downsize to be less than 640 x 480, then convert to JPEG, the file properties remain at the larger size. Convert to JPEG first then downsize works.

Rambunctious - I'm assuming drive ratio of 1.5 and weight of 6500 lbs. Here's your new graph. How about a pair of HP500s?
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