bbladesprops

You're initial question was..

what set up will perform better?

Gear ratio out weighs pitch when it comes to throttle reponse!! Stick to the 1.5's as long as you have that much room to grow with the Bravo 1 propeller family. Your speed will maintain itself. We're not talking triple digits here.

Bumping the rev limiter with a light load with a 30 Bravo 1 Lab Finished will provide 87 and change with a reasonable slip % like your current 10.18%.

Props are alot cheaper than gear replacements.

Brett Anderson / BBLADES Professional Propellers

tomcat

I'm with Throttle Up on this one. I don't expect much more than 10 mph with the increase in HP, so 83 would be good. This would work well with 30 Bravo and 1.5 ratio. Is it true that the center prop sometimes needs less pitch to RPM the same? Your slip numbers are a bit high. Have you considered 5 blades?

bbladesprops

Let's look at this logically, if the current application can turn 5200rpm with Labbed 26 bravo1's, adding the anticipated power will surely push the boat past the need for 2 additional inches of pitch. 13000lb deep v running under 11% slip is not just acceptable, it's good! Remeber 26 Bravo 1's aren't exactly 26 pitch.

Yes center engines commonly run different pitch, or as or more commonly, different diameters. We set them up like that all the time. Depends on drive heights and hull configurations. Then there are big differences with cats or V's and with steps.

Matt had great advice advice, Run the 28's you have, then determine by the way the boat talks to you, how much more your going to need. That's free.

Brett Anderson / BBLADES Professional Propellers

WETTE VETTE

Just curious, but does a 1.5 ratio truely offer better acceleration / throttle response? If a 1.36 ratio is used a lower pitch prop is required for the same top speed / RPM. To me it seems it would make no difference. Lets use a theoretical 5200 RPM target. A 1.5 ratio and 30 pitch yields a theoretical top speed of 98.5 MPH, at 4000 RPM 75.76 MPH, at 2000 RPM 37.88 MPH. Now with a 1.36 ratio the same theoretical top speed would require 27.2" pitch at 5200 RPM. With that pitch at 4000 RPM would be 75.76 MPH, and at 2000 RPM the speed would be 37.88 MPH. What am I missing?

Craig

throttleup

Wettevette,
Your logic is sound, changing the gear ratio with the proper prop pitch is a wash, theoretically.

Some of the issues that come into play is the pitch of the prop if the pitch gets too tall it starts to lose efficiency. Another factor is the rpm of the prop, or prop tip speed. As the gear ratio is raised (lowered numerically) the prop turns faster. The tips of the blades travel faster (feet per second) which can have either a positive effect or negative effect depending on the application.

The best way to find out for sure is to try every possible combination and see what works best. Of course thats not really practical so it's best to go with sound and practical advice.

Matt

bbladesprops

This question has been debated for years.

More engine revolutions per prop shaft rpm means more tourque. From my experience the throtle reponse is stronger. Not the speed. The important thing here is to put the engine rpm in the correct place for the manufacturers specs. Either ratio will work.

Ask a H2O drag racer what g.r. he would prefer.

Brett Anderson

WETTE VETTE

Thanks for the replies! I'm glad my theoretical thinking is correct, but all the other factors mentioned is what makes you prop shops the experts.

cobra marty

My point exactally that the taller pitch props will loose some efficency with a bell shape curve with the 28" at the top.

WETTE VETTE

Good point on the drag racers Brett. Most drag cars and boats aren't geared for a max MPH at a sustained WOT, but the torque required to turn gears, bearings, and the shaft through 90 WT oil is still going to be more when things are spinning faster.

tcelano

Goofy1103,

I should have gotten more info from you before posting the 90mph estimate. That was based on the assumed parameters shown, and not specifically on your hull configuration and setup. I wholeheartedly agree that if the boat will run 73 right now with the 425's, it probably will go around 84-85 with the 525's. Was the 13000 lb figure you quoted all up with fuel? The boat seems to behave analytically as though it were closer to the 14000 mark. What kind of shape is the bottom in? I've seen on my own boat that less than sharp corners results in 3-4 mph speed loss. Drive height on a triple can really affect you as well.

Here is a more calibrated estimate (i.e. assumed 14K for fuel and friends...) for your baseline with the 496's, 26P props, and 1.5 gears.

SPEED(mph) ENGINE RPM REQ'D BHP AVAILABLE BHP PROP ADVANCE WETTED LENGTH(ft)
60.00 4367 827 1112 0.826 13.69
61.00 4427 849 1122 0.828 13.48
62.00 4492 877 1133 0.830 13.44
63.00 4552 901 1143 0.832 13.23
64.00 4613 924 1151 0.834 13.03
65.00 4673 949 1159 0.836 12.84
66.00 4734 973 1166 0.838 12.65
67.00 4795 999 1173 0.840 12.47
68.00 4855 1024 1178 0.842 12.29
69.00 4916 1050 1183 0.844 12.11
70.00 4977 1077 1186 0.846 11.95
71.00 5038 1104 1189 0.847 11.78
72.00 5099 1132 1191 0.849 11.62
73.00 5160 1160 1192 0.851 11.46
74.00 5221 1188 1191 0.852 11.31
74.05 5224 1190 1190 0.854 11.16

Now, same boat, 525's, with 29P props (or maybe 28's with a little extra cupping...)

SPEED(mph) ENGINE RPM REQ'D BHP AVAILABLE BHP PROP ADVANCE WETTED LENGTH(ft)
60.00 4002 799 1232 0.808 13.69
61.00 4056 821 1249 0.810 13.48
62.00 4115 848 1267 0.812 13.44
63.00 4169 871 1283 0.814 13.23
64.00 4224 894 1298 0.817 13.03
65.00 4279 918 1313 0.819 12.84
66.00 4333 942 1328 0.821 12.65
67.00 4388 966 1342 0.823 12.47
68.00 4443 991 1356 0.825 12.29
69.00 4498 1017 1369 0.827 12.11
70.00 4553 1043 1381 0.829 11.95
71.00 4608 1069 1393 0.830 11.78
72.00 4663 1096 1404 0.832 11.62
73.00 4718 1123 1414 0.834 11.46
74.00 4774 1151 1424 0.835 11.31
75.00 4829 1179 1433 0.837 11.16
76.00 4884 1208 1441 0.839 11.02
77.00 4940 1237 1448 0.840 10.88
78.00 4995 1267 1454 0.842 10.74
79.00 5051 1298 1460 0.843 10.61
80.00 5110 1335 1465 0.844 10.61
81.00 5166 1366 1468 0.845 10.48
82.00 5221 1399 1470 0.847 10.36
83.00 5277 1431 1472 0.848 10.23
84.00 5333 1464 1472 0.849 10.11
84.19 5344 1471 1471 0.850 9.997

Definitely let us know how this turns out!!