Changing Prop or gear ratio
 

Which setup would perform better or would it be almost equal ?

Bravo 1 Prop 28 Pitch Lab Finished and Bravo 1 XR 1:35
or
Bravo 1 Prop 30 Pitch Lab Finished and Bravo 1 XR 1:50

Triple HP 525 on a 13000 pound V Bottom with Boxes and High X Dimension

Re: Changing Prop or gear ratio
 

I would call Chris Martin . HE KNOWS DRIVES. he rebuilds them and ships them all over. Pm my if you what his number.

Re: Changing Prop or gear ratio
 

I assume that you have results with one of these combinations. If you can tell me current top speed and RPM with your current combination I can shed some light on this with a graph. Are these 525SC or 525EFI engines?

Re: Changing Prop or gear ratio
 

I say the 28" prop because I think that it is the most efficient pitch but you will probably get a 50/50 split. To each his own.

Re: Changing Prop or gear ratio
 

My suggestion would be to go with the 30's and the 1.5

Your throttle response will be sharper. Your speed will be the same taking into consideration all box stock props are different although they are not supposed to be.

Brett Anderson / BBLADES Professional Propellers, LLC.

Re: Changing Prop or gear ratio
 

Goofy1103,

My simulation software shows that 30"s with 1.5 gears are not quite enough for your setup. I have made some assumptions in the inputs about your boat and motors, but below is close to what your boat should look like with 31"s on 1.5 gears. (Assumed power curve for HP525SC...) Note that the power curve assumes you aren't dragging the trim tabs.

You can run the code yourself at www.speedboatperformance.com.

Performance Prediction Input Summary

Manufacturer: X

Model: X

Length: 42 ft

Beam: 8 ft 6 in

Weight: 13000 lb

Deadrise Angle: 24 deg

Hull Depth: 6 ft

Number of Running Strakes: 4

Number of Transverse Ventilated Steps: 1

Hull has a keelpad

Boat has an open cockpit

Engine information:

Boat has 3 x 502 c.i. Normal engines installed

Engine Redline: 5400 RPM

Peak BHP is 525 BHP occuring at 5300 RPM

Peak Torque is 550 lb-ft occuring at 4200 RPM

Drive information:

Boat has 3 x Bravo One, X, XZ, XR Drives Installed, with 1.5:1 gears

Propeller information:

Propeller: 4 bladed Bravo 1

Prop Diameter: 15.25 in

Prop Pitch: 31 in

Prop is set 30 inches behind, and 5 inches below the running surface

Environmental Conditions:

Boat is to be simulated operating in Salt Water, at 0 ft above msl, with an ambient air temperature of 80 degrees F

Program Completed

Computational Results
Your boat's computed performance is as follows:

SPEED(mph), ENGINE RPM, REQ'D BHP, AVAILABLE BHP, PROP ADVANCE, WETTED LENGTH(ft)

25.00 1988 212 522 0.632 22.72
26.00 2042 223 541 0.640 22.88
27.00 2097 234 560 0.647 23.09
28.00 2154 247 579 0.654 23.37
29.00 2206 258 598 0.661 23.19
30.00 2254 266 615 0.670 22.54
31.00 2305 276 633 0.677 22.16
32.00 2353 285 651 0.684 21.57
33.00 2405 296 669 0.691 21.24
34.00 2453 306 687 0.698 20.69
35.00 2505 318 706 0.704 20.40
36.00 2553 328 724 0.710 19.90
37.00 2602 339 743 0.716 19.42
38.00 2655 352 762 0.721 19.19
39.00 2704 363 781 0.726 18.75
40.00 2753 375 800 0.732 18.32
41.00 2803 387 818 0.737 17.92
42.00 2855 401 838 0.741 17.74
43.00 2905 414 857 0.746 17.36
44.00 2955 428 876 0.750 17.00
45.00 3005 441 895 0.754 16.65
46.00 3058 458 916 0.758 16.51
47.00 3108 472 935 0.762 16.19
48.00 3159 486 954 0.766 15.87
49.00 3209 501 973 0.769 15.57
50.00 3259 517 992 0.773 15.28
51.00 3310 533 1011 0.776 14.99
52.00 3361 549 1030 0.780 14.72
53.00 3415 568 1050 0.782 14.64
54.00 3466 585 1069 0.785 14.38
55.00 3516 602 1088 0.788 14.13
56.00 3567 620 1106 0.791 13.89
57.00 3618 638 1125 0.794 13.66
58.00 3670 656 1143 0.797 13.43
59.00 3721 675 1161 0.799 13.22
60.00 3772 694 1179 0.802 13.00
61.00 3824 713 1196 0.804 12.80
62.00 3878 737 1215 0.806 12.76
63.00 3930 757 1232 0.808 12.57
64.00 3982 778 1249 0.810 12.37
65.00 4033 799 1265 0.813 12.19
66.00 4085 821 1281 0.815 12.01
67.00 4137 843 1297 0.817 11.84
68.00 4189 865 1312 0.819 11.67
69.00 4241 888 1327 0.820 11.50
70.00 4293 911 1342 0.822 11.34
71.00 4345 935 1356 0.824 11.18
72.00 4397 959 1369 0.826 11.03
73.00 4449 984 1382 0.827 10.88
74.00 4501 1009 1395 0.829 10.74
75.00 4553 1035 1407 0.830 10.60
76.00 4606 1061 1418 0.832 10.46
77.00 4658 1087 1429 0.833 10.33
78.00 4711 1114 1439 0.835 10.19
79.00 4763 1142 1449 0.836 10.07
80.00 4819 1175 1458 0.837 10.07
81.00 4872 1203 1466 0.838 9.954
82.00 4924 1232 1473 0.840 9.833
83.00 4977 1262 1479 0.841 9.716
84.00 5030 1292 1485 0.842 9.600
85.00 5082 1323 1490 0.843 9.488
86.00 5135 1354 1494 0.845 9.378
87.00 5188 1386 1496 0.846 9.270
88.00 5241 1418 1498 0.847 9.165
89.00 5294 1451 1499 0.848 9.062
90.00 5347 1485 1499 0.849 8.961
90.38 5368 1498 1498 0.850 8.862


Thank You for using www.SpeedboatPerformance.com!!

We appreciate your business and hope we can meet your needs again soon

Re: Changing Prop or gear ratio
 

Engines will be 525 EFI
I have now 496 HOs with 26 Bravo I lab finished props at Bravo 1 X running 73 MPH at 5150 RPM (rev limiter)
I already own a set of 28 lab finished Bravo 1 Props - so when buying new engines I might keep the 28 props with 1.35 gears

Thanks for the input so far

Re: Changing Prop or gear ratio
 

Given your current numbers with the 496 HO and anticipating the increase in power with the 525 I would definitely go with the 1.5 gears. I would estimate your top end speed to be about.83-85. If you had a very efficient set up you should be able to run this speed with the 28 Bravo 1 lab props with 1.5 gears. More likely you will need either a stock 28 Bravo 1 or a lab finished 30 that may need to be down pitched a little or your 28 Bravo 1's could be up pitched a little.

I would start with the 1.5 gears and the lab 28's you have now to get a baseline number and go from there.

Matt

Re: Changing Prop or gear ratio
 

Interesting that speedboatperf predict 90mph and Throttleup 83-85. You actuat results will be very interesting.

Re: Changing Prop or gear ratio
 

To gain almost 20 by adding 125 hp per motor would be highly unlikely.
We have a several customers with boats in the 42-47 foot range with triple 525's and they are in the 82-87 mph range.

Re: Changing Prop or gear ratio
 

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

Re: Changing Prop or gear ratio
 

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?

Re: Changing Prop or gear ratio
 

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

Re: Changing Prop or gear ratio
 

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

Re: Changing Prop or gear ratio
 

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

Re: Changing Prop or gear ratio
 

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

Re: Changing Prop or gear ratio
 

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. :drink:

Re: Changing Prop or gear ratio
 

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

Re: Changing Prop or gear ratio
 

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.

Re: Changing Prop or gear ratio
 

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!!

Re: Changing Prop or gear ratio
 

You guys are really great
Thanks a lot for the input
I will let You know the results

Best regards

Goofy1103

Re: Changing Prop or gear ratio
 

tcelano: Because guys are usually adding power to a hull with known "before" speed numbers, and because you don't always have accurate weights, hull conditions, drive heights, C of G etc., models are usually being calibrated with before numbers. As long as the power increase and associated speed gains are relatively small, this seems to work. If something else has changed, like a shorter drive, then it may not work as well.

I like your model; the prop advance and wetted length numbers are very informative. Do you use prop efficiency curves? Have you considered a graphical output? This is useful for seeing where the curves cross, and can give the user a better feel for how choosing prop pitch for absolute top speed will affect acceleration.

Welcome to the board, get used to having your brain picked! :D

Re: Changing Prop or gear ratio
 

Tomcat,

Thanks for the welcome! My model is based on Savitsky-based mathematics, with lots of extra hydrodynamics and aerodynamics added in to account for aerodynamic lift and drag on the hull as well as gearcase drag.

The basic problem with most available methods is that you have to choose from an infinite combination of running trim angles and keel depths. My research yielded a method for doing so, and is based on longitudinal/vertical stability (porpoising) limits. Essentially, my code is showing you what you would get if you could balance your boat right at the threshold of porpoising for every speed.

I also developed a propeller algorithm based on everything I could find on supercavitating prop tests. Being an engineer, I know that no matter how complex I make it, there will always be factors that can't universally be explained, and the marine environment has more than its share of those.

I agree with your assessment on my output needing to be more graphical and user-friendly. It is hard to articulate to people just how much is behind it when all you get is a lousy string of html numbers..

I have been spending my time studying for the PE exam, so when I get that off my plate, there will be more upgrades to this code. I seem to have it dialed in very well for boats with #6's, and pretty good with other SSM's. Bravo drive boats seem to have the widest variability in results, and I think it is simply the fact that the hulls those drives are on just aren't built and maintained with the same care.

The code will make "blind" predictions for many boats quite well. Most that are off a bit predict a bit faster than what users actually see, which makes sense. How many boats out there do we really think are actually set up perfectly? How many props have pitch distributions which are dead-on? Heck, its even hard to get the same boat to repeat a performance exactly. Also, there are always variations and errors in the way parameters are measured. If it was an exact science, it wouldn't be any fun!