GPS Question
 

Yesterday we decided to compare mph readings between my mounted Garmin 192 GPS with antenna mounted just front of small Cock pit wind screens in front of driver side, and a hand held Garmin GPS.
Smooth water, no wind!
They were dead on until we hit the mid 80s, soon I saw 92mph on the small unit,later with a max speed of 94.4 mph during several miles of trimming her up, while my 192 only recorded a max speed of 88.7mph.
We were spinning 29s with a 1.35 ratio at 5500 rpms!
What could give us a 6 mph difference between the two units?

Re: GPS Question
 

I've seen the small hand held units give readings that are off quite a bit. I would think your 192 is the accurate one. You could test with another unit or test it against radar.

Re: GPS Question
 

Ditch the 192 :D

Much better bragging rights with the handheld :drink:

Re: GPS Question
 

My handheld and my GPS speedo read almost dead on. I wouldn't think the 192's method of recieving the signal and calculating speed is any different than the handheld. I think with the higher end GPS units you are paying for the built in functions and mapping rathing than the quality of the receiver. Maybe a malfuntion?

Re: GPS Question
 

there are two different techniques used to derive speed from GPS.
The first technique takes consecutive position measurements and calculates delta distance vs delta time to arrive at speed.
The second technique actually measures the Doppler off the carrier waves from the GPS and calculates "instantaneous speed"
Given the error associated with the GPS position it is quite possible that the calculated velocity can jump momentarily.
The instantaneous technique is much more accurate and will not have as much error associated with the measurement.
Furthermore, a momentary jump in velocity can occur due to the constellation of the GPS satellites. If one of the satellites used by the GPS receiver is no longer in view than the position and subsequently velocity will jump. and conversely if new satellites come within line of sight the calculated position and velocity will also be affected.

Two different GPS receivers can also have differing sensitivities causing one unit to see more satellites than the other, thus, yield a more accurate reading.

in summary, GPS receivers capable of providing instantaneous velocity are more sophisticated then the derived velocity GPS units and most likely more expensive. I doubt any of the GPS receivers we are talking about actually do this. The problem is most likely a combination of the error associated with GPS position and changes in the GPS satellites the unit can see.

Re: GPS Question
 

Sorry don't want to be Mr. Obvious but just borrow a third unit find out wich one is broke and get rid of it.

Re: GPS Question
 

Glenn, I have a hand-held you can try, 94.4mph that Gun is flying! Thanks for the everything at Rock The Bay. Bill

Re: GPS Question
 

Glen If you were to do the math 89mph is 20% prop slip 95 mph is 15% prop slip. 95 seems about right. I know you hull is kevlar, do you know what it weighs?

Re: GPS Question
 

My Top Gun is approx 9250lbs dry with 800+ hp @ 5600rpms per side.
We have pics from this weekends poker run and @ speed the only thing in the water are her drives, after 80 or so she is light as a feather but if it's rough enough she is very stable.
As I don't often run her trimmed up and wide open I would like to really know what she will honestly do .
If I can get my hands on a third unit, Fri eve we will give it another shot down the bay, Rudy our engine builder/ rigger and Billy from Diamond performance have this boat dialed in and really ripping, I believe we should see 95mph out of her in the rough.
This GPS thing has me a little perplexed as all summer a few people have questioned it's accuracy after 85mph @ approx 5000rpms vrs another 500 trimmed up rpms for only 3 mph or so GPS
Thank you all for your input and the truth is that other than for 5 seconds of bragging rights that know one cares about anyway, weather she can turn 88 or 95mph she is a true blast to run and with these new mufflers is also a real sleeper we can actually talk and here what people are saying now @ cruise speeds , I love it!

Re: GPS Question
 

:drink: :drink: :drink: :drink:

Re: GPS Question
 

Glen if you go to bam marine's site, go-fast.com. click the knowledge base and then speed calculations. Enter your spec. you will see 94mph is Just about right. I am curious what a different GPS indicates

Re: GPS Question
 

That calculator doesn’t account for prop slip. You need to subtract your slip to get a good speed rating. This is according to Mercury Racing.

Re: GPS Question
 

My Garmin 172c and Gaffrig analog GPS speedo with recall usually read within 1 MPH of each other. What was your speedo reading?
The only difference I see between mine is that the Gaffrig updates slower, and on a short top end burst may read slower.
Regards,
Steve

Re: GPS Question
 

My speedometer quits at 80mph, on the small unit last I looked I read 92mph and rising and on the 192 the recall later read 88.7mph .
We were running way to fast for me to pay a lot of attention to both. I saw my tacks @ 5400+ and my small GPS towards the end of our few mile run.
On Fri I will hook up a new 100mph Livorsi recall GPS speedo and give it another shot along with the other units!

Re: GPS Question
 

Glen Well ? did you install the GPS speedo? What is your real speed?

Re: GPS Question
 

Unfortunately I never did install the new GPS last weekend ,to busy with other things. I will surely up date this thread when she's in as I am curious as well..

Re: GPS Question
 

WW:
One other thing you might check. My 172c was shipped from the factory with the speed reading preset in knots per hour.
It was doing the same thing as yours, and really f'ing me up until I noticed it said kt beside the readout.
When I reset it to mph, I noticed there was 3rd option of statute miles which would also result in goofy readings. You've probably already reset yours to MPH, but FYI anyway.
Let us know how it turns out. By the way, I mounted my antenna under the deck, and it works perfectly.
Regards,
Steve

Re: GPS Question
 

I use the GPS to check top speed but monitor my Gaffrig(non-gps) for trimming, cruise, etc.. Its always 1-2mph faster then the gps but its much quicker to react to small changes in trim or throttle.

Re: GPS Question
 

Without knowing too much about the boat, here is what the performance of a boat such as this should be:

Manufacturer: Cigarette

Model: Top Gun

Length: 38 ft

Beam: 8 ft 6 in

Weight: 9800 lb

Deadrise Angle: 24 deg

Hull Depth: 6 ft

Number of Running Strakes: 2

Number of Transverse Ventilated Steps: 0

Hull does not have a keelpad

Boat has an open cockpit

Engine information:

Boat has 2 x 572 c.i. Supercharged/Intercooled engines installed

Engine Redline: 5800 RPM

Peak BHP is 800 BHP occuring at 5600 RPM

Peak Torque is 850 lb-ft occuring at 4400 RPM

Drive information:

Boat has 2 x Bravo One, X, XZ, XR Drives Installed, with 1.35:1 gears

Propeller information:

Propeller: 4 bladed Bravo 1

Prop Diameter: 15.25 in

Prop Pitch: 29 in

Prop is set 30 inches behind, and 6 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 SLIP WETTED LENGTH(ft)
25.00 1989 219 432 39.17 21.81
26.00 2037 228 447 38.20 21.34
27.00 2085 237 462 37.29 20.91
28.00 2133 246 478 36.42 20.50
29.00 2182 256 494 35.60 20.13
30.00 2227 264 510 34.75 19.56
31.00 2277 274 527 34.01 19.23
32.00 2323 282 542 33.23 18.71
33.00 2372 292 560 32.57 18.43
34.00 2418 301 576 31.85 17.95
35.00 2468 312 594 31.25 17.70
36.00 2515 322 611 30.60 17.27
37.00 2562 331 627 29.97 16.85
38.00 2612 344 646 29.45 16.65
39.00 2659 354 663 28.87 16.26
40.00 2706 365 681 28.32 15.89
41.00 2753 376 698 27.79 15.54
42.00 2804 390 717 27.36 15.38
43.00 2852 401 735 26.87 15.06
44.00 2899 413 753 26.39 14.74
45.00 2947 426 772 25.94 14.44
46.00 2999 441 791 25.58 14.32
47.00 3047 454 810 25.16 14.04
48.00 3095 468 828 24.75 13.76
49.00 3143 481 847 24.36 13.50
50.00 3191 495 865 23.98 13.24
51.00 3239 510 884 23.61 13.00
52.00 3288 524 903 23.26 12.76
53.00 3340 543 923 23.00 12.69
54.00 3388 558 942 22.67 12.47
55.00 3437 574 961 22.35 12.25
56.00 3486 590 979 22.04 12.04
57.00 3535 606 998 21.74 11.84
58.00 3583 623 1017 21.46 11.64
59.00 3632 640 1036 21.18 11.45
60.00 3681 658 1054 20.91 11.27
61.00 3731 675 1073 20.65 11.09
62.00 3784 698 1093 20.47 11.06
63.00 3833 716 1111 20.22 10.89
64.00 3882 735 1129 19.99 10.72
65.00 3932 754 1147 19.76 10.56
66.00 3981 774 1165 19.53 10.40
67.00 4031 794 1183 19.32 10.25
68.00 4080 814 1200 19.11 10.10
69.00 4130 835 1217 18.91 9.965
70.00 4180 856 1234 18.71 9.825
71.00 4229 877 1251 18.52 9.689
72.00 4279 899 1268 18.33 9.556
73.00 4329 921 1284 18.15 9.427
74.00 4379 944 1300 17.98 9.300
75.00 4429 967 1315 17.81 9.178
76.00 4479 990 1330 17.64 9.058
77.00 4529 1014 1345 17.48 8.941
78.00 4579 1038 1359 17.33 8.827
79.00 4630 1063 1373 17.17 8.715
80.00 4684 1094 1388 17.08 8.720
81.00 4734 1119 1401 16.93 8.612
82.00 4784 1145 1413 16.79 8.507
83.00 4835 1172 1425 16.66 8.404
84.00 4885 1199 1436 16.52 8.304
85.00 4936 1226 1447 16.39 8.206
86.00 4986 1254 1457 16.27 8.110
87.00 5037 1283 1467 16.14 8.016
88.00 5087 1311 1476 16.02 7.924
89.00 5138 1341 1484 15.91 7.834
90.00 5189 1370 1492 15.79 7.746
91.00 5240 1401 1498 15.68 7.660
92.00 5290 1431 1504 15.57 7.576
93.00 5341 1463 1510 15.47 7.493
94.00 5392 1495 1514 15.37 7.412
94.71 5429 1518 1518 15.27 7.333

Looks like the 94-95 GPS unit might be the winner. If your temperature was cooler than the 80F I assumed, or if your motors are actually 800 "+", then you should see a little more. Note that my predictions always adjust the motors' output based on ambient temp and altitude. The wind drag also changes some with these parameters. This program actually calculates propeller slip based on individual blade loading. The amount of cavitation and ventilation vary with prop height. The code computes exactly how high the water will rise from the transom to where it meets the drive unit and propeller based on trim angle, drive height, setback, and speed. So, not only can I predict propeller performance using the method, but I can see how much drive drag you will get with changes in installed height for each different type of drive unit.

It sounds complicated, and it is, but it works.

Go to http://www.speedboatperformance.com, and run the calculator there. It does account for propeller blade loading, drive height and setback, as well as hull configuration. Give it a try while it is free!

Re: GPS Question
 

Isn't the gun an 8' beam?