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Author Topic:   UPDATE 3/28/01-GPS bought -40hp 4 stroke mph verified and confirmed.
awares posted 03-28-2001 08:51 PM ET (US)   Profile for awares   Send Email to awares  
Today was a very cool day and the water was running pretty good on Lake Austin after the 3" rain we had yesterday. Lake Austin is the Colorado river that runs between Lake Travis and Town Lake.

I wanted to be sure I got all of the variables for Clark's formula. I went to the Central Transportation Systems certified scales today to weigh the rig so we could get the actual weight less the driver.

I did not have time to unload the boat and get the trailer weight. I will try to do that this weekend. The boat with the trailer weighs in at 1300 lbs. That is full of gas and the exact way I ran it last time. Plus I had alot more clothing on tonight.

The only missing variable now is the trailer weight that needs to be subtracted from the 1300 lbs.

Also my weight is about 205 lbs. Depending on how much mexican food I had at lunch.

Tonights results are as follows:
Boat ran great and gave me the exact same readings at 5900rpm's, I achieved 45 mph per the Lowrance X65 paddle well speedometer.

The new Gps I bought is the Garmin 3 Plus. I could not wait for the new series Garmin's to become available. This will be my back up unit. Very easy to use and matches my truck speedometer. Plus I got an additional 10% off because I opened a charge card with West Marine.

Readings between the gps and X65 are not consistent unless I go perpendicular to the current. Lake Austin is way to narrow for a good test.

The readings with the gps falls in line with Clark's formula I think. The gps top end speed at 5900rpms indicated between 34.6 and 35 mph.

Conclusion: The stock prop of 15" is diffently not a good match with or without gps readings. The hole shot was terrible and my X65 speedometer would not read above 37mph. After the new prop 45mph.

Do not buy the X65 speedometer option because based on the gps it is worthless.
I have not tested the odometer function for accruacy. So my mpg is now in question also.

Question: What was the original performance of the stock prop based with the gps readings? I will retest with old prop.

Clark please run the numbers for us on the new known factors.

Additional question: What do I do now? I am happy with the performance.

Clark Roberts posted 03-29-2001 08:27 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
OK, add 205lbs and subtract 500lbs (my guess at trailer weight) and gross weight is 1000lbs... with 180 constant the predicted top speed comes out to be 36mph and with 200 constant it's 40mph... futher testing will reveal which consant is closest! and remember that I'm guessing at trailer weight, and that 100lbs is about 1mph either up or down!! Measure your speed with at least runs in oposite directions.. A GPS measures speed over the ground (not through the water) and tide/current can add or take away from "water speed"... Clark
awares posted 03-29-2001 10:26 AM ET (US)     Profile for awares  Send Email to awares     
What other variables do you need other than the trailer weight?

I am trying to keep everything constant from one test to the other. Same things in the boat with exact same amount of gas and equipment. This is also the way I had it weighed.

Another question: What are we actually trying to achieve? Is are final goal 100% propeller to motor performance.

Remember that my one and only goal was the hole shot issue. That problem was corrected with the Pro Tech prop. Nettle's Prop Shop hit that goal right on the very first time with the prop they selected. As I stated earlier top end speed is not a concern. I am not trying to build a racing machine.

I used only the indicators I had until I bought a gps yesterday and it was the X65 speedometer function. The other indicator was the actual time it took to bring the boat on plane.

Let me know what else you need or what variables I need to keep constant. I will keep testing and reporting this if it brings value to us and others. Otherwise I am more than satisfied.

Clark Roberts posted 03-29-2001 10:48 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Awares, the formula only allows one to determine what optimium top speed should be! If a boat's top speed is way off then it is an indicator that improvement is possible! That's all.... you decide! Seems like you have done a good job and no improvement may be possible without sacrificing hole shot... which you don't want to do! Happy Whalin'.. clark.. Spruce Creek Navy
bigz posted 03-29-2001 10:50 AM ET (US)     Profile for bigz    
Allen, if your happy with the set up as it stands, which in my opinion sounds pretty darn nice, then leave well enough alone and move on to bigger and better things!

You can only "tweak" so much and as Clark has stated most of this exercise is still just a good bit of "SWAG" or as he says "rule of thumb". One can drive themselves nutty --- basically you have a 13 boat which appears now to be performing very well, enjoy it!

Just my two cents worth Tom

Lil Whaler Lover posted 04-03-2001 12:13 AM ET (US)     Profile for Lil Whaler Lover  Send Email to Lil Whaler Lover     
Belatedly I would like to thank Alan for getting back to the forum so promptly with the GPS information on his boat's performance. I, too, think that the new figures speak of an excellent performing boat and congratulate him on its performance.
jimh posted 04-03-2001 09:13 AM ET (US)     Profile for jimh  Send Email to jimh     
This is all interesting stuff. Perhaps I can make this Part-2 of the Propeller article, showing how Clark's forumula works, etc.

How about a few pictures to document/illustrate.

Don't sell the paddlewheel speedo short. It seems very accurate at trolling speeds, and fishermen tell me that getting the trolling speed right can be important.

Keep us posted on further tests and results.


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