Just replaced the l/u on my 1998, 225 rude with a 2005 factory l/u...the water intake is different and there is no speedo hole...I have lost some speed, around 5mph, but run the same rpm at wfo (6000)....could it be the hydro-dynamics of the old l/u are different then the new one....or is it the 95+degree temps here in fla thats got my extra 5mph...<>rob<>..
I would tend to believe the latter..SMG
Does the new gearcase have the same gear ratio as the old one? If not, that could be one explaination for the difference.
Are you measuring speed with a GPS or your speedo? Regular speedos aren't very accurate so I wouldn't trust one to measure true speed.
Good Luck!
IF the replacement gearcase was a Magnum series, it would be about 3/8" larger bullet diameter which slows down fast boats around 5mph.
If it has plastic replacable water inlets and your old gearcase had the round cast holes, then there will be change in speeds due to bullet length and the hydradynamics.
You could have gotten the Lightning gearcase which would have bolted right on.
-----
A Technical troubleshooter possessing more tools than talent !
It's probably a combination of the larger gearcase and the high temps. See if you can trade it back for the lightning case, it will be a couple of mph faster than your old case and will be more fuel efficiant than the larger case you have now.
A little speedo theory, a pitot style speedo is actually a better test instrument than a gps as it measures speed over water and not speed over ground like a gps does. The pitot speedo may be off by say 5mph but it will always be off by that much. So if you change props and your speedo shows 2mph more than it did before, then you know that you picked up 2mph. While a gps is extreamly accurate on flat water with no current it can be off on speed over water and your prop change may actually show more or less speed than you actually gained due to current and wave direction. Now for the disclaimer, if you went from a gearcase mounted pitot system to a boat mounted pitot system then you will see a change in speed because of the different water preasures of the two different pick-up areas. This may be where your 5mph went.
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Neveredge »</TD></TR><TR><TD CLASS="quote">A little speedo theory, a pitot style speedo is actually a better test instrument than a gps as it measures speed over water and not speed over ground like a gps does. The pitot speedo may be off by say 5mph but it will always be off by that much. So if you change props and your speedo shows 2mph more than it did before, then you know that you picked up 2mph. While a gps is extreamly accurate on flat water with no current it can be off on speed over water and your prop change may actually show more or less speed than you actually gained due to current and wave direction. Now for the disclaimer, if you went from a gearcase mounted pitot system to a boat mounted pitot system then you will see a change in speed because of the different water preasures of the two different pick-up areas. This may be where your 5mph went.</TD></TR></TABLE>
I'm far from an expert, but I'd say there are some holes in your Theory. First off, water conditions, wind, boat setup, etc. will definitely effect the pitot speedo. None of these conditions should effect GPS as it's based on satelite positioning. I'm not saying the GPS is as accurate as radar, but it's as close as it gets. I don't see where your getting your info that water conditions will affect gps accuracy. How's that possible?
Also, changing props could potentially have an effect on the pitots accuracy. Different props cause the hull to run at different heights (stern and or bow lift) and since the pitot is mounted to the hull, its contact with the water could change. Therefore, 2mph may not actually be 2mph. Sorry to be so disagreeable, but your theory just doesn't make sense to me.
We should probably make a thread on this on the general board but I'll give you one example wich is a little extreame but is proof pudding!
I was doing a couple of photo runs in my Bullet and running against a 2mph current. My gps read 83.6 mph but my actual speed over water was 85.6 mph. Had I been going downstream with the current my gps would have read 87.6 mph but I would still only be going 85.6 mph over water. The gps speed is speed over ground and had there been no current at all the gps would have shown 85.6 mph because speed over ground and over water would be the same. Most boating isn't done in such currents but the gps cannot compensate for any current or cross currents (lakes and waves) and the differences are enough to effect test results. A pitot system would have shown no changes in speed whether going up current or down current. Or across current.
You are correct that different props will affect the pitot reading but not as much as you are thinking. If the pitot is in clean water (from the boat bottom) and is set up correctly it will not show much change due to different transom angles. Water density can play a role in pitot accuaracy but you are testing in the same density so your results are still reliable.
I forgot a couple of points, water conditions, wind and boat setup do not effect pitot speed in the test enviorment.
Waves will effect the pitot speed reading but you aren't going to be doing high speed testing in heavy waves that are big enough to effect the pitot. Wind has no effect on the pitot at all. Boat set up will not effect pitot readings unless the pitot is not set-up correctly to begin with.
All that said there is a way to use a gps accuatly for testing. You must use the same stretch of water and drive in the exact same direction for each test run. If you do not change this parameter then the gps will work fine as a test instrument. I just find it hard to always have the same stretch of water open and free when I want to test.
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Neveredge »</TD></TR><TR><TD CLASS="quote">I was doing a couple of photo runs in my Bullet and running against a 2mph current. My gps read 83.6 mph but my actual speed over water was 85.6 mph. Had I been going downstream with the current my gps would have read 87.6 mph but I would still only be going 85.6 mph over water. The gps speed is speed over ground and had there been no current at all the gps would have shown 85.6 mph because speed over ground and over water would be the same. Most boating isn't done in such currents but the gps cannot compensate for any current or cross currents (lakes and waves) and the differences are enough to effect test results. A pitot system would have shown no changes in speed whether going up current or down current. Or across current.
</TD></TR></TABLE>
Given your scenario, your gps reading would be the correct speed that your boat was traveling. All else being equal, your boat would actually go faster down stream than up. How could it not? The pitot is only measuring your speed relative to the water you are in, not from point A to point B. The current actually affects how fast you get from A to B. I understand how you could arrive at your point of view, but where you're getting off track is with how the gps measures speed. The gps measures the location of your device relative to satelites in space. When your device moves, it displays the speed, which it calculates based on your change in coordinates as measured by satelites ( not by points on land).
As far as I know, it is commonly accepted that radar is the most accurate way to measure the speed of a moving object. When the guys on Scream & Fly do speed runs, the gps and radar measurements are usually very close AFAIK. I think if you were to compare pitot tube speedos to radar, in general you'd find that they'll vary much more. In summary, pitot speedos are just inconsistent. Some are fairly accurate and some are not. It's a crap shoot as to whether you end up with a good one or not.
For most folks, the old pitot tube speedo is a perfectly fine tool to use. But for the gentleman who originally posted, it sounds like he could use something a little more accurate to determine if he has really lost 5mph by swapping gearcases.
I am very familiar with how a gps works as I was using them in the military before they were available to the public. They can do some other things that you wouldn't believe possible but that is not available to the public yet!
I am not saying that the pitot is more accurate than the gps, I am only saying that for measuring work done by the boat, motor, prop combination the pitot is a better test instrument for the exact fact that it doesn't measure point A to B. As you said my Bullet was actually going fatser downstream, that is true, speed over ground. However speed over ground is useless in determining work done. The 2 mph current was giving me 2 free mph on my top end and that my set-up didn't earn (no work done to accomplish the added 2 mph) A radar would also show this added 2mph. But if I were going upstream the radar and gps would both show a 4 mph slower speed than my downstream run. This is pretty cut and dried when running straight with or against a current but it is hard to quantify when running cross current wich most of us do.
Only an accurate speed over water can measure work done. You need to measure work done in order to detrmine if the prop or engine height change or whatever you did actaully worked. If I were testing on that river again I would sure not want a gps with a possible 4 mph swing. Give me the pitot system so I will know the effects of my changes.
I see your point. I think it's fair to say they are both useful measuring tools for different situations!
if you gain 35 mph going down river will that not show on the GPS? this is too funnygoing down current and up current- does that not put a different pressure on the line from the pitot- and why- are you going faster or not? does not make much sense what you say
now if you want to gain some speed easily, get you a B&WB gps- that's an instant 5mph
Phoenix 721 Pro XP, 250 Pro XS, aluminum gravel bar modified prop, paper sack tackle storage, ugly stik pro team, color c-lector pro team, TEMU Fishing Team
What does work done have to do with anything? You are looking for an accurate speed measurement, SOG isn't your issue with a pitot tube, in theory yes a pitot tube is a great instrument and under labratory conditions would be just as accurate as a GPS, maybe! But on a boat, it won't be accurate, measurements from run to run can very, water density, bubbles, angle, a lot of things can very! A GPS is the most accurate and accepted way of measure speed over the water, in situations where there is current, make a run up stream and a run down stream and average the two for the most reliable speed estimate.
I disagree, a pitot tube that is set up correctly (assuming you are using a quality gauge) will have very good repeatability from run to run.
While it sounds good to just average two runs in opposite directions with a GPS, unless you are on a straight stretch of river you will have to contend with cross currents. There is a way to quantify the cross currents but you would need to know the exact direction and speed of the currents to do so. Just like adjusting for wind direction and wind speed in an airplane.
Water density will not change enough from run to run to make a difference. It might make a difference going from summer water temps to winter water temps but that would mean that you are testing under diferent circumstances which is a no no. Of course if you run from a fresh water river to the sea you will encounter changes but how many of us test our boats in the ocean?
how can current affect a GPS reading? if it is pushing you to the side it will either add or subtract- that is a real speed difference. the GPS measures the sog no matter if you're going down current, up current, in the ocean or on a magic carpet. the first 3 will affect a pitot, the latter is only affected by the quality of the smoke you bought
Phoenix 721 Pro XP, 250 Pro XS, aluminum gravel bar modified prop, paper sack tackle storage, ugly stik pro team, color c-lector pro team, TEMU Fishing Team
You are the only one who thinks that. A pitot tube will not be as accurate, I don't care how well it's setup, that is just how it is. Run your boat on a lake, then you don't have to worry about current.
Alliss, You need to know speed over water in order to determine work done. Speed over ground is what a gps measures and will only show work done when it is equal to speed over water. Basically when there is no current at all. As for the pitot reading higher or lower due to current flow that is not possible. The motor is working in a fluid envioroment, and you are measuring speed in that enviroment. I can't explain it very well but if you want you can borrow my pilot dvd's and it explains it really well.
massbasser, even lakes show currents and I do run on them but I test in the canal behind my house and the local river, both of which have current.
The gps is extreamly accuarate for measuring SOG, far better than a pitot system but while a pitot system may not be as accurate as to true speed it will be the same run after run. It may read 5 mph higher than actual speed but it will read that for all runs done under the test conditions.
Basically we're splitting hairs here as most people don't need super accurate test results to determine if a combination is working for them. But if I was setting up a race boat I would deffinatly want the more accrate test instrument.
yeah, I'd like to borrow that. I'd like to learn what you're talking about.
Phoenix 721 Pro XP, 250 Pro XS, aluminum gravel bar modified prop, paper sack tackle storage, ugly stik pro team, color c-lector pro team, TEMU Fishing Team
While I try to see both sides of this discussion, I would like all of you to remain friends.
Reply With Quote
