Speed Loss From Speedo Pitot?
 

I plan to do a with and without comparison later when the water is warm enough for swimming but right now I'd like real world experience if anyone has experienced top speed loss from dragging a mechanical speedo pitot like this one on my boat. My average top speed is around 83 mph on gps.
http://i763.photobucket.com/albums/x...pse7d5dc34.jpg

I didnt even know people still used pitot speedos. Is 83mph from the pitot reading?

Just curious, is that speedometer or GPS speed ? MT, I guess we're wondering the same thing.

You will see a speed loss when you go to GPS . :frog:

No. I have gps on my chartplotter and also a handheld. 83 is average top gps speed. 85.8 is highest ever recorded speed. (no, not on the river!)


I want to keep the stock analog speedo and it only reads about 2 mph high on the top end. Looks more impressive to passengers too :) . If that pitot is scrubbing a couple mph though I will install a gps speedo in the dash. I've heard that a pitot can rob up to 5 mph. Has anyone seen that? I can't do the back to back runs until the water warms up.

I was gonna say not if you asked the speedo. it is amazing how much drag a GPS signal induces. slows the boat down 5 mph sometimes...

I doubt you'll see more than a half mph, if that.

that pitot is less area than a barnacle. pushing a hull thru water at speed is a whole crap load of drag. that's why 100 hp for 10 mph increase is optimistic. you will never measure a diff from something that small. but you will pick up 5 mph just by believing what that pitot has to say.

OK, I have heard people claim to have seen a measurable loss from a pitot. Try to hold your finger in the water at 80+ mph! One person claims to have lost 4 mph which didn't seem possible to me. That's why I was looking for other input here. Here is a pic of my dash and the original speedo I'd like to keep working and a pic of my best recorded top speed.
http://i763.photobucket.com/albums/x...g212Medium.jpg
http://i763.photobucket.com/albums/x...Picture020.jpg

The issue with a Pitot is not the drag of the Pitot itself, it is the result of the water disturbance caused by the Pitot that can affect the prop.

It looks like it would be easy enough to remove it - just plug the tube and pull it up out of the way. That way you can use your GPS to settle the argument once and for all. Don't forget to post your results up on OSO for us! :D

I dont think you have to remove it, just loosen the set screw slide it up.

It is very easy to remove the tube. I just have to get in the water to do it!

If you prove it makes a difference, I will be removing the pick up for my live bait tank. I don't use the tank and it's pick up is about 4 times the size of any pitot tube pick up!

RR

I had one similar to that on my 24 Superboat. No speed difference. I got tired of constantly fixing or replacing it so I went to a GPS speedo. It got bent all the time from chit in the water hitting it.

If you already have the GPS, why not switch to a GPS speedo?

I also saw no differance in speed with or w/o it.

Wile proving to the keyboard reacers and "skeg guard will loose you speed" I tested in real life on my boat. With and w/o my pick up, speeds were the same. With and w/o skeg guard, speeds were the same. But I only play in the mid 50's MPH (55-57 depending on load) , so maybe at 80+ mph, but it made no differance in my boat.

OK, sounds like it's not hurting my speed. Thanks for the info. I don't want to replace it because I already have the giant Livorsi liquid filled speedo that works fine and I have two gps's on board now for true speed readings. Can a dash mount gps speedo be piggybacked to my chartplotter? I don't think it can so it would need it's own puck. I had to install the pitot years ago when I put on the Imco shorty. I got the spring loaded one so it can't get torn off the boat from debris or seaweed.

You can use the NEMA output from your chart plotter to provide a signal to the GPS speedo. That way, you only need to buy the gauge for about $200-$250 or so.
Eddie

I have always been curious about the same thing. I used to hear all kinds of stories. I took my boat apart before I ever had a chance to test with/without. Something to remember about GPS is reading speed over the ground versus a pitot which is showing water speed.

Where I grew up (Lake Michigan) in most cases the difference was of no consequence as there was not much current or tide. However down here the gulf stream average is about 4 mph, so running in opposite directions would give you an 8 mph spread.

Gaffrig (the original, may he RIP) tested his first speedos on my old Cig. We bought a radar gun and did runs up and down the Chicago River. They were amazingly accurate. It all depends on the quality and calibration of the gauge that is used.

It was also lots of fun to cruise down LSD, turn the radar on and watch the sea of red tail lights as everyone's radar detector maxed out.

Good stuff. Thanks Eddie, Marc.

I had a Norskog years and years ago that had a similar pickup, (well the pickup tube was identical, it was just hard mounted to the transom--bought many a pickup tube) anyway, Norskog told me that total topend would be effected 1/10th to 1 MPH. When GPS became affordable, I never really could tell much difference at 80mph, maybe a mile an hour? When I tore off the tube (again), I just used the GPS. They do make analog looking speedos that are GPS. I think on my next remodel that's what I'm going to do. I like the speedo sitting there, but I'm old school! haha!

That outdrive dragging in the water is slowing you down too, get rid of that BAHAHAHAHAHA :bigbird:

Yes, big time, but an arneson is not in my near future!

Speedo tube used to cost me 1 mph at 100.....bitd

I'm betting drive showers do the same...

Pilot tube never seemed to effet speed on my boats much if any. Drive showers seemed to though on several of the boats I installed them on.

A lot of the drive shower installs I see have the tube sticking down WAY further than it needs to be.

I actually did. 3mph. It had an adjustable pick-up. I did not notice it lowered itself and I spent a good amt of time trying to figure out what was causing the extra prop slip.

Finally noticed it while not looking at it specifically...LOL.

I had mine about 1/4" below the cav plate. Slowed me down ~2 mph. Cut the tubes to raise them 1/4" above the plate, and got the speed back. Video shows the shower works just fine at that setting.

Just read an old article out of hotboat magazine this morning while I was well you know...... They calculated the pressures on that kind of pitot tube at different speeds. Pressure was dramatic, 146 lbs. at 100mph, over 200lbs at 120 if I remember correctly. I'll take a shot and post it tonight. That will translate into some kind of significant speed decrease.

I had a customer bring his boat to me to tune. We met at the launch and he was already in the water. We tried to get the boat on plane and the prop would break loose every time as soon as the nose started to break over. We tried 10 times to get it on plane. It was a new prop and he thought it was the problem. We put the boat back on the trailer so that I could see the prop. I noticed his drive shower pickup was less than 1/2" from the prop. He told me that he just installed it. I pulled it out and cut it right there at the launch. He told me that there was NO WAY that was stopping the boat from getting on plane. We put the boat back in and it promptly jumped on plane. The pickup was just to close to the prop and was disturbing the water so much that it caused the prop to cavitate when it needed to bite the most.
Eddie