I have to ask this question. This weekend, I installed halo type drive showers on my boat. I have two pick-up ports that extend just below the cavitation plates on each drive. Here is my question: I lost 4 mph on GPS on my top end? Has this happened to anyone else? They said that this would not happen. Did I install these wrong? Any thoughts?

I put a Simrek halo on my 28' Powerquest and lost not .1 mph - there was no difference in the performance of the boat, other than my drive was cooler. Hard to believe that those little tubes could cause enough drag to notice, let alone cut 4 mph....

Depending how fast you are going some speed could be lost with the installation of a drive shower. 4 MPH seems like a lot, but I have heard the speedo pitot tube is worth 1 MPH on boats going over 80, so if the showers have 2 pick ups per drive I suppose it is possible to lose a couple MPH. Just get rid of the showers and dump the water out of your pressure control valves on your drive tops.

Hmmm, that's wild - I would've never thought it could have that sort of effect. Probably depends on hull, speed, etc etc.

Learn something new every day, I guess!

I'm like vette I have never heard of more than one mph which extra gas or the weather could cause ,but it does work better to use the dump water

Which speed was your GPS reporting ???

Over water or over ground ??

Maybe the original number was reported over ground with a 4 mph current

G

GPS readings are from a satellite and are not affected by current, the boat may be, but a GPS is as accurate as you'll get, land, sea, air!

I don't know........Could the props be cavitating at speed ?
i know the pickups on drive showers can cause cavitation at
slow speeds {coming on plane} if they are to deep......

It depends on what you are measuring

Tell us the scenario behing the GPS readings.

I loose 4 MPH on the GPS every time I turn around and drive up stream.