Gps

[ZUL8TR]

pickup drag

How much drag do the water pickup's add? Can they slow you down?

More drag than you might expect. When I changed my rear transom pickup to the fin pickup the mph went up about 1mph at 60mph with no other changes when changed in a very short period during testing. Note that the Keller transom water pickups are about 4" below the bottom and are thicker than 1/8", see here for dimensions (larger pickup pic down the screen:

http://www.portagebaysystems.com/marine/products32.htm

At the fin there is already fin drag and the small 1/8" tube is equal to or less than the fin thickness at the rear and the small tule extension beliw the fin is a much reduced drag increase than the rear pickup. Of course if testing in rough water the rear pickup will provide a more consistant reading since it will remain in the water more often. However the fin pickup provides really good readings when the fin remaines in the water. Note that if all else remains constant (area, density, coeff of drag) drag increases as the V^2 and power to overcome that drag increases as V^3.

[sam]

Its not a scientifically valid test unless you switch back and get the original speed back. There could be another factor changing the speed.

[ZUL8TR]

pickup

Its not a scientifically valid test unless you switch back and get the original speed back. There could be another factor changing the speed.

Did that and the gained speed reading was essentially back to the rear pickup value. No doubt the exact conditions can never be met but over the short time (minutes) to change over I assumed that the conditions were essentially the same.

However in the grand scheme of things it doesn't matter since the spedo readings are to be used for relative changes in speed.

[Dave M]

Over Thinking

Don't over think testing. All the pressure guage is going to do is tell you what prop is faster on top end, same as a GPS. You can time your ET if you pick a point to point run and time yourself. Crude, yes, effective yes.

I can change a tenth of a mph by peeing before I get in the boat.

Do whatever you feel comfortable with and what your budget allows. I used the bolier pressure guage fo rmany many years, and I seem to have won a few championships with it. I don't race with it, nor did I ever try to, when racing it could bend the pickup as you are driving hard through a corner. I also never worried a lot on race speeds verses test speeds.