View Full Version : speedo pick-up

07-05-2011, 01:18 PM
where does the factory dash speedo in an 01 classic get its pick-up from?I have a 6.2 with a bravo drive, also i hear alot about different pitches on the props, i have a 25, i can get just over 60 on the speedo and i dont have the drive trimmed out right i'm still learning that, how does the pitch affect speed and rpm? does the 25 pitch prop sound right for my application?

07-05-2011, 01:34 PM
Bravo drives have a pickup built in, so you should see the tube running from the drive itself

As for pitch, first off what is your drive ratio? My 6.2's are mated to 1.65 drives. Some seem to have 1.5 ratio like the 350's.

What RPM are you seeing at 60? What is the prop type, Mirage Plus?

07-05-2011, 02:06 PM
its a mirage plus, and the rpm's were about 4200 i'm not sure about the ratio i will have to look that up in my paperwork i will have to look for the tube,,, what color is the tube, and is it plastic or rubber?

07-05-2011, 02:30 PM
Raise the drive up all the way, bend over and look on the front of the drive about half way up. You will see a black hose of about 1/4" in diameter coming from the drive and going to the transom.
It is fairly common that they come off the small stem that it is mounted to and will need to be put back on and secured with a small zip tie.


07-05-2011, 02:59 PM
just to add, if you look in front of the water pick up on the front edge of the drive you will see a little groove..this is where the speedo gets the water through the drive,they get pluged with silt and may need to be cleaned or maybe someone forgot to hook the quick disconnect back up when and if the drive was removed..

make sure you have no tabs down and the boat is properly trimed before thinking about changing props..run the boat about half throttle then raise the trim just until you hear the prop slip,bump the drive back down until you hear the prop bite then ease into the throttle @ wot..read your rpm's..mirage + props like a little slip which may increase your mph of your boat..

07-05-2011, 03:17 PM
22's like trim too. The 6.2 has a WOT operating range of 4800-5200 RPM and you are WAY short on that. Trim out more and try again and see what you come up with.

4200 @ 60 MPH I'd have to take a guess that the drive is 1.5 if those are accurate numbers. 25 is on the large side for a 6.2 with 1.5 drive. if it's a 1.65 drive I'd think you are fine.

The ratio is stamped into the side of the drive, look at the Mercruiser label, I think as you look at the stern it is on the right side.

07-05-2011, 03:23 PM
i will check on the info u guys gave me tonite and get back to you tomorrow thanks for the info,

07-11-2011, 12:04 PM
ok i finally got around to gettin the info on my drive,,, 1.65 ratio and a 25 mirage plus i seem to get 60mph on the water pick-up speedo and 4400 to maybe 4600 on the rpm's how does that sound? 6.2l engine

07-11-2011, 01:48 PM
ok i finally got around to gettin the info on my drive,,, 1.65 ratio and a 25 mirage plus i seem to get 60mph on the water pick-up speedo and 4400 to maybe 4600 on the rpm's how does that sound? 6.2l engine

I don't know what speed you should expect with that boat/engine/drive/prop. But to your point about whether it is giving you the right performance, the first question is how fast is it really going now.

Of course if possible, I'd grab a handheld GPS to get a firmer number there. And because the GPS will save your highest speed without you having to watch it like a hawk, it will free you up to play with trim and get a more clear reading off the tach, to see your real peak RPM.

That said, maybe these numbers will act as a bit of a sanity check on the existing pitot speedo's 60 mph reading.

With 1.65 drive ratio, 25" pitch, 4500 RPM:
60.0 mph ---> 7% slip
59.4 mph ---> 8% slip
58.8 mph ---> 9% slip
58.1 mph ---> 10% slip
57.5 mph ---> 11% slip
56.8 mph ---> 12% slip
56.2 mph ---> 13% slip
55.5 mph ---> 14% slip

Not sure if 7% slip is likely to be accurate, but maybe. Seems like about the lowest slip numbers I've ever heard claimed for a "normalish" Classic were down in the 7-8% range, but I'm stretching my memory on that.

Steve (Osur866) and some others have probably measured this stuff very closely.

I don't know what speed your setup should make, and won't even guess. But maybe the slip #s will give some folks here an idea whether the speedo reading is accurate or not.

07-11-2011, 02:07 PM
Ghost,,, what are the slip #'s? the amount the hub slips??? i think my boat with the package i have is probably all it will get, the boat is sooo smooth running at WOT, i'm still gettin use to the performance of it, one other thing, there is no rooster tail at all, just long and flat, i'm not sure if that means anythin at all or even if im still not trinning the drive correctly yet

07-11-2011, 03:22 PM
The hub should not slip. It should turn 1:1 with the driveshaft, which turns 1/1.65 times per turn of the crankshaft.

Slip is the inefficiency in the prop's interaction with the water. An example will make more sense.

Picture a Jello shot the size of a toolshed. Now, take your propeller and start screwing it into the jello shot. The definition of prop pitch is how far the propeller moves forward through the jello in one revolution of the prop.

In your case, your 25" pitch prop should move forward 25 inches per revolution. In a perfect world, that would mean the boat would be pushed forward 25 inches per prop revolution. But of course, liquid water is not Jello, and the prop won't actually screw forward through the water 100% efficiently. The difference between the theoretical 25 inches per revolution and reality is your prop slip. So, if you got 23 inches of forward motion per revolution, that would be 8% slip, because 8% = (25-23)/25.

The slip numbers I listed above are calculated by taking your 4500 RPM, dividing by your 1.65 gear ratio to get prop revolutions per minute, multiplying by 25 inches per revolution, and then converting all the units to miles per hour. At zero slip, this comes to 64.6 miles per hour. That's the fastest your set up could possibly be pushing the boat at 4500 RPM.

I think that explains slip in terms of math. As for WHAT is "going wrong", I suspect it simply has to do with the dynamics of a solid screw in a fluid. If you turned your prop VERY slowly for one revoluton, by hand, in water, the water would just get pushed away from the prop and your boat would hardly move at all (FAR less than 25 inches). Your prop slip would be nearly 100%. The faster you turn the prop, the less the water can "get out of the prop's way" and thus the more the prop will screw forward through that water. To a point, the faster the propeller spins, the more the water will act like jello, giving the prop something to push against. So, in general, slip numbers tend to decrease as speed increases.

I think that kind of explains what is happening. The reason for my little table of slp calculatons was to check on your speedo's reading. We came up with 7% slip, based on your best reading of the gauges (60 mph). Imagine your speedo had said 80. Your slip number would be negative. We know that something's wrong then, likely the speedo. Suppose your speedo said 63. That would give a slip of 2%, which we also know is too low, and again, we'd think your speedo was wrong.

With the 7% number, I wasn't sure if that indicts your speedo, or if it is likely to be about right. I am hoping folks will chime in and let us know if it sounds like they'd expect, or if your speedo is perhaps a little optimistic. But with:

200 RPM of uncertainty (you said your tach was somewhere from 4400-4600)
the possibility that the prop pitch isn't exactly 25"
I don't think we're going to have a terribly firm answer about whether the speedo is right. Rather, I think we're in the "nothing is clearly wrong" range.

Meaning, we think your speedo is probably accurate or at least in the ball park. And so, you're probably going close to 60.

And so now the question is, with that motor, should you be seeing much more speed than that, and similarly, should you be able to spin your 25 inch prop faster than 4600?

07-11-2011, 03:51 PM
so i can try a different pitch and compare the difference, i think i will get more seat time for now and enjoy the boat, what about the rooster tail?

07-11-2011, 03:56 PM
so i can try a different pitch and compare the difference, i think i will get more seat time for now and enjoy the boat, what about the rooster tail?

Rooster tails sure look like lost energy to me. (I won't say 'wasted' energy because they're fun to look at. :) ) My guess is no rooster tail is a good sign, not a bad one.

07-11-2011, 04:15 PM
thanks ghost, i am enjoying my boat, there are lots i need to learn about it, sounds like lots of geometry in the proper pitch, i'm pretty sure i'm at the limit on the trim at a short WOT run and tabs all the way up, and its flyin.

07-11-2011, 04:16 PM
I'll take an educated guess that your boat should run low 60's. your RPM's are currently too low so I'd go with a 23" Mirage Plus. That should add the RPM you need to get into the powerband of the engine, which should add some speed.

07-12-2011, 10:57 AM
Just stumbled onto this post, In stock form the 6.2 with 1.65 gears running a 25P M+ will and should run 67-69 mph in very good conditions running light on fuel if everything is stock and the motor healthy.

First I'd grab yourself a GPS speedo, theres a free Iphone app for that but wont log your fastest top speed, grab a passanger for that, with the 6.2, 1.65 gears and 25p M+ you should run 67-69 at around 5000-5050 rpms when trimmed to the edge and light on fuel in a little chop, my slip #'s where down in that 6-7% range with the stock set up. Question, does the boat have trim tabs on it and are they retracting in a true negative angle when raised all the way? You should be able to run W.O.T. with a bunch of trim and no tabs in the stock form, most of our classics trim tabs don't retract in a true negative angle and scrub some speed on the top end.

How many hours on the motor? Seat time can gain you some speed also, verify the gear ratio, most of the 18's with the 6.2 did come with 1.65's but some also came with 1.5's it sounds like to me if you at WOT and only turning 4,400-4,600 and your not dragging the tabs and trimmimg at least 1/4 your either short on ponies or have the 1.5 gears, doesnt matter what the speed is on gps that set up with 1.65's and 25p M+ should turn 4900-5000 rpms, do we know if the tach is at all accurate?

The picture albeit poor shows a straight edge against the bottom of the boat and the negative angle of the trim tabs when fully retracted, most leave the factory almost parrell to the bottom ans when up on plan carrying the bow they are actually still wet scrubing speed.

If you get bitten by the speed bug I've taken my 18 pretty far in the handeling and speed department from its stock form, and I'll tell ya when my boat was stock it handled like a POS compared to now, theres a few little things that will help handeling and then you can add more HP for added speed and do it more safely and more in control.

07-12-2011, 12:04 PM
I think he's got a 22 with the 6.2

07-12-2011, 12:39 PM
Sorry if I confused the issue and wasted anyone's time bringing Steve's 18 up. I mentioned that because I know he has been meticulous about making observations with many props, and thus he might have some ideas about what prop slip might make sense at around 60 mph. Possibly with a similar prop.

Still, these are more good data for the board... :)