WOT RPM too high

[Barracuda]

I have a 1974 Glastron Carlson CV-16SS that I purchased and fixed up over the winter. 140hp mercruiser engine with mercruiser outdrive. <br />Engine Serial3597778 and Outdrive Serial 3587533. 21" Pitch Aluminum prop. It needed a rebuilt carb, head, and a good basic tune up and timing. I did all of this and it runs great. It starts with a flick of the switch and accelerates smoothly to WOT. However, it takes 9 seconds to plane out and hit top speed of 38mph, BUT, at WOT it reads 4800 rpms on my shop tach. This is where the problem lies. I would have assumed that pitching down to a 17" is what I want for skiing, but then it would overev at WOT. Everyone I spoke with says the tach must be wrong, but I am confident it is correct. (Yes, I checked the selector switch for 4 cylinder), it is a handheld tuning tach with dwell, etc. Could the lower end be geared different? I also had someone suggest that maybe someone rebuilt it at one time with an automotive cam. Can this even happen since this particular engine was never put in a car? I also checked the clearance of the prop hub to the lower unit housing to ensure that the exhaust was not leaking out in front of the prop causing ventilation problems. The prop is new. Thanks in advance for any ideas...

 

[Barracuda]

Re: WOT RPM too high

I Forgot to say that I am running the boat at about 5500 feet above sea level.

 

[QC]

Re: WOT RPM too high

This is weird. Take a look at this speed calculator. You would have to either have a gear ratio of 2.14 with 15% slip or a slip of 34% with a 1.65 ratio. I am not clear what your ratio really is, but I doubt it is over 2 and I also doubt you're slipping that much. Could be a prop hub though . . .<br /><br /> http://www.rbbi.com/folders/prop/propcalc.htm

 

[waterone1@aol.com]

Re: WOT RPM too high

Looking at this from another angle, have you weighed the boat ? Any chance you have water logged foam or some other water issue causing the hull to perform poorly ?

 

[QC]

Re: WOT RPM too high

waterone,<br /><br />I was thinking something similar, but the RPM and pitch combo says the boat is still pretty light. You would think from the poor hole shot that it wouldn't make that RPM at the top end. Especially at 5500 ft. Something is just not right . . .

 

[waterone1@aol.com]

Re: WOT RPM too high

Hmmm, how about trim and weight distibution ? Is the bow plowing the water at speed ?

 

[tommays]

Re: WOT RPM too high

i find on my 3.0 even at sea level a have to make a Compromise on prop sizes if i want to get up on plane quickely<br /><br />the motor does not have much low RPM torque and that makes it hard for it to push a big prop out of the hole<br /><br />with the power loss at 5500 ft it sounds like its doing well<br /><br />i just run a small prop and dont rev the motor to high because getting on plane quickey is more inportant to me than top speed<br /><br />you have to keep in mind that your hull is a much older design and is most likely much harder to push through the water than a newer one<br /><br />tommays

 

[Barracuda]

Re: WOT RPM too high

Thanks for all the replies! The boat is actually very light and the foam is in excellent shape, no waterlogging. I removed a small section of floor "just to see". It was bone dry. It came from Arizona and was hardly in the water. I have played with the trim as well and it seems to do exactly what it should. I start out with the trim all the way down and then trim up a few degrees once I get on plane. By triming up I pick up 3 mph. Could the prop simply be to high pitched and causing it somehow cause cavitation? I will take the prop in today to see if the hub is spinning, but I kind of doubt it. I want the boat for skiing. Top speed is not that important to me as long as I can get it up to about 32. I am just worried about blowing up the motor with a smaller prop.

 

[tommays]

Re: WOT RPM too high

as long as you use a tach and dont run the motor over its recomend WOT it will not get dammaged<br /><br />i use a 15 x 17 for skiing it limits speed to about 27 MPH at 3800 rpm<br /><br />i am going to try a 14 x 18 4 blade prop this season to see if is a better all around prop<br /><br />i dont like to use the motor above 4000 rpm because i just dont feel it will have a nice long life if i run it 4500 to 4800 rpm for long periods of time<br /><br />its all trial and error to find the best all around prop for your boat and how you want to use it<br /><br /><br />tommays