rpm issue/spun prop?

[turbo_pete]

Just got back from my first day on the water this season! 17' fish and ski (~1700 lbs loaded) with a 1982 50 hp Evinrude, 11.75" x 17 pitch prop. I did a lot of work on the boat in the last month, such as new steering, fuel tank and installing gauges, including the boat's first tach. Upon reaching 5000 rpm, the revs would drop to ~4000 rpm suddenly without any noticable change in boat speed or engine sound. Is this an indication of a spun prop? Cavitation? Or is this maybe a case where I have to add a resistor across the tach terminals? The cavitation plate is a couple of inches above the keel, and the engine is mounted as low as the mounting holes will allow. This was at approximatly WOT (I know, I should be getting 5500-5800 rpm - swapping the prop is another topic). I was traveling at only around 25 mph (gps verified). Besides the odd tach behavior, I think I should be getting more speed out of the boat. It was kind of choppy and windy today, so I might have been able to squeeze a few more mph on a calmer day. Any and all suggestions appreciated.<br /><br />Pete

 

[turbo_pete]

Re: rpm issue/spun prop?

After reading some posts here about spun props, I went out to the boat and put it in gear and tried to turn the prop by hand. There was some resistance initially, but it does spin. It seems the hub also is turning (at least what I imagine is the hub - the central body that the prop is bolted to). Does this indicate a spun prop? <br /><br />Pete

 

[walleyehed]

Re: rpm issue/spun prop?

Does this indicate a spun prop? <br /><br />Pete

No....Sounds to me like you have some venting until the bow breaks over solid, then the prop hooks up...IF you notice a drop in RPM by sound..<br />If not, it may be a tach issue...are you using a Teleflex tach???

 

[turbo_pete]

Re: rpm issue/spun prop?

I didn't notice the rpm drop by sound. And yes, it's a new Teleflex tach. I'm not sure if I understand your venting comment. As in the prop clearing the water? I don't think that happened, even thought it was choppy out. The rpm decrease was not intermitent. The rpm reduced to ~4000 and held there until I reduced the throttle. With reapplication of throttle, it would accelerate back up to 5000 rpm, then drop back to 4000.

 

[walleyehed]

Re: rpm issue/spun prop?

So you're telling me you notice no sound change when the tach changes????<br />Do you notice any speed change at all???<br />Venting is air in the prop...it can happen for many reasons...surfacing is just one of them.

 

[turbo_pete]

Re: rpm issue/spun prop?

I noticed no change in sound, at least none that I can recall. No speed change, just couldn't accelerate further.

 

[walleyehed]

Re: rpm issue/spun prop?

#1., Do you have ANYTHING mounted to the transom such as a transducer, pitot tube, speed wheel????<br />#2., Does it act normal from a stop...add power, the tach goes up with the engine sound, then at speed, the tach drops 1000RPM, and you don't feel just a slight increase in speed?<br />Sorry for the questions...just hard to determine, not being IN your boat...<br />BTW, 4000RPM at GPS of 25 is negative slip,(not possible) so there is a good chance the tach is not right...now, there could be stator issues, or even reg/rectifier issues causing the faulty readings.<br />Another thing that just hit me is that 50 is very small for a 17ft F/S type boat....

 

[turbo_pete]

Re: rpm issue/spun prop?

#1. Yes, there is a speedo pitot and a fishfinder transducer on the transom.<br />#2. your description is correct. I agree that 4000 rpm can't be correct. And I definitely agree that the boat is underpowered. The 50 hp is what was on the boat when I bought it (got a great deal - boat, engine and trailer for $1800). Motor runs well, I'll upgrade to 90-115 hp someday.<br /><br />After researching some more, I think it might need a 2k ohm resistor across the tach. This seems to be common on old evinrude engines of that size range. Should be easy to try anyway. Also, I'm thinking of re-propping to a 13 pitch since the motor is loaded heavily.<br /><br />What do you think is the main side effect of being underpowered? Increased slip (resulting in reduced top end speed)? Poor hole shot? Poor gas mileage? I'm a little new to this and appreciate your time in this thread.

 

[walleyehed]

Re: rpm issue/spun prop?

What do you think is the main side effect of being underpowered? Increased slip (resulting in reduced top end speed)? Poor hole shot? Poor gas mileage? I'm a little new to this and appreciate your time in this thread.

.....yes....<br />The reason I asked about anything mounted to the transom is make darn sure it's outboard of center over 12-14" in either direction...these can be causing air pockets until the bow drops over and sometimes it shows up REAL bad in a slight turn in 1 direction or the other.