Slipping through acceleration and VERY high revs at WOT

[boatingnow]

Hey my first post on here sorry sorry if I lack details. I brought a second hand s/s 14 1/2" / 22" prop for my 15.5 foot pleasure boat and all I can describe the feeling as is "clutch slipping" it will slip then grab all the way through acceleration and when I get to WOT im revving at 7100rpm where my optimal is 5000. the motor is a 175 evinrude 1977 motor, motor and gearing seem in good nick but I cant understand why so high rpm with a relativly high pitch, blades have a couple tiny cuts n dents but very small from what I thought???? thoughts?

 

[BonairII]

Re: Slipping through acceleration and VERY high revs at WOT

You've probably got a spun prop.

 

[jestor68]

Re: Slipping through acceleration and VERY high revs at WOT

Now you know why it was for sale. A spun rubber hub? :facepalm:

If you're lucky, it will be one with a replaceable rubber hub, like Mercury's Flo Torque hub.

 

[steelespike]

Re: Slipping through acceleration and VERY high revs at WOT

What was the original prop size,rpm, speed and performance with the original?
Usually when a hub fails it starts at a certain strain and gradually gets worse with use.
But it could be the hub. Take the prop off and put a water proof line across the hub.
Do a couple of runs and check the line. If its broken its the hub.

 

[boatingnow]

Re: Slipping through acceleration and VERY high revs at WOT

the prop i took off was 14.5"/19" it also revved up around 7000 with max speed 66kmh the interesting factor is it also slipped (not as bad) but its edges were severly damaged and i put that slipping down to cavitation. is there another factor that would give the slipping effect ? or is it more like the slipping from prop 1 was cavitation? little bubble holes were also formed on this prop

 

[Texasmark]

Re: Slipping through acceleration and VERY high revs at WOT

the prop i took off was 14.5"/19" it also revved up around 7000 with max speed 66kmh the interesting factor is it also slipped (not as bad) but its edges were severly damaged and i put that slipping down to cavitation. is there another factor that would give the slipping effect ? or is it more like the slipping from prop 1 was cavitation? little bubble holes were also formed on this prop

Well sir, true cavitation, not ventilation most folks experience will drill holes in your prop as you mentioned, and a rough perimeter on your blade will/can cause cavitation. BTDT

However, unless the dings were such that the bubbles could come across the thrust area of the blade I doubt that they would make all that much difference in performance....like on the first couple of inches of blade.

I had a new '71 Chrysler 85 with a very funny shaped leading edge on the stock alum prop. It was similar in design to the black prop at the top of this page (iboats ad) but more pronounced with it's reverse curvature right at the blade root. That sucker kept the paint gnawed off just behind the root of the blade and kept digging out the alum. Didn't have a tach so I don't know if it cost me mph/rpm's or not.

My 2c,
Mark

 

[steelespike]

Re: Slipping through acceleration and VERY high revs at WOT

It appears that the long shaft motor uses a 2.42 ratio which calculator indicates 41.01mph (66kmh) mph produces about 6,100 rpm w 10% slip.
the Short shaft is 1.86 ratio and calculates at 4700 at 41.01 mph.(66kmh)
Max rated rpm a 5,800.
You need to do the hub test.
If it is slip the motor may be too high on the transom.
is the anti vent plate just above the prop about even with the bottom of the boat?
Is it a long 20" shaft or short 15" shaft motor?

 

[boatingnow]

Re: Slipping through acceleration and VERY high revs at WOT

Unsure of shaft length sorry but both those rpms are well off mine, possibly a faulty taco reading? the anticav plate looks fairly close to bottom of hull and on plane at a guess there would be appox 100mm of water above top of prop? Am taking boat out tomorrow for hub slipping test, would over trimming motor cause some of these effects? I have been under the assumtion i should have it full trimmed down but what ive read last couple days maybe not the case?

 

[boatingnow]

Re: Slipping through acceleration and VERY high revs at WOT

So ive been out looking at the tacho and i believe it is for a car therefore 4stroke engine. It has settings for 4 cyl 6 cyl and 8 cyl , does anyone know if boats need a specfic 2stroke tacho?

 

[steelespike]

Re: Slipping through acceleration and VERY high revs at WOT

As far as I know an I/O tach uses a completely different source for a tach.I believed coming from the distributor where the outboard is sourced from the stator where voltage is generated for charging/ignition.
There are others here with a clearer understanding of tach operation on the 2 types of engines.
I believe if your tach is clearly marked 4 cylinder 6 and 8 cylinder it is for a 4 stroke automotive type setups.
2 stroke tachs are marked for the stator pole count.
It is possible to over trim and cause ventilation with resultant high rpm. Typically ventilation will cause a lack of thrust where slip is just inefficient prop operation.
Usually recommended to trim down for hole shot and trim up as speed increases. to an ideal rpm and speed.
If you measure from where the mounting bracket would rest on the top of the transom to the anti vent plate;;
short will be about 15" and long will be about 20".You can measure the height of the transom it should give a clue to long or short shaft
Being a relatively small boat it may have a short transom.
Of course the motor copuld still be a long shaft if someone has jacked it up to fit. with the 5" difference it will be obvious.

 

[boatingnow]

Re: Slipping through acceleration and VERY high revs at WOT

Hub wasnt slipping i tested serveral times and checked several times after "slipping" effects, i brought a 2 stroke tacho and at wot with 14.5/22 prop i was only actually hitting 4500rpm at 75kmh. but a definit slipping effect was felt particually at 3000rpm it would slip and jump to 5000 thn grip and go back down while accelerating, the transom height if anything was slighty low when i took to my local dealer. Any other likely reasons I would get these symptoms? once at wot slipping seems to stop

 

[steelespike]

Re: Slipping through acceleration and VERY high revs at WOT

OK we still haven't confirmed for sure if it is a long or short shaft motor/transom".Slightly low" doesn't tell us much. We need to know the actual height of the transom. You don't need your dealer to measure.
Measure vertically not along the angle of the transom Should be about 15-17" for a short shaft transom or about 20-22" for a long shaft transom.then we need to know if the motor fits. Usually the anti vent plate(just above the prop) should be about even with the bottom of the transom.
All this measuring is to confirm the actual size of the motor and if it fits the boat.
We also need the size of the motor to confirm the gear ratio so we can do your prop calculations.

 

[boatingnow]

Re: Slipping through acceleration and VERY high revs at WOT

Ok so after measuring I believe I have the long shaft. please note that it only feels like excessive slipping through acceleration espcially at 3-3500 rpm it will jump 1000 or 1500 then jump back down with no increase in speed, once at WOT it doesnt feel like slipping anymore

 

[steelespike]

Re: Slipping through acceleration and VERY high revs at WOT

Your experiencing venting and some minimal venting is normal on a good on the edge setup.
Tell us about the height of the anti vent plate I mentioned.
The boat can't go 46.6 at 4500 rpm Is it gps speed?
Most tachs have a selector switch on the back yours may be set wrong.
Guessing a reasonable slip of 10% the rpm would be about 6000 at 46.6.
6% slip would be about 5700.

 

[boatingnow]

Re: Slipping through acceleration and VERY high revs at WOT

yes the speed is GPS, anti vent is about 1 - 1 1/2inchs underwater on the plane, tacho is defintitly set to 6 cyl and is a marine tach? iv got no idea what isnt adding up?