Random change in rpm at wot...BIG CHANGE

[facelift]

hi there. i have a 1999 bayliner capri ss 3.0lx. i crunched my stock aluminum prop and bought a used one on ebay. I didn't do my homework and was too easily attracted to how nice and shiny it was. so i ended up with less than ideal prop. and i realize that, and realize that my wot is too high at 5000. and so i dont ever go there. should be 44-4800 and i try to stay around 4-4200 - which is about 3/4 full throttle. now, all that said, here's the thing. all factors being equal....same day, same wind, same waves, same load, same absolutely everything, i can stop, shut off boat for a few. when i start back up, i am at that 4000-4200 rpm range before i even hit 40% full throttle. have no idea what wot would be in these instances, dont go anywhere near it. just thinking proportionately, it would be well over 6000!! then upon shut off & restart, everything is back to normal. there is no pattern, at least not that i can discern, as to when it occurs, nor for how long it last. it may remain through numerous restarts or it may not even occur at all. i've read a lot about spun hubs. that seems to me to be an all or nothing prospect. do hubs, can hubs very sporatically and randomly slip??? that is the only that seems to me that could create my issue.

 

[alldodge]

Engine couplers and prop hubs don't slip one time and not the next, so I would think your tachometer is the issue

Assuming your trim angle is staying the same

 

[QBhoy]

Hi. Other than what’s said above and hubs etc. Do you have a merc prop with the pvs vent holes in it ? These react like this when no plugs or if they have a vent hole in the plugs. Horrible things.

 

[Texasmark]

You said shiny and Stainless Steel props are shiny. What's the history on the prop you bought?

How about a photo of the transom, lower unit in the normal trim position where you have this problem and the prop (all in one shot) from the side of the lower unit, down on one knee (2-3' off the ground for camera position) with the camera off to the side of that combination about 5'.

Then a second picture from the rear, same height 3' back from the lower unit 2' off the ground. Looking for port holes in the prop, relationship of the outdrive to the hull and type of hull

 

[QBhoy]

Still thinking this is caused by the plugs being out a merc pvs prop. ?

 

[Texasmark]

Still thinking this is caused by the plugs being out a merc pvs prop. ?

No experience with ported I/O props but I do recall postings on iboats some time back that mentioned problems.

Going to guess that unlike a Bass Boat that has the hole shot and is up and away with adequate water flowing across the ports to seal them off, I/Os, especially deep Vs with high dead rise at the stern, tend to stay in the water for longer periods of time and don't get up and get going all that fast meaning that the ports don't get sealed off and as a results RPMs remain high and boat speed less than what it should be (otherwise) for the application.

 

[facelift]

thank you to all for your answers. i think i can rule out tach, as i can physically feel the change, ie i clearly am not going as fast as usual at the same rpm. sometimes there's a surge thats even more discernable. as if i just crossed a big wake when in fact i did not. this occurs independent of trim. can happen with consistent trim setting. trim setting doesn't seem to cause, nor correct issue. i've experimented with it. not sure about pvs vent holes but i'm going to go look later today. i cant post requested pics until i take it out again, but i will post a pic showing why. as for prop history, i have no idea. turns out wasn't such a "great deal". i suspect (was told) it has lack of cupping issues and maybe even material missing. like the blades ground down some. the other problem i face now is everything i've learned about prop selection is based on or relative to what i have now. this problem prop is 14x19rh, but with wot all over the place, i dont have a base line. i'm pretty sure i replaced like-for-like. but the original prop got tossed. any suggestions there? i'd be most interested in top speed (over holeshot & fuel economy). the boat has a straight 4, 3.0 liter, 135hp, alpha one drive. usually very lightly loaded. and i usually only go early, early before all the weekend warriors come out. and only when no wind....so what i'm getting at is velvet smooth, glassy water. thanks again. i'll post those pics tonight

 

[TunaFish389]

Not sure if the prop is the problem but with a good running 3.0 a three blade Solas 14.5"×19p for elevations above 4500ft or 14.3"x21p for below 4500ft is a good place to start.

Really sounds like you may have damaged the drive when you crunched the original prop. You can find a new Solas Amita 3 in the sizes above for under a hundred dollars if you look and see if that changes anything and go from there.

 

[facelift]

no vent plugs

 

[facelift]

as for suggested views in additional pics, well, you can see its kinda a tight squeeze. i have about an inch on each side coming through the door, and only about 30 outside in front of door. i have to drop boat perpendicular, move truck, swing the tongue around by hand and winch back in. apartment living sucks. i make too much noise and have too much $#!+

 

[facelift]

TunaFish389 is sure hope your wrong. but i'm afraid i might be holding on to false hope. a few years back, i melted the upper gears. (who knew you're supposed to fill them damn things bottom up?)

let me ask you, do you have any theory or hunch as to what in the drive could fail in such a way as to produce the symptoms i'm experiencing? what i'm getting at is it seems to me, failure in the drive is an all or nothing prospect. there are no varying degrees of in gear, no levels of neutral. there is a clearly defined event horizon between the two. and should that separation fail, its not going back to normal without outside intervention.

my symptoms are sometimes present, sometimes not. when it occurs, there is a very smooth transition between it not occuring and it occuring. like a surge. or like someone is riding the clutch.

unless i didnt get one of the gears pressed on its shaft good enough, what else could "slip" a little? thats why my original question/thinking was the hub, because of the rubber component, its the only thing i could think of that could "give" a little.

i only rebuilt the upper half. so i didn't become intimately familiar with the lower like i did with the top. is there something in there that could fail in such a way?

thanks again to all of you for your time and input.

 

[TunaFish389]

I would try a known good prop and go from there. It could be the hub they are made to slip at a point. Look to see if you can borrow a prop or like I said get a new one for under a hundred. If problem persist look into the drive which is beyond my knowledge.

 

[Scott Danforth]

prop hubs either slip or they dont. they do not slip to a point

3.0's dont like to turn more than 4800 RPM for very long, the head gasket goes

you can not turn a 3.0 to 6000 RPM, the valves float long before that

when buying props, first focus on diameter and pitch. what was the diameter and pitch of your first "crunched" non-shiny prop. what is the diameter and pitch of your new shiny prop?

then focus on blade technology (vented, cupped, bow-lifting, stern-lifting, etc.) what did you have? what do you have now?

random changes in RPM would most likely be fuel system or ventilation

 

[TunaFish389]

I have to disagree on slipping to a point. They have a rubber contact point that can grip and let slip. The main job is to grip but they can slip to a point. What that point is will depend on load and temputure. A colder rubber will slip more than a warm rubber(expansion). It is possible that you are slipping.. The ventilation is another possibility but since you said you can't control it unlikely.. For example if I run my boat fully trimmed at sharp turns I will ventilate, once I straighten out or lower rpms issue ceases.

 

[Texasmark]

Slipping hub detection is a no brainer. You have the drive.....the splined inner hub.....and the driven, the outer barrel and the blades. Scribe a line from the drive to the driven. Take the boat out and romp on it a few times getting it to act up. Take the prop off and look at your scribe line. If still a straight line look elsewhere for your problem besides prop slip.

 

[facelift]

the marks are already there, i used an engraver and put two tiny little hatch marks. i wasn't sure if a sharpie would stay. i just haven't had a chance to take it back out yet.

i'm not sure of how a hub is constructed, and how it joins to the prop. but if it is just compression and friction that gives it its bite, i would think there would be a point it could theoretically "slip". and "slip" would be the best way i can think of to describe what occurs. as improbable as it may be, its seems it would be the most likely link in that drive train.

but that point is moot as the definitive will come when i see if those marks are still aligned or not after taking her out.

i dont know what the specs of the original prop were. it got thrown out in a move, and i stupidly did not take note before doing so. though i cannot say for a fact that i recall this specifically, knowing myself, its a pretty good bet i searched for a prop with the same details. and the shiny one on the boat is 14x19.

my plan at this point, i think regardless of hub test results, is pick up a new prop. and those rpms simply have got to come down one way or another. period. so i think i'm gonna go with a 3 blade 14.3x21p, as suggested. that then really puts the prop/hub issue to rest.

i could see how ventilation could cause the smooth surge effect, but i dont see how it could occur. this occurs with no change in variables. no wind, no wakes, glassy smooth water. same load, same trim (which is no where near the surface), same throttle setting, running straight, not turning, no catalyst at all i can discern, everything being equal, except the boat performance. though sometimes stopping, and shutting down can correct it, that is not always the cause. it can occur well underway and on plane.

does ventilation only occur from sucking air from the surface? coming too near or out of the water? or are there other ways, like exhaust or something?

one more time, thanks to all of you for sharing your time, knowledge, and experience. i will report back when i have some data. i'm still going to run the shiny prop once with the marks on it out of curiosity, even if i'm able to pick up a prop prior to the next outing. but that probably wont occur for a couple 2 or 3 weeks as we're going on a family vacation before school starts back up which has me strapped for time and dollars. so if you dont hear from me for a minute, i haven't flaked. i will report back for the benefit of the community, and out of respect and appreciation for the help all of you have freely given.

 

[jimmbo]

Are there a lot of weeds in your lake?

 

[Texasmark]

We'll certainly be interested in your results so post back in 2-3 wks.

 

[facelift]

no. no weeds. its pretty deep. i typically am in 80-100++ of water the majority of the time. except when i'm easing up into a cove to fish or finding a sandy spot to beach. and even then the "weeds" are more like submerged tumbleweeds and desert bushes. i really dont come into contact with anything (except for the time that tore up prop. and i swear, there were extenuating circumstances on that one!) even when i beach it, i coast in slow and easy motor killed outdrive up in trailering position. Never run her up on or off. and only nice sandy spots, or extremely fine gravel, sometimes.

 

[facelift]

i promised a follow-up report. its been longer than expected, so i just wanted to let yall know i haven't flaked on said report. there's just been some hiccups on the road, and i haven't had a chance to get on the water yet. really hoping and expecting this weekend. will give report then if so.