Random change in rpm at wot...BIG CHANGE

[QBhoy]

no vent plugs

Hi
not sure if it’s been mentioned, but the picture looks like you may be missing your thrust washer behind the prop. You should have a little clearance enough to see a little of the thinner shoulder section of the inner side of the prop where is sits into the prop shaft casing area.

 

[Texasmark]

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.

In answering that question: Ventilation is sucking air in from above. Air being lighter than water is easily attracted and the reason for the Anti-Ventilation plate in the first place. I really doubt you are ventilating and causing your existing problem with ventilation as you described your test criteria herein.

I think if your prop hub is slipping, causing what you are saying it's the old design, pressed in rubber type, not consumer replaceable...usually. A slipper can slip and catch and slip and catch. Usually if it starts slipping, it stays slipping at that throttle setting. You have to shut her down, shift to N and back to F and reapply power. If it's not too worn it will catch and that will be the end of it. As wear increases it will slip more frequently untill it just goes into a free spon.

A new style prop with separate plastic/brass insert would shear, causing the plastic to break away, twisting the brass inner core (the splined part) that has spokes that stick into the plastic for grip.....those are one time whack and you're dead....course the whack has to be lethal.

 

[facelift]

Hello all!!! I realize how old this thread is. I can't believe it's been 5 years. Life is crazy. Sometimes you think you're down, but you don't see the cliff up ahead and realize how far down being down really is. But with family, a little luck and by the Grace of God, you keep going. One foot in front of the other. You don't quit or lay down and eventually you'll start to climb out of the abyss.

What I'm saying is I hit a rough spot. You can't imagine what I went thru to hold on to my little boat. But I did!!! And it finally just saw the water again for the first time since this post. I made a promise back then to report back with the outcome for the benefit of the community. I am finally keeping my word.

So for those of you whose lives or minds were shattered by having to wait five years for this post, I'm very sorry. Ha!

Long story short....it was that shiny prop. I took tuna fish's advice got a Solas 14.5x19 and she runs perfect. She's up on plane in a jiffy, and at about 85% of WOT she's running about 4000 rpms. Right where she's supposed to be. And I just love when I'm right at those cruise specs and I trim her a little and she speeds up with a little lunge with no change in throttle position or rpm.

These are the facts I can relay with 1000% confidence because I observed them first hand. Why, what the mechanics or dynamics were I can't say. Other than I made a $#!++y prop purchase and let somebody pawn off their mistake on me. It's my hunch that it had to somehow be ventilating.

All I know is I'm so glad I was able to hang on to my boat. Its one of the few things that makes me happy. There's some folks whose dogs sleep in their bed and some whose dogs sleep outside. I'm the prior. For the first 20 years of its life that boat lived the good life in the garage. The last 5 she's been in pawn shop parking lots. Friend's side yards and backyards and driveways. She even stayed hooked up to the truck for a few months and just came along with me everywhere I went.

Let's put it like this - she went from $50 salon insanity to .99cents store sauve shampoo. It wasn't easy for her. I had to really work hard to make it up to her, but we had the best weekend this past weekend, and believe it or not, even though I may not deserve it, she still loves me!!!






Which made the wife just f**king hate me even more!!! Ha!!! I LOVE MY LITTLE BOAT!!!

 

[jimmbo]

What does it do at WOT, rpm wise? 85% throttle means nothing, as the Rpm change is Not Linear to Throttle change.

 

[facelift]

What does it do at WOT, rpm wise? 85% throttle means nothing, as the Rpm change is Not Linear to Throttle change.

Sorry I just saw this reply. I never realized it is not linear. That's interesting. I'm guessing it would top out somewhere around 4800, but I understand and believe your statement, which means I also understand my 'guess' worthless. I've just enjoyed being back out on the water after so long so much that I've been really babying the girl and going easy on her. I just don't want f nothing else up. That said, I'm heading this weekend I think. So for the sake of science and The Shared Knowledge, I will report back with definitive, empirical data to that query.

 

[facelift]

What does it do at WOT, rpm wise? 85% throttle means nothing, as the Rpm change is Not Linear to Throttle change.

Sorry for delayed response. On my next trip out after last post, I put in single handed. I took her off the trailer by rope and tied her to the dock while went to park the truck. Came back and opened engine compartment to find she was taking on considerable amounts of water. I turned on the bilge and booked it to the truck. Once out of the water back on the trailer I discovered the issue - torn shift cable boot.

So it took a couple weeks till I could get around to fixing that. Went back out last weekend and it wasn't until I was leaving the no-wake zone that I discovered I didn't have shift interrupt adjusted correctly. I had to resolve that one in the water on a night so dark I couldn't even make out the horizon (the moon didn't come out till shortly before sunrise).

I got it resolved just before day started to break and it was gorgeous. Not even a whisper of a wind and barely a soul on the water. Silky smooth glass. I just didn't want to push my luck at that point. Not even for the sake of science. Long story short, I never pushed the throttle all the way over to WOT. I cannot say definitively what it would be or what it is. Acknowledging your statement of the non-linear nature of the rpm curve and thus the lack of usefulness of my estimation, I'm still gonna go out on a limb and say I'm guessing it's going to end up somewhere around 44-4500 rpm.

I found the sweet spot on a perfectly calm day, very lightly loaded - 2 adults, other boats wakes few and far between, on plane, trimmed out to be about 37-3800 rpm doing about 29-30 mph. I was perfectly content there. I did something completely out of character for me - I tried to exercise good judgement and not push my luck. I ran her a good 30 miles round trip like that and she performed flawlessly. Ran cool as a cucumber and barely even sipped the gas tank.

Sorry I have nothing really of use to report. But I will say me and my boy had a glorious morning and we were back loaded up and gone before all the yahoos started to come out and the chaos at the ramp began! Got some sausage biscuits with egg and hash browns on the ride home. Yep.

 

[jimmbo]

Without knowing what the WOT Rpm is, you can't arbitrarily claim a sweet spot. You do not prop for a Speed at a certain/preferred Rpm, regardless of how the Engine Sounds