Low rpm and porpoising help!!

[ghuru45]

I just bought a 2005 carolina skiff flats boat with a 2005 yamaha 150 4stroke.. I did the unthinkable and bought it without taking for a test drive.. I know, i know.. well, i took it out and this is my problem.. It has a jackplate and the boat gets out of the hole fine.. but once on plane it is very boggy and only opens up to 3900 rpms wot.. If i try raising the motor or trimming up at all the boat porpoises terribly.. I mean bad.. I know that the low rpms is contributed by having to keep all of the motor in the water but surely this can't be the complete reason?? The motor sounds fine and runs smooth.. Just boggy on plane.. My immediate thoughts were prop since this is a very nice/newer rig and it has a terrible looking 17 pitch aluminum prop on it.. I'm guessing the guy probably just put on whatever he had and took off his nice stainless one before i picked up the boat.. I'm about to die wondering if the wrong prop can make the boat perfom this terribly? Please help..

 

[ghuru45]

Re: Low rpm and porpoising help!!

sorry JB.. the boat/motor/gear weigh approximately 2500 lbs.. prop is a terrible looking aluminum 14x17..

 

[steelespike]

Re: Low rpm and porpoising help!!

Did you read the stickies at the top of the page?
We need your present max rpm and speed.Weight info is good but boat size will help.From my limited reading it appears flats boats can be difficult to setup.You need to start with a good prop.The turning pointHustler gives close to stainless performance at aluminum prices.It would appear a 17" pitch. The experts may have some better suggestions.

 

[ghuru45]

Re: Low rpm and porpoising help!!

I was skeptical about even joining on here because i read a ton of posts from NEWBIES that seemed to get blasted over simple posts.. I realize you guys that are Junkies over boating topics don't wanna answer the same questions over and over but don't be a jackazz either!! I did read the stickies and i posted all the information i had available! DID YOU READ MY POST??? If you did you would have seen my current max rpms!! Some of you guys are just sitting waiting on something you can blow up so it makes you look like a super hero of forums.. Well back off a little dude!! I'm here for help not hassle!! If there is something i can answer specifically to my setup that i haven't already let me know and i will gladly try to answer it.. If i had the current mph i would've posted it. I know you want to be "technical" but i'm not an idiot.. The boat maybe runs 30-35 mph at best.. My main question is still "Can a prop really make that much of a difference?" If anyone is a prop guru they will probably be able to tell me..

 

[stevens]

Re: Low rpm and porpoising help!!

Nice comeback, Ghuru.

And to answer your question; most likely it is a combination of things that brings about your porpoising - jackplate height & setback, weight distribution, trim setting, trim tab application (if any) and possibly also a ****ty prop with little ability to hold the bow up. The prop is at least easy to fix. Worth giving it a try, but it may possibly not solve your porpoising if the problem really lies elsewhere.

 

[ghuru45]

Re: Low rpm and porpoising help!!

thanks stevens and i appreciate the reply.. I figured that a flats boat would be a little tougher to get straightened out just because of it's awkward shape.. It just baffled me that this rig has everything it should to perform top notch but doesn't at all.. The only thing that stuck out like a sore thumb was the prop.. I just wanted to see if anyone else (possibly another flats owner) had similar problems. It seems odd to me that a prop would or could make that significant of a difference in performance.. Especially when it's the correct size prop.. I just didn't know if a prop could get worn out enough that it could lose it's ability to perform properly.. That is what i am hoping is the case. Again, thanks for the help.

 

[CATransplant]

Re: Low rpm and porpoising help!!

Well, you've just about guaranteed that the very people who can help you with your question are not going to answer. If they did, they'd be asking even more questions.

Like the length of your boat, whether the engine is running perfectly, and what you mean by a "crummy-looking" prop.

You think you have given the necessary information, but I can guarantee you have not. Before your problem gets analyzed you're going to have to provide a lot more information. Jumping down the throats of the people who are trying to help you is not going to work out well.

Since you didn't take the boat for a test drive, your problem will probably be tough to diagnose, since it could be an engine issue, a boat weight issue, a prop issue, or any of a number of other issues.

Of course, you could take it to a good marine service place and leave it there, after describing what the problem is. Since they could test all the things we're asking about, they could probably find and fix the problem. For a price.

Here, the advice is free, but it has to be based on the information you supply, and so far we don't have enough information to offer decent suggestions. So, you get more questions.

It's up to you.

 

[CATransplant]

Re: Low rpm and porpoising help!!

Another note:

You said that the engine would only turn 3900 rpm with your prop. That says to me that either the prop is not right for your setup, which is probably unlikely, according to your description, or the engine is running poorly and can't spin the prop up to the proper RPM.

Couple of other issues to check on and get back to us: Where is the anti-ventilation plate on your outboard in relationship to the bottom of the hull at the stern? When you trim out, does the prop blow out and ventilate? You have a jackplate, you said. What happens when you jack the motor up? Down?

Porpoising is a complicated issue, not a simple one. But, I'm most alarmed by the max RPMs you're getting. Nowhere near where they should be. That's the first thing to investigate.

Again, what do you mean by a "crummy-looking" prop. Is it dinged up. Bent? Missing chunks of prop? Get precise on us.

I could go on and on with questions that may be relevant, but this is a start. Fill us in with the answers, and we'll ask some more questions. Eventually we'll drill down to what your problem is.

 

[ghuru45]

Re: Low rpm and porpoising help!!

hi.. I'm pretty sure i gave boat length, weight, etc. in the post but it's a 2005 carolina skiff 20' flats boat.. weight with motor and gear is approximately 2500 lbs. The boat performs pretty much the same whether i have the jack plate all the way up or down.. of course i don't take it so far up that the prop is coming out of the water. The boat takes off and revs pretty nice out of the hole but once it gets up on plane it kinda bogs and you can tell the engine gets much quieter.. by saying the prop looks like junk i'm saying the blades are getting pretty thin and it just looks pretty crappy to be on such a nicely outfitted boat. i will post pics later

 

[CATransplant]

Re: Low rpm and porpoising help!!

No, you didn't give the boat length or weight. Not in this thread, anyhow.

OK, your prop is just worn, then, and not bent up or with chunks out. So, if it was the prop on this boat, it was probably close to being right.

From your description, it sounds like your engine is dropping a cylinder at WOT. You said it gets quieter. It shouldn't do that, frankly. So, it's likely that your low WOT RPMs are an engine problem. Could be ignition. Could be something else.

Your worn prop would not cause low RPM. Rather, it would probably do the opposite.

Getting the outboard to put out the proper RPM at WOT is the first step, then. Once that is repaired, then you can actually address the porpoising. Right now, you're not getting maximum performance out of the combination, so there's no point in trying to figure out trim issues and the like.

I'll assume that your boat isn't waterlogged and heavy, given it's age. That would also slow things down and affect how it runs out. You gave the weight. Is that the published weight or scale weight.

Since you can jack the outboard up far enough to expose the prop, then your outboard is not the wrong length for your boat. I didn't think it would be, since it sounds like the combo is original.

You're going to need to get the engine running properly. Then you can address the other problems, if they persist. It may just be that once the engine's running right and turning the proper WOT RPMs, your porpoising will just go away, since the attitude on plane will probably change with the additional power and speed.

 

[hwsiii]

Re: Low rpm and porpoising help!!

Ghuru, you blasted steele for just asking information that we need to help you with, you aren't qualified to know what information is needed to help assist you or you would have given us that information to start with, you didn't even give us the length of boat and hull form or the speed which we need to be able to predict prop slip. You don't recognize that but we do, you bought the boat without testing it, we didn't, so don't take it out on people who are just trying to help you.
Having said all that I have filled out my forms with what I think is reasonable weights for an average load on your boat and done some preliminary prop picking with that setup. I have a Sundance Flats boat myself.

Boat and Motor

Prop Picker

Prop Change

And your boat should be turning close to maximum RPM with a 17" aluminum prop and you should be going about 42 MPH at those RPM. Cat is right you have a motor problem of some kind and you need to have it looked at to find the problem.

H

 

[steelespike]

Re: Low rpm and porpoising help!!

I was skeptical about even joining on here because i read a ton of posts from NEWBIES that seemed to get blasted over simple posts.. I realize you guys that are Junkies over boating topics don't wanna answer the same questions over and over but don't be a jackazz either!! I did read the stickies and i posted all the information i had available! DID YOU READ MY POST??? If you did you would have seen my current max rpms!! Some of you guys are just sitting waiting on something you can blow up so it makes you look like a super hero of forums.. Well back off a little dude!! I'm here for help not hassle!! If there is something i can answer specifically to my setup that i haven't already let me know and i will gladly try to answer it.. If i had the current mph i would've posted it. I know you want to be "technical" but i'm not an idiot.. The boat maybe runs 30-35 mph at best.. My main question is still "Can a prop really make that much of a difference?" If anyone is a prop guru they will probably be able to tell me..

It seems you came in with a chip on your shoulder.I asked one very legitimate question based on the fact that at least 70% don't read the stickies.It was a question and only that.I followed that with 3 or 4 lines
of constructive information. And would have added more.if I had more info.
We all understand you may not have all the info we need We just inform what is needed.When I work on a question I do research.If someones looking for outboard specs/help I look in one of my 6 aftermarket service manuals
If someone asks if a prop number will fit or what it is instead of saying look it up here I look it up. I look it up here or on the net.when I do a answer it takes forever with my one finger style and constant proof reading to try to be sure I'm making sense.99% of us do this without jabbing anyone.And we're
here because we love boating and helping boaters.If you just read the answers I think you will see what I say is true.Stop seeing only the mean posts and make note of the many,many helpful answers.We are all members just as you are and In a way we all represent iboats if abuse was rampant by anyone here do you think iboats would let us participate any more?
Please don't prejudge and keep in mind we are all here to help.
And ask as many Qs as you want even the ones you may think are dumb.
There are no dumb Qs only Qs from the experienced.How else do you learn?I've been boating 60 years and just the other day I asked what a bolt on a old Johnson TD15 leg did.
Phew! thats enough.Imagine how long it tookj me to write this? Please stay with us; there is an amazing amount of helpful accumulated knowledge here.
Spike

 

[ghuru45]

Re: Low rpm and porpoising help!!

I apologize for being rude to anyone.. I certainly don't mean to blast anyone that is truly on the other end offering help to us less informed boaters. I read hundreds of posts before i decided to even register and lots were very helpful just by reading and reading.. I did see a few posts that i thought were kinda taking advantage of the newbies and making them feel stupid. I guess that kinda put a small chip on my shoulder.. I'm sure there are tons more helpful people than those i saw taking advantage of an opportunity to slam a newbie. I apologize. I just thought something said to me was a little rude but i guess some people are just "blunt" and it appears rude when it's just being simple. Lets move on and try and fix this mess i'm in with the boat. It really is puzzling the heck out of me and a couple of my more serious boater friends.. I was curious about the post telling me that it sounded as if one or more cylinders were dropping off. Does "dropping off" mean they may go dead after i get up on plane?? I decided to go outside just now and hook up the muffs. I pulled off each plug wire one by one and each cylinder is alive and well at idle. I don't know how much that tells you but at least it rules out dead cylinders all the time. thanks to everyone for all the info/help!!

 

[steelespike]

Re: Low rpm and porpoising help!!

I think I would start with a new 17" pitch Turning point Hustler.
It's obvious we need a prop change by your description. Historically
these give an overall improvement even against a decent regular prop.
I think it will be a good start.Did we discuss checking that the throttle is opening all the way?With a new prop you may get more meaningful results from the jack plate.

 

[hwsiii]

Re: Low rpm and porpoising help!!

Ghuru, can you borrow another 17" Pitch prop from someone you might know. There is a small possibility that the prop you are using has something wrong with it and is causing your low RPM and your porpoising because of a slightly bent blade or something that is hard to see. As a normal course of events an aluminum prop has the least chance of causing porpoising because the normally have no rake to raise the bow and start the porpoising. But porpoising is actually caused by the center of lift on the boat being moved back and forth form the center of gravity of the boat, a lot like chine walking. On some boats if you throttle up slower when you come on plane it will slow the effect down a lot, but on others it doesn't matter if you do it slow or fast it still happens. It is a basic instability in the boat that can be caused by many reasons, including hull features but usually it is caused by weight placement in the boat. Moving some weight forward or backwards some of the time can totally cure the problem, but not always. This problem can be exacerbated at times by using a prop that has some rake but not enough to lift the bow as required, and at other times it is best to use a zero rake prop that lifts the stern only and thus eliminates the tendency of the boat to propoise because it keeps the center of lift on the boat in the stern and it never passes the center of gravity farther forward in the boat. I know that is a very lengthy and detailed explanation, but that is the root of the problem with porpoising, and I hope the reason for your low RPM as well, as that is a cheap fix.

H

H

small​

 

[CATransplant]

Re: Low rpm and porpoising help!!

That was me. Plugs cause more problems under load than at other times. You said that the boat jumped onto plane pretty easily, but then "bogged" down. Bad plugs or a weak ignition system can easily only show up as an engine's speed increases and power goes up.

If you're only reaching, what was it, 3800 RPM, then you're nowhere near the normal WOT RPMs. I'm assuming that the prop on your boat is probably the one it came with, given its worn state. It's unlikely that the engine would have performed that poorly for four years without someone wondering what the heck is going on.

I'm suggesting that a complete diagnostic routine be done on that outboard. Compression test, a readout of the computer codes, and whatever else is recommended in the service manual for diagnostics. I don't know your engine, but I know engines.

You might try an experiment, though. Put in a set of new spark plugs, the exact ones recommended by the manufacturer and gapped to specs. Then go try it out. It could be that simple. It could very easily be that simple.

If that doesn't do it, then it's time to figure out why your engine isn't reaching its optimal revs. On today's modern 4-strokes, that's not as easy as it was on the old outboards.

I suspect that once you get your outboard turning proper revolutions, many of your handling problems will simply disappear.

But, it's up to you. I'm just suggesting that you get someone to run a complete diagnostic on your outboard. What you do is your decision.

 

[ghuru45]

Re: Low rpm and porpoising help!!

I just thought of something it may be.. well, not completely but it may be part of the problem.. The throttle handle on the boat is very, very stiff.. It could be that the boat is simply not reaching wot via the cable.. I know thats exactly what it acts like.. The boat will plane out without porpoising.. It's just that i have to have the trim all the way down and if i try to trim up the slightest bit i start the rought bounce. But if i had more rpms pushing at the stern maybe i could trim a bit without the porpoising. Everyone is right.. I need to figure out why the low rpms first, then the porpoising because the low rpms may be causing the porpoising. That very stiff throttle handle has me thinking that now.. Is there a simple way to test this? I know where the linkage is but really don't know how to tell if it is maxing out.. i guess the best thing to do is take it to the dealer and give them all my feedback from everyone.. I will tell them what its doing and about how stiff the throttle is. I will also see if they will let me try out a loaner prop to see if that helps. I will post the outcome.. Thanks for all the help!!

 

[Gremlin]

Re: Low rpm and porpoising help!!

You had posted on my thread that you were having similar problems with your Yamaha that I was. I have the '99 Key West 20' WAC w/150 Yamaha 2 stroke. I contacted Key West to see what the originally set up the boat and motor with, discovered previous owner put a 19" pitch, when Key West recommended a 17" pitch. Based on other recommendations on this forum I have ordered a Turning Point Hustler 17" pitch. Don't know if this will solve my problems, but cost was $105 and change, will be here this week, and will post on my thread the results.


Good luck!

 

[CATransplant]

Re: Low rpm and porpoising help!!

Yah, it's possible that you aren't getting full throttle movement. Pop off the cowling and whatever else is needed to get a view of the throttle plate(s). I don't know if you have carbs or fuel injection, but there is at least one throttle plate you'll be able to see.

Move the throttle lever to its maximum and look at the throttle plate(s). It (they) should be horizontal, fully open. If not, then you probably have your answer for the low RPMs. If it is, you'll still have to look further.

Again, I emphasize that you need to figure out why your engine isn't getting its maximum RPMs. If that prop is the original one, or the same pitch as the original one, something's not right with the engine.

I don't know what the optimal WOT revs are for that engine, but I know it's way over 3800.

 

[ghuru45]

Re: Low rpm and porpoising help!!

ok.. so my local repair shop/dealer can't look at my boat for a week so i thought i'd hit you professionals up for more great advice/suggestions!! I have sorta reduced my thoughts on a prop issue and have leaned more towards maybe a fuel problem. I have checked for cable kinks and there are none. I checked the cable and linkage and the linkage is opening completely. I have been reading in the forums about clogged injectors causing the same problem i am having. Is there anything i can do to check them simply? Is there an additive i can try that may show some signs of improvement? I'm still just trying to rule out things and/or give the mechanic feedback from all of you guys.. thanks