New Mercury 20hp hitting the rev limit

[Jimbeau]

Hello.

I put the boat in the water yesterday. First time with the new motor installed! Accomplished two hours of the break in procedure. She ran great except it reaches the rev limiter about 1/8” to 1/4” prior to reaching the mechanical stop. My dealer’s mechanic says to lower the tilt. I tried that yesterday but I did not go to the lowest possible hole because it seemed plenty low enough. I started out with the trim in the level position (middle hole), then tried one hole up and one hole down from the middle. No luck. The mechanic says to lower the tilt all the way down to the lowest hole.

I’m considering, based on another tinboat thread recommendation, going with a small jack plate. Does anyone know if this will help or hurt my RPM situation?

Thank you.

 

[FuzzyGrub]

What was your gps speed and rpm’s at just below hitting the rev limiter? I’d think more of too low of prop pitch.

How was the boats orientation while at that speed? Was bow riding high, maybe porpusing? If it was running level, I wouldn’t go lower on trim.

 

[Jimbeau]

I don’t know the speed. It felt pretty fast but don’t know how to check that. Also no way of checking RPM’s. There’s no gauge. It’s just a tiller model. The bow was riding plenty low and is the main reason I didn’t try to lower the trim anymore.
The mechanic says if a lower tilt doesn’t fix the problem it could be the prop. I definitely don’t want the bow down any lower.
I’ve had a lot of boats but never had one do this with a standard prop installed.

 

[FuzzyGrub]

It is probably worth getting a Tiny tach/hour meter or equivalent. I use them on my small outboards. A wire just wraps around one of the spark plug leads.

There is probably an app for your phone to get gps speed.

While I don’t have the experience of using a jack plate, I suspect you will hit the rev limit sooner, given you are raising the motor higher and will have less drag.

I don’t agree with your mechanic, to force the bow lower, to prevent hitting the rev limiter. Just turn the throttle back a touch when you do. If you are going to jack plate, wait and retest, before going higher in pitch.

 

[DaleH]

The mechanic says to lower the tilt all the way down to the lowest hole.

Your mechanic is a complete IDIOT and you must tell him that I told you so! That can cause a "bow steer" trim condition and on small boats - that can KILL!

Please wear your ignition kill lanyard ...

Use the Prop Calculator here NOW ... https://forum.tinboats.net/viewtopic.php?t=37433 ... and put in YOUR hull/motor and actual/honest weights for all gear carried, or pick a hull that is closest in match for what YOU have. See what that calculator tells you that YOU should be using for your exact rig and how equipped.

 

[MrGiggles]

I agree, while the mechanic's idea is a "fix", you would sacrifice the boat's handling/ride and be leaving quite a bit of efficiency on the table by slamming the bow into the water and slowing it down that way.

First step in prop selection is getting a tach rigged up. Generally an inch of pitch is worth ~200rpm, you want it to land between the min and max rated RPMs, it's better to be closer to max than the other way around. Lugging is not good for it, and it's good to have the headroom if you ever load it down heavily. I'd venture to guess that you only need to go up an inch, maybe two depending on the rev limit of that motor, since you are hitting the limiter pretty deep into the throttle.

Pitch is the angle of the prop blades, if you imagine the prop turning in a solid like a screw, the pitch (in inches) is how far it would travel in one revolution. Prop diameter is inversely related to pitch, the more pitch, the smaller it will be.

The calculator is a great starting point, but since you already have data and a baseline, I don't know that it will be all that helpful to you. I only use it if I'm starting completely from scratch with a new boat or motor and don't have a prop to start with, for that it has been great.

Your prop should be stamped something like 10.3x15. The first number is diameter and the second is pitch. Using that example, say you wanted to drop 400rpm, a 9x17 would be appropriate, giving 2 more inches of pitch at 200rpm per.

A 3 blade aluminum replacement is fine for that combo, should be pretty cheap too.

Jack plates create some pretty drastic changes with how the outboard applies leverage and force to the boat. I would put that on first before doing any prop changes. In theory it should raise the RPM a little bit since would be less drag, but they can also cause it to drop since the prop is running in cleaner water, being further away from the hull. So it's definitely a try and see sort of thing.

 

[airshot]

The " first" thing you need to do is check the height and angle of the cavitation plate. Use a long straight edge on the bottom of your hull sticking out back towards your motor. The cav plate needscto ve approx same height as boat bottom and the cav plate should be parallel to boat bottom. Once this has been done, then try moving trim pin one hole in and one hole out....go with the hole that gives the best performance. Now you are ready to consider props. Getting the proper height and trim angle should be done first. Then only a tach and gps will get you fine tuned for best performance.
Unfortunately todays new engines can have most any prop on them, as most engines no longer come with a standard prop anymore, you pick out the one you need..

 

[Jimbeau]

Great info here! The anti-ventilation plate is slightly low. It’s sitting about 3/4 below the bottom of the boat as measured with a straight edge. This is also one reason I’m considering a Jack plate. It allows me to move the motor up without drilling a bunch of unnecessary holes in the transom during testing.
Ok. I’m convinced that I need to get a tach of some sort. And a gps. I might have a handheld auto gps around here somewhere. If not I’ll get that also.

It’ll be a few weeks before I can get back to the lake but I’ll be sure to report back with results.

Thanks much!!

 

[GYPSY400]

Get a tiny tach off Amazon and a GPS app on your phone.. then change the prop.

Sent from my SM-A526W using Tapatalk

 

[FuzzyGrub]

It sounds like the prop included with your motor is for a larger, heavier boat. I'd contact the dealer and see if they would exchange it for one with a little more pitch, say 1-2".

I think it worthwhile to still run the numbers in the prop calc and see how close it comes to what you have.

Raising 3/4" on the existing transom, should not be an issue. Going further than 1" would probably neccesitate a jack plate. Need to decide if its worh the trouble. Do you make long runs where the slight efficiency or slightly higher speed will pay off for you? If that answer is "yes", then need to look at what Pappy mentioned, about props that can work good closer to the surface. That usually means a more "cupped" prop and different rake. Not sure if there are "performance" props readily available for these small engines.

 

[FuzzyGrub]

The mechanic says to lower the tilt all the way down to the lowest hole.

Your mechanic is a complete IDIOT and you must tell him that I told you so! That can cause a "bow steer" trim condition and on small boats - that can KILL!

Please wear your ignition kill lanyard ...

Use the Prop Calculator here NOW ... https://forum.tinboats.net/viewtopic.php?t=37433 ... and put in YOUR hull/motor and actual/honest weights for all gear carried, or pick a hull that is closest in match for what YOU have. See what that calculator tells you that YOU should be using for your exact rig and how equipped.

Yea, I was giving him the benefit of the doubt. I assumed he may have known something about his boat and set-up, that I didn't, or could think of.

Every operation manual I've seen includes pictures of boats under and over trimmed, with big "X"'s through them.

 

[DaleH]

Not sure if there are "performance" props readily available for these small engines.

Likely limited availability, but Turning Point does make a new line of what they call as"pressed" (forged?) aluminum props where the blades are almost as thin as SS props, but costing $80 or so, not $100s of dollars. I config'd a few 40hp and 60hp rigs with them and the results were impressive!

Using that calculator by them that I linked to, you'd get notice if they have a premium prop available for your rig.

 

[Jimbeau]

I’ll definitely check Amazon for a tiny tach and get a gps app also. Does anyone know what RPM the rev limiter should activate? Just want to make sure my limits are correct. I’d be surprised if there’re off but worth checking.
I doubt the dealer will swap a prop out. They certainly didn’t offer anything. The mechanic said they have to tilt motors down (all the way) sometimes to get this to stop. He’s never seen my boat! They just sold me the motor. Maybe he is an idiot, I don’t know. He probably just has bigger problems than my little boat. I know he loaded a dang nice 25hp in my truck instead of the 20HP that I paid for and tried to send me on my way, even after I questioned the part number on the box. There’re lucky I caught that or it would have been a costly mistake! There’re a high volume dealer. Nice folks. Can’t beat the price, but it’s a 100 mile drive and not exactly impressed with the attention to details.

 

[MrGiggles]

I’ll definitely check Amazon for a tiny tach and get a gps app also. Does anyone know what RPM the rev limiter should activate? Just want to make sure my limits are correct. I’d be surprised if there’re off but worth checking.
I doubt the dealer will swap a prop out. They certainly didn’t offer anything. The mechanic said they have to tilt motors down (all the way) sometimes to get this to stop. He’s never seen my boat! They just sold me the motor. Maybe he is an idiot, I don’t know. He probably just has bigger problems than my little boat. I know he loaded a dang nice 25hp in my truck instead of the 20HP that I paid for and tried to send me on my way, even after I questioned the part number on the box. There’re lucky I caught that or it would have been a costly mistake! There’re a high volume dealer. Nice folks. Can’t beat the price, but it’s a 100 mile drive and not exactly impressed with the attention to details.

I don't know about smaller motors but the rev limit on big Fourstrokes is 6200.

It should be listed in the owners manual.

 

[DaleH]

I don't know about smaller motors but the rev limit on big Fourstrokes is 6200.

That's the same as my vintage '98 175hp. But I will say I get 2 different readings between the ECU (the OB 'brain'', from reading the computer history) and the tachometer!

I do believe however, that my tach is accurate @ lower RPMs, as that's where I set the idle speed in gear whilst using a timing light. But I sure ain't hanging my arse off the back of the exposed motor @ 6K RPMs to verify the TOP end, LOL!

Opinionated? You betcha! Biased? Yup,that too. But stoopid? Nope ...

 

[RaisedByWolves]

The mechanic says to lower the tilt all the way down to the lowest hole.

Your mechanic is a complete IDIOT and you must tell him that I told you so! That can cause a "bow steer" trim condition and on small boats - that can KILL!

Please wear your ignition kill lanyard ...

Use the Prop Calculator here NOW ... https://forum.tinboats.net/viewtopic.php?t=37433 ... and put in YOUR hull/motor and actual/honest weights for all gear carried, or pick a hull that is closest in match for what YOU have. See what that calculator tells you that YOU should be using for your exact rig and how equipped.

If there were ever a time to call a stranger you never met (Wait, are there strangers you have met) an Idiot it is a time like this.
I can just see Dale up out of his chair and shaking his fist.