3 different props and still over reving

[89retta]

I just bought a 2004 Caravelle 187 it has the 4.3 gxi. The guy I bought it from had a 17 ss 3 blade on it and was spinning at 5800 plus hitting the rev limiter. Tried a 3 blade 19 ss prop brought it down to 5400. Last prop is a 21 pitch 5 blade revs are 5200 and hits the limiter when I trim up. Speed is 43mph with the 5 blade. Should I try a 4 blade ss in a 20 pitch that is cupped. Thanks for any info

 

[SkiDad]

i would think you would hit the rev limiter way before 5800 - i wonder if you tach is reading high.

 

[steelespike]

I'm pretty sure your tach is reading high. Your rpm and gps speed with each prop would help.
but here is what I got with the 21" 5 blade using a 1.81 gear ratio and 13% slip Speed should be 50.
If we go the other way 43 at 13% slip,1.81 ratio,21"= 4500 rpm.wot rpm needs to be about 4800 rpm
The 5 blade will hold down your rpm and probably won't produce the best speed.It should have amazing grip and may actually be faster than my estimate because the slip may be lower than 13%.
The 5 blade is usually suggested for more powerful boats.

 

[Chris1956]

Gee, I would not think that boat would even hit 50MPH. It should make the mid-40s, I would think. Sounds like the Tach is incorrect.

 

[89retta]

It was hitting the rev limiter. Hits it at around 5400. Speed was done with gps. Not really concerned with top speed just like over reving and when I trim up with the 5 blade hits the limiter pretty much right away. I have an sx outdrive with a 1:66 ratio

 

[Maclin]

Using a prop calculator, at 43mph a 1.66 drive ratio with 21 pitch and 11% slip shows 3887 rpm. A 1.8 drive with same numbers is 4373 rpm.

So... thinkin' tach is set wrong or showing wrong.

If the tach is set to read coil pulses for a 4 cylinder but fed by a 6 cylinder then 6000rpm reading is actually 4000 rpm, just as an example.

 

[89retta]

It is the tach that was in the boat since new. I was thinking it was off also. Does the tach control the rev limiter and could I unplug it to see if it still hits the limiter? Thanks again

 

[jestor68]

A 1.66 ratio turning a 19 pitch prop at 45 mph(10%slip) works out to about 4613 rpm.

Your tach is wrong. You might try pulling the connector off the back of the tach and cleaning the pins(3?) to restore proper operation.

I need to do that on some of my tachs once a year or they'll start goofing up; usually reading too high.

According to the prop selector a 19 pitch should be the correct prop, yielding the aforementioned rpm and speed.

 

[89retta]

The limiter was kicking in at a little over 5200 which is where it is suppose to kick in. So thinking the tach is right. I'm going to try an Apollo XHS 4 blade in a 20 pitch. Hoping the larger diameter and cupping will grab more water and keep the rpm's down.

 

[Maclin]

89retta, the point about the tach we are trying to make is that the with a certain drive ratio and MPH the numbers (RPM/MPH/RATIO) have to jive. Your Tach number is way off if the other numbers, like the MPH, are accurate. If the Tachometer is integral to the rev limiter circuit then sure the limiter will kick in even if the actual RPM is off.

 

[Barramundi NQ]

Hi 89retta, Listen to the wise words above, because much of what you've said is based on a presumed accurate gauge, the basis for changing the prop is wrong. Start at the beginning and confirm the tachometer is accurate before all else. Then come back. Remember we're all on your side

 

[Texasmark]

A 1.66 ratio turning a 19 pitch prop at 45 mph(10%slip) works out to about 4613 rpm.

Your tach is wrong. You might try pulling the connector off the back of the tach and cleaning the pins(3?) to restore proper operation.

I need to do that on some of my tachs once a year or they'll start goofing up; usually reading too high.

According to the prop selector a 19 pitch should be the correct prop, yielding the aforementioned rpm and speed.

I ran the same numbers on my BAM calculator and got the same answer. So if the limiter is limiting and the tach is off, where is the smoking gun?

I realize that this is an I/O and I don't know where the limiter and tach get their signals. OBs get theirs from the battery charging rectifer/regulator module. They count the sine waves coming in from the alternator under the flywheel which are dependent upon how fast the crankshaft turns, aka revolutions per minute.

In running the numbers on the posted 21 5 blade at 5200 and 43 that looks like about 31% slip which is a lot if you ask me unless you were coming out of a hole shot, before the boat leveled off and the prop got a good solid bite.

BruceB 58 can probably provide the origin answer.

Mark

 

[Maclin]

Yo TexasMark, on most I/O engines the number of signals to the coil, sensed from the negative terminal, feed the tach and it meters out based on a division of the signals by the number of cylinders the tach setting is at. Same as most car engine tachs worked up until the proliferation of electronic engine management with crank sensors and the like.

 

[Maclin]

Oh yeah, on the Rev limiter for the OP's boat, I do not know if that is separate unit from the tach or if the tach is a contributing component.

 

[89retta]

Okay will go out tomorrow and check the tach to confirm if it is right. Bought a new one to see if I get the same numbers. I haven't checked the oil in the outdrive perhaps something is slipping in there. The rudder has a curve to starboard and there is a piece missing at the leading edge. Also the 17" prop had a good ding in it. Thanks for all your help.

 

[89retta]

Should I also check the exhaust? This boat is louder then other similar models. If my bellow is broke would this not cause aeration in front of the prop. And in turn cause the prop to slip. I have an hour drive to where I keep my boat stored so I want to make sure I check it all when im there.

 

[Scott Danforth]

check the RPM with a hand-held tach. you can burrow one from your local auto store.

yes, leaking exhaust could cause blow-out. fix your bellows

 

[Tail_Gunner]

Yes the 17 is far to small....You have a electrical issue somewhere your engine could not reach 5800 rpm that was a bad read and a 4blade 20 ss should be about right.

 

[Texasmark]

If the bellows is cracked open, at rest the boat can fill with water so you will know right away if there is a problem with that. Besides it is out of the water when running an any kind of speed.

Mark

 

[NYBo]

Bought a new one to see if I get the same numbers.

That was a bit premature. Your old tach may be set for the wrong number of cylinders or just have a bad connection somewhere. Keep the new tach in its packaging; maybe you can return it if the old one turns out to be okay.

I haven't checked the oil in the outdrive perhaps something is slipping in there.

There is nothing to slip in the outdrive. A bad coupler could, though.