First boat, rpms too high?

[ba1335]

Hey guys,
First time boat owner. Bought a boat which just had the engine rebuilt. Took it out to work out the kinks and hopefully you can help me get this one worked out. I have an 18ft fiberglass sylvan with a 4cyl 3L mercruiser. When I open it up it takes about 6-8 seconds to get on plane which seems fine but the engine climbs to 5500-6000 rpm. I would like to think the tac is off but the engine deffinitely sounds like it's working way to hard. Again this is my first boat and I'm not sure what it's suppose to sound like but the fact that it made me uncomfortable is cause for concern. I never ran the engine like this any longer than a 30-60 seconds. After each run like that the engine would start to smell, which I assume is just burning off some spilt oil and lube, all of which has been changed before I took it out. The temp gauge never climbed from where it was during idle. What are your thoughts? I'd rather not take it to the shop until I have to. Would adjusting the throttle and carbs help? I don't think this will do anything for my rpm issue, and don't need to mess with something that's not broken. I also have a 19 pitch prop. Would getting a 21 lower my rpms? Also, I only had one other person with me. The boat is rated for 8, would riding closer to capacity effect the rpms or just the amount of time required to get on plane and top speed? Any ideas or additional information would be greatly appreciated. thanks

 

[QC]

Re: First boat, rpms too high?

Let's get you to the prop section first for this one. I am thinking Tach still, but please read the stickies above. Good luck!!!

 

[steelespike]

Re: First boat, rpms too high?

As suggested get us as much info as possible using the stickys at the top of the prop forum..If your motor was just rebuilt you may need to go through a break in period.
I too believe your tach is reading high.The info you get us will determine whats up.

 

[ba1335]

Re: First boat, rpms too high?

Thanks for the heads up, here is some of the information, will provide the rest later this evening.

1) Performance issue you are trying to correct. - High rpms, don't want to blow up the engine
2) Current prop manufacturer, model, aluminum or stainless as a minimum. - will provide later tonight
3) Current prop diameter and pitch (required). - Tonight and 19
4) Wide open throttle RPM and speed with an average load (very helpful) - 5800rpm going ~32mph
5) Engine/drive make, model, year, and HP - GM 4cyl. 3L, tonight, 1996, and I would assume 135hp
6) Boat make model, year, length and weight - 96 sylvan V178, 18ft, ~3000lbs

 

[QC]

Re: First boat, rpms too high?

If the MPH is correct (GPS preferred) then the tach is Waaaaaaaay off. Since you are going to see the boat, there is typically a switch in the back of the tach for 4 , 6, or 8 Cyl. Check that while you're there. You should be going around 46 MPH if the data was correct (assumes 12% slip and 2:1 ratio), so we can assume the data is bad at this point. Of course any one bad piece of data can mess this up, but a 3.0 spinning that fast, well it wouldn't . . .

The slip comment is for any others paying attention; don't get hung up on that yet!! Long discussion.

 

[ba1335]

Re: First boat, rpms too high?

The mph felt right, the gps is a good idea though. The rpm's also felt and sounded accurate. I will check the tach switch but I doubt this wouldve changed as these are all in dash gauges which I assume are all original. I never thought about calculating how fast I should be going, but thats all theoretical and doesnt take into affect other things such as drag and so fourth. From my research this boat, with this set up should only hit about 35 max anyways, unless I'm mistaken. I assume slip is calculated but can i check to confirm the 2:1 ratio?

Another what if: What if the tach is correct and the engine really is spinning that fast, would this mean that there is significantly greater slip? Where does this slip occur, is it a simple look and see, or is it involve ripping everything apart?

 

[Tail_Gunner]

Re: First boat, rpms too high?

The above posters are correct you have both a tach problem and speedo. a little 3.0 cannot rev that high pushing a boat...Maybe in netural it might migh reach 6000

 

[QC]

Re: First boat, rpms too high?

If the data is correct, and I agree with TG, you have 39% slip. Oh, and slip is "drag and so forth" . We're looking for slip numbers from 10 - 15% in your application . . .

 

[Idlespeedonly]

Re: First boat, rpms too high?

Another what if: What if the tach is correct and the engine really is spinning that fast, would this mean that there is significantly greater slip? Where does this slip occur, is it a simple look and see, or is it involve ripping everything apart?

The slip occurs in the water. You pitch is 19" ,so for every revolution of your prop it travels theoretically 19" forward. Which never happens. The slip is due to drag basically.

 

[ba1335]

Re: First boat, rpms too high?

Assuming the info i get from the boat isn't out of the ordinary, is my next step to take it to the shop and get the tach qualified? (I assume the speedometer is more or less useless and probably a pain to calibrate.)

If the tach is reading high and the engine is actually only running 4500 rpms is this the ideal way to run the boat? Even if it does sound like its going to blow up......again first boat/engine which isn't muffled by a firewall.

 

[ba1335]

Re: First boat, rpms too high?

Aluminum prop. Aprox 13 1/8" diameter.

 

[QC]

Re: First boat, rpms too high?

If the prop is in good condition, and a true 19 inch pitch, the missing pieces of data are gear ratio (I have been assuming 2:1), confirmed RPM , and confirmed speed. Don't get hung up on Diameter at this point, but thanks for the info. Mo is betta!!!

BTW, you can get a tiny tach cheaper than a trip to a shop

 

[ba1335]

Re: First boat, rpms too high?

Where can I check the gear ratio on the mercruiser?
And if I get a new tach can I assume the wires are correctly hooked up to the motor? Or how hard is it to hook those wires up to the engine?

 

[QC]

Re: First boat, rpms too high?

Let's forget the gear ratio for now. If it was wrong, the indicated speed would be even higher, so I am still confident the tach is off even if the ratio is wrong. Let's check the switches in the back first, and go from there. It doesn't matter how old or new the tach is. Could've been set wrong forever. I've bought a new boat and had the tach set wrong, and many a guy here has found theirs wrong too for exactly the same reason you are . . .

Do you have year model? S/N? Anything else from the drive?

 

[fishrdan]

Re: First boat, rpms too high?

Aluminum prop. Aprox 13 1/8" diameter.

13" diameter and 19 pitch, that doesn't seem right as it should be around 14" diameter for the 3.0 Mercruiser. Is the prop barrel the same size as the drive case, 4 3/8 diameter? Reason I ask is there are 13Dx19P outboard props. Something just seems off with a 13" diameter prop. What identification and numbers are on the prop, you might have to remove the prop to get the numbers.

I think something is off with the RPM's too, doesn't seem right being able to turn a 3.0L 5800-6000RPM under load

Oh yeah, your WOT (wide open throttle) RPM for the 96 3.0L is 4400-4800RPM. So, with an average load you should be able to run between that range when holding the engine balz out, with a light load 4800RPM. 4500 WOT is good, but you don't want to run there all the time for reliability sake.

 

[steelespike]

Re: First boat, rpms too high?

Sometimes you can operate the tach switch to clean the contacts.
My info indicates 1.94 gear ratio,4800 max rpm guessing 10% slip 40 mph could be possible.

 

[QC]

Re: First boat, rpms too high?

I missed the '96 part. I see 1.94:1 as well. So the slip gets a little worse, and the finger continues to point to the tach. I agree on the gearcase to barrel comparison.

 

[ba1335]

Re: First boat, rpms too high?

I checked out the back of the tach las night and there are no switches to be adjusted. There are only 4 wire connections and the light bulb.
I will provide the prop catalog number and hopefully the mercruiser number, but i didnt think i could find any markings on the outdrive.

I really appreciate all the help and knowledge you guys are sharing with me!

 

[airdvr1227]

Re: First boat, rpms too high?

Why was the motor rebuilt? If it was falling off in power perhaps the previous owner compensated for it with prop pitch.

 

[QC]

Re: First boat, rpms too high?

I don't think there is a healthy 3.0 that can pull a 19 to 5800 RPM. Even if it didn't come apart, there is no power up there, so I think that's unlikely.