Where did my top end go?-gctid383512

[Guest]

So after a few outtings this season, I've noticed that the power plant package of my boat is not doing what it's supposed to.

Standard merc cruiser 3.0L w/o alpha 1 gen 2 here. Last season I was able to do about 30mph @4000rpm and about 40mph WOT @ 4800 RPM, speed is measured with GPS so it's pretty accurate.

This season, I'm doing about 25mph @ 4000rpm and WOT I cannot get above 33mph @ 4800RPM.

I've ruled out the prop/hub slipping since I switched to an identical back up prop / hub and it's still doing the same thing.

I've noticed no obvious leaks or anything wrong anywhere. I did the standard winterizing last year, changed the impeller at the beginning of this season and everything went back fine and dandy.

Fuel filter was changed last season.

The load is the same in terms of weight, 2 adults, 20 gallons of fuel.

So I'm at the end of the line here in terms of my ability to diagnose issues.

Help please!

[Guest]

info on the boat might be helpful, please fill out your signature so its like mine and has your details.

That being said, is the bottom clean? Motor trimmed properly?

[Download_Complete]

Time to do a FULL tuneup.

[Guest]

Download_Complete wrote:
Time to do a FULL tuneup.

Seems extreme for a 2 year old boat no?

The boat is a tahoe q4, basically the same as a bayliner 175. I found this forum when I was researching tahoe vs bayliner and stayed as a lurker :arr

[dmcb]

Download_Complete wrote:
Time to do a FULL tuneup.

I don't think that is the problem. He isn't getting the speed for the rpms he is turning like he was. If he got 30 mph at 4000 rpms before, he is still able to turn 4000 rpms but in only going 25 now. His top rpms are down but there is weight or drag somewhere that could account for this.

Bottom condition would be first thought. Trim as stated another.

Weight in the form of water in a place that can't be pumped is another place to look.

Doug

[Guest]

dmcb wrote:
I don't think that is the problem. He isn't getting the speed for the rpms he is turning like he was. If he got 30 mph at 4000 rpms before, he is still able to turn 4000 rpms but in only going 25 now. His top rpms are down but there is weight or drag somewhere that could account for this.

Bottom condition would be first thought. Trim as stated another.

Weight in the form of water in a place that can't be pumped is another place to look.

Doug

Forgot to add, the bottom is clean (trailered boat), the trim is perfect as can bear, usually on a smooth day, I trim out to near cavitation and still speed doesn't go above 33-34 where i used to get 40.

I have smart tabs on the boat, at the end of the season last year, the boat was still working perfectly and it was covered over the winter.

I can't imagine water getting in through the tabs some how and wetting the foam.

[Guest]

Mordekai wrote:
Forgot to add, the bottom is clean (trailered boat), the trim is perfect as can bear, usually on a smooth day, I trim out to near cavitation and still speed doesn't go above 33-34 where i used to get 40.

I have smart tabs on the boat, at the end of the season last year, the boat was still working perfectly and it was covered over the winter.

I can't imagine water getting in through the tabs some how and wetting the foam.

bad gas.....did the gas tank get winterized also???...cuz that will slow you down alot!!!

:arr arr

[Guest]

I say full tune up as well, You should of seen my cap and rotor after two year new boat, not to leave out the plugs. Replaced all and preformed way better! I was loosing power as well. Cheap and easy fix!

[Guest]

Mordekai wrote:
I've ruled out the prop/hub slipping since I switched to an identical back up prop / hub and it's still doing the same thing.

Are you sure it's 100% identical? There are many attributes to a prop other than diameter and pitch that will affect how the prop performs on your boat. Attributes like actual blade area, the shap/outline of the blades, cupping of the blades.

If your other prop had been repaired, the diameter may be a little smaller than the diameter according to the specs for the model number of prop you have. Also if repaired the cup of the blades may have been changed during the repair process.

[Guest]

Are you sure the smart tabs are actuating properly? If they were stuck in the down position, it might explain the extra drag and loss in speed.

[Guest]

myflies wrote:
Are you sure it's 100% identical? There are many attributes to a prop other than diameter and pitch that will affect how the prop performs on your boat. Attributes like actual blade area, the shap/outline of the blades, cupping of the blades.

If your other prop had been repaired, the diameter may be a little smaller than the diameter according to the specs for the model number of prop you have. Also if repaired the cup of the blades may have been changed during the repair process.

Nope I bought 2 identical prop w/ same pitch / diameter and manufacturer.

[Guest]

I think lots of good guesses but still lots of unknown factors.

No location stated. Where are you boating? salt or fresh? tides? current? River? Lake? Same lake? Different altitude?

As has already been suggested 2 years is a long time to leave spark plugs in without checking and either re-gapping or replacing.

Same with cap and rotor and fuel filters.

[Chief_Alen]

Engine Won't Reach Operating RPM. Check

1. Fuel condition. Type and Octane possibly old fuel

2. Propeller pitch or diameter, damaged blades

3. Restricted fuel pickup tube or anti siphon valve Fuel System Test

4. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse

5. Marine growth on hull and outdrive

6. Wrong gear ratio in outdrive

7. Restricted carburetor air intake (clogged flame arrestor)

8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive

9. Poor cylinder compression Compression Test

10. Carburetor defective, or wrong type.

11. Fuel pump pressure and vacuum

12. Boat overloaded, improperly loaded, or improperly trimmed.

13. Engine Overheating

14. Engine timing and ignition system operation

15. Remote control cables and linkage for proper travel to open throttle plates fully.

Above copied and pasted from another website Don s is the author.

[dmcb]

I could be wrong about this but I think you guys are missing a big clue.

1. At 4000 rpms he was doing 30 mph.

2. Now at 4000 rpms he is doing 25 mph.

The tune of the engine doesn't matter. When it turns the same rpms it should be doing the same speed.

Its not.

Whatever is preventing that most likely is also preventing the engine from reaching the top rpms.

The tune up of that engine doesn't matter a bit when the same rpms don't get the same results. At 25 mph its on plane.

Its something else. Drag or weight or trim.

Doug

[Guest]

dmcb wrote:
I could be wrong about this but I think you guys are missing a big clue.

1. At 4000 rpms he was doing 30 mph.

2. Now at 4000 rpms he is doing 25 mph.

The tune of the engine doesn't matter. When it turns the same rpms it should be doing the same speed.

Its not.

Whatever is preventing that most likely is also preventing the engine from reaching the top rpms.

The tune up of that engine doesn't matter a bit when the same rpms don't get the same results. At 25 mph its on plane.

Its something else. Drag or weight or trim.

Doug

Which is why I was asking about the water. Depending on the water conditions I can be slower or faster at the same rpm on any given day. A 5 MPH loss is not a drastic loss.