WOT max rpm loss

[pilot2fly]

So we've been noticing lately that its been really hard to get up on plane without putting one or two people at the front of the boat. I took some readings on the tach and at WOT my port engine RPM's are 3200 and my starboard is 3300. When we went back and looked at the marine survey we got done before we bought the boat in March, it maxed out at 4400 rpm's and 41 MPH. I can only get it to 31.5 MPH at WOT, even when I experiment with the trim tabs. We noticed yesterday there is barnacle growth on the trim tabs, but we don't believe there are barnacles anywhere else on the boat (needs further inspection to be sure). I'm thinking that might have something to do with it, but would it cause a near 1000 rpm loss? There also seems to be a thin layer of gunk on the bottom of the boat from sitting in the water all the time.

One thing I read on here is that bad ignition coils could contribute. We had a bad one on the starboard engine last month and replaced it immediately. Port engine ignition coil hasn't been changed, since it didn't give us any trouble. The fuel lines are a little kinked as well.

We do keep the boat in salt water near Wrightsville Beach, NC, but the previous owner bottom painted it just before we purchased it in mid-March. We always try to take a scrub brush and brush off any gunk on the bottom when we go and anchor out, but its hard to get it all off when the boat is in the water all the time.

Any ideas? Happy to give more info if needed.

 

[blaster]

Even with bottom paint you can get growth which can slow you down considerably. The paint slows the growth but doesn't eliminate it. Check and see, then if its dirty, haul it out for a cleaning. Or rent a scuba tank and get a spatula.

 

[Lazy Daze]

Get the bottom and running gear clean before you do anything else. From what you posted, a dirty bottom sounds like the most likely culprit. Are the props perfect?

FYI, 4400 RPM's is the bare minimum for where that engine's max RPM should be. It should really be as close to 4,800 as possible.

 

[Chief Alen]

Don't want to step on any toes here, seeing as i am relatively new here.

If this were my boat ...

I would first make sure the bottom and drives are clean.

I would confirm by looking on the cover or engine recommended wot, rpm for your engines and year.

Say your recommended wot, rpm is 4200-4600 as mine is; a 1994 5.0l engine 4400 would just about be perfect.

My first step would to make sure of what my recommended wot, is for the engines in the boat.

This would be critical and easy, should have a sticker right on the engine or plastic cover.

Have the bottom cleaned and in my mind there could be restrictions in the raw water pickup.

 

[Lazy Daze]

Say your recommended wot, rpm is 4200-4600 as mine is; a 1994 5.0l engine 4400 would just about be perfect.

My first step would to make sure of what my recommended wot, is for the engines in the boat.

The newer the engines, the higher the WOT RPM. His year is 4400-4800. But, sure, can't hurt to double check.

 

[pilot2fly]

Thanks guys. Yeah its 4400-4800. We talked to the marina owner and they are going to haul the boat next week for a proper cleaning. Will post more then.

 

[Dsybok]

I don't know about the waters of NC, but here we have to clean our hulls monthly, and have inexpensive dive services to do it in the water. I can tell you that the difference between a clean hull at the beginning of the month and a fuzzy hull at the end of the month can easily be 1,000 RPM and a lot of knots.

I run 4600 RPM the day or a few days after my cleaning and I can drop all the way down to 3400 with a dirty bottom.

 

[mquiet]

Pilot, we are just north of you. I took my boat for a ride on the water yesterday to clean the bottom. Prior to cleaning, I was able to get 3600rpm and 2mph at WOT. We then cleaned running gear and bottom. On the way back we turned 4200rpm and cruised at 32mph. If you have any bar ice growth then your performance degrades. Also note it had only been 1.5 months since I last cleaned the bottom and gear.


Sent from my iPhone using Tapatalk

 

[Lazy Daze]

Thanks guys. Yeah its 4400-4800. We talked to the marina owner and they are going to haul the boat next week for a proper cleaning. Will post more then.

Good plan. Let's hope that's all it is.

 

[pilot2fly]

So we had the boat cleaned and there was a bit of barnacle growth on the trim tabs and props. Tested today and went from 3200/3300 before the cleaning, to 4000 max rpm's at half tank of fuel and 2 people. Got on plane quick. That definitely helped alot but there is still a layer of gunk on the props that will need to be sandblasted. Planning on doing that during the winter haul out. Hopefully that and a more detailed cleaning/new bottom paint will help improve rpm's to the 4400 range.