Need help with Trim tabs and Engine Trim - Guzzling Gas!


3

34PCinMA

New member
Recently bought a 34 PC with 496 Seacore/Axius. I'm having a tough time getting her to efficient cruise. at 3500 she's at about 23 mph and 44 gph, 3/5 tank of fuel, 1/2 tank water, 2 adults.

I believe the drive ratio is 2.0:1 and standard 3x3 B3 prop set. This seems way too slow and way too much gas. I put about .5 up trim on engine (as shown by vesselview) or maybe 1/8 up on the analog trim gauge. Trim tabs were in the middle of deployment. Looking for anyone who has nailed the best cruise with this set-up between trim and tabs. Anymore up trim and the bow comes up, and rpms race up. down trim tabs seem to help speed up to a point.

Thanks to anyone who answers.
 
Can you share what year, although not sure it matters.

Something does not seem right though....

What is your WOT

Is this boat new or new to you ?
 
toofast said:
Can you share what year, although not sure it matters.

Something does not seem right though....

What is your WOT

Is this boat new or new to you ?
Click to expand...

It's new to me. 2009 boat with 450 hours. WOT was only 4250 with engine trim down. Surveyor got 4600 rpm.
 
Yeah something's really wrong have you checked the bottom for growth

Sent from my SM-G960U using Tapatalk
 
WOT was only 4250 with engine trim down. Surveyor got 4600 rpm.
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Surveyor got 4600 because he is a DA. I can take any boat and trim the drives up high enough to get what ever rpm I want. Your getting 4250 so either your under propped or the motors are tired, or need to be tuned up.

Would start by getting some baseline numbers.
Compression test
Leak down test
Scan motors for codes and hours

If this checks out then need to look at props
 
Thanks, guys.

I had her back out this weekend and got about 26mph at 3500 and 40/42 gph after I scrubbed some mild growth off of hull, drives and props that I could reach.

I'm getting ocassional whiffs of gasoline while running at cruisem, sometimes this is strong, barely noticeable other times - bilge is dry, and left engine seems to be burning more just by gas levels in tank. I can't tell in the VesselView display what each is burning that I know of. Could there be a leak or fuel injector/pump issue that would result in that kind of fuel burn/smell?
 
Could there be a leak or fuel injector/pump issue that would result in that kind of fuel burn/smell?
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Many things can cause increase fuel consumption. Your going to need a scanner to look at running parameters. If the the ECT sensor is telling the PCM the motor is cold, then it will be giving the motor more fuel, same goes if the MAP is not reading correctly
 
I have the same boat w/o Axius. we didnt purchase the boat for economy. my numbers are similar. 27-29MPH (depending on tides) @ 3500 RPM. mine came with 22Pitch props. Most come with 24P. I bought a set of 24P props last season... and changed them back. much more lively and economy was better with the 22P props. still burning 38-40GPH. On the bright side, when i purchase a 45 yacht, it will have better economy with diesels.
 

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FWIW I have 2003 37PC 496(non HO) 660 hours and cruise 3600 @ 30.8-31.2MPH and 38-39GPH (self calculated but verified pretty close over 3 years). 4600 WOT @ 43MPH. I usually carry 2/3+ fuel, lots of water, gear and beer and we are not small people. I typically use very little drive trim(@ 3600 but up to 1/2 analog @ 4100+) and no tabs unless quartering or beam wind) Fresh clean water and I routinely scrub bottom paint at waterline so no growth issues. Yours just doesnt seem right. I have tried less than 3500 cruise many times and find need tabs to stop wandering about the keel so maybe you might try nudging up the RPM, back off the drive trim and get off the tabs? Overall, I have come to accept this boat does not like to dawdle.
 
FWIW I have 2003 37PC 496(non HO) 660 hours and cruise 3600 @ 30.8-31.2MPH and 38-39GPH (self calculated but verified pretty close over 3 years). 4600 WOT @ 43MPH. I usually carry 2/3+ fuel, lots of water, gear and beer and we are not small people. I typically use very little drive trim(@ 3600 but up to 1/2 analog @ 4100+) and no tabs unless quartering or beam wind) Fresh clean water and I routinely scrub bottom paint at waterline so no growth issues. Yours just doesnt seem right. I have tried less than 3500 cruise many times and find need tabs to stop wandering about the keel so maybe you might try nudging up the RPM, back off the drive trim and get off the tabs? Overall, I have come to accept this boat does not like to dawdle.
 
409 said:
I have the same boat w/o Axius. we didnt purchase the boat for economy. my numbers are similar. 27-29MPH (depending on tides) @ 3500 RPM. mine came with 22Pitch props. Most come with 24P. I bought a set of 24P props last season... and changed them back. much more lively and economy was better with the 22P props. still burning 38-40GPH. On the bright side, when i purchase a 45 yacht, it will have better economy with diesels.
Click to expand...

What are you doing for trim for this? You're still doing a bit better than me. We have the same SmartCraft gauge. Anything over .3 and I hear the props over-running. Can you tell me what the trim display is to get 27-29 mph and 38-40gph? It seems any up trim I give her makes it slower.

Since this was a Florida boat, I'm going to get a full sanding and paint over the winter. Any suggestions here? Also, what do you think about the Mercury XR props? I don't know what pitch I have, I'm guessing it's the factory 24 but who knows - another thing to figure out this winter...hahaha. It never ends.

Agree about diesel. Next will be a 37 PC with VP D4 or D6 diesel, unless I re-power this beast. Maybe try the VW/Mercruiser diesel. Any thoughts?
 
Finallya37pc said:
FWIW I have 2003 37PC 496(non HO) 660 hours and cruise 3600 @ 30.8-31.2MPH and 38-39GPH (self calculated but verified pretty close over 3 years). 4600 WOT @ 43MPH. I usually carry 2/3+ fuel, lots of water, gear and beer and we are not small people. I typically use very little drive trim(@ 3600 but up to 1/2 analog @ 4100+) and no tabs unless quartering or beam wind) Fresh clean water and I routinely scrub bottom paint at waterline so no growth issues. Yours just doesnt seem right. I have tried less than 3500 cruise many times and find need tabs to stop wandering about the keel so maybe you might try nudging up the RPM, back off the drive trim and get off the tabs? Overall, I have come to accept this boat does not like to dawdle.
Click to expand...

Good idea. So you hit 4100 with no tabs and no drive trim, then you trim up halfway to get to WOT at 4600?

So far i’ve only hit 33mph at about 4200 and 56gph. I’m going to focus on growth, bottom paint and maybe new props. This is driving me crazy!

What pitch props are you running? Any bottom, drive or prop paint? Realize you are in fresh so we are going to be different there. Plus the 2000 or so pounds hahaha.

Hardtop?
 
34PCinMA said:
Good idea. So you hit 4100 with no tabs and no drive trim, then you trim up halfway to get to WOT at 4600?

So far i’ve only hit 33mph at about 4200 and 56gph. I’m going to focus on growth, bottom paint and maybe new props. This is driving me crazy!

What pitch props are you running? Any bottom, drive or prop paint? Realize you are in fresh so we are going to be different there. Plus the 2000 or so pounds hahaha.

Hardtop?
Click to expand...

For clarity - I do run some drive trim but not much, less than 1/4 at optimal 3600RPM cruise and progressing to 1/2 at WOT. Never any trim tabs unless needed for weight imbalance or sea state.

24P props - keep them very clean and balanced every 2 years
Bottom painted but clean water and fortunate to be able to clean often so bottom is clean and smooth
No drive paint - drives are new and I keep them clean
No hardtop
 
34PC

You really need to figure out why you can't get the minimum recommended wide open throttle.... cuz that could be a big reason why you're having these issues.

Could be many things that eliminate all those variables and getting to recommended wide open throttle will allow you to really figure out what's going on so I will try to focus on that if you can.

If you're getting there but I missed it I apologize but to me it seems like that's still part of the root cause

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Thanks, yeah I think more trouble shooting, but mechanic found port fuel pump tegulator stuck and bad plug and wire. Hoping the repair will help a little. And soda blast and repaint bottom before spring. So lucky you’ve got diesels. What is your mpg at cruise?!
 
I had a 2004 34PC with 496 mags/sterndrives. At 3500 RPM you should be over 30 mph no problem! Something is def wrong. At WOT i would see almost 50 mph in that boat.
 
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