Declining Efficiency

J

jeff_c

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Hi all,

I need your help.

I have a 2007 Formula 31PC with 400 hrs on the Mercruiser MX 6.2L MPI Horizon engines.

I'm looking to build out a list of things (that I can hopefully check one by one) to determine why my boat has progressively become less efficient as it relates to fuel burn. This is my fifth season with the boat (I bought it new) and in season 1 I was burning 9+ gph, during season 2 it went to 10+ gph and the 11+ in season 4 and now it's at 12.5 gph and trending up (overall I am at about 12 gph - 5000 gals over 400 hours). Some important facts: 90% of my boating is running east-west on the Great South Bay (a rather tame and predictable body of water) and I like to run at 24 knots as this is where the boat runs the most comfortably. I used to be able to hold this speed at 3200-3300 rpm but now I need to hold the engines at 3500-3600 rpm to maintain 24 knots (in similar conditions). I tried running her up this weekend and was only able to bring it up to about 34 knots at 4200 rpm max. It's been a while since I've done something similar to that but I have vivid recollection that the last time I did the same I was able to bring her up to about 36 knots easily and still felt that there was more there (I just couldn't bear to burn more fuel for no good reason).

Anyway, I just did the math this weekend so I haven't been able to investigate much but here's the list of causes/cures/suggestions I've compiled so far from various dockmates:

1) get back on your diet, Jeff (working on it)
2) finish those bottles of Kettle 1 already (took care of that this weekend)
3) When was the last time you had the waste tank emptied? (were they telling me I'm full of crap?)
4) check the props (stainless duo-props - we did a visual on them during the winter and other than tarnish they looked perfect)
5) check the bottom paint (it's supposed to be ablative)
6) have the hull checked to make sure it's not absorbing water (yikes!)
7) when was the last time I did a tune up? (never...the plugs are rated for 5 years and we pull 4 each year to check them as well as the cap and wires - they all look like new)

Does anyone want to weigh as to other possible causes and/or the possibilities on any of the above being a likely culprit?

TIA,

Jeff
 
At what speed are you getting these gph figures. Or is it an overall average.

Regardless, I want to know just how you are getting such good mileage on twin inboard engines.

George
 
Our last boat was bought new in 2005 and in the 3rd season the 350Mag running a B3 like yours started to have some of the same symptoms you had. I changed the fuel filter (gen 3 design sucks) first but it didn't help. I called in a mechanic and he discovered the distributor cap had some green on the contacts so it was replaced. Things were better but it was still wasn't right and couldn't get to 5200 rpms where it used run up to. I had the mechanic come back and he changed the plugs and the problem was solved. They left me the old plugs and I didn't think they looked bad but on the first trail I ran right up to 5200.

Good luck
Niles
 
These are overall numbers including 5 mph zones (there's at least 1/2 hr down/up my canal at no wake for each trip) and cruising in various conditions on the GSB (90+% of the time) and occasional near shore voyages (like heading to NJ or going from Jones Inlet to FI Inlet). Part of the reasons that I tend to get better 'mileage' is that a) it's a Formula and b) I work the engine trims constantly to maximize my speed/rpm at a particular throttle point with the baseline being 24 knots.

BTW, here's the spreadsheet: https://spreadsheets.google.com/spr...dU5oZkFJVjRUbEtvNnc&hl=en_US&authkey=CMC5od0O

Also, I just spoke to my marine mechanic and he suggested two things...one is the tune-up and the other was the increasing amount of ethanol that is in the fuel that we use. Both good points. So while I can't do much about the fuel I can finally bite the bullet and do the tune-up.
 
quote:
Originally posted by j-d

Sounds like weight to me.

Click to expand...


me too...but I can't figure how that would increase enough to matter each year with such consistency unless the hull is absorbing water.

So a good question is how much weight would it take to make a difference?
 
You mechanic is spot on. Get a full tune-up, Ethanol is ruining everyone's mileage...
 
Certainly need a tune-up. What it the rated WOT RPM's (wide open throttle) for the engines? Are you hitting that? You should take a run up to WOT at least once if not more times per year. You burn some fuel but it's a great check of how the boat is running. If you're not hitting WOT there's only a few things it could be... Props (dinged, nicked, dirty etc), Engine (tune, preformance), Bottom (dirty) or weight (water, gear, people).
 
Have you ever sent your props out to be "prop-scanned"?
 
My number one thing would be to look at weight. Look at how the boat is sitting in the water. I started experiencing performance issues, mostly slower to plane, and found that there was water getting trapped in the forward part of the hull. There was some dirt blocking the weep holes. Whenever I got water in the bilge it would work its way forward but couldn't find it's way back to the bilge pumps. I noticed that the bow was down in the water a bit compared to the bottom paint line. I cleaned out the crud and got rid of the water and the problem went away.
 
Jeff, I frequently run a friend's 2006 31PC. (love that boat btw)

His didn't come with flow meters. How are you calculating the burn rate?
 
You don't mention if you are putting bottom paint on her. If so, that's going to add a lot of weight annually.

Agree with a tune-up. You need to be able to hit the Merc WOT recommendations. You are adding a lot of unnecessary wear on your engines by not doing so.
 
If you haven't had a tune-up in 4-5 years you shouldn't be surprised at the results you are seeing. Not everthing may need to be replaced but at least checked, cleaned and adjusted. Cap, rotor and Dwell and timing.
 
I am definitely going to do a tune-up...top of the list now. It wasn't done before because the plugs are rated for 500 hours and everything else is pristine. My mech confirmed that each year much (if not most) of the paint is removed from the bottom as I often have bare spots (down to the barrier coat) so I think I am good there. I will check the area under the v-berth (where the H20 tank resides) for hanging water as well as check the anchor locker to make sure I am not carry anything I don't need.

Also, I don't necessarily note it here but I often take the last mile or so of my return trips (from the second Wantagh Bridge to the first) to open it up as the water there is often like glass but I don't always note the RPMs and speed. But I would say that today I can't reach the same RPMs at WOT that I could in the past.

Hogan, good idea, I will look into have the props "prop-scanned" the next time it is pulled.

SLW, thanks, we love it too. I do have smartcraft gauges but those numbers are all instantaneous. I am using real numbers based on actual fill-ups and hours: https://spreadsheets.google.com/spr...dU5oZkFJVjRUbEtvNnc&hl=en_US&authkey=CMC5od0O

Thanks, all.

Keep the ideas coming.

Jeff
 
Change the plugs and filter! You live up north so I am guessing you have the engine fogged every fall? That is not good for the plugs.
 
weight, drag, and bad props added to decresed fuel power content all contribute to the problem.
 
quote:
Originally posted by Padraig

Change the plugs and filter! You live up north so I am guessing you have the engine fogged every fall? That is not good for the plugs.

Click to expand...


Hummm...which filter are you talking about? All filters from gas to air are changed each spring.

Full tune-up will be next week at the latest so hopefully I'll see a difference after that.

Thanks,
Jeff
 
I wouldn’t expect much change , let us know.
 
Hogans got it, ethanol in fuel lowers mileage in cars or boats. 10-15% ethanol equals less mileage. Add in the weight factor of "items" added. Get a full tune up, remove what you don't use, and look for fuel without ethanol.
 
quote:
Originally posted by pdecat

weight, drag, and bad props added to decresed fuel power content all contribute to the problem.

Click to expand...


+1

more than likely a combination.

Look at the numbers and youi can see the trend started almost immediately.

Each of the items discussed is a factor and added together they become significant.

The good news here is that no one is ranting about the 6.2's, the props the manufacturer and the dealer.

Please post your results for us all.

RWS

RWS
 
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