I had posted this on another thread, but since the conversation seems to be continuing here, the weather looked pretty good on Thursday about 3pm so we headed out on the big lake. Still pretty calm.



We had about 3 miles visibility, and the fog horns were running at the pierhead. I only saw maybe 4 boats all day. Maybe it was because of a weekday or maybe the foggy conditions, but 3 miles visibility is not bad at all.

I got a chance to do a few 5 mile runs for fuel mileage purposes; using different throttle and trim tab settings. Maybe ran 25 miles in all.

I got at best around .9mpg, and at worst, about .5mpg - at different crusing speeds. I didn't even try to get a fix on WOT as it is usually so short, and well, kind of useless information.

But there were some interesting results:

At 3,000 RPM, boat got about 15mph and about .4mpg. At 3,400 RPM, the boat was doing about 20mph and .9MPG, which was the best of the day. At 3,600 RPM, boat got about .5mpg and about 23mph.

At 3,000 RPM the bow ran a bit high. At 3,400 RPM, with those big 5ft wide trim tabs on my boat, at about 50% down, the bow settled down and became the best mileage.

This is kind of a bit counter-intuitive, because logic says that with the trim tabs deployed, it causes more drag. But the deployed tabs cause the boat to run at a much better angle, and especially with inboards, the thrust angle becomes closer to parallel. So the loss of drag in the hull and better thrust angle of the props more than makes up for the increased drag of the tabs, me thinks.

The 3,000 RPM mileage was the worst, and even though the boat was on plane, it seemed to be plowing through the water rather than planing at a good angle. I don't think I got the boat up out of the water enough.

But this was the first time I have tried this, and is only possible since I added NMEA2000 fuel level sensors (not fuel flow sensors), which allow me to see actual gallons left in the tank after each run. What I did was to throttle down at minimum speed, so to keep the boat from rocking from side to side, run for a few seconds, then take a fuel reading. The sensors gave me an accuracy probably +/- a half gallon, much more accurate than a standard fuel gauge.

There were some hysteresis issues with the fuel sensors, but I was able to get it pretty darn close, I think.

Then I made a run for 5 miles, then settled the boat back down again and took another reading.

Now, there might be some error in the reading with the boat still under power, but it seems more accurate than rocking the boat side to side. And since I put the boat into the same state each time, I figure the error will be offset the same each time.

Previously, I concentrated on RPM without as much concern about tab settings, but it appears now that the boat is quite sensitive to trim tab setting, so I need to make a few more runs with different tab settings.

I still want to put in fuel flow metering, but with the EIF fuel flow system I have, I can only use the one made by FloScan. And I don't have the dash room for two 3.5" gauges; at least not yet.

I did find that I had to raise the dinghy on the mounts a bit as when coming off plane, the bottom of the dinghy kind of whacks the incoming back wave. Also I think the boat still squats a bit when coming off plane.

Al--At 15 mph, you're more of "on plow" than on plane--while never in a boat your size, pretty much every boat I've owned, 15 mph seems to be the off/on plane threshold. While I have no data to back it up, best MPG will be obtained between 2800 and 3400 RPM AND at the lowest speed the boat is "freely" on plane. I guess most would agree with that...

As for "logic" saying trim tabs cause drag, my experiences tell me that the opposite may be true. My theory: If you take time to really tweak the drive trim and trim tabs, you can get a substantial portion of the bow out of the water with drive trim, and successfully limit porpoising with the trim tabs. The bow is now slightly higher out of the water, and so is the stern...the result is the net amount of boat displacement - and drag - is reduced.

Like I say, just theory. And might not hold water with a shaft drive boat. But, our old Sea Ray had a drive trim/trim tab sweet spot that was very evident from the speedo and tach numbers when I hit it, and it was like the boat was hardly touching the water. During one run from Wahweap to Bullfrog I got over 5 mpg

FR- I am sure that the 15mph speed was plowing, because when I looked for where the wake kick-out was, it was almost at the bow. But this boat can plane down to about 10mph, much slower than any of my smaller boats.

From this photo:



of the sister ship in my marina, it was going about 15mph - as calculated by MARPA on my radar.

So I think 15mph might be fine, but when I ran at 15mph, I didn't mess a lot with the trim tabs, and only after calculating the mileage figures did I realize that the trim tab settings are going to be quite important.

So I do have work to do. And I do know that 3600 rpm is out - if not for the mileage figures I already have, I am not comfortable long-range cruising at that rpm.

So I am going to take the boat out again and try both 3000 and 3400 rpm with different trim tab positions and see what I get.

One thing I do notice is that at around 3400 rpm, its hard to keep the boat at that rpm. It either wants to slow down or speed up. But at either 3000 rpm or 3600 rpm, the rpm and speed stays put. So its like I am right at an edge performance-wise at 3400 rpm.

Fortunately, I have a trim tab gauge as part of the NMEA2000 sensors I installed last year, so I should be able to get some pretty consistant tests, and I am going to work with it some more.

I did talk to the prevous owner, and he thought I could get up to 2mpg out of the boat. I find that hard to believe, so he must not have taken the boat out a lot.

Its my guess that if I can get the boat at a better planing angle with the tabs at 3000 RPM, that might be the best I can do.

However, Carver's published specs show .9mph at 3400 as ideal, and I did match that. But the previous owner had the larger trim tabs installed, and the normal size is about half as long as the ones I have. So I should be able to really get the bow down.

Its just going to take some more testing.

Al, Just curious if you can hear the differance in how the engines sound. If you take away all the gauges (throw a towel over them) and make a run, adjust till it seems right, how close are you to the settings that give you the best gph burn? All the larger twin engine boats that I have riden in all had that pitch point that just sounded right.

They definately have different pitch sound depending on RPM, but I am not sure I have heard a distinctive sound. Biggest difference I can tell with engine sound is if the synchronizer is not centered.

What kind of carbs you running? I'll bet your 3400 rpm 'problem point' is where your carbs are on the enrichment threshold. They enrich and you speed up, or they don't and you slow down. Sounds like 3600 is a sweet spot, but it won't likely be a fuel saving one. Try shooting for 3200-3300, if I'm right, that may be the most economical spot as you're getting max prop speed before the carbs enrich. You could consider playing with carb tuning, but that's probably overkill...

And where did you get a sync gauge, I can't find one anywhere!

FR: I think Al has EFI engines.

I suppose there could still be critical stages in there, but I would they would be much less noticeable than secondaries opening up.

I bet it deals with the hull and ride conditions more than it does with engines.

AL: are you comfortable at 3500 rpms? It might be the most economical. It sounds like your hull is hitting a critical planing point at 3400rpms, running "cleaner". 3600 might be pushing the boat a little hard. 3500 might be the sweet spot?

FR; they are EFI.

And that is one of the problems - at least from the fuel flow gauge perspective.

The EFI design is called "open loop" I think, where each engine has two low pressure fuel pumps and one high pressure pump. At least that is how I think it is.

One pump brings fuel into the engine, and fuel that is not used is circulated back to the tank.

Maybe I am not describing it correctly, but there are two fuel lines to the tank.

Anyway, to get an accurate fuel flow reading, you need a sensor on the inlet line, and a sensor on the return line that subtracts its reading from the inlet sensor.

I'm told this is what diesel engines use, so I have to use a diesel fuel flow metering system.

FloScan only makes one unit that will work, and it takes 3 1/2" gauges, and I really don't have a lot of room on the dash for them.

What I really need to do is to make a few more runs, and concentrate more on the trim tab settings, to see if it gets the hull on plane at a particular RPM easier.

Crusaders says that I should run my engines at cruise at no more than 75% to 80% of WOT. The engines are 1995 models, so WOT for those are 4,400 RPM, so 3,300 to 3,520 RPM is the maximum RPM I should be running at cruise.

I'll let you guys know.