Most economical cruising speed

[MyPleasure]

Thanks for the input guys! I know all about over propping! When I purchased my Uniflite 36 aft cabin, it came with 19” X 20” 3 blade props. WOT was 3500RPM at 19 knots with 0 inches of vacuum. I wound up with 17” X 16” 4 blade props and WOT was 4400 RPM at 26 knots and 5” of vacuum. My cruising RPM went from 1800 to 2200, but had 10” of vacuum and used less fuel at 9 knots. I always prop my boats so that the engines will get to their max RPM with the boat full of water and fuel.
As far as the Carver 36 aft cabin we currently have, with the big blocks, I guess that it’s going to be cruising at hull speed for us. At almost $10 for a US gallon of gas here in BC, slower is better!
Has anyone ever installed the Holley fuel injection on their engines? Just wondering what the difference would be in fuel consumption over the Edelbrock carburetors on the engines now?
Thanks again fellas!

[MyPleasure]

Thanks for the input guys! I know all about over propping! When I purchased my Uniflite 36 aft cabin, it came with 19” X 20” 3 blade props. WOT was 3500RPM at 19 knots with 0 inches of vacuum. I wound up with 17” X 16” 4 blade props and WOT was 4400 RPM at 26 knots and 5” of vacuum. My cruising RPM went from 1800 to 2200, but had 10” of vacuum and used less fuel at 9 knots. I always prop my boats so that the engines will get to their max RPM with the boat full of water and fuel.
As far as the Carver 36 aft cabin we currently have, with the big blocks, I guess that it’s going to be cruising at hull speed for us. At almost $10 for a US gallon of gas here in BC, slower is better!
Has anyone ever installed the Holley fuel injection on their engines? Just wondering what the difference would be in fuel consumption over the Edelbrock carburetors on the engines now?
Thanks again fellas!

[Rocketman]

I agree, if you drop to 6-7 knots you will see a decrease in consumption.
If you want to run on - my experience is that you need to get up on plane a little more so your not dragging as much.
Typically this ends up being about 3/4 throttle to get the boat really up on plane and running efficiently.
With your fuel flow meters you should be able to do some trials and find your sweet spot for hull speed and on plane.

[km1125]

Source of the post
.... Has anyone ever installed the Holley fuel injection on their engines? Just wondering what the difference would be in fuel consumption over the Edelbrock carburetors on the engines now?
Thanks again fellas!

Going to fuel injection won't really make the engines more efficient over a carb, unless your carb is having issues and not operating correctly. It's much easier and cheaper just to fix the carbs.

Fuel injection can be more efficient, but you need to add a lot of other features to do that other than just injecting the fuel. You need to add a closed-loop mechanism (air/fuel ratio sensors in the exhaust, normally O2 sensors) and need to optimize the timing and the fuel amounts for all speed/load ratios more accurately. Normally on marine engines they run a little richer and with a little less spark advance so they are more tolerant of all kinds of issues (like non-optimum fuel) so they don't destroy themselves under load.

[Cooler]

Most of the boaters in our area that converted from carbs to fuel injection have gone back to carbs. er

[MyPleasure]

Wow, well so much for that idea! Both Edelbrock carbs are new, so I’ll stick with those for the time being! The engines had one of the original Holley carbs on one engine, and a new Edelbrock on the other, so I replaced the Holley with a new Edelbrock. I guess that it’s been a while since I bought a carburetor, but $800 for a carb, and then a new fuel line for another $100 was a bit of a shock. The last time that I bought a new Holley carb and an Edelbrock intake for my 69 Camaro, I think that I paid less than $500 for the 2 of them, but then that was over 40 years ago!

[bud37]

Those eddies are very tuneable carbs....metering rods/springs/jets etc, with tuning kits available that have all the bits.

Hopefully those carbs you have are the marine versions.

[Seaplane Steve]

Following this discussion.

We just did our first fueling. We were not keeping great track of run times for the first part of these tanks, but we are now. We typically cruise around 1500-1900RPM, guesstimate we used about 150gal in 15-18 hours of cruising. Seems to match the flow numbers provided earlier.

It has been fun when we get new people out on the boat to show them what planning looks like. Wide open we hit about 22kts at just over 4,000RPM. Come back to about 2800RPM where I have been told the extra two bbls close and we are down to about 14kts and start to need some bow down trim to keep the ride comfortable.

[kgarguilo]

I almost forgot to post the MPH until I just saw your post.

Gallons per hour obtained underway on Diacom software
RPM / GPH per Engine /Total GPH / MPH GPS
1050 - /1.57 /3.14 /7.26
2000 - /4.54 /9.08 /10.00
2776 - /9.46 /18.92 /12.5
3449 - /14.95 /29.90 /20.7
3950 - /21.14 /42.28 /22.5

[km1125]

Source of the post I almost forgot to post the MPH until I just saw your post.

Gallons per hour obtained underway on Diacom software
RPM / GPH per Engine /Total GPH / MPH GPS
1050 - /1.57 /3.14 /7.26
2000 - /4.54 /9.08 /10.00
2776 - /9.46 /18.92 /12.5
3449 - /14.95 /29.90 /20.7
3950 - /21.14 /42.28 /22.5

There was probably a spot between 2776 and 3449 RPM where the fuel economy would have popped up a little higher. It doesn't look like you were all the way on plane at 2776, or needed a little tab there to help. The fuel economy is lower than it was at 3449.

carver_fuel_economy.jpg