More people onboard means more GPH, right ?

[baz]

This past weekend we had a small 1-day cruise. Normally there's just 1st mate and me onboard and moving along at 8 to 10 kts we burn around 1.5 to 2 gallons/hour. This is very typical and constant.

For the 1-day cruise we had two more adults, two small children, extra luggage and ourselves. I estimate the increase in payload was no more than 400 lbs. During our cruise I held the speed at 8 kts and the fuel burn rate was close to 3 gallons per hour. Our speed did fluctuate between 7.4 and 8.1 kts but the fuel burn always stayed at around 3 GPH.

Do you think the extra 400 lbs caused almost a doubling of our typical fuel burn of 1.5 to 2 GPH ?

There was no wind and tide was out during this observation.

 

[walldog]

Just my guess, but I think that the extra weight did not have THAT much effect on your GPH. I would suspect condition of your bottom and below water external items have more effect. Very little growth on bottom, trim tabs, and rudder will have a noticeable effect on speed and therefore the power required to maintain speed.

 

[baz]

Thanks Herb... that leads me to think that if I can offload 400 lbs from the boat I might get the GPH down to zero with just the normal two adults onboard. :lol:

 

[JamesTXSD]

I base fuel burn on RPM rather than speed (hold over from my pilot days)... too many variables with speed: current, wind, water conditions, bottom condition, trim. Weight definitely does make a difference, too, though. Generally, as you add more weight, it takes a higher RPM to reach the same speed (all conditions being equal, and they rarely are).

When traveling at displacement speed, a small increase in RPM (causing the boat to have to work harder as it tries to climb the bow wave; aka drag) increases the fuel burn at a steeper rate than the speed increase.

Using the formula for determining displacement speed (yes, I realize the Rangers aren't a full displacement hull), you come up with HS (hull speed) = the square root of the length of the waterline X 1.34... in the case of the R25:
HS = 4.98 X 1.34
HS = 6.67 knots

8 knots puts you into an inefficient speed; increasingly so as you make small speed increments.

For perspective: on our boat, 1800 RPM gives us hull speed (usually) about the same as yours. We burn around 1 gallon per hour at that RPM/speed. Increase the RPM to around 2,000, and the fuel burn goes to 1.5 gph, but the speed only increases a couple tenths of a knot. When we ran with Herb and Wilma on the south Texas Gulf ICW, we had to burn about 2 gph to stay with them at the displacement speed on the 27 they had at the time... a 100% increase in fuel burn to gain about a 20% increase in speed.

At those slow(ish) speeds, the fuel burn is far from linear with any increase in speed.

Hope that helps.

 

[baz]

Yes, that does help for sure. Our RPM for the ~8 Kts was at 2500 and the fuel burn was around 3 GPH. There was a very light chop and very little swell as the boat stayed quite flat without hardly any rolling.

 

[pleeson]

At 2500 rpm you are definitely not going to do better than 3 gph. I agree that you need to ignore the speed and focus on fuel burn and RPM if you are interested in most economical cruise. Think of it as placing your hand in the water and moving it forward palm first. Move slowly - no problem. Move quickly - a little more apparent resistance. Same as walking versus running uphill. More calories (energy) required even though you get there faster. If you watch your GPH it is easy to see at what point the "penalty" for a slight increase in speed starts to really have an impact. It will likely be in the mid to late teens somewhere.