Calculating range for gas consumption...

I have a mercruiser 165 hp 6cyl in a 17.5 tri-hull with a 20 gallon fuel tank.

How would I go about calculating how many nautical miles my boat can go?

Thank you for your help.

Re: Calculating range for gas consumption...

Considering your location I am concerned that you are thinking the Catalina Channel . . . With that noted, the way most of us figure this is 1/3 out, 1/3 in and 1/3 reserve. Nautical mile (knot in speed) is 1.15 miles. Most of these smaller runabouts cruise at just around 3 MPG. So it is 3/1.15 = 2.61 Nautical MPG. Sooooo, 20 gallons divided by 3 = 6.67 gallons, which gives youi 17.4 Nautical miles out, 17.4 Nautical miles in and 17.4 Nautical miles in reserve.

The fact is that weather conditions can change this a lot, so the 1/3 safety factor needs to be considered as non-negotiable. The 3 MPG may be low, but without a lot of data, it might even be high, sooooooo . . . The only way to really know is a fuel flow meter and a GPS for distance and speed, or lots of experience with this boat in a variety of weather conditions.

Sorry it is so complicated, it just is . . . Welcome aboard!!

Re: Calculating range for gas consumption...

there are way too many variables, because fuel consumption varies widely with speed on a boat, and current/wind play a big factor.

There is the old rule of boating 1/3 out 1/3 back 1/3 reserve.

There are charts that show a boat/motor projecter performance at GPH and MPG

Re: Calculating range for gas consumption...

Oh, what he said too ^^^^^

Re: Calculating range for gas consumption...

not difficult to understand at all. I should be able to make it to Catalina without issue and can refuel in Avalon.

Re: Calculating range for gas consumption...

bridweezul said:

not difficult to understand at all. I should be able to make it to Catalina without issue and can refuel in Avalon.

Click to expand...

Please take these comments as constructive. The Catalina Channel can be a very dangerous crossing. In the AM it can be reasonable, even in a low freeboard boat like yours, but it can get nasty anytime and if you don't spend the night, the return can be a disaster.

The general rule is 25 ft., but there are many exceptions. I have taken my 23 footer over a few times, and have been over in a 20 footer, but it was a very sea worthy hull and we went with a group. At a minimum you need a VHF radio. Some would say Radar is required as pea soup fog is very common.

It is the weather and unforeseen issues that need to be understood. What's your experience level? And what's the condition of your hull, engine and drive?

Re: Calculating range for gas consumption...

I have a 19.5' tri-hull with the same motor which is doing 14 litres/hour at 3200rpm

Re: Calculating range for gas consumption...

Good idea to keep charts in general, but they're great for penciling in how long it took you from A to B, a quick note about the conditions, and how much fuel you started with and ended with. After a couple trips in different conditions, you'll get an average of what to expect.

If thats too much clutter on your chart, then theres always the log book option. Makes you look official too..

Re: Calculating range for gas consumption...

Agreed you'll probably get about 3mpg with your rig.

But if you are considering Catalina in a tri-hull, you are a crazy person, IMO.

Re: Calculating range for gas consumption...

How would I go about calculating how many nautical miles my boat can go?

It's the same as a car. You have to know what your miles per gallon is, then multiply that by 20 for maximum (tank empty) range. Remembering MPG is an ever changing number when boating, if you think you're getting 3 and only get 2, your range just went down by 1/3 ...