gph

With a gasoline engine, you can estimate it at 5 gallons per hour for every 100hp produced. That gives you a pretty good ballpark.

GPHs

Run the boat 100 miles and top off. My 36 al w/454s gives me about .75 mpg at 2800 and 22 mph. ws

Best thing to do is get something like a Floscan which can also interface with you GPS data and give you real time info including if you have enough fuel left to make your destination so you can adjust your speed or destination accordingly.

http://www.floscan.com/html/blue/index.php

My experience with gas engines is that they burn close to 10 gallons per 100 HP produced.

Hi,

Here is some interesting data on working it out for yourself based on your exact HP etc.

http://www.epi-eng.com/piston_engine_technology/thermal_efficiency.htm

That's fantastic if you know either fuel flow or thermal efficiency already. Not so fantastic if you're trying to determine both variables at once (in advance of real world experience or metered consumption).

Floscans only work well, once they are calibrated. I've found that most heavy boats with 454's will burn around 40 gph at 3200 rpms......closer to 50 gph if you have to cruise 3500-3800rpms.......

Your typical marinized automotive gasoline engine you can use 25% for your thermal efficiency value and be within +/- 1.5%. If you have an old long rod engine like the International truck engines and other WWII vintage heavy equipment plants, use 27%. No gasoline engine gets much better than that except aircraft engines, and very few are worse than 23%. Any of these number sets get you well within the ballpark for planning purposes because you should always plan with a 30% reserve based on nominal weather usage.

Are you talking about 2 stroke outboards? Never mind, sorry, you're right, I was thinking cruise on a 100hp engine...duh...

On a note somewhat less speculative and significantly more relevant than the usual pissing contest around here, from The Chris Craft Commander Forum...

You're also encountering old tech that may have been rated with optimistic gross HP vs. pessimistic modern net HP - a difference than can easily exceed 20% and drastically effect fuel burn (calculated theoretical and observed actual) depending on gearboxes and 3, 4, or 5 blade props, shaft angle, weight of the boat, many things. Each of those considerations changes the math to the point where working that equation with two variables results in nothing more than a WAG, or semi-SWAG at best.


Additionally an oversquare short stroke plank head engine like the 431 is also going to have different fuel consumption characteristics than an undersquare long stroke wedge head engine, and where an oversquare engine usually loses the low end torque war against an undersquare engine, the 431 is a slow turning torque monster - up to 500 lbs-ft in marinized version while only turning under 4500 redline and rated just 275hp gross.

There are just too many variables and unknowns to get anywhere closer than inside a very large ballpark with boilerplate equations.

I agree, I would say to take that chart, run the vessel and see if it achieves the same cruising speeds (which I doubt after so many years, boats always tend to get heavier and slower), and take 20% efficiency off of it and that should get you close.

If you run the boat and generate a new set of tables, you will have the answer and surely don't need to do any further guessing and pseudo engineering calculations.

Hello happy hour, Welcome to the forum!

Congratulations on finishing Your rebuild, pictures are always welcome.

I find from experience the above posted gph chart to be about 10% Optimistic but a fair guide in a perfect world. Please let us know of the real world findings You come up with.

Kiwi