Need second opinion regarding fuel Consumption

[luckystriker]

I am in the process of purchasing my first cabin boat, it's a classic 6,3m Chris Craft 1966 in perfect condition with a VOLVO PENTA AQ 110 HK, bensin(Petrol) engine

I have worked out the fuel consumption to around 27,6 Litre's of fuel per hour, that's 7,3 Gallon's per hour, which seems way to high, I would really appreciate a second opinion on this, I my be working it out wrong.


March Speed is 18 knots
Top speed is 22 Knots

Current fuel capacity is 100L

Thanks in advance!
Jason

 

[Mischief Managed]

Re: Need second opinion regarding fuel Consumption

Seems like you are basing your calulation on .1 US gallons per hour per horsepower. The reality is that petrol engines typically use .08 GPH per HP.

Knowing that drag is a function of the square of speed, and that HP required is proportional to drag, here's my math: 22x22 = 484, 484/110= 4.4; 18x18=324, 324/4.4=73.63

Your 18 knot cruising speed should require 73.63 HP, if the boat does indeed achieve 22 knots with 110 HP. If the engine is in proper tune, 73 HP would require about to 5.6 US gallons per hour.

 

[proxyx]

Re: Need second opinion regarding fuel Consumption

that is some nifty math... I like it

 

[bruceb58]

Re: Need second opinion regarding fuel Consumption

Fuel consumption is almost impossible to calculate without knowing way more information...weight of the hull, deadrise angle...just to name a few.

 

[luckystriker]

Re: Need second opinion regarding fuel Consumption

Fuel consumption is almost impossible to calculate without knowing way more information...weight of the hull, deadrise angle...just to name a few.

I agree. @Mischief Managed, my original calculation was 0,45 pounds per HP per hour, but either way your calculation boasts better consumption than my original calculation which must obviousl be wrong, let me explain.

assuming a in-tune besin engine will consume 0,45 pounds per hour x 110 HP = 44 pounds per hour / 6 (6 pounds = 1 gallon) = 7,3 Gallons per hour. Is this wrong ?

@Bruce, i agree, i do, i just want some idea really. It's nice to own a boat but if i can rarely afford to use it, it's a bit of a shame

 

[proxyx]

Re: Need second opinion regarding fuel Consumption

on 20ft, 205hp 4.3L br i'm getting about 5 gph (plane), there are some boat test sites that will give you an idea of what similar boat and engine could do

 

[Silvertip]

Re: Need second opinion regarding fuel Consumption

Install a fuel flow monitor. It will give you instantaneous and average fuel flow numbers (g/hr or l/hr) as well as total over time, fuel remaining, etc. Northstar and Navman (now Northstar) have relatively inexpensive units.

 

[Mischief Managed]

Re: Need second opinion regarding fuel Consumption

I agree. @Mischief Managed, my original calculation was 0,45 pounds per HP per hour, but either way your calculation boasts better consumption than my original calculation which must obviousl be wrong, let me explain.

assuming a in-tune besin engine will consume 0,45 pounds per hour x 110 HP = 44 pounds per hour / 6 (6 pounds = 1 gallon) = 7,3 Gallons per hour. Is this wrong ?

@Bruce, i agree, i do, i just want some idea really. It's nice to own a boat but if i can rarely afford to use it, it's a bit of a shame

Most GPH calculations are based on .08 gallons per HP per hour. So 110 HP would require 8.8 gallons per hour.

You would normally not run an engine at fully rated output power all the time, because it will quickly fail, so most people throttle back to about 70% power. Based on you max speed and cruising speed numbers, cruising speed was attained at very near 70% power.