30 ft Commander

[PassingWinds]

Hi Guys

The 30 ft Commander with two 5.7 gas pots. I’m guessing at 20 knts the fuel burn would be about 80 litres/ hr.
But what about hull speed ? Would 8 knts and 15 litres/hr be close ?
What are your thoughts?

Mike

 

[ab1752]

That's a heavy boat, you'd want to sea trial to know for sure. My 26 hourston with a 320hp vp duo prop at 20 knots is about 7 to 8 gallons an hour. What drives are on the boat? If I had to guess at 8 knots it would be turning about 1500 to 1600 rpms so I think 15l an hr is on the low side.

 

[Eden Island]

Check out the thread - https://sportfishingbc.com/threads/commander-30-versus-26-and-few-questions.93417/

Includes this quote: I've got a podded 30' with a pair of 350 Verados, it does about 45 knots if I need it to (that's way toouch boat for that speed). It cruises happily at about 25 k it's and has a com ones burn rate at that speed of about 68 L/H.

There's quite a bit of discussion of fuel economy and range - a lot regarding diesel.

 

[fishin_magician]

No way you'd cruise at 20 knots burning 80 litres an hour with that rig. More like 110-120 litres an hour and anyone who is telling you less probably wants you to buy the boat. Add full fuel, gear, passengers and it adds up in a hurry.

Take any boating mag running a sea trial and add 15 to 20 percent to those numbers for real world consumption.

There are some simple laws of physics which lots of manufacturers claim to break...but the reality is...while they may make it more efficient- the air to fuel mixture needs to be the same to make power.

 

[Rain City]

I burn 60-80 (depending on conditions) at 22 knts with diesels. Probably upwards of 120 at 28 knts, which she will do at 3600 RPM. I'm usually somewhere in between. Mine is a 92.

 

[Coyote Spooner]

My guess would be 60-70 litres per hour, for each engine. FM’s estimate is bang on. A diesel in these heavy boats would be the way to go, if you can afford the diesel repair bill.

 

[BCI]

Old rule of thumb;

4.3L 8gph at cruise.
5.0L 10gph at cruise.
5.7L 12-13gph at cruise.
7.4L 15-16gph at cruise.

Cruise for the V8's is usually 3200-3500rpm. I have never had 4.3L so I can't tell you the cruise.
Question is: what does the 30 Commander with twin 5.7L outdrives cruise at, at 3200-3500rpm? If 3200-3500rpm gets you the speed you need, expect to burn 24-26gph total... 95 L/hr at 25gph.

 

[ab1752]

Really depends on the hull. I can cruise easily at 2800 rpms at about 18 knots and 20 knows right at 3200. Things open up fuel consumption wise past 3200 with a 5.7 Vortec rated for 320hp.

 

[BCI]

While there isn't a 5.7L 320hp Merc here, this is some good data here including a 6.2L 320hp Merc and a 5.7L 320hp Volvo. We can interpolate between them as required.

1. https://www.boatgph.com/mercruiser/
2. https://www.boatgph.com/volvo-penta/

In summary;

Merc 6.2L 320 hp, 3000 rpm, 3500 rpm, 4000 rpm, 9gph, 11.5gph, 14 gph respectively.
Volvo 5.7L 320hp, 3000 rpm, 3500rpm, 4000 rpm, 10.2 gph, 13.9 gph, 17.2 gph respectively.

The back of the napkin math is generally solid over the years - at 3500 rpm, 5.0L 10gph, 5.7L 12gph, 7.4L 15ggph, if propped correctly. If I recall the 4.3L is 8gph at 4000rpm or something...

Every boat is different - speed will vary between each hull's efficiency... fuel burn should remain consistent at these RPM as long as propped to reach WOT... a big heavy hull with a deep V may only do 18-20mph at 3500 rpm... while a light shallow V may do 30mph at 3500rpm... but if propped correctly to reach rated RPM at WOT... the fuel burn should be accurate at the rated RPM.

 

[ab1752]

I think any way you slice it up, a 30' Commander with twin 5.7s is going to blow through fuel, particularly when planning. Health of those engines will have some impact too on fuel burn.

 

[Sttr]

Just drop the top on that bad boy and save yourself a few gph

 

[Rain City]

I think any way you slice it up, a 30' Commander with twin 5.7s is going to blow through fuel, particularly when planning. Health of those engines will have some impact too on fuel burn.

Commanders seem to have cracked some miracle code of efficiency. I'm not joking, this has been verified by a couple repowers and podded ones. It's also why they slap. Shockwave solves that problem though.

 

[BCI]

Commander seem to have cracked some miracle code of efficiency. I'm not joking, this has been verified by a couple repowers and podded ones. It's also why they slap. Shockwave solves that problem though.

The secret is the 17 degree dead rise at transom. Very fuel efficient. Also why they could slap.

The less the dead rise at the transom, the less the fuel needed to push the boat through the water in regular operation. As long as the hull isn't departing from the ocean at the transom, 17 degrees is fine.

You'll find large sport fishers have this kind of dead rise at the transom.... the idea is to let the forefoot eat the sea, hoping that the overall length of the hull will keep the butt end in the water. Also the plan would be to travel at a speed that keeps the transom in the water so it isn't re-entering.

Go fast boats, light boats, shorter boats, where air-time is not unheard of... need more dead rise at the transom for cutting and for smoother re-entry.... more dead rise there equals more wetted surface, equals more fuel to push through the water all the time.

 

[Aces]

The secret is the 17 degree dead rise at transom. Very fuel efficient. Also why they could slap.

The less the dead rise at the transom, the less the fuel needed to push the boat through the water in regular operation. As long as the hull isn't departing from the ocean at the transom, 17 degrees is fine.

You'll find large sport fishers have this kind of dead rise at the transom.... the idea is to let the forefoot eat the sea, hoping that the overall length of the hull will keep the butt end in the water. Also the plan would be to travel at a speed that keeps the transom in the water so it isn't re-entering.

Go fast boats, light boats, shorter boats, where air-time is not unheard of... need more dead rise at the transom for cutting and for smoother re-entry.... more dead rise there equals more wetted surface, equals more fuel to push through the water all the time.

Great explanation

 

[PassingWinds]

No way you'd cruise at 20 knots burning 80 litres an hour with that rig. More like 110-120 litres an hour and anyone who is telling you less probably wants you to buy the boat. Add full fuel, gear, passengers and it adds up in a hurry.

Take any boating mag running a sea trial and add 15 to 20 percent to those numbers for real world consumption.

There are some simple laws of physics which lots of manufacturers claim to break...but the reality is...while they may make it more efficient- the air to fuel mixture needs to be the same to make power.

Hi

Thanks for the reply. Sadly I think you are closer the the real fuel burn for going 20 knts.