Ocean Alexander 423

I am purchasing an OA 423 and will be bringing it up the AICW from Florida to New York. I am having difficulty finding out fuel consumption on the Cat 3208 NA. If I run at 1500 rpm giving me about 8 knots what should I expect for fuel consumption/hour on each engine or for both. Thanks for any feedback.

Capt. Pete

Congrats on the new boat. Haven’t had those engines but I would guess around 8 to 10GPH at that speed.

Sounds like you are on a “delivery schedule” so at 8 knots Comodave’s estimate is probably accurate. Along the AICW, some days you will be running against the tidal current and burn a lot more fuel and at higher rpm. Other days, the tides work in your favor.
Your new OA, like many trawlers/ cruisers is rather inefficient if you are running at or near theoretical “hull speed”. If fuel burn is important to you, slowing to 6-7 knots will reduce your fuel burn significantly.

Hull speed is the speed at which the boat starts to climb out of the water on to plane. It's a mathmatical formula described as 1.34 x squareroot of the waterline. Assuming your OA 423 has a waterline of 38-feet, that works out to 8-1/4 knots or so. As Comodave notes, I would expect fuel consumption to be at least 8 gal/hr (efficiency of 1 gal/mile). Drop the speed to around SL = 1.1-1.2 (approx 7-knots), and the efficiency should improve by 15%-20% - around 0.8 gal/mile. In real terms, over the course of 1000 mile trip, you'll consume 200 fewer gals of diesel and save around $800. It will take an extra 17 engine hours (142 @ 7 kts vs125 @ 8 kts), which could be an extra 2-days so a chunk of the $800 may go to marina fees and a couple nice dinners which sounds like a better use of money to me.

I love trip logs - hope you post your adventures. Best success-

Peter

Peter’s numbers are correct especially for hull shapes like his Willard, KK’s etc. Other hulls like my DeFever and the OA under discussion, unfortunately aren’t that efficient, probably need at least 1.5 knot reduction from theoretical to reduce the stern squat & bow wave ime.

We have 3208s and displace 44,000 pounds. We cruise at 1400 RPMs (8.5 knots) and have seen 5 - 6 gals an hour while also running the generator for a time every day.

I have a 66,000 lb 55' with Cat 3208TAs and get 1 mile per gallon at 10 knots and 1750 rpm. I actually average lower consumption of 7.77 gallons per run hour because of time running at idle, 5 knots in the harbor, and anchoring etc.

I appreciate the feedback. I am going to try to run outside weather permitting at least from Jacksonville to South Carolina and then again from Cape May to New York.

Capt.Pete

I have a single 3208 that I generally run at 1200 rpm. That gives me six to seven knots with a burn rate of about two gallons per hour.

3208 210 NA 1500 rpms = 2.0 1600 rpms = 2.4 1800 rpms = 3.2 1900 rpms = 3.7 2000 rpms = 4.3 2100 rpms = 4.8 2200 rpms = 5.5 2300 rpms = 6.2 2400 rpms = 7.0 2800 rpms = 11.9

mvweebles said:

Hull speed is the speed at which the boat starts to climb out of the water on to plane. It's a mathmatical formula described as 1.34 x squareroot of the waterline. Assuming your OA 423 has a waterline of 38-feet, that works out to 8-1/4 knots or so. As Comodave notes, I would expect fuel consumption to be at least 8 gal/hr (efficiency of 1 gal/mile). Drop the speed to around SL = 1.1-1.2 (approx 7-knots), and the efficiency should improve by 15%-20% - around 0.8 gal/mile. In real terms, over the course of 1000 mile trip, you'll consume 200 fewer gals of diesel and save around $800. It will take an extra 17 engine hours (142 @ 7 kts vs125 @ 8 kts), which could be an extra 2-days so a chunk of the $800 may go to marina fees and a couple nice dinners which sounds like a better use of money to me.

I love trip logs - hope you post your adventures. Best success-

Peter

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If I read these numbers correctly, at 8.25 knot the 1000 mile trip will take about 121 hours, and at 7 knots it will take about 142 hours. 121 hours at 8gph uses about 96 gallons, and 142 hours at 7gph uses about 100 gallons. No major difference on overall fuel consumption, but at the higher speed you save about a day of travel time and likely a day or two of marina costs.

seabum said:

If I read these numbers correctly, at 8.25 knot the 1000 mile trip will take about 121 hours, and at 7 knots it will take about 142 hours. 121 hours at 8gph uses about 96 gallons, and 142 hours at 7gph uses about 100 gallons. No major difference on overall fuel consumption, but at the higher speed you save about a day of travel time and likely a day or two of marina costs.

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Efficiency (mpg) decreases 15%-20% when speed is decreased from SL 1.34 down to something closer to SL 1.1-1.15. While related, it's different than consumption (gph). But you are correct, if cost is the #1 criteria, you have to factor other costs too.

Peter

I think you should plan for 1 gallon per mile with a likely possibility of achieving 1.25 miles to the gallon. If you plan for 1.25 miles to the gallon and you end up using 1 gallon per mile you could end up in trouble.

The Cat 3208 typically burns 5 gallons per hour at 1750 RPMs. Times 2 if you have twins. This is not a guarantee as prop pitch can affect this. How fast you go at 1750 RPMs depends on the length and weight of the boat. 8.5 kts is most likely the sweet spot for a 42’ boat. If you can make 8kts with only 1500 RPMs you will use significant less fuel.

tiltrider1 said:

I think you should plan for 1 gallon per mile ....

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When I was delivering, this was exactly the assumption I made (1 gpm at 8-ish knots) for most motoryacht style boats. Worked out fine. As I've mentioned before, I never asked customers what their fuel consumption was because I never felt they really knew. Their experience was day trips and would simply take the gallons they took on divided by engine hours. Risk is this is optimistic due to low speed and low consumption for quite a few engine hours, which is typical with day trips. Some owners - Nordhavn types - had reliable consumption curves for long passages that were reliable, but that was uncommon. Few owners make long passages

Peter

Osprey69 said:

We have 3208s and displace 44,000 pounds. We cruise at 1400 RPMs (8.5 knots) and have seen 5 - 6 gals an hour while also running the generator for a time every day.

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I’m getting the same with my 44K pound, semi D with NA 3208’s @ 1500 rpm - 8 knots and gennie three times a day.
At sea when taking a beam sea, I lift rpm to 1750-1800 to stabilise the roll, 9 knots with a slightly higher fuel burn rate, but far more comfortable.

I did the loop in 2018 on OA 423 with twin Cat 3116's, We traveled 5539 mi using 2670 gal. and 668 engine hours. Ran the generator a couple hrs most days. Usually ran at 1500 rpms. That includes idle time traversing 103 locks and a lot of no wake zones on the ICW. About 2 mpg.