D6 370 vs. D6 435

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Eagle 2

Active member
Joined
Jul 19, 2025
Messages
35
Location
Long Beach CA
Fluid Motion Model
C-30 CB
Vessel Name
Eagle 2
I have the D6 435 in my CB 30. I average about 1-1.1 KMPG at cruise 2950 RPM, however, I"m towing a 350# Dingy 100 feet behind me most of the time, cruising at about 15.5-16 knots. I would surmise that I'd get about 1.2 KMPG without towing. Is there anyone that has either the CB30 or the Sedan 30 with the D6 370 that can share their cruise numbers? Keep in mind my use is 100% offshore so seas, tides currents and wind varies the numbers. Is the D6 370 simply 65 CI smaller displacement with a smaller turbo?
 
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I have the D6-370 in the sedan sport top, most of my cruising is between Newport and two harbors or out to San Clemente/Tanner etc., so basically the same crossing you're running most of the time.

The 370 is definitely under powered for the vessel... I typically run very heavily loaded so that is a factor as well (full fresh water, full fuel, 5-8 passengers + dog, multiple SUPs, 100# dinghy, and provisions for all passengers for 1-2 weeks.

Cruising speed on the way out with full load is 14.7-16.5kts depending on wind/swell with RPM at 3150 burning 1.0-1.1mpg
On the way back with typical is 17.5-18.5kts (generally down swell) at the same 3150rpm burning 1.2-1.3mpg.

With a very light load 1-2 passengers, 1/2 fuel, minimal water and provisions I can run 16.9-17.4kts at 2990rpm burning 1.4mpg.

WOT is 3550rpm and anywhere from 19.1-24.0kts burning 1.0mpg
 
Above 20kts is when the vessel noticably is fully up on plane and where it wants to be for the hull steps to come into play. Fuel economy is completely flat from 19-24 but the 370 just can't go any any faster. It's "M" rating is technically a 4 whereas the 435 is a 5 so it's theoretically a slightly longer lasting motor, but only if operated within the nominal load specs.

I would almost rather have the D6-280 and just operate in displacement for much longer range without worrying about carbon buildup from low rpm/load.
 
Interesting input,especially covering the same waters. Although I would surmise the CB 30 probably weighs, guessing 1000#s more +/- with the bridge, we too always run full/full,water,fuel, 2+ big dog, and all the other crap that goes on an island trip, ie: paddleboards, floating dock, ect ect. Plus towing, we can't really exceed 16 knots, so that part is limiting for us. I understand that a bit faster would be more efficient due to hull design. I was just curious as to the difference in the 370 vs. the 435 in general performance. Sounds like based on your facts that fuel burn is about the same due to higher RPMs to keep your speed at about an 89% Load vs. about an 84% load that the 435 runs at, while towing. My interest was peaked at my last $300 RT fuel bill running to the island. Our Mainship Pilot 30, that we just sold was about 1/2 the burn rate, 315 Yanmar, typically towing at 15knots, 3000 RPM, full/full. Cameron out of Dana on his CB 30 435 gets about the same at 1.1 RT but he cruises a bit faster since he's not towing, but carrying the dingy like you are. Thanks for the input.




















































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Jesperjo said:
Caveat: I’m not a Penta expert, just an amateur engine geek.

My understanding is that the 370 has a turbocharger, while the 435 has both a turbocharger and a compressor. The displacement is 5.5l on both. I was looking for specs on them but I only found the ones on the 380 v 440 that replaced the other models in 2019 but it should be similar: https://www.volvopenta.com/en-us/marine/all-marine-engines/d6/#:~:text=D6 - 300 hp D6 - 340,Rating
Click to expand...
Thanks for the link. I got a kick out of the part where it said in the link "Fuel Efficiency" Compared to what? LOL
 
With the giant stainless steel structure of the sport top I would estimate it's roughly the same weight as the command bridge, but you certainly have more windage which is definitely a factor though not huge at our speeds.

I have contemplated reducing the water tank to ~25gal and just running the watermaker daily since I have plenty of solar to cover its draw, dropping the weight and hopefully getting my 16kt comfortable cruise closer to the 80% load factor where I would prefer to be for longevity.

My last trip over I just ran at 1050rpm averaging 5.8mpg and only burned 29gal of fuel RT including generator runtime while over there.
 
Full_O_Fish said:
With the giant stainless steel structure of the sport top I would estimate it's roughly the same weight as the command bridge, but you certainly have more windage which is definitely a factor though not huge at our speeds.

I have contemplated reducing the water tank to ~25gal and just running the watermaker daily since I have plenty of solar to cover its draw, dropping the weight and hopefully getting my 16kt comfortable cruise closer to the 80% load factor where I would prefer to be for longevity.

My last trip over I just ran at 1050rpm averaging 5.8mpg and only burned 29gal of fuel RT including generator runtime while over there.
Click to expand...
1050
 
Now the watermaker is a whole nother subject! So far carrying 80 gallons has not been an issue for us, but we are still learning the boat, but that is an interesting addition, I’d be interested in hearing more about that. Where you installed it, ect, when you get a chance!
 
Not having to carry the extra 300-400#s of water would be quite advantageous for me other than having to limit guests shower consumption onboard or have them stagger throughout the day as more is made since I won't have as much reserve.

At this time I've gotten in the habit of only keeping 20gal or so in the tank during crossings for economy, then get to producing as soon as I'm on the mooring till the tank is full.

Generally this works just fine, but I do get complaints from the guests about the amount of noise the tank makes throughout the night from the water sloshing so much if the tank isn't above 3/4 full since there are no baffles in such a large shallow tank. Which is only made worse on the last night when I'm trying to run the tank down to have less weight for the crossing back.

Honestly, biggest benefis to adding the water maker have been being able to freshwater flush the main engine after arrival at the island, and rinsing the whole boat off after a wet crossing without worrying about needing to go into the fuel dock to refill with their heavily chlorinated water.
 
I attach a hose to the outdoor shower aft, that allows me to rinse the boat down when I get on the mooring. I don’t have a fresh water engine flush system though. That would be a nice addition.
 
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