Cruising RPM for Mercruiser 8.1 HO engines

RiverGalsMan

Member
Nov 3, 2006
327
Shady Side, Maryland
Boat Info
2007 44 DB
Engines
Cummins QSC 500
Hello all. I'm new to the board and new to my boat. I've only had her out once and the smartcraft was showing .8 MPG from 3200 RPM all the way to 4000 RPM. The boat felt really happy at 3800 RPM. Will this be a reasonable cruise RPM if I don't want to abuse the engines? I know about the 80% rule and I could slow down to 3600 or 3500 RPM, but she just felt right at 3800. Does anyone have experience with the HO version of the 8.1 liter engines?

Thanks in advance.
 
370 or 420 hp?

Here are the Boattest results for 370; looks like best cruise is 3500:

Test Power: 2 x 370-hp MerCruiser 8.1 S Horizon RPM MPH Knots

RPM MPH Knots GPH MPG NMPG Range NM dBa
700 3.5 3.0 2.5 1.38 1.20 311 270 67
1000 5.8 5.0 4.2 1.39 1.20 312 271 66
1500 8.1 7.0 6.2 1.31 1.14 295 256 71
2000 9.6 8.3 9.9 0.97 0.84 218 190 74
2500 11.4 9.9 17.4 0.66 0.57 147 128 77
3000 17.6 15.3 28.6 0.62 0.54 138 120 79
3500 24.3 21.1 34.4 0.71 0.62 159 138 83
4000 29.3 25.4 44.0 0.66 0.58 150 130 86
4500 33.1 28.7 57.0 0.58 0.50 131 114 87
4750 34.7 30.2 61.8 0.56 0.49 126 110 87

Speeds measured with Stalker ProSports radar gun. Fuel consumption (gallons per hour) measured with Floscan digital fuel-flow meter on gas inboards, sterndrives, and outboards, TechMate electronic scan tool on gas EFI inboards, Caterpillar digital fuel-flow meter on diesel engines, or permanently installed fuel-monitoring equipment. Range is based on 90% of published fuel capacity. Sound levels determined using Radio Shack digital decibel meter on A scale. 68 dBa is the level of normal conversation.

Performance Chart

Test Conditions
Test Power: 2 x 370-hp MerCruiser 8.1 S Horizon Time To Plane 6.6 sec.
0 to 30 12.1 sec.
Test Power 2 x 370-hp MerCruiser 8.1 S Horizon
Transmission ZF 63
Ratio 2.5 :1
Props 4-blade Nibral
Load 2 persons, 1/2 fuel, no water, minimal gear
Climate 82 deg., 71% humid.; wind:10-15 mph; seas: 1-2'

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The words from the video (since now in Archives)
New 38 Features More Room Throughout

Sea Ray raises the bar again, and leaves nothing out with this new model for 2006.

By Capt. Bob Smith

Now you may remember the old 380 from a few years ago, and you may think there isn’t much more to bring to the market, but Sea Ray really outdid themselves with their new release of the 38 Sundancer for 2006. The 38 SD fills the space between the 36 Sundancer and the 40 Motor Yacht.

Thoughtful Features

Walk up to this new model and two things will immediately grab your attention; first, the standard fiberglass hardtop with aft sunshade offers excellent protection from the elements and secondly the optional sun pad on the front. This unique pad has a frame system that pops up the back end of it to become a backrest for sipping your favorite beverages, or reading the novel you haven’t been able to finish.

Stern cleats are placed on two levels to allow easy line handling from any dock. The extra large swim platform graces the back of the 38 and provides a wonderful place to enjoy the day when at anchor. Tucked underneath the swim platform is a deep reach telescoping swim ladder for easy boarding. On either side of the stern are lockers for gear and connections for shore power, telephone and cable. Organizers keep the dock lines ready. The center walkthrough takes you into the cockpit where Euro styling and comfort are abundant.

Cockpit Layout

The aft seating nearly wraps around the entire cockpit with the added filler cushion. The cushion fills the space for the side step when docking alongside a pier. The cockpit table fits neatly below one cushion when not in use and sets up quickly for serving snacks. Lower the table and add filler cushions and this becomes a lounger for soaking up some rays or letting the children nap topside.

If you enjoy watching videos, then opt for the 17†flat screen TV with built-in DVD player for the cockpit. And with the topside wet bar you don’t have to keep going back to the galley. It includes a sink with facet, trash receptacle and a NorCold refrigerator. There is also dedicated space for the carry-on cooler to keep a bag of ice handy for the drinks.

Helm Features

The helm is set up to capture the feel of luxury at sea. The wood accented tilt wheel is the center of attention with tons of room for optional equipment to supplement the standard engine gauges and Mercury SmartCraft instrumentation aboard. Optional equipment such as the Northstar 6000i Radar/GPS/Chartplotter lets you take your home on the sea anywhere you like. Autopilot helps keep you on target while you find your way on the long runs offshore and it also allows you to take your hands off the wheel with some ease of mind.

The helm seat is a swivel-bucket style with flip-up thigh-rise bolster, armrests, slide adjustments and a neat pneumatic vertical adjustment. Having banged my knees on many helms, I really appreciate the addition of the knee bolster at the helm for more comfort when stand-up driving. Even the co-pilot seat is comfortable and extra wide so another rider can join you at the front.

Moving Forward

Rather than breaking up the dash with steps leading to the bow, entry to the bow is down the sides of the 38 DA. There are plenty of hand-hold rails, but the walkway was pretty narrow for my size 13D foot. I believe that it’s a good thing to give up some space here to keep the cockpit and salon width, just use caution when dashing forward to grab an errant fender or line. The windlass is covered to keep the bow clean smooth and the line locker has a washdown connection inside to clean the anchor. An optional dual spotlight with a turning radius of 360 degrees will help with spotting the buoys and safely getting through tight spots in the dark.

In the Cabin

Heading down below, entry is gained with flying wood steps that keep the space open and large. Cherry wood finish creates a luxurious look to the salon and galley. The salon couch has Ultraleather with extra thick cushions for long lasting and extreme comfort. The bottom slides out to reveal more storage and filler cushions can convert this into another berth. Above the couch are cabinets boxed in by the speaker system. The aft cabinet houses the sound system components. The table uses the same beautiful cherry wood and stores neatly under the V-berth mattress in dedicated space.

The mid-cabin has tremendous overhead room when seated. The conversation area becomes a large berth at night when needed, and has a wraparound privacy curtain to separate the space from the salon.

Practical Galley

The port galley has the necessary space and equipment for you to keep the meals going even at sea. Dishes can be stored neatly in cabinets above with pegs to keep them from sliding around. The countertop has plenty of food preparation space as well as a sink and double-burner cooktop. Sea Ray also outfitted this model with a microwave, refrigerator, freezer and a large cabinet for the cookware with four drawers for utensils and silverware.

Owner’s Stateroom

The main stateroom is located forward and features a V-berth. The pocket entry door saves valuable floor space and inside you have a comfortable area to rest. This stateroom is complete with two lighted hanging lockers for your clothes and your own private 17†flat screen LCD TV.

The head on the 38 is split with the shower to starboard and the toilet to port. This arrangement allows them to be used separately and speeds up the process of everyone getting ready in the morning. The toilet can also be accessed from either the salon or the forward stateroom.

Specifications and Performance

This all-new entry from Sea Ray measures 39 feet length overall and has a beam of 12’6â€. She weighs 18,500 pounds and draft requirements are 37 inches with the standard inboards. She carries a total of 250 gallons of fuel and can carry up to 55 gallons of fresh water.

We tested this new model is some pretty tough chop near San Destin, Florida. Under these conditions the 38 proved the ride can still be comfortable even in rough water, and handling was smooth. With twin 8.1 MerCruiser Horizons, we found her best cruise around 3500 rpm running 24.3miles-per-hour for a range of 159 miles on a full tank of gas. She topped out at 4750 rpm traveling 34.7 miles-per-hour with a fuel burn of 61.8 gallons-per-hour. The 380 planed in 7.2 seconds and passed the 30 mile-per-hour mark in only 11.9 seconds.

This new 38 Sundancer will be a big hit for Sea Ray. If you are ready to move up to the next level of comfort, or need to come down in size and don’t want to give up the luxuries you’ve been used to, then this model will make you feel right at home.
 

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Wesley, I've got the 420 HP High Output (HO) version. The only difference is that they have a higher Wide Open Throttle rating and they achieve the higher horsepower at the higher RPMs. I just wonder since they're designed to turn higher RPMs if they'll last just as long running them at slightly higher speeds?
 
I am sure you are correct and fine, others will chime in that have actual experience with the 380 but hopefully the information from Boattest helps (they only provide the test information for the 370HP). Others will get involved shortly again welcome to Clubsearay.com!!!! :thumbsup:

Wesley
 
RiverGalsMan said:
Hello all. I'm new to the board and new to my boat. I've only had her out once and the smartcraft was showing .8 MPG from 3200 RPM all the way to 4000 RPM. The boat felt really happy at 3800 RPM. Will this be a reasonable cruise RPM if I don't want to abuse the engines? I know about the 80% rule and I could slow down to 3600 or 3500 RPM, but she just felt right at 3800. Does anyone have experience with the HO version of the 8.1 liter engines?

Thanks in advance.

RGM .... you are in the ball park in regards to rpm, though on the high side for max. engine longevity. the 496/8.1 engines would be best run between 3200 and 3400 for max. engine longevity. The HO has a more agressive cam than the regular 8.1S and does as you say reach more hp at higher rpm. If I remember right the HO and the regular 8.1S have the same torque, around 500 ft./lbs. at somewhere 3200 rpm. A BB should not really want 3800 rpm to be happy, but in your boat that is probably true. The HO is also an even worse gas hog (and I mean there is a noticeable difference between the 370 hp and the 420 hp HO) in fuel consumption at higher rpms.

0.8 mpg looks like a very good number for your 38DA at 3800 rpm. It's a heavy boat for gassers and will need the rpms to run "right" and is just as bad of a gas hog as an old 380 or 410 with gassers. My 340 with the regular 8.1 cruises at 3400 rpm getting around 1mpg and is happy at that rpm. Up to 3600 rpm my engines feel in the right range, above that you notice that they are getting "unhappier" and above 4000 rpm they sound out of their "sweet spot".

Your numbers in regards to rpm and mpg look very reasonable and spot on for the engine/boat combo you have.
 
River Gals Man:

It makes sense that you "feel" the boat is happy at 3800RPM. The 420HP 8.1HO reaches its max torque (503 lb-ft) at 4000 RPM http://www.gm.com/automotive/gmpowertrain/engines/specialized/marine/2006_8100_hp_marine.pdf where as the 370HP (rate 392HP by GM) 8.1S reaches its max torque (503 lb -ft) at a much lower 3200RPM http://www.gm.com/automotive/gmpowertrain/engines/specialized/marine/2006_8100_marine.pdf . With the 8.1 HO version you are also close to max HP (approx 375HP) at 4000 RPM so the "happy" feeling is even more justified. The 8.1 S is making right around 300 HP at 3200 RPM.

With my 2002 380 DA, I have made numerous fuel burn runs. My average on a trip will approach .95- 1.0 MPG. However,, that is a misleading number. It is derived by simply dividing the Miles traveled (as per the GPS NOT the ST60) by the number of gallons to refuel along the trip. Those miles traveled include No Wake Zones, slow speed travel coming into and out of ports, traveling slowly for the enjoyment of the ride, etc. According to the instantaneous GPH on my SmartCraft, I have determined that my "sweet" spot for cruising is 3400 RPM where I achieve approximately .83 - .85 MPG depending on conditions and load (not the captain, the boat :grin: ).

One more note on the SmartCraft. The instaneous MPG is only rounded to the nearest tenth. IMHO, it is not very accurate in determing your sweet spot. The GPH is a more accurate number, however it requires that you have a helper with a calculator handy to punch in the numbers in order to determine your MPG.

I frequenty travel with a neighbor who has a 2006 38DA with the 8.1HO 420HP engines. On every trip we took this year, he burned more fuel than I did.
 
osd true ... still part of the difference in max. torque is academic. You can see from the torque cruves that both engines have above 500 ft./lbs. of torque from 3300 rpm or thereabouts to 3800 (8.1S) or 4000/4200 rpm (8.1S HO) ... they are not far apart.

The HO engines does carry the abulity to increase HP to a higher rpm than the non-HO. THe HO has also a little less power on the bottom end, but again that would be academic in the real world. Only difference between a HO and non-HO equipped boat is a little more top end speed.
 
Well, I just got back from the boat. We ran her yesterday... a bit cold, but a beautiful day nonetheless! Some interesting things to report.

Smartcraft still reading .7 MPG for anything between 3200 and 4000 RPM. Also, the same at all trim tab angles. Fully deployed to fully retracted. The running angle would change pretty dramatically, but the Smartcraft was glued to .7 MPG. I'm still chewing on that one.

I then broke out the calculator and started calling out the RPM and fuel burn numbers while my wife would do the quick math. It looks like the peak MPG was at 3600 where we calculated .75 MPG so it's looking like 3600 RPM is the optimum cruising RPM. That's still 75% of WOT so it meets the 80% rule.

I think I couldn't recreate the .8 MPG reading I had on the sea trial for a couple of reasons. First, the boat is now fully loaded with all sorts of stuff my wife says we can't live without. Second, there may have been some current helping me out on the sea trial that just nudged the Smartcraft to read .8 MPG.

All in all, a great weekend on the new toy. Thanks to everyone for the information.
 
Is it using any oil? Do you need to add a quart or so every other time out. If so, you're pushing the engines too hard. You shouldn't use ANY oil.
 
Chef Doctor, that's a good tip. Thanks.

I just changed the oil a couple of weeks ago so it will be early next season before I can tell if she's burning oil.
 
RiverGalsMan said:
Well, I just got back from the boat. We ran her yesterday... a bit cold, but a beautiful day nonetheless! Some interesting things to report.

Smartcraft still reading .7 MPG for anything between 3200 and 4000 RPM. Also, the same at all trim tab angles. Fully deployed to fully retracted. The running angle would change pretty dramatically, but the Smartcraft was glued to .7 MPG. I'm still chewing on that one.

I then broke out the calculator and started calling out the RPM and fuel burn numbers while my wife would do the quick math. It looks like the peak MPG was at 3600 where we calculated .75 MPG so it's looking like 3600 RPM is the optimum cruising RPM. That's still 75% of WOT so it meets the 80% rule.

I think I couldn't recreate the .8 MPG reading I had on the sea trial for a couple of reasons. First, the boat is now fully loaded with all sorts of stuff my wife says we can't live without. Second, there may have been some current helping me out on the sea trial that just nudged the Smartcraft to read .8 MPG.

All in all, a great weekend on the new toy. Thanks to everyone for the information.

I also think that the "Max. cruise rpm at 80% of WOT" is somewhat academic in this case. The rotating assemblies of the base and HO engines are identical, except the camshaft, but the max. rpms are different because of the power curves only. Both engines have the same identical max. cruise rpm from an engine longevity perspective because this rule is based on the dynamic loads on rotating engine internals, piston speed and piston side load. Whether that is 80% of 4600 rpm or 80% of 4800 rpm for a 496MAG/8.1S style engine ... Take your pick.

My non-HO 8.1S engines top out at 4800 rpm, but that might be tach inaccuracy. Top speed of my 340 at 40 mph SOG would indicate to me that she actually turns 4800 rpms.
 
Alex D, I understand what you're saying. I think you're right about taking 80% of the base engine's WOT RPM. 3600 RPM still sounds reasonable (based on 4600 RPM on Mercruiser's web site.)

The previous owners say the boat will do 39 MPH at WOT (5200 RPM) so that must be where the HO helps. I've yet to confirm this and I'm not going to try until it's a perfect windless/currentless day. I'm not really concerned about top speed, though. I just like to get there in style and comfort! :thumbsup:
 
RiverGalsMan said:
Alex D, I understand what you're saying. I think you're right about taking 80% of the base engine's WOT RPM. 3600 RPM still sounds reasonable (based on 4600 RPM on Mercruiser's web site.)

The previous owners say the boat will do 39 MPH at WOT (5200 RPM) so that must be where the HO helps. I've yet to confirm this and I'm not going to try until it's a perfect windless/currentless day. I'm not really concerned about top speed, though. I just like to get there in style and comfort! :thumbsup:

I agree RGM ... for me the most important thing is that I can cruise anywhere between 28 and 31 mph, WOT is pretty academic to me too ... your 38DA is a wonderful boat !!
 

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