Improved fuel economy

[enjoy2]

I used to run my 1993 370 sedan bridge ( twin 454's ) at 3600 rpm and curise at 18-19 knots and burned 45 gals a hour . I chatted with some of you a year ago and was told to love my achor and slow down . I did . I know run 22-2300 rpm's and curise at 7-8 knots - big savings in fuel and relaxed , letting my smaller boat friends get their and set up the morning rafting plan before I show up 3-4 hours latter .

I was at ther Boston Boat show over the weekend and chatted with the Mercuriser Rep who suggested re-powering with their new Reman 350hp 383 carb engins , their are lighter , more hp and more torque , suggesting a potential improvement of 1/3 in fule ecomony at half the cost of repowering with the big blocls ? Sounds inviting - Has anyone heard or did such a repower conversion like this . Any suggestions ?

 

[MasterFab]

As with most things in life, there are no free rides. Moving a boat through the water at a given speed requires a certain amount of horsepower, and that horsepower requires a certain amount of fuel to generate - regardless of what displacement engine does the job. You may gain an incremental increase in efficiency with these engines, due to their lighter weight, and because they are fresh, but it will be nowhere near 33%. Unless, your current engines are tired and running poorly?

If not, I would recommend you save your money, stay with the engines you have, and seek other ways to improve efficiency. Is the bottom smooth and free of growth? Are your props tuned, and sized properly for your combination? Have you removed excess weight from the boat?

If your engines are running properly, and you are seeking a 1/3 increase in efficiency, then a diesel conversion is the only way yo make it happen. The money spent doing it would take about 30-50 years to make up.....

Dale

 

[HUMPH]

45 gallons per hour seems like an awful lot for that size boat. As mentioned, how is the condition of your bottom paint? Any growth at all on the bottom will really hurt your performance. Get a smooth, clean bottom and get your props tuned then find the sweet spot and get you burn down to less than 40 GPH most likely. No way should your boat burn 45 GPH at cruise speed, WOT maybe but not at cruise.

 

[CV-23]

I really like the 383 and I know Boaters who have repowered a single 454 to 383. Some feel they have gained and some feel they have lost some power. Torque is the key with horse power being second. If I updated my CV-23 from stock 350/260hp to 282/320hp, I'm sure I'd go from 55mph to 60mph. If I swapped my 454/330hp to a 383/320hp in the 270DA, I'm sure I would loose some mph. Twin small blocks will give you more room to work in the bilge but I doubt fuel savings will be that great and mph will drop a bit. Less stern weight may get you on plane a bit faster and allow lower "at plane" speeds. Fuel Injection would use less fuel but the 4bbls get up and go quicker. The 320hp I'm talking is IO at the prop....are they saying 350hp at the prop?, Mike.

 

[Phlorida]

Torque is the key with horse power being second.

Since torque isn't a published specification from Mercruiser for these engines how does one determine that?

 

[CV-23]

I'll look around for that. Here's some results from a 290 Formula with twins, Mike.

http://www.mercurymarine.com/engines/engine-tests/third-party/?ID=196&

 

[CV-23]

Here ya go, Mike.

http://www.michiganmotorz.com/compl...eplacement-mercruiser-applications-p-806.html

 

[Captain Crispy]

Here ya go, Mike.

http://www.michiganmotorz.com/compl...eplacement-mercruiser-applications-p-806.html

That 429 lb-ft of torque comes at 3800 rpm. Not a good replacement of a big heavy boat with lots of windage. Your most effective and cost-effective repower options [in this order] are 1) a pair of new 454's with fuel injection [if you can find a pair], or a pair of 496's or 502's.

 

[Red Horse]

WIth a boat sometimes you need to think about MORE horsepower and torque to get the economy you want. With a larger more powerful engine you can run a larger prop and then lower your RPM to get around the same speed. I have a 21 footer with a 350 Mag. If you are running a 4.3 or 5.0 then you will have to apply more throttle to go faster. Smaller engines have smaller props and have to run higher RPMs to get the same MPH. I worked this all out with my Smartcraft fuel flow meter and GPS to find my sweet spots. I am not sure how this works out on a 18K plus boat but might be worth looking into. Besides, I have yet to run into someone stating that they wish they had less power.

 

[Whitenights]

I burn around 38 gph with mine running around 3400 rpm, doing around 23 mph. When I slow down to 1050-1100 rpm I am running around 7+ mph and getting 1.25 mpg. Sounds like 2200 rpm is digging a huge hole with little gain over a fast idle speed. Granted your boat is heavier than mine but you might try have half the rpm and see how it does.

I would love to have fuel injected motors but doubt I will ever wear these out and need to replace them.

 

[dpmulvey]

I burn 22 gph at 22 knots. Granted I'm hauling a LOT less weight, but I make the statement for a reason. I used to burn close to 30 at the same speed. I offer this as an initial and cheaper than new engines start. See if you need to resize your tabs. I did and it made all the difference in the world. As stated, make sure the bottom is clean. Have her If necessary, have her soda blasted to get rid of excess layers of paint (weight). Make sure your props allowing you to reach the mfg WOT for your engine. Preferably the high side of it to 100 RPM's above. When was the last time you had them Propscanned. If you can afford it, go to 4 blade props.

I see you are from Mass. If you feel new engines are a must, check out a company in Portsmouth, R.I. If my engines needed replacing, these would be a consideration.

http://www.boatstoreonline.com/420_main.html