Changing a petrol engine for Diesel

Good evening all

I have a Fletcher Zingaroo 26 foot motor cruiser, it came with a 7.4 litre long block chevrolet mercruiser V8 engine which gave approx 370 hp.well it goes with out saying that when we did the calculations it worked out at a massive amount of money to fill the tanks so the question is.

What deisel engine could i install to replave the petrol version giving me a similar output

Thanks all in anticipation of someone out there knowing

Womble3amp

 

This is not as simple a question as you may think. There are a lot of factors to consider. The added weight of the diesel can create draft problems on small boats. The balance of the boat will be different in most cases. In rare cases you may develop some unpleasant sea keeping traits due to the added weight. If the boat company that built your vessel offered you boat with a diesel option I would try and talk to some one who selected that option and see what they have to say. The choice of engine packages offered at the factory may be a good place to start with engine selection. I have had this discussion with a number of people about repowering there boat to “save money on fuel”. If you operate you boat say 200 hours or more a year then a repower is worth considering. At 200 hours a year (make sure you calculate your running time not time spent on the boat) or more then the pay back on the $20,000. to $30,000 coat of a repower can be justified. However if you operate you r engine like many of us say 50 hours a year or less then you would be looking at many (10-20) years before any savings in fuel cost. But having said that and you are still a player then read on.


The horsepower to weight ration is different for a diesel over a gas engine. The power curve is different also. Your gas engine is rated at 370 HP at around 4000 RPM I would think. A diesel of the same HP will weight 30-40% more than your gas burner. The truth is that you will almost never use the full 370 HP that your gas engine produces. Ask yourself how often do you run it at that full speed? If you answer very little to never you are like most of us.

A diesel is designed to produce most of its HP at its recommended running speed (RPM). As an example say brand x has an engine that produces 270 HP @ 2800RPMs. You can see this is 100 HP less than what you have now. If you fitted your vessel with this engine and a proper gear and prop. You may see a performance close to what you had with your big block GM at say 60-70% of WOT (WOT= wide open throttle) but at half the fuel burn. The diesel produces less HP than your Gas engine. The difference is almost all of the 270HP is delivered to the prop all the time. It also burns less fuel in doing so. This is the attraction of a diesel to most of us but it comes at a price.


To change out your Big Block for a diesel you are looking at big money. The engine and gear will run you 15,000.00 to 25,000.00 or more depending on what make of engine you select. If you have to hire the work of changing them out plan on another $5,000.00 minimum. You did not say if your boat has a shaft drive, jet, out drive or what. If it is a shaft you may need to increase the shaft size to handle the added stress of a diesel, this may apply to the engine stringers also. The fuel system will need to be modified and possibly the exhaust. The air inlets to the engine room may need to be increased, diesels use a lot of air. The physical size of your engine compartment may limit your choice of engines you can use also. If you have a gen set you will want to change it also to avoid having 2 separate fuel systems gas and diesel. This can run another 4-6 thousand or more.



You have to ask yourself at what speed will I operate this vessel at most of the time. That is the goal of what you want to try and design a repower project for. For most of us a speed that gives us a decent cursing range for the weekend with a fuel burn that we can afford. Selection of an engine manufacture is like buying a car. You need to shop around, look at who offers the best service network in you area. Talk to owners and see what they have to say. In most cases you will want the people you buy the engine from to also install it. You will need to work with prop manufactures to select the correct gear ratio and prop package to get the desired results. The engine people should be able to give you information on expected speed and economy of the package they offer as well as a reference list of happy customers (if not run) you can contact. Your engine choices may be limited by the physical properties of you vessel. Don’t let that cause you to make a bad choice. If you can’t find an engine package that offers you what you want out of you vessel them maybe trading vessels would be a better option. Do you research well you will be investing a lot of money in you boat. Make sure you will get what you want in return.

 

Petrol to diesel engines

Flybaby Jim Hi
Thanks foir that indepth response, as you say there is a lot to consider, we are in position to do the work our selves its just a case of finding the power unit .

We currently have a mercruiser outdrive leg at the moment and are looking at using this.. do you consider this to be the best option or not ????

In the mean time i will peruse the power units avaiable and look at what is about , once chosen we can look at the drive


Once again thanks for your response , talk soon

Regards

Womble3amp

 

With a mercruiser sturndrive or any other outdrive you will be concentrating a lot more weight at the stern. Be sure and check to make sure that you can carry the addition weight. As to using your existing out drive. Well maybe, the gear ration is different to operate with the higher RPM of the gas engine. You will most likely have to change the gears. The drive should be tough enough. Mercruiser offers several different Diesel/outdrive packages. Talk to a local dealer and see if they will take your engine and drive in on trade. Some will some want. Good luck and let me know what happens.


FlybabyJim

 

Why couldn't you put a 6.2 or 6.5 chevy diesel out of a pickup in place of that 454? The weight difference wouldn't be any more than having ole uncle Bubba sitting in the back of the boat. And yes they are made for pulling 30,000 lbs of cows and trailer up mountains with your right foot plastered on the floor all day long so the somewhat questionable marine durability queston shouldn't be an issue. What I could envision as a problem would be the increased vibration over the gas motor, but that could be dealt with better vibration damping motor mounts. Time, skill, innovation and hard work are still preferable in my mind to spending big bucks to have someone else do it for you.

 

Peninsular has the 6.5 in a marine version with a mechanical injection pump versus the problematic electronic one. It will bolt right in place of the 454, though I agree the gear ratio will have to be changed.

 

If you figure the fuel consumption of normal operation underway , your using about 10 hp for every gallon an hour you burn.

EG 20 Gph would be 200HP (rather high ) , I would guess 12gph (120hp) would be closer

The diesel you select should be rated at your cruise HP at the diesels 80% power point.

Use the cont duty HP table ( there are 4 or 5 tables) and your speeds should be the same.

YOU will take longer to get up on the plane as there is only 20% excess hp rather than 100% from the gasser.

A 150 - 200Hp Yanmar should fit the bill and weigh no more than the iron monster.

Biggest hassle is the gas engine would cruise at 2800 to 3200rpm , many diesels prefer 1800 or so .

FAST FRED

 

not mentioned here yet is diesel fuel weight witch is more than gas. dont ask me how much but remember reading this in boating magazine years back comparing pro's and con's on marine gas and diesel engines.

 

Dont remember the exact numbers Yip, but the weight difference is pretty negligible, something like 7.6 versus 7.7 PPG. Not a lot unless you're talking thousands of gallons.

 

Sounds high is right, considering my twin Crusader 270s burn 20 GPH at 3500 RPM, probably somewhere around 170 HP a piece at that RPM. (Had a torque/HP curve for these engines but can't find it now). Anyway, that's 340 HP at 20 GPH, according to my Flowscans, which are dead on.