planing gas to displacement diesel conversion

Gentlemen - I have found that the mid-80s aft cabin motor yacht is a great value on the market, but I do not need the speed (even though 17 kts is pretty anemic) and therefore the volume and weight of the typical twin 454s. Am considering what it would take to convert to smaller gas or diesel engines. Is this economically sensible? My guess is I only need ~60 to 80 hp per engine (24000 # displacement) - could 4 or 6 cylinder engines - yes? Would this destroy the value of the boat? I guess I am essentially making a sundeck trawler-type boat, but on a (formerly)planing hull. Comments? JeemboNC

 

It would be a lot cheaper to leave everything like it is and throttle back to 8 or 9 mph. The small diesels will do about the same with a boat that size. Fuel would be about the same. Good luck on whatever you do.

 

Thanks for the input. The common wisdon is that diesels use half the fuel, last forever, and are safer. Hence gas engines are considered second string. Affects resale. Formal analysis of the numbers does not substantiate the magnitude of the advantages, but analysis and real world are sometimes different. I'm still chewing. JeemboNC

 

My guess is I only need ~60 to 80 hp per engine (24000 # displacement) - could 4 or 6 cylinder engines - yes? Would this destroy the value of the boat? I guess I am essentially making a sundeck trawler-type boat, but on a (formerly)planing hull.

Hulls are designed to be efficient at ONE speed a slowed down plaining hull may use 10 to 20% MORE FUEL At low speeds than a hull designed for displacement cruising.

For resale you only need ONE buyer , so the value in a "special" vessel may take more advertising , and time , to find the purchaser.

FF

 

As to the issue of converting to diesel, go have a look at the articles by David Pascoe on marine diesels, you'll find them very revealing.

 

I suppose a good way to get part of this clarified would be to find two people with the same boat, one with gas and the other with diesels. That would at least clarify some of it. (OR, get the manufacturer to share performance data) It is reasonable to accept that FF is right when he says a planing hull at 8 kts will not be as efficient as a displacement hull at that speed. The dilemma for me is fuel economy, cost of purchase, maintenance, engine room, etc. Somebody said it before to me - it's all about the compromises. I read the Pascoe articles - very informative. Does anyone know of a site where boat tests are available for older boats/yachts? I cannot find much. I'm gonna start a new thread to see if someone knows. Thanks to all. JeemboNC

 

I'm in the process of repowering a 34' Silverton - here are some things I've learned:
You are only going to go about 5 to 6 kts with any kind of fuel economy.
At 5-6 kts. you will be pretty economical on fuel - maybe over 2 mpg.
If you try to push the boat to 8-10kts, you'll probably burn like 2gpmile or more.
If you get into rough seas in a planing hull boat and you don't have enough hp to power your way around, your ship may end up on the sea floor.

You may be able to find some post with helpful info if you search for my post - I've had a lot of good responses from others on this website. Hope this helps!

 

http://www.yachtsurvey.com/GasNdiesel.htm

I cannot say that I agree with him though, and gas prices will probably break $4 per gallon within 2 to max 3 years, what then? Eurpoean prices are over $6 per gallon in many countries. Your most economical choice would be a displacement hull of course, fancy that?

 

OK, let's just talk replacement costs. A crated GM 350, all decked out, ready to install, in my area, costs between $5K to $7K. The equivalent Volvo diesel, with aprox. the same characteristics, in my area, costs between $27K to $30K. So... even if you did get double the "mileage" on diesel (which hasn't been my experience so far), and with both fuels within pennies at the pump, how much more fuel could I buy with the $23K difference? Oh, actually, it would be more like $46K difference, because I've got "twins".

Actually, if money is no object, just install whatever power gives you the biggest thrill. Remember, if you need to ask how much it costs, you can't afford it.

Pascoe's "point" is that most boats are "marina queens", getting far less than 100 hours of use each year. Spending megabucks and rationalizing it by "extended longevity" or "better fuel mileage" is primarily moot.

Having said that, I can report that I just happen to be "fortunate" to have bought my boat with twin diesels already installed. What will I do when they are shot? Probably rebuild 'em, at aprox. $1000 per hole. Why? Because it's easier to work with what I've got than to start from scratch.

 

cannot say that I agree with him though, and gas prices will probably break $4 per gallon within 2 to max 3 years, what then? "

Happy to report its now under $2.00 a gal for unleaded.

And still falling.

FF

 

I love this forum! The reputation of diesels lasting forever is compelling, but the actual life achieved in 'typical' recreational use is far less than what they are capable based on stress and wear. Pascoe makes this point. Repair of gas engines is so much cheaper it becomes a huge factor in a life time cost analysis. Further, my experience says that if you limit power use on a gas engine to the kind of stress margins diesels enjoy, they last longer, as well (I bet this will be controversial). I have no definitive data to support this - wish I did. I will say that if the hour meter was able to base use on a weighted basis, with 'longer' hours for low power time, this would be much more accurate. My time as an engineer in the gas turbine world and our development of 'life used' algorithms took this in to effect (also included throttle transients, temperature limits, fuel used, etc and was NOT linear).

In the aircraft world, recip egines have a definitive TBO (time between overhaul) that is defined by the OEM with FAA oversight/approval (note - this is only a hard rule when the aircraft is in commercial use, private operators can use an 'on-condition' limit). But there is a caveat - the operating time must be within a certain calendar time or other factors (rust, seal deterioration, oil acidity, etc) can become the limiting factor. A typical IO-470 overhaul (260 hp fuel injected, 6 cylinder horizontally opposed, air cooled, 2800 rpm max) is $20,000 and the tbo is 1500 hrs. However, almost none make it to full TBO without some substantive repairs, such as individual cylinder repair (they each can be removed individually), accessory repair, etc. Therefore, the $13.33 hourly 'set-aside' for engine life consumption is actually much greater, potentially by 20-50%. This has to be factored in to operating costs, just like fuel burn (typically 10-14 gph @ $3.00-$5.00/gallon for avgas).

The point?! If diesels really do cost 5 times that for a gas engine for acquisition and repair/overhaul, the fuel savings must be tremendous if the criteria is cost alone. Safety, reliability, resale, weight, fuel availability, emotional gratification, bragging rights, etc all come into play, but some of those are hard to weigh and give a value when there are so many other variables. And for me, the smell of diesel fumes is a real problem (I didn't used to be this way, getting old, I guess).

Question - my limited research (in NC) puts diesel fuel at the dock @ $2.15/gal and gas @ $2.30. Is that a good reference? Also, has anyone had any substantive experience with the issue of higher alchohol content in gasoline and it's affect on fuel system components? It can be really problematic in aircraft systems, and I hear this will be a bigger and bigger issue in the future, especially for older engines not designed to handle ethanol (remember the lead issues and valve seat wear?).

So, I am still confused! Too much analysis - I should just enjoy being on the water!!!!!!! Thanks to all. JeemboNC

 

Let's see now... (gotta get my calculator out for this... gas price right now is running about 1.02 per litre x 3.8 to convert to USG = aprox $3.88 CDN per USG x 0.85 exchange rate = $3.29 USD per USG. Diesel is running 0.91 per litre x 3.8= $3.46 per USG x 0.85 = $2.94 USD per USG.

Owwww!! My head hurts! (And my wallet has gone into shock). This means that our 145 USG diesel fill-up will cost about $426.30 USD

Jeez... you're damn right I better have a good time...

With regard to ethanol in fuel, the only problem I'v heard of is the potential for absorption of water into the fuel if it is stored for any length of time. Common practise here is to treat the remaining fuel at the end of the season with a fuel stabilizer to prevent problems from this and also to prevent "varnishing" of the fuel.

 

Here's my general guideline on the subject:
If the boat is mainly going to sit at the dock as a floating cottage, it should be beamy and voluminous with cheap engines.
If the boat is going to be used as a boat, it should be as efficient a hull shape as feasible, and have reliable, economical engines.
Fuel costs are on the way down this month, because (a) it's winter, (b) it's a warm winter on the whole. Long-term trend, on an annual basis, is up, up, up. $1 CDN a litre is typical in summer for either fuel. The oil industry in the US is pretty heavily subsidized in various ways, so you guys pay a fair chunk less down there for it. (Then there's those 3rd-world oil producing republics where you can get it for a fraction of N.Am. prices....)
The diesel engine, by the nature of its thermodynamic cycle, is inherently some 25-40% more economical than a gasoline motor of comparable power.
As far as biofuels go- 10% ethanol is indistinguishable from pure gasoline in virtually all engines. 20% or higher and you might have to worry about moisture absorption, and some older engines (esp. those with finicky carbs) will balk. To use the new 85% blends requires an engine designed with ethanol in mind. Biodiesel, even pure, will burn in almost any diesel but expect to go through a lot of fuel filters in the first month if you switch an old motor over; good biodiesel is very, very clean and a powerful solvent, to the point where it will actually clean out all the gum and varnish residue in the system that's left there from the old oil fuel, leaving all that stuff in the filter.

 

Marshmat - thanks, good points. I have a friend that has been running old frying oil in his Mercedes cars with great success, and he relates the same 'cleaning' action. He really gets immense pleasure from using old oil and is religious about filters.

You are right, compression engines are thermodynamically superior to spark ignition engines, and modern examples of each in the under 1000 hp motive power class have sfc's (specific fuel consumption) of .35 lbs/hp-hr and .42, respectively. Also, diesel fuel is heavier (~7#/gal vs 6#/gal) than gasoline. Gas turbines? Horrible efficiency in small sizes, but light and simple as hell.

For my anticipated use the additional costs for fueling a gas engine are projected to be 10-20% greater (based upon the entire cost picture - insurance, slip fees, value of money, etc) than diesel power, but acquisition and potential maintenance costs are much greater for the diesel option (I have to admit inexperience in diesel maintenance and am terrified of a $16,000 bill to overhaul even a little one like the 120hp Lehman). The bugger is that fuel is a variable cost and will hinder my hopping in and going for a cruise because I'll be looking at a $100 bill every time I fire it up and go more than 100 yards! This very thing limits my use of my Cessna airplane.

I am still worried about the ethanol issues. Though some of it in fuel is good (what do you think dry gas is?), more is bad. I have heard that there are additives that will ameliorate the effects that are being worked by the miracle fluid guys, but we'll see.

Anyway, thanks to all. After all the numbers, I bet I go with an unpredictable emotional reactio!! JeemboNC

 

Also, has anyone had any substantive experience with the issue of higher alchohol content in gasoline and it's affect on fuel system components?

IN the North East US there have been great problems with intergal gas tanks actually dissolving and leaking from the Alcohol.

For a cruiser the very short life (absorbes water from atmosphere) of gasahol might be the best reason for a diesel, or removeable tanks.

No problem tossing 12gal of gas left over from a weekend in the family buggy, but the larger boats that eat 10gph per engine will have tank sizes that do not lend themselves to feeding the family car.

Yet the fuel is going bad , day by day ,thanks Tree Huggers!

FF