Hypothetical Gas Engine Question

[moparharn]

Lets say you own a 45-50 gas powered planning hull flybridge cruiser that needs repower, or will some day. If you had no real interest in going faster and could live with 7-9 knot cruise and 10-11 knot WOT could you consider re-power in some kind of unique gasoline engine? If you did not have the money to spend for diesel conversion, and could remain with gasoline everything. Is there such a thing as a high torque low rpm gasoline engine. A Gardner in gasoline so to speak. High displacement, high quality, good longevity, high torque, lower rpm? Replace a big block Chevrolet with some kind of farm or off-road gas engine that would keep the fuel bill low yet still spin a good size prop at trawler like performance. Can you re-power a gas boat and get some of the trawler like benefits of diesel and keep your cost down. I realize a transmission is part of the change and you still have the potential fire hazard of gas. Is this a well hashed out question already?

 

[rslifkin]

I don't think there's a good off the shelf option, but sticking within base engine designs with plenty of marinization parts available, you could easily build something durable, long lasting, and at least somewhat more efficient than most off the shelf choices. Mind you, it wouldn't be all that cheap (but still cheaper than diesels).

I've given some thought to this for my own big block powered boat, although I wouldn't want to sacrifice ability to plane in my case. When the day comes for a repower, there are basically 4 options:

• New long blocks of the same thing ($)
• New some other kind of gas engine ($$)
• Custom build a pair of big blocks with carefully chosen internals, fuel and ignition management, etc. to target better efficiency and durability ($$$)
• Diesel repower ($$$$)

 

[tiltrider1]

The answer to this is fairly simple. The cheapest repower for big blocks is another set of big blocks. If you don’t want the speed but want economy, you can rejet the carburetor and advance the timing leaning out the engine at 2200 rpms. Now doing this will likely increase your fuel economy by 50% but you will want to put in a throttle restriction. Pushing a leaned out advanced engine beyond 2200 rpms will likely blow it up. What you have done is optimized the engine for the low end giving up ever being able to access the higher end. No one does this but it has been done in the past.

 

[moparharn]

Thank you for the replies. I wonder if there was ever a low production six cylinder beast made for torque in an off road application. The idea of leaning out any gas engine is a little scary. Some six cylinder made with massive bores, pistons and rods, and crank. Some over built low rpm six cylinder. Maybe a tractor motor? Something with crazy reciprocating and rotating mass. Maybe even a big hit and miss engine. Just thinking out loud.

 

[jleonard]

I had a Chevy in line 6 292 cu in in a 1967 pickup truck. It was a low rpm torque monster. It liked 1200 to 1800 rpm and towed trailers better than my son's Chevy 350 powered early 80s pickup.
I have seen boats with them but don't know how they performed.
The old Ford 300 in line was pretty torquey as well, but don't know if it was ever marinized.

 

[jungpeter]

Hi Mopararn,

If you had no real interest in going faster and could live with 7-9 knot cruise and 10-11 knot WOT could you consider re-power in some kind of unique gasoline engine?

Sure. One could "consider" it. But why "unique? There are ample small-displacement gasoline engines suitable for repower in a large flybridge cruiser. Mercury Marine, Crusader, et al.

Can you re-power a gas boat and get some of the trawler like benefits of diesel and keep your cost down.

Uh, no. Not in my opinion. You may well be able to repower with some form of inexpensive gasoline engine(s) for less initial cost than diesel. Will that "...keep you cost down?" Maybe, up until resale. Then you've got a unicorn on your hands. Likely a unicorn with NO buyer. And your original intent of "keeping your cost down" goes right out the window when you have to give your unicorn away (or scrap it, should the unicorn buyer not exist). Depreciation is a very real cost of boat ownership.

The cost to repower ANYTHING (gas or diesel) is usually not a cost effective solution to boat ownership. A repower is typically only pursued by those with a long-standing love relationship with their boat, whose love they can use to justify the expense. And who typically see many, many dead presidents vanish as sunk costs, never to be recovered upon resale.

I realize a transmission is part of the change and you still have the potential fire hazard of gas.

Absolutely. For those (and MANY other reasons), your idea is "well hashed out", and soundly rejected by most. A simple search of the WWW for large flybridge cruisers with tiny gas motors will very likely be fruitless. That's a good clue.

Regards,

Pete

 

[rslifkin]

If keeping big blocks and wanting more efficiency, I'd personally build them with aluminum heads, a good, fully forged bottom end, and shoot for more compression than the typical 8.5:1 or so of a marine big block. Something more like 9.5:1 or 10:1. Change to a roller cam of not too long duration, but plenty of lift. Then add EFI and electronic timing control, so you can run lots of timing and fairly lean under light load, but still keep it safe when you want to throttle up.

That should cut low speed fuel burn significantly, keep durability up, and still make as much or more power than they did originally. If you wanted to go even further and can figure out an EFI setup that will handle it, consider adding EGR to further reduce pumping losses at light load.

 

[tiltrider1]

Replacing a 454 with a 292 is not going to get you any significant efficiency. Changing from a carburetor to fuel injection will get you some efficiency. If you have a 50 hp mission then using the lightest 50 hp engine will gain you some efficiency. In the end you will spend way more money trying to make something else work then you will ever receive in fuel savings.

If you plan to run 2,000 hours a year, you can easily justify repowering with diesels, you will make the money back easily over ten years. If you plan to run 100 hours a year, your best option is a pair of long blocks.

 

[rsn48]

Unfortunately I'm not a techie but I do recall reading - I think in Sea magazine - why taking a truck or car engine and using it rather than a marinized version is not a good idea, why escapes me now.

You are probably best to get a remanufactured Merc engine . The 3.0 Alpha TKS might fill your bill, remanufactured, 135 hp.

https://www.mercurymarine.com/en/ca...emanufacturing/plus-series-alpha-sterndrives/

I'm not clear on what size boat you have but I just replaced a 270 hp Merc remanufactured motor and I know my old engine only got my boat up to 11 knots. The previous owner made it "more fuel efficient" by over propping. So you might consider an lower horsepower remanufacture and over prop. PS: me personally, I'm not a fan of over propping.

 

[Bill V]

In the industrial world there are some great high torque gas engines but none have been marinized. The best option in my opinion is to install one of the many MPI engines that are available. None will be cheap though. Your least expensive option is a rebuilt long block from Mercruiser, Cruisader and a host of others.

 

[moparharn]

Great responses. Thank you. Let me tell you why I am asking. I will be purposely vague to keep others out of this.

A while ago I sea trialed a big block crusader powered fly bridge cruise in the 45-50 foot range. The sea trial was a disaster. I specifically asked for a ten to fifteen minute open water full throttle run to see how the engines would handle the load and heat. We did not go to open water, but to a very short back water open throttle run. One engine was 800 rpm's less than the other 44 versus 36. One engine was blasting water all over the engine room from a riser (the 4400), the other engine was emitting a hot goo from the shaft log that was 60 degrees hotter than the other log. Windlass did not work, one A/C unit kept blowing its fuse, etc, etc ,etc. They tried to tell me they would fix everything, but I told them I wanted to use the boat soon and did not feel any level of confidence in what I was seeing (long ride home). Did they care at all about the boat presenting itself well? Did they care about my written and verbal insistence on an open water 10-15 minute WOT run? Enough.

So the boat is still there. I want to call and ask if they repaired it. Not sure what I am thinking. That boat should have diesels. I am not paying for diesels. Worst case if the motors are not up to repair I could replace with more efficient gas motors and keep costs down. For all the good reason presented here that is likely good money after bad. I liked the hull, but clearly there was no person giving it the love it deserved. Maybe a top end rebuild with EFI and electronic ignition and computerized control could work. I was just thinking of ways to rationalize making this lost opportunity work. Not likely. I need to stay focused on the trawler I have in mind, but with Covid going on I have too much time on my hands and came up with this hair brained idea. It really is mostly about the available BTU's of diesel versus Gasoline and nothing is going to change that. There is no semi free lunch. Thanks again for the input.

 

[rubenova]

I'd hang a pair of outboards off the swimstep, pickle the engines, remove the existing prop's and have fun. This would allow the boat to be put back the way it was easily (resale). I would also route the throttles/gear change to the helm and drive it like a bulldozer.

 

[moparharn]

At first I laughed at that assuming you were kidding. Then I remembered that a 35 Bertram was purchased locally with just that in mind. They are going with a bracket and triples I think. You could hang diesel outboards even. Not going there, but very creative thinking.

I'd hang a pair of outboards off the swimstep, pickle the engines, remove the existing prop's and have fun. This would allow the boat to be put back the way it was easily (resale). I would also route the throttles/gear change to the helm and drive it like a bulldozer.

 

[RT Firefly]

Greetings,
Mr. m. You'd probably be better off with one stream engine driving the two props. Almost unlimited power with steam.

 

[rslifkin]

A 15 minute WOT run on a seal trial of any boat without continuous rated engines is an absolute non-starter in my book. Even more so with most marine gas engines, where max continuous power is a heck of a lot less than full power. 1 - 2 minutes at WOT to check temperatures, RPM, etc. is fine, but anything more is just abuse. For a 454, for example, max continuous is typically regarded as 3400 RPM, which is only about 180 - 220 hp depending on where in the 4000 - 4400 RPM range it sits at WOT with the current props.

Unfortunately I'm not a techie but I do recall reading - I think in Sea magazine - why taking a truck or car engine and using it rather than a marinized version is not a good idea, why escapes me now.

There's nothing inherently bad about a marinized car engine, but how well it works really comes down to how good the marinization parts are, if anything was modified or done differently for better durability at high load, and whether the basic design is suitable for the desired max continuous power output (regardless of peak power output). Some engines (like the 454) translated pretty well into marine use, for example.

 

[Solly]

Used to be you could take a small or big block Chevy or Ford and probably Mopar too and build an engine to do most anything you wanted. Cam, heads, pistons, carb or injected. Want torque, build a truck motor. Want HP go for it. All the marinization parts are out there and proven too.
But the short story is replace what's there now. That's what was designed into the boat.
Or take a mechanic to look at the motors. What you described might only be maintenance.

 

[Lepke]

The Chevy inline 6 of the late 1960s did have better torque and pulling power than a v8, but at the expense of mileage. Towing or heavily loaded got you about 6mpg, but gas was 25¢/gallon.

 

[Ski in NC]

They used to make big, low rpm, long lived gasoline engines, mostly in the 40's, 50's and 60's.

But this is exactly where diesel shines based on fuel efficiency and long life. As soon as diesels were able to fit those apps, the gassers were retired.

There are no gas engines on the market now that fit that bill because it can be done much better with diesel.

 

[rubenova]

I'm very serious. I chartered a trawler that busted a gut 4 hours in and got towed back. They gave us a SeaRay 370 to replace it. Pretty much the opposite of a 7 knot single diesel trawler in every way. Including comfort. By that I mean a real mattress. We ended up running at a fast idle (6kts and 1300 rpm) most the week and outran a squall (20kts). 20kts gave 0.5 nmpg and 6kts gave 0.7 nmpg. Cheap outboards, or replacing the 454s with 18-27hp Yanmars would make a plastic-football shaped boat into the a blasphemous interpretation of a slow boat...only comfortable. A lot like putting a Ford 302 in a GM with a Chrysler 4 speed.
Signed: an ex-purist wooden trawler guy

 

[Iggy]

Well, I understand what you are trying to do.

But I would repower with diesel if I could. Most reasons go with out saying. But going from gas to diesel for me, I felt a little safer. Now I don't have to worry about equipment being ignition proof.

 

[Nomad Willy]

I can think of one engine that may fill the OP’s requirements.

It’s a 302 cu in V 6 truck engine. Made by GMC for hard working trucks like big vans ect.
It was designed to operate very close to max power continuously. Or maybe even full bore all the time. The cooling system was quite different from the usual. And there was at least a small price to pay ... they were known to burn more fuel than the car derived engines.

Availibility of both engines and parts could be a problem though. They were sixties something engines. Also finding marine manifolds could be the biggest problem.

A serious gas truck engine that’s much like a diesel. Very durable at high power output.

 

[guy with a boat]

Consider that if the drivetrain is in such terrible condition that the overall maintenance has probably been equally bad. Everything on a boat wears out, and if they didn’t maintain anything then you will be fixing and replacing things for the next few years if you buy it. My advice is find a boat that has had a reasonable level of maintenance. You will still have to fix and replace, but it will be a fraction of the time and expense that a boat will be that is obviously neglected.

 

[RT Firefly]

Greetings,
Mr. m. Just noticed the typo in my post (#14). Should read steam instead of stream...BUT, what about the option of a single diesel driving the two prop shafts? Potentially cheaper than 2 independent engines, increase in ER space and IF the correct size, planing may be possible.

One last "out of the box thought" Single diesel generator and twin electric motors.


Edit: Just had to do that "Captcha" thing to post this.

 

[Okydowky]

Mopar, I will give you some real numbers of our last eight months of cruising exactly how you see yourself.
We are 40’ and 50’ over all with dinghy. 30,000 full fuel, water, gear! We have twin 454 ‘s and cruise religiously at 7 knots. And always hoping on a following current.
We are getting 4.1 gal/hr for both engines, we know this mileage well, because we start every trip full of fuel and end topping off tanks. So we have a pretty good accurate fuel burn with no fancy gauges.
Buuut if we screw up and have to push current or wind or running late for a passage, well hell yah we can burn a couple dinosaurs. We like this option in our back pocket knowing we can beat a weather front or just get to where we need to be.

 

[rslifkin]

Mopar, I will give you some real numbers of our last eight months of cruising exactly how you see yourself.
We are 40’ and 50’ over all with dinghy. 30,000 full fuel, water, gear! We have twin 454 ‘s and cruise religiously at 7 knots. And always hoping on a following current.
We are getting 4.1 gal/hr for both engines, we know this mileage well, because we start every trip full of fuel and end topping off tanks. So we have a pretty good accurate fuel burn with no fancy gauges.
Buuut if we screw up and have to push current or wind or running late for a passage, well hell yah we can burn a couple dinosaurs. We like this option in our back pocket knowing we can beat a weather front or just get to where we need to be.

Out of curiosity, what rpm are you running for 7 kts? I've been using 5.5 gal/hr (total) as an estimate for 1300 rpm / 6.7 kts in calm water.

 

[Okydowky]

We typically run 1350-1600 rpm. And again where we run, we are at the mercy of current, there is no way around it.
If you take your time and plan your trips, there is no reason why you can’t hop on a 3-4knot current if not higher to pull you along for hours.

We have been in neutral going 9-10knots surfing along saving fuel.

We are in the midst of re propping right now! Hopefully bettering our mileage.

 

[tiltrider1]

Buying a gas boat with the thought of converting it to diesels is a mistake. If you want a diesel boat, by a diesel boat, it will be cheaper than converting a gas boat.

 

[Art]

Calculated hull speed on 40' LWL is 8.74 knots.

IMO - Put in a couple is 350 ci Mercruisers or Crusaders and cruise at 7 to 8 knots.

Repowering cost will be about the most minimal available and general cruising will be fairly cost efficient in fuel. Depending on hull bottom design and weight of boat... a pair of 350's would likely put it up on plane if you needed to get into the 16 knot range. Fuel use would become costly then.

 

[Nomad Willy]

If you do go diesel FF knows about International Harvester truck engines he thinks highly of and knows where to get them cheap.

tiltrider,
Is there a problem switching fuels in the tanks?

 

[FF]

"I wonder if there was ever a low production six cylinder beast made for torque in an off road application."

Detroit Diesel made a version of their series 50, a 4 banger version of the series 60 that was set to operate on natural gas for the La La Land bus companies. Great torque at 1200-14--.

These are true industrial rated engines and built to have a carb , not fuel injection diesel style.

Might be a source of a heavy duty engine?

The simplest way to help mileage on a V8 is to install a 2 BBL carb made for some racing rules .

Quick and dirty is to simply disconnect the linkage that operates the big secondaries on a 4 BBL carb.