Engine Swap help needed

[Cigatoo]

My boat has an older 375hp Cat engine. Cruising at seven knots, I average a burn rate of 2 gph.

Nice boat Seighlor. We looked at a couple of Wilbur’s. One was overly modified the other above my pay grade. Beautiful boats!

 

[rsn48]

You might want to read this article which will irritate some about gas versus diesel, you might be better with gas. One stereotype is that gas is the bomb in the back, guys who write this here on these forums after writing it, get in their car with a gas bomb in the back and drive all over. Gas explosions percentage wise on a boat is far less than gas explosions in a car. But in a car we have become used to gas in the back.

What you want to pay attention to is the servicing of diesel engines and the fact that it is expensive and often left undone.

https://www.yachtsurvey.com/GasDiesel.htm

 

[jimisbell]

It takes the same amount of energy to push the boat at Hull speed, whether using 300 horse or 75 horse engines. I doubt you could motormuch more economically with smaller engine. 3 gallons an hour is not bad.

Gordon

Well, a NA Perkins 6-354 will push a 35 to 45 foot boat at 7 knots at about 1 gallon per hour.. My Bruce Roberts 44 Ketch used 3/4 gallon an hour on the NA 6-354 You can get a Mainship 34 with that Perkins engine (turbocharged) already installed (factory original) If you keep off the Turbo I would think at 6-7 knots you would get about 1 gph.

 

[jimisbell]

You might want to read this article which will irritate some about gas versus diesel, you might be better with gas. One stereotype is that gas is the bomb in the back, guys who write this here on these forums after writing it, get in their car with a gas bomb in the back and drive all over. Gas explosions percentage wise on a boat is far less than gas explosions in a car. But in a car we have become used to gas in the back.

What you want to pay attention to is the servicing of diesel engines and the fact that it is expensive and often left undone.

https://www.yachtsurvey.com/GasDiesel.htm

BUT the gas bomb in your car is not enclosed in a space ventilated by an electric fan!!!!! In fact it is very purposefully hung UNDER the car so fumes are dissipated safely!!!!

 

[Gypsyjon]

Perkins

Back in the 80's I had a 34 foot steel tug that I converted to a cruiser. It has a 6-71 Detroit in it which was way more engine than I needed. I found a Massey Harrison 1106 up in Canada that had a 6-354 Perkins in it. I bought that engine for a little bit of nothing. Then I bought a water cooled manifold from Perkins, bolted that on, added a Vetus Water Lift muffler, piped it out the side of the boat and it worked great for many years. The boat had a dry stack exhaust that was hot and incredibly loud before the switch. In fact I kept the boat at Foxhaven Marina on Catawba Island for a long time. The Perkins burned about 1.5 gph running at about 6.5 knots. Lots of fun. Lakeside Marine, on Marblehead, which I think is gone now did a bunch of interior finish work for me. It was a great sea boat and paid no attention to the Lake Erie slop. It weighed 34,000 pounds.

One thing to keep in mind is that yes the engine swap is doable, but I was working with a steel boat which lends it self to shall we say, "cut and paste" type work, rather than fiberglass.

Do some more research but dont let the naysayers here scare you away from a cool project like that.

 

[sunchaser]

BUT the gas bomb in your car is not enclosed in a space ventilated by an electric fan!!!!! In fact it is very purposefully hung UNDER the car so fumes are dissipated safely!!!!

For grins I looked up BoatUS fire statistics. Dependent upon the period you look at, electrical fires outnumber all fuel related fires by a 10:1 margin.

 

[RickyD]

There is a reason Mainship installed a 300hp+- engine, and not a 77hp Perkins. I recently saw a 395 with a 385hp Yanmar which at displacement/trawler speed was quite economical. Are you sure you know more about engine requirements than the builder/designer?
And balance the cost of the changeover against the possible fuel savings, a kind of cost benefit analysis.You won`t be just swapping engines, there are engine bearers,mounts, exhaust, gearbox mounts, fuel supply, raw water supply, prop shaft modifications, instrumentation and senders, lots of knock on changes to consider.
Maybe run it for a while after you buy one, and see how it looks then.

I agree with Bruce. Just cut back on the throttle and enjoy the economical ride. I had a 47 Lien Hwa with twin Ford Lehman 375's and at 10 knots I got about 1 gallon per mile. Now I have a Californian 55 with twin Cat 3208's with 425 horses each and at about 10 knots I get...….about 1 mile per gallon. I can crank it up to 18 knots. Lord knows how much fuel I'd be burning. When I had the boat hauled for purchase they said it weighed 64,000 lbs. I did have completely full fuel tanks at the time. 650 gallons.

Have fun with what ever you decide to do.

 

[BIG CAT]

my 2¢.

changing the engine is not going to change the amount of fuel needed to move the boat at x speed. the only advantage going to a smaller engine would be in the fuel required to turn the engine. bigger engines take more fuel to turn at x rpm. now with that said, the amount of fuel you will save will be very minor compared to the cost of swapping from one engine to the other. if you were starting from a blank slate it would make more sense but on an already running boat with no know issues you would be pissing away money that could be better spent stocking the bar.

 

[Nomad Willy]

I still think kooking for a boat more suitable to your needs would be best.
Short of that running the existing engine slow should work fairly well.

 

[OldDan1943]

BUT the gas bomb in your car is not enclosed in a space ventilated by an electric fan!!!!! In fact it is very purposefully hung UNDER the car so fumes are dissipated safely!!!!

If necessary, one can leave the car that is on fire and run on solid ground to escape. I still cant run on water nor change water into wine.

Your money, your boat, your choice. Enjoy what ever you buy, please.

 

[Woodland Hills]

If gasoline is so deadly, why aren’t we seeing an uptick in boat explosions with all the new multi outboard CCs? Sure, the powerhead is in the open air, but there are hundreds of gallons of gas under the floor. And they are never grounded when fueling like an airplane.

 

[Rosborough 35]

Like most everyone that has responded, I too would stick with the larger engine that the boat has in it now. I have a Rosborough 35 trawler, with a Cummins B210 engine. The Rosborough 35 is similar to the Mainship 35, but maybe a little heavier. The Cummins B210 has been a good, economical engine for me. To give you an idea of fuel consumption versus speed for my boat with the Cummins B210:

1000 RPM, 77 HP 1.7 gph 4 Knots

1600 RPM 161 HP 3.5 gph 6.5 Knots

1800 RPM 183 HP 4.6 gph 7.4 Knots

The hull speed for my Rosborough 35 is 7.5 Knots (8.6 mph), so to cruise at hull speed, I actually need 183 HP. I never run over 1,800 RPM, unless I'm trying to blow the soot out of the turbo. The engine makes a lot more noise and burns a lot more fuel at higher RPMs (11.9 gph at 2600 RPM), but because it's a displacement boat, it doesn't go much faster at all. Running at the higher RPM is just a waste of money. We normally cruise with the engine running between 1600 and 1800 RPM. I would not want a 77 HP engine in my boat and be stuck at 4 knots. Running at 7.5 Knots, I can cover 86 statute miles per day and I have enough power to deal with an incoming tide and also to get out of the way when I need to.

 

[BruceK]

If gasoline is so deadly, why aren’t we seeing an uptick in boat explosions with all the new multi outboard CCs? Sure, the powerhead is in the open air, but there are hundreds of gallons of gas under the floor. And they are never grounded when fueling like an airplane.

From explosion reports I`ve seen,some were associated with refueling,some with work being undertaken,others just while running. I think the replacement of carburetors with EFI helped by eliminating some sources of leakage.
Gas mixture can be too rich or too lean to explode,sometimes moving the boat after refueling admits more air and the mixture becomes potentially explosive.
All said, I would not have enclosed gas engines. I`ve seen the aftermath,met survivors (a baby died, all the adults survived), seen the injuries of people fired 20ft into the air, the huge changes it made to their lives,it`s not a risk I`d take.

 

[rsn48]

Larger engine, lower RPM's necessary to push boat at a given speed. I just had Volvo give me a hard time about wanting to put a V6 into my boat, they felt the motor (240 hp) would be too stressed pushing my heavier 29 foot boat. Read the article I linked to above, as the old saying goes - "there is no replacement for displacement."

I got a Mercury 6.2 litre 350 hp stern drive gas and lets say at 7 knots, the 240 Volvo engine would have higher rpm's to get the same speed versus the Merc. Again read the article.

There is more to an engine than just the ability to push the boat, there is also longevity.

 

[Gordon J]

Ski could tell you better and has

Well, a NA Perkins 6-354 will push a 35 to 45 foot boat at 7 knots at about 1 gallon per hour.. My Bruce Roberts 44 Ketch used 3/4 gallon an hour on the NA 6-354 You can get a Mainship 34 with that Perkins engine (turbocharged) already installed (factory original) If you keep off the Turbo I would think at 6-7 knots you would get about 1 gph.

Not sure what you are trying to say. 35 - 45 foot boat covers a lot of ground. Your ketch is a full displacement boat which is not what we are talking about. Putting more hp on your sailboat would not give you much more speed, just noise.

Turbos improve fuel efficiency as well as output. That is why engines come with them. More bang in a smaller package.

 

[tn6437]

first of all, you didn't state that you were a long time boater. if you and your wife are new to boating I would buy the boat you like, see if you enjoy boating. at the same time, cruise a lot at a snails pace, 6-7 knots. check your fuel burn then decide if you can save enough by repowering to make it worth while. remember to factor in the loss of resale value.

 

[Ski in NC]

Like most everyone that has responded, I too would stick with the larger engine that the boat has in it now. I have a Rosborough 35 trawler, with a Cummins B210 engine. The Rosborough 35 is similar to the Mainship 35, but maybe a little heavier. The Cummins B210 has been a good, economical engine for me. To give you an idea of fuel consumption versus speed for my boat with the Cummins B210:

1000 RPM, 77 HP 1.7 gph 4 Knots

1600 RPM 161 HP 3.5 gph 6.5 Knots

1800 RPM 183 HP 4.6 gph 7.4 Knots

The hull speed for my Rosborough 35 is 7.5 Knots (8.6 mph), so to cruise at hull speed, I actually need 183 HP. I never run over 1,800 RPM, unless I'm trying to blow the soot out of the turbo. The engine makes a lot more noise and burns a lot more fuel at higher RPMs (11.9 gph at 2600 RPM), but because it's a displacement boat, it doesn't go much faster at all. Running at the higher RPM is just a waste of money. We normally cruise with the engine running between 1600 and 1800 RPM. I would not want a 77 HP engine in my boat and be stuck at 4 knots. Running at 7.5 Knots, I can cover 86 statute miles per day and I have enough power to deal with an incoming tide and also to get out of the way when I need to.

Those HP numbers are are way off. A B210 makes about 17-19 hp per gph consumed. 183hp at 4.6gph is almost double that.

 

[Nomad Willy]

tn6437,
A smaller engine could be a gain in resale value.
There was a GB 36 that had been repowered w two 55hp Yanmars a few years back and it seemed to sell rather quickly. The smaller engines have important benefits over just low fuel consumption. Our poster here may not be the only one wanting a boat powered to go the displacement speed. Most trawlers go slow 95 to 99 % of the time. Or 100% of the time.


Ski,
Almost everyone under reports their fuel consumption. Only one thing to do ... ignore it.

 

[MICH MIKE]

for sale, Marine Trader 32. less than 150 hours on completely overhauled Lehman 80.

 

[Riverguy]

Need to rethink before repower.

New guy here from Ohio.
The Wife and I are Ready to buy a boat. Our intent is to motor around
here and there and all over.
Narrowed it down to a 35 0r 39 Mainship single engine. <snip>

Re: "Problem...300 and some horses apparently eat 3 to 13 gallons per hour. I am thinking maybe an engine swap down to Maybe a 4 cylinder Perkins."

The idea that a smaller engine will burn meaningfully less fuel is a very common misconception.

Engines burn fuel depending on how much horsepower they are producing.

A single-engine Mainship 390 running at 7 knots is going to burn the same amount of fuel regardless of what the size of the engine is.

That is because the horsepower required to move the boat through the water at 7 knots is the same no matter what the engine size is.

Yes...this seems counter-intuitive, but it is true, and it bears repeating...

Engines burn fuel depending on how much horsepower they are producing.

Now...there will be a miniscule (low-single-digit percentage) improvement due to the weight savings.

Just because a particular engine is capable of producing, let's say, 370 horsepower (like the Yanmar 6LYA in our MS390) does not mean that it will be doing that all the time, it only means that that power is available when needed.

As you work through the details and think about how you will use the boat, and learn about operating in adverse weather and rough seas, you will quickly discover that there are times you are going to need WAY more than 77 hp in a Mainship 390.

Not to worry though, have a look at these fuel-burn numbers:

Note that the numbers at 900 and 1,200 rpm are wrong due to a spreadsheet error. I learned this in correspondence with Capt. Bill Pike, who wrote the article. You need to double the gph at the 900 and 1,200 speeds, but overall this picture tells the story.

If you really want to save fuel, spend the money on a slick bottom-job, propeller balance and tuning and (most of all) GO SLOW.

 

[OldDan1943]

When it comes to burn rate vs speed, they are the manufacturer's and builder's numbers and may not relate to YOUR reality.
When you get the boat loaded with your stuff, make your own fuel curve noting sea state, apparent wind direction and speed plus other information you feel important.

 

[thirsty boy]

If you repower a boat with something different than OEM, than you should do it for yourself and not care about resale. I believe most prospective buyers will walk away quickly, when they see what has been done.

 

[Mako]

Not sure if the OP is still following this thread. If so, what did he decide to do?

BTW that boat only displaces about 8 tons. He really doesn't need much more than 40hp if he is cruising at 7 knots or less. But with all that top hamper he'll likely have to add some ballast down low.

 

[Driftless]

I have repowered boats with odd engines and it can be done very economically if you're willing to get your hands dirty a little.

For example, I repowered a 40' trawler with a 120 hp turbocharged Isuzu to replace a Lehman-Ford 6 cylinder. What that involves, though is putting a non-marine spec (i.e. non water cooled exhaust turbine) engine in, and you and your insurance company will have to be cool with that. This repower completely transformed that old boat - the Isuzu is so smooth and quiet, it seems like an electric motor is propelling the boat, plus the Isuzu never leaks a drop of anything and starts instantly.

I have made water injection elbows using threaded stainless fittings, and welding in sea water injection pipes. I've done this with the Isuzu, a Perkins 6-354 Turbo and a 33 Hp Yanmar.

The part that requires the most engineering is making a mount for the sea water pump. I use a fanbelt drive sea water pump - Jabsco makes some - and take some time to get the bracket really rigid and the alignment of the drive pulley right.

The heat exchanger can be easily mounted to the hull and only needs radiator hoses to connect to the engine. You should buy an expansion tank with radiator cap (Summit Racing has some, also Jeg's).

Mounting the transmission is not all that hard - just be sure you get an engine with an SAE standard bellhousing and flywheel. You'll need to get a bellhousing adaptor to mount the gear to the housing - Hurth/ZF have the necessary adaptors.

Good Luck!

 

[Orest]

Emergencies

Didn’t finish: I spent 8 k which is about 15% of what the whole boat is worth. Did I give it more value for resale? I don’t think so. Find the right boat !! When I think of it ... cushions 2 k. Fridge 1.5 , heater 3 k, electric toilet , new top 6 k, electronics with radar 5 k, this is just scratching the surface. To go to a smaller engine and gamble with my safety after all this??? And I did Most of it other than cushions myself. Buy the best equipped vessel you can find

 

[Juggerknot]

Well being a backyard mechanic I called around the block and a guy with supposedly 38 years experience with cat diesels, in my instance, said that basically you are just removing all of the turbo and afterburner stuff and plugging some holes left by the removal. I am sure you would have to re-tune the injector pump. As for resale you still will have all of the factory parts to put back on. Yes you may want to get a different prop for low end torque.
I am planning on travelling around on this boat not sitting so I can appreciate 1/2 half gallon or more per hour!
In researching online I came to the conclusion that they are taking a standard non turbo engine and pumping it up to way more horsepower causing longevity issues.
I also called Caterpillar Technical Line and they said Oh No we don't recommend changing the factory motors because they are specially built with stronger pistons, connecting rods, cylinder heads to accept the extreme pressures.... Me being an old time Drag Racer I am thinking Oh Boy I would have a much stronger....more well built motor....with suddenly much less strain put to it....In my thinking that makes for a much longer lasting rig.
Correct me if I am wrong.
regards, Gary

Having owned and run more diesels than any sane man should be allowed, one must grasp the principles of the design. If you have ever had a td that lost its boost, you end up with a slug, not an economical n/a oil burner. I am a huge advocate of a well designed diesel and the best overall economy to smiles ratio has come from an intercooled turbo diesel.
You asked for educated opinions on your swap and now a detuning, many old salts have shared their wisdom and experience with you, it doesn't get better than that.
My advice, buy something you are happy with and use it, trust me, you'll find plenty of things to work on in your new to you 20+ year old boat...
Like my dentist in Managua once explained to me "I can probably make a pair of shoes and they may be ok but I'd rather have the shoemaker make a pair and know they are correct" he's not a maxillofacial surgeon but a very good dentist.
Trust the marine architects and engineers that designed the boat, buy what you want as it was designed, rube goldbergs are generally a disaster. Sail is another option and you can get some incredible economy with that propulsion design. Good luck.

 

[rsn48]

I don't know the water conditions in your area, in my area currents can be a significant issue. I want a larger engine so it isn't as stressed at the possible top speed of the boat. But I also want extra Oomph when more power would get me out of a stressful situation.

In the video below, the first half shows a boat successfully and without much stress making it through a rapid current condition in Dodd Narrows in British Columbia. But jump to the 1:05 minute mark, and you might question the sanity of the skipper of that boat. What these mental midgets don't realize is that an engine is designed to handle so much stress, then it over heats, or has the possibility of over heating.

I have a retired Captain from the BC Ferries assist me in decisions with my boat refit and when I was discussing a larger engine (actually the engine is smaller than the old Merc engine but cranks out an additional 110 hp), he said the extra hp would be of benefit in "iffy" currents. He then related a story of one of the larger BC ferries being caught by a swift current and swung 360 degrees before control of the ship could be re-established.

Beginning at the 1:05 mark, you can see the skipper has no margin of error safety in his motor, its going full tilt and we can safely assume being stressed by forces it wasn't designed for:

 

[Bigsalmonfish]

I don't know the water conditions in your area, in my area currents can be a significant issue. I want a larger engine so it isn't as stressed at the possible top speed of the boat. But I also want extra Oomph when more power would get me out of a stressful situation.

In the video below, the first half shows a boat successfully and without much stress making it through a rapid current condition in Dodd Narrows in British Columbia. But jump to the 1:05 minute mark, and you might question the sanity of the skipper of that boat. What these mental midgets don't realize is that an engine is designed to handle so much stress, then it over heats, or has the possibility of over heating.

I have a retired Captain from the BC Ferries assist me in decisions with my boat refit and when I was discussing a larger engine (actually the engine is smaller than the old Merc engine but cranks out an additional 110 hp), he said the extra hp would be of benefit in "iffy" currents. He then related a story of one of the larger BC ferries being caught by a swift current and swung 360 degrees before control of the ship could be re-established.

Beginning at the 1:05 mark, you can see the skipper has no margin of error safety in his motor, its going full tilt and we can safely assume being stressed by forces it wasn't designed for:

Although you may perceive the person at 1:05 as a mental midget I see it as one of the ways to get through Dodd before all the morons try it at slack water. I once transited as your "midget" did against a full run. I have a 65' single, full displacement hull. Ran the back eddy as far as I could and dove out into the current. Crawled through at full rpm with the Graymarine Detroit screaming.

 

[Rosborough 35]

See the attached data sheet for the Cummins B210

I've attached a copy of the data sheet for my Cummins B210. Specifically, it is a 6BT5.9-M. Based on the data sheet, I think my RPM, HP, and fuel burn numbers are correct. I know the RPM and fuel burn numbers are correct, because I have a tachometer and a fuel flow meter. I don't have any reason not to believe the HP numbers from the Cummins data sheet.

 

[Ski in NC]

Your mistake is reading the hp #'s off the full load table and reading the fuel burn off the prop load table.