Pros and cons of single or twin engines for GB 36s?

We have owned a GB 36 single Ford Lehman for the last 25 years. The last 15 years were spent cruising Southeast Alaska. Our average fuel burn is 1.9 gph which includes the gen set and diesel heater. We generally cruise at 1700 rpm' at a speed of 7.2 knots, if we are not fighting a current, and faster if a current is in our favor. In all the years we have owned Interim we have never had any type of engine failure. I must admit that I'm very conscious about oil changes and yearly oil samples. However this year I'm removing the transmission to check for worn parts as I have 7k hours. I especially love the single screw keel protection, twins leaves your props and struts hanging in the wind.

Per NW's comment, I looked online today at three 1990's era GB36's.

One had a single 210 hp Cat 3208.

One had twin Lehmans - 135 hp each (270)

One had twin Cummins- 210 hp each (420)

Quite a range...I'm not real sure I understand GB's thinking but maybe some buyers like to get up and go.

But for the OP, it would seem that you need to determine how you plan to use the boat. Because you mentioned fuel efficiency and 420 hp would seem unnecessary if you aren't concerned about planing very often.

For Gbimterim: can you hold a plane with that single, without being firewalled on the throttle?
What hp is it?

I've had twins most of my life and am very biased.
I came in twice on one engine in about 50 years of boat ownership. One of those times with a partially jammed rudder from a bundle of discarded crab pot lines. Crab pot lines in general are a real problem in the PNW. Shaft line cutters help, but didn't for the bundle.
Two engines don't mean twice the costs. In my experience, about 10-20% more depending on speed and engine type. But I use engines known for reliability and zero electronic controls. With twins driving an wood, 80 ton, 83' boat I burn as little as 6 gallons an hour @ 7 knots. I usually cruise at 10 knots for 8.5 gallons an hour. These are open ocean speeds in average conditions.
Two engines give many maneuvering options. I have never used a bow thruster and never thought I needed one.
I enjoy very remote places, sometimes hundreds of miles from a port. I like the reliability best.

cold water

If this inside passage is bad for floating lines, then consider the issue of clearing a single screw vs having one side fouled on a twin. Are you going down to clear the shaft, or call for help? This may determine "reliability" more than the engine(s) themselves.

MANY thanks to all who weighed in on this. The pros and cons are very clear, and very useful to us as we go deeper into our search. I'm glad I posted the question and am grateful for the thoughtfulness put into answers. Let me ask, for GB 36 owners with either a single or twin: What is your gph at 7 or 8 knots? I understand there are other factors that affect this but would appreciate seeing what you have experienced.

Since trap lines have been mentioned, I too recommend cutters. I have Spurs brand and I'm happy with them. In almost thirty years I've never been stopped by line. I did once find a short piece hung up on something down there so I know I've hit at least one line and probably more.

If you go for the single, have a back up plan. It could be call Sea Tow. It could be Tow with your dinghy. You could do what Murray did and mount an Outboard bracket on the swim platform.

In more than fifty years of driving single engine boats, I've only been towed in once. I didn't really need the Tow that time but I got tired of paying for Unlimited Towing and not using it. The belt broke. I had a spare. I had the tools. I had the knowledge. It was just too damn hot. I was too damn tired and my friends were sitting on the bridge drinking beer, so I called for a tow and enjoyed the ride home.

There's an age old saying for cars and boats. If you're worried about its fuel consumption costs then either it's too costly over all for you... or... simply don't make use of it so much that it gives you a fuel cost headache.


You can also anchor and enjoy a boat. And, a little tow behind runabout can afford you great economical gunk holing while your "mother ship" languishes at anchor.


There's more than one way to skin a cat... err, float your boat!

Fuel rates vary by boat and speed

Delta Dog said:

We boat in the Sacramento Delta, which is prone to shoaling a lot. Our single screw is well protected so if and when we touch, we aren't doing a whole lot of damage. We burn 4.3 gph with our single Cummins at 8.5 knts. Not bad for a 43ft boat. As others have said, if two engines makes you more comfortable, by all means get two. For me, being prepared with spare parts and an extra case of fuel filters allows me to sleep well at night. It really is a personal preference.

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My fuel burn rates on my Ocean Alexander 456 approximated 4 gallons an hour when bringing her from Fort Lauderdale last year to Norfolk Virginia. I typically ran at 8.5 kn. My engines are twin cummins 330s.

I can tell you I think the boat would be absolutely perfect if it had only one engine. I can imagine all the extra space I would have in the engine room.

The boat before this one was a beneteau423 with a single diesel engine. We ran that often times like a Trawler and never had a problem with the engine not working when we needed it too.

I do like the maneuverability of having two screws. Combine that with the bow thruster and I can do just about anything with the boat.

Gordon

Were I in your shoes I would look at how I planned on using the boat for many years.


If I planned on doing a lot of long distance cruising vs doing a lot of short distances with more docking I would likely choose a single. If the opposite were planned I would likely choose a boat with twins.


We have twins. Twice in the past 5-6 years we have had to come back to the marina on one engine. There's a lot of confidence in knowing you have a second engine should one stop running.


Also, the ease in docking a twin engine boat is much greater than a single. I've driven and docked both. Singles can be learned but they're still more difficult in a wind or current than a twin.

Gordon J said:

My fuel burn rates on my Ocean Alexander 456 approximated 4 gallons an hour when bringing her from Fort Lauderdale last year to Norfolk Virginia. I typically ran at 8.5 kn. My engines are twin cummins 330s.

I can tell you I think the boat would be absolutely perfect if it had only one engine. I can imagine all the extra space I would have in the engine room.

The boat before this one was a beneteau423 with a single diesel engine. We ran that often times like a Trawler and never had a problem with the engine not working when we needed it too.

I do like the maneuverability of having two screws. Combine that with the bow thruster and I can do just about anything with the boat.

Gordon

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Gordon

You do mean 4 gals. per hour - per engine... for twin cummins 330s, at 8.5 kn. Right?? Or, are they using only 2 gals. per hour each???

"My fuel burn rates on my Ocean Alexander 456 approximated 4 gallons an hour"

Yes, 4 gallons for both

Art said:

Gordon

You do mean 4 gals. per hour - per engine... for twin cummins 330s, at 8.5 kn. Right?? Or, are they using only 2 gals. per hour each???

"My fuel burn rates on my Ocean Alexander 456 approximated 4 gallons an hour"

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Art,

Yes, 4 gallons for both engines. It was a small fraction over 4 gallons. This included some slower speed's in the Florida ICW for manatee zones and no wake zones. But it also included a couple of ocean Jogs, Fernandina beach to Charleston, and Wrightsville beach to Beaufort, where our speeds were closer to nine knots.

I was quite pleased with the boat's fuel consumption rate.

In a couple weeks we will start out for the Bahamas. This will give me another opportunity to monitor fuel burn rates over long distances.

Gordon

I can see that English grammar got in the way again. What I meant to say was 2 gallons per hour, per engine.

Gordon J said:

I can see that English grammar got in the way again. What I meant to say was 2 gallons per hour, per engine.

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Very economical!

The fuel burn single v/s twin difference isn't worth talking about and can swing either way despite what people say. However the twins w double the power will almost always burn more fuel because the owners will run them faster. And if you have a GB36 w twin yanmars that are slightly more efficient .. that boat may burn less than a GB36 w a single FL or Perkins.

If I was buying a GB36 I would only look at singles for a number of reasons but if I could choose the engines I'd definitely buy a twin.

Also IMO the old wifes tale about twins halving twice the maintenance is just passing along a rumor or wifes tale. But the tale applies to twin engines v/s single whereas the engines all are the same size. And a proper twin would have engines half the size of it's counterpart single and maintenance would be almost exactly the same. But for the boats we're talking about most of the time .. maintenance is actually about double.

One of the advantages of a single engine boat is the protection the keel gives to the propellor and rudder.

In the PNW, with the amount of floating logs and crab pots, I'd be a little concerned about having 2 unprotected props. But if your lucky, you may still be able to get home on one engine.

Nomad Willy said:

IMO the old wifes tale about twins halving twice the maintenance is just passing along a rumor or wifes tale.

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I think twice the maintenance is about right. Double the engine oil change, double the oil filter change, double the fuel filter change, double the transmission oil change, double the oil cooler replacements, double the stuffing box overhauls etc.

Repairs are another story. It is highly unlikely for both alternators to croak simultaneously. ditto for raw water pumps, fresh water pumps, transmissions, injectors, starters.

I have driven many hundreds of thousands of miles in a single engine car. I'm still alive. I don't see a boat engine as all that different. I am happy with the increased reliability and better maneuverability that my twins offer. The cost of the extra fuel is not a major factor. I could always anchor out more if I needed to keep expenses down. Fuel expenses are tiny compared to summer/winter storage fees.

Re. The GB36 specifically, owners report a solid continuous Max cruise of just over 16kts on twin 210hp Cummins ; impressive for a 12 tons trawler. Its a surprisingly efficient hull at 30hp/ton equaling the new Swift trawler34, as it actually planes without pushing a massive wall of water in front of the bow.

With twin 120hp Lehman's it will still manage 11-12kts for 'a run for shelter' option if you get caught out in worsening conditions.

The 32' is a displacement hull, and the 42' is a huge bulldozer; the 36 is the ballerina

PS: from my recent research, I don't own one.

Rusty wrote;
"With twin 120hp Lehman's it will still manage 11-12kts for 'a run for shelter' option if you get caught out in worsening conditions."

Made possible probably by the well fwd heavy engines. May be a handful in following seas though. The 32 is a SD hull.

AusCan said:

One of the advantages of a single engine boat is the protection the keel gives to the propeller and rudder.

In the PNW, with the amount of floating logs and crab pots, I'd be a little concerned about having 2 unprotected props. But if your lucky, you may still be able to get home on one engine.

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I`ve not seen one here but if memory serves there are US twins boats with rudder/prop protection. Gulfstar and maybe one other come to mind, just from pics and posts on TF.

We have twin 330's and at 8K burn a total of 4 to 4.5 gallons/hour depending on conditions. LWL 46.8, weight 32,000 lb. So long as we stay under hull speed economy is good. I doubt a single of any HP would do better enough to make fuel burn an issue. Weight extra maintenance and room in the engine room and keel configuration with prop protection would be the basic pros of the single not fuel. Flemmings cross oceans with twins and when they tested single against twins found fuel a non factor.

BruceK said:

I`ve not seen one here but if memory serves there are US twins boats with rudder/prop protection. Gulfstar and maybe one other come to mind, just from pics and posts on TF.

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There are a few twins around with protected props but not GB's.

Here's a nice Tassie built boat with twin keels.
https://yachthub.com/list/boats-for-sale/used/power-boats/high-seas-63/206958

AusCan said:

There are a few twins around with protected props but not GB's.

Here's a nice Tassie built boat with twin keels.
https://yachthub.com/list/boats-for-sale/used/power-boats/high-seas-63/206958

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Marlow explorer

Want to mention a pretty sizable misconception regarding calling it 2X the maintenance work-time for twins compared to a single.

When you do service needs such as oil change, coolant refreshment, fuel filter change etc... it's the time spent purchasing products to use, gathering tools to do the project, getting up close and personal in the engine compartment, cleaning up at end of service performed, dumping old product for recycling or flat out trash and putting tools away... that takes just about the same amount of time to do for two engines as it does for one.

That said - I agree that during the actual service performance on each engine it doubles that time spent. But... much of the time spent to accomplish service happens before and after the service itself instead of during the actual on engine service procedure. Therefore twins are no where near 2X maintenance time compared to a single. Of course, service product cost is 2X.

So, I conjecture... for twins it takes maybe 1.2 to 1.3 more time to perform entire general service procedures than on a single. To say twins are 2X the time for servicing is way overboard. Pun intended!

We have twin SP 225's in our President 41. As to prop protection with a single or twins, I always thought that twins were very easy to damage the props with submerged debris as compared to a single. We have a fairly large keel with exposed props. Last year coming up the Hudson River, we hit submerged debris at least 12 times that could be felt all the way up to the flybridge. There was a tremendous amount of floating debris as well as submerged. I was sure that our props would be mangled and have to be rebuilt. After the 1400 mile trip home and countless strikes of submerged debris, when we hauled the boat for the winter, there was absolutely no damage to either prop. Now I am not so sure about the single vs twin prop protection theory...

Comodave said:

We have twin SP 225's in our President 41. As to prop protection with a single or twins, I always thought that twins were very easy to damage the props with submerged debris as compared to a single. We have a fairly large keel with exposed props. Last year coming up the Hudson River, we hit submerged debris at least 12 times that could be felt all the way up to the flybridge. There was a tremendous amount of floating debris as well as submerged. I was sure that our props would be mangled and have to be rebuilt. After the 1400 mile trip home and countless strikes of submerged debris, when we hauled the boat for the winter, there was absolutely no damage to either prop. Now I am not so sure about the single vs twin prop protection theory...

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That's quite the "lucky" story. How fast were you traveling through the water when items hit the boat? Slower the better... as the items will tend to be pushed to the side unless you were traveling quite fast. Semi submerged items are less likely to be pushed aside and can become a problem regarding hitting a prop.

In 1960's I traveled through Hells Gate quite a few times heading up the Hudson for delivery of twin screw boats. Man, there was a lot of junk of every type and the current caused eddies that some times swirled debris in circles. I can still clearly see the poles, pallets, tires and other debris in my mind's eye, Reason I recall that so vividly is because being a young teenager I was often on the bow leaning on a bow rail yelling back to the captain, my dad, what was coming up and which way best to slightly turn the boat. Many times I just pointed and he turned that way.

I do not recall any prop damage. I do know that we were going slow as possible while still making headway against the strong current. Obviously slow enough to push debris aside.

We were doing 10 knots.

twin owner here.

next boat will be single. have run home after a minor failure too.

fuel not a consideration can run 1 engine at a time and get close..

top speed, meh, thought it was important.. nice to have but not required.

2x the maintenance is a consideration that can't be ignored.

kev_rm said:

twin owner here.

next boat will be single. have run home after a minor failure too.

fuel not a consideration can run 1 engine at a time and get close..

top speed, meh, thought it was important.. nice to have but not required.

2x the maintenance is a consideration that can't be ignored.

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You mean 2X parts cost [which is minimal regarding annual expenses] or is it time spent you mean by 2X?

Please read Post # 23.