N/A vs forced induction

[Capt Pappy]

The captain in the YouTube videos I watch to educate myself goes on about how you best stick with the NA diesel engines unless you have quite deep pockets.

if we do immerse ourselves into life aboard it comes along with a drastic change in finances, so we must be cautious.

and since we plan to pay cash for a vessel, we are of course looking at quite older models

so i wanted to see what those with more knowledge thought about the running and maintenance costs of N/A vs forced induction motors

long time 'boater' but only outboards and a couple gas I/O boats several decades ago

so don't know much about the diesel engines

also, do the N/A motors perform about the same as the higher HP motors during low speed maneuvers and docking?

many thanks

 

[sean9c]

I have to think that these days the vast majority of diesels out there are turbo'd and go many thousands of hours without a lot of maintenance. Unless you have a boat with low power requirements or the ability and space to have a large heavy engine naturally aspirated is not really practical. Everyone seems to love the FL, it's 1100# and puts out 120hp.

 

[Capt Pappy]

thanks for the info.

I take it FL = ford lehman? that happens to be one of the engines the youtube capt seems to like. (FYI he calls himself the No Regrets Lifestyle or something to this effect and says he went to Tech school for diesel mechanics)

And I do recall it produces 120hp but at 1100# that is one heavy engine -- not exactly an ideal weight/hp ratio.

 

[Portage_Bay]

Like all things boating it's largely a matter of choice. And if you're shopping the lower end of the budget scale and older boats you won't have as many choices. Better to look for a well maintained boat than shop a specific characteristic such as N/A vs T or TA.

My opinion, not fact, just opinion is that turbos on well maintained and properly operated engines last a long time without problems. But when they fail it can be $$$. Add aftercooling and now you are adding maintenance costs to maintain the cooling system. Failure to maintain the system is inviting huge costs.

My experience with N/A vs TA has been mostly on CATs from the mid 60's to the mid 80's vintage. You asked about low speed performance maneuvers and docking. I don't find a lot of difference because the Turbos haven't produced enough boost to matter if they have produced any boost at all. Before turbo boost kicks in the same block and pistons are going to produce the same, or very similar, toruqe and HP. I'm speaking about older iron here, the newer stuff may be considerably different.

When I say a properly operated T or TA engine I'm speaking of running it hard enough to get the turbos working well. If you don't, depending upon the engine, you may have issues including oil build up in the turbo, carbon build up as well. And with Aftercoolers all of that crud gumming up the aftercooler. I'm not talking about running it at max power, that just wears things out quickly.

Something I'm learning about mid 80's "trawlers" is that they can be both underpowered and overpowered at the same time.

Two examples.
#1 My last boat a Californian 42 LRC with twin CAT 3208 N/As. 210 HP each, 420 HP total. It cruises very nicely at 1400 RPM, smooth, quiet, acceptably low fuel burn. But I've been warned that running that RPM is too low and the engines may suffer. So I ran her at 1600 RPM. Noisier, higher fuel burn for only about 1 kt gain in speed. Going any higher RPM was a wast of $$$ unless I needed the speed. I did run her at about 2000 RPM for a brief period each day to clean things out. At max RPM she was making 12 kts, a huge wake and burning a LOT OF FUEL. So, at 1400 RPM she wasn't working hard enough, she was overpowered. At max RPM she was working her guts out, stern squatted, big wake and not able to climb out of the hole.

#2 My current boat a West Bay 4500 with twin Cummins 6BT. 210 HP each, 420 HP total. Running her at 1400 she is sweet as can be. Very smooth, very quiet, very small wake. I have been advised that that will not get the turbos boosting and cause problems. Run her harder, 1600 min, 1800 better. Well now the same thing happens as with the Californian. Squatting, big wake. At max RPM not climbing out of the hole and burining too much fuel. Same problem, both overpowered and underpowered.

A very long winded opinion of saying if you're shopping engines shop the boat performance and your desired operating characteristics as well. If you wanna go slow buy a boat designed and powered to go slow. If you wanna go fast buy a fast boat. A sorta fast hull with not quite the right HP will present challenges. Or, buy the boat you like and learn to live with performance constraints.

 

[KnotYet]

thanks for the info.

I take it FL = ford lehman? that happens to be one of the engines the youtube capt seems to like. (FYI he calls himself the No Regrets Lifestyle or something to this effect and says he went to Tech school for diesel mechanics)

And I do recall it produces 120hp but at 1100# that is one heavy engine -- not exactly an ideal weight/hp ratio.

That weight is typical for an old-school 120hp 6-cylinder diesel.
The similar Perkins 6.354 tips in at 1225 lbs, minus the transmission.
The Gardner 6LW that only makes 112hp lists its weight at 0.67 tons.

 

[rslifkin]

In general, low speed performance will be fine regardless of engine. But the faster the boat is powered to go, generally the more thrust you'll have at idle without needing to throttle up. Assuming you have 2 engine that both rev to 2800 RPM at WOT and one makes 150hp while the other makes 250hp, the 250hp engine will be turning a bigger prop and at the same idle RPM it'll produce more thrust.

As an example of that, some big sport fishes idle at 7+ kts in gear due to big, low revving diesels (where idle may be 25% or more of WOT RPM) and being powered and propped for a high top speed. At the other end of the spectrum, many sailboats are doing under 2 kts idling in gear.

 

[Capt Pappy]

Like all things boating it's largely a matter of choice. And if you're shopping the lower end of the budget scale and older boats you won't have as many choices. Better to look for a well maintained boat than shop a specific characteristic such as N/A vs T or TA.

My opinion, not fact, just opinion is that turbos on well maintained and properly operated engines last a long time without problems. But when they fail it can be $$$. Add aftercooling and now you are adding maintenance costs to maintain the cooling system. Failure to maintain the system is inviting huge costs.

My experience with N/A vs TA has been mostly on CATs from the mid 60's to the mid 80's vintage. You asked about low speed performance maneuvers and docking. I don't find a lot of difference because the Turbos haven't produced enough boost to matter if they have produced any boost at all. Before turbo boost kicks in the same block and pistons are going to produce the same, or very similar, toruqe and HP. I'm speaking about older iron here, the newer stuff may be considerably different.

When I say a properly operated T or TA engine I'm speaking of running it hard enough to get the turbos working well. If you don't, depending upon the engine, you may have issues including oil build up in the turbo, carbon build up as well. And with Aftercoolers all of that crud gumming up the aftercooler. I'm not talking about running it at max power, that just wears things out quickly.

Something I'm learning about mid 80's "trawlers" is that they can be both underpowered and overpowered at the same time.

Two examples.
#1 My last boat a Californian 42 LRC with twin CAT 3208 N/As. 210 HP each, 420 HP total. It cruises very nicely at 1400 RPM, smooth, quiet, acceptably low fuel burn. But I've been warned that running that RPM is too low and the engines may suffer. So I ran her at 1600 RPM. Noisier, higher fuel burn for only about 1 kt gain in speed. Going any higher RPM was a wast of $$$ unless I needed the speed. I did run her at about 2000 RPM for a brief period each day to clean things out. At max RPM she was making 12 kts, a huge wake and burning a LOT OF FUEL. So, at 1400 RPM she wasn't working hard enough, she was overpowered. At max RPM she was working her guts out, stern squatted, big wake and not able to climb out of the hole.

#2 My current boat a West Bay 4500 with twin Cummins 6BT. 210 HP each, 420 HP total. Running her at 1400 she is sweet as can be. Very smooth, very quiet, very small wake. I have been advised that that will not get the turbos boosting and cause problems. Run her harder, 1600 min, 1800 better. Well now the same thing happens as with the Californian. Squatting, big wake. At max RPM not climbing out of the hole and burining too much fuel. Same problem, both overpowered and underpowered.

A very long winded opinion of saying if you're shopping engines shop the boat performance and your desired operating characteristics as well. If you wanna go slow buy a boat designed and powered to go slow. If you wanna go fast buy a fast boat. A sorta fast hull with not quite the right HP will present challenges. Or, buy the boat you like and learn to live with performance constraints.

Many thanks for the detailed info.

 

[Capt Pappy]

That weight is typical for an old-school 120hp 6-cylinder diesel.
The similar Perkins 6.354 tips in at 1225 lbs, minus the transmission.
The Gardner 6LW that only makes 112hp lists its weight at 0.67 tons.

geez, far cry from our 500lb Yamaha 200hp outboard

 

[Portage_Bay]

geez, far cry from our 500lb Yamaha 200hp outboard

HP is only part of the equation. Torque especially at lower RPMs is very important. A diesel of most any kind will develop more torque than an outboard of most any kind.

An old skool engineer I used to know said by the heaviest diesel you can. More iron, lasts longer, fewer failures.

 

[Capt Pappy]

In general, low speed performance will be fine regardless of engine. But the faster the boat is powered to go, generally the more thrust you'll have at idle without needing to throttle up. Assuming you have 2 engine that both rev to 2800 RPM at WOT and one makes 150hp while the other makes 250hp, the 250hp engine will be turning a bigger prop and at the same idle RPM it'll produce more thrust.

As an example of that, some big sport fishes idle at 7+ kts in gear due to big, low revving diesels (where idle may be 25% or more of WOT RPM) and being powered and propped for a high top speed. At the other end of the spectrum, many sailboats are doing under 2 kts idling in gear.

thanks -- makes sense.

never piloted anything over 28' so the whole docking issue is a little worrisome

 

[rslifkin]

HP is only part of the equation. Torque especially at lower RPMs is very important. A diesel of most any kind will develop more torque than an outboard of most any kind.

An old skool engineer I used to know said by the heaviest diesel you can. More iron, lasts longer, fewer failures.

Torque can be made up by gearing. The only time low RPM torque really matters is on a boat propped to run fast on plane, where you need enough power at lower RPM to shove the boat up onto plane in the first place. Otherwise you can never use the full power output at low RPM anyway with a fixed pitch prop due to how prop loading drops off as RPM reduces.

 

[ranger58sb]

The captain in the YouTube videos I watch to educate myself goes on about how you best stick with the NA diesel engines unless you have quite deep pockets.

and since we plan to pay cash for a vessel, we are of course looking at quite older models

so i wanted to see what those with more knowledge thought about the running and maintenance costs of N/A vs forced induction motors

also, do the N/A motors perform about the same as the higher HP motors during low speed maneuvers and docking?

Like all things boating it's largely a matter of choice. And if you're shopping the lower end of the budget scale and older boats you won't have as many choices. Better to look for a well maintained boat than shop a specific characteristic such as N/A vs T or TA.

I agree. Find the candidate boat first. Consider engines afterwards.

Reaction at the dock is about more than just horsepower. Torque, props, boat weight, maybe hull shape all play a role.

There's a whole lot of a boat that you'll interact with more often than engines. Space, in general? Helm stations (bridge, pilot house, etc.). Boarding, stairs, ladders, etc.? Heads? Galley appliances? Berths and other furnishings? Lockers? Tender-related systems? Et cetera.

Back to candidate shopping, first.

-Chris

 

[Capt Pappy]

I agree. Find the candidate boat first. Consider engines afterwards.

Reaction at the dock is about more than just horsepower. Torque, props, boat weight, maybe hull shape all play a role.

There's a whole lot of a boat that you'll interact with more often than engines. Space, in general? Helm stations (bridge, pilot house, etc.). Boarding, stairs, ladders, etc.? Heads? Galley appliances? Berths and other furnishings? Lockers? Tender-related systems? Et cetera.

Back to candidate shopping, first.

-Chris

Understood ---thanks

The better half is more interested in those other aspects anyhow

 

[heysteve]

That weight is typical for an old-school 120hp 6-cylinder diesel.
The similar Perkins 6.354 tips in at 1225 lbs, minus the transmission.
The Gardner 6LW that only makes 112hp lists its weight at 0.67 tons.

Our Cat 3208 NA is 1740 lbs dry weight, 1980 lbs with the TD 506 gear attached. The thing is a beast but does help provide a nice, sea kindly ride

 

[twistedtree]

I think turbo or no turbo is a giant Red Herring. Turbos are everywhere, including engines with continuous duty ratings that operate 24x7 and run for 10s of thousands of hours between overhauls. Turbos aren't the weak link, nor are they bellwethers of longevity.

I think duty cycle rating is what you really need to look at. Different manufacturers call it different things, but it's basically the portion of time that you can run the engine at full power. This is the real indication of how much the power package is being pushed to its limits, with corresponding compromise to longevity. Continuous duty is a robust as it gets, and it doesn't make a rat's ass bit of difference if it's got a turbo or not. These engines can be run at full power continuously, non stop, forever, until you need to change the oil. Next will be something like 16/24hrs at full power, then 8/24, and some even limit full power to 1-2 hrs. IMO, that's how to judge the longevity of a power plant. This full range of ratings can typically be found in the same engine where the basic package is pushed to different HP levels. By way of example, my main engine is available in a 400hp continuous duty rating, or in a 750hp 1h out of 12hrs at full power rating. I'm confident which of those will last longer, even though both have turbos.

 

[O C Diver]

As others have said, find boats you're interested in first, then enquire about specific motors.

I've owned 5 diesel engines that were all turbos. Never had a problem with the turbos. If you're looking for long life from naturals and turbos:
Change the oil on shorter intervals.
Keep the fuel clean.
Don't overload the engine.
Don't overheat the engine.
Keep the engine load 99% of the time below 70%.

As many naturals get older, it will be harder to find parts that don't normally wear out. FLs are an example where a few parts are only available as used from junkers.

Understand that if you're buying inexpensive used, a reputable engine survey will be extremely important. Having to remove, rebuild or replace, and reinstall could cost more than your boat might be worth.

Lot to be said for going slow with a small engine. Almost everything is less expensive.

Ted

 

[DDW]

By way of example, my main engine is available in a 400hp continuous duty rating, or in a 750hp 1h out of 12hrs at full power rating. I'm confident which of those will last longer, even though both have turbos.

Actually I'd think either of those engines would last about the same amount of time, under the same operating conditions. Of course the 400 hp version can't operate at 750 any of the time, but from 400 hp on down, they are the same engine, doing the same thing, and lasting just as long. The 750 version, if actually run at 750 for its rated hours, will not last as long - but the aggregate power output over its lifetime might be the same. This is why the Tony Athens idea of engine longevity - how many gallons of fuel you can run through it before overhaul - makes sense, as it accounts at least to a large extent for operating conditions and is somewhat constant over a range of use.

 

[rslifkin]

Actually I'd think either of those engines would last about the same amount of time, under the same operating conditions. Of course the 400 hp version can't operate at 750 any of the time, but from 400 hp on down, they are the same engine, doing the same thing, and lasting just as long. The 750 version, if actually run at 750 for its rated hours, will not last as long - but the aggregate power output over its lifetime might be the same. This is why the Tony Athens idea of engine longevity - how many gallons of fuel you can run through it before overhaul - makes sense, as it accounts at least to a large extent for operating conditions and is somewhat constant over a range of use.

That's mostly the case, especially if the 750hp engine revs higher and still has a similar power curve at lower RPM. However, if the 750hp engine also makes more power at lower RPM and would be turning a different RPM (lower or higher) to get 400hp (accounting for prop load curve) then longevity may be somewhat different.

But yes, in the end, fuel used is a decent metric to go by. And even the higher HP engines should last well if not abused and rarely operated beyond their "max continuous" rating. In a planing hull, for example, you need enough grunt to push the boat over the hump onto plane. Depending on the cruise speed target you may end up with a significant power surplus at cruising speed (as in fast cruise being at or below max continuous) just to get adequate performance coming over the hump. That's where a turned up version of an engine can do well, as you only need the extra power for a few seconds.

 

[DDW]

That's mostly the case, especially if the 750hp engine revs higher and still has a similar power curve at lower RPM. However, if the 750hp engine also makes more power at lower RPM and would be turning a different RPM (lower or higher) to get 400hp (accounting for prop load curve) then longevity may be somewhat different.

They would naturally be propped differently (otherwise why specify 750 hp?), but if both are propped for the 400 hp case, operating torque is set by the prop and would be identical for the two engines, as would the longevity. Basically, you have a 400 hp continuous output engine that, by changing fueling and perhaps boost characteristics, can to produce more at the cost of a shorter life, if (and only if) it is asked to produce more.

 

[Capt Pappy]

Our Cat 3208 NA is 1740 lbs dry weight, 1980 lbs with the TD 506 gear attached. The thing is a beast but does help provide a nice, sea kindly ride

Yeah, that's what I was thinking after decades of banging around in lighter outboard powered boats.... the weight of these vessels surely provide a smoother ride

 

[Capt Pappy]

As others have said, find boats you're interested in first, then enquire about specific motors.

I've owned 5 diesel engines that were all turbos. Never had a problem with the turbos. If you're looking for long life from naturals and turbos:
Change the oil on shorter intervals.
Keep the fuel clean.
Don't overload the engine.
Don't overheat the engine.
Keep the engine load 99% of the time below 70%.

As many naturals get older, it will be harder to find parts that don't normally wear out. FLs are an example where a few parts are only available as used from junkers.

Understand that if you're buying inexpensive used, a reputable engine survey will be extremely important. Having to remove, rebuild or replace, and reinstall could cost more than your boat might be worth.

Lot to be said for going slow with a small engine. Almost everything is less expensive.

Ted

understood. thanks