Displacement vs. Semi Displacement, Plaining

Question that has come up.....

Regarding fuel and speed.

Is there a big difference in fuel burn with any hull when operated at hull speed, everything else equal?

I hear that the bigger the engine, the more fuel burn. I believe they can be just as efficient as a small engine, if operated at the same power, perhaps better. I've also heard that displacement hull is much more efficient than the other hull designs. Now extremes like comparing a 1300 hp engine to a 200 hp may be totally different.

But, my friend that operates a 455 Meridian gets 8.4 mph on 2.1 gph (4mpg), with twin Cummins diesels 370's. He's on his 3rd loop and a very detailed person. But sounds exceptional.

I'm using this to decide what boats to look at and seems it really doesn't make a lot of difference. It's more operator technique than design.

However, another friend who has done a half loop and a lot of boating gets 8 mph on 1.5 gal in his 32 Senator (Grand Banks lookalike), but a much smaller boat.

Gut feeling, I'll be in the 35 to 42 foot size and engines aren't significantly different.

For the technical folks, what do you say?

I think your friend's Meridian gets that 2.1 gph @ 8.4 mph on each engine, not both.
Not sure exactly what your question is, but I will say that hull and powertrain design do have everything to do with fuel economy.
A full displacement hull with properly matched motor/trans/prop will, in terms of fuel efficiency, outperform a semi disp or planing hull of the same size at the same speed.
All that aside, people who "do the loop" usually buy their boats based more on creature comforts than a few bucks worth of diesel.
Full disp hulls usually pay a draft penalty over their flatter bottomed cousins, and do not offer any significant speed for situations where that is desired.

Generally speaking a smaller engine is more economical. But it depends on the boat hull and speed in question and many other things.

The core of the issue is the surface area of the cylinder BDC to TDC plus the area of the combustion chamber and the area of the piston crown and the temperature of the combustion. The latter variable is the one that most often breaks the rule of thumb that the smaller the engine the greater the economy. But at the same combustion temp the smaller engine will or almost certianly will be more economical. Heat loss is a negative re economy but w turbocharging the increase in pressure on the piston crown more than makes up for the heat loss.

Looking at specific fuel burn charts to produce a specific amount of power will help. This will show that underloading is not good for economy. So if economy is your goal a small turbocharged engine (or engines) running fairly hard (say 60% to 75% load(depending on engine)) with relatively high power per displacement will produce good numbers.

GPH dock talk can be informative but usually is misleading.

Planing hulls come with large amounts of horsepower to get the boat on plane, displacement boats comes with enough horsepower to get the boat up to hull speed. If you run a planing hull slow it is hard on the engines turning such low rpm's, manufacturers recommend 70% throttle 70% of the time for diesels.

My friend who ran his Nordic Tug 32 at displacement speed (6 knots @ 1 gph) had issues with soot and carbon buildup with his Cummins 220. I was able to tow him in at 1 gph making 5.5 knots with my Yanmar 4jh, my normal speed with that fuel burn being 7 knots. If he powered his NT for the speed he wants to run he would have needed only 50 hp and could achieve 70/70.

This conversation goes on and on with endless considerations of running gear drag, running one engine at a time, shaft seal issues, and an endless litany of other considerations....

I'm guessing that a purpose built "hull speed" type trawler will have an edge. How much of an edge is what might be interesting to see...
Our sailboat could burn over 2 gallons an hour at 7 knots for 3.5 ish nautical miles per gallon. That was a pretty efficient little 40 hp Yanmar. It was working hard to keep that speed on.
This picture is a fuel burn chart for a new 40' "semi-displacement" type boat. At about 7 knots the fuel burn is pretty close to our sailboats. Sure the sailboat actually used a little less fuel but it was much less pleasant than the powerboat is at those speeds.
My in laws swear their Willard 40 uses no fuel at all! They cruise to Maine and back at 7 or so knots and use very little fuel. They will not push their engine!

We have another buddy who swears on a bible that his 40 hp sailboat engine only burns 1/2 gallon an hour. I like to kid him that it is in fact burning all of that 1/2 gallon and a bunch more at speed. He doesn't go very far though so he probably has no real idea...

My best, non professional guess is that to move a boat through the water at displacement speed requires a certain amount of fuel for a certain displacement.
Sure there will be differences but until you begin to push that boat uphill things are more even than not...

Remember, I said it was my best guess!
Bruce

Dont think so

AKDoug said:

Planning hulls come with large amounts of horsepower to get the boat on plane, displacement boats comes with enough horsepower to get the boat up to hull speed. If you run a planning hull slow it is hard on the engines turning such low rpm's, manufacturers recommend 70% throttle 70% of the time for diesels.

My friend who ran his Nordic Tug 32 at displacement speed (6 knots @ 1 gph) had issues with soot and carbon buildup with his Cummins 220. I was able to tow him in at 1 gph making 5.5 knots with my Yanmar 4jh, my normal speed with that fuel burn being 7 knots. If he powered his NT for the speed he wants to run he would have needed only 50 hp and could achieve 70/70.

This conversation goes on and on with endless considerations of running gear drag, running one engine at a time, shaft seal issues, and an endless litany of other considerations....

Click to expand...

My 50,000 pound 45 1/2 foot ocean Alexander gets to miles to the gallon at 8.5 kn point point point on flat water. It is all about the motive force it takes to push the boat through the water. My friends KK 48 uses half as much fuel, and his boat is heavier, but he has a displacement hole while I have a semi displacement hole. By the way, 2 miles per gallon is with both engines running.

On another site, boat diesel, the experts say that it does engines no harm to run at slow speed for their entire life. I have seen nothing in any literature on my boat engines, Cummins 330, that tells me to run the engines 70% throttle 70% of the time. According to the site moderator on boat diesel, Tony Athens, an expert on boat engines, engine life should be improved by running the engine slower. I think Tony monitors this site and will probably correct me if I have missed quoted him.

I typically run my 330 hp engines at 14 to 1600 RPM which gets me 8 1/2 to 9 kn on flat water. I seriously doubt that I'm harm in my engines.

Please excuse the typos, I am dictating this on my cell phone.

kapnd said:

I think your friend's Meridian gets that 2.1 gph @ 8.4 mph on each engine, not both.
Not sure exactly what your question is, but I will say that hull and powertrain design do have everything to do with fuel economy.
A full displacement hull with properly matched motor/trans/prop will, in terms of fuel efficiency, outperform a semi disp or planing hull of the same size at the same speed.
All that aside, people who "do the loop" usually buy their boats based more on creature comforts than a few bucks worth of diesel.
Full disp hulls usually pay a draft penalty over their flatter bottomed cousins, and do not offer any significant speed for situations where that is desired.

Click to expand...

Kapnd,
I'm trying to get a realistic feel for economy and speed parameters, in looking for a loop boat.
After starting my search, I had focused only on trawlers, but later learned that a MY or cruiser can give quite good economy at hull speed. So I've included more boats in the search.

But I'd sure like to get closer numbers. I realize the hull design, motor design, etc., all have a factor. But would like to know what the difference might be in a cruiser, swift trawler and displacement trawler, all at the same weight, all the same engines and all operated at hull speed.

If the cruiser burned twice the fuel as the displacement trawler, I might think differently about the cruiser. However, it doesn't seem that it's much difference than perhaps 25% or so, which in the whole scheme of things makes no difference.

Just trying to get better stats on this. I would be nice to hear a bunch of responses that had logged their numbers and had fuel flow meters to get exact numbers. Then one could compare exactly. But I'm probably the only one to keep fuel logs. Do you perhaps keep one?

Yes, comfort is one thing in choosing a boat, as well as a ton of other things. Thx for the comments.

My previous 37' single engine displacement hull at 8kts with a 165hp engine burned 1.6gph. A twin engine MY of the same size ..... not even close.

Yes, a FD boat with a engine that is running in the most efficient portion of its power curve will provide better fuel economy at displacement speeds.

That said my Bayliner 4788 with a 44' WLL powered by twin Cummins 330's gets very close to 2.0nmpg at a tad less than 8 knots. At 9 knts I get 1.5nmpg, at 8.3 knots I get 1.75nmpg.

That is running both engines of course.

The choice in hull form involves allot of trade offs. Draft, speed capability, fuel economy, rough water handling, maintenance, the list goes on and on.

But again, a FD boat with a properly sized engine will get better fuel economy.

Fuel is the least of it.

Seevee said:

Kapnd,
I'm trying to get a realistic feel for economy and speed parameters, in looking for a loop boat.
After starting my search, I had focused only on trawlers, but later learned that a MY or cruiser can give quite good economy at hull speed. So I've included more boats in the search.

But I'd sure like to get closer numbers. I realize the hull design, motor design, etc., all have a factor. But would like to know what the difference might be in a cruiser, swift trawler and displacement trawler, all at the same weight, all the same engines and all operated at hull speed.

If the cruiser burned twice the fuel as the displacement trawler, I might think differently about the cruiser. However, it doesn't seem that it's much difference than perhaps 25% or so, which in the whole scheme of things makes no difference.

Just trying to get better stats on this. I would be nice to hear a bunch of responses that had logged their numbers and had fuel flow meters to get exact numbers. Then one could compare exactly. But I'm probably the only one to keep fuel logs. Do you perhaps keep one?

Yes, comfort is one thing in choosing a boat, as well as a ton of other things. Thx for the comments.

Click to expand...

I think you will find throughout your ownership that fuel costs will be the least of your worries. Lets think about that: An 1100 mile run down the ICW will cost me about 550 gallons of fuel (4 gal/hour at 8.5 knots, or roughly speaking 2knots/gal. At $2.50/gal, my fuel cost will be $1375. Now, lets say I dawdle and spend four weeks underway, of which, half will be in marinas, the rest at anchor or on balls. My marina costs will be about $100/night, close to what I pay for fuel. Now figure in repairs, provisions and restaurants, rental cars... Well you get the idea. If your new boat burns twice as much fuel, it will only ad $1375 to your total cost. It seems like a lot when you pull up to the pump with empty tanks, but when calculated in to the total cost of boating does not seem terrible to me.

And, I just traded a sailboat in for my trawler, and still have a bit of the sailer-save-a-dime mentality. All my friends with motor vessels tell me the same thing, however, fuel is the least of it.

my 250 cents/gal worth
Gordon

twice as much

Seevee said:

Kapnd,
I'm trying to get a realistic feel for economy and speed parameters, in looking for a loop boat.
After starting my search, I had focused only on trawlers, but later learned that a MY or cruiser can give quite good economy at hull speed. So I've included more boats in the search.

But I'd sure like to get closer numbers. I realize the hull design, motor design, etc., all have a factor. But would like to know what the difference might be in a cruiser, swift trawler and displacement trawler, all at the same weight, all the same engines and all operated at hull speed.

If the cruiser burned twice the fuel as the displacement trawler, I might think differently about the cruiser. However, it doesn't seem that it's much difference than perhaps 25% or so, which in the whole scheme of things makes no difference.

Just trying to get better stats on this. I would be nice to hear a bunch of responses that had logged their numbers and had fuel flow meters to get exact numbers. Then one could compare exactly. But I'm probably the only one to keep fuel logs. Do you perhaps keep one?

Yes, comfort is one thing in choosing a boat, as well as a ton of other things. Thx for the comments.

Click to expand...

Seevee,

As I said, my friend's heavier, larger, single engine Katy Krogen burns half the fuel I do 2 gal/hour versus my four. Of course he never gets above 7.128 knots on flat water doing 1600 RMP. At those turns I am going 8.5 knots and can turn the boat on a dime because of twin engines. He is currently headed down the ICW, and will spend $1500 less than me for the season. Over time, it just doesn't seem like a big deal to me. KNOT that I am made of money . ..

SeeVee,

One other thing, the trade off in potential speed. I can speed up to make a bridge opening if I need to, up to 15Knots, my friends KK48 might squeak out 8 knots in a pinch.

Gordon

If you are concerned about fuel consumption, you should keep in mind that modern diesels are much more fuel efficient than older ones. The Perkins and Lehman engines from the 70's, 80's and 90's are fuel hogs by modern standards. And the two cycle Detroits are in another world all to themselves.

But as someone else said, fuel consumption should be a minor consideration when choosing a boat for the loop. Other factors are much more important.

Seevee said:

Question that has come up.....

Regarding fuel and speed.

But, my friend that operates a 455 Meridian gets 8.4 mph on 2.1 gph (4mpg), with twin Cummins diesels 370's. He's on his 3rd loop and a very detailed person. But sounds exceptional.

Click to expand...

It is. I'm sceptical. Kevin's boat is similar to the Meridian and his figures are more believable to me. I get appx 1.8 GPH at 7.5 kts, KK42, full displacement.

Jim

Big engines on planing hull boats will burn more fuel. It takes fuel to simply spin the engine and pump its juices. Called parasitic losses, and bigger the engines, more they are. And planing hulls are not as slippery in the water at slow speed as a displacement hull.

But the difference can be trivial. A big heavy Hatteras with Detroit two strokes running at 8kts can be a fuel pig. But a Bayliner/Meridian with modern 5.9 Cummins is actually pretty good at hull speed. And the engines are loaded enough to keep them pretty happy.

I have a 38' with a single Cummins 8.3liter 450hp. It is capable of 20kts at 11gph, but at hull speed of 7.7kts, it is about 1.9gph. Not great, but still pretty good. If I was using a say 80hp engine to make the requisite 30-odd hp, I'd probably burn 1.5gph. I don't mind as I like the ability to run quick.

First you will never get the best economy at "hull speed", whatever that actually is, too fast.
Keep the bow down and the stern wake to a minimum and with a 40'+ 30,000 boat you will probably get 2-3 MPG, in flat water, with twins regardless of rated HP of the engines.
up the speed to 9+ kts and fuel use will increase significantly. Go on plane and expect less than 1 MPG.
Smaller boats will get better fuel mileage because they displace less water.

Hull shape windage etc and reasonable engine size are secondary to speed and displacement.
As stated above fuel is a smll part of the expense. Buy the boat you like and dont worry about minor fuel changes.

Seevee said:

If the cruiser burned twice the fuel as the displacement trawler, I might think differently about the cruiser. However, it doesn't seem that it's much difference than perhaps 25% or so, which in the whole scheme of things makes no difference.

Just trying to get better stats on this. I would be nice to hear a bunch of responses that had logged their numbers and had fuel flow meters to get exact numbers. Then one could compare exactly. But I'm probably the only one to keep fuel logs. Do you perhaps keep one?

Click to expand...

I suspect you're homing in on it. Yes, there are more and less "fuel efficient" hull forms, but I'd guess the actual degree of fuel efficiency at various RPMs and in boats of various weights and shapes... may or may not be all that important to every given buyer.

I don't keep a perfectly-detailed fuel log, and don't have fuel flow meters... but data from some of our two-way speed runs and using Cummins' fuel burn charts for estimation...

Looks like I get about 2.3 NMPG @ about 7 Kts @ 900 RPMs...

And looks like about 1.5 NMPG @ about 9 Kts @ 1200 RPMs.

This with twin 450Cs, i.e., total. (Decreases to about .7 NMPG at normal higher-speed cruise RPMs, of course.)

If we had a full displacement hull of approximately the same size as our current ride, and if it got closer to 2.5 or slightly better NMPG at 7 kts... or maybe 2.0 NMPG at 9 kts... no big whoop. For us.

Especially since we'd be losing our 20-22 Kt cruise capability for those times when that seems useful.

-Chris

After doing a bit more research on the 70/70 "rule", it seems the biggest consideration is the operating temperature of the engine. At low rpm's there is incomplete fuel combustion and soot and carbon are formed, as well as some corrosive compounds.

That said, I mostly motor at just above idle speed since I troll for fish a lot and the slower I troll the more fish I catch. I run the engine up to pretty close to max rpm's at intervals and watch the oil burn off in my wake. If I am damaging my engine, it's just a part of why I own a boat, I fish.

Every marine diesel information source I have ever encountered recommends adhering to the 70/70 "rule" from Nigel Calder on down...

To the OP the question you should be asking is the difference in the fuel burn really significant. I and most others will concede there is a difference based on hull type and relative motor size and more important overall weight of the boat. Fortunately for most of us who do not have the ideal combination the practical answer is it rarely matters relative to other issues like comfort and time allotment issues. If you have a hang up about ultimate economy of fuel burn you will need a long narrow light boat with just enough HP to attain sl over hull speed and a large slow turning prop. This type of boat comes with a down side. The extra $ you spend on docks will blow away your fuel savings and comfort in a long low light boat is compromised and the rarity of such boats speaks to the down side.

AKDoug said:

After doing a bit more research on the 70/70 "rule", it seems the biggest consideration is the operating temperature of the engine. At low rpm's there is incomplete fuel combustion and soot and carbon are formed, as well as some corrosive compounds.

That said, I mostly motor at just above idle speed since I troll for fish a lot and the slower I troll the more fish I catch. I run the engine up to pretty close to max rpm's at intervals and watch the oil burn off in my wake. If I am damaging my engine, it's just a part of why I own a boat, I fish.

Every marine diesel information source I have ever encountered recommends adhering to the 70/70 "rule" from Nigel Calder on down...

Click to expand...

I would like to point out that modern common rail engines have changed the game plan and have solved most of that soot and overload issue as part of the ecology standards they must meet. Therefore these engines can be run at lower loads wasting less fuel and increasing the engine life. It is suggested that at the end of a run or on a long run RPM be upped on a hourly basis with a load>80% for ten minutes . For those with older engines best to do little idling and blow out the soot and carbon running at higher loads. My twin JD 6068 engines are often cruised at under 40% load. They never smoke No stain on hull and no oil in the water. I am purposefully under propped and can barely get over 80% load at max RPM of 2600+ rpm at 18K. I do run them frequently and run up the rpm to max for a few min each time they are used. I do not think these engines are going to die or need major work from overloading or soot there are many other maintenance issues and marine age that are more likely culprits. So if you have a boat with common rail engines I think the old iron rules get lost with the smoke soot and hull staining.

eyschulman said:

I would like to point out that modern common rail engines have changed the game plan and have solved most of that soot and overload issue as part of the ecology standards they must meet. Therefore these engines can be run at lower loads wasting less fuel and increasing the engine life.

Click to expand...

I agree completely. The old rules don't necessarily apply to the modern common rail diesels.

From a practical standpoint for the OP, my opinions are:
- Not a lot of difference between FD and SD hulls if you are operating below hull speed.
- The bigger the engine, the less efficient you will be running at slow speeds.
- A single will be more efficient than twins, all else being equal.

As others have mentioned, fuel burn will likely be just a tiny fraction of your total ownership costs so don't stress about it too much.

Fuel costs are minor considering my fuel consumption compared to other boat costs.

At maximum throttle, my 2010-built, naturally-aspirated, 80 h.p. JD4045 has a fuel burn at 4 GPH at maximum RPM of 2400. Hull-speed, however, is achieved at 2200 for my fat, 14-ton, displacement-hull boat. Fuel-consumption rate at about one knot below hull speed is less than two GPH, reached at 1800 RPM, my typical throttle setting. This providis well over 160 hours of operation from four fuel tanks totaling about 320 gallons.

I typically fill two of the fuel tanks every 9 to 10 months. Thus, annual fuel costs are a lot less than two month's berthage. Even annual property taxes on the boat are several times fuel costs.

Fill-ups can be a shock, however. My sister offered to fill my fuel tanks shortly after I acquired the boat. I refused her offer. Didn't want to sock her with a $1200 bill.

You frequently hear the comment that fuel costs are a minor factor in boat ownership. Probably true overall unless you are doing a multi-thousand mile trip e.g. the Great Loop. Or a trip to the Caribbean.

However, in my experience fuel costs are frequently cited in why someone does not want to make a trip. I think of a friend who sold a Carver after it cost him $800 one way Michigan City Indiana to Leland Michigan. Our yacht club use to have a Michigan City to Chicago trip. A number of boaters used the fuel costs as the excuse for not going.

Fuel costs and out of homeport marina fees are one of the few expenses which are discretionary in boat ownership.

The purchase cost of a boat is a minor part of boat ownership expenditures, over the years. If people had taken that to heart, many boat owners wouldn't have "taken the plunge."

Dave wrote;
"A single will be more efficient than twins, all else being equal."

"all else" should be total power.
Sure if you compare a 120hp single to a 240hp twin. As most do. But if you compare a 120hp single to a 120 hp twin (obviously w two 60hp engines) the burn rate difference will be hard to detect and if detectable the twin may often be more efficient. The OP needs to be aware of that IMO.

A GB36 w two 55hp Yanmars would probably be slightly faster and burn less fuel due mostly to the much more modern engines and lighter weight. Re a GB36 single.

I'd suggest your friend is a little optimistic with his Meridians fuel figures. We have a Meridian 459 with twin 370B Cummins. When we recently did a 600nm trip I kept a detailed fuel log. Most of the way we did 7 knots at 1200RPM. If you look at the torque curve for these engines that is 98hp and the Cummins fuel burn figure if correctly proped is 14lph. I found we got slightly better than that at 12lph and our overall fuel burn came in at 2.2 nautical miles per US gallon.
I think you will find that with modern engines running at a couple of knots below hull speed the difference between a full displacement a semi displacement and for that matter a planning hull will be small. Ask people who have actually had both like the folks who own Dirona and they will tell you the same thing.
We love our Meridian. Very economical with lots of room and comfort.

Jetstream wrote;
"If you look at the torque curve for these engines that is 98hp and the Cummins fuel burn figure if correctly proped is 14lph."

I think you'll find that 98hp is at WOT. No wonder you burned less fuel. And of course there's the no wake zones, engine warmup and coming and going from harbors where you're burning far less fuel but the hour meter is marching right along.

Re the SD FD question opinions have been posted. My opinion is that a typical SD trawler will have about 1.5 to 1.8 times as much resistance as a FD trawler. A 40' Willard requires 23hp to run 7 knots.

Jetstream is that the Meridian 459 in your avatar? Looks like fun!

Planning hull at slow speed

I have a planning hull that I use at trawler speed. It is a Tiara 3700 Open. Crane lift weight and verified again on a travel lift, is 27,400 lbs. It is powered by 2 - 435 hp Cat diesels. The boat will top out, in fresh water, at 30 kts. @ 2850 rpm. I can cruise comfortably at 21 kts. @ 2200. It is equipped with digital flow scans and I keep an accurate fuel usage log for each engine and generator. My low speed operation, 90% of operating time, is at 950 – 1000 rpm. This rpm translates to 7.8 – 8 kts over the ground. Boat tracks true with no wandering at this speed. I am able to maintain 190 degree operating temperature down to 800 rpm but I do run engine up to 2200 for about 5 minutes every 3–4 hrs to spool up turbos and blow out carbon. I have no soot on hull sides or transom. Engines are not slobbering at low rpm operation. Over the 245 running hrs since February of 2016 I have burned 762 gallons of fuel. My GPS trip log indicates 1780 miles (1548 nm). I average 3.1g/h or a little over 2 nm/g. I make no extravagant claims to fuel economy. My only objective is to provide information of other boat configurations and operation parameters.

It is all about the size of the wave you make.

I usually try for a wave less than 6" tall. That gives me 8 knots and 2 mpg with a SD hull. If I want to go faster, I will make a taller wave, until I run out of hp at about 10.5 knots and 2' tall wave, using all 400 hp and getting close to 0 mpg.

At the other end of the scale, at 5 knots, the wave is indiscernible, as is the fuel consumption.

Nomad Willy said:

Dave wrote;
"A single will be more efficient than twins, all else being equal."

"all else" should be total power.
Sure if you compare a 120hp single to a 240hp twin. As most do. But if you compare a 120hp single to a 120 hp twin (obviously w two 60hp engines) the burn rate difference will be hard to detect and if detectable the twin may often be more efficient. The OP needs to be aware of that IMO.

A GB36 w two 55hp Yanmars would probably be slightly faster and burn less fuel due mostly to the much more modern engines and lighter weight. Re a GB36 single.

Click to expand...

Nomad,
Good points. That's what I thought with twins, however one does double the maintenance. And with twins at the SAME hp as one single, I'd expect speed to be close to the same. I don't imagine that one would gain more speed with either, using the same HP... but don't know.