GPH Information

Hello everyone. I recently retired about two weeks ago after over 30 years in the fire service. I currently hold a 100 ton master USCG license and am very familiar with boats and their maintenance schedules. I am very handy with electrical, mechanical, and most do it yourself projects. I am planning to purchase a live aboard within the next six months and "live the dream" traveling from Texas, around the east coast, around the great loop, over to the Bahamas, then on down to the Caribbean. (I lived aboard a trawler for several years in St. Thomas in the 90's). My Trawler in St. Thomas (35 foot Chien Hwa) had a single Perkins with no generator or AC. This time round I want to live on the hook so I would like to find something that has generator, minimum of two cabins, full size refrigerator, ice maker, water maker and a washer/dryer. I don't want to have to go to a marina every month to take on water. I don't mind going three miles out to dump the black water tanks either.

My concern is that many of the boats I look at (not all trawlers, some motor yachts) have the Detroit diesels or 3208 caterpillars in them (I would prefer ford lehman or perkins). I'm not worried about speed because I'm not in a hurray to go anywhere fast. I am worried about fuel consumption and don't want to burn 10+ gph. Fuel prices are better now than they have been in years but I know that this isn't going to last. Not very many trawlers meet all of my wishes whereas several motor yachts do. I am looking at the 80's models from 44'-55' and under 100K (remember I am a retired fireman not a doctor or lawyer). I'm trying to see if anyone knows what the GPH usage is on some of these vessels that are made to cruise at 15+ knots but only used in the 7-9 knot range. Is this something anyone has done? How hard is it on the motors? I do know they will have to be blown out occasionally. I will not be traveling everyday just when I feel the need to go somewhere new.

Any information would be greatly appreciated. I have been reading this forum for over 3 months now and anything else I can get my hands on. My biggest problem in finding the best boat is that I live in the Dallas/Ft. Worth area which makes in a minimum of a 4 hour drive to get to the closest coast. My mom does live in Florida and I could use her house as a home base when I get really serious about finding a boat since a majority of boats seem to be located there.

Greetings,
Welcome aboard and happy retirement.

Congrats on your retirement!

We live aboard a 1989 PT52 52' yachtfisher that has most (if not all) of the requirements you list:

• 2 cabins (forward and aft)
• washer/dryer
• ice maker
• 12kW genset
• 3/4 size fridge
  

On our short list is the watermaker and bow thruster. There's plenty of living and storage space.

We are powered by 375hp Cat 3208TAs. Top speed is 22-24 knots; our normal cruising speed 7-9 knots. We idle at 600 RPM/approx 4.5 knots, and our cruise RPM is about 900-1000. Fuel burn rate combined is about 5 GPM at that speed. We can freewheel one engine and drop the fuel usage by 1 to 2 GPM.

For every 8 hours of run time, we'll take her up to 80% for about 10 minutes. As you many know, the Cat 3208 is a smoky joe when cold, and burns clean once up to temp. Ours run smooth, and I fully expect to see 10k+ hours out of them.

fireman749 said:

My concern is that many of the boats I look at (not all trawlers, some motor yachts) have the Detroit diesels or 3208 caterpillars in them (I would prefer ford lehman or perkins). I'm not worried about speed because I'm not in a hurray to go anywhere fast. I am worried about fuel consumption and don't want to burn 10+ gph. I'm trying to see if anyone knows what the GPH usage is on some of these vessels that are made to cruise at 15+ knots but only used in the 7-9 knot range. Is this something anyone has done? How hard is it on the motors? I do know they will have to be blown out occasionally. I will not be traveling everyday just when I feel the need to go somewhere new.

Click to expand...

Fireman, I think you will find that most here use speeds that do not reflect the power they have installed and have the hp & hull design to go faster than they normally do. There are some exceptions here, like myself, that have full displacement hulls that are not able to go above their hull speed. This is a primary choice to be made when starting to look. If you truly will never want to exceed your hull speed, then indeed there is little reason to purchase the HP and hull type that can do so. Having said that, however, 1) you will find the marketplace much smaller both when buying and when selling. 2) although displacement hulls usually have lots of volume and always have more weight carrying capacity, they normally lack the space for an aft cabin.
In the 44-55' range you are looking at, 7-9K speeds you mention are displacement speeds so you will have no trouble obtaining them with low HP. But for the absolute economy of a full displacement vessel, you give up, for always, the ability to semi-plane and move out more smartly if needed.

As a first approximation, nautical miles per gallon (NMPG) which is the measure of fuel efficiency, is all about speed. GPH by itself isn't very interesting since you really need to know how fast you are going for it to be meaningful in any way. How far you can get on a gallon (or liter) is what matters. The slower you go, the higher the NMPG, and the faster you go, the lower the NMPG. Speed is the dominant factor.

There are secondary factors too, but they typically can be swamped by a slight adjustment in speed. These other factors are:

Boat length and displacement: A longer water line improves efficiency, and higher displacement reduces it. Since longer water lines usually drag along higher displacement, these two tend to cancel each other out.

Engine efficiency, and engine operating point: Older engines tend to be less fuel efficient that newer designs, and 2 strokes tend to be lower efficiency than 4 strokes. Also, every engine operates over a range of power output levels, and is more efficient at converting diesel into output power at different points in the range. If you are re-powering or designing from scratch, then picking an efficient engine and sizing it to be operated mostly at its optimum power level makes sense. But most of us are buying boats that are already designed, built, and powered, so although these factors make for interesting forum discussion, it's completely rhetorical.

Hull form: This too can make subtle differences in efficiency, but like engines, most of us have the hull that we have, so changing it's efficiency is a moot topic. And I don't think it really matters much anyway, at least not compared to speed.

So for the most part there is one knob/control that we can play with, and that's our speed.

It is important to make in advance the correct decision about the use of a vessel, next step is engine size.

The specific fuel use for a diesel engine is;

At 90% load approx. 200 gr/kW/hour the efficiency is then 30 to 35%

At part load it can be as high as 300 gr/kW/hour efficiency less then 15%

CeesH

twistedtree said:

As a first approximation, nautical miles per gallon (NMPG) which is the measure of fuel efficiency, is all about speed. GPH by itself isn't very interesting since you really need to know how fast you are going for it to be meaningful in any way. How far you can get on a gallon (or liter) is what matters. The slower you go, the higher the NMPG, and the faster you go, the lower the NMPG. Speed is the dominant factor.

Click to expand...

Agreed ... GPH ... a common shorthand and misconceived expression of fuel efficiency. My (planing) boat/engine is more fuel efficient at 13 GPH than at 3 GPH ... when you consider the distance covered over the time (speed).

30 MPH / 13 GPH = 2.3 MPG ... at 3300 RPM
6 MPH / 3 GPH = 2.0 MPG ... at 1000 RPM

Some builders offer the engine/boat fuel data over the range of RPMs that can be used to really evaluate the potential fuel efficiency and range ... two examples of semi-displacement hull boats:

BENETEAU Swift Trawler 42

Standard Power: 2 x 300 HP
Maximum Speed: 26 knots
Cruising Speed: 18 knots

Engine: 2 x Yanmar 6LYA-STP 370 HP
Gear: HSW 800 1.96:1
Prop: 22.8x22.4 4 blade bronze France Helice



FATHOM Element 40

Standard Power: 1 x 425 HP
Maximum speed: 18.75 knots
Cruising speed: 6.5 - 16 knots

Engine: 1 x Cummins QSB 425 HP
Gear: ZF 280-1A Ratio 2.478:1
Prop: LH 27x24.5 5 blade Nibral Class 1




Hope this helps ....

I think we are all saying the same thing, that there are three basic factors that affect fuel consumption: speed, weight of the vessel and hull form in that order of importance. Number and size of engines has a smaller effect.

So you are looking at 44-55' boats, and all except a very few makes (that you probably can't afford) will be twin engine, semi-displacement hull types and you want to cruise at 7-9 kts. Then you are looking at 3 to 5 gph on the low and high end of these parameters.

David

Reiziger said:

It is important to make in advance the correct decision about the use of a vessel, next step is engine size.

The specific fuel use for a diesel engine is;

At 90% load approx. 200 gr/kW/hour the efficiency is then 30 to 35%

At part load it can be as high as 300 gr/kW/hour efficiency less then 15%

CeesH

Click to expand...

Right you are and seems to be lost on most people (especially boat sales people)

Another way to look at it is that around 8 kts you will get about 1 NMPG

I ran two hypothetical boats through a prop calculator (not the final word by any means) one a semi-displacement 36'W/L, 12 beam. 1.5 molded draft, displacement 24000. The second, full displacement, same same except 4000 lbs more for ballast and a 3:1 transmission instead of 2:1.

The first vessel will make 15k using 310 HP and 18GPH for .83 NMPG
The second " will make 7.6k using 45 HP and 2.5GPH for 3 NMPG

This gives some general idea of the cost of speed on water

We have a Jefferson 52 MY weighing in at about 50,000 pounds. Twin Detroit 6V92s 550 HP. We cruise at 7.5 to 8 knots slight seas and a combined burn of 7 to 8 GPH. At 9 to 10 knots it jumps to about 12 to14 GPH and don't even think of going faster unless you won the lottery! But at the lower speed it is not too bad. Top end is about 19 knots at 60 GPH.

Definitely gph is not the best indicator. Consider fighting a 3-4 knot current. Gph is the same but you are covering a lot less ground in one hour.

Richard W said:

Agreed ... GPH ... a common shorthand and misconceived expression of fuel efficiency. My (planing) boat/engine is more fuel efficient at 13 GPH than at 3 GPH ... when you consider the distance covered over the time (speed).

30 MPH / 13 GPH = 2.3 MPG ... at 3300 RPM
6 MPH / 3 GPH = 2.0 MPG ... at 1000 RPM

Click to expand...

This is one of my pet peeves. In the old days before GPS, fuel flow meters and reliable speed logs, when most boats were full displacement hulls, you used GPH because you always set your throttle for a certain RPM using the tach. After factoring in the effects of wind and current, you'd always go the same speed at a given RPM, burning fuel at the same rate.

GPH was a handy number to know back then.

Now, we always know our speed with a high degree of accuracy. We can calculate exactly how far we can go on a given amount of fuel. The number we really care about is MPG (statute or nautical, whichever you're using.)

So why does everyone still go around talking about X GPH at Y knots?

Don't make me do the math, just tell me how far you can go on a gallon at that speed!

Sorry. I'm done now. Thanks for letting me vent.

So far us it is at 7.5 to 8 knots we go 1 nautical miles per gallon. At 9 to 10 knots we will go .75 nautical miles per gallon. Now we have it both ways.

Thanks for all of the information. All of this is very helpful. This is going to be a fun experience trying to figure all of this out.

Happy New Year to each of you!

Craig

I also have a 56 with 1 cat 3208 and at 6.5 to 7.5 , i can burn 3.5gph .
You must take care if you use just one e gibe because the other one can't lubricate the reversor


Sergio "Alemao" Sztancsa
Sent from my iPhone, using Trawler Forum

Owned a Duffy lobster yacht with a 375 3208, would cruise at 18 K and top out at 22 K. Did not run it at 8 K. Also owned a 37 foot SD with a 210na 3208 with Max speed of 14 k with a burn of 11 gal/hr. Usually ran it at 8K with a burn just under 3 gal/ hour. Have owned three boats with 6 cyl Perkins and current boat with Ford Lehman SP 135. The 3208 is by far the smoothest running of all the engines owned and the easiest to live with. The V8 configuration is inherently smooth.