A question about cruising at "Trawler Speed" MPGs

[thataway]

There are a number of constantly adjustable pitch props--not aware of any for outboards however. The boats we built had a Sabb engine with only a dog clutch, and a constantly adjustable pitch prop, from full feather to full flat fore and aft--in both forward and reverse. This was accomplished with a hollow prop shaft, and an inner shaft, which could be worked back and forth, catching ears on the inner aspects (inside the hub boss) of the prop--thus changing the pitch when running. I would dock with the engine running at 1000 RPM--and then just use the pitch to walk the boat sideways, or move forward or aft. The design of this prop system is close to, if not more than 100 years old. Some very large props in commercial applications use exactly this same technique. There are some available for the largest of cargo ships.

In outboard props, those which flex, may work well, but are never quite as effecient as a well designed and specificated prop for that engine and specific application.

 

[Matt Gurnsey]

There used to be a couple of manufacturers of two speed props. Started out at one setting, and then as load increased would "shift" to the other setting. Slowing down would allow the prop to "downshift" again.

Available for I/O's, and outboards using the same lower unit as the I/O (merc / Evinrude / Johnson and some others) these were made by Land and Sea and Quicksilver. There may have been others. They were expensive, a bit complex, and never really caught on. probably didn't hold up well in salt water either.

 

[centerisland]

I had a Heimdahl CPP (controllable pitch prop) on a small Romsdahl trawler a number of years ago. As Bob described, you could turn the boat in just over it's length by oscillating between forward and reverse (the prop always turns in the same direction). Cruising was a little different - you basically set the throttle to a working RPM and then adjusted the prop to reach an optimum temperature on an exhaust gas pyrometer (analogous to cylinder head temp).

 

[hardee]

Bringing this thread back to life.

Has anyone actually calculated the true hull speed of the 22 Cruiser, or for that matter any of the other C-dory craft?

I have been looking for the actual waterline length, but have not found that today, or lately. Need it for the true hull speed. Thanks in advance.

Harvey
SleepyC :moon

 

[Chester]

An efficient speed is usually from 1.0 to 1.3 times the sq root of the waterline length (in feet) in knots. 1.1 or 1.2 usually being more efficient than 1.3. Hull speed is not always a specific fixed equation. Hull shape, prismatic coefficient, immersed transom and so on affect the entire business.
As Capt. Jack Sparrow might say 'Hull speed is more of a guideline'.

 

[hardee]

Chester said:

"An efficient speed is usually from 1.0 to 1.3 times the sq root of the waterline length (in feet) in knots. 1.1 or 1.2 usually being more efficient than 1.3. Hull speed is not always a specific fixed equation."

Steve, I understand the equation which is technically 1.34X. I have been looking for the waterline length, and will have to go measure a boat, (Mine's not at home), and then do the calculations. I agree that the 1.34 is probably the max, and lower will be more efficient. I don't have fuel flow on twin 40's, and want to do some trip calculations, so have been looking for the numbers.

In practice, I believe the most efficient speed is at the point of just before the bow wave curls and falls, so you should not hear the water splashing on the hull. For my boat, my load, I believe that is about 5.25knots. We spent about 4 hours one day, with 4 on board, running a single down, and didn't seem like we burned any fuel. Sorry I couldn't measure it, that was putting around in Princes Louisa inlet, not able to refuel after the session.

Harvey
SleepyC :moon

 

[Notayot]

I have used 20 feet as the LWL in my calculations. This gives a max displacement speed of 7.2 mph, so I try to stay below that when trying for "good" mileage. My experience is that about 1200 rpm gives 10 mpg at 4-4.5 mph, and at about 1500 rpm you get 5.5 mph and 8 mpg.

I didn't have enough data to do this with my own cruising, so I have borrowed fuel logs posted on this site by others to gin up a spreadsheet. From that I just follow the engine and hull speeds when I want to go far on less!

 

[Yellowstone]

Harvey - See my comments of page 1. Also, I note in Trailer Boating magazine, whenever they conduct fuel efficiency tests on their featured boats, 1,000 rpm seems to be the beginning of the testing, and it always yields the best mpg. (talking about gas not diesel motors of course) I always use my gps when underway and since it gives speed over ground, (or bottom), holding at a constant rpm and track shows the slight variations in speed brought around by a beam wind, or coming out from behind an island and running into bigger waves.

In the ocean where currents are always in play, one should experience the additive speed when going with the currents, but I have never attempted to measure this.

Again thanks for helping me get the C-Dory calendar. Is the first year for this? A really nice accessory.

 

[Yellowstone]

Dennis - I found with the flo scan that moving up to 1,200 rpm from 1,000 with the Honda 90 (old carb. model), began to start a very slight decrease in fuel efficiency. One of the variables which is really an independent one in this discussion is the actual displacement taking place when measuring hull speed and efficiency. Some folks are carrying much heavier loads, and if I have found out there is an immutable characteristic about the 22', it is highly sensitive to actual weight loaded aboard.
John

 

[Levitation]

In the 20-25 foot range running at hull speed it appears that roughly 6mpg is what you will get - regardless of engine type... My R25 with a 150hp Cummins diesel, at 6mph burns exactly 1 gallon an hour - and I am heavier than a C-Dory of comparable length...

 

[hardee]

Harvey - See my comments of page 1. ...... I always use my gps when underway and since it gives speed over ground, (or bottom), holding at a constant rpm and track shows the slight variations in speed brought around by a beam wind, or coming out from behind an island and running into bigger waves.

In the ocean where currents are always in play, one should experience the additive speed when going with the currents, but I have never attempted to measure this.

This may be the reason I need to do some work on getting the paddle wheel speed calibrated. Running in the ocean, there is almost always some current in one direction or another. Hull speed is relative to the water the hull is going through, not speed over ground. For instance, at the entrance to Sequim Bay, the current can run at 5 knots. If the tide is coming in, and I am going out, a 5 knot hull speed, would give me a speed over ground of 0 knots, but if I was coming in on the tide, that 5 knot hull speed would be 10 knots speed over ground. So for hull speed cruising, I think the GPS is great if you are on a lake, but for ocean or river travel the paddle speed is the one to watch.

(BTW I just tried Rogerbums quote method, and Thanks, It is good.)




Bouy 7 Sequim Bay, on a flood tide. Speed over ground inbound = 7 knots, after a 180 turn, without touching the throttle 2 knots, for a 5 knot tidal flow.

Harvey
SleepyC :moon

 

[Grumpy]

Harvey,

Put another way, using your example. Against the current your milage sucks and you will never get there, with the current you can turn the engine off, get there at 5 kts and get infinite mpg.

Problem with all the lower speeds is, as you say, there is always some current and the lower your speed the bigger % difference it makes on your ETA and economy.

You can do some great comparisons in programs like Coastal explorer. Plot a course to somewhere, input your boat speed and try it at different times of day.

Sometimes faster is better.


(That will keep you awake for a bit :shock: )

Merv

 

[NORO LIM]

. . .




Bouy 7 Sequim Bay, on a flood tide. Speed over ground inbound = 7 knots, after a 180 turn, without touching the throttle 2 knots, for a 5 knot tidal flow.

Harvey
SleepyC :moon

Unless your post-180 turn speed was minus 2 knots, I think the tidal flow was actually 2.5 knots, and your boat STW was 4.5 knots. (i.e., 2.5 + 4.5 = 7 going one way, and 4.5 - 2.5 = 2 going the other? I'm still thinking about this!) But the point is an excellent one. I am constantly having to remind myself of this necessary adjustment when using GPS speed. Unfortunately, either pitot or paddlewheel speedos always seem to crap out on me one way or another - intermittently, or permanently. As a consequence I do rely primarily on my GPS and try to do some best guess adjustments. (I was just rereading an account of the Shackleton/Endurance expedition's open boat crossing from Elephant Island to South Georgia. Now there were some guys who knew how to make adjustments for wind and current - all in their heads!)

 

[Hunkydory]

Harvey, your points about boat ground & water speed combined with current along with wind and boat load make your point of paddle speed verses ground speed on a gps valid, but if not wanting to add one more thing out of the kiss principle just knowing on an average at what speed the boat should be with no wind or currant at a given rpm along with sog with the gps should give a close approximation of water speed. Whatever speed its over or under the known rpm speed should be the amount of wind or current at bow or stern. Of course the other variable being the boat weight difference changing the drag as you take on or off fuel, water and other supplies. Took some cruising last year in Southeast Alaska to fix that rpm and of course I thought I would remember it and now can't recall exactly, so will have to do it all again next cruise. Maybe with plans on allot of displacement speed cruising it could be worth one more addition. Probably the thing that added most to extra fuel cost via loss of mpg on our last summers cruise was just forgetting to get those trim tabs and motor trim back up. With the heavily loaded stern of the boat from the extra fuel had to keep both maxed out for better mileage on plane and riding comfort in chop for the first couple hundred miles after a fueling. Several times I was thinking the head wind or current was stronger than I thought it should be before it dawned on me about forgetting the tabs and motor trim. At 6 mph a 1 mph speed loss at the same rpm results in a increased fuel usage of close to 17% and another good reason if considering displacement speed cruising to take in account the tides and ride with them whenever safe to do so.

Jay

 

[hardee]

Jay said,

At 6 mph a 1 mph speed loss at the same rpm results in a increased fuel usage of close to 17% and another good reason if considering displacement speed cruising to take in account the tides and ride with them whenever safe to do so.

Jay, you are right on. At displacement speeds there are times when I might be going backwards. Even at planning speeds I still consider the "Washburne's Tables" before crossing the strait. Having been a long time sailer, and almost always preferred to ride my bicycle down hill rather than uphill, I like the even slightly improved economy of Mother natures push, (riding with the tides and currents.)

Bill on the Noro Lim got me on this:

Unless your post-180 turn speed was minus 2 knots, I think the tidal flow was actually 2.5 knots, and your boat STW was 4.5 knots. (i.e., 2.5 + 4.5 = 7 going one way, and 4.5 - 2.5 = 2 going the other? I'm still thinking about this!)

Bill, I'm no mathmatatisian (shucks, can't even spell it), I was just reporting what I saw when we did it. :smiled

Merv said:

Put another way, using your example. Against the current your milage sucks and you will never get there, with the current you can turn the engine off, get there at 5 kts and get infinite mpg.

Problem with all the lower speeds is, as you say, there is always some current and the lower your speed the bigger % difference it makes on your ETA and economy.

Merv, Right on, the lower the speed, the higher the current speed percentage is. More good reason to go with the flow. I have not tried Coastal Explorer, but that sounds like fun, and useful.

Harvey
SleepyC :moon

 

[NORO LIM]

Harvey said:

"Bill, I'm no mathmatatisian (shucks, can't even spell it), I was just reporting what I saw when we did it. CoolSmile"

Neither am I! ':?' I had to edit my post three times in about two minutes just on the addition and subtraction! I think staring at the "7" on the buoy mesmerized me. I kept subtracting 4.5 from 7, instead of 2.5 from 4.5, and couldn't get the coming and going numbers to agree.

You know what they say: "There are three kinds of mathematicians - Those who can spell and those who can't." ':lol:'

 

[thataway]

Interesting that we seem to have two parallel threads going at the same time about LWL and speeds; Such is C Brats.

The most important for navigation is speed across the bottom. The practical reasons for thru the water speed are sailboat racing, fishing, and figuring out your fuel economy at one instant--but for more accurate figuring do runs in both directions and use GPS or speed across the bottom. Before GPS, all we had were paddle wheel , or chip logs--to find out the really important information: where we were we had to look at charts, meterology and our senses.

As for currents in oceans--Most of the world's oceans do not have swift currents--there are exceptions, such as the Gulf Stream--with an average current of 4 knots and max of about 5.6 knots. There are many parts of the oceans which have far less than .5 knots and are well shown on Pilot charts. What folks are talking about here as "ocean" are really bays, straits, reaches etc--where there is a high tidal range and some restriction of water flow. This is of interest for low powered vessels for example in the PNW and certainly can have an influence on the fuel concerned and the ride--but on the Gulf Coast, S. Calif. and many other places in the world currents are of little concern.

There is no magic about 6 knots--getting 6 miles per gallon, but I have had some very different boats I have owned get that seemingly magic number: one was a boat with a LWL of 50 feet, beam 14 feet and displacement of over 65,000 lbs--and another boat with LWL of about 41 feet, beam 12.5 feet and displacement of 35,000 lbs. The thing they had in common was an 80 hp diesel, and a place on the LWL vs hp curve, which gave 6 nautical nautical miles a gallon. On the other hand, I have had some deep v types which would be pushed to get 6 miles per gallon even at 4 knots.... The 1.34 x sq rt LWL is a generalization for the "average" boat--where it is not starting to climb over its bow wave to plane, and digging a deep hole in the water (displacement boat which will never plane). Catamaran and other long skinny hulls can be exceptions to this number.

 

[journey on]

There's another thought, mentioned above by Chester, that there are additional factors in play other than running at hull speed. On Our Journey, a 36' sailboat, with a clean bottom, the mileage was 10 nm/gal, with a 30 hp diesel. Parting the waters cleanly and bringing them back together smoothly counts, too. The C-Dory hull is a good hull, but not in that sense. And the mileage drops considerably as you pack more cruising gear on board, since weight jams that hull deeper in the water requiring more effort to part the waves.

The moral I'm trying to get to here is that traveling at "hull speed" does get better fuel mileage, but there's little one can do to raise it over the nominal figures reported above (for a C-25, ~5 nmpg.) And being precise as to hull length or prop pitch has less effect than the gas you buy, which now has 10% alcohol. I figure that loses me about 10% in mpg. Unpacking the gear off your boat probably would help more than a new prop and cost less.

In the 20-25 foot range running at hull speed it appears that roughly 6mpg is what you will get - regardless of engine type... My R25 with a 150hp Cummins diesel, at 6mph burns exactly 1 gallon an hour - and I am heavier than a C-Dory of comparable length...

What also impresses me is that a R25 isn't getting more than 6 mpg. Heck, given the cost of that diesel motor and the price of diesel at the pump, running an outboard on a light C-Dory is the way to go.

Boris

 

[NewMoon]

On calibration of paddlewheel speed:

We've noticed that the paddlewheel-reported speed can be very accurate at planing speeds, and very inaccurate at displacement speeds, or vice-versa.

For our purposes, it seems more useful to have the paddlewheel calibrated to show accurate speed over water at displacement speeds. This way, by comparing paddlewheel speed with GPS speed, we can accurately gauge how much current we are running with or opposing, at the slow speeds where current makes the most difference.

When we're going faster, we're more interested in GPS speed over ground.

 

[Hunkydory]

Harvey,

Put another way, using your example. Against the current your milage sucks and you will never get there, with the current you can turn the engine off, get there at 5 kts and get infinite mpg.

Problem with all the lower speeds is, as you say, there is always some current and the lower your speed the bigger % difference it makes on your ETA and economy.

You can do some great comparisons in programs like Coastal explorer. Plot a course to somewhere, input your boat speed and try it at different times of day.

Sometimes faster is better.


(That will keep you awake for a bit :shock: )

Merv

Quite correct Merv, and that's why when going against the current especially if over 2 mph more often then not we will get on plane. Of course is even more important when going through the passes where the current can run 5 to 10 mph or more and like you said will never get anywhere at displacement speed. All the more reason to follow the tides, but what I meant as only when safe to do so, examples are in places like the long stretches between safe harbors where just a small change in the weather can make a big difference in a 22 foot boat. I sure didn't feel comfortable with displacement speed cruising on the Gulf Of Alaska side of Baranof and Chichagof Islands, the long stretch of Lynn Canal without a very good weather window or passing with the ebb of up to 14 mph in South Inian Pass between Icy Straight and Cross Sound. Riding the ebb there into the wind and swells of Cross Sound could be a very poor way to safe a few dollars on fuel.

Jay