I have a 38' sedan running twin 350CID gas. Thirsty bugger at speed. My question is - is there a firm formula for finding the most economical displacement speed? I can be running at 1200RPM or 1700RPM and not be on the plane, with very little difference in speed, but a huge difference in fuel consumption. How do I find my sweet spot?
Trawlers vs. Cruisers
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I don't know if it's just me or what but our 25' express has no sweet spot where you can maximize econonomy. It really doesn't seem to matter if we do 6kts at 1100 RPM or 23-25 kts at 2800 - 3000 rpm our fuel consumption for distance over the ground is pretty much exactly 1 liter per kilometer. The only benefit of 23 kts is you get there a lot faster. We've done this test a few times and it pretty much plays out the same every time, albeit, by non-scientific means. I would think that with a larger planing hull some sort of fuel consumption monitor would be best. That way you could graph speed/rpm vs GPH and find the optimum settings.
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Kevfra, hull speed (1.33 x sq root of Waterline length) is usually the fastest displacement speed you can go at... on my boat it's about 9.2 kts and i burn about 10gph (total)
running at slower speed does increase fuel economy but trying to push faster result in much higher fuel burn as you're just pushing a bigger wave, until you get on plane.
KB, try finding fuel test for a similar boat (LOA and engine), usually it shows better economy at low speed (prob 6 ot 7kts on a 25 footer) and sharp increase in fuel burn until you get on plane.
running at slower speed does increase fuel economy but trying to push faster result in much higher fuel burn as you're just pushing a bigger wave, until you get on plane.
KB, try finding fuel test for a similar boat (LOA and engine), usually it shows better economy at low speed (prob 6 ot 7kts on a 25 footer) and sharp increase in fuel burn until you get on plane.
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That's right Pascal. She starts to fall into her own stern wave at about 6 kts. She's only 20' at the waterline. What I was getting at was more fuel burned for distance covered. If we're up on plane at 23 kts we'll burn approx. 30 lt/hr (approx. 2.5 mi/gal) and at 5 kts we'll burn 7-8 lt/hr. That's pretty much equal burn for distance covered it just takes a lot longer to get where you're going. We did the eastern Ontario Golden Triangle (Kingston, Ottawa, Montreal) route this summer, 640 kms covered and 650 litres of fuel burned so it's pretty consistant. I can't wait till I get my trawler built! ;o)
Hey Bruce, sorry, just got caught back up with this thread. You can and will absolutely crap up high horse diesel engines by running them at or close to idle speed (thus attaining hull speed on a flat bottom boat) for prolonged periods of time. They have to be spooled up to cruise speed and temp on a regular basis, on the same day-long trip, or you will get excessive carbon build up on the pistons. I have seen several examples of this, for instance, on "training" boats that are used primarily to teach docking and low speed handling. The new electronic diesels counter this to some degree, but like so many things, there is no free lunch. Engines that are used consistently at cruise (75-80% of WOT) will outlast those used at WOT and those used predominantly at idle, given proper prop loading.
True displacement boats are used for cross-ocean travel for a few reasons, including: 1) a bigger, deeper hull for tankage, quarters and provision 2) the ability to get weight down low for greater ultimate stability (as opposed to initial stabilty of a flat hull) 3) slightly more fuel efficient at hull speed, or more typically 2/3-3/4 hull speed. Yes , they are "rolly" and need some sort of stabilization to make humans comfortable over long, multi-day non-stop runs. Their motion will be more gradual and less snappy than a flat bottom boat.
Again, I agree with you 100% about the marketing hype around so-called "fast trawlers". And I happen to like many of them!
George
True displacement boats are used for cross-ocean travel for a few reasons, including: 1) a bigger, deeper hull for tankage, quarters and provision 2) the ability to get weight down low for greater ultimate stability (as opposed to initial stabilty of a flat hull) 3) slightly more fuel efficient at hull speed, or more typically 2/3-3/4 hull speed. Yes , they are "rolly" and need some sort of stabilization to make humans comfortable over long, multi-day non-stop runs. Their motion will be more gradual and less snappy than a flat bottom boat.
Again, I agree with you 100% about the marketing hype around so-called "fast trawlers". And I happen to like many of them!
George
I aggree, Camano has not designed anything radically different. The have tweaked a bit what is basically a downeast hull, whatever the thew most basic of those may be, given it a catchy name & marketed the boat with it. Nice boat too. Had a poke around a few at boat shows.
What is a trawler? Who knows! Could you put a 'bubbleboat' topside on a full displacement hull & call it a trawler? Imagine that! How about a Nordhaven topside ontop of a Fountain hull? What's that?
How different is this from the Camano, & others? The first production of the hull was in the early 80s
At speed (13kts on a good day) with only a 100hp diesel. No tabs. At displacement speed it gave < 1 1/2gph. The $ per hr cost was less than a 13' Whaler with a 30hp on it.
Is it a trawler? Not by my definition. A 'pocket' trawler maybe since it is only 27' long.
What is a trawler? Who knows! Could you put a 'bubbleboat' topside on a full displacement hull & call it a trawler? Imagine that! How about a Nordhaven topside ontop of a Fountain hull? What's that?
How different is this from the Camano, & others? The first production of the hull was in the early 80s
At speed (13kts on a good day) with only a 100hp diesel. No tabs. At displacement speed it gave < 1 1/2gph. The $ per hr cost was less than a 13' Whaler with a 30hp on it.
Is it a trawler? Not by my definition. A 'pocket' trawler maybe since it is only 27' long.
The most economical speed will be below the point at which you experience any bow rise and stern squat. If you have constant fuel use over all speeds I suggest that your lower speed is still too high unless you have a totally planing hull, see below. The formula is correct but bow rise is easy to detect.
While I cant see the stern the Camino looks to be a semi planing hull depending on power while there is no doubt that that Willard is a true trawler. As noted those round chines reduce the snap roll expected with sharp chines. The idea is simple if you consider the different behavior of a flat box vs round ball on the surface of the water. The box will react to the surface wave as they pass under by tilting the ball just rises and falls. Of course, an actual hull is a compromise between the ball and box and rides somewhat lower nevertheless the concept is informative.
I did find an interesting graph on the Camino site showing similar fuel use for semi displacement?? (Semi Planing?) and the Camino at speeds less than 10 kts. It also shows a true planing hull curve and gives some insight into the puzzling observations that true planning hulls have similar fuel use over the entire speed range. The curve shows fuel use is a straight line if you eliminate the hump between about 7 and 18 KTs, a speed range most people never see.
While I cant see the stern the Camino looks to be a semi planing hull depending on power while there is no doubt that that Willard is a true trawler. As noted those round chines reduce the snap roll expected with sharp chines. The idea is simple if you consider the different behavior of a flat box vs round ball on the surface of the water. The box will react to the surface wave as they pass under by tilting the ball just rises and falls. Of course, an actual hull is a compromise between the ball and box and rides somewhat lower nevertheless the concept is informative.
I did find an interesting graph on the Camino site showing similar fuel use for semi displacement?? (Semi Planing?) and the Camino at speeds less than 10 kts. It also shows a true planing hull curve and gives some insight into the puzzling observations that true planning hulls have similar fuel use over the entire speed range. The curve shows fuel use is a straight line if you eliminate the hump between about 7 and 18 KTs, a speed range most people never see.
George:
Diesels will usually announce when they are unhappy by producing smoke. Smoke from being too cold or too much fuel. Both are correctable problems though some old Detroit’s just seem t like making a mess. I suppose we should distinguish between and old Detroit and more modern 4 cycle diesels. I don’t think you would have seen any problem with anything modern and a brief WOT operation will usually clear any carbon that might develop. I’ll guess those engines you ran were not at required operating temp.
Diesels will usually announce when they are unhappy by producing smoke. Smoke from being too cold or too much fuel. Both are correctable problems though some old Detroit’s just seem t like making a mess. I suppose we should distinguish between and old Detroit and more modern 4 cycle diesels. I don’t think you would have seen any problem with anything modern and a brief WOT operation will usually clear any carbon that might develop. I’ll guess those engines you ran were not at required operating temp.
Bruce
Here are 3 pic showing the bottom of my boat.
The boat is a 38 Ocean Alexander Aft cabin it has twin 135 Ford lehman diesels. Is the boat cruises at 8 Knts 1800 rpm. Can you tell if this is concidered a true trawler ?
Here are 3 pic showing the bottom of my boat.
The boat is a 38 Ocean Alexander Aft cabin it has twin 135 Ford lehman diesels. Is the boat cruises at 8 Knts 1800 rpm. Can you tell if this is concidered a true trawler ?
Not in my opinion. There is a some hull rise toward the stern for reduced drag but those hard chines and square transom are intended for speed. IMO yours is a hybrid like may others that would plane if given enough power. A true displacement hull such as the Willard pictured above would not plane regardless of power. Bert that doesn’t mean that you dont have a nice boat that is great for the type of boating most of us do. You could even add active stabilizers or paravanes (ugh) to reduce rolling.
I think that in a naturally aspirated engine there has to be some consideration also for when the primary kicks in. As long as the carb is drinking air from only the secondary throats there is better fuel performance. So some combination of hull speed and avoiding primaries may be a good indicator for us gas guzzlers. I can rig a hose from each carb and listen under power to learn when I am opening the primary then make an RPM reference, see where it relates to speed. As long as it isn't prohibitively slow...that may be my best fuel performance speed.