Gas economy

[firstlight]

Ok.....gas is $3.75/gal at the marina. I'm thinking the Honda 4 cyl (specifically a 90hp)is a marine bred Civic motor. Most cars get their best gas mileage at 2500 rpm....is this also true for outboards? Would it be most economical to put a high pitch prop on and keep rpms low. I realize this would kill top end, but at my age, I'm not really in a rush to get anywhere anymore. Any other ideas on fuel economy (besides getting a sailboat) ??

 

[ziemann]

Re: Gas economy

Lugging your outboard is not going to give you the results you desire. Prop your motor for peak RPM then experiment until you find its "sweet spot". All outboards have an optimum cruising RPM that gives its best fuel mileage, and I am going to guess that the "sweet spot" for that Honda is somewhere around 3500-3700 RPM's. <br /><br />You can install a fuel flow meter if you like to find that exact spot. Hondas are fantastic on fuel to begin with, so we are really splitting hairs here.

 

[MajBach]

Re: Gas economy

Firstlight. You're failing to realize that at 2500 rpm in a car at the 'sweetspot' (I've never heard this what I suspect to be a myth), the car is NOT running at full throttle, which you propose to do. The car would neither be generating anywhere near it's top horsepower. In a boat, since therer is no transmission, your speed is almost directly dependant on your throttle and thus your rpm. This is why mileage is so bad in a boat and a marine engine never lasts as long as a car - the loads are SO much greater on the engine. <br />YOur idea to max out at 2500 rom would be not unlike taking all but 10th gear on a ten speed bike.Imagine how your legs would feel if you took off from a stop in thenth gear everytime. This is what Z-man is referring to as 'lugging'<br />Your outboard achieves (at full throttle ONLY) its maximum horsepower at 5400 rpm. In other words, if you find a prop that maxes out at 5400 rpm, thats your fastest top end ( on my boat - also a 90hp HOnda - this is 41 mph with a 19" prop). But becasue i rarely drive my outboard at WOT, I give my engine a break and run a 17" prop most times.

 

[imported_Curmudgeon]

Re: Gas economy

I'm thinking the Honda 4 cyl (specifically a 90hp)is a marine bred Civic motor.<br /><br />Well, they're both four strokes, but that's about the only similarity! <br /><br />Most cars get their best gas mileage at 2500 rpm....<br /><br />Most outboards don't, even if there was a collary, which there isn't! <br /><br />Prop your motor for redline RPM at WOT, then run it at about 66% of that ... you'll be closer to best economy than you can measure.

 

[scotty c]

Re: Gas economy

i think you mean "corollary" but i knows <br />what y'all talkin'about !

 

[vikingham]

Re: Gas economy

I think the "sweet spot" will vary considerably depending on the hull....for a trawler it could be at 1500, for a semi-displacement 2500, and a deep V 3500 or more. If your engine will reach the proper RPM wide open, I'd leave the prop be and do a little experimenting!<br /><br />In the Coast Guard course I took, an engine designer/mechanic taught the session on engines. He suggested that everyone try to find the lowest planing speed for their boat (assuming a planing hull) and use that as their baseline cruise speed, since it's the most efficient high speed mode. His method for finding it was to get the boat up on plane at moderate speed, then keep backing off a few hundred RPM at a time, adjusting trim as you go, until you can't go down any further.<br /><br />After trying this, we've found that our Honda 90-powered modified V (which is designed to plane at relatively low speeds) does very nicely at about 3200-3300, running around 11 knots or so. Better than 5nmpg in decent conditions.<br /><br />Which, now that I think about it, it pretty darn close to Old Curmudgeon's 66% of WOT, which in my case is about 5200!

 

[MajBach]

Re: Gas economy

Vikingham: You brought up some interesting points which I 'm glad to have read. When I first took possession of my boat, I researched several other boats through books and magazines, the same size and hp, all with speed, throttle and mileage tables. I noticed that their best range was slightly above lowest planing speed [ not unlike yours ], which, to this day I have had a hard time comprehending. My boat is quite short and wide and also very stern heavy(due in no small part to the max hp O/B and the fact that it is a 4-stroke). For this reason, it does not plane at slow speeds. I'd say 15-18 kts (around 3200 rpm) when lightly loaded - even with a hydrofoil. I've never believed this to be anywhere near an efficient speed to run despite what I have read and the example you have cited. My basis for this is because the transition from ploughing to planing is not stark plus - even though it is clearly planing, there seems to be quite a bit of wake. <br />But regardless of how my particular boat behaves at slow speeds, I still find it a tough concept to grasp. Is it not true that any planing hull would likely require a full trim-in [bow-down] attitude in order to achieve its lowest planing speed? Is this not inefficient since you are pushing more water? I have always assumed when driving my boat, that its most efficient speed [best range] while on the plane is at the point where, as I reduce throttle, I have to trim bow-down in order to keep it from dropping off the plane without adding a bit of throttle. In my boat, this is around 20kts. I can keep my trim the same as I would for twice that speed at this throttle; any slower and I would have to trim the nose down. <br />I suppose I judge efficiency as being directly related to the wake I throw and to some degree, my trim setting. I am not disputing what you stated. Rather, I'd like a little more insight into determining all the variations that affect low speed efficiency.

 

[vikingham]

Re: Gas economy

This is an interesting topic, Maj - glad your raised it! <br /><br />Some of this gets beyond my limited knowledge of hulls and hydrodynamics....but hey, that shouldn't stop anyone from commenting, right? <br /><br />The *most* fuel efficient speed will always be displacement speed - when you're not planing, but just pushing the hull through the water without making a big bow wave. Trawlers always operate in this mode. Only trouble is that you're limited to pretty low speeds (say 6 knots for a 20-foot boat), which are a function of the length of the hull (i.e., a longer hull can go faster in displacement mode than a shorter one). That's why planing hulls have been developed - you use some of the engine's power to generate lift, which then reduces the amount of water you have to push out of the way. I think the reason a low planing speed is most efficient is that you're giving enough gas to generate the necessary lift, but not spending extra to increase the forward speed, which is almost never a good deal in terms of efficiency.<br /><br />You mention:<br />>Is it not true that any planing hull would likely require a full >trim-in [bow-down] attitude in order to achieve its lowest >planing speed?<br /><br />I can't say definitively, but I'm pretty sure that this would vary from hull to hull - there's so much variation and so many tradeoffs at work. Typically you do trim the bow down to go on place, but there can be a difference between getting on plane and staying there.<br /><br />Depending on the shape of your hull, planing at 15-18 knots might be perfectly reasonable (if it's got a deep V with lots of deadrise, for instance). Hull shape also affects what sort of wake you generate - a flatter shape will produce less than a V-shape. <br /><br />You also mention that there's not a crisp transition into planing - again, this is a function of the hull and how it's trimmed. Lots of boats, especially deep Vs that are designed to run fast in heavy seas, have a significant "in-between" zone where they simply squat and dig a hole in the water. You might try putting some weight up in the bow and seeing if this helps; this can also help a flatter boat cut through chop more effectively. <br /><br />Anyway, sorry for the rambling respoinse...but hey, what kind of hull are we talking about here, anyway?<br /><br />All the best, and good luck!

 

[Silvertip]

Re: Gas economy

There are two ways to view fuel economy. 1) gallons/hour and 2) miles/gallon and the type of hull certainly makes a difference as was suggested. A 50 HP Yamaha four stroke for example on an aluminum hull fishing boat shows 3.65 MPH at 1500 RPM. It burns .265 gallons/hour at that speed which equates to 13.77 Miles/gallon. Its sweet spot is at 4000 RPM where it burns 1.95 gallons/hour at 20.5 miles/hour. That equates to 10.51 miles per gallon. So -- the story here is that at 3.65 MPH it would take 5.61 hours to travel that distance. Over that time you would burn 1.48 gallons of fuel. 1.95-1.48=.47 gallons saved by running slow, but you spent four hours more getting to your destination and added that running time to the engine. That's not economy in my book. By the way, the figures came from Yamahas web site.

 

[MajBach]

Re: Gas economy

UPinsmoke: That's why I referred to 'while on the plane' in my post, since I do know that at idle, range is typically better. But there too, many more factors come into play.<br />In aviation, we make the distinction between 'best range' and 'best endurance', the latter being how long you can stay in the air without worrying about how far you can go. I would submit that measuring engine output in terms of gal/hr is more synonomous to best endurance and only useful in displacement modes.<br />How far one can get while idling [trolling] isn't always dependant on gal/hr and thus the math can be flawed. The reason for this is due to wind and currents. If you are travelling at 3.65 mph into a 3 mph current, you are getting far worse 'range' than if you were at WOT. Of course, if the current is in your favour, your range might rival that of a good car. But I also believe that while in the displacement mode, a boat's 'efficiency', i.e distance travelled vs. fuel burned drops very rapidly as increase your speed, - especially if it is a planing hull - and one's mileage, or range, drops to it's worst point even though gal/hr are still quite low.

 

[vikingham]

Re: Gas economy

Maj, you're exactly right when you say:<br /><br />"...while in the displacement mode, a boat's 'efficiency', i.e distance travelled vs. fuel burned drops very rapidly as increase your speed, - especially if it is a planing hull - and one's mileage, or range, drops to it's worst point even though gal/hr are still quite low."<br /><br />As you go from zero to top displacement speed, mileage slowly increases, peaking at top displacement speed. Then, the hull starts digging in and generating a wave, which eats up energy (supplied by gasoline) without moving you much faster. This situation gets worse and worse, until you hit the point of worst possible gas mileage.<br /><br />But then, if you add a tiny bit more push, the hull finally generates enough lift to get up on plane. All the water resistance goes away, and you zoom along twice as fast while burning roughly the same amount of gas (somewhat less effiicently than at top displacement speed, but with a much better tradeoff, as Upinsmoke notes). As you accelerate past this point you gain speed, but lose fuel efficiency.<br /><br />And YES, wind and current have a huge effect, whatever mode you're running in - sea conditions, too.<br /><br />To get back to your original question...I hadn't ever thought about this, but I bet you're right that the ideal would be for the engine to put out its maximum horsepower/gallon right about at the planing speed of the hull. And it's interesting to think about doing this by re-propping! I am NOT a mechanic, but I suspect that to work right it would require a motor that was designed (or at least tuned) to operate over the narrower RPM range. <br /><br />Outboards have evolved over the years to be used on many different hulls, so we're all probably compromising one way or another, but we're also getting the advantages of relatively high-volume production.

 

[firstlight]

Re: Gas economy

Sorry to have posted this topic and then taken off fishing for the weekend. I did test my theory with a the installation of a Navstar fuel transducer and a new tach. My Honda 90 is propped (by the previous owner) to max out at 5900 rpm with a 13 pitch prop. My gas usage at 2500rpm is about 1.6 gph at 6.2 mph. Up that to 3500rpm and I use about 4.2 gpm and go about 9.5 mph. I get on plane (I've a downeast design that fights getting on plane) at about 4500 rpm and can trim up and keep her there at about 4200 using about 5.3 gph at about 16 mph. Any higer and fuel economy goes down the toilet. My basic math tells me (at least on my semi-displacement hull) to keep her slow. By the way, at trolling speed, my fuel transducer barely registers and I've got to look at the tach to be sure the motors even running. Sorry about the confusion...I certainly wouldn't prop a motor to max out at 2500 rpm...my gut feeling was that if my best economy was in that range, perhaps going from a 13 to a 15 pitch might be more economical. Right now I'm feeling pretty good about my Honda so I think I'll leave things alone and concetrate on getting by the bluefish and into the bass..... Thanks for the responses.

 

[MajBach]

Re: Gas economy

How much are one of those fuel transducers? Sounds like a good toy to have? EAsy to install?

 

[firstlight]

Re: Gas economy

MajBach....here's a link to where I bought mine. I originally had one on my inboard Sisu which worked great. The Honda sips so little gas at idle that the flow meter doesn't register. This might skew your readings a bit. All in all, a handy device for fuel flow and economy (and one more cool thing to look at.) On a scale of scale of 1 to 10, a 5 to install.<br /> http://search.store.yahoo.com/cgi-b...n+fuel+flow&.autodone=http://boatersland.com/

 

[ziemann]

Re: Gas economy

I am always amazed at the collective intellect that resides here on these boards. There was some fantastic analysis in this thread- perfect for a geek like me. Thanks guys!