ETEC Low Speed Operation

I am building a displacement boat with outboard power. Hull speed formulas/ hp required say 10 hp for 6 knot hull speed for a straight shaft diesel. I know it is more than I need but the Evinrude etec 40 has 3:1 reduction. The efficiency of the reduction and running at low speeds would be very quiet and effieient. The engines are fuel injected of course but how would they perform at low speeds for extended periods?

 

News to me that it is 3:1.

 

ETEC engines run fine at low RPM for extended periods. If your boat needs 10HP it doesn't matter where they come from. It is inefficient to grossly overpower a boat.

 

The online spec sheet for the 40 lists gear ratio as 2.90:1. Gonzo do you think what I propose is grossly overpowering?

 

Personally I'd look for fuel efficiency in the speed range targeted, this will give some idea of where an engine is 'happy'. Mostly this is around max torque, or just under in fact, so a well designed (non race) engine should be pretty useful from low down to that point.

I don't want to preclude Gonzo and his wise advice, but I think that is what he iss getting at. This means not having too large an engine burning more fuel at very low rpm compared to a smaller one running faster but in a more efficient window.

Sorry if I have contradicted you Gonzo.

 

OK, that does surprise me, I'd assume from that they are using a larger gearcase than previously.

 

the oil injected outboards reduce oiling at idle/low rpm to reduce smoke. read as no oiling, you would eat up the engine. like was stated you'll want engine operating mid throttle for desired speed to gain reliability and economy. I would look for trolling motor engine setup.

 

Thanks everyone. I was exploring possibilities and it doesn't look like the 40 ETEC is a good idea. I will probably go with a Yamaha High Thrust 25.

 

I went through all the 25Hp motor specs a couple of years back researching the best for fuel efficiency at low rpm. Not necessarily 'high thrust' but a lot can be done by prop selection. At the time the then new Suzuki triple was the best, claiming about 11-15% less fuel at idle and first 1500+ rpm up. After some 700+ hours with said engine type, i can confirm it is pretty good on fuel, will accelerate a light hull or heavy (800Kg Whaler type) one extremely well to planing speed and beyond.

Data from engine use prompted the search, as I knew we were using engines for 70-80% of the time at idle or within less than 2500 rpm.

It may well be another company has a better engine now. Weight, charging ampage etc all come into play as well as low speed operation. Previously I found the Honda a little low on charging, important if you power an onboard radio or other supplementary drains - tilt, echo sounder? etc.

My personal experience of somewhat older Yamaha engines has been that the gearboxes are prone to damage if not driven by competent drivers. Otherwise they seem fairly competent, but I'm not au fait with the very latest offerings of theirs.

Of course I also realise that access to a dealer and support is aslo a factor so consider as many things as possible. I'm sure you'll be OK whichever manufacturer you choose.

 

The Yamaha T25 has a 2.42:1 gear case versus the Suzuki 2.09:1 case. The Yamaha will also accept props to 12.25" versus the Suzuki 11". I do like the fuel injection on the Suzuki, the Yamaha is carbureted. EFI might give the Suzuki an edge it is hard for me to say.

 

Eat up the engine ? I don't see that happening, what about boaters who clock up lots of hours trolling ? Running at low speeds lightly loaded the need for lubrication is greatly reduced. Not that I think it is a good idea, but if he just happens to have the engine lying around, it could do the job. It had certainly piqued my interest that this engine now runs a 2.9 to 1 box. It was 2.66 to 1 not long ago, I assume they have a larger box off the higher hp ranges now. Which would make it a good one for slower heavier boats.

 

If you can get to try them, in terms of different engines do so. The triple cylinder set up is definitely smoother at idle and low speed than the rocking couple inherent in a twin set up. The Suzuki is a 120 deg crank arrangement, I've been through the parts list/manual as it's not obvious (or in the general literature) and could have been a 180. It is certainly very quiet and smooth compared to any twin 25 I've used. I also value the battery less ignition pull start as standard, very useful if you have a flat battery. Similar to one of my old Italian M/cycles... BTW the older V twin Suzy 25 is a bit of a dog, as they never got the carburation right on the V twin, it really needed 2 carbs not one! so do not go there.
A lot of slightly larger engines say 40Hp like Mariner, Yamaha etc are triples too, partly for sweetness and balance as well as compact arrangement.

As Mr E says it is possible that the g/box is from a larger engine model, some guys use the same for 40 as 25. If your powering a heavy double ender say, at constant low speed in a seaway you probably won't beat an inboard diesel though with a chunky flywheel!. As long as the 'box is up to the job and the change sweet, surely the box ratio is less important? The ratio is chosen by the manufacturer to optimise several factors, economy, acceleration and constant speed running being among them. The trade off is good at low speed but too slow or struggling on acceleration and over geared at top end. I've not known of any gearbox wear problems from constant low speed running though.

I would definitely test the quality of gear change on a second hand Yamaha....

Yamaha tend to use Mikuni carbs. They clean out and set up OK, similar to the Keihan ones on some older Mariners. AFAIK they are OK on the twin cylinder. The 4 cylinder 4 carb versions can have problems with the single accelerator pump after some years. General access on the Yam's is mostly pretty good for getting at things, some of the newer Mariners are a PITA.

Had one guy with big Yam and a rough idle problem a couple of years back.
The fun bit was finding on a 115Hp 4 carb engine that one of the mixture screws was missing the conical fine end... about the last 4mm of it....

 

Etec will idle forever using way less fuel than anything else

 

It would help if you knew the prop diameter, pitch, and rpm used with the diesel. On a slow boat, if you are swinging a smaller prop, chances are you will need more horsepower and rpm to get the same performance.

other factors:
-efficiency of prop design I'm pretty sure there can be big differences here, though I can't quantify them. This is apart from the diameter issue.
-what's upstream of the prop? In general, the more uniform the water flow coming into the prop, the more efficient. Also, if the stern tapers in steeply, having a prop in the right place may prevent separated flow, which can cause drag. Not a problem, I imagine, with most displacement hulls, but perhaps with some*. For inboards, something like a lobster boat or a sailing auxiliary is likely to have a whole bunch of stuff just upstream of the prop, and I'm sure that doesn't help.

If 10hp can get your boat to hull speed, a 40hp motor is going to be far less efficient. Generally, internal combustion engines are going to be more efficient, in terms of fuel flow per horsepower, when the throttle is open. In this case, though, a small engine might need to be geared down so that it can swing a big prop, or the inefficiency of the small prop will eat up the gains from the small motor. I am now imagining a small Briggs and Stratton industrial engine sitting on top of the lower unit from a large outboard motor, geared down with a belt drive. Probably not worth the trouble, and noisy. And the lower unit will be oversized. Sigh.


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* Warning: going off topic here: Papers by a guy named Goldschmied explore how to make this work to lower drag even more, but I think he only analyzed racially symmetric hulls. Some of his papers are at the CAFE foundation web site, but I doubt this is of any relevance unless you do a submarine! Fun to think about, though, at least for me, though I can't say I've really read through with complete understanding.

 

Usually, engines are specified slightly of more power than necessary. However, that is 15-20% at most. Overpowering by 400% is a lot. It is not economic to run a large engine at low loads. The friction of the large engine, plus the extra weight will cause it to use more fuel than a small engine. I have seen the ETEC engines at the Sturtevant facility idle for days. They have engine mounts on the water where they run them for testing.