Powering a slippery cat with 20 hp. Fuel injected or carb.

Discussion in 'Outboards' started by Russell Brown, Feb 15, 2020.

  1. Russell Brown
    Joined: Jul 2012
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    Russell Brown Junior Member

    I'm nearing completion on a 23' very lightweight camp-cruising outboard powered catamaran. It was designed with the older Yamaha F-20 in mind. Now the 20 is fuel injected and heavier. The new 25 is the same size and weight as the new 20. 20 hp is plenty of horsepower (I have experience with a slightly smaller version), but using a heavier outboard doesn't sit well. Fuel efficiency is super important with this boat, but it's really hard to find data on outboards this small. Is the fuel injected 20 really more fuel efficient? I'll be operating at half to 2/3rds throttle a lot.
    I found a new 2017 carb model that I could buy. My buddy put 10,000 hours on one.
    Thanks for your thoughts.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

  3. DCockey
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    DCockey Senior Member

    According to this article the Mercury 20 HP and Suzuki 20 HP both have fuel injection and weight substantially less than the new Yamaha F20. Yamaha F20 and T25 Outboards Revealed - boats.com https://www.boats.com/reviews/yamaha-f20-and-t25-outboards-revealed/
    Key competitors in the 20-horsepower class include the Mercury FourStroke 20 and the Suzuki DF20, both of which are also equipped with an EFI system that does not require a battery. Both also weigh less than the 126-pound Yamaha F20—the Mercury is 99 pounds and the Suzuki is 97 pounds (all weights are for the lightest available model), or about 25 percent lighter than the Yamaha, a significant difference on a small boat, or if you really need the motor to be portable. The Yamaha advantage is displacement, with 432cc compared to 333cc for the Merc and 327cc for the Suzuki. I would expect the Yamaha to offer more low-end power to move a heavily loaded boat on plane.

    The old F20 was 2 cylinders and had a lot in common with the F15.

    The Mercury 15 HP and Suzuki 15 HP are both 2 cylinder engines. Specs are similar but one is a 2-valve engine and the other is a 4-valve engine. The Mercury 20 HP specs are the same as the Tohatsu 20 HP and I've previously heard that Mercury sources their small engines from Tohatsu.
    FourStroke 2.5-25hp | Mercury Marine https://www.mercurymarine.com/en/us/engines/outboard/fourstroke/25-25hp/
    Suzuki Marine - Product Lines - Outboard Motors - Products - DF20A_EFI - 2012 - DF20A_EFI http://www.suzukimarine.com/Product%20Lines/Outboard%20Motors/Products/DF20A_EFI/2012/DF20A_EFI.aspx
    TOHATSU outboard motors https://www.tohatsu.com/outboards/20_4st.html

    My guess is an EFI engine should provide substantially better efficiency at part throttle than a carbureted engine. The EFI system should be able to more determine and deliver the amount of fuel needed than a carburetor. Also carbureted engines tend to run somewhat richer (more fuel) than EFI engines are part throttle. These are general trends and controlled tests would be needed to make any definitive statements.
     
  4. Richard Woods
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    Richard Woods Woods Designs

    I would have used the Suzuki 20 efi on our Skoota had they been available when we built. so we used the Tohatsu 20s instead, conventional carbs. Never had any problems with them. But they weren't strong enough to survive Dorian unfortunately - one leg broke in half. I agree with Davids last para above, but I'm not a marine engineer

    Look forward to seeing pictures of your boat afloat, sadly probably not in real life as not sure when we'll get back to PT

    Richard Woods
     
  5. Russell Brown
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    Russell Brown Junior Member

    Wow, what a response! I didn't know people even looked at this thread.
    My inclination was to try the Suzuki DF 20 because of they seem the most serious about weight and fuel economy, but in the remote (not tiller handle) they only offer power trim/tilt. The power tilt bracket is much deeper, which gets in the way of the fairing built around the motor. This is a single engine catamaran with the engine in the middle mounted to a sled with a fairing attached which protects the motor and splits the water for the leg of the motor. It's mounted on a sled so that it can be lowered or raised to adjust the prop immersion. Anyway, the trim/tilt bracket won't fit. I have thought about modifying a Suzuki, but not sure about that. I do need electric start and shift/throttle controls, but not steering. This boat steers with small rudders.

    The Suzuki is about 10 pounds lighter than the older Yamaha 20, but your last paragraph, DCockney, makes me lean toward fuel injection. I just wish there was real data for small motors. I tested our F-20 at 2 gph at full throttle (6000 rpm), 1.36 gph at 5000 rpm, .89 gph at 4000 rpm. This was our testing.

    Hello Richard, good to hear from you. Where are you? What do you hear about the small Suzukis? Some cooling issues with them in my neighborhood.

    Thanks so much to all of you for the help. I will post photos.
     
  6. Richard Woods
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    Richard Woods Woods Designs

    As you know, we lost Skoota in Dorian when it was stored ashore in the Abacos. The cabin was ripped off. It was strapped down but the deck ripped out round the cleats so at some stage the port hull flew. I knwo that because there is an engine UNDER the port hull. I went to see it a couple of weeks ago

    Right now I am on a Searunner 36 catamaran built by Jayne Marine in the Chesapeake. I assume a Marples design?? The owners lent it to us after hearing about Skoota, we are currently in St Augustine but start heading north back to NC in about 10 days. It has a Honda 15 on a sled, 5.9 knot cruise speed. Ropes to tillers to turn it

    we did 6000 miles on Skoota no problems with the engines except a powertilt solenoid failed after 3 months, replaced under warranty. I can understand the potential for cooling problems, everything has to be very smallbore as its so light and thus needs cleaning/flushing regularly.

    Our Tohatsus were a bit more economic than your Yamaha.

    RW
     
  7. DCockey
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    DCockey Senior Member

    Suzuki may offer a relatively inexpensive kit to modify the tiller version of the engine for remote steering and control. I know Mercury offers such kits for some of their engines. Modifying a rope start engine to electric start may be expensive.

    Tohatsu 20 HP dimensions and drawings are available at https://www.tohatsu.com/outboards/img/2be08e052937b1155f56a272c90353a24af771ce.jpg

    Fuel consumption varies with load and speed. Load varies with boat speed and depends on the boat. Propeller differences can also affect fuel consumption with boat and engine speed. So it may not be meaningful to compare fuel consumption data for different engines on different boats, particularly at less than max engine speed.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    I pointed you to "real data" and you seem to have not looked at it.
     
  9. Richard Woods
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    Richard Woods Woods Designs

    With respect, I imagine Russell has looked at that page. I certainly did when I was first designing low power catamarans. The catamarans listed are all planing high power types. Pontoon boats are nearer the mark, but as we all know they are horribly inefficient.

    And of course it only compares one Yamaha to another, not a range of different outboards

    RW
     
  10. DCockey
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    DCockey Senior Member

    The only data for a 20 HP engine in your link above is for a carbureted engine on a Zodiac inflatable. PB_ZNA_310BayRunner_F20LPA_2014-11-18_INF | Yamaha Outboards https://yamahaoutboards.com/en-us/home/outboards/25-2-5-hp/25-15/f20/pb_zna_310bayrunner_f20lpa_2014-11-18_inf

    There are 13 tests of 25 HP engines. 8 of the tests are of the F25LA which is list on test as having carburetor. One is of the F25LC which is listed as EFI. Four tests are of the F25LEHA but no indication if it has a carburetor or EFI. The boats vary from a 312 lb aluminum skiff to a 1545 lb pontoon boat. For the reasons I mentioned above I don't think it is possible to draw any conclusions about the difference in fuel consumption between carburetor and EFI engines from this data.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Naturally EFI is going to be better fuel consumption, but in the range he says he uses (1/2 to 2/3 throttle) it is more advantageous, as the boat test comparisons show. Maybe 20% better. Throttled up close to maximum revs, less than 10% better, probably. The actual fuel cost difference would be minimal, you might use a gallon more on the day, but engine choice might be a factor if range is the issue. You are paying more for EFI, and it is less DIY friendly.
     
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  12. Russell Brown
    Joined: Jul 2012
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    Russell Brown Junior Member

    Richard, I had no idea that you had lost Skoota. I'm so sorry. That was a really great boat and a good home too.
    I don't seem to have an e-mail address for you. I'm working on a design I'd like to get your take on.
    The boat I'm building now makes me really appreciate plywood. It's all composite and I'm really not impressed with how much work it is for such a small weight savings. All the plywood boats I've built are still in fine shape and make so much more sense.

    Mr Efficiency, the data on small outboards is so rare and questionable. Apparently the fuel flow sensors are better for gas guzzlers. Small outboards are such a small part of the market and it's hard to find data. My buddy put over 10,000 hours on an older F-20 Yamaha before parts started falling off of it, which is why I was thinking about buying the older carb model, but my experience with that motor is that it's a bit of a guzzler for 20 hp. I will be using this boat for long range camp-cruising, so both fuel consumption and reliability are factors. Both old and new (EFI) Yamahas should be good, but not so light. I'll look into modifying a Suzuki.
     
  13. Richard Woods
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    Richard Woods Woods Designs

    Russell, here are a few recent Skoota photos....

    skoota wreck1.jpg

    skoota wreck 2.jpg

    skoota wreck 3.jpg

    But its worse for those on shore especially in Marsh Harbour

    My email address is woodsdesigns@gmail.com so hope to hear from you soon

    Richard Woods
     

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  14. Mr Efficiency
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    Mr Efficiency Senior Member

    The risk with EFI is fuel contamination with water and sediment, I think you'd need better protection with traps etc. compared to carbs.
     

  15. fallguy
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    fallguy Senior Member

    Not to muddy the water, but what about Evinrude 15 h.o.?

    I think it has most of your needs.
     
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