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Thread: Dual vs single OB of same total HP?????

  1. #1
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    Default Dual vs single OB of same total HP?????

    OK guys, I'm boat stupid but trying to learn.
    A friend from out of state has an older 21' Starcraft that he leaves with me to babysit for him the 51 weeks a year he's not using it. He keeps telling me to use it & I finally want to start taking advantage of the situation, probably using it for black bear out of Homer or Wittier more than anything else.
    I'm sure I'll have a slew of questions for you but here are a couple.

    It has twin 40 hp Yamahas on it right now.
    In actual performance how will the two 40s compare to one 80hp?

    The props are 11.5 x 11. One guy a couple of years ago that did the annual tune said that's the way to go. The guy that's getting it ready for winter this year said that's nowhere near enough pitch for what he's using it for (running out of Homer, Seward, or Wittier for halibut silvers, etc.) & recommended a pitch of at least 13, maybe 15 to make best use of the boat & get reasonable fuel consumption.
    When he's out he usually has 5 folks in the boat, along with about 50 gal of fuel & misc.

    Thanks for the help guys.
    Vance in AK.

    Matthew 6:33
    "But seek first the kingdom of God and His righteousness, and all these things shall be added to you."

  2. #2
    Member Alaska Gray's Avatar
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    The best thing about twins is, If one goes bad on you while you are out there you have a back up. Good question, I feel that with twins you have better control over your boat. I don't know if both engines will way more then the single 80 hp.
    Living the Alaskan Dream
    Gary Keller
    Anchorage, AK

  3. #3

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    A single 80 hp outboard will always give you better top speed than twin 40's. This is because twins double the drag and increase your transom weight. Twin 40's would probably give you a better hole-shot, if water-skiing is what you do.

    In Alaska, the main advantage to twins is safety. But...keep in mind that many outboard failures are fuel or battery related. True safety with twin outboards rely on isolated fuel and battery sources for each. Also, unless one of the twins can get you on step alone, you are no better off than a single with a kicker for back-up. Twins require double the maintenance, and double the repairs if you hit something. Don't forget more fuel consumption too. On a boat your size, maneuvering advantages aren't an issue, as the outboards are too close together.

    Correct propping is a factor of rpm, load, and what you intend to do with the boat. It can be complex, but basically you want to run the maximum pitch possible while maintaining the upper end of your outboard's rpm range. A Yamaha 2-stroke 40 hp has a maximum rpm of 5500. A Yamaha 4-stroke 40 hp has a maximum rpm of 6000. If you are achieving that rpm at wide-open throttle with your load, trimmed out, then your propping is correct. If you are exceeding that rpm, then you need more pitch. If you are not reaching that rpm, then you need less pitch. Sometimes you can increase speed without decreasing power much by increasing pitch and decreasing diameter. But that is a matter of trial and error propping, as most perfect propping is.

    A 21' Starcraft with twins, 50 gallons fuel, 5 people, and gear, is a pretty good load. So your 11 pitch prop looks ok. Going up might drop your rpm.

    New outboards are extremely reliable. Especially the 4-stroke fuel injected. If that were my boat, I'd have a single 90 hp Yamaha 4-stroke with a Yamaha 8 high thrust kicker. Nobody makes an 80.

  4. #4

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    Well said Big Water.

  5. #5
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    Thanks for the help guys.
    Big Water, I know nobody is making an 80hp, but wanted to compare apples w/apples
    I understand your answer that unless you can get on step you are better off with one larger engine & a kicker. I tried toi follow an earlier discussion on hull displacement speed or some such. I'm not sure if the boat will or won't get on step with one engine when trimmed right. I've only had it out once (On Skilak lake) & at that time it had one engine that wasn't running great (long story but bent crank). The engine that was running well decided to have intermittent electrical problems & wouldn't atart at one point so we headed back to the dock on the one poor engine. Couldn't get on step, but had 1200# of people, & 30+ gal of fuel on board along with the dead engine trailing in the water (didn't think to lift it).
    The dead engine restarted a little later & ran great saving the day (it was a starting problem, not a running problem) & letting us play as long as we didn't shut down. It was a rare perfectly flat day out there.
    All that to say I've always liked the idea of twins for safety as Alaska Gray mentioned but understand the single with a kicker.
    The point is moot though as it's not my boat & I can't afford to repower it for him. His theory (and I can't fault it) is there are getting to be a bunch of these 40hp Yamaha 2 strokes around cheap because of the rules on the Kenai so cheap spares abound for him. The one with the bent crank is getting a new (to him) power head for $400 including labor & I just picked up (for him with his $) another one that runs good for a spare for $600.

    Thsnks for the info on props. The engines don't have tachs, but if a guy could find a couple of cheap ones they would probably be worth having to get it figured out. Right now the boat will cruise close to 30kn.
    Vance in AK.

    Matthew 6:33
    "But seek first the kingdom of God and His righteousness, and all these things shall be added to you."

  6. #6

    Default Hi Vance...

    I was a pretty staunch proponent for twin outboards, but it seems I've been swayed as lots & lots of boats in Alaska go with a single w/kicker I've seen. It's common here in Hawaii for lots of boats to go with twin outboards, not sure why, but it just seems like that's what a lot of guys do. It may have something to do with the style of fishing, as trolling big lures is common here, and usually that's done at 7-10 knots. Maybe the twins work easier at that speed than one single. I do see a few boats with a main/kicker. I'm always contemplating getting a saltwater boat for Seward, Homer, Deep Creek and I'd probably just stick with a single w/kicker. I guess it depends on the boat and how it's rigged, as my budget would most certainly lean me towards a used boat. The point of one of the twins being able to get the boat on plane makes a ton of sense. As was mentioned before, it would make the most sense if running twins to have dedicated fuel tanks, batteries, etc. for each motor.
    Jim

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