Best 10hp and prop for kids hydroplane

Discussion in 'Outboards' started by MattM, Nov 20, 2010.

  1. MattM
    Joined: Aug 2004
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    Location: oregon

    MattM Junior Member

    My 10 year old son and I are going to build a wood hydroplane for him over the Christmas break. In Oregon, he can only drive up to 10 hp. I'd like something reliable. I don't have much outboard experience. Can I get a 20 year old 2 stroke and be OK, or should I get a newer honda or other 4 stroke? Any specific recommendations?

    Also, I imagine I'll need a higher pitch prop than will come w/ a used trolling motor. What do folks do for that?

    thanks, Matt
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Reliability is purely a function of how well you take care of it. A 50 year old 2 stroke, like my 1960 Evinrudes can be perfect running little machines. You keep them clean, repaired, in tune and regularly used and they'll keep you happy.

    Anything less then this and they will become progressively more cantankerous, eventually refusing to start. Pretty much the same thing if you neglect your wife's needs.

    The 4 strokes are clean, reliable, much less noisy and easier to maintain (to some degree), but they're also heavier and more costly then a used 2 stroke. Depending on when your hydro was designed, it may not like the additional weight of a 4 stoke, though in recent years the manufactures have been reducing the weight difference a good bit.

    If it was me and the design could tolerate the weight, I'd go with a 4 stroke, just because they're clean and soft spoken.

    You may have to play with different props, to get which ever engine you select to hit her target RPM. Look around in your area and see if any class racers are active (they probably are), these are the folks that will get you "dialed in" with props and other gear.

    As far as brands, they all preform well if cared for, though some are engineered better then others. A few Chinese brands have cropped up and these seem to be real junk after some hours on them. So, stick with the major brands and you'll have a good time.
     
  3. michael pierzga
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    michael pierzga Senior Member

    Its been a few years since I bought a small outboard. For lightweight yacht tenders you need a lightweight powerplant. The two stroke 8 hp Yamaha outboard was the highest power to weight outboard available at the time...say 4 years ago. 26 kgs. and a great reliable little motor. A small hydro would also want the highest power to weight ratio powerplant. If you see one for sale pick it up.

    http://www.outboard-engine.com/images/yamaha8T-1.jpeg

    http://www.outboard-engine.com/enginespecs.php?recordID=231
     
  4. Yellowjacket
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    Yellowjacket Senior Member

    Unfortunately most of the newer motors aren't well suited for small hydroplanes or runabouts. They are too heavy and make it really hard for a small boat to get on a plane with that added weight aft. They are quiet, reliable and eco friendly, but they are ill suited for a hydroplane.

    If you are going to build a small hydro, either a three pointer or a small runabout, you want a light powerful motor for it to be any fun at all. I had a Yellowjacket racing utility as a kid and we ran a lot of different motors and the 10 hp Johnrudes were pretty anemic. We always came back to the tried and true Mercury Super 10. Super 10's don't have a gearbox, but if you put a hydro or runabout in reverse it will just get swamped anyway so it isn't an issue. Just pull the rope and go. Most of the small hydroplanes that you can buy plans for were designed around the Merc Super 10 Hurricane motor.

    The Mercury Super 10's from the early 1950's were actually 18 hp motors. They were rated at 10 hp to beat the requirement in Florida and some other states that you had to license a boat with more than 10 hp, so lots of people would get a Merc and then they could have a small boat that would go like stink and not have to go through the hassle of getting it registered an renewing the registration every year. They are physically the same block that were in the later Mark 20 engines. With a good prop a Merc Super 10 powered hydo will be a lot of fun. You can find nice unrestored engines for less than $500. If you really want to go fast you can put on a quicksilver lower unit and a hot prop and go 50-60 mph. That will cost over $1,000 but you will have the real thing.

    There are still a lot of Merc Super 10 Hurricane engines around, find a good one and set it up with a deadman throttle. Those engines are pretty much bulletproof, simple and easy to work on they are what you want for a small runabout or hydro.
     
  5. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    Brings back childhood memories !!! I had a mercury Hurricane when I was a kid. Powerful motor. Had it on a big chunky crab skiff. Start that baby up, give it the gas and Id fly across the water in a cloud of smoke. Great because the motor is so stripped down , basic and simple that kids learn engineering when messing around with it. If you come across one , buy it. Its wonderful just to look at !!!
     

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  6. Yellowjacket
    Joined: May 2009
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    Yellowjacket Senior Member

    What boat is it for?

    BTW, What boat are you planning on building?
     
  7. Submarine Tom

    Submarine Tom Previous Member

    Surely there is a modern design for 4-stroke weight distribution.

    Matt, I wouldn't focus too much on the speed until the pilot has a few hours under his belt.

    I don't mean to harp, but please keep safety in mind. Positive floatation, safe attitude, etc.

    Great project, have fun!

    -Tom
     
  8. Submarine Tom

    Submarine Tom Previous Member

    Just one of many hits I got when I Googled: 4-stroke 9.9 hydroplane

    AcidTouch7's Full Review: Mercury 9.9 Big Foot 4-Stroke
    Wow!!!! This thing is silent! Never have I operated such a quiet out board, well except for my electric trolling!-) I first had the little monster on a small hydroplane, for those of you unformiliar with the term "hydroplane," it simply is a type of boat that is designed for all out speed, and actual has two ski like projections in the front called sponsons that it rises upon when moving at higher speeds. This ability to rise out of the water reduces drag tremendously.
    The Big Foot was enough! The tremendous low torque of the engine popped the small boat right up onto plane! Not only was it silent, but the engine was clean! The fermiliar blue/grey smoke of 2-stroke outboards was gone for the most part! The odor of the exhaust was also less noticed.
    The engine was also placed onto a small aluminum fishing boat. The engine was once again shocking me! The silent, low vibration capabilities, of the overhead cam allowed me to troll for salmon unhindered.
    The overhead cam also was excellent in providing a low idle. The engine could be throttled way down during warm-up or when I was casting into my favorite bed of lily pads! This conserved fuel, allowing a small gas tank to be carried, thus making more room for all of my tackle! I could go all weekend on a single tank, which made the engine very cost effective during these gas price hikes!
    Over-all, I was very impressed! If you are looking for a good small engine that is quiet, clean, and efficient, the Big Foot is the way to go!



    Recommended:
    Yes
     
  9. PAR
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    PAR Yacht Designer/Builder

    I'm not sure of your expertise Yellowjacket, but clearly it isn't with "newer motors". As I pointed out, recent 4 stroke designs have reduced weight differences, but the other attributes of 4 strokes are very desirable, particularly the available torque at RPM ratios in comparison to 2 strokes.

    Some of the now dysfunctional hydro classes wouldn't be able to tolerate the weight of the first generation 4 strokes, but if these classes were still active, newer hull designs would permit their use. This is primarily because the 4 strokes are better in most every way you gauge an outboard.

    Yes, some of the engines over a half a century ago where well designed and more importantly way under rated. We forget how difficult it was to keep these engines working and producing ample power. They were finicky, touchy and blew up a lot more then most seem to remember.
     
  10. Submarine Tom

    Submarine Tom Previous Member

    Oh how I hope this thread doesn't turn into another fist fight.

    Both 2's and 4's have their attributes and drawbacks.

    I wouldn't even consider a 2-stroke on one unless I just couldn't find an appropriate hydro design.

    But, once again, only my opinion.

    -Tom
     
  11. Yellowjacket
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    Yellowjacket Senior Member

    A KG7 Super 10 weighs 63 pounds. With a quicksilver lower unit it weighs even less. A present day Mercury 15 hp, which has slightly less power than a Super 10 makes weighs 111 pounds. If you can afford almost twice the weight of motor hung over the back of the boat, then go for it, but most of the hydroplanes that you can get plans for wouldn't work well with that much weight hung on them.

    If you cut down to a 9.9 hp you will have something that weighs about 83 pounds (for the Merc, which is lighter than the Yamaha), which is probably ok, but the boat is going to be lucky to go 25 mph with that motor and it's gonna get old with something that slow. Decent lake toy for the first season, but after that the kid is going to outgrow it and want something faster.

    While four strokes are more environmentally friendly, the old 2 strokes have the new engines beat on power to weight ratio every time. A typical 20 hp four stroke weighs 115 pounds, and an older Merc Mark 55 40 hp weighs exactly the same. That's a power to weight ratio that is twice that of 4 stroke. Sorry, but there isn't any way a small 4 stroke will compete with a 2 stroke. If you run the RPM way up on the 4 stroke, it is possible to make as much power, but keeping valves in the 4 stroke at those speeds is going to be an issue. For real world engines that you can buy the 2 strokes still win hands down.

    You can get a Super 10 and use the original prop and the boat will go about 30 mph, and then as he gets older you can put on prop with some more pitch and the boat will be faster. You can always put in a throttle stop if you think the boat is too fast.

    The old Merc's were, as noted above, very simple engines. Turn off the fuel and run the engine until the carb is dry at the end of the day and they will be about as reliable as a stove bolt. If you leave gas in the carb they will gum up, but there isn't much else that can go wrong on them. Check the points and plugs at the beginning of the season and they are about as reliable a machine as you can get.
     
  12. RHough
    Joined: Nov 2005
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    RHough Retro Dude

    Merc 9.9 2-strokes are restricted 15's. Some of the most popular Merc parts are the stickers etc. to label your 15 as a 9.9 (or so I'm told) :D

    When I was looking at top speed, the highest pitch prop available was good for about 27 knots at max RPM due to the gear reduction lower. I asked my Merc guy and he said that to go faster with the 15 I needed to find a racing 1:1 lower unit.

    I don't remember if they are still available new or if you have to find a used one.

    I agree with the posts that suggest you check the designed weight of the engine. The Glen-L 9'6" Picklefork specifies motors up to 80 lbs. A new Yamaha 9.9 weighs 91 lbs. A Mercury 9.9 or 15 2-stroke weighs 77 lbs.

    I'd go with a Merc 15 and change the stickers to 9.9

    R
     
  13. PAR
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    PAR Yacht Designer/Builder

    If you want to use a cantankerous 60+ year old engine, then go for it. Good luck finding parts and I hope you have a good buddy that's a fine welder and careful machinist.

    Lastly, if you want to run in a 50 or 60 year old 3 point or runabout hull design, I guess you're limiting yourself there as well. About a mile from here are a few dozen of these class racers, that still blast along on the local lakes. All but a few die hards are using the 60's hull designs and 50' - 80's, of course they never win. Most recognize the benefit of modern hull design and outboard engineering, though no one is forcing you to if you'd prefer to remain entrenched.
     
  14. MattM
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    MattM Junior Member

    Thanks for all the feedback. I wasn't familiar w/ the specifics of the trade offs between the 2 and 4 strokes regarding weight.

    In our local craigslist there are 5 or 6 motors that would probably work. Starting at about $700. There is one exception. A '90s Nissan 8 hp that has a new tuneup and waterpump. They want $300 for it. That sounds very interesting even if we use it for a season and then upgrade. I don't see very many Nissans. Is there some reason to steer clear of this? Any experience?

    thanks, Matt
     

  15. michael pierzga
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    michael pierzga Senior Member

    Get it... simple ,lightweight two stroke and Nissan is a good product. Seems every poor fisherman in the world runs a Nissan. Not so popular in North America because folks are rich and can aford all the bells and whistles offered thru the big name producers.
     
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