Small plywood ski boat?

Discussion in 'Powerboats' started by ThomD, Sep 2, 2010.

  1. ThomD
    Joined: Mar 2009
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    ThomD Senior Member

  2. tunnels

    tunnels Previous Member

    I like power on tap so a boat that size it be looking at a 50 hp motor , not a fan for loading little motors and wringging there necks everytime you try to pull a over weight non skier out of the water . Like pulling a bag of wet sand .
    A 50 hp will dig deep and pull hard . :p
     
  3. srimes
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    srimes Senior Member

  4. lewisboats
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    lewisboats Obsessed Member

    My personal opinion...you are going to need something a bit more substantial than what you have shown I think...Don't forget that a skier is going to affect the boat as they go from side to side, over the wake etc. If the boat isn't hefty enough to absorb the momentum it imparts to the skier suddenly yanking it off to the side it could quite difficult to control. I would go with a hull that is no less than about 350 lbs for a skier in the 150 lb range. 35 horse would be a decent starting point as far as an engine goes but up to 50 would be OK.

    This one looks about right: https://www.boatdesigns.com/13-Tuffy-runabout/products/367/

    [​IMG]
     
  5. apex1

    apex1 Guest

    And die fast.........

    Outboards need to be operated near wot, to stay alive. A oversized engine is a costly mistake in this case.


    Although this thread is already a bit old, I like to recommend the tiny little Bullet design liked ny the OP.

    It seems to fit the bill and with a full load of sanity and awareness it will do the job faily well.
    As Lewis mentioned care has to be taken due to the rather vivid nature of such a light and short boat.

    Regards
    Richard
     
  6. waikikin
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    waikikin Senior Member

    That's it. How big a 4 stroke minimum?
    Looks like a heap of fun!
    How much power for this "Bullet boat?

    regards from Jeff
     
  7. apex1

    apex1 Guest

    Well the plans ask for 35/40hp and I do not call that the minimum. The 40hp will more likely be the max. for the 12" boat.

    Both should tow a average skier at 25hp already.
     
  8. Yellowjacket
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    Yellowjacket Senior Member

    Well Richard it seems that you've jumped in to a subject that you obviously have little or no experience.....

    The Bullet isn't a very good boat for what the OP wants to do for about a half a dozen reasons. While you could ski from it, it's too light, too short, too aft loaded, and isn't going to pull the skin off of a grape with a 25 hp engine.

    Lewisboats is right, you want something like the "Tuffy" and here's why.

    First, the Bullet is very short coupled and the driver and passenger seats too far aft. Second, if you put a pylon on it that is high enough to clear the outboard, coupled with the fact that outboards are heavier (third reason) than they used to be, you will struggle to pull the skier out of the water and get the boat on a plane at the same time. The added moment from the pylon, the higher engine weight aft and the seating position all conspire to make it drag the skier for about a week before you get him out of the water.

    Fourth, if you use a bridle for the towrope, the boat is so light that the skier will pull it around if he cuts on a slalom ski, so a pylon would be better, but you can't use it so you are stuck with the bridle.

    Fifth, you want to be able to carry at least three people in the boat, not two like the Bullet. You need to be able to carry an observer as well as the driver. You can legally do it with a wide angle mirror, but it is really dangerous to do and an observer is really a good idea. When I was a kid my old man wouldn't let us ski with just a mirror for good reason. Ok sounds like the Bullet is still Ok, but what do you do if the skier get tired, falls or just plain has enough? Put him in the boat right... Not likely in the Bullet, with a ski or two and two people already in there. Add the fact that you could easily swamp the Bullet trying to have a third person climb into it.

    Sixth, you really want and need at least 35 to 40 hp to pull a skier out and not drag him. Inexperienced skiers do better if you get them up quicker, and it takes 35 hp or more to do it. Yes, I've skied behind a Merc Super 10 (16 hp) but you aren't going to pull up a slalom skier in a deep water start (like if he falls) without dragging him half way across the lake with 25 hp.

    And what is this hooey about 2 cycles wanting to run flat out or they won't live. That's nonsense.

    Perhaps you should know something about what you are talking about before you offer your expert opinion.
     
  9. apex1

    apex1 Guest

    Perhaps you should focus on the OP´s requirements before you trample on my toes!

    Though several of your comments are without any doubt valid and sensible, you miss as many points as you address here.

    And your offending tone does not make it better.

    Have not seen Lewis recommendation by now, seems the Tuffy fits the bill better, thats right.
     
  10. tunnels

    tunnels Previous Member

    Well Richard it seems that you've jumped in to a subject that you obviously have little or no experience.....Perhaps you should know something about what you are talking about before you offer your expert opinion.

    Hes at it again !!!!!Mr know it all !!!!!!The hamburger guru !!!
    :p ]
     
  11. Yellowjacket
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    Yellowjacket Senior Member

    Richard,

    The OP wants a boat to ski with kids and use with geriatric adults. He doesn't want high performance (so no real high power needed), but you have to pull the skier out of the water and kids can weigh between 100 and 150 pounds, and it is going to take more than 25 hp and a good size planing surface to do that. The boat may go faster than he really wants, but pulling the skier out of the water sets the power requirement.

    You are absolutely right, you didn't mention 2 strokes. I assumed you were referring to 2 strokes since you said outboards, and most outboards, until very recently were 2 strokes. My bad, and I'm sorry, but what ever makes you think that you have to run any engine 2 stroke, or 4 stroke flat out or it won't live. You run your car at part power all the time, it's a four stroke and it it lasts just fine. The statement that you have to run an outboard hard or it won't last is just baloney.
     
  12. Yellowjacket
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    Yellowjacket Senior Member

    Lastly the OP is stating that he wants to use a 4 stroke, and it will likely work on ok the Tuffy with some adjustment, but forget it on the Bullet. A 40 hp 4 stroke Merc weighs 205 pounds where the typical 2 stroke that the Bullet was designed for, 35-40 hp, weighs about 125 pounds. That’s 80 pounds more hanging over the transom and that's going to be an issue with any of these small boats.

    Remember that most of the boat plans that you can buy are pretty old and these boats were designed around the older 2 strokes. Not that you can't use a 4 stroke, but you just have to account for the fact that you are hanging almost twice the weight of the motor over the transom. That means that you need to push some weight forward, likely the driver and passenger to keep the same weight and balance or the boat will be slow to plane, possible porpoise, throw a large wake when you are pulling a skier, and generally be a mess if you don't adjust for it.
     
  13. apex1

    apex1 Guest

    Flat out was not said and not meant. But these engines need to be operated at realtively high rpm to live, thats just fact. Comparing them with car engines shows the insight is limited.

    Unfortunately I typed my post while Lewis had already posted his recommendation, I would have skimped on that when I had seen it. Sure that boat fits the bill better.

    Don´t again put words in my mouth:

    2stroke was not mentioned here before you came up with them! And 4strokes are on the market almost as long as the bullet plans exist. A recent 25hp outboard weighs about 150lb just to inform you.
     
  14. Yellowjacket
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    Yellowjacket Senior Member

    So explain this statement. What basis in fact are you quoting other than some preconception that you have.

    Here are the facts:

    Higher throttle openings and higher operating speeds reduce the life of ANY reciprocating engine. Run them harder and they don't last as long. Whether you consider cylinder wear, bearing wear, valve life, or the life of any parts that have a life limit, higher speed and operating temperatures have a huge impact on the life. Rotating stress is equal to speed squared, so run 10% faster and the stress will be 20% greater. Higher speeds increase cam follower wear and increases valve spring fatigue since the springs run hotter at higher speeds.

    Higher throttle openings increase pistion pressures, which results in higher side loading on the pistion during the expansion stroke, = more bore and ring wear. Same thing with higher RPM's, = higher pistion speed, more ring and bore wear.

    Higher throttle openings result in higher combustion temperatures which increases metal temperatures throughout the engine and has a huge impact on hot parts like exhaust valves. Stress rupture life of high temperature materials is a strong function of temperature, 25 degrees F temperature increase will cut the life in half. Do a search on Larson Miller parameter and learn something about creep rupture life, which is what kills exhaust valves.

    Higher maximum rpm's increase bearing wear and reduce seal life.

    By any measure, if you run any reciprocating engine at higher load factors and higher rpm's the life will be lower. This is just as true for diesels, 4 cycle gas engines, and 2 strokes.

    If you run an engine near wot it won't last nearly as long if you run it at a reasonable power factor. Some engines are more robust, and running harder is less of an issue, but run them harder and they won't last as long, it's just that simple.

    Oversizing an engine will cost you a bit more in fuel consumption, but it won't be a big deal compared to what you will gain in life. Putting a 25 hp engine in a setting where you are running it at wot all the time is a sure way to use it up a lot faster than getting a 35 or 40 hp engine and not using it hard except when you are putting the boat on a plane or pulling up a skier.
     

  15. srimes
    Joined: Sep 2008
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    srimes Senior Member

    if you have an axe to grind a new thread on the subject might be a good idea...
     
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