Need Help w/ a Channel Hull for Outboard Jet

Discussion in 'Boat Design' started by Duckhunter, Dec 26, 2007.

  1. Duckhunter
    Joined: Dec 2007
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    Location: Idaho

    Duckhunter New Member

    Hello, I'm a new member and was hoping for some help. I own a 1978 Alumaweld 16ft flat-bottom sled. I run a 1978, 150HP Merc outboard w/ jet currently. I'm going to tear it down this summer for a 2nd overhaul. I want to cut the hull and fab a channel down the center. I'm assuming it will help feed water to the pump, allow me to raise the jet, and elliminate cavitation.

    I run this boat duck hunting here in Idaho, and it goes extremely shallow. I'm hoping to improve performance even more. I don't like the jet shoe sticking down below the bottom of the boat. Larger rocks will cause major damage some day.

    I'm also hoping to widen the hull ~10" per side to add flotation and stability. I'll be installing trim tabs on the back as well.

    Can someone please help me with the channel design? What type / shape / length / depth should it be? How far up the boat does it need to go? Does it need to have a slight taper to force / direct water to the jet shoe?

    Any advice or links to other information would be great. Also, is a 1978 in-line 6 cyl. worth re-building? It's ran great for me so far....but I'm thinking I might want a new motor?

    Thanks guys...
     
  2. Village_Idiot
    Joined: Oct 2007
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    Location: USA

    Village_Idiot Senior Member

    You want the depth (height) of the tunnel to be no more than three inches. Deeper tunnels are made for props and will make a jet cavitate. If I were designing it, I'd make the tunnel wider at the front to help force water into the tunnel (cross-sectional area [width x depth] of tunnel should always be greater as you work toward the bow). I'm not sure what tunnel length should be, as I'm not too familiar with jet setups, and they are a different animal than tunnel prop setups. Take a look at other jet boats in your area and see what their tunnel dimensions are. Check out Wooldridge, Marathonmarine, Roughneck, Alaskan, etc.

    Adding a jackplate will also get you some additional height on the jet foot, even without modifying the boat hull. The additional setback will allow the motor to run higher and still get adequate water feed.

    You may want to consider flotation pods on the transom as opposed to the side of the boat. They are readily available( http://www.fisherbeavertail.com/floatationpodsintro.html ) or easily fabricated and can enhance planing efficiency without losing hull efficiency.

    Also, check out http://www.fish.state.pa.us/boatcrs/jet/jetboard.htm

    I'd keep that old motor well-maintained and running. I think in some ways they are better than modern engines.
     
  3. Duckhunter
    Joined: Dec 2007
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    Location: Idaho

    Duckhunter New Member

    Village_idiot - Thanks a lot for the info! Much appreciated. I will check out the sites and info you mentioned.

    Regarding the old Merc 150 in-line 6 cyl... I'm considering attempting the re-build myself? I think it could cost as much as $3k to have some local shops do it. Hard to know what makes the most sense. If I buy a used motor, you just never know what you're getting...

    Thanks again.
     
  4. dick stave
    Joined: Dec 2004
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    Location: MISSION B.C. CANADA

    dick stave Senior Member

    Go to the" Outboard Jets "website and check under boat selection. There is a description of tunnel parameters there.
     
  5. Duckhunter
    Joined: Dec 2007
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    Location: Idaho

    Duckhunter New Member

    Thanks - Outboard Jets website says jet tunnels don't work well on flat-bottom boats... Bummer. There's got to be a way to channel the air bubbles away and get clean smooth water to the intake on a flat bottom boat? I'd still like the try and cut a channel into mine, but sounds like I need to do more research first.. The beavertails look like a great idea though.
     
  6. Quackenator870
    Joined: Oct 2009
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    Location: Idaho

    Quackenator870 New Member

    Let me know

    Duck hunter,

    I also hunt Idaho. I have just gotten a similar year 16 ft alumaweld sled that I am looking to put a tunnel is as well. research isn't looking the best for it but I too think it can be done. Let me know if you have since completed the tunnel and how it works!
     
  7. lat 64
    Joined: Aug 2009
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    Location: alaska

    lat 64 Junior Member

    Jet tunnel

    Hey,
    if anyone on this thread is still interested, I successfully installed a tunnel on my old tin flat-bottom jonboat. I made air-bleed vents and a slight vee within the tunnel to deal with the air. I can snap some pics and chat about it more if there is a response.
    I'm in Alaska. The water goes up and down with the weather and lot of thought has gone into the problem up here.

    Think spring,
    Russ
     
  8. Quackenator870
    Joined: Oct 2009
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    Location: Idaho

    Quackenator870 New Member

    Russ,

    I am still interested! I have since went ahead with a design very similar to the Lowe roughneck jon boats. I'm still working on the motor, floors, console etc. so I haven't got to test it yet. If you don't mind I would like some pics of yours just for future reference!

    Thanks,
    Jason
     
  9. lat 64
    Joined: Aug 2009
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    Location: alaska

    lat 64 Junior Member

    tunnel design pics

    Here's those pics I promised.
    The aluminum plate the welder used is a little thicker than the original.
    You can see the air bleed vents. They are just creases at the top corners that the welder made on his brake. I had him put a slight vee in the top of the tunnel to help push air toward the vents. This works well as a tunnel to keep the jet foot up out of harms way. I hit much fewer rocks this way, and I suck alot less gravel when trying to get around in shallow bars.
    On a choppy lake or river it still sucks air if I go too fast. I can keep it just on step without too much air problems if I keep speed down in waves and serious ripples.
    The trim tabs helped quite a bit to get it up on on plane faster. I'm planning to make them bigger yet. I also want to try fins on the jet shoe.
    I can go through water that won't float a canoe, but I have to be loaded fairly light to show off my skinny-water fly bys. It's very fun!

    Cheers,
    Russ
     

    Attached Files:

  10. Village_Idiot
    Joined: Oct 2007
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    Location: USA

    Village_Idiot Senior Member

    Interesting design, looks pretty ingenious. Keep us posted on how it works out over the season.

    A hydraulic jack plate would let you lower the motor to reduce cavitation in choppy waters, but would add some weight. If you're happy with how it works now, I'd keep it that way.
     
  11. lat 64
    Joined: Aug 2009
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    Location: alaska

    lat 64 Junior Member

    After I posted yesterday, I bought a second hand 6-7/8-inch impeller to try. Right now it's 6-5/8.
    My '86 Evinrude 48spl is a good runner but I want to bring the revs down a bit.
    I have been cautioned against going too far in this direction. (the next step would be a stainless impeller).
    My boat is light and my motor is small. I hope to keep it that way. It's much easier to shove off a bar when the water runs out. It's quite the rat boat, but it does bag groceries once in a while.
    Also, my parts contact said some guys use UHMW polyethelene to make the wings on the foot intake. That sounds easy to do. The plastic will make a good rock bumper too. I wonder if an impeller or sleeve or any other parts could be manufactured out of plastic or composite?
    I was thinking I could touch up the leading edge of the impeller with epoxy. Some of the stuff will stick pretty good. Just a thought.

    OK, back to work.

    r.
     
    Last edited: Apr 9, 2010
  12. Riverboy
    Joined: Oct 2010
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    Location: Arkansas

    Riverboy New Member

    jet tunnel

    Hi i'm thinking about modifiying my jon like the one you showed in the pictures. Have you installed fins on the jet intake or done other mods that helped the boat run better. I'm going to run a 50hp mer on a 1648 jon. what size of motor are you running
     

  13. lat 64
    Joined: Aug 2009
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    Location: alaska

    lat 64 Junior Member

    Riverboy,
    Geeze, forgot I even posted this. No I haven't put any fins on. It's getting snowmobile season, so it will be next spring before any new mods.
    The boat works quite well the way the photos show, but you have to think about the weight all the time. Three people, 10 gallons of gas, a picnic cooler, spare 5hp motor; and it's over loaded. Chuck the third guy and go fishing.
    I have a 1986 Evinrude 48 hp. I wish it were a little bigger.
    I think my boat is about the same as yours, 16' x 48"
    I had access to a very talented welder who could weld thin aluminum. He is clever with a brake too.

    Russ
     
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