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Thread: To Tunnel or Not to Tunnel

  1. #1

    Default To Tunnel or Not to Tunnel

    Thinking about taking my 20ft SeaArk and having a tunnel put in. What are the cons for putting one in? What will I be giving up? Will I lose tight turning capabilities? Are there any tunnel design recommendations? I live in Fairbanks so any recommendations on a welder/tunnel designer would be appreciated. Thanks for the help. J

  2. #2
    Member Akgramps's Avatar
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    Karolds welding does a great job at fabrication.
    I like the way the tunnels worked on the two boats I have had, if done right it can raise the jet foot up so its slightly above the bottom of the boat, which will help to keep it from banging on rocks.
    Following is some info about tunnels I copied and pasted from speciality jets website:
    (Keels can be a problem, introducing air into the jet intake. Center keels vary in size and may introduce air. If this is suspected, the keel should be removed 2-3 feet forward of the transom. Other keel arrangements which tend to funnel air to the jet intake should be avoided.
    A properly designed tunnel, combined with a slight V bottom hull can greatly enhance jet boat performance. It should raise the motor 2-3 inches and place the heel of the jet intake flush or slightly above the bottom of the boat. A jet tunnel doesn’t work well with a flat bottom boat due to air ingestion. It’s imperative that the hull is designed correctly with the tunnel for the boat to operate properly.
    The tunnel needs to be just large enough to feed the jet drive its water requirements. A tunnel that is longer, wider or deeper than necessary wastes power in lifting excess water, tends to suck the stern down when planning and sits deeper in the water at rest due to lost buoyancy.
    The top of the tunnel width should be about 1 ¾ times the width of the jet drive water intake. The tunnel length doesn’t have to be longer than about 2 ½ times the water intake width. )
    What they dont say much about is the top of the tunnel, from my experience it can be critical, depending on the hull design, I have found on my boat it is important the top doesn not angle down, if it does (even 5 degrees) it will act like a big trim tab and push the bow down, generally this is undesirable, or at least it was with my boat. I think a up angle of less than 2 degrees is ok, any more and it makes it diffucult to get the jet set right as the water continues to rise as it exits the tunnel. Another thing I learned is the top of the tunnel should be extended back so its very close to the front of the jet foot, it may need to be trimmed to allow the foot to swing and typically a piece of UHMW is needed to minimize spray.
    Hope this helps some

  3. #3
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    I will second Karolds for doing the welding, he did a great job on all the modifications i wanted for my boat. i do agree if you have a tunnel put on use some UHMW to go from the back of the tunnel to the top of the foot to help reduce spray and it really helped my boat reduce cavitation. i had a tunnel added to my first boat, Sea Ark 1852, and got a pre made tunnel from Sea Ark for that size boat. Compeaus orderd it for me and it added $800 to the price of the boat, i dont know what the price would be now. i wouldnt buy an outboard river boat without a tunnel. Now i have a 2072 Sea Ark with a tunnel and it saved the foot on my motor when i dead centerd a rock on the Gulkana.

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    Default to tunnel or not

    Anyone out there run a flat sled with a tunnel ? How do they preform ? I have read that OB/Jet's on flat bottom boats are not recommended and a 6 to 10 deg deadrise is best. But theres alot of flat sleds with no tunnels outthere like Alumaweld,DW,Fish-rite.
    I am also thinking of installing a tunnel as a winter project,I was going to use the info on there site to design a proper tunnel, length,width etc.
    But now I am wondering if I'll be introducing even more air because it a flat sled . I know that there's a plus to rising up shoe but don't want to make it worst by introducing air .

    Want do you guy's think ?
    Tks

  5. #5
    Member Wallbanger's Avatar
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    Default I had a tunnel built

    into my 18' Grayling flat bottom boat/Yamaha 90 Jet Drive. The Grayling guys copied a Wooldridge tunnel exactly but it did not work on my boat. It cavitated big time in turns or in any more that a 2" ripple. I even had to put a new shaft on my motor because of it. The boat shop was so frustrated with the poor performance they filled in the tunnel and replaced the transom again. $3,000 experiment gone bad.

  6. #6

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    i put one in my 18 foot alumaweld with a flat bottem along with some reverse chines and added about 8 inches to back of bottem on it and it runs really shallow and handles well. i did the work myself and i cut what i burned in fuel in half running a early 90s 140 on it and burning 6gph so i am happy with that.
    Attached Images Attached Images

  7. #7

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    here are a couple more
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    Default to tunnel or not

    Wallbanger, that what I am worried about
    But now, seeing Rmk4x4 post I am thinking about the rev chines helping keep the flow in the tunnel. Looks like you did a great job there Rmk4x4 and wow cutting down the gas. Whats the OAL of that tunnel and why did you feel you needed to put a extra 8" on the back? My rig has 6" weld on tabs already, so with a bit of fitting can make it like your's only 6" long.
    Still wondering why Alumaweld, DW and fish-rite don't put tunnels on as std, after all those boat are top of the line and we pay good $$ for them.
    Anyone care to comment? Maybe some boat maf, they should be reading these forum's since we use there products.
    Tks

  9. #9

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    Hey all, thanks for the info. it definitely has given me some food for thought.

  10. #10
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    Default tunnel design

    Any chance you could pass on some dimensions of your tunnel? Your boat is about two feet longer than mine, but I would think that the tunnel would fit. Also, are there any gussets or supports to the 8" extensions? Whats to keep them from bending up or coming off alltogether when jumping beaver dams?
    Your design looks as good as any I've seen and the performance of your sled sounds pretty good also! Thanks for any info you can pass my way.

  11. #11

    Default ...what is the question?

    I am running a 16' Seark with a jet tunnel and I love it. The boat has more potential than I've got skills...I truly believe the tunnel hs kept my mistakes from being big mistakes. That being said, Seark makes two types of tunnels...the jet tunnel, short (length wise- to minimize entrained air while accellerating and turning) and narrow (to accomodate the jet-foot only). The other is a prop tunnel which is longer and wider. If you are going to run both a prop and a jet, go with the jet-tunnel. If you are going to be running a jet go with the jet tunnel, if you are going to be running a prop ONLY, go with the prop tunnel design.

    It shouldn't be too tough to get the specs/schematics from another seark owner,directly from seark or from one of their authorized dealers. I think another difference between the jet tunnel and the prop tunnel or no tunnel is the heighth of the transom to allow for higher jet-foot placement. Personally I, or preferably a guy from the chosen shop, would make some measurements of a seark in the chosen configuration to base retrofit off of...Seark has experimented with enough of their boats to know what dimensions work best on their boats...

    OR, just buy a new hull in the configuration you want...you'll probably make enough on the trade-in to make-up the difference you're going to spend on the mods, plus you'll have a brand new hull with a brand new warrenty (which you'll probably void by modifying your existing hull) and you won't have to poke anymore holes in your transom when you need to relocate your motor mounting...

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  12. #12

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    Quote Originally Posted by Coldjoe View Post
    Any chance you could pass on some dimensions of your tunnel? Your boat is about two feet longer than mine, but I would think that the tunnel would fit. Also, are there any gussets or supports to the 8" extensions? Whats to keep them from bending up or coming off alltogether when jumping beaver dams?
    Your design looks as good as any I've seen and the performance of your sled sounds pretty good also! Thanks for any info you can pass my way.


    i sent you a pm but on the extensions they are gusseted on the sides just was not done in that one pic that was after a test run. i took it out with just me in it and balanced right then i bent the extension to make the boat run flat side to side then i welded in the gussets

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