pocket tunnel - thoughts

Discussion in 'Boat Design' started by fjlegend, Apr 4, 2008.

  1. fjlegend
    Joined: Apr 2008
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    fjlegend Junior Member

    I picked up this 1975 20' proline flatback that was re-worked in 2003. The rebuilder installed a pocket tunnel. I have only run it a few times and it is really touchy to trim and lift. It tends to blow out if not in the right position. Needs a lot of positive trim at cruise speed or the leg sprays a ton of water and the prop losses bite. I have run the jack plate up to about 5" while on plane and the tell tale was still strong. No water pressure gauge at the moment but one will be going on shortly.

    I did not buy this boat from the rebuilder but from the next owner. I did however contact the rebuilder and he stated that the tunnel gave them some trouble at first as it was shooting to much water up onto the leg of the motor. They had to install a aluminum trim ring on the backside of the transom effectively making the tunnel a bit longer and directing the water further back. He told me that they rebuilt the boat to be a shallow water tow boat for a local franchise in and around Crystal River Florida.

    I am heading out for a week on the water tomorrow so I will be able to test her real good. The Suzuki DF140 has a 14x17 prop and runs mid 30's trimmed out at about 6,000 rpm. She proably could run a 18 or 19 pitch. Current hole shot is like a bullet. What about a cleaver or chopper prop given how high the motor is mounted and the water jet effect of the tunnel? 1st picture is with jack plate full down.

    Floats in about 11" and should be able to run in 5" if you have the balls to do it.

    Wost case scenario I fill in the tunnel and remount the motor a bit lower.

    All thoughts are greatly appreciated.

    [​IMG] [​IMG] [​IMG]
     
  2. fjlegend
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    fjlegend Junior Member

    anybody

    Just looking for thoughts and ideas on how to tweak this design. I have a full week on the water starting tomorrow.

    kls
     
  3. Pericles
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    Pericles Senior Member

    Lengthen the ring/shroud a bit further?

    Pericles
     
  4. Village_Idiot
    Joined: Oct 2007
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    Village_Idiot Senior Member

    check your prop

    You might look into getting a better prop. Pocket tunnels perform best with props that have a large blade area and lots of cupping. Four-blade props seem to work better on pocket tunnels. More blade area will give a better bite on the water and reduce problems with the prop ventilating. If you want to go with a factory prop, I'd recommend something like the Mercury Trophy Plus 13.75 x 17p along with seal ring (p/n 878421). Merc seems to make the best factory props. However, for a really nice prop, I'd recommend a custom prop from a custom builder, such as Louie Baumann www.baumannprops.com
    Explain to Louie what your setup is and how it performs, and he should be able to hook you up with a much better prop than you have now. It will probably make all of the difference in the world in how your boat performs.

    As an aside, if the back of your boat squats down in the water upon takeoff, that is a good indication that you have vacuum in the tunnel. If that is the case, many folks who run pocket tunnels have drilled a hole (about an inch to an inch and a half diameter) through the bottom of the boat at the leading edge of the tunnel, then weld (or fiberglass in your case) a one-way ball valve or a pipe to the vent hole to keep water from coming back into the boat. This allows air to be sucked into the tunnel to break the vacuum and enhance the boat's holeshot and top speed performance. However, you may be able to simply alleviate such a problem with judicious use of your trim tabs.

    Btw, what are the dimensions of that tunnel (L x W X H)?
     
  5. fjlegend
    Joined: Apr 2008
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    fjlegend Junior Member

    Village & Pericles,

    Thanks for your responses. The boat has been running great all week as I have now have a command over where the trim and jack plate need to be positioned. The motor likes about 2-3 inches of vertical lift and about 2-3 seconds of trim. If i make a high speed turn I always have to remember to drop the trim back down otherwise the prop blows out.

    It does appear to sink down upon take off. I have not been able to get on plane in skinny water due to this so I just motor out till I get at least 3' then depart. I have a floor hatch directly over the tunnel so adding a vent will be a pice of cake. I need to add a bilge to this area anyway.

    Village, do you agree with Pericles thought of adding a longer trim ring. I can get a local fabricator to make a new ring that will simply bolt to the exiting ring. I can get this made of any length so where should I start? Longer is better as I can always trim it down myself.

    When I am trimmed out and running bow high I really have a jet of water about 15' behind the boat that is about 2 foot high. Not sure if this helps with the analysis.

    I am also considering adding a hydrofoil to the lower unit. Any thoughts on this.

    Once again thank you for your reply as I attemtp to make this tunnel work for me.

    I will get the tunnel measurments when I pull the boat out of the water tomorrow. We head home on Satruday.


    Kevin
     
  6. Pericles
    Joined: Sep 2006
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    Pericles Senior Member

    It's best to change one thing at a time. Lengthen the trim ring as close to the lower leg as reasonable and them measure any alterations in performance. Then fit a hydrofoil with the original ring in place and establish whether that modification improves or worsens matters. Then try both modifications together.

    As for your 15' of water fountain when trimmed out, that appears to suggest that the transom or the engine mount may be slightly too vertical as the propeller wash is breaking the surface. This would only be of concern if you find that when the leg is trimmed in fully, the bow is still running too high. Otherwise, you can probably leave is "as is" and be pleased with the large range of trim the Suzuki has.

    Good luck.

    Pericles
     
  7. Village_Idiot
    Joined: Oct 2007
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    Village_Idiot Senior Member

    As Pericles mentioned, it is best to change one thing at a time. As far as lengthening the ring, I don't think it could hurt - it should feed a more solid stream of water to the prop. Only downside I can think of is possible loss of turning (steering) performance, but that is not too good to begin with on pocket tunnels anyway.

    If you don't wan't to drill a hole in the hull at the leading edge of the tunnel, you can test what would happen by adding a vent this way: Temporarily glue some pipe (I'd guess a 2-inch diamter half-pipe would work well) to the inside of the tunnel with the outlet coming out at the transom just above the water line. You could also try a hose in lieu of a pipe, again shooting for 1.5-inch i.d. or so. It is important to feed the air into the tunnel at the very front of the tunnel.

    Again, I think you'd see biggest gains from a well-designed prop with a lot of blade area and a lot of cupping.

    As for a cavitation (anti-ventilation) plate, I'd recommend the plate that Glen Boatright makes http://www.boatrightmarine.com/accessories.php Note that they call it a compression plate. It works much better than your standard commercially-available plates.

    Check out the forum at http://2coolfishing.com/ttmbforum/forumdisplay.php?f=36 - they have some pretty good info on getting pocket tunnel boats to run right.
     
  8. fjlegend
    Joined: Apr 2008
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    fjlegend Junior Member

    Here are the measurements on the tunnel.

    Lenght: 41"
    Width at transom: 20.5"
    Height at transom: 8"
    Trim ring addtional lenght: 4"

    I had forgotten that the engine already has a Bob's stabilizer plate installed. The Bob's plate is not very large compared to the other custom copression plates on the market.
     
  9. Village_Idiot
    Joined: Oct 2007
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    Village_Idiot Senior Member

    That tunnel seems a little short and big for that size of boat. I'd rather see something like 60" L x 14" W x 7" H. The smaller diameter would give less ventilation but could possibly also hurt steering performance.

    With the abrupt rise in the tunnel that you have (8 inches over 41 inches), I'd bet you're getting a lot of ventilation. I think the first step is a big prop (large diameter with large swept blade area) with lotsa cupping. Surface-piercing prop may work well, also.

    I've had experience with a jon boat (1860) that had similar tunnel dimension problems (36"Lx16"Wx7"H). Only way it would get on plane is with a 60hp Bigfoot swinging a 13.75-inch prop. The non-Bigfoot 60hp engine maxed out at 12" prop diameter and could not bite enough water to get the boat up.
     
  10. fjlegend
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    fjlegend Junior Member

    She gets up on plane really well in deep water. I am going to find a local dealer who will let me demo some props.

    I was running on Saturday with 4 adults, fising gear and a 1/2 tank of gas and hit 6100 rpm. So I know think I have room for some more pitch.

    There is a 13.75 x 19 Merch Trophy Plus for sale on the Floridasportsman for $200. All I would need is the hub kit.


    Kevin
     
  11. fjlegend
    Joined: Apr 2008
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    fjlegend Junior Member

    I probably could get another 6 inches in length to the tunnel with a longer trim ring. Will this still allow me to jack plate up as high. I know there are trade offs with all changes.

    Prop change is first on my list of things to change. Do you guys agree.


    Kevin
     
  12. agturbay
    Joined: May 2010
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    Location: argentina

    agturbay Junior Member

    Pocket tunnel

    Hello Fjlegend, I was doing the same project( pocket Tunnel) on my 15 ft fishing boat.Now is in test runs. My outoard is a Yamaha 40hp 2 stoke, original prop 11-1/4 x 14. The boat get on plane good, but i can turn at relative high speed (about 25-30 mph) Is this normal on pocket tunnel hull boats when the motor is on the high position?
    I want to know if do you tested with a vented tunnel.Can you post photos of your boat?
    Thanks
    Alfredo
     

  13. Village_Idiot
    Joined: Oct 2007
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    Village_Idiot Senior Member

    Most pocket-tunnel boats will slide through a turn at high speed when the motor is elevated, due to very little of the motor being in the water to "steer" the boat. In severe cases, usually with a v-bow and/or inexperienced captain, the boat can swap ends, a potentially dangerous phenomenon at higher speeds.

    I have seen boats with "ice runners" on the bottom that steer very well at all speeds, despite elevated motor height. These "ice runners" are just vertical longitudinal ribs, about two inches deep, running the length of the boat. Their downfall is that if you ever ground the boat, it is much harder to get the boat un-stuck. Since the purpose of a pocket tunnel is generally for running shallows, sooner or later you will ground the boat and wish you didn't have "ice runners", so best to learn to drive/slide in the high-speed turns.
     
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