Aeration in tunnel

Discussion in 'Boat Design' started by weldandglass, May 31, 2013.

  1. tom kane
    Joined: Nov 2003
    Posts: 1,767
    Likes: 48, Points: 58, Legacy Rep: 389
    Location: Hamilton.New Zealand.

    tom kane Senior Member

    Some basic information where the position of propellers should be to get good results
     

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  2. Village_Idiot
    Joined: Oct 2007
    Posts: 382
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    Location: USA

    Village_Idiot Senior Member

    The easiest way to deal with aerated water in a flats-type tunnel boat is to use the correct prop. Since you are in TX, look up baumannprops.com - they specialize in these types of props. The props typically have a large swept blade area and double- or even triple-cupping. The cupping has the same effect as increasing blade area. Lots of blade area grabs the aerated water better than your typical off-the-shelf prop. A secondary issue is that you lose much of your reverse thrust with a heavily-cupped prop, a trade-off that most folks in this situation are willing to put up with.
    Baumann will work with you to get the correct prop for your application, and you can swap out props until you get the best one. There are also other good custom prop shops along the coast that specialize in flats props.

    Along this topic, there are many who add vents to their tunnels to add MORE air into the tunnel. This has the effect of breaking the vacuum that the tunnel creates, allowing the hull to "pop up" and operate much more efficiently, often adding 5mph to the top end speed with no other changes. The vent is typically a one-inch-diameter hole drilled at the leading edge of the tunnel, and often with a valve so that it can be opened/closed at will. Closing it allows better shallow-running capability, opening it allows faster and better boat performance in deeper water. Several boat manufacturers are now offering this "venting" option on their boats.

    As for reducing turbulence in the tunnel, I've often thought about adding some vanes into the tunnel to help "straighten out" the water flow before it exits the tunnel. Just haven't gotten around to doing it. Seems a lot of tunnel "experts" agree that a rounded tunnel is best (like you would find in a good TX flats boat), and if you can't do round, five-sided is better than three-sided. If you can find a Majek RFL boat (popular on the TX flats), study their tunnel design, and just as importantly, the overall design of the boat bottom. It helps to be able to funnel some water from underneath the boat toward the tunnel to keep it primed and filled with water. In this respect, positive deadrise is good for jet motors and their associated tunnels, but I think negative deadrise may be more beneficial for prop tunnels.
     
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  3. tom kane
    Joined: Nov 2003
    Posts: 1,767
    Likes: 48, Points: 58, Legacy Rep: 389
    Location: Hamilton.New Zealand.

    tom kane Senior Member

    Hydrodrive Surface Propulsion patented by a local Marine Engineer in Hamilton New Zealand and sold localy and in America.
    I had the pleasure of travelling at 75 MPH burning two gallons of fuel a minute in an ocean race boat equiped with one of these tunnel drives with fins to streighten out the water flow and using a venturi principle. like jets there is not much action at low RPM and WOT is a must to go anywhere and like most tunnels and pods they have very little steering other than full ahead and when a turn is required the edges of the tunnel interfere with the water flow to the prop and trouble arises. Even fitted in a deep V hull these problems arise and are worse in a shallow draught hull.
     

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