Design aluminium shallow water

Discussion in 'Jet Drives' started by Gian Milan, Jun 20, 2011.

  1. pistnbroke
    Joined: Jan 2009
    Posts: 1,405
    Likes: 33, Points: 48, Legacy Rep: 404
    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    AIR is forced into the tunnel....howdo you match a tunnel with a jet ?? Note in diagrams outboard prop is out of the water ...site not touched since 2002
     
    1 person likes this.
  2. Gian Milan

    Gian Milan Previous Member

    My english is bad.
    I don't understand :" Note in diagrams outboard prop is out of the water ...site not touched since 2002".
    Sorry, can you rewrite?
     
  3. mohawk
    Joined: Jun 2011
    Posts: 24
    Likes: 1, Points: 0, Legacy Rep: 19
    Location: New Zealand

    mohawk Junior Member

    The only way to make a tunnel hull work with a jet is too have a jet in each tunnel ,or 2 tunnels and the jet in the middle.
    It is common to have the jet intake grill higher than the keel line like the sjx, but alot of work is put into shaping the areas leading too it too insure the water flow is solid and totaly free of air.
     
    1 person likes this.
  4. Village_Idiot
    Joined: Oct 2007
    Posts: 382
    Likes: 14, Points: 18, Legacy Rep: 138
    Location: USA

    Village_Idiot Senior Member

    There seems to be a little confusion in this thread, so I'll add to it ;)

    Regarding tunnels, there are three types to talk about here:

    1) full tunnel - this is what you find on catamaran boats and pickle-fork racing boats (and, somewhat, on pontoon boats). They really have no advantage in shallow water when boat is at rest, although they can run shallow at speed. The Flatscat boat that the OP linked to uses this type of tunnel, although the canted-wall design of the tunnel does help direct water up to the elevated motor/prop. Yes, the Flatscat can run in just an inch or two of water, as long as it is running (water-ski effect). It is able to do this because the entire propeller is running above the bottom of the hull with only the skeg hanging below the hull. However, if you stop momentum, the boat will be grounded as it does require about eight inches of water to float when static. There are other flats boats designed/built in Texas will do this (run in 2-4 inches of water, but be grounded when you stop) - they typically will need 10-12 inches of water to get the boat back up on plane. Most of the flats boats are designed around the following (pocket tunnel):

    2) pocket tunnel - this is what you find on some shallow-water river/fishing boats. These boats are designed to float shallow, putt-putt around in shallow water, and run on plane through shallow water. There are many, many designs for prop motors, but some of the most efficient tend to be about one-third the length of the boat, 14-18 inches in width and around 5-6 inches in depth. Texas flats boat mfrs. have developed the design over time, and some of the better tunnels are either rounded (half-pipe) or designed with a flat roof (not a V-shaped roof). The buoyancy that the tunnel takes out of the hull is often offset with floatation pods attached to the transom on either side of the outboard motor - these pods also allow the boat to get on plane in shallower water since they do not allow the outboard motor/prop to dig down under full throttle acceleration. These tunnels have problems with aerated water since there is much disturbance in the water as it enters the tunnel - a good tunnel prop (double-cupping, large blade area) is a must for optimum performance with these systems. Some tunnels even incorporate an air vent to allow air to be sucked into the tunnel through a fitting in the hull/floor/sole of the boat to break the vacuum created in the tunnel and pick up an extra 3-5mph top end speed. Additionally, many of these boats are outfitted with hydraulic jack plates that allow the engine height to be fine-tuned on-the-fly for various water conditions. These boats will also run shallower than they will float, but do draft less water at rest than the full-tunnel boats. I run such a boat, and it will float in eight inches water, requires twelve inches to plane, and will run through 4-6 inches of water - the boat is 24 feet long with a 60-inch flat bottom and a six-foot long tunnel. If I stop it in less than three inches of water, it is very difficult to drag to deeper water.

    3) jet tunnel - this is a pocket tunnel, like for a propeller, but the dimensions are different (VERY IMPORTANT!). A jet tunnel is typically 2-3 feet long and maximum three inches in depth. This shallow tunnel is just enough to allow the jet foot of an outboard to remain above the bottom of the boat to protect it from groundings, rocks, etc. Lowe Roughneck offers the jet tunnel, as does SeaArk (I think SeaArk supplies the hulls for the SJX). These boats will run as shallow as the good tunnel prop boats mentioned in 2) above. The difference is that a well-designed tunnel jet will plane in water that it floats in, whereas the tunnel prop will generally require 4 more inches of water to get moving. You do not want to use a jet in a tunnel that is designed for a propeller as you will have cavitation/performance problems. The only reason I don't run a jet is because the river I run in has a lot of fine sand, so fine that much of it is suspended in the water itself, and this really accelerates wear on the impeller/housing. Vegetation/debris also tends to play havoc with jet systems. They really excel in clear rocky rivers.

    HTH (Hope That Helps)
    VI
     
    1 person likes this.
  5. mohawk
    Joined: Jun 2011
    Posts: 24
    Likes: 1, Points: 0, Legacy Rep: 19
    Location: New Zealand

    mohawk Junior Member

    Exellent post Village.
     
  6. Gian Milan

    Gian Milan Previous Member

    Eccellente Post, Village! Eccellente!

    But Ancient Romans said...
    Video meliora proboque, deteriora sequor.

    There are good reasons for the tunnel.
    1) The boat must slow rivers surf without waves, with gravel bottom, and many points to overcome almost dry with the bottom of small stones that moves.
    2) A flat bottom would be perfect: little draft and little power needed.
    3) more boats are long and narrow, better go.
    Instead of a boat, 7 meters long and 1.3 wide, with cat have 5 meters with 2 "hulls" of 0.6.
    The result is a boat 10 meters long with a large 0.6 meters
    4) apply a jet, two jets, an outboard jet or propeller surface does not change anything for me.
    5) the draft required from empty cat is 4 cm (2.5 inc.)

    QUESTION: Is there an outboard propeller surface?
    Thank you soon
    Gian
     
  7. Village_Idiot
    Joined: Oct 2007
    Posts: 382
    Likes: 14, Points: 18, Legacy Rep: 138
    Location: USA

    Village_Idiot Senior Member

    There are a few good reasons to go with catamaran (stability, efficiency, good ride in rough water), but they will require more water to float than a similar-sized monohull (even with pocket tunnel).

    Do you have width/length requirements/restrictions? A narrow boat is more efficient at displacement speeds and has a better ride in the waves (less frontal area for slamming into waves), whereas a wide boat of the same length will draft much less water, get on plane and stay on plane at slower speeds and will generally be more stable. If you are after a planing hull in calm water, go wide. If you are seeking a displacement hull, then stay narrow.

    As to overall size: speaking from my experience with aluminum boats, the larger boats will draft less water than smaller boats if you are carrying any kind of load; however, if you become grounded, it is much easier to drag the smaller boat to deeper water. If you travel light, go small.

    As to propulsion: if you do not have a lot of vegetation/debris/very fine sand to deal with, jet would likely be your best option. The best jet boats, IMHO, have a small deadrise to the hull and the small jet tunnel at the stern. They will not draft as shallow as a true flat-bottom at rest, but if you can pick the areas where you will stop, they would be your best bet.

    If you want to go with an outboard/propeller design, things can be tricky. A good (pocket) tunnel prop boat will run as shallow as a jet, but it takes some trial and error to get set up correctly. Good tunnel designs (talking pocket tunnels here) can be difficult to find. For a propeller, you want blades with a large swept area (long-eared blades) and cupping - cupping has the effect of enlarging the blade. The larger blade area is necessary for getting a good bite on the aerated water coming from the tunnel. I don't know what is available in Italy, but there are several good prop builders in Texas, USA that design propellers for tunnels - I use baumann propellers myself, shipped cross-country. They are much better than stock or factory propellers. Also be aware that tunnel boats generally have poor reverse thrust, due mostly to the required cupping on the prop and somewhat to the tunnel design dynamics. If you have a very limited choice of propellers, four-blade is generally better than three-blade for tunnel applications, although a custom three-blade can outperform a factory four-blade. Mercury has some good factory props.

    Having run shallow rivers for the last 25 years, my suggestions would be
    1) jet with jet tunnel (unless you deal with a lot of vegetation/debris)
    2) prop with prop pocket tunnel
    3) flat-bottom monohull
    4) catamaran with very shallow full tunnel (with deadrise being equal, the catamaran will always draft more at rest than the monohull for equal l/b dimensions)
     
  8. Gian Milan

    Gian Milan Previous Member

    TU Village.
    In your opinion jet outboard in shallow water is ok?
    An old riverman let me see how he put his johnson 25 with jet. jet is 2 cm (1 inc.) over the keel.
    Gian
     
  9. pistnbroke
    Joined: Jan 2009
    Posts: 1,405
    Likes: 33, Points: 48, Legacy Rep: 404
    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    Outboard must pick up cooling water .....
     
  10. Village_Idiot
    Joined: Oct 2007
    Posts: 382
    Likes: 14, Points: 18, Legacy Rep: 138
    Location: USA

    Village_Idiot Senior Member

    Yes, jet outboard is OK in shallow water, as long as you are getting water to the outboard's intakes. You may consider a shallow-water pickup (moves intake to nosecone). See http://www.bobsmachine.com/Products/cones.cfm for an example. A good fabricator shop should be able to put one of these together for you, or your guys might even be able to do it. You might also consider an anti-ventilation plate, or compression plate, to help keep water around the lower unit: http://www.boatrightmarine.com/cuerpo_gallery.php?from=accessories&modelid=55

    I should mention that if the outboard is mounted further back, for example on a setback plate or a jackplate, it can be mounted higher since the water coming out from under the hull will be shooting up to meet the lower unit on the outboard. You could probably mount the outboard 1 - 1.5" higher for every 5" of setback, but the weight offset will cause your stern to draft more. Then you add pods to get your level floatation back. IMHO it is easier, in the long run, to go with a true jet tunnel.

    Although probably bigger than you need, something like the Mercury SportJet is more robust and efficient than the outboard jet foot.
     
  11. Gian Milan

    Gian Milan Previous Member

    Incredible Village! Now i reduce my tunnel and try the plate...TU very much.
     
  12. Gian Milan

    Gian Milan Previous Member

    Dear Village,
    I send a new keel design.
    The boat is a flat/cat with two jet and central tunnel for air way out.
     

    Attached Files:

  13. Village_Idiot
    Joined: Oct 2007
    Posts: 382
    Likes: 14, Points: 18, Legacy Rep: 138
    Location: USA

    Village_Idiot Senior Member

    I see no design, just a brochure for the Castoldi...

    By your description, I presume you mean a jet in each hull, similar to a traditional twin-powered catamaran. This would work for the cat-style. However, if you were to instead decide to put an outboard between the hulls, similar to the Flatscat, then I think you would be better served with a propeller as they can be designed to deal with aerated water, whereas the jets have a propensity to cavitate and lose power with aerated water.

    Again, your simplest design would be something like the SJX with the Castoldi rather than the Merc SportJet that they use. You should investigate the boats constructed for jet purposes by companies such as MarathonMarine, Wooldridge and even SeaArk.
     
  14. Gian Milan

    Gian Milan Previous Member

    Hi village, hi mokawk.
    I posted picture of the work in progress of the jet shallow boat.
    Was not possible to have a tunnel and 1 jet.
    Jets needs a V keel, and I need a keel most possible flat.
    So I designed a keel as a trimaran at 10°, as want jet provider and i'm most flat possible and I can have 1, 2 or 3 jets.
    Next week I go in water.
    God save the Queen, me and my boat!
     

  15. mohawk
    Joined: Jun 2011
    Posts: 24
    Likes: 1, Points: 0, Legacy Rep: 19
    Location: New Zealand

    mohawk Junior Member

    We need pictures ;)
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.