penetrating standing waves

Discussion in 'Boat Design' started by river runner, Oct 23, 2011.

  1. DCockey
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    DCockey Senior Member

    Two other illustrations from Powell's book:
    [​IMG]
    [​IMG]
     
  2. Ike
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    Ike Senior Member

    I guess that's my AAADD kicking in.

    Age Activated Attention Deficit Disorder.
     
  3. Ike
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    Ike Senior Member

  4. thudpucker
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    thudpucker Senior Member

    You need a little bit of keel up front. not enough to let your bow down on a rock. If you don't have that little fold down the center of the bottom, your boat will just skid away from your target.

    If you get the bow over the wave and the stern lifts, you'll lose the Jet propulsion, and will stop right there, the wave will turn you sideways and back down the Wave you'll go.

    If the Wave is so wide you cannot go another way round it, your only option is to center the load and hit that wave a a high rate of speed.

    Even the really expensive Aluminum jet boats have a 'fold' from bow to stern. You can't quite call it a keel, but without it, a flat bottom, even with ribs, is tough steering in turbulent water.
     
  5. river runner
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    river runner baker

    I've been looking at photos of the USCG lifeboats that operate in the massive surf at the mouths of rivers along the Oregon coast. From what I can see and read they have a V shaped bottom forward and the bow has significan flare and, in this case, it realy is flare and not flam. Seems like a pattern is emerging. I wouldn't say the boats that currently operate in the Grand Canyon got it wrong, but I would say that it is obvious that you can create a better bow shape, for penetrating waves, than what can be built using single chine, flat bottom construction. I think the ideal shape is a rounded or V shape bottom forward with a rounded transition to the stem, only moderate overhang, and a fair amount flare.
     
  6. Ike
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    Ike Senior Member

    I have a lot of familiarity with those Motor Surf Boats. You would be better off looking at the old 44 foot motor surf boat than the new 47 footer or 30 footer see http://www.uscg.mil/d1/stachatham/Boats.asp

    [​IMG]

    Reasons. The old surfboats were displacement hulls. They weren't very fast but could handle most anything. They were self righting, and could do a 360 degree roll and keep right on going. the engine wouldn't even quit. It could run upside down.

    The new ones are a combo semi-displacement hull. They have the same hull shape forward, but have a planing hull aft so they can get up and go in smoother water. This has resulted in a number of quirks in their handling, and the USCG trains boat coxswains specifically on how to deal with it. The bow sections are convex. This can create negative pressure at certain speeds which results in the boat suddenly turning to the right and nosediving. Not a characteristic you want in a surfboat. The Naval Research Laboratory did an exhaustive study of this and it is caused by the convex sections in the bow.

    Adding to that the designers anticipated that the torque from the single engine and big prop (they have a much larger prop than a comparably sized motor boat because they do a lot of towing) would tend to make the boat veer to the right (actually the stern moves to the left) so they put a small hook in the planing surface just forward of the transom to counteract this. This only made the problem worse. To make matters worse, most planing hull boats have the engine as far aft as possible to move the Center of Gravity aft, to be over the center of pressure of the planing surface. These boats have the engine further forward in the longitudinal center of gravity to aid them when in displacement mode with their ability to handle large breaking waves. That doesn't help when at planing speeds and adds to the nose diving problem. Fortunately this only happens at about 30 mph, and the coxswains are trained not to go that fast.

    For the types of water and speeds you are talking about you would do better with the older 44 foot motor surf boat that had rounded bottoms and displacement hulls.

    But the one thing both of these boats (all motor surfboats have) to make them self righting is weighted keels, like in sailboats. That would not be good in a drift boat.
     
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  7. Ike
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    Ike Senior Member

    Just another thought. If speed is what you want, jet boats have been running the western rivers for many years, both up and down. They are pure planing hulls. Speed and power is important for these types of boats, and they have no rounded sections. here's an example http://www.roguejets.com/
     
  8. river runner
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    river runner baker

    I saw a boat like that USCG lifeboat in the museum at Astoria (my brother lives in Portland). It still looks like it has some flare in the bow.
    No, speed is not a factor, at least not that much speed. I'm used to going down rivers in fairly fast canoes. In a dory you are mostly just floating. It might frustrate me. If you do a trip down the Green or Colorado to the confluence, which I've done quite a few times, you have to take a big jetboat back up the Colorado (see my thread on the subject). There is a lot of sticks floating down the river so periodically they stop and open a little hatch to clean out the intake ports. Takes several days to float down the river in a canoe and a few hours to ride back up in the jetboat.
     
  9. DCockey
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    DCockey Senior Member

    The 30 foot Surf Rescue Boat,prototype in service in 1977, production boats in service from 1982 to 2006, had a documented high speed instability. It's described in American Coastal Rescue Craft: "All of trhe production boats exhibited some degree of dynamic instability when planing at high speed, which could result in the loss of rudder control plus a tendency for bow depression to the port side (resolved by slowing down to idle speed)". Lou Codega, a naval architect who formerly worked for the Coast Guard and the Navy, was involved with the investigation of the 30 Ft SRB. He dicusses it in his overview of dynamic instability which appeared in Professional BoatBuilder #31, pp 20-28. Donald Blount and Dean Schleicher have a more recent article on dynamic roll instability in PBB #84, pp 26-34. (Professional BoatBuilder back issues can usually be viewed at the PBB site.) It's more complicated than just convex sections in the bow.

    Ike - do you know of any similar problems with the newer 47 foot class? I have not heard of any.
     
    Last edited: Nov 12, 2011
  10. Ike
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    Ike Senior Member

    It is more complex than that but I didn't want to get into equipping the boat with pressure sensors and pressure gradients, tank testing and so on. If they are interested, the study done by NRLB is available. Here's the link to the PROBOAT article http://www.proboat.com/predicting-bad-behavior.html I knew Steve Cohen who was the primary NA on the design and Lou. I am not an NA even though I have spent most of my life working in Naval Architecture. I and an NA that I worked with, Peter Ball (unfortunately deceased), discussed the 30 foot SRB with Steve on several occasions although we had no input into the design or the subsequent studies.

    I was first introduced to this boat in 1980. We had one where I was stationed at USCG Base San Francisco, that along with the rest of them, had been hauled out and was sitting on the hard about 100 yards from my office. I had a lot of time to look it over. I also got to know Master chief Boatswain mate Rogers who was the person who first tested these boats for the Coast Guard. he was also the first person to be tossed out of one when it nose dived. (a dubious honor)

    After I transferred to Boating Safety, Standards division in HQ, I and Peter Ball had many discussions about this boat and it's unfortunate characteristics.

    Meanwhile the 47 was on the drawing boards and if you look at the hull lines it is just a blown up 30 foot SRB. It is an excellent boat and has been involved in a lot of dramatic rescues. It is just a matter of making sure they get the right training.

    By the way, if you do a google search on dynamic instability one of the hit's is Lou Codega.
     
  11. DCockey
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    DCockey Senior Member

    Ike's link is to a third, more recent, ProBoat article about dynamic stability.

    I'm very skeptical that after the exprience with the bad behavior of the SRB the Coast Guard would build the 47 footer without ensuring it was free of that behavior which was known by the time the contract for the 47 footer was issued.
     
  12. river runner
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    river runner baker

    Checked out the proboat link. A bit above my head, and I have no plans to build a powerboat, but I plan to study the article and see if I can learn anything from it that would relate to dories or sailboats. If nothing else, it might help me ward off alzheimer's.
    Ike: do you have any links to photos of the older design, or the 47 footer? I still think something can be learned from it's bow shape. A photo of the bottom would be helpfull too.
     
  13. Ike
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    Ike Senior Member

  14. Ike
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    Ike Senior Member

    Sorry David. 34 years with the Coast Guard left me rather skeptical of in house design work. Not that they are incompetent, on the contrary. But they have to deal with many competing concerns, which often makes the design a compromise. Sometimes that compromise doesn't work out. Look up the USCG 110 foot Patrol boat life extension (and length extension) http://www.uscgnews.com/go/doc/786/138897/

    And that is a rather nice article on it. You can find some that really blast the USCG for this boondoggle.

    That is not to say that everything the naval engineering division does is no good. Most is really outstanding. But it seems the goofs are just as spectacular.

    But as I said before the 47 has so far been a very successful rescue boat.
     

  15. river runner
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    river runner baker

    Is that as dented as it looks? That skeg looks balasted. Yes?

    For quite a while I was unable to look at the replies to my own thread, so getting back to some comments made a number of posts back: I talked about raft vs. dory in a previous thread. I once owned an 18' cataraft (inflatable pontoons). Inflatables can be a real pain in the behind. I'd prefer not to own another one. Catarafts are fairly responsive, but you'd be surprised at how little even an eighteenfooter can carry. The oval shaped rafts, that most people are familiar with, row like garbage scows. If you hit a big rock in the middle of a big rapid, you are in a world of hurt no matter what you are paddling. I'd rather be in a boat that I can maneuver around the rocks and not worry so much about what happens when I hit one. In the Grand Canyon, if you follow someone that knows the lines through the rapids, hitting a rock is not as likely as flipping.
     
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