Adding lifting strakes??? help

Discussion in 'Jet Drives' started by Akgramps, Aug 24, 2008.

  1. Akgramps
    Joined: Aug 2008
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    Location: Alaska

    Akgramps Junior Member

    This spring I acquired a 20 foot aluminum riverboat, and have some questions and concerns about the hull design. The boat is actually 19’-8” with a 60” wide bottom, outboard jet powered with a forward walk thru windshield, very similar to a Wooldridge Extra-plus. I would like to improve the handling if possible and would like to know if adding lifting strakes to the bottom would be of any benefit.
    I believe the hull to be a “warped hull design” as the deadrise at the transom is about 5 degrees and increases thru the planing surface to about a 11 degrees when it starts to curve up to form the bow.
    It has 1” X 1” center keel that ends just forward of the tunnel. It also has a 1” X 1” aluminum angle that serves as a reverse chine. The bottom is 1/8” aluminum and smooth with the exception of a 1” X 1”X 60” aluminum angle welded longitudinally from the transom towards the bow; it is located inside the chine by 15””. That is the way the boat was sold by the builder, I have added, starting at the transom, a 1-1/4” X 18” long “T-bar” within 3” of the chine, this was added as a recommendation by the builder as a “fix” to keep the back of the boat from sliding to the point of impending “spinout”, which can be a bit unnerving. The T-bar has helped considerably, however learning how to control the slide is the order of the day, and I am starting to get the hang of it. My prior boat was a Wooldridge 17 foot Alaskan and it rarely slid. The new boat will slide easily and I feel as if the deeper V of the bow cuts too hard when turning, when I initiate a turn the bow will dip slightly, while I am not concerned about this behavior in any way, it is just an observation and perhaps an indication about what is going on. I feel that the forefoot digs in more aggressively than it should and is possibly unloading the stern to some degree?
    I suspect this is due to the relationship between the fore and aft deadrise in the warped hull. I also suspect more deadrise in the back and less in the front would diminish this reaction. Neither is easy to change. I can live with the boat the way it is setup, however I cannot help but wonder what possibilities exist.
    My thoughts are to add one or two lifting strakes to each side. What I have in mind looks like something Harbercraft is currently using on their Jetcraft series. I have also seen a very similar design used on older Hewes craft, essentially it appears to be about a 3” X ½” (with 1 edge removed) channel starting at the transom and continuing through the planning surface ending midway towards the bow. My thoughts are these lifting strakes would help push the bow up to minimize the effect the forefoot has at that moment, and stabilize the rear. Will this help? Am I chasing a ghost? Are there other solutions to improve the handling? The reaction is the same lightly loaded and with a load.
    Looking forward to any advice..
    I will try to attach a few pics, the last one shows the stake design I am interested in using
     

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  2. lazeyjack

    lazeyjack Guest

    there is a whole thread on strakes - search for it here
     
  3. Akgramps
    Joined: Aug 2008
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    Akgramps Junior Member

    lazeyjack,
    Thanks for the reply, there is fair bit of info on strakes, I cant say that it gave me a clear answer to my particular question, hopefully someone can steer me in the right direction.
     
  4. lazeyjack

    lazeyjack Guest

    they have that strake on totally wrong, let me sketch it, that will not do a thing, you have monohydron with little deadrise, hard enough to make track with outboard, jet is more difficult again
     
  5. lazeyjack

    lazeyjack Guest

    this would be what i would do, abt 7 feet long, your boat is a very hard riding lake boat, normally strakes add to hardness of ride so try keep to min
    do this pair close in to start
     

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  6. Akgramps
    Joined: Aug 2008
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    Akgramps Junior Member

    Holy Mackrel..! Lazeyjack,
    You mean to say harbercraft has been doing it wrong all this time?
    I thought monohydron meant the chines and keel are parallel to each other?
    virtually impossible in a warped hull w/o constant deadrise?
    It appears what you have sketched would come together at some point on the keel, I suspect that design would be more benefical on a boat with a deeper keel. Also the boat is designed for shallow running in Alaska rivers, so not exactly a lake boat by any means, and yep..rides hard..perhaps the grog down under is taking its toll.......:D
     
  7. Ike
    Joined: Apr 2006
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    Ike Senior Member

    You have what is basically a jon boat with a little bit of Vee to it. Jon boats are notorious fro sliding whne on a plane because their bottoms ar flat. There is practically nothing on the bottom of your boat to keep it going in a straight line. Lifting strakes won't help much because from your own dsecription the bow dips as you go into a turn. This causes bow steering which leads to spin outs. Lifting strakes would just force the bow down even more.

    What you really need is a keel, maybe two to three inches or so deep, along the bottom of the hull to give it directional stability and resist sliding. Or a couple of strakes running absolutely straight from about 1/3 the length, aft to the transom. What you want is something that will resist sideways motion and keep it going in a straight line. That little bitty keel you have there is just too small to have any real affect.
     
  8. Akgramps
    Joined: Aug 2008
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    Akgramps Junior Member

    Ike,
    Thanks for your reply, To understand all this completely takes me awhile,
    The Wooldridge Extra-Plus has a larger keel, but still only 1-1/2 X 1-1/2, and otherwise the bottom is smooth, they have less d-rise then mine in the front, even though my boat is a sled style it still has considerable d-rise in the front.
    Are you saying that running lifting strakes all the way to the transom will pick up the back and not the bow when on plane?
    I am afraid more of a keel would cause the bow to dip more than it already does, as I think the existing keel caused by the D-ries and reverse chine is what is causing the bow steering?
    It seems perhaps what you are saying is the keel is more important aft for turning and stability, unfortunately on a jet sled the keel needs to end 2 feet short of the transom to provide smooth clean water to the jet foot.
    I have considered adding sister strakes parellel to the keel as you suggested and they may be my best option. The added T-bar helped considerably.
    Thanks again for your thoughts
     
  9. Ike
    Joined: Apr 2006
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    Ike Senior Member

    Yes, lifting strakes will cause the stern to rise but not help the bow. Because when you are on plane the bow is not in the water, so the lifting strake there has no effect. As you turn, the stern will be higher, the bow will lower and dig in.

    Directional stability is the ability of the boat to go in a straight line and turn without sliding. Lateral resistance is what keeps the boat from sliding sideways. You have very little lateral resistance. Lateral resistance is provide by the under water shape; a keel, fins, or the chines, if they are in the water when the boat is on plane. You need to increase the lateral resistance.

    I realize that the aft few feet must be clear to feed water to the jet pump. That's why I said " Or a couple of strakes running absolutely straight from about 1/3 the length, aft to the transom" These would not be lifting strakes, but just a piece of metal about 1 or 2 inches deep welded to the bottom running to the transom. A long short fin as it were. Go to your local dealer that sells john boats. Look at the bottom of the boats. Almost all of them have strakes running the length of the bottom. This is to keep the boat running in a straight line and not skidding in turns. You need something similar.
     
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  10. Akgramps
    Joined: Aug 2008
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    Akgramps Junior Member

    Ok, I finally found some time to do some expermenting, I have been examining the tunnel design and with the keel level to the world noticed the top of the tunnel was inclined down about 5-6 degrees. It should be flat or so I am told.
    Orginally the builder installed the tunnel with a continous rise to where it exited the transom, this made it very difficult to get he jet set right because the water coming from the tunnel was still rising as the tunnel did not flatten out.
    Anyway I had the tunnel extended about 4 inches and supposedly flattened, the problem with the 5-6 degree down angle is it was acting like a big trim tab and pushing the bow down. so a week ago I cut the sides of the tunnel and bent it up to a 2 degree rise. and then raised the motor about 3/4 of an inch to get the jet foot close, I may need to fine tune that some more.
    So, when testing, HUGE improvement, no big gains in speed but wow! is it a lot easier to steer, the helm was very heavy before, also turns better and seems to be on plane much better as the wave coming off the bow is much further back than before. Obvious a step in the right direction.
    What I have is a hull design that has a real propensidy to "bow steer", I suspect this is due to the relationship of the deadrise through the planing surface.
    I know this looks like a relatively flat bottom boat, and in many aspects it is, but the warped hull makes for some interesting issues. now I know why the builder said "My boats like some weight in the back", they certainly do as it helps keep the bow up! I could live with the way it is now, but I believe it can be better. No ez way to correct unless I am willing to alter the deadrise at the transom and give it more v thruout the planning surface.
    I am still fixed on lifting strakes running full lenght parellel to the keel about 9" off centerline, this will put them very close to the edge of the tunnel, possibly will help funnel water into the tunnel.
    Also I am considering enlarging the reverse chines from 1" angle to a 1-1/2" angle, I know the strakes will pick up the stern as well as the bow, however anything that will raise the bow seems like a postive reaction?
    Any help??? or advice, particuarily from some one that knows, loves understands riverboats.
    Thanks
     
  11. Akgramps
    Joined: Aug 2008
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    Akgramps Junior Member

    IKe, I have previously added two fins from 1-1/2 -bar about 20 inches long, these are at the very back of the boat near the chine, and they did help in the sliding department.
    Many jon style boats, and this is really not a jon style do have full lenght ribs.
    However there ar many sled style riverboats with smooth bottoms, Wooldridge has made a number of boats with this design, both IB and OB, and have heard they handle quite well.
    Not many boat builders build a warped hull design riverboat, and therin lies the problem that I need to overcome, the problem is the bow IS in the water when it is on step and it should not as most would assume, i am presumong with the bow in, the lifting strakes will help pick it up?
     

  12. Akgramps
    Joined: Aug 2008
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    Location: Alaska

    Akgramps Junior Member

    A couple of pictures I took with the boat on step and lightly loaded, I also expermented with trimming the motor while a buddy monitored the GPS.
    The boat ran best in a variety of RPM settings with the foot all the way down (forward), I could watch it slow down as I trimmed the foot out.
    The only throttle setting where trim made a diffrence was WOT, I suspect at WOT the propulsion from the motor is trying to force the bow down, when I started to adjust the trim the boat speed would drop until I had alot of trim, then the speed would almost match the what it was with the jet down, at that point the bow was starting to porpoise and would cavatate ez on a turn.
    The pictures were taken at various throttle settings with the foot all the way down.
     

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