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Thread: Sorensen long haul center fin

  1. #11
    Tim Weber
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    You are not maintaining the plane. Either you have a rocker in the bottom or the plane is too short.

    The bottom needs to be dead flat for 36" or so before it breaks. If you have a rocker " bump " the boat will react to it. If the plane is to short that can cause it to oscilate.

    Tim

  2. #12
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    Tim is of course correct, if the bottom isn't flat it will porpoise. In addition you have the engine mounted too low, if the shaft centerline is 35mm below the bottom, it's almost twice as deep as it should be. First thing to do is check the bottom and raise the motor up at least 12mm. You may also want to add some tuck into the setup (which will also drop the motor down more so you will need to raise it up some more after you do that.

  3. #13
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    Last 30" is dead flat, then it start to break "v", just like plans show. I found hydroracer.net thread where they mention 1/8-1/4 tuck. I assume that means negative trim but how it is measured? 1/8 from where?

  4. #14
    Team Member zul8tr's Avatar
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    A trim tool is usually used that attaches to the prop shaft that has a long straight piece that goes under the bottom. The trim is measured from that straight piece that is parallel to the prop shaft center line. You would measure up from that to the bottom at 2 points separated about a foot apart, the difference in the measurements is the amount of tuck or kickout. Here is a pic of such a tool, it is the one on the left. The other measures the depth of the rear end of the prop shaft center below the bottom.

    http://hydroracer.net/forums/filedata/fetch?id=345350

    They are very accurate if made right with a good snug fit on the prop shaft and the long piece truly parallel to the prop centerline with the machining. The tool needs to be checked for truly parallel of the long piece to the centerline of the prop shaft. A little bit of slop in the fit on the prop haft and off a bit on not parallel construction will greatly magnify away from the support at the prop shaft especially with the measurement taking farthest from the prop shaft attachment when dealing with 1/16 and 1/8 inch differences that mean a lot on these race boats.

    There are other ways to measure the trim if you do not have the tool.
    " Three may keep a secret if two of them are dead" Ben Franklin
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  5. #15
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    Default about the ride

    Quote Originally Posted by Jippe View Post
    First test drive done. Shaft is paraller and 35mm below.
    Boat porpoising very much (bow up and down). When moving my weight to forward it helps little bit but not much.
    What causes this? Prop, trim angle, motor height?

    Water was dead flat and prop 3 blade dewald cleaver.
    Jippe, hopping is not uncommon with kneeler runabouts at all, as long as it smooths out once you push the throttle open. You might need to go to about 3/4" shaft depth (center shaft end, to surface) once everthing is worked out. I think that's what the rules allow on "C" class Yamato powered runabouts "if I'm not mistaken"....It would be a good idea to double check the stock outboard tech manual on the APBA website about this stuff to get it right. It is important set up info that could help. It is on the sight and you just click and load it to read.

    My "B" mod runabout does the same thing until I open it on up...then it shoots on past 65+mph on real quick. I'm running a nice built Yamato 80 with a custom gearfoot of my design. I have mine at 1/2" below with a 2-blade cupped and edged prop that I had done. I'm running neutral (level) lately. I started out deeper and realized that I could get an extra couple or more MPH raising it on up. I followed ZUL8TR's advice on this and a few other tech info a while back, and it helped me out.

    When I start off, it cavitates. I just hold it at about 4000 or so RPM, then after a short bit, the RPM drops as the prop grabs and it accelerates hard. The ride involves a bit of galloping until I squeeze the throttle to smooth out and huddle myself down back and low. The ride is so fast, its hard to remember it.

  6. #16
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    Thank you all. I try 3/4 shaft depth next.
    Do you use some non-stock thrust brackets on yamatos?
    I dont have that trim tool.

  7. #17
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    20mm below and little bit negative trim. It porpoise less than yesterday but still too much..

    4 min. video
    http://youtu.be/pGfsdnRPbGI

  8. #18
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    Default running

    Quote Originally Posted by Jippe View Post
    20mm below and little bit negative trim. It porpoise less than yesterday but still too much..

    4 min. video
    http://youtu.be/pGfsdnRPbGI
    It looks like its running pretty good to start!....There is another video on tube that has a fellow "Charlie Smith" testing a DSR racer and it hops a bit more than your doing as is now. If you were racing other boats at the time, the water would be a bit more rough, and your hull would be running perfect in that kind of water. As far as tuck, what that means is 1/4" or 1/8" difference in height from the center end of shaft to the center nose of the gear case (where the other shaft end would be). This is in relation to the level keel line. As far as the bottom design, yours is correct. What those rules mean is that the keel has to be level for at least 36" forward till it starts the rake up toward the bow. It could be more if one wants, but no less. The planing surface has to be flat at least 18" forward from transom. It could be more, but not really necessary. When your boat is running at racing speed, it should only be running on around the last foot of pane surface anyway if its doing good. You followed the plans and your hull is just right.

  9. #19
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    Looks like you are getting it dialed in. A bit more tuck and you should be fine. You need to also make sure you are running fast enough, the faster you go the more it will smooth out. What prop pitch are you running, how fast and at what RPM's are you turning it? Also, you said you had a clever on it. What rake do you have as this makes a big difference. Most older runabouts didn't use cleavers, they tended to use round ear props, but I don't know what the current thinking is on that. The amount of bow lift that the prop contributes can make big changes so that could be your issue. I think if you get it running fast enough you should be fine but if this prop has too much bow lift it could be the biggest cause of the porpoising. I would talk to the Sorenson folks and see what prop the recommend.

  10. #20
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    Yes, almost there I just tried two, two blade prop and porpoising was totally gone around 40-50 mph.
    I think those should be run deeper than 3/4, because engine was screaming and there was huge water spray behind.
    I have one more prop to test, 3 blade ron hill cleaver 6 1/2 @ 11"

    First prop was 3 blade dewald cleaver, marked "osy 400 qualifier", I dont pitch or rake..

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