Porpoise/Possible Trim Tab Problem

Discussion in 'Stability' started by hrdcor32, Jan 1, 2007.

  1. hrdcor32
    Joined: Jan 2007
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    Location: Wilmington, NC

    hrdcor32 Junior Member

    I am having a problem with my new 29' center console custom boat. I have having a stability and porpoising problem with the boat. I haven't had a chance to get it out in flat calm conditions but I have had it in 2' swell and 3-4 foot chop. It is a heavy boat and I think my porpoising problem comes from the transom being too heavy with the twin 250 motors on the bracket and the 155 gallon fuel tank being aft of the console. Also I think the trim tab deisgn is not well designed but I don't see how else the could be designed. The middle of the end of the tabs move 2 1/4" down when in full down position. When in motion they do not seem to do much for the stability of the boat or the porpoising of the boat. I realize I have not had enough on water time with the boat to get a feel for it in all conditions but I know something is not right. Sand bags have been added in the console, in the bow cuddy cabin, and in the bow anchor locker. I have not tried the boat without these as the previous owner added them. I have ran the boat in calm water for a short distance on a lake and it did not seem to have the porpoising problem but did have the same stability issues that the trim tabs seem to do little to correct. I have searched this website and done much research but have only found most revelant info to be on much smaller boats so I was just wondering if some of you more knowledgable folks could give me some advice on this. Thanks a lot in advance.
     

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  2. SuperPiper
    Joined: Jan 2003
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    Location: North Of Lake Ontario

    SuperPiper Men With Little Boats . .

    The 16' runabout at the cottage does not have trim tabs. But it does have power tilt on the 85hp outboard. Would trimming the motors DOWN reduce the porpoising?

    A ski boat we owned as teenagers would chine-walk. Wow, that was scary.
     
  3. hrdcor32
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    hrdcor32 Junior Member

    Trimming the motors all the way down will reduce it but not eliminate it. Plus this is not fuel efficient at all and I still get little to no response from my trim tabs. Thanks
     
  4. nevd
    Joined: Nov 2003
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    Location: Australia

    nevd Junior Member

    The last photo indicates the trim tab is relatively large, but if it is in the lowest position, it is approximately parallel to hull and stepped above the hull planing surface.

    If my interpretation is correct, then I recommend the cylinder mounts be packed so the trim tab can be lowered further to allow an angle of at least 3 degrees to hull planing surface.

    The problems you are describing can also be caused by having the motors mounted too low. Can you supply more photos showing mounting height with the AV plate parallel to hull bottom?
     
  5. RMSOSF
    Joined: Mar 2007
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    Location: Florida

    RMSOSF Junior Member

    What props are on the boat? Make and model of the prop please....

    Porpoising is the result of the engines trying to lift the bow, it gets to a certain point andthe weight overcomes the engines and the bow drops. This is generally due to 1) engines trimmed out too much, 2) poor weight distribution (that fuel tank being too far aft), 3) The hull having a "rocker" to it (see note below), 4) engines mounte too low or 5) improper type of prop. A bow lifting prop can cause this.

    You can check for a rocker hull by putting a straight edge perpindicular to the keel near the outside strake at the last 5 feet of the boat. If there is a hump there, u have a rocker hull which can magnify any other problems such as porpoising.
     
  6. hrdcor32
    Joined: Jan 2007
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    Location: Wilmington, NC

    hrdcor32 Junior Member

    The props I have on now are 3-blade Viper 14 3/4x19

    I also have a pair of 4-blade Turbo Ultima 4's 14 1/4 x 21

    I would like to try the 4 blades, but when I tried to put them on the splines did not come far enough out of the prop for the bushing to lock onto them? Does that mean these props will not work on these motors?

    I think I have the porpoising issue to a manageable state. The last time I took the boat out it handled good in 3 foot seas. However i have another issue: when in calm water (on the ICW) after getting the boat on plane and gaining some speed the boat will turn itself. It seemed to go starboard more often than not, but it did take atleast one hard turn towards port. To stop the turn I basically have to slow the boat down to the point where it comes off of plane. This is a serious issue and I don't feel comfortable operating the boat at any kind of speed with this issue. It did not seem to do this out in the ocean though and we were cruising at 25-30mph. I know this boat will go 50+mph but I'm sure not going to try it until I get this handling issue figured out.

    Thanks for all the help thus far guys!
     
  7. RMSOSF
    Joined: Mar 2007
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    RMSOSF Junior Member

    The Viper prop has a lot of rake to it, meaning it is a bow-lifting prop. the more power you put to those props, the more they want to lift the Bow. You need a prop with less rake, one with Transom lifting characteristics. With a prop of that design, you will be able to trim the motors out to acheive a better riding angle.

    The steering problem you have is related to your solution for the porpoising,,,,,,, you are trimming the engines and or the trim tabs all the way in to reduce the porpoising and at the same time creating "Bow Steer". Bow Steer is when you have too much bow in the water at higher speeds. It is a very dangerous situation, at higher speeds, it can easily flip the boat. The boat has some obvious setup and balance problems from the factory (sand bags installed when you got the boat).

    One other thing I saw in your pictures..... the trim tabs look like they are not installed right. check your tabs by placing a straight edge (4' straight 2X4) along the bottom of the boat to the trailing edge of the trim tab with the tab full retracted (up). With that size of a tab, you should have at least 3/4" - 1" gap between the straight edge and trailing edge of the tab. If not, adjust as necessary.
     
  8. hrdcor32
    Joined: Jan 2007
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    Location: Wilmington, NC

    hrdcor32 Junior Member

    Thanks for the great info!

    So what is your opinion on the 4-blade props I have, are they "transom lifting"? and what about the splines not coming far enough out of the props?

    I agree about the trim tabs, I will do the straight edge test when I can. Is there a suggested angle they should be to the transom when all the way down? I wish I had the armstrong style bracket so I could get wider and shorter trim tabs mounted correctly, but I guess I'll have to make what I got work the best I can for now.

    Thanks again!
     
  9. RMSOSF
    Joined: Mar 2007
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    RMSOSF Junior Member

    The trim tabs will only go as far down as the actuators will allow. Those tabs are not exactly set up the way they are supposed to be. The actuators that you have are designed to go on a much shorter tab. With the shorter tabs you would have more travel. If I see it right in your picture, the tabs are set up with a slight downward agle with tha actuators retracted (up), that is a huge no-no. The tabs need to be at a slight angle upwards when the actuators are retracted. The resulting down angle with the actuators extended is all your going to get due to the incorrect geometry of the tab. With the way that boat is set up, installing the tab properly will be a challenge.

    Now, with that said, if the tabs are indeed in a down ward angle, then they will push the bow of the boat down, causing bow steer. They could also be the source of your porpoising; with the tabs pushing the bow down and the props pushing the bow up, there is a constant fight there. If you correct the trim tab angle, you may find the boat to become too "flighty" meaning the bow will really want to soar at higher speeds, this will be due to the high-raked props pushing the bow up. You also might find the boat to handle a lot better and decide a prop change is not necessary.

    Trim tabs on this type of boat are only meant to level the boat from side to side and to help get her on plane, they are not meant to adjust the angle of attack at higher speeds, that is what the engine trim is for. Quite frankly, on a boat like that, I would never use the trim tabs except in heavy seas or strong winds.
     
  10. RMSOSF
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    RMSOSF Junior Member

    Those Turbo Ultima props you have look like the right prop for that boat except the pitch might be off.... one way to find out LOL. The reason for them not fitting may be due to something as needing a different thrust washer in front of the prop. On this web page: http://www.boatownersworld.com/Props/turbo/turbo_ultima_4.htm there is a link for you to ask tech questions on the prop.
     
  11. RMSOSF
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    RMSOSF Junior Member

    Any joy?
     

  12. hrdcor32
    Joined: Jan 2007
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    Location: Wilmington, NC

    hrdcor32 Junior Member

    Thanks for the great help. I'm still looking into it and will be taking the boat out this weekend to experiment with a few things. Thanks again!
     
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