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Thread: Donzi 16 sacrilege - dolphin tail??

  1. #1
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    Donzi 16 sacrilege - dolphin tail??

    I think I know the comments I am going to get before I ask, but I have to. Has anyone ever tried a hydrofoil on a 16 or 18. I know there is a lot of negative press about "gimmicks" and don't work, but hear me out. I have a 16 with 350 retrofit. And I am not sure if it has to do with the extra weight or just part of the 16 hull design, but porpoising is pretty bad at about 30 MPH. I have Bennet TT's so I can adjust it away or drive through it, and it goes away at about 35MPH and stays away.

    I previously had a 18 Baja and it already had one on. My friends made fun of me so I took it off. Well it was night and day. With the dolphin tail, it came up on plane much quicker, little bow rise, and did not effect top end one bit. And it actually felt more stable at top end (on a good day 54). So here is the question. Has anybody tried this and would they recommend one, and if so what brand? Or is there anybody out there that would like to talk me out of this because it's a bad idea. I have heard negatives that it hurs outdrive and may become unstable at higher speeds (p.s. I am still exploring that area so I don't want any surprises!!

    Thanks for any insight into this.

  2. #2
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    My 16 had a Dolefin on it the season before I bought it, it was on the boat for a few weeks. My buddy thought it would help with skiing, it did it popped up quite a bit quicker. Now the BAD, when he trimmed for speed the azz end sucked down. The trim had to be left in the neutral position cutting top end quite a bit, He pulled it off and gave it away. I filled the holes in the cav plate when I re-painted drive after I bought it, IMO don't waste your time go for tabs that is the one thing 16's needs. The addition of tabs makes the boats so much better in many respects, some say they don't need them me I like full control of my boats all have ended up with tabs.

    Phil
    No matter what your beliefs are "GOD BLESS AMERICA"

    Fully retired marine tech near 60 years in the biz.

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    Whatever works... I almost bought a fin then went to the Nauticus Trim Tabs and love them for my boat. No big deal to install and they seem to give the boat a much better feel in the choppy stuff and no porpoising whatsoever. They are adjustable (not with controls like your bennetts) but set and forget. They have a shock actuator and adjust up a little when you are at planing speeds.

    But you already have trim tabs so you probably don't want to replace them with cheaper versions.

    Maybe somebody else can jump in with the right answer.
    Bob

    Covington, Washington
    1966 Donzi 16' Hull #16-226



    here are some short videos...

    a little video clip of our boat idling over to the boat launch so you can hear it.
    http://www.youtube.com/v/GSx8zAmi3R0

    Eaton outdrive rotating 101
    http://www.youtube.com/watch?v=auaFB...yer_detailpage

    thread on our boat
    http://www.donzi.net/forums/showthread.php?t=61975


    Video of the boat on the trailer

    http://www.youtube.com/watch?v=S9qXz4CJS8U

  4. #4
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    Quote Originally Posted by BobinCovington View Post
    Whatever works... I almost bought a fin then went to the Nauticus Trim Tabs and love them for my boat. No big deal to install and they seem to give the boat a much better feel in the choppy stuff and no porpoising whatsoever. They are adjustable (not with controls like your bennetts) but set and forget. They have a shock actuator and adjust up a little when you are at planing speeds.

    But you already have trim tabs so you probably don't want to replace them with cheaper versions.

    Maybe somebody else can jump in with the right answer.

    MOP did jump in with the right answer......

  5. #5
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    yes he did, but when I was forming my words of wisdom to reply, I had not seen his post yet.
    Bob

    Covington, Washington
    1966 Donzi 16' Hull #16-226



    here are some short videos...

    a little video clip of our boat idling over to the boat launch so you can hear it.
    http://www.youtube.com/v/GSx8zAmi3R0

    Eaton outdrive rotating 101
    http://www.youtube.com/watch?v=auaFB...yer_detailpage

    thread on our boat
    http://www.donzi.net/forums/showthread.php?t=61975


    Video of the boat on the trailer

    http://www.youtube.com/watch?v=S9qXz4CJS8U

  6. #6
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    I had one on my 1994 16 classic - can't recall the brand of fin. It made a huge difference for getting on plane quickly. There was no impact to top end speed gauging by the top mph that others have posted over the years for their 16s.

    Any porpoising at I was able to control with the trim switch - but I guess this won't help you out with your Eaton drive. I never felt the need for trim tabs as the boat was always stable, other than chine walk at very high speed and high trim.

    And...who really cares what anyone has to say about whether you have one on your boat if it sorts out a performance or handling issue.

  7. #7
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    Ignore the Eaton drive comment, I mistakenly thought this was Bob's original thread.

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    Thanks for the feedback. I think I will spend another season playing with the trim tabs first, before I drill holes in the outdrive for the fin. Has anyone heard that they actually can cause damage or stress to your outdrive?

  9. #9
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    The 16 is a seasaw at lower speeds, and the tabs work. The best thing I have found is to drive fast, really fast!

  10. #10
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    We had one on the 16. It worked great but in time we accomplished the same effect by reworking the prop and picked up a few mph. Not a bad short term fix but there are better ways to go.
    2001 35 Fountain Lightning w500 EFI's
    1973 X-18 - Merc 383 Magnum
    1974 Chris Craft Roamer with 1271 TI's

  11. #11
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    Quote Originally Posted by The Hedgehog View Post
    but in time we accomplished the same effect by reworking the prop and picked up a few mph. ..... are better ways to go.

    this is true for every 16 well maybe except an Eaton 16( not sure if there are any props that will help ) The best money spent will be on a prop. the porpoise cycle can be limited to a small speed range or almost eliminated with the correct prop. The cause is the prop to begin with so why try to cure the symptom and not the cause???( porpoise is caused by the presence of bow lift then the absence of bow lift , some props cycle is very large and can take away all mid range cruise without some other force like tabs. I guess this happens at certain speeds were the prop cannot maintain the lift the bow falls and and as the prop builds lift the cycle repeats) I have seen props that will do it from 25-35 mph and others that only do it at 25-27 mph

    Tabs are a good investment for rough water handling and load balancing but get used less and less once the boat is propped correctly. so stop and prop before you drill

  12. #12
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    I'd rather die than being seen with one of those on my boat....
    Lake St. Clair, Lake Huron, Lake Erie, Lake Oakland

  13. #13
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    Quote Originally Posted by Carl C View Post
    I'd rather die than being seen with one of those on my boat....
    Carl, if you had a really nice Bimini, no one would notice the flippers on the tail. DJ
    Avatar: RELIANCE (1903) The Largest single masted sailing yacht ever built (Until 2004)...with over 16,000 square feet of sail. Successful Americas Cup Defender.
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  14. #14
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    My boat had the phins on when I bought it.
    Made it real easy to climb the drive getting out of the water.
    Keeps your feet off the prop.
    I took them off and got 2-3 mph faster.

  15. #15
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    So maybe you can solve some with prop selection. In our case, the old Eaton had the shaft replaced with a stronger inboard shaft and better prop and after a little trial and error, it took the porpoising totally out of it (according to my dad). The original Eaton prop hubs were twisting apart with our motor. It doesn't look like it should work, but it does.

    Attachment 60993



    When I added the trim tabs (Nauticus) they helped it come up on plane faster and also nearly eliminated the chine walk effect
    Bob

    Covington, Washington
    1966 Donzi 16' Hull #16-226



    here are some short videos...

    a little video clip of our boat idling over to the boat launch so you can hear it.
    http://www.youtube.com/v/GSx8zAmi3R0

    Eaton outdrive rotating 101
    http://www.youtube.com/watch?v=auaFB...yer_detailpage

    thread on our boat
    http://www.donzi.net/forums/showthread.php?t=61975


    Video of the boat on the trailer

    http://www.youtube.com/watch?v=S9qXz4CJS8U

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