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Thread: Boat Porpoising

  1. #1
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    Oct 2007
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    North end of Vancouver Island
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    Default Boat Porpoising

    On Sat, when I went out to wet the lines, the boat would porpoise when ever I reached anything above 3300 rpm. Last time I had it out, it did not do this. The only difference was that I filled up the boat. It was about a 1/4 empty. I tried to move up all the weight I could into the bow, but it changed very little. I have researched a few different options, including going to the boat tomorow and ensuring that the Cav plate is equal to the bottom of the boat. Also adding a "Whale tail, Sting Ray, Etc." or even adding trim tabs like these
    http://www.boatownersworld.ca/smart_tabs/sx.htm.

    When I took it out for the sea trial, it never did this, but also there was two of us and he weights about 280 (power lifter shape) and I am 220 (.......Shape). Could it just be a weight problem, too much in the stren and because it is aluminum, too light in the bow?

    Any help would be appriciated.

    Boat - 18" Hewescraft welded aluminum with a 34 gallon fuel tank, 115 yammy four stroke and 8 HT yammy 4 stroke. Also two batteries and downriggers, all in the stern.

    Sorry for the long post.


    Thanks

  2. #2
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    Apr 2006
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    Default load and trim

    Sounds like a trim issue... You did'nt mention about whether the motor has power tilt/trim...

    I run a 20' Hewes craft Sea Runner soft top w/ a Yam 115 and a Yam High Thrust 8... If I trim the motor "too much" my boat will do just like yours....

    I have to back the trim off and it will usually settle right down.

    The other issue has to do with the water conditions... Where they really the same?

    Read the below link.. I installed a Stingray on my boat. It helped and I found I can get on step quicker and hold step at slower speeds...

    But load placement and trim are really the key..

    http://forums.outdoorsdirectory.com/...ght=hydrofoils

  3. #3
    Member Mort's Avatar
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    Apr 2006
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    Default Trimming motor vs tabs

    Concur with Winmag, sounds like a trim issue. If the motor is trimmed up (toward the "trailering" position) too much, porpoise. Trimmed too far down, the bow digs in. Typically, I trim higher on calm water. Trim down a bit to keep from bouncing off the waves a bit.

    Trim tabs should generally only be used once the motor is trimmed properly, and then the tabs are used to right the boat from listing to account for uneven loads on the port/starboard side. Using trim tabs to get the bow down is just gonna cost you fuel.

    Good luck with a frustrating problem!

  4. #4
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    Default

    I have played with the trim (Power) and no luck. I have been around boats all my life and have never had this problem.

    The water was like pi$$ on a plate for most of days, with a small ripple the odd time.

    I am looking into the "Sting Ray" type addition as one option.

    Thanks

    SD

  5. #5
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    Default More info

    Some more info to ponder....

    I started out of the hole with the boat in the lowest trim (closest to the boat) and got up on step, I then started to increase the trim, at around 2800, the boat starts to porpoise, backed off on the trim, boat settles down and plows water, increase rpm's and boat starts to porpoise again, no adjustment on the trim, yet. Try to adjust trim and the boat becomes a carnival ride. The best I could stand and still keep a decent speed (26 mph land) was around 3700 rpm, but soon backed down to 3300 as my back was getting a good work out. One thing I did notice was that there was three tabs still left on the trim tab indicator on the yammy guage. Not sure if they are supposed to go all the way down to none or stay at three. It will another thing to check out today.

    I have had the boat out a couple of times and did not have this problem. I will check the motor to ensure that it trims all the way down, although I do not see how it will make a difference as I will then be plowing a whole lot of water. I will look into the extra fin as well. Any ideas on where and the best ones.

    Again thanks for all the help so far...

    Cheers

    SD

  6. #6

    Default

    Don't know if this will help or not, but here is my two-cents;
    When I bought my boat (Seasport) I could hardly keep it level or control it at any kind of speed. The trip tabs and outdrive trim are very responsive and I was all over the place regardless of the water condition. Each time I used it I got better at handling it, I could feel changes in the way I loaded it and soon was able to make it go faster and faster the more I practiced.
    Do you think its a issue of learning how it handles or have you had it long enough that its something else?

  7. #7

    Default Sounds arse heavy

    I had a similar boat (18' Alumaweld Intruder). It sounds like your a little heavy in the back, especially once the tank is filled. Before you go putting a bunch of money out for trim tabs, take a look at the bottom of the boat at the transom. Does the bottom of the boat extend past the transom a little? If it does, you can use that like a trim tab. Once you figure out how you USUALLY load your boat, you can modify that "ledge" which is actually the bottom of the boat. Bend it down a little bit, and the bow comes back down. You can also make adjustments for side-to-side load placements, bend the port side down a little bit and the port side of the boat wil ride higher. When I first ran my previous boat, it would porpoise terribly when I loaded it to go somewhere, a little re-working of that ledge and it's finally running how I like it. You can use a crescent wrench to bend that ledge a little at a time.

    Did I make that clear as mud???

  8. #8
    Member AKBighorn's Avatar
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    Default

    I have used the stingray and another brand of fin and they do what they were designed to do. Having said that I wouldn't put one on a motor that has power trim unless I was using the boat in shallow water and wanted a better holeshot. I have to agree with most of what has been said, play with the trim not necessarily looking at the gauge. Start with the motor in the straight down possition (parallel with the transom usually) and get on plane at a good clip. Then start adjusting by feel. It sounds like you have a fair amount of weight at the stern but you were able to overcome that if you were plowing water at one point. There should be a happy median in there.

    I had considered putting tabs on my boat but that was just so that I didn't have to tell people where to sit to level the load side to side.

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