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Thread: porpoising / prop issue?

  1. #1
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    Default porpoising / prop issue?

    I just got a new 17 foot wooldrige sport offshore with the offshore bracket and a 115 etec. The boat has a deep vee and propably weights almost 2000 pounds with the motor fuel and two people.
    The issue appears to be that it porpoises a little when the motor is trimmed up. The seats are forward and I put the anchor and line in the bow so there is weight in the bow but there is a lot of weight aft.
    The prop is an aluminum 15X15. I can stop the porpoising but it appears to slow the boat and I think it would burn more fuel. I can turn 6000 rpm but do not have a speedometer so am not really sure of the speed. I think I can take the gps from my car and get a speed reading but have not done so.
    I go on and off of a rocky beach regularly so I did not get trim tabs and would like to stay away from a really expensive prop because there is always the danger of hitting a rock.
    Any suggestions for a prop change that might reduce the porpoising?

  2. #2

    Post Dolphins

    You could add a pair of dolphins on the lower unit. They work well on my wooldridge deep water plus. More affordable than trim tabs. Good luck.

  3. #3
    Member SusitnaAk's Avatar
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    Trimmed up? The motor should be trim down or in to keep bow down,Trim up is bow up. I don,t think a new prop would change this condition. You have to trim in Or move your weight forward, I friend of mine had a issue like this and the motor wouldn,t trim in any further, What we did is shim the engine on it,s mounts.They make angled little metal shims for this. Remove top bolts and put shim in and replace bolts, Might have to buy longer bolts worked perfect, Also look up those dolphin fin things they could help too.Me and tikahtnu typing same time
    Last edited by SusitnaAk; 05-05-2009 at 21:36. Reason: spelling

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    I was not very clear in my first posting, I can stop the porpoising by trimming the motor down. but assume it does increase the drag in the water and either increase fuel consumption or slows the boat. I need to use the gps to check the impact on the speed.
    I used a dolphin on my previous boat. I may have to do this again.
    I may remove the trolling motor to see if removing some weight far aft reduces the motion.
    Thanks for the information and ideas

    [QUOTE=SusitnaAk;490814]Trimmed up? The motor should be trim down or in to keep bow down,Trim up is bow up.

  5. #5
    Moderator Paul H's Avatar
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    Too much weight aft. I had that problem with my boat when I trim the engine up for max speed/economy and fixed it by shuffling gear forward and moving the fuel tank forward.

    Boats have a hydrodymanic balance point based on the shape of the hull, and then there is the center of gravity based on where the weight is distributed on the boat. When the weight is too far aft, the hull will porpoise at speed.

    There are dodges to deal with the issue, trim down, trim fins and dual fins, but if you can move the weight forward it is the best approach IMHO.

  6. #6
    Member Dan in Alaska's Avatar
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    Changing the prop work do anything to stop the porpoising, but slowing down helps smooth out the ride.

    I put a hydrofoil on my 22-foot SeaRunner, and in this specific situation, it helped with the porpoising. I wish I would have put it on sooner.

    There are local Anchorage dealers that sell them: West Marine, Deweys, and Sportsman's Warehouse. Here's a link to Cabelas
    so you have an idea of what I'm talking about. I put the SE 300 on my Yamaha F115. If you're not keen on drilling holes in your expensive outboard, they offer a "no-drill" mounting bracket now (Sport Clip).

  7. #7
    Member AKBassking's Avatar
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    I find if I trim the motor "up" too much I start to porpoise. I just back it down a little until it stops.

    ALASKAN SEA-DUCTION
    1988 M/Y Camargue YachtFisher
    MMSI# 338131469
    Blog: http://alaskanseaduction.blogspot.com/

  8. #8

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    Same as bassking. Trim to far up (nose down= porpoise) same as a floatplane.

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    Member breausaw's Avatar
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    Default Permatrim Hydrofoils

    If youíre considering installing a Hydrofoil consider the Permatrim, http://www.permatrimmarine.com/

    First time we took out our 22ft c-dory it had a tendency to porpoise, after installing a Permatrim you canít make it porpoise. Really satisfied with the performance and trim enhancement achieved with the Permatrim; its bullet proof.
    Jay
    07 C-Dory 25 Cruiser
    OurPlayground.


  10. #10

    Default

    I'll second the permatrims. I haven't used them but have a friend who loves them. I've read a lot on other forums and they appear to be great. They aren't plastic like other brands either, either aluminum or stainless(I think). I'll be putting two on our tomcat this summer. I don't think trim tabs will work on that hull, so hopefully the permatrims will help.

  11. #11
    Member Rod in Wasilla's Avatar
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    As far as hydrofoils go, the Permatrim is expensive (relative to other hydrofoils, not to other boat stuff) so I made my own using some dimensions I found on the interweb. I had some sheet steel, so all I had to buy was bolts and paint. It cost me about $10 total. It's not fancy stainless or aluminum, but I've been using it for a couple of years and it works great. On the down side, I should probably take it off and repaint it this year...

    Anyway, I recommend the Permatrim... or at least the Permatrim design. It really helps with the attitude of the hull in the water. With it on, small trim adjustments make a bigger difference. In smooth water I can trim the bow higher without porpoising and gain a little speed. Or I can really nose in to the chop for a smoother ride. Other benefits (on my boat) are that I picked up about a half a mile per gallon in fuel economy at cruise, and it lowered my speed to plane a couple of miles per hour. Competing with the $2.50 bubble level I mounted on the dash, my Permatrim knock-off is likely the best bang-for-the-buck modification I'll ever do to the boat.

    For comparison purposes, I pilot a 17 foot Arima Sea Ranger. It's a modified-V fiberglass hull with an 8 foot beam and a hard top. It's currently powered with an old 90 hp 2-stroke. And I've figured that we load up right near max load capacity when we go out for the weekends.
    Quote Originally Posted by northwestalska
    ... you canít tell stories about the adventures you wished you had done!

  12. #12
    Member Sobie2's Avatar
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    Definately I'd go with a Doel Fin or some type of Hydrofoil (I like and have used 3 Doel fins with great results). You can buy them at Harri Plumbing, Western Auto, and maybe Willies... I think even Tanners (where you got your boat). Any where you can get them for about $35.

    I bought one for my sister in-laws boat (18.5 North River Sea Hawk 115 yamaha) and it helps the boat plane out at much slower speeds so when the Taku chop or other nasty Southeasters come up they do well.

    Additionally you ought to be able to try different props from Tanners since you bought the motors from them. Sounds like your prop has a lot of bow lift. Any boat light enough with enough power will do what you describe its a balancing act and a foil and the right prop will take care of it.

    The standard budget props usually provide stern lift and you usually pay more for props with bow lift or stainless props (both being high performance).

    At any rate a you'll appreciate a foil regardless of what prop you end up with.

    Are you going to post some pictures to the forum of your new boat?

    Sobie2

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    Default wooldridge sport offshore picture

    Sobie2 will try to post picture see how it comes out.
    It sounds like I will need to get a fin of some type. Will also try another prop.
    Attached Images Attached Images

  14. #14
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    Default put on a permatrim

    I had a permatrim mounted today on the etec 115. Hopefully will launch tomorrow and try it out. I also had the battery moved forward in the boat. Have not tried since this change either.
    Will post how all this works after using the boat in varying water conditions.

  15. #15
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    Default etec permatrim

    I launched the boat today with the permatrim on it. The boat does keep on step at a lower RPM and lower speed. The porposing was greatly reduced.
    Top speed seemed to be down a bit as did max rpm. Speed 37 MPH and RPM 5800. I was alone and did not have anything on board. I did go back and look at the permatrim and it was slightly under water. I believe I need to raise the ETEC, may try one hole at a time.
    tomorrow will head out for the weekend with more weight on board. Will try set up as it is and see what happens.

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    Default

    How did your test go?

  17. #17
    Member Dupont Spinner's Avatar
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    Your cavitation plate should be just slightly above or at the bottom of the hull.

    My IB jet porposises after 35+mph and I have now figured out how much weight to move forward to get it to stop. A 150 quart cooler full of water on the bow solves the problem and I can easily adjust the weight by adding fish and letting water out if I am running light. I changed the nozzle around because I didn't like the fishtailing I got when running.

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    Quote Originally Posted by Alaskan XL View Post
    How did your test go?
    The boat was level in the water when anchored after moving the battery.
    Underway with more weight, wife and gear for the weekend, it kept on step at a lower speed and did not porpoise as much as before the permatrim was installed.
    I still believe the engine needs to be raised but is acceptable as it now performs.

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