Porpoise Perplexed?


Longtime fellow Woody Boater Brian Swanson sent us in this question. Now of course this is the internet where everyone has an opinion. Which of course is part of the advice roulette, and somewhere in the comments is an idea you may not have thought about. Like could using Marvel Mystery Oil cause your boat to porpoise? Here is Brian’s note.

Hello Woodyboaters, when I went out for my first run, my 331 Hemi only gave me 1500 rpm at max throttle.

Brians Hemi

I did various things to correct the problem: changed the plugs, changed the electric fuel pump and even tried some additives to help compression. My first compression test gave me readings in the 60 – 80lb range. So, thinking that maybe some rings were sticking because it sat for a season or two or that it needed some lube to loosen things up I tried that Marvel Mystery Oil. I put some in the cylinders and in the fuel. After doing that I was told that I might have done the compression test wrong the first time. I did it with the other plugs in and the throttle wasn’t wide open. So I removed all 8 plugs and opened the throttle and did the test again and then got compression readings in the 150 – 160 lb range. Before putting it away for maintenance, the ranges was in the 110 -120 lbs or so.

At Smith Mountain Lake – 2009

After doing all that, I finally figured out that one of the carbs was clogged. I unclogged it and now it runs like a Hemi should. However, now the boat porpoises at high speed. If I push it over 3300 RPM it starts to bounce and it gets progressively worse. Any more RPM and I think I may bounce it out of the water. I’ve had this 22′ Shepherd since 2007 and It NEVER did that before. I used to get 3800 RPM or so and about 40 mph. Now anything over 3300 and it’s Mr. Toad’s Wild Ride.

Old can photo for illustration purpose? Porpoise?

Could my Mystery Oil have increased the compression above what it used to be? Am I getting more power at 3300 RPM and lifting that boat out of the water? Any thoughts? You know of anybody in the hobby that may know?

Brian does knows his stuff and is confused about this and thought it might be a fun Woody Boater story.

So to help our readers, I did ask these questions.

1. New Prop? NO

2. Hull been worked on? – NO

3. Engine moved? – NO

4. Hull damaged, sitting wrong on a trailer? – NO

5. Have you thought of just changing the name of your boat to FLIPPER and move on? – NO

So there ya have it. A true Woody Boater question.



25 replies
  1. Ian
    Ian says:

    Off loading from the trailer you may have banged bent prop or prop shaft – carefully inspect

  2. m-fine
    m-fine says:

    Even if the oil changed the static compression readings, it will not give you additional horsepower. Also, same hull, same weight, same prop, at the same RPM is the same hp being delivered no what the engine is capable of delivering.

    So, definitely not an engine issue. The most likely candidates are prop/shaft damage or hull warping/damage. Is anything loose or flexing, especially near the stern?

  3. Gary Van Tassel
    Gary Van Tassel says:

    OK, I have to respond to this one! As some of you know I am a Naval Architect. I look forward to reading the comments. Someone may have experienced this and found a unique and practical cause. Love to hear about it.

    A couple of points about porpoising.
    1. It is longitudinal instability. The mechanics are somewhat complex and not all that easily evaluated.
    2. It is speed and trim dependent. Not power dependent. So what would be interesting to note is the speeds that it occurs at. Obviously power changes effect speed, my point is knowing what RPM it occurs at isn’t helpful unless a speed for that RPM is known.
    3. The LCG (longitudinal center of gravity) is a key factor. Too far aft boat will porpoise. Move LCG forward to cure. Or artificially lower the trim by adding trim tabs or wedges.
    4. Porpoising is trim dependent. The higher the trim the greater likelihood of porpoising.
    3. Props can have an effect as they can be configured to provide lift, but usually only on high performance stuff. You said no prop change. If you had a prop generating some lift and changed to one that didn’t, that could cause the boat to now porpoise.

    The odd part is that the boat did not porpoise and now it does. You answered the questions I would ask, but one. Did anything change, moved or took out weight in the boat, etc. moving the LCG aft. What were the sea conditions you were in?

    I have a KAVALK Classic 19. It has a modified deep Vee hull and the straight buttock planning surfaces aft are a little short (INMHO). It is an outdrive boat with trim tabs so I have a lot of control over running trim. With drive in neutral trim, shaft parallel to water flow, at below 40 mph it takes some amount of trim tab to keep the boat from propoising. At speeds above 40 mph up to 55+ no trim tab. As the trim decreases with speed porpoising tendencies decrease. What is odd to me is in your case porpoising is increasing with speed when the trim should be decreasing.

  4. Kw
    Kw says:

    Boat was stored wrong bottom has a hog a couple of years not supported properly can do this

  5. syd
    syd says:

    Another thing on the lcg, do you usually have people in the aft seat but did not this time?

  6. Gary Van Tassel
    Gary Van Tassel says:

    KW, your point is well taken with one exception Hogging will not cause porpoising, but sagging will. When the hull sags, lower in the middle, the buttock line rocker aft will increase. That will cause high trim angles.

    Another question for Brian, is the running trim of the boat different from past experience, trim noticeably increased? Do you have a distinct hump speed? Trim increasing then noticeably drops with an increase of speed and no increase in throttle as the boat gets on plane?

  7. John Rothert
    John Rothert says:

    Gary said it all. This seems more mystery than mystery oil….

    tabs would look awful unless you got or fashioned some of the really neat period type….and would make a big difference.

    otherwise ???? John in Va.
    headed to Harborfest in Norfolk…rain be damned!

  8. Mark in Ohio (sometimes da U P )
    Mark in Ohio (sometimes da U P ) says:

    I agree with some of the other comments. I think it is a bottom problem. How was the boat stored? Did you change anything on the bottom over the winter? I would think if the prop-shaft was bent bad enough to make it purpose it would shake. Good Luck

  9. floyd r turbo
    floyd r turbo says:

    I don’t see how a bent shaft or prop would cause porpoising unless it created too much pitch in the prop. Bent shaft would cause vibration. Something in the above narrative must be false regarding the bottom and it being changed due to storage for it to porpoise. We had a customer’s 19′ double cockpit Garwood porpoise because the original tired Chrysler Crown was replaced with a 360 c,i. Chrysler V8. At higher speeds which were unobtainable with the old 6, the boat would porpoise more and more the faster you would go. The Crown was rebuilt and reinstalled removing the V8 solving the problem. I’m wondering if there is a little more horsepower that the owner would not necessarily be able to quantify from last layup to this point. That and the bottom changes are the variables that haven’t or can’t be quantified without technical tools to evaluate those variables as to their impact on performance. Yet the owner says he got higher RPM in previous years without porpoising and now gets it at a lower RPM which, by process of elimination, points to a bottom change, namely bellying somewhere on the planning surface.

  10. Andy C
    Andy C says:

    I put a wedge on the bottom of a chris craft on the side that it was leaning to. It did the trick. Also, make sure that there is no play in the running gear.

  11. Brian of Swanson
    Brian of Swanson says:

    Thanks for all the comments!!

    I suspect it must be a bottom issue then.

    A few years ago, I sustained some pier damage that required the replacement of a few side planks. It was large enough that I decided on a whole refinish job as well. For me that was an expensive repair so I did it in stages. In short, the boat sat on a trailer for three seasons with, who I thought were, competent experts. I got the boat back and the repair and finish look good, but the motor didn’t work, two gauges don’t work and now it porpoises.

    If the bottom did some shape shifting while sitting, now that it is back in the water, what are the chances that it will work itself out?

    Thanks all – appreciate the help.

    B. Swanson

  12. floyd r turbo
    floyd r turbo says:

    Shingle wedges as Andy indicated are worth a try. One 3 or 4 inches wide about 6-8 inches long 3/8’s to 1/2″ on thickest end. Try to place over a location at transom using 2 existing planking screws fore and aft so you don’t create extra holes in the planking, Placement and thickness will be a trial and error process. But that’s a starting point

  13. Briant
    Briant says:

    Uh yeah. Since nothing in the boat has basically been changed….and since the Naval Arch mentioned it….it clearly is a weight/center of gravity issue.

    And the only things possibly different now as it porpoises is the amount / weight of fuel in the tank and the um…uh…maybe a new and improved weight of the operator. I mean it has been three years or so…..

  14. Dan T
    Dan T says:

    If it sat for a couple years, the hull may have dried out and lost a bit of weight in the bow. I’d add a little weight forward and see if it makes a difference. Good Luck!

    • Phil Jones
      Phil Jones says:

      This boat is already carrying an extra 320 plus lb. in the bow as that is where the fuel cell is located. 33-3400 rpm should be top RPM. This is a strong boat not easily affected by weight change or distribution. She is built like a tank. Hate to say this but you have a hull problem. Check for sag is my bet. Also I know she sunk. That can do strange things to a hull, sinking or when she is raised, may have pushed the sides or transom out of whack. Best bet is to try wedges, and would suggest larger shims in the 8-12″ length, and 6″ wide, 3/4″ to zero. These boats can handle much more trim than a CC or similar designed boat.

      Brian good to here from you after all of this time. The reason I became a Shepherd fanatic is your fault. We met at Smith Mt. Lake years ago and I got a ride with you on yours. I was hooked. I searched for one until 2011. If you get the Rudder, Purple Haze was a feature boat in the spring 2018 issue. She’s a 1948 22′ runabout model 110, with an upgraded 354 dual quad Hemi. Yes I shoehorned her in but not a simple or easy task. I have eaten and slept Shepherds ever since 2011.
      Give me a call or email me we can talk.

      Its strange I have not been on here for quite a while and run into this post, must have been fate.:)

      Phil Jones

  15. Troy in ANE
    Troy in ANE says:

    Sounds like the bottom has sagged from sitting on the trailer. I would be curious to know what kind of trailer it is sitting on.

    Another thought to not overlook would be movable equipment that may not have been put back in the same place. (IE: A 30 pound anchor that used to be stored in the bow that is now in the engine compartment or not in the boat at all with make a huge change in the LCG)

  16. Dave Hughes
    Dave Hughes says:

    Brian – have you checked your rudder shaft and steering gear to ensure there is no excess play in any of the steering mechanism?

  17. tom
    tom says:

    I own a’57 22′ Shepherd with a 354 (M45s-3),since the mid 1980’s.Two single barrels, like yours.I’ve seen alot of marine hemis in that time,but never saw one with the flame arrestors like yours.They’re always a single piece cast body with the brass arrestor screwed to the intake.They just slip over top of carb,held in place with a thumb or wing bolt.Your carbs have a different top,choke set-up to.Was this how it was when you aquired the boat?Great looking boat,by the way.

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