Pourposing

Discussion in 'Powerboats' started by CaptPPan, Oct 22, 2003.

  1. CaptPPan
    Joined: Oct 2003
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    Location: Bellport,N.Y.

    CaptPPan Junior Member

    My 27' center console inboard fixed surface drive boat pouposesat some speeds and RPM I can usualy correct and eliminate it with the trim tabs, but alot of trim tab deflection seams to slow the boat. I am considering installing longer tabs, or rearanging the waight somehow. Any input would be appriciated.
     
  2. mmd
    Joined: Mar 2002
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    Location: Bridgewater NS Canada

    mmd Senior Member

    There are several possible reasons for the porpoising that your boat is experiencing. Porpoising is caused primarily by speed, excessive bottom loading (a heavy boat for a given waterplane area), longetudinal centre of gravity (LCG) location, or a combination of these factors. Some solutions are available, and of these, some are easy and some are difficult and costly.

    Unfortunately, the easiest, cheapest, and most likely to be effective is the least palatable solution - slow down. Almost all dynamic-instability problems can be eliminated with a reduction is speed. Not what you want to hear, I'm sure, but sadly true.

    You can use external devices such as trim tabs or transom wedges to alter the running angle of the hull, but these add drag and slow the boat down, as you have discovered.

    You may want to shift some weight in the boat aft to see if this will reduce the problem. Check to see if this will work by putting a few hundred pounds in the form of buckets of water or fishing buddies at the transom and going for a spin. If you have a large fuel tank, do this when you are low on fuel so that you are shifting the LCG aft without adding weight to the boat. If this works, look for heavy things that you can permanently move aft, like batteries and fuel tanks.

    If your boat is steered by a rudder(s), they may be cavitating at speed and causing the porpoising. The cure here would be to replace it (them) with rudders that have cavitation plates at the top.

    If your outdrive itself is too near the surface of the water and consequently cavitating, it could be the problem. The fix would be to add an aftermarket cavitation plate above the prop.

    There are further things to do, but these entail re-working the hull, and I'm betting that you don't want to go there, at least not yet.

    Keep us informed about any further developments, and good luck.
     
  3. CaptPPan
    Joined: Oct 2003
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    CaptPPan Junior Member

    Thanks mmd
    Are you saying that moveing the LCG aft will most likely help the situation? If that's so then longer tabs may not help me.Tabs wil only give me bow down trim, I thought some long ones would give me some more planing area in the rear. The drive is a fixed surface piercing so it's always cavatating or more accuratly superventalating.
    I will try some water balast today.
     
  4. gonzo
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    gonzo Senior Member

    Porpoising is normal with fixed drives at some speeds. That is what the trim corrects, at a price.
     
  5. CaptPPan
    Joined: Oct 2003
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    CaptPPan Junior Member

    The water balast may have answered some questions. I used two 10 gal pails filled with water. I started with them in the back and the pourposing was worst then ever. I moved them to the bow and It was like a different boat alltogether, just a beutiful smooth ride. In a folllowing sea with about 25knot wind and 2-3ft swell the bow did plow alittle.
    I think the answer will be longer thiner tabs, I have short fat ones now. That way I'll get the trim I need and with alittle less drag. The short fat tabs are like puting the flaps down in a plane while flying.
     
  6. Willallison
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    Willallison Senior Member

    That makes more sense to me - I was always under the impression that porpoising was the result of the LCG being too far aft......

    Changing the tabs may well fix the problem, just as adding weight fwd will do. But how about onboard weight.....why not look at moving batteries, stores and the like further fwd. Fuel and water tanks can be shifted too, though its best to have them located as close to the LCB as possible, so that trim isn't affected as they are filled / emptied
     
  7. Guest

    Guest Guest

    I agree with you Will. I think MMD had jumpy fingers on the keyboard since I'm sure he knows this stuff.

    Tom L
     
  8. gonzo
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    gonzo Senior Member

    What was the behaviour of the boat at different speeds? Did it solve the porpoising at the top end and make it worse at cruising speed?
     
  9. mmd
    Joined: Mar 2002
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    mmd Senior Member

    "MMD had jumpy fingers on the keyboard " - <lol> more likely jumpy brain functions from trying to post a serious answer at 2:00 a.m. Sorry for the confusion, glad a solution presented itself.

    The balance of a boat on plane is such a delicate balance between LCG, centre of dynamic lift, and the hull form. All three must work in concert to provide a stable planing condition. Such subtle changes as a small change of a running strake's size or angle can alter the whole character of a hull's handling behaviour. What I have always found fascinating is that what is a reasonable solution for one hullform is an anethema in a different hull. As a book-end to the solution found for CaptPPan's boat, consider the following anecdote:

    A local boater had a porpoising problem with his 7-metre speedboat that was aggravated by light chop. I looked at the boat and thought that there was a preponderance of weight forward, which might be causing the hull to cycle between accelerating up the dynamic lift C of E until CofE passed under the LCG and the boat "fell off the wave". It would then immerse more hull causing additional drag and slowing the boat down, but the shape of the hull quickly added more lift and the bow went up, the boat accelerated, and the cycle began anew. In light chop, when the bow dropped into a wavelet, the additional lift and buoyancy seemingly "bounced" the hull back up with increased vigour. In the proper conditions, the magnitude of the porpoising increased, even though the throttle setting was constant, until it became quite alarming. We used the highly technical and sophisticated water-bucket trial method, and determined that by placing about 50 kilos about a meter ahead of the transom the resulting shifting of the LCG aft eliminated the problem. He relocated his batteries and solved the porpoising problem.

    CaptPPan, if you can still trust me (it is 2:00 p.m. now, so I am more awake <others may refute this>), I'd look at relocating some weight rather than swapping trim tabs. More tab - regardless of shape, will just add more wetted surface and appendage drag and though it may solve the porpoising problem, it will come at the cost of additional expense and lower top speed. Shifting some weight already in the boat probably won't cost near as much as a set of trim tabs, and the top speed should remain the same or maybe even get a bit better.
     
  10. Guest

    Guest Guest

    Sure enough MMD,

    In engineering, this porposing behavior would be classified as simple harmonic motion. The solution is to damp out the oscillations and the best solution is to do this without causing more drag. As you said, this sometimes calls for moving weight forward and sometimes aft. It's more common to have weight too far aft though. The number I've heard and used as a guide is to have the CG at least 1 1/4 times the max waterline beam forward of the transom. (I think I got that right)

    Of course, the hull shape forward and aft will have an effect too.

    Tom Lathrop
     
  11. CaptPPan
    Joined: Oct 2003
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    CaptPPan Junior Member

    The pourposing was mainly at cruising speed, when i gave it more throtle the bot flattened right out and the bow stayed down, at speeds much not to much slower then cruise the prop(about 2'behind the transom) would fully submerge and it does'nt seam to like that.

    I can move the batterys, that would be easy and I think thats a good idea. I'm also adding some more encloseure that will add some weight.

    If the weather is better tomorrow I'll try and take some pictures of the boat at speed.
     
  12. CaptPPan
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    CaptPPan Junior Member

    Boat Picture

    Sea Venture at speed
     

    Attached Files:

  13. gonzo
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    gonzo Senior Member

    The picture doesn't load. Could you post it again?
     
  14. Jeff
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    Jeff Moderator

    CaptPPan - for some reason the photo you posted got truncated and the bottom didn't make it to the server - I resaved the top half in your post above at least - hope that's ok.
     

  15. CaptPPan
    Joined: Oct 2003
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    CaptPPan Junior Member

    Jeff, I don't know what any of that means, but thanks.
     
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