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Author Topic:   porpoising?
alukban posted 06-26-2002 11:41 AM ET (US)   Profile for alukban   Send Email to alukban  
What causes porpoising and how do you stop that?
JohnAz posted 06-26-2002 11:47 AM ET (US)     Profile for JohnAz  Send Email to JohnAz     
how about more info on what boat , motor, and when it hapens....porposing can be eliminated with tuna nets,,,,
alukban posted 06-26-2002 12:02 PM ET (US)     Profile for alukban  Send Email to alukban     

Sorry... same mistake.

It's a '68 Currituck with 90 hp Evinrude. The water was flat, maybe 3/4 full throttle, one person, low on gas. Could the fuel supply have been bouncing around whenever the trim would change?

Bigshot posted 06-26-2002 12:02 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
good one John! I was gonna comment on Dolphin procreation but no need now.

Trim the engine in more or move weight fore or aft.

Jerry Townsend posted 06-26-2002 02:05 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Alukban - the porpoising is caused by the changing dynamics of the boat hull on the water. Virtually every boat has the same or related response. These forces include the hydrodynamic lift and trim of the hull and gravity. As someone pointed out, changing your trim in or out just a bit will correct it. Another benefit of powered tilt and trim. ----- Jerry/Idaho
Jay A posted 06-26-2002 05:57 PM ET (US)     Profile for Jay A    
alukban: If you got a fat mother-in-law,seat her on the bow! Make sure she's not too big or your engine might need a longer shaft! Now you know why they call it a "Whaler"!
SSCH posted 06-26-2002 08:59 PM ET (US)     Profile for SSCH  Send Email to SSCH     

Jerry's right of course. For each different type of loading in your boat the trim will need to be adjusted to compensate for the porpoising. If you are bow heavy, your boat may not porpoise at all. Stern heavy, and you may get this condition way too often. There is a sweet spot for each boat at each total load. You want the weight distributed such that you can trim the engine out away from the hull and reduce the hull contact with the water to obtain maximum speed without porpoising. If you load the bow too much, you'll lose speed and efficiency. With the stern too heavy, the engine cannot be trimmed out enough to get an optimum drive angle without inducing porpoising.

Just my two cents worth,

Boston Marine posted 06-26-2002 10:47 PM ET (US)     Profile for Boston Marine    
Buy a propeller that has more bow lift.
EddieS posted 06-27-2002 01:11 AM ET (US)     Profile for EddieS  Send Email to EddieS     
I dont think he wants his bow lifted if he is porpoising. A new propeller is overkill, or perhaps whalerboy is trying to be funny. The simple answer has already been given, trim the bow down slightly and the porpoising will go away.


mudpuppy posted 06-27-2002 09:42 AM ET (US)     Profile for mudpuppy  Send Email to mudpuppy     
I just move the Labrador closer to the bow and ask him to stay there.
Reliant posted 06-27-2002 12:55 PM ET (US)     Profile for Reliant  Send Email to Reliant     
Could the engine be mounted too low on the transom? A friend of mine who knows more about these things, explained to me that if the cavitation plate is dragging in the water while planning, it will cause the transom to lift and fall causing the boat to porpoise Try raising the engine up a hole or two on the transom.
Bigshot posted 06-27-2002 01:02 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I have seen that before. the reasons doel-fins keep you from porpoising is they keep the plate up. i have never had a porpoising issue in a whaler that was not curable with weight redistribution or trim/engine height adjustments. My Hydra-sport on the other hand had a factory hook that was not curable without a doel-fin or trim tabs if you wanted to trim it high. My Montauk with all that stern weight will trim until it cavitates before she'll porpoise, hence no reason to use fins.
dogface posted 06-27-2002 08:41 PM ET (US)     Profile for dogface  Send Email to dogface     
A Doe Fin or StingRay Fin works quite well to help calm down a jumping boat. I put a small StingRay Fin on my 10 HP 4 Strk Merc on my RIB.
Boston Marine posted 06-27-2002 08:53 PM ET (US)     Profile for Boston Marine    
By changing to a bow lifting propeller (By adding more "rake")this boat will be able to sustain the weight carried forward without having to trim down and as a result will attain a smoother ride.This same principle applies to any boat capable of higher speeds. His boat is not able to hold the bow up and as a result the propeller loses lift,boat comes down,boat hits water with forward part of keel,equal reaction,boat goes back up,propeller tries to keep bow up,not enough influence and bow comes down again.As a former tunnel boat racer I just thought I would add my .02
Jerry Townsend posted 06-28-2002 12:11 AM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
The subject of 'bow lifting propellers' has been addressed in other subjects of the forum some time ago - and I still question the concept. That is, with any prop, the prinicpal resultant force is thrust on the prop shaft axis. There is also a moment couple (viewed as an force normal to each prop blade located at a fixed radius from the prop axis - which effectively balance out) which will vary with rake and rpm. Of course, changing the rake will affect the thrust and I would expect some given rake angle to produce an optimum thrust/rpm performance.

Therefore, looking at the dynamics of a rotating prop, I don't see any bow lifting forces and therefore question the concept - but I have been wrong before. Previously, I mentioned that I wanted to really take a hard look at propeller design and talk with some recognized experts - but haven't had the opportunity to do either yet. ----- Jerry/Idaho

Boston Marine posted 06-29-2002 09:59 AM ET (US)     Profile for Boston Marine    
Call Roger Koeplsel At Mercury Hi Performance.
He Is The Senior Hydrodynamics Engineer.
bigz posted 06-29-2002 11:27 AM ET (US)     Profile for bigz    
That older Evinrude may not have enough negative trim consequently you can purchase simple wedges which will increase the angle of entry which then will give you the ability to increase the negative trim element forcing the bow DOWN eliminating most if not all your porpoising problem. These will also afford more latitude in trim available at various speeds with a standard prop.

Bow lift hogwash you want just the reverse for porpoising.


PS the Renegade prop for offshore Bombardier (OMC) produces is one of the few though Mercury has a similar design that will bring more control. Problem is they don't make it for the smaller outboards.

Tom W Clark posted 06-30-2002 02:53 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

Without getting into what the physics of porpoising are I can tell you what causes it on a 16' Whaler hull with an Evinrude 90. It is either too much weight in the stern or a combination of too much weight in the stern and having the motor trimmed out.

If there is a lot of weight in the stern and little in the bow the boat may porpoise even with the motor trimmed to a normal, (i.e. prop shaft parallel with the water's surface).

On the 16' Whaler hull there is plenty of transom angle for any outboard ever made. No need for transom wedges which were sometimes necessary on the first generation of the V-20 hull with OMC motors. You should never have to have the motor trimmed in so far so the the prop shaft and propeller thrust is pointed down.

The Currituck model is the lightest of the 16' models and if the fuel tanks are behind the stern thwart seat and you are sitting on that stern thwart seat without any other gear or persons up front then I would almost expect the boat to porpoise at higher speeds. 3/4 throttle with an Evinrude 90 hp (at 300 lbs) on the back will go pretty fast. I really think this is normal behavior for your boat. If it bothers you, the first thing to do would be to add weight up front.

You do no want to create more bow lift. This will exacerbate the problem.

With regards to raising the motor on the transom as reliant has suggested, the reason it may help is that if you can get the cavitation plate out of the water it will no longer be having any influence as a trim tab so if the motor is trimmed out it will not be able to "dig in" and pull the stern down and tilt the bow up. You could try it but I know from personal experience that the 16' hull is pretty happy with an OMC 90 mounted all the way down.

If you have power trim and tilt try playing around with it. If your Evinrude is a manual tilt then be sure the tilt pin is in the second hole from the transom. This will put it in the most useful overall position.

alukban posted 07-01-2002 07:58 AM ET (US)     Profile for alukban  Send Email to alukban     

I put a 6 gallon fuel tank up front - no more porpoising. That helped a lot more than the power tilt.

A Currituck with 90 hp goes WAY TOO FAST. I thought I was WFO utnil I realized I had another 1/8 more nudge of throttle to go... the Evinrude gave me even MORE! This was on Friday, New Haven harbor, with storm clouds rolling in over me from the west - simply god-freakin' beautiful. I was so much at peace - it would have been a good day to meet my maker...

Andrew :)

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