Porposing Four Winns

If you read the Katayama papers, you see what is needed to quite accurately model porpoising. Porpoising is self-exiting the amplitude grows from a small initial disturbance.

I guess you could force it e.g. by rocking your upper body in a small boat. We used to use that for stopping the porpoising a bit quicker than it would without it. Thus it works just on the limit.


Savitsky method is not used for porpoising. It is used for calculating the balanced position (vertical + trim) of a boat and its drag in steady state. Then from this information the porpoising regime can be approximated from a graph (or a curve fit to that).

However Savitsky method does show how to calculate all the static forces in any trim and vertical position. So it may be usefull in developing a time dependent model. It does not include the damping caused by water and I have not seen a model for that (Katayama measured them for an individual boat).

 

Basic trim settings

Two things i would check right away!! one look along the keel line while the boat is in the trailer and see if the boat has a curve up along the keel line even a slight curve! This is called hook and yes it will make the boat porpiose but you can still trim so it dosent do it .in some boats hook is introduced to make them go faster and get up on the plane quicker !
Second motor height , this is easy to set . Get a straight piece of wood or what ever and put along the bottom of the boat against the keel slowly slide it back and along side the outboard leg , It should be exactly inline and just touching the underside of cavitation plate !!.
With this set then you have to use the powertrim on the motor to find a point at which it will ride with out going up and down . Yamaha have the best trim them omc and merc last ! Theres no need to look at the rim gauge, the trim can be done buy feel , sight and sound.
In other words you need to get intouch with your boat and learn from what it wants not what you want .
This is easy !!
Go out alone in the boat and get on the plane at about 1/2 to 3/4 throttle ! tuck the trim right in as far as it will go, now listen to the sound of the motor and watch the tacho to see where it is . very slowly work the trim back up a little at a time listening to the sound of the motor and taking particular notice of where the spray is coming out the side of the boat !
When it reachs a particular point of trim the outboard motor will sound differant and it could have also picked up 100 or more rpms and the spray from the chine will have shifted back from the bow . You have reached the sweet spot for your hull trim over that and you will porpoise !!. trim back under just a little and the motor will start to slow down again and the spray from the chine will have gone forward again trim up and find that spot again !!! . take particular notice of this position ,that is the spot where it needs to be trimmed each time you go out .
No two boats are ever the same ,even the same type of boat with a differant outboard will not be the same setting .
It is very important to check the motor height before you start . To me it sounds like its to high or you have your trim set wrong right from the start .
If the porpoising is there when you turn the wheel the trim could be to far up and the motor height is to high as well
Do things slowly just a little at a time dont just hold your finger on the trim button and expect magic things to happen !!
If the boat has been doing this since day one the motor was never set properly when it was put on the boat . Some of the guys that fit outboards dont even know what day of the week it is and in a lot of cases just plain dont care , its just a job !! who cares !!

 

"dont even know what day of the week it is and in a lot of cases just plain dont care !!"

I wish I'd said that Thanks for the hands on tuning tips, I've listened before but have't paid enough position to the spray patterns. Have you any thoughts about adjusting trim tabs on inboards?

 

Adjustable trm tabs ! This depends on the size of the boat and how you load it . If you are trying to get the nose down reguardless of how the boat is loaded then the boat has a small problem ! The very aft end of the hull all the way across give a sand and build up the last 50 mm wide strip of the hull about 5 mm with auto filler simply to deflect the water down and lift the back . Take the boat and try it . if the nose has come down but not enough add another couple of mills untill you have found what you want in the way of fore and aft trim at all speeds . if the nose has gone down to much simply sand some of the thickness off ! Remembering the faster you go the more lift it will greate so if theres not enough at slow speed shift some of the heavy things in the boat and trim it better .

My case i have a 14.6ft fast fishing boat with a semi cabin .
Had a old 70 hp and went reasonable good but the motor was way to high and the transom needed a lot of work, so chopped the lot off , Rebulit a complete new transom in solid glass on my work bench and stitched on the back . Now it has 450 mm set back ,and has a 25 inch ,115 yamaha v 4 on the back and fly's almost .Trim is critical and the set back has made it very sensitive and responsive . When i put the boat in the water and get going i set the tacho on 2500 prms with the trim right in and them slowly bring it out till it feels like i have bumped the trottle lever it is so noticeable the increase in speed and comfort of ride and thats the sweet spot . It just comes natural and i never look at the trim gauge any more . One person or 4 i always do the same thing!. I carry almost 70 litres of fuel when all tanks are full and the tanks are way up forward , one is completely under the fore deck to counteract the extra weight of the big motor . As the weight OF FUEL disapears from fuel usage the powertrim gets to tuck the motor a little more but i'm always looking for the sweet spot throughout the days fishing . Yamaha speedos dont go high enough! it goes right off the clock at 6400 rpms on the tacho when its full out !! I only go fast when i am by myself and in real smooth water and no head wind as the bow becomes a little flighty!!
But a hell of a ride if you have the feel for that kind a thing!! . Yeah im 66 years old and my mind never did grow past 40!! Live on the edge ! its where the best veiw is !!
My boat and i are on my Avatar top of the page !!

 

add on to Basic set up

Just a little snippet to add to basic setting the hight of an outboard.
For every 150 mm your outboard is away from the back of the tramsom the outboard can be lifted 3 mm up above the line of the keel.
So if the motor is 300mm away from the tramson it can be raised 6 mm above the level line of the keel .
Setting the motor on the high side will give a slightly higher top speed but with everything you gain you also loose something as well, that being the likely hood of cavitation when you turn either way at speed , and when you take off quickly (hole shot) !! not forgetting steering will be lighter to turn !.