Lowspeed tracking problem

Hi all
My name is Mattias

I have a princess with twin kad 42 and duoprop drive
Its realy hard to drive it slowly
I know this is problem with some planing hull and drive
But it must be a soulotion how to get it better so my autopilot can handle it

I thinked to try and put 2 extra skegg on the stern or bigger on the drive
Do anybody have a sugestion?

 

If you add skegs, high speed handling will change. It may even became dangerous if there is too much lateral resistance. The boat may heel over or get out of control.

 

Mattias,
look at the quickshift from twin disk gearbox, http://www.twindisc.com/MarineProducts/MarineQuickshift.html
rgds
Bertho

 

Presumably the problem is "wander" at low speeds, the cause being there is not enough vertical surface area in the water aft.

 

If you could post a picture of the boat's underwater areas it would be helpful, but as has been eluded to, the hull form itself is likely the issue, not the drives or sufficient lateral plane. Additional lateral plane area may help, but it will be a marginal improvement at best with likely an adverse high speed affect.

 

Here is a bad pic but as u see its tunnels to get water to the drives
this boat is not build for shafts

The boat is realy heavy in the stern and i need to use lot of tabs and max in with the drive to get the most speed in planing

Maby i should be better to take away the tunnel and put on a drive extender?

 

No, there is nothing wrong with the tunnel shape: it is even beneficial for low speed stability.
Check the tie bar adjustment, there should be a few degrees of toe-in to take up any free play in the pivoting points.

At the stern there isn't much you can do, it would take a skeg or keel to really improve low speed handling.

 

There is 2.5cm toe in from the shield to the prop

 

With a shape like that it will wanting to suck the stern down and make the back squat almost untill you got going fast enough for the tumbling water at the transom to let air in under the hull so to speak . what a terrible design !!! If it was me i would fill it in and extend the hull bottom right to the transom like a ordinary boat . If its heavy in the back need to look seriously at getting shot of some of the weight and moving somethings that are heavy forward to balance the boat a bit better . . wandering is from all the swirling confused water flow in and around the back of the boat what else can you exspect !! .will be like a washing machine under the back !!

nEED SOME BETTER PICTURES ITS ALL GUESS WORK SIDE SHOTS AND STERN SHOTS SHOTS OF THE BOTTOM LOOKING AFT AND FROM THE BACK LOOKING FORWARD NEED THEM WITH LOTS OF DETAIL

 

"At the stern there isn't much you can do, it would take a skeg or keel to really improve low speed handling."

How about a drop down stabelizer between the drives.

Think dink kick up rudder, but stronger, might have a tie bar to help steer.

 

Princess Yachts is a serious UK based company. They would have made a different hull shape for displacement, but this one is optimized for gliding.
Slow speed handling is poor, not just for this hull but for the whole range of fast cruisers. In fact the whole Bavaria sports series and all small Sunseekers are even worse.

My Draco behaved exactly the same when it was powered by twin stern drives, going straight ahead was impossible. Installing large trim flaps didn't make any difference. Now it has two tunnels, a lot more surface under the stern and prop shafts and runs like on rails, but is very reluctant to turn at slow speed.

 

i have just converted my boat from twin shaft drives to a single volvo 270 stern drive and it does the same thing. within a week i have got used to it and corrrect the steering without even thinking about it. the first time i took it out i was going all over the place. i do find it is better also with the tabs right down at displacement speeds.

 

There's not much effective to do except changing weight distribution. In smallish boats it's easy, bigger ones not so. An additional water ballast tank maybe?

 

2.5cm toe-in is excessive. You only need enough to take in the slack.

 

1.5-2.5 is what Volvo recomend i have tryed 0.5 cm no diffrence

The water tank is under the bed in front and fuel i in middle
so the weight is hard to change
Here is more pic