Help Tuning a Powercat

I have a Skoota 32 powercat with twin Yamaha 90hp 4 strokes, both right turning. The boat is a little squirrely at her top speed of 22 knots running just over 5800 rpms on each. I am afraid to hand over the helm to anyone at speed.

A boat captain I know told me that my steering is probably taking the boat a little sideways and as soon as I hands off the wheel; the boat wants to head to the right and this is why I am fighting the wheel some.

Is this correctable by some method or strategy?

He suggested pointing both tabs so the boat presses to the middle more, or the left tab to the left (viewed from the back) and the right tab to the right.

I do know how to tweak a single engine boat, but not the double.

 

It sounds like you have a slight directional stability issue.
Just add small skegs aft, that should correct it.

How big..well, that depends how much stbd turn she has....so a bit of trail and error may be in order. Since you don't want to over do it, then she'll be too hard to turn.

 

But I have those massive beaching keels 12' long.

My friend suggested it is caused by P force or both props rotating same?

 

They may well be the source of the instability.
Since those beaching keels, have moved the hull's neutral point and it is now much closer to the LCG.
So, if you add skegs aft, the same area as a minimum, as each beaching keel, then that may well correct it...

 

Uff da. The beaching keels were a ton of work to fab, tab, and fair.

I do have the mould left, but am literally days away from cooking it. Maybe I'll hold off!

The beaching keels are 12' long about 5x5", with a 4" by 1.5" sacrificial timber under.

And that would add more displacement aft....to compete with our other changes..

I thought about hooking a camera up and watching...maybe I need to for sure.

But the master captain seems to think the boat is constantly pulling to the right and I am unwittingly fighting it.

The more conservative approach for now would be to try to tow the steering in..he was talking about P force? He told me I might even need to shim the cylinders and get each motor pointed in some.

I wonder what power cats with four rotating props same way do..

 

The only way to know..is go out on a flat clam day..no wind...just the helm to zero heading...open her up....and do nothing...don't try to correct any course deviation.
So..how much rate of turn do you actually have?
A 'thought' is sadly insufficient for anyone to work out what is truly going on.

So, set her on a straight course, let go of the helm..other than the throttles, and see what occurs.
Does she turn to stbd?..if so..how much rate of turn.
And then...when she is in said turn....how much corrective helm is required to bring her back on the zero heading again?
And when correction is applied, does she maintain her straight heading?

Otherwise there is lots of speculation.

Generally one fits opposite turning props, where possible... but in a catamaran, is shouldn't matter too much, given your size and power.

The "P" force sounds like a red herring to me.

 

Is this the boat that was a few inches out of trim?Resolving that would be high on my priority list and then experimenting with motor tilt.Never having dealt with power cats,I wouldn't know if there ought to be an amount of Ackerman geometry in the system.Perhaps our expert advisor could comment.

 

Yes, but that is static trim.

When underpower, the hulls have rocker and she levels out at a modest speed.

To be honest, I have been too afraid to let go of the wheel at the top. I think I need to do some of the suggested trial and error at lower speeds, probably downriver. We have current as well which would also be a factor.

If the boat turns hard sideways at 22 knots, I doubt I'd put her over, but it is a little unnerving to consider.

She is on the hard until June, now.

I spoke with the Master Captain yesterday about her handling, put 35 hours on last fall before October 24 pullout, and did not top her out very much as this was also 30% break in period at lower rpms, and friends cruising on the river versus lotsa testing. I tried to setup the autopilot and did not even test that much due to the width of the river. So, I did not talk a lot about the handling with anyone. I did not notice a pull one way, but he said I might not unless I let go of the wheel. It is hydraulic steering and surprisingly easy to steer..

I would describe the behavior like chine walk on a modest scale.

The beaching keel sacrificial timbers are a little undersized to the keel and a bit like waterskis with rocker. I think @AdHoc is onto something because the boat is probably rising up and riding on all these hard edges midships.

Next summer, I'm going to make some videos and do more testing.

Up until yesterday, I didn't think I had any ability to change anything.

My jackplates also need to be covered on the bottom.

https://youtube.com/shorts/udvt35fGLyg?feature=share

https://youtube.com/shorts/W1jHF7c3Ka8?feature=share

 

Now that she is on the dry, take some pictures showing the wetted parts of the hull. One from the side, if possible covering the hull length. I cannot see that propeller rotation should have any significant impact here, but if the engine trim fins are not properly adjusted, you would feel a bias in steering effort.

You say she is "levelling out" with increasing speed, but since she is also "climbing" the bow wave, there is an increase in forward lateral area all the same. The result is that the center of lateral area is still too far forward. Your beaching skids could alter the performance of the original lines considerably, if not set up correctly. In addition, the tunnel flow is slightly different from the outside flow, and it might call for a slight toe in ("heels out") (or vice versa) in the straight forward position to get a fair balance. Engines should be trimmed out a few degrees to increase hull trim.

Edit: you say she is heading to stbd when you let go of the helm. That can not come from the propeller rotation, since the normal prop rotation is clockwise; ref to Ad Hoc's "red heering"......

 

Nope, I believe I said or meant to say I have not noted a starboard heading.

Just noticed she is a little squirrely left or right meaning it feels a bit too easy to steer either way or that I am afraid to let others take the wheel because the helm requires constant attention. The river is not super wide either, so at 22kts, going sideways is the bank fast..

 

Well, this is what you actually wrote in your initial posting. The symptoms you describe there and later point towards an effective lateral center too far forward, possibly even moving fw with a small heading change. Please note "effective", since flow detachment ahead of a fin/lateral plane can change the pressure distribution enough to make the boat unstable. Are there any additions to the hulls that were not included in the original drawings from the designer?

Come to think about it; have you checked that the steering is topped up with oil and all air vented out? There must be no elasticity at all when you try to turn the engines manually!

 

Sorry @baeckmo

The captain made an assumption the boat pulling to the right. I have been too nervous to let go! I got you stuck in my bad grammar.

 

I did bleed the system, but before we launch in spring, will rebleed. Just to be clear, you mean elastic as in springs back or a bit loose?

It steers so easy and I am not accustom to a boat steering so easily.

I plan to add bow extensions which may also shift that lateral center? Ad Hoc has offered to help me, so any steering effort will follow that change.

The boat is in negative trim 8.5" on 32' lwl but it is rising up and leveling out when throttles up.

 

If someone holds the wheel, can I check for elasticity by moving the engines by brute force?

 

Normally with hydraulic steering, there are s.c. block valves that prevent rudder cylinders to force oil through the steering pump. The valves only open when the pump is moved. In this case you don't have to hold the wheel when checking. If the system is lacking block valves, I recommend that they are fitted. When you apply the "brute force", the maximum in/out movement of the cylinder ram is about +/- 3 mm.

Any pictures/drawings of the wet parts of the hulls??