Catamaran Planning Angle and TTop Angle question

I have a 22' Seacat Power Catamaran that will not run at WOT at less than 6-8 degrees, even when the motors are trimmed down excessively.

The load distribution makes the hull bow happy even with 11 degree transom mounted outboards that also have wedges to increase stern lift.

Moving the console and fuel tanks forward is not a job that I will take on, and the 30" wide transom on each sponson does not afford enough width on either side of the twin 115 outboard 14" dia. props to add standard trim tabs. So I will be adding stiff aluminum Permatrim Doel Fins to each motor instead.


I believe the hull is planning at 6-8 degrees instead of 3-4. The semi displacement sponsons have flat keels and rounded sides, and do not appear to have any rocker.

The 8' x 6' wide umprella sided TTop is mounted parallel to the deck, and the deck is parallel to the water surface when the hull is at rest. I therefore believe that the trim top tilted up in the front when the hull is on plane and is
slowing me with excessive drag
that its surface, along with the slanted tunnel top is making the hull porpoise.

Light boat tests with same props, and no top or bottom paint get 45 mph and I am getting 39 mph trimmed up in smooth water.

2 questions
Is 3-4 degrees proper trim angle for this hull with props running parallel to water surface??

If top and hull is running at 8 degrees high neutral trim, would you lower the front of the ttop 8 degrees in realation to the deck so that it runs parralel to the water surface to to reduce drag??

 

Yes and almost yes but not quite. I'd try 4 degrees negative angle of attack first. Too much negative will create downforce. Not what you want.

The top is essentially a wing. It creates lift. Anything that creates lift also creates drag.

You could temporarily remove the top and try the boat. If it behaves differently and your speed increases.....then you have your verification. If that turns out to be the villian, then experiment with angle to find the right one.

Certain race cars, mainly USAC type sprint cars use wings to induce downforce so that they can negotiate corners faster. Most of them have adjustable struts to change the AOA easily. Some have pneumatic gimmicks that level the wing down the straightaway so that drag is minimized.

 

How are you measuring the trim angle, it seems improbable to me you could get 40 mph with 6-8 degrees of trim. If your speed info is accurate, that is very good pace for a cat that size with twin 115's. I doubt the T-top angle of attack would be having any deleterious effect on your speed, it may even be helping ! Fitting the permatrims will be a relatively cheap and easy way to see if you can change the porpoising behaviour, but I don't see anything sluggish about your boat's performance, as is. If your boat is trailered, you could try rolling it back along the trailer to see at what point it starts to tip, not before supporting the back end of the trailer so the whole thing doesn't tip, of course ! If the tipping point is less than a third of the boat length, your boat is likely stern heavy, and maybe the permatrims will mitigate the effects of that.

 

The spray exiting the bottom of the sponson is about 2.5' aft of midships on a 16' keel line that's in water at rest, so my calcs are done on a 1' rear transom draft at speed using math.I believe I can also measure this with an inclinometer on the deck and top while under way.

I do plan to temporarily remove the cloth to confirm the speed with and without theory prior to cutting struts to modify its attack angle, after I install the permatrims. But as I said, I am more concerned about my priority of reducing the amount of Bow Porpoise in head chop than gaining another mph in wot speed.

The top is too heavy to contemplate forward hinges and rear adjustable struts (like Volkswagen Shocks) to find its sweet spot while under way, but I had the same thought too.

So do you think that 3 degrees deck angle with not cloth at neutral trim is the correct angle for a cat with no rocker?

 

The engines are not podded ? Another thing that may affect your boat's behaviour is that it is a centre console, in head winds the upswept tunnel forward creates aerodynamic lift, and there is no upperworks with surfaces that could create an downward pressure, to counter that underbody lift. Combine that with the cat's lack of pitch stiffness, and porpoising appears. The permatrims may help, they are almost standard equipment on one brand of podded cat here.

 

Here is a link showing the hull without the top, and how it performs, according to the OEM. Boat tests are usually fudged to look better and it is common to state speeds just running with the tide, low fuel, no tttop or bottom paint, fresh engines and cool temps with flat seas, so performance in real life is not my concern, handling in head seas are.

http://www.suzukimarine.com/Product...oats Inc/Sea Cat 22 Center Console Twin DF115

The splash from my sponsons exits 2' farther back than shown in the photo even with the motors trimmed down making for a very dry ride, but porpoisy in head seas.

There are no pods on the motors, so there is flotation and planning surfaces under them with this eurotransom design. The gas tanks and console are just mounted too far back IMO to get a flat attack angle.

The front underside slanted surface the tunnel is also acting as a wing, but the bow cap slants down, probably negating a lot of lift created below decks.

So if the hull tilts up or should tilt up at 3 degrees, I am stating that the ttoop installer should have mounted the top with a negative 3 degrees of tilt in the front so that it would run flat on plane catching less air, and to my it looks like it is tilted up 8 degrees at the moment.

I'll remove the cloth to test and also measure, not calculate, the angle, prior to proceeding with the saw and ttop welding.

 

It is a good idea to remove the fabric top to test the difference., as you plan to do. Your boat's design is quite high at the bow, that in itself encourages bow lift running upwind, especially in a centre console. If things improve running with the chop and wind, running angle wise, that would tend to confirm excessive aerodynamic lift forward is part of the problem, which would be worsened by the LCOG being too far rearward. Permatrims may help.