mmwalters

I have some interesting results on my 30' motion regarding spinning IN and OUT I originally rigged my twin 300pro max spinning out best speed was over 109mph and it would run 104 any day with full load 4 people +100gal. fuel. Well at the very beginning of this season I changed the lowers to spin in but at the same time I lost a prop[15x34 3bl lab cleaver] Any how no matter what prop I put on the boat It would struggle to run much over 100mph with a load and I never saw over 105mph all summer. I figured must be props. Well just for the hell of it I changed drive rotation back to spin out. Results.. with a not so great set of props and3 people and 1/2 fuel this boat ran just a little over 100mph spinning in Next day same props with 2 people and spinning out we went 106mph. So I must conclude prop rotation was worth at least 3-4 mph. With props spinning in had much more bow lift and could not use much trim . With props spinning out needed more trim and gained 250rpm. Go figure.

mcollinstn

all hull/motor/weight combos react differently.

Spin IN = more bow lift. That much is a given. Digging in the tail to lift the bow, though, is not necessary for some combos and anything that pulls the boat INTO the water creates more drag. Cats commonly fall into this category.

merlin540

so do you think my 32 skater would pick up spinning out?? or do you think it will get to much tail lift. it has ran 144mph w/30 3 blade clevers spinning in at 7100rpm with 1.25 gears on speedmaster 5's what do you think??????????

mcollinstn

merlin

I guess it falls into a weird category. Running totally on theory only, we need to look at one thing inparticular:

With your current setup, at top speed what is your trim setting and how is the boat running (flat, light, skittish, et..)?

Also, is the centerline of the gearcase ABOVE the bottom of the hull?

At first thought, theory tells me that your cat will run faster spinning out, cause your bow lift is ENTIRELY based on air pressure making prop leverage to lift the bow unneccessary. Spinning out would pull more tail out of the water. HOWEVER the whole bow lift/stern lift part of propellor rotation is based on how the boat cuts the water into a VEE shape for the props to engage into - this VEE shaped groove the boat cuts thru the water leaves the props "more" in the air towards the centerline of the hull and "more" in the water towards the outsides. The "climbing" of outward rotation gives stern lift and the "digging" of inward rotation pulls the stern down. At the extreme speeds you are running I'm just not sure what kind of "path" your hull is "cutting" thru the water for the props. My guess is that you ain't doing hardly anything to the water since you aren't hardly touching it at all. This may make your personal results less dramatic than a 90mph boat.

My guess is still that "out" will leave less wetted surface in the water.

The propshaft height thing is a concern for "chopping the throttle" at speed. If the centerline of your gearcase was "even" with the bottom of the hull, then running "out" with a little positive trim may be dangerous if you lost power suddenly. As the tail sits from power loss, the squat could cause the gearcase to get a better bite of water and pull the tail down at speed, causing the air pressure to spike which could result in the nose coming up and blowing over. Just a cautious thought.

At your high speeds, I would think that whichever rotation direction allowed you to run closest to neutral trim would give best speed.

Again, this is all theory.

Cleavers are stern lifters by design, so you may already be running perfect.

mmwalters

On my motion it has a 44'' tunnel with a full tab[44''] The boat does gain about 1-2 mph at full speed with the tab full down. But the ride is starting to feel a little light. It semms with the props spinning out I gained more RPM due to the fact I need to use more trim. Much trim with the props spinning in felt like the bpat could blow over, really high in the front scary!

John B

Mike,

Don't forget---when you run a cat into the wind you gain speed, and loose going with it. I find a difference of 4 to 6 mph with as little as a 10 mph breeze. This makes testing more difficult. After you add atmospherics on different days well .

John B

mmwalters

Thanks John You are correct I always try and run both ways. I think the current has effect also as it reads speed over ground not water. It is funny that these merc. outboards do not seem to like the cool weather. I don't know if something in the brain is calibrated to go to rich or what but the just run better in 75* temps. I think you would find that interesting.

Gary Anderson

In my experience cats dont work like V hulls. The effect of drive rotation will depend on how high the drive is mounted (as mcollinstn wrote), hull design (pads, notched transom), and drive location relative to the tunnel (often overlooked). I found with my Chris cat, the drives were very close to the tunnel. That gave the effect of much more bite (cleaner water through the tunnel) when turning props in. The whole boat ran much higher out of the water and MUCH faster. It was running SSM IIIs 1" above the bottom.
Gary

Brad Zastrow

My 30 Motion with triples ran much better turning in. Noticable difference was it would not corner as well by turning them in. Also had less propoise turning in. The boat had more stern lift and felt looser turning in. I have found on my last 4 cats that turning in is faster. 22' Rapid Craft (twin o/b), 25 Warlock (twin o/b), 30 Motion (triple o/b), 35 Motion (twin I/O) I have tried both ways on them all.
I like em in.

Brad Zastrow

I have to ask what do you consider rotation? I have read threads where some thought it was the direction of the bottom of the prop.

Turning in is: right hand drive on left side (port)
left hand drive on right (starboard)