TomCat Propeller Rotation

J

John T

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C Dory Year
2018
C Dory Model
255 Tomcat
Hull Identification Number
NWM25T01D718
Vessel Name
Nashua
My wife and I took delivery on our new TomCat, "Nashua", last Friday, 8/25 and spent a few nights aboard in Anacortes and Friday Harbor before bringing it down on its own bottom to our home in Gig Harbor. This boat rides more smoothly and quietly than anything we have ever owned. We already love it! However, it is a real challenge to dock due to its INWARD counter-rotating props. I'm told that some TomCats are set up this way, while others have the more common (and very easy to maneuver) twin engine configuration of outward counter-rotating props. I'm told that outward rotating props produce cavitation issues on the Tomcat. I wonder how bad those issues are.
Can anyone speak to the pros and cons of each setup on the TomCat?
Thanks much,
John
 
Hi John, Congrats on the new TomCat. You are going to love it.

AS to the counter rotating props, I have to pass, but on twins I might be able to help.

Were you using the steering wheel while docking? If so, here is a hint. :wink: Don't :!: :shock: Leave the wheel with the OB pointing straight ahead, and use the throttles, on forward one reverse to turn the boat. Go slow, (and maybe practice out in open water) and feel the difference. You can spin that boat around in it's own length without using the wheel. Start now, practice and it will fix your pain. 8) And yes, You are welcome.

Harvey
SleepyC :moon

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I have owned a large MY with outboard rotating inboard props--there are lots of things you can do with these, including walking the boat sideways.

The Cats get more stern lift from the inward rotating props--and thus handle better at speed. Docking not quite so as with the outboard rotating props.

The boat can be made to walk--but it is different, and more difficult. I also found that the hard chine of both fairly narrow hulls, make the boat more difficult to "handle" as you would normal counter rotating twins. The engines are widely set, and thus give far better "leverage" than the conventional narrow spacing of the C Dory motors. You have to use more power than you might expect.

What I have done with many new or unfamiliar boats, is make an "artificial dock"--I happen to use two fenders, or two Chlorox bottles lashed to each end of a boat hook or a closet rod--whatever you have. On a clam day, with no current, practice maneuvering--come bow to the "dock", stern to the "dock". bring the boat along side the dock. Practice getting away from the dock.

With outward rotating props, you would use your outboard near the dock to bring the stern in, with inward, it is the outside prop. Practice seeing what you boat will do as you use one side at a time, with the engine straight ahead, and see which way it pulls the boat. Then you can add turning and the second engine.

I still used the helm when docking--I don't agree with Harvey--but he has experience with his specific boat--the Tom Cat will handle differently. For coming into a dock on the starboard side, head in at your angle, then as you get the bow a couple of feet put the outboard--outboard. (port side) into reverse, with the engines turned slightly to pull the boat to the dock. --not too much, or it will pull the bow out.

If you are attempting to do a 360 in place, then you will have to be aggressive with the power. Practice, and practice some more.
 
Thanks Harvey and Bob,
I have had 32 and 42 foot monohull twin screw boats and always loved the ease of pivoting and maneuvering, often with the helm straight and using only the transmissions. For the last eight years, I've had a 23' single I/O. I was looking forward to returning to the ease and versatility of twin screw operation, only even easier on the relatively small TomCat with its widely spaced engines. But so far, in each of my docking efforts I have looked much more like I've never run a boat before in my life than like the 40-plus-year boater than I am! Yes, I definitely need practice with this boat, as the prop walk on each side, in both left and right turns, counteracts the thrust of each motor and completely annihilates attempts to run it like a typical twin screw vessel. Even the folks at NMI were surprised at how it behaved. So far, I'm finding it best to use more helm and treat it like a single screw boat, though I noticed what you said Bob; very much helm at all in reverse brings the bow off the dock. So, as you said, practice, practice, practice! Or, might it be a worthwhile trade-off to reverse the engine locations so as to have outward rotating props? Or is it more important to have the stern lift under way that you said inward rotation provides? NMI said the drawback to outward rotation with the TomCat has been "some cavitation issues."
 
John T,
I'd advise staying mellow.
You are not a beta-tester for this 2006 design (Bob was in 2006). Switching engines would most likely be on your dime and not worth it. Of 13 trailer boats, this is my first with twins, and it IS very different than any of the others, which took some getting used to. As little as 2-3 seconds in gear on ONE engine at the dock at idle speed results in more EFFECT than any trailer boat I've ever owned (4 blade 17P props, 2 other sets in boxes and still experimenting).
Yes, she prefers to be an arse-heavy squatter if allowed. I forget whether the LF or TXR Yamaha is the counter rotator, but mine was rigged the opposite of most (by Mobile East Marine, a reputable and C-Dory dealer). I'm confident I could 'get used to it' if they were switched...2 widely spaced CR outboards are better than my Regal 26 I/O Duo-Props or any other combo, regardless (I've never had inboards).
There is NO SUBSTITUTE for many hours at idle experimenting with how YOUR boat responds to helm/throttle inputs. You'll likely never need over 1500 RPM on either engine even docking in adverse wind. (We have that at 10-15K on the port side most outings. A 'chicken line' helps). In 12 months you'll be smiling at your own post here.
I agree with Bob that most 255 posts on the issue have concerned stern squat (including Marc Grove)...I don't recall anyone mentioning 'cavitation issues'. Ventilation is annoying but harmless, cavitation not so much.
It's not 'practice' you need, it's 'familiarization' with your new boat.
Try some 'minimalist' helm at your fake 'dock' like you had a team of 300 healthy horses pulling you in (you do, unless you got 175's).
It's a great boat, and you're going to love it.
Heaving her on and off the trailer is like launching Jabba The Hut, so you're golden.
Happy Familiarization!
John
 
Congrats on your new Tomcat and I am envious! I have a 2007 Tomcat with Honda 150’s and I absolutely love it. While I may have owned too many boats, I have to admit the Tomcat is the most forgiving, capable, and fun boat I have ever owned. As I look back on my two plus years of ownership I am amazed at how little I understood what the Tomcat is capable of. Every day I use it it seems I learn another ‘secret’ to it’s capabilities/handling. As an example, on my past boats the inclination was to slow down when the chop got rough, but with the Tomcat I have found speed (air cushion) and trim is often the secret to a smooth ride.

FWIW- The only thing I can’t do with my Tomcat is move sideways. I can turn it on a dime and give you change, but sliding sideways I have yet to accomplish. I have no idea whether my props turn in or out but having two outboards so far apart makes docking a pleasure. That said, when it comes to approach angle to the dock I treat the Tomcat like a single engine boat (except for not using the wheel) and I use the engines to spin it parallel to the dock (may take a little more throttle than you are use to) and stop movement just prior to ‘touch-down’.
 
John, See, you are getting lots of help, and I agree, in part, with all. Both Dr Bob and Gulfcoast John have TomCats so they know whereof they speak.

Consider the physics. Your outboards are widely spaced. That makes them able to rotated the boat with less effort than if they were close together, right? All maneuvering based on proportional engine throttling should be based on the OB's pointed straight ahead. Turning the "steering wheel" will change where the center of the boat is according to the OB's, and that makes things more confusing. (Yes my boat is smaller, with a flat bottom, and my OB's are closer together, and less powerful, which should make all maneuvering for you easier though there will be a bit more resistance due to the two hulls instead of one.) Do the process in slow, it works, and if there is a bump, it won't show as much ;-)

With your twins on the stern, you have two motion options depending on your use. 1. Rotation, which we have addressed. One throttle forward, the other in reverse and the boat will spin the bow towards the side of the one in reverse. Consider that use as your bow thruster. 2. Prop Walking, (moving the stern of the boat sideways relative to the bow.) Consider that your stern thruster. Again, steering wheel straight ahead, and since your boat has counter rotating props, one engine will move the boat to port, the other to starboard, so ONLY ONE engine can be in gear when doing Prop Walk maneuvering.

There are few laws in this world that always apply, (as some know, if you have a good lawyer you can do almost anything) BUT, the Laws of Physics always rule. Gravity always wins.

As John said, You don't need practice as much as familiarization with your boat. As Bob said, the engines are wide apart so they have more leverage. It is a bigger and heavier boat, but your engines and props have more power and bite. You can do it, and it will be fun. Enjoy.

Harvey
SleepyC:moon

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