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Old 04-04-2016, 01:18 PM   #1
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Prop rotation

Just how much difference would it make on a twin engine fd trawler if both props rotated the same direction. I'm planning to install new (rebuilt) smaller engines in my boat that are both standard rotation. I would like to rebuild my current VD gears and reuse them, they are 72s I think, 2.91 ratio. But, they cannot be used for counter rotation. I could change to a newer type like the Hurth 630 in a 2.87 ratio (read $$$) or Twin Disks (more $$$). I dont need a down angle gear or a drop center, the Velvet drives are perfect for my setup. I have friends (charter captains) that run twin outboards with the same rotation, so they only need one spare lower unit and prop. They dont notice any difference. Any thoughts, preferably actuall experience ?
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Old 04-04-2016, 01:23 PM   #2
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Can be done...prefer the counter rotation for control...


Just depends on what you want to give up...but with twins..more power will easily overcome the prop walk issue...just a little less comfy..


Wouldn't even compare to outboards....different animal...inboard diesels and you definitely would know the difference...but it's not essential.
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Old 04-04-2016, 01:25 PM   #3
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I've never experimented with it. I ran a planing sporty that had same rot props due to a gear problem. Really could not tell much of a difference, other than it felt a little "weird" maneuvering. I never ran it with CR props, so could not compare directly.

On a trawler it is much lower hp/ton, so it would probably be undetectible.

Just make provisions when designing the install that it would not be a huge deal to swap out with 630's in case it ends up bothering you.

Just remember that all single screw boat props spin only one way!!! And plenty of those out there doing just fine.
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Old 04-04-2016, 01:37 PM   #4
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The shaft angle is such that a 10 degree down angle gear (like the 630) would put the front of the engine lower than the rear. I could use a drop center gear but it would raise the engine a bit. A straight thru gear is best in this situation, I just dont know of any that are counter rotating.
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Old 04-04-2016, 01:41 PM   #5
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630 comes in drop-center, but with no angle. ZF63A is down angle. ZF63 is drop center, no angle. Same with 220 and 220A. If hp is low, ZF45 may be an option, but don't know that gear well.
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Old 04-05-2016, 07:07 AM   #6
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I did a repower with a Cummins 6BTA with a nose down attitude. Was not a big deal, as long as the cooling system is properly vented. Cummins has a bulletin on how to do this. I'm sure it's the same for other manufacturers.
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Old 04-05-2016, 08:22 AM   #7
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I will look into the 630, sounds like it would work. The engines are fairly low power and small. 122 cubic inch, 50 hp now but may get turbod and turned up to 80 hp if needed. I'm just now getting around to having the blocks rebuilt, new pistons/rebore, bearings, etc. while I have the engines completely apart I would like to fit everything up on the engine beds. Much easier with a bare block and bellhousing/gear. I will need to build or reposition the beds. The boat had DD 4-53s. Man that space is nasty, spent most of yesterday wallowing in it.
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Old 04-05-2016, 08:58 AM   #8
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Thanks Ski, I looked into the 630 drop center gear, it would work. 2.8 ratio is ok but not great. Also looked up the HBW series gears. 360, 450 and 630. The HBW 450 would work fine for this application. However, I have never used a mechanical gear. Any thoughts there ?
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Old 04-05-2016, 09:45 AM   #9
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Avoid the mechanical gears. Loud, harsh shifting and often don't live long. Ok for sailboat aux, you want hydraulic.

Hurth HBW- mechanical. The B stands for "bang" as in the noise it makes when shifting.

Hurth HSW- Hydraulic. The S stands for "soft" shift. That's how I remember the model numbers.

Hurth made the hbw and hsw series like the HSW 630, then ZF bought Hurth out and model numbers became ZF 63, basically the same gear.

Be careful shopping these gears as some have slightly different ratio in A and B direction, avoid that. Most of the new hydraulic gears A and B direction are exactly the same.
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Old 04-05-2016, 09:59 AM   #10
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Thats kinda what I thought, "bang" but didnt know for sure. I had a Volvo MS gear once that was ,,,abrupt, for lack of a better term. Look like I will be searching for a pair of 2.8 ratio 630s. I have a pair of 2 to 1 630 down angle gears. Trade ?
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Old 04-05-2016, 06:30 PM   #11
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Twin Disc 506's are straight through and will CR. That's what I have, My ratio is 2.96 but they came in other ratios. Old, basic but pretty good. Maybe you could find a used pair?

Check your engine manufacturer whether forward down is OK. For my JD 6068's it is not, rear down only.
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Old 04-05-2016, 06:41 PM   #12
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Ran a crewboat as a charter boat and did a little towing with it. The orginal owners bought 6-71s and standard gears to reduce the number of spares for their fleet. Ran fine with 2 rights. Backing could have been better but still manageable.

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Old 04-05-2016, 09:32 PM   #13
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I'm about to purchase a 2005 Mainship 43' trawler with twin Yanmar 370 hps. Should I expect one engine to rotate to the right and the other to rotate to the left? and I'm guessing the starboard engine to rotate right (clockwise) and the port engine to rotate to the left (counter clockwise)?

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Old 04-05-2016, 09:42 PM   #14
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That is correct Jim. Both engines are standard rotation and the counter rotation is done by one gear turning that prop opposite. Lots of older engines in twin screw boats were left and right rotating and used gears that were not made to take care of it. Most gas boat engines and some diesel engines were counter rotating. Detroit Diesels were almost all CR in twin installations. Newer engines are standard rotation.
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Old 04-05-2016, 10:25 PM   #15
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Kulas44, Thanks for the quick response.

So I'm clear, both engines will operate in the same direction. The transmission in one (port side) will have a gear that drives the prop in a counter rotation direction.

Is that correct?

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Old 04-06-2016, 03:37 PM   #16
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Both engine CCW as viewed from aft. Port prop LH, also CCW viewed from aft. Stbd RH, CW. Most gears the favored direction is RH as only one set of gears in play. For rotation to be same as engine, two sets of gears must be in play with a little more friction.
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Old 04-08-2016, 09:40 PM   #17
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Not sure about smaller transmissions like the ZF Hurth, but my company overhauls a lot of larger Twin Disc for commercial fishing vessels. They all turn the same way. When you want one to counter rotate you run it in reverse and use an opposite rotation prop. If your control is with a cable you use a bell crank so the control's move the same way. You're moving the lever forward but you actually are shifting the transmission into reverse. About 20 years ago when we first got into marine gears I had a customer that sent in a Twin Disc 527 and wanted us to set it up for opposite rotation. Not knowing better I called Twin Disc engineering to see how to do this since they only had one part number for each transmission. He said just change the prop and put one lever in forward and one in reverse then hook the control up so they both move in the same direction. Its easier if your controls are electric or hydraulic. Just a matter of sawing hoses or electrical connections.
I may be wrong with the smaller gears but I don't think so. we do several Twin Discs and about 20 Velvet Drives each weak.
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Old 04-08-2016, 10:08 PM   #18
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Another minor point of interest. On V-hull boats the top of the prop spins outboard, normally. On newer high speed catamaran boats both props spin inboard, for whatever reason. As far as the shifting on our gears is concerned, usually just change it at the shift lever, push or pull, simple.
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Old 04-08-2016, 10:35 PM   #19
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With twin props turning in the same direction, I'd expect the rudders would need to offset the prop walk when going forward. My single-engine boat requires a three-degree rudder angle to compensate for the forward walk. So, twins turning in opposite directions should be more efficient than turning in the same direction.
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Old 04-08-2016, 11:00 PM   #20
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I have known three boats that were twin screw, right hand wheels. They were always know as 'dogs' around the dock. And were never good handling boats. Did they get the job done? Yes. But not with any appreciation or fond remembrance.
Check with the gear manufacturers to see if the gears can be run backwards long term and if the gearing is the same fwd as astern. Some gears have the same ratio ahead as astern, others don't. simply switching one side to astern (by hooking the lever pintle to the opposite side, or swapping hydraulic hoses) would be the simplest solution.
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