Borg Warner 72c gearbox ratio mystery

Bill W

Bill W

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I have a 72C Borg Warner Velvet Drive Box. The little brass plate on it tells me that the ratio is 1.6. I have looked high and low on the internet and nowhere can I find the existence of that particular ratio, the nearest being 1.52:1. I'm wondering whether this box is a "special order" ratio, or whether the 1.6 designation is just a rounding-up from 1.52:1. The boat is laid up for the winter and even if she was afloat, I don't have the equipment to check rotations against engine revs. Any gearbox gurus out there who can solve the mystery for me?
 
I don't see a 1.6:1 mentioned in my 72C workshop manual, 1:1, 1.523:1, 1.91:1, 2.100:1, 2.57:1, 2.91:1 are listed, the manual is from October 1969
There are some other ratios available
In the boat owner's manual it mentions that the gear ratio is 1.5:1 also written on the cover is AS12-72C, the forward gear is 1.523:1 and in reverse gear the ratio is 1.68:1 maybe that is it?
 
In practical terms the difference between 1.6 and 1.52 is irrelevant. The difference in shaft speed is less than 100 rpm at maximum - not enough to justify a different size prop.
 
Tranona said:
In practical terms the difference between 1.6 and 1.52 is irrelevant. The difference in shaft speed is less than 100 rpm at maximum - not enough to justify a different size prop.
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Thank you for bringing me to the point of my question.
The gearbox in question is on my port engine. My starboard engine has a PRM 750, which is a 1.459:1 ratio. It has a spacer to make it a direct replacement for a Borg Warner box. Now, 1.46:1 to 1.52:1 is not a significant difference but 1.46 to 1.6 might be. Currently, neither engine reaches Mermaid's rated WOT rpm of 2600. The port engine will hit 2300, the starboard struggles to hit 2100 at start of the season. I am considering a slight reduction of pitch, maybe 1.5 inches on the starboard and 1 inch on the port, to bring the engine WOT revs closer to manufacturer's spec and "synchronise" the prop revs for a given engine revs. It would really be useful to know for sure what ratio the BW box actually is, so I could ask a prop shop or other knowledgeable person if this is sound thinking.
I'll see if I can get the rest of the info from the BW brass plate to see if it can be worked out that way.
 
Bill W said:
It would really be useful to know for sure what ratio the BW box actually is, so I could ask a prop shop or other knowledgeable person if this is sound thinking.
Click to expand...
Easy enough to check, mark the fly wheel and the prop shaft put it in gear and turn the engine one revolution with a spanner.
 
The plot thickens. I got the numbers from the brass plate 10-14-000-001, which appears to indicate that it is a 72CR. My understanding of what I've read is that the CR stands for counter rotation, i.e. output shaft rotates the opposite way to the input shaft. Not so in this case. The engine is a Ford Mermaid four cylinder and rotates anti clockwise looking at the flywheel end. The prop, being on the port side, rotates anti clockwise when in forwards gear, looking from astern.
If it is indeed a CR, not a box made from two halves with the CR plate left on, it should be 1.58:1.
Doing the sums, If the both engines are doing 2000 revs the starboard engine will be turning the prop 105 rpm faster than the port engine. To achieve balanced prop revolutions it would be necessary to run the starboard engine at around 153 revs less than the port engine (at 2000 rpm), which is a fairly significant difference.
Neither engine is currently able to make the stated 2600 at present and we have pretty well eliminated all the other potential causes. There is about a 200 rpm difference in WOT and the starboard engine makes black smoke at higher revs, so I'm inclined to think that both sides may be over-propped and because of the lower reduction ratio, the starboard one even more so. That's why I'm thinking it may warrant a reduction in pitch, with maybe an extra half-inch being taken off the starboard prop.
 
This is from the BW manual the part number you quote is halfway down the page, it should be an opposite rotation, could it be that the gear selector is connected in reverse? then you would have gearbox in what should be astern to give drive ahead and the gear ratio is then 1.74:1

72CVD2.jpg
 
Alicatt said:
This is from the BW manual the part number you quote is halfway down the page, it should be an opposite rotation, could it be that the gear selector is connected in reverse? then you would have gearbox in what should be astern to give drive ahead and the gear ratio is then 1.74:1

View attachment 201453
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Thanks for posting that chart. In fact, it shows exactly what I have. The port engine (in fact both of them) is a Mermaid turbo Four (Ford 2722E), which is LH or anti clockwise rotation. The 10-14-000-001 box in forward rotates with engine (LH) and drives the port (LH) prop. So what I have is a 1.58:1 box to port and a PRM 750 1.459:1 RH box to starboard.
 
Bill W said:
The plot thickens. I got the numbers from the brass plate 10-14-000-001, which appears to indicate that it is a 72CR. My understanding of what I've read is that the CR stands for counter rotation, i.e. output shaft rotates the opposite way to the input shaft. Not so in this case. The engine is a Ford Mermaid four cylinder and rotates anti clockwise looking at the flywheel end. The prop, being on the port side, rotates anti clockwise when in forwards gear, looking from astern.
If it is indeed a CR, not a box made from two halves with the CR plate left on, it should be 1.58:1.
Doing the sums, If the both engines are doing 2000 revs the starboard engine will be turning the prop 105 rpm faster than the port engine. To achieve balanced prop revolutions it would be necessary to run the starboard engine at around 153 revs less than the port engine (at 2000 rpm), which is a fairly significant difference.
Neither engine is currently able to make the stated 2600 at present and we have pretty well eliminated all the other potential causes. There is about a 200 rpm difference in WOT and the starboard engine makes black smoke at higher revs, so I'm inclined to think that both sides may be over-propped and because of the lower reduction ratio, the starboard one even more so. That's why I'm thinking it may warrant a reduction in pitch, with maybe an extra half-inch being taken off the starboard prop.
Click to expand...
A 1" reduction in pitch will increase engine revs by 200-250 in that shaft speed range. Suggest you seek advice from one of the prop specialists like Hamble Propellers or Castle Marine who will do the sums for you and repitch the props.
 
Bill W said:
Thank you for bringing me to the point of my question.
The gearbox in question is on my port engine. My starboard engine has a PRM 750, which is a 1.459:1 ratio. It has a spacer to make it a direct replacement for a Borg Warner box. Now, 1.46:1 to 1.52:1 is not a significant difference but 1.46 to 1.6 might be. Currently, neither engine reaches Mermaid's rated WOT rpm of 2600. The port engine will hit 2300, the starboard struggles to hit 2100 at start of the season. I am considering a slight reduction of pitch, maybe 1.5 inches on the starboard and 1 inch on the port, to bring the engine WOT revs closer to manufacturer's spec and "synchronise" the prop revs for a given engine revs. It would really be useful to know for sure what ratio the BW box actually is, so I could ask a prop shop or other knowledgeable person if this is sound thinking.
I'll see if I can get the rest of the info from the BW brass plate to see if it can be worked out that way.
Click to expand...
Are the engines handed, or are you running one gearbox in reverse?
 
Had a pair of 72C attacted to Perkins M135 engines . One of the boxes had been set up to run permanently in "reverse" to achieve R/H or L/H rotation, also seem to recall some debate even regards the best direction of rotation for both props as well.
Cannot recall if both props were of identical pitch or not., one was 22" x 17P.
Never noticed it failing to achieve recommended RPM at WOT on either engine.
Loaded or light it never seemed to make much difference at all.
 
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The engines in my boat are handed with the starboard engine running in reverse rotation, I have tried reading the ID plates on the 72C gearboxes but they are too worn, I will be back at the boat on Monday and try another look.

oldgit said:
Had a pair of 72C attacted to Perkins M135 engines . One of the boxes had been set up to run permanently in "reverse" to achieve R/H or L/H rotation, also seem to recall some debate even regards the best direction of rotation for both props as well.
Cannot recall if both props were of identical pitch or not., never noticed it failing to achieve recommended RPM at WOT on either engine.
Loaded or light it never seemed to make much difference at all.
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If you look at the table I posted above you can see not all gearboxes had the same ratio in forward and reverse, also the ratio of my gearboxes are not listed on the above table either, they are in the workshop manual I have (1.523:1)

BW_70C_series.jpg
 
May well have note somewhere as to prop size and if pitch of R/H and L/H props were different and direction of rotation.
The boat was a Transocean 37 basically a home build on modified Aquafibre hull frequently used by Broom et al.
It may have seen 15 knots on very very good day, rolled like a happy pig in all sea directions except head on and then about as wet as they come.
Loved it to bits.
 
Tranona said:
A 1" reduction in pitch will increase engine revs by 200-250 in that shaft speed range. Suggest you seek advice from one of the prop specialists like Hamble Propellers or Castle Marine who will do the sums for you and repitch the props.
Click to expand...
I think I'll phone a couple of prop shops tomorrow for their advice.
BB1 said:
Are the engines handed, or are you running one gearbox in reverse?
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No, both engines are anti-clockwise looking from the flywheel end. The port engine has the aforementioned BW box, the starboard engine is equipped with a PRM 750 with a spacer behind the box. The PRM is set to reverse the engine rotation, giving clockwise shaft rotation.
My problem, if it is that, is they have different ratios. The BW on the port engine is 1.58:1, the PRM on the starboard is 1.459:1, a significant difference but the closest available between the two makes. My guess is that the PRM box was fitted when she was re-engined with two left-hand engines, to give correct "outward" rotation of the props.
The boat had new stern gear fitted on the insurance prior to me buying it, following an accident. One prop was repaired, the other was brand new. I wonder if the props had previously been de-pitched but left with the original numbers stamped and the prop repairer/supplier has set them to the stamped numbers.
With a clean bottom, port engine gives WOT at about 300 revs short of the rated 2600, starboard engine WOT is about 450 revs short and lots of black smoke.
 
oldgit said:
May well have note somewhere as to prop size and if pitch of R/H and L/H props were different and direction of rotation.
The boat was a Transocean 37 basically a home build on modified Aquafibre hull frequently used by Broom et al.
It may have seen 15 knots on very very good day, rolled like a happy pig in all sea directions except head on and then about as wet as they come.
Loved it to bits.
Click to expand...
Same Aquafibre hull as will be on our Ocean37, with the twin145hp HT6.354M she made 17knots with a bit of throttle still to go and another 400rpm to the max recommended for those old Perkins. Wife was not happy about going so fast, she was quite stable - unlike the better half ;) but seeing on the AIS that a freighter was coming into the canal I backed off to pass it at a more reasonable speed, that plus we were coming up to the end of the highspeed zone anyway.
I have heard that the boat rolls a bit in anything other than a head sea, but so far not had her out on any large expanse of water yet.
 
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