Transmission Replacement: What I learned (long)

[Patches]

With roughly 2500 known hours on my M25XP and original Hurth HBW50 transmission, it has become time to replace the Tranny. I didn't think this would be a big deal because I had purchased a "rebuilt" HBW50 on Craigslist ($300), complete with invoice from Harbor Marine in Everett, WA.  Only it probably wasn't rebuilt by them. 

As I learned from Mike in the tranny shop--super guy, and very knowledgeable--it is a not uncommon scam to sell one of these representing it as "rebuilt' and with a Harbor Marine invoice showing the rebuild.  I live close by and brought in my transmission so he could verify whether it had, in fact, been rebuilt by them.  He showed me why (output shaft had some play and no evidence of the sealant they use when putting the transmission case back together) they probably did not do the rebuild.

So the choices come down to (1) rebuild or (2) replace.  Very few people recommend rebuilding a 30 year old Hurth HBW50.  They don't enjoy a great reputation, and the rebuild cost I was quoted from Mike was roughly $1800 and 30 days out.

As for replacement, it is either stay with an upgraded Hurth (ZF12m) or go to one of the new transmissions with the cone-type clutches.  Both are bolt-on replacements.  These are either the Newage PRM-60 or the TMC40P.  The PRM and TMC are very, very similar but with small dimensional differences.  It bears mentioning that Harbor Marine used to sell the Newage PRM60s but no longer does.  Ask Mike about that.

Common issues to both:

--you've got to remove the bell housing to unbolt the HBW50 and to install the new Transmission.  At a minimum, that means I'm going to lift out the engine into the galley area to do this.
--you've got to replace the damper plate with a new Sachs while its out.
--as long as its out, replace the 32 year motor mounts with new.

Cost/Benefits of the ZF12m:

--gear shift on the same side as the HBW50 (no surgery on the engine bed)
--its more expensive ($2300)
--its bigger dimensionally than the PRM or TMC which means both raising the engine higher and cutting down your prop shaft as the "overhang"--distance from front of prop to cutless bearing/P strut--will increase by 1.5 inches.
--I couldn't find anyone who recommends them over the PRM or TMC alternatives. In a telling post from 2016 or so, Maine Sail basically endorsed running away from the new ZFs and to the TMC40.
--gear ratio is 2.14:1 vs. original 2.05:1 for HBW 50.

Cost benefits of the PRM/TMC:

--less expensive ($1260 for the TMC)
--smaller dimensionally.  When installed you have to raise an M25XP 5.5mm due to lower output shaft on the TMC 40, and the "overhang" (distance from prop to strut--usually a shaft diameter (1")-- increases only 4mm).
--shift lever on the port side.   This means a little more effort to adjust the shifting cable and/or adjust the cone clutch (which is represented as being "self adjusting" but which may need to happen sometimes).  The adjustment of the cone clutch is actually covered in the TMC40 and PRM60 manuals.
--Have to cut away a small area of the port engine bed rail, just aft of the rear port motor mount, to make room for the port side shift lever.  (roughly 1" deep and 3" long)
--Have to alter the location of the Heat Exchanger to access the dip stick and fill.  As you know, that is already difficult with the HBW50.  I have to loosen my 3" Sea Kamp HX to fully extract the dip stick.  Noah has a great post on how and where to relocate your heat exchanger for better maintenance access on the tranny, so I am going to do that anyway.
-gear ratio is 2:1 vs. 2.05:1 for HBW 50

Choosing the TMC40:

Based on overwhelming positive reviews vs. the ZF12m, I chose the TMC40P.  This was reinforced after I spoke with Joe DeMers at Sound Marine Diesel (CT) who is a Beta dealer and who sells a lot of TMC transmissions as well.  Generous with his time over the phone, he went through all the considerations when putting in a replacement transmission for my engine.  There is also a Beta Marine document titled "Conversion from ZF5 (Hurth HBW50) to TMC40" which fully explains those considerations.

I ordered my TMC transmission ($1259), 4 new motor mounts at $49/each, and new shift bracket and hardware ($98) from Joe.

(FYI: The motor mounts are dimensionally the same ones that Catalina Direct sells for $180/each!)

By the way, I also went down the road of doing a complete re-power with a Beta 25.  I'm pulling the engine anyway to replace the transmission.  Problem is that on the left coast there are no Beta 25s in stock.  Serious supply chain issues in getting Kubota cores to Beta in England.  Earliest "projected" arrival--which is very soft--is March 2023.  If you're not on the left coast, they have stock out of North Carolina.  I operate as a charter boat so I need better definition than that and don't want to interrupt the season for an engine replacement.  But, when the supply increases I can buy a new Beta 25 "bobtail" and attach the TMC40 to it.

Hope this helps anyone having similar issues/thoughts.

Patches

[waughoo]

Thanks for the thorough analysis.  If you can remember, post a few photos when you have it installed.  It would be interesting to see the modifications in situ.

[Noah]

Thx Patches! Great info. I rue the day when my H50 eventually dies....

[Noah]

Patches; Just to clarify: the brand/model you selected is also known as "Twin Disc" — and previously mentioned/used by some other Forum members. Correct?

[Patches]

Yes, TMC = "Twin Disc."

[Jon W]

Great write up, thanks. Do you keep or exchange the old HBW50 and m25xp? If repowering, why not upgrade the hp to the Beta 30?

[Patches]

I'm continuing to use my M25XP engine--only changing out the transmission for now.  Not much, but some, value in the replaced HBW 50 as a rebuildable core.

My understanding is the Beta 30 uses a different (and larger) Kubota core and would be tighter in my Mark 1 engine compartment.  The local Beta dealer came down to the boat and confirmed this.

[Noah]

Just have LogoFreak build you a bigger engine compartment!
Comment strictly in fun...

[waughoo]

Good one Noah!!  Logo is definitely making space about his boat.  Perhaps we should having him look into the engine bay!!

[LogoFreak]

I heard that 
I can attest to the twin disc, replaced the hbw50 in my m25 I had in my Catalina 30, it's still going strong!

I actually looked into the beta hybrid engine, can't remember which model, likely the smallest one. It meant that you'd have to cut out the rear "shelf" up in order to fit that engine, no thanks!

A better option was to stick a panda dc genset in place of the engine and install an ocean volt leg under the aft bed, however that just doesn't make ANY financial sense and the advantages of having the ocean volt setup aren't quite achieved with our hulls, minimum water speed for getting some regenerative charge is 6kn, I'd prefer having a boat with that setup that could do 7~8 average.

[Ron Hill]

Noah : That's why I went with the M25XPB engine.  Even then I had to do some rewickering of the engine mount stringer to get everything to fit and line up!! 

Would have loved to install a M35BC engine, but that would have been a rebuild of the interior!!

A few thoughts

[Bill Shreeves]

For what its worth, I replaced my HBW50 on my M25XPB with a ZF10 back in 2017 myself.  I did not pull the engine or need to pull the bell housing to remove the HBW50.  I did, however remove the bell housing after pulling the transmission to replace the damper plate.  Pulling the tranny was a bit of boat yoga but, not that big of a deal for me.  A few bolts after pulling the control cable, coupling etc. and out she came.   BTW, while I was at it I replaced everything all the way back.  New SS shaft, stuffing box, prop and added  a flexible coupling.

[Ron Hill]

Guys : If you want to use to your old shaft with a new ZF10 or Hurth 100; you are going to have to shorten the shaft.  I see that Bill installed a new (shorter) SS shaft!!

A thought

[Bill Shreeves]

Ron, If I recall it was an 1" longer.  Do you really think that would make that much of a difference?

[Patches]

Or, replace it with a Twin Disc TMC-40 as I am and it is only 4mm (>3/16") longer and 1/4 of the price of a ZF10 ($1259 vs. $5000 on CD).  I'm mid project right now, having removed the exhaust riser and heat exchanger, drained coolant form the motor, and removed the alternator and wiring from the motor.

I'll report back when I've successfully swapped the gearbox and relocated the heat exchanger.

Patches