Sumara of Weymouth

23rd August 2025

Reading Time: Five Minutes

The first thing I would like to say is that this engine has been wonderful since its installation about five years ago. The extra 4 hp above my previous 12hp Kubota has made a great difference to the ability of the yacht to punch into sea when needed. It has started and run faultlessly – until last week.

Next year, 2026, I am intending to sail to Scoresby Sund in Greenland. Although the wind can be fierce in the Denmark Straight, once Sumara gets into the fjord the weather is generally windless. The fjord is the largest in the world and I would like to try to circumnavigate Milneland, an island in the fjord. That could involve motoring for 350 nautical miles in an area where spares and repairs will not be possible.

I have built up a huge faith in the new engine and transmission over these last five years so you can imagine my disappointment when the gearbox started to slip.

The thing is that this has happened to me before in the Westman Islands off the south coast of Iceland. It was my old engine with a Hurth gearbox. I left the mooring and noticed something wasn’t right so I just turned around to tie up again and investigate. A fender had blocked the gear lever from fully engaging. Problem solved, so I thought, as I moved the offending fender away, but sadly not. The few minutes of the gearbox being half engaged had polished the cones and the gearbox would not grip. We sailed back to Scotland and just used the engine to sneak into the harbour at the very end. The gearbox was rebuilt and refitted at considerable cost. I couldn’t replace the gearbox because no new gearbox would match.

We first noticed the new gearbox was slipping in the Caledonian Canal. I first thought maybe we had a bag around the prop but the canal was litter free and Sumara has never had a fouled prop. I then thought perhaps the prop is slipping. I sheared the brass keyway once in Guadeloupe and had to do some diving. To check this prop was secure we used an underwater camera and could clearly see the prop was firmly fixed to the shaft, besides, in forward gear the prop would generally grip the tapered shaft and only slip in reverse. We finally checked the flexible coupling but that was good. It must be the gearbox.

The following day it behaved fine but the next day it began to play up again and again, finally resulting in me deciding it was too risky to continue cruising and I retired the boat to Dunstaffnage Marina.

I checked the change lever was achieving its full stroke and it seemed fine. The ATF was at the correct level and I had changed it in Corpach. It looked clean to me.

If I was cruising around the UK I would try a fix. Beta Marine have been their usual helpful selves. That makes a mammoth difference! They suggested a possible quick fix which may have resolved the problem and probably got me out of trouble if I was coastal cruising but I was concerned that I may have worn the cones and I don’t want any extra anxiety whilst in Greenland. You can download the adjustment instructions in the “Downloads” section of this website under “Instructions” then “Beta16 Engine and Gearbox Instructions”

I decided that the best course of action was to purchase a new gearbox. With VAT and delivery it cost £752.40. It looks like the engine will need lifting out to fit the box so the costs will mount up.

I am concerned that the “Bathtub Theory” might apply. This is when most failures happen in either the commissioning period just after installation or at the end of the expected lifespan of the product with a long period of reliability in between. After the installation the boat will be on the hard until the spring so the commissioning period will be dangerously close to the embarkment to Greenland. We will need to triple check everything to ensure we are totally happy with the installation.

I will update this as we progress. Fingers crossed it will all be fine.

28th August 2025

30th August 2025

Update 21st September 2025

I few things that I have learnt

I have delayed fitting the new gearbox but have continued to have chats with people who know more about these things than me. I am beginning to build up a picture that there is more to marine gearboxes than meets the eye.

Here is a section from an email I recently received:

I just tuned into your blog and read about your gearbox troubles this summer!

I don’t know if you remember but I also had TMC gearbox troubles when my new Nanni 14 was fitted. My engine had to come out a further two times and then I switched to a PRM box.

Initially it had an annoying whine coming from the box and although I wasn’t aware of any slip when I changed the gearbox oil at the end of the season it was full grey metallic looking particles, they just exchanged the box and fitted a new one, then the following season the same also happened and the boat yard took the engine out again and I drove the gearbox to (company mentioned) who stripped it on the bench with me, we found bolts loose that shouldn’t have been! I then suggested we fitted a different gearbox, there was a PRM model, which we agreed on, which also had a better gear ratio. Which touch wood has been fine 350 hours later, but I do still drain the box each year and check for particles 😊

I’m sure your new gearbox will be fine and that TMC must have overcome any issues now, I think you had your new engine fitted just after I had mine fitted, so perhaps they had a production issue around that time and its why they where happy to try a different gearbox for me, I notice that Nanni still supply TMC’s, so I expect my PRM box is a bit of a one off, particularly as the serial number is stamped with something like ‘Test Model’.

What I learnt from my gearbox troubles was;

Make sure you can get full revs in gear (3600 Nanni 14), it always seems a bit cruel running the engine at these revs, but it only needs to be for a short while, but it determines that the prop and engine are in happy partnership. When the yard changed the engine originally, they said my old prop pitch was ok, it turned out it could have been a reason for the cones slipping and have I’ve reduced the pitch.

Oil level – the dipstick is based on the engine fitted level, so depending on the angle of the engine this could be wrong, so I always drain completely and just add (0.2 lt for a PRM box).

I think draining the oil into a jar might spot any cone slippage early, as I couldn’t see the contamination on the dipstick when it was failing, even though I didn’t notice loss of power. I guess these little gearboxes have to work hard on a 5-ton boat if the prop and gearbox is not matched perfectly, although when I had my 10hp Yanmar for 20 years, and probably 1000 plus hours later, I didn’t ever change the oil and only probably only checked the level twice! (Once I new it was ATF oil, I assumed that was good for a longtime just like in a car with an auto box)“

That was very interesting stuff. During a quick chat afterwards he mentioned his Vertue was 1 knot faster with the Yanmar 10 hp than with the Nanni 14 hp and it was put down to the fact that the Yanmar had three gearbox options and one was specifically for heavy boats like the Vertue.

Chatting to a boatyard owner the other day he said that he had been told by a gearbox supplier that they tried an experiment on a perfectly functioning gearbox by checking the input and output revs with two calibrated tachometers and at the end of a full days motoring they showed a notable difference. This was indicating that even a good gearbox may have slight slippage issues during normal use.

There will no doubt be more words of wisdom from those who know!