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Old 12-08-2019, 22:47   #1
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Repowering a Tolly

I would like to repower my 40 Tollycraft Aft cabin from the current 454 CDI gas engines with Yanmar Diesel Engines. Has anyone had experience with aproject like this?
Apparently the switch is relatively simple with just a drop in replacement using same gears and exhaust system. Is this correct? Any guidance and advice would be appreciated.
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Old 14-08-2019, 00:31   #2
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Re: Repowering a Tolly

Gas engines are usually higher revving than diesels and diesels of equivalent horsepower generate more torque. Consequently replacing with the same gearbox ratio may not work.

In order to get similar hull speed performance you may need to change the props or use a lower gearbox ratio which will spin the props at the same speed with lower diesel engine revs.

Most engine manufacturers publish a series of plots which illustrate the relationships between power and engine speed.
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Old 14-08-2019, 04:56   #3
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Re: Repowering a Tolly

I'd have expected different gears, maybe larger shafts, different props... but that's without any tech knowledge at all, just from reading.

If you're near where you could have somebody like Mack Boring (Yanmar dealer) advise, and maybe even do the work, I'd think that'd be less risky.

You'd want to convert a genset, if installed, to diesel too.

A dock neighbor converted a 35 Betram Convertible from gas (blew an engine) to 370-hp Cummins B engines (and converted genset to diesel too)... and they loved it. I heard it cost him about $100K (including some relatively minor refurbishment in their head). He ended up with a boat that was eventually listed as about $105K asking price when they decided to sell afterwards. (Latter was due to a change of residence and circumstance, nothing to do with the engine conversion.)

Unless you're completely sold on Yanmars, you might also consider the Cummins options... especially if you have easier access to serious Cummins gurus versus Yanmar gurus nearby.

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Old 14-08-2019, 05:59   #4
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Re: Repowering a Tolly

Make sure they change the engine mounts, too. Repowering from gas to diesel is pretty common and the techs doing the work often save a little time and money by reusing the engine mounts. Diesels are much heavier than their gasoline-powered counterparts and will often cause the old mounts to sag over time. This can lead to misalignment of the shafts and stuffing boxes, leaking, premature wear, etc.

John
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Old 14-08-2019, 10:05   #5
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Re: Repowering a Tolly

Thank you gentlemen, for the helpful hints. I do have a selection of Yanmar gurus along with Cummins where I live on the west coast of Canada. I'll post once I have it done. The Yanmar guys are suggesting new gears for the reason given by Raymond. The Gen Set is on the list for replacement since it is an old Kohler with no parts available anywhere.
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Old 15-08-2019, 08:23   #6
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Re: Repowering a Tolly

If you want to repower with a decent chance of keeping the shafts and props, look at the V8 diesels from Yanmar and Mercury. They redline at 3800 and 4200 respectively, so pretty close to the 4000 - 4400 a lot of the older 454s spin. And with a lot of the older 454s in the 330 - 350 hp range, the 370 hp rating of those diesels is also a pretty good match. From the numbers I can find, both options are around 100 lbs lighter than the 454s as well, so probably the easiest diesel setup to fit, although other engines may be better in other ways.

If you currently have counter rotating engines (rather than counter rotation handled at the trans), you'll most likely need new transmissions to go with the repower.

I've had similar repower thoughts for my boat, but it's not underpowered with the current 340hp 454s, and the repower costs would likely be near $100k. Even though the numbers show I'd see about a 50% mpg gain at cruise, with $4 / gal fuel prices it would take 2500 hours of cruising time to make up that $100k in fuel costs.
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Old 15-08-2019, 10:25   #7
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Re: Repowering a Tolly

Thanks rslikin
Your reasoning make s sense from an economic perspective. have you done any work on your 454's such as head gasket replacement? How many hours can one expect from these engines. Mine have 1700+ hours.
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Old 15-08-2019, 11:02   #8
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Re: Repowering a Tolly

My stbd engine was replaced a couple of years ago (prior to the boat being mine) due to an oil cooler line failure that wasn't caught fast enough wiping out the main bearings. Port engine is still original and healthy, never been opened up beyond an intake manifold gasket replacement and it's got about 1500 hours on it. Risers, manifolds, etc. have all been replaced a few times, of course and both carbs have been rebuilt.

The older engine doesn't run any differently than the new one and it still compression tests well, so it should have a good bit more life left in it.

I brought the boat up from Long Island Sound to Lake Ontario a couple of weeks ago and put just shy of 50 hours on the engines in the process without a single hiccup, including one day where we had over 11 continuous hours of engine runtime.
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Old 19-08-2019, 10:49   #9
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Re: Repowering a Tolly

I have a simple formula, add $15,000 to whatever you think its going to cost. A formula one of the members here offered for his refit experience, and my situation has born his estimation out - multiply by three times the amount of time you think your repairs are going to take, and multiply by three the amount of money its going to take.

And on older boats, whatever item you think you can re-use, you can't.

A modification will need to be made unanticipated by anyone.

If you are over propping or modifying your prop in anyway, you can't. Modifications from manufactures specs affects warranty, for me that included using a high output alternator.

On the positive side, besides standing in food lines because you are poorer after replacing everything, you will worry much less about your power plant, etc.
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