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Message board > Engines > Replace inboard diesel with outboard engine.

Message 5 of 5
Posted by member Henry on Monday 11 March 2024

Hi Col

I've come to the same conclusion: better the devil that I know, so I'm sticking with the existing set-up. As you say, Sonic spares are readily available, and I have some ideas which might reduce engine noise.
I'll put a note on the board once I've got a success to share / brag out (or a catastrophe to grumble about).

In case anyone else has a Lister Petter, I found the people on the Lister facebook page very helpful.



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Message 4 of 5
Posted by member Colin on Wednesday 31 January 2024

Hi Henry,

Fair comments all!

Personally, if I were you, I would be looking to keep as close to your existing installation as possible, ie, retain the Sonic stern drive mated to a suitable inboard diesel engine.

I know the Sonic sterndrives are no longer available but as far as I can see, there are available spare parts that could recondition your existing unit, then it would be a case of finding a suitable inboard diesel engine that could be mated up to the drive flange of said unit, doubtless changes would be needed for the engine mounts to make it fit etc.
None of this sounds inexpensive but that's boats for you! Outboard engines are not cheap and bring their own 'problems' well that's my experience.

Maybe someone with more experience with these matters will come along to give you better advice (wouldn't be difficult as my knowledge with these things is probably limited)

I would be looking to tap into local knowledge in the marina, there's usually someone who knows someone etc who can advise.

Good luck and keep us advised of how you resolve your issues, maybe a written feature of how you achieve this by way of a future article for the Club magazine for Marie the Editor to include in a future magazine, I am sure like me, other owners would be interested to hear!



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Message 3 of 5
Posted by member Henry on Wednesday 31 January 2024

* Hi Col - sorry for the late reply - my original response got lost somewhere in "the system".
Yes your eyesight is spot on. She's currently got a Lister Petter Alpha LPWS2 20 hp diesel plus a Sonic stern drive.
My reasons are about noise and reliability: currently I can't talk on the VHF with the engine running, and there is a fair bit of wear on both engine and stern drive.
I'd prefer a splash well, but as you say it's pricey (quotes from a marine contractor). I'm currently getting quotes for a replacement inboard diesel as a Plan B. Not sure how much quieter this would be though.
For the outboard option, my noddy displacement calculations predict that the stern would be up by less that 1 inch. due to the lower engine weight: I reckon this wouldn't affect the handling, and would compensated for anyway by the outboard engine trim setting.
So, options still open. Thanks for your thoughts - and more are always welcome.

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Message 2 of 5
Posted by member colin hewitson on Tuesday 16 January 2024

Hi Henry, good looking Navigator by the way!

You don't state your reasons for doing this, is the existing set up worn out for example? At the end of the day, the cost of achieving what you wish to do will likely exceed replacing/improving 'like for like'.

There are a few considerations here, some of which you have touched upon, if my eyes don't deceive me, the existing set up is some form of stern drive leg coupled to probably an inboard twin cyl diesel engine with an auxilliary transom bracket mounted - I may be wrong as my eyesight isn't what it was!

Are you talking about reconstructing the transom to incorporate a splash well for the outboard mounting, or a Transom bracket with out board mounted like your auxilliary?
The former will probably be quite expensive to achieve I would imagine and I doubt it would be gaining any considerable saving in cockpit space if that is what you are looking to achieve.
The latter will probably be more difficult for engine access etc once the boat is on the water. Our main engine on our MS18 is so configured (as is the auxilliary) and it can make things awkward.

Personally I would think your existing set up is better and more desireable than any outboard configeration - but that's just me, I don't know how many others would agree with that!

Are you looking to get this done by a marine contractor? if so maybe you should get some quotes first as that may help you decide, I wouldn't have thought this would be a DIY type job, anyone attempting it would need to know what they were doing, at the end of the day, it could seriously affect the value of your Hardy. Unfortunately over the years I have seen a few 'cobbled' Hardy boats that left a lot to be desired and it did put me off considering them when we were looking to purchase, rather like Kit cars - it depends who has done them and how they have been done!

Really not trying to put you off but I imagine there are more questions than answers involved in such an undertaking!



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Message 1 of 5
Posted by member Henry on Tuesday 16 January 2024

* Hi - I have a Navigator, and I'm considering replacing the current inboard diesel engine with a 25 HP outboard.
There are issues to be addressed around trim, engine mounting and strength of the transom, plus the general pros and cons of doing this anyway.

Anyone ever done this ? I'd be really keen to hear about how it went .... Cheers Henry

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