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Twin Engine to Single engine re-powering question.

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moresparks

Regular Contributor
Hi, looking for advice for anybody that has replaced 2 aging sterndrive engines to a single sterndrive engine.

Had a catastrophic failure on an old Volvo engine where parts are unavailable. I decided the only option was to re-power and have since removed the engines and drives. Here in the UK options are limited, we don’t have the suppliers of complete refurbished turn-key engines.

So… one option was to replace the aging engines with a new modern single V8 with same or more HP. Modern twin V6’s are a lot bigger than my old one and most will not fit.

I have had conflicting advice that by changing to a single engine, the balance will be all wrong and rolling could increase. I have trim tabs and wondered if that would correct the rolling? The single engine will be as much as 100kg lighter than the old engines. I could have a “get you home” kicker on the stern platform and balance the other side by repositioning batteries etc.

Before anybody shouts “Outboards” this is a design no go on my boat!

The boat originally had the option of a single diesel but they were bigger and heavier.

Still contemplating my options, but welcome opinions.
 
It would help me if you gave a better description of what you have...length, beam, weight...and exactly what you have already removed.

I doubt rolling would be a big deal unless you spend a lot of time dead in the water...trim tabs will likely only matter when under forward power.
 
Hi, Thank you for responding,

It is a 27foot sports cruiser (30+ knots) mainly used in estuaries or coastal waters it originally came with twin Volvo Penta AQ171c’s around 167 hp each. Beam is around 10 foot and weighs roughly 3.5 tonnes. There were a number of options which included a single 200hp Diesel and a single gasoline 275Hp and single gasoline 3320HP as well as various twins up to 200hp each.

I was looking at the option of putting in a 350HP V8 duo prop. Now all the earlier engines are a lot heavier than a modern V8. The problem I have is that the original AQ’s had a small footprint so whatever I choose, major transom work will be required, even if I go for twins.
I was told by a respected engineer that going from Twins to Single, the balance will be all wrong and yet it had that as an option when new!
 
As I read your posts, I'm thinking that you are planning to replace one of the engines with a bigger one in its existing location on the transom and ballasting the location of the other engine???? USUALLY when folks go from DUALS to a single, the single engine is located on the centerline with MUCH work done to the transom to "plug" BOTH old holes in the transom and cutting a new mount in the center. This is NOT for the feint of heart. I live 2 blocks from a marina and watched them do that to a 25ft Bertram.... or try to... YEARS later and many $$$$, they scrapped the unfinished "conversion. And this was a project attempted by several guys who restored boats for a living.
 
Hi, That wasn't my intention. If I go the single engine route it will be plugging both holes, reinforcing the transom and cutting a new hole on the centreline. Whatever route I choose the transom holes will need to be re-cut anyway, as they are currently too close for ANY twin engine installation.
The transom, bearers etc work, will be undertaken by a specialist GRP Engineer. I know only to well it doesn't make economic sense, but to a certain degree I have no choice. I have seen this done on a similar boat quite successfully
 
moresparks said:
Hi, That wasn't my intention. If I go the single engine route it will be plugging both holes, reinforcing the transom and cutting a new hole on the centreline. Whatever route I choose the transom holes will need to be re-cut anyway, as they are currently too close for ANY twin engine installation.
The transom, bearers etc work, will be undertaken by a specialist GRP Engineer. I know only to well it doesn't make economic sense, but to a certain degree I have no choice. I have seen this done on a similar boat quite successfully
Click to expand...
Ayuh,...... I think you'll be fine with a single BBC, at nearly the horsepower you had with the twins,.....

Donno what yer engineer is thinking,.... with a single in the center, the balance should be fine,.....
 
Hmmm… I wondered if the Engineer thought the same as SandKicker? What seems obvious for those that have been around boats for many years, the fact that I didn’t mention the fact of redoing the whole transom led him to believe I would use one of the existing cut-outs. That would then make sense!!

Thank you for your responses.
 
Using a single engine on the center shouldn't induce any balance issues...

I'd agree a BBC would give you the max speed/best economy for a minimal investment. If you cruise long distances between petrol stops, the diesel may be worth considering.

Either way, if you can 'copy' one of the factor configurations, you shouldn't have any major problems....
 
moresparks said:
Hmmm… I wondered if the Engineer thought the same as SandKicker? What seems obvious for those that have been around boats for many years, the fact that I didn’t mention the fact of redoing the whole transom led him to believe I would use one of the existing cut-outs. That would then make sense!!

Thank you for your responses.
Click to expand...
Re: "have had conflicting advice that by changing to a single engine, the balance will be all wrong and rolling could increase. I have trim tabs and wondered if that would correct the rolling?"
This is what lead me to think you were going to use one of the existing transom holes... Changing from duals to a single CENTER mounted engine would not impact balance to the point of causing rolling... some fore/aft trim, (static) sure.. "Rolling" is side to side... AND BTW, "Sandkicker" is a retired R&D engineer.
 
“Sandkicker” – I hope you did not take offense as I highly value you responses the fault is entirely mine for not being concise enough. The single engine option did look a good option, if the preferred twin option will not work - both economically and physically.

As you said, the Fore and aft trim will be affected slightly as the installation will be around 100kg lighter at the stern – hence the batteries etc being moved further aft.etc.

Here in the UK we have to have a survey if any major modifications are made, including a valuation survey, all for insurance purposes. Just when you think you have one option, someone throws a curved ball to make you doubt your plans.

Thank you.
 
moresparks said:
“Sandkicker” – I hope you did not take offense as I highly value you responses the fault is entirely mine for not being concise enough. The single engine option did look a good option, if the preferred twin option will not work - both economically and physically.

As you said, the Fore and aft trim will be affected slightly as the installation will be around 100kg lighter at the stern – hence the batteries etc being moved further aft.etc.

Here in the UK we have to have a survey if any major modifications are made, including a valuation survey, all for insurance purposes. Just when you think you have one option, someone throws a curved ball to make you doubt your plans.

Thank you.
Click to expand...
NO!, was not offended at all, rather was amused... You just pointed out that most engineers, myself included, have a very "different" way .... ( pity my poor wife!) :)
 
moresparks said:
Hi, looking for advice for anybody that has replaced 2 aging sterndrive engines to a single sterndrive engine.

Had a catastrophic failure on an old Volvo engine where parts are unavailable. I decided the only option was to re-power and have since removed the engines and drives. Here in the UK options are limited, we don’t have the suppliers of complete refurbished turn-key engines.

So… one option was to replace the aging engines with a new modern single V8 with same or more HP. Modern twin V6’s are a lot bigger than my old one and most will not fit.

I have had conflicting advice that by changing to a single engine, the balance will be all wrong and rolling could increase. I have trim tabs and wondered if that would correct the rolling? The single engine will be as much as 100kg lighter than the old engines. I could have a “get you home” kicker on the stern platform and balance the other side by repositioning batteries etc. https://assignmentbro.com/us/economics-homework-help is the source that offers me engineering and economics homework help from writing experts. Before anybody shouts “Outboards” this is a design no go on my boat!

The boat originally had the option of a single diesel but they were bigger and heavier.

Still contemplating my options, but welcome opinions.
Click to expand...
Hi Moresparks,

I wanna share my experience and a few things I did to mitigate the risk you're concerned about.
First, you can install trim tabs. Trim tabs are hydraulically operated plates that can be extended or retracted to change the angle of the boat's hull in the water. This can help to counteract the roll caused by the single-engine.

Second, you can install a "get you home" kicker on the stern platform. The kicker will help to balance the boat out and make it easier to control.
Finally, you can reposition the batteries and other heavy items on the boat to balance out the weight of the single-engine. This will help to keep the boat from rolling too much.

In my experience, replacing two engines with a single engine has been a great decision. It's more fuel-efficient and easier to maintain.
I hope my advice is still helpful.

Best regards,
Solomon
 
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