Single or Twin Sterndrive?

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Scarron

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Just trying to do a cost/benefit analysis on re-engineing with single or twin sterndrives. Currently running twin 170hp petrols in a 25' boat - what is the efficiency loss/gain of one drive rather than two, I recall reading somewhere that there is an approximate formula to calculate the power loss of a second sterndrive?

So; if I were to replace my 2 x twin carbed 4cyl petrols [total weight for the pair inc drives 340hp total, 578kg] with a single fuel injected V8.

Would a 5.7gxi small block[320hp, 469kg] do the job

Or would I need a 8.1 gxi big block[420hp, 570kg]

Also would I see an improvement in fuel efficiency at cruise, currently about 2mpg, 14gph?
 
Think there was a report recently in MBM between 2 similar boats (something like a Botnia targa but not..) 1 had a single Volvo D6 the other either twin D3's or 4's

Think they were rated at similar HP's but the single was a fair bit faster and more economical!

I'd assume there would be broadly the same for petrol, so my money would be on the small single all other things being equal!

320hp sounds like plenty in a 25fter as well /forums/images/graemlins/cool.gif
 
Twin engines every time.

The only factor to consider is - if your single engine fails mid channel with a super tanker bearing down on you - "What now, Skip" - wheras you can always limp out of the way the remaining engine if you have a pair.

I know there are circumstances where both engines could fail at the same time - but you're decreasing the chances of being adrift at sea with a pair.

The peace of mind is worth the cost. I once found myself drifting across Southampton VTS in the path of a tanker after a single engine failed on me - and don't believe those people who tell you that their 15hp mariner outboard which doubles up as a tender motor will get you out of the way either.

It may push you slowly across the tide - but will not push against the current - even if you can deploy it properly from your bathing platform in a force 4.

Nope - single engines on cruising boats are fine for rivers - at sea - I'd go for 2
 
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don't believe those people who tell you that their 15hp mariner outboard which doubles up as a tender motor will get you out of the way either.

It may push you slowly across the tide - but will not push against the current - even if you can deploy it properly from your bathing platform in a force 4.

[/ QUOTE ]

I was going to suggest 1 engine for faster speeds, better economy and cheaper and easier servicing. I was then going to add a bit about having a decent aux outboard in case of trouble.

I have a single engined 25 fter and have tried a number of different outboards on the back. I found that something around 10hp, mounted correctly so that the prop is deep enough in the water will move a boat of this size at a modest speed. By that I mean around 5 knots. I tried it in the Mudway against the full flow of the tide and managed 3 knots.

It won't get you out of the way of a tanker in a hurry, but then the answer is to not put yourself in that position in the first place.

On reflection, I accept the twin engines are safer argument, but on a boat of this size, I would prefer to have one well maintained engine and a decent, well thought out aux.
 
[ QUOTE ]
I accept the twin engines are safer argument

[/ QUOTE ]I for one don't.
That's just one of the most common myths of pleasure boating world.
Mind, just pleasure boating - and that says it all.
 
[ QUOTE ]
[ QUOTE ]
I accept the twin engines are safer argument

[/ QUOTE ]I for one don't.
That's just one of the most common myths of pleasure boating world.
Mind, just pleasure boating - and that says it all.

[/ QUOTE ]

I am not sure it is a myth. Being around boats alot, I can think of a few occasions when a twin engined boat has lost both engines, but to be fair, 9 times out of 10, there is a problem with just one of them.

Bear in mind I would go for the single, what I am accepting is that there is an extra element of safety in having twins.
 
Manoevering with twins is much easier.
Do you currently have two fuel tanks- if only one tank then the the reliability factor for twins is reduced.
 
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Leaving aside the arguments about efficiency vs safety, how do you plan to patch up the transom if you go from twins to a single? That's going to be a major exercise, I'd have thought.

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fully agree,
additional to the transom patchup, have you looked at the engine mountings on the hull bottom. Some are specifically made for a twin engine, and can't be safely modified to single.
Choice for single or twins has pro's and cons, for you to decide, but if you want to change from twin to single I would go for another boat. /forums/images/graemlins/blush.gif
 
I see what you mean, but statements like "single engines on cruising boats are fine for rivers - at sea - I'd go for 2" clearly show that there is indeed a mythological component in such belief.
So many boaters are not even aware that most of the safest bluewater MoBos around, capable of oceanic passages in the roughest conditions, are single screw.
 
Another point to add to the two vs one argument is if the engine with the steering pump fails, your have no (little) steering anyway. Not something that a good aux suffers from!

My tweopenneth!! /forums/images/graemlins/wink.gif
 
Safety/redundancy - yes the peace of mind that comes with having twins is in principle worth having, although going from two currently reliable [touch wood] 17 year old carburetted Volvo petrols fed from one tank to one new injected V8 may not be a hugely retrograde step.

Handling - yes twins will definately be be more handy at close quarters

Rigging - yes filling two holes in the transom would be a pain and changing engine beds etc. but she was built in 69 to Lloyds A1 so it would be like rebuilding a brick shithouse [the transom is over 3" thick].

The build weight is actually fundamental to the problem, she is only 25' long with a very deep 25° Vee, yet weighs between 3.5 and 4 tons with 600kgs of machinery in the stern. Therefore she needs quite a bit of power to drive - but probably wouldn't take to kindly to having engines that are a lot heavier than at present.

Diesels rule themselves out either on weight or cost grounds leaving me thinking that one much higher power to weight ratio engine may be better - ideally I'd like one of these Raylar 550 /forums/images/graemlins/cool.gif

We are not doing too badly at the moment;cruising at 27 knots and reaching 34 knots flat out, but because of the deep vee she doesn't really settle nicely till above 30ish knots. Hence wondering if 320hp and lots more torque on a single would make an appreciable difference.

I recall hearing somewhere that you should discount the effective horsepower of the second engine by 50% or something? anyone know?
 
Conked out once, wind over ebbing tide, very scary.
RNLI to the rescue.
Now have twin diesels, bit of a problem coming back from Cardiff the other week but didn' flap as S/B engine still going fine!
Actually the problem was something around the prop rather than the engine, but point is I still had another.
2 for me!
 
2 for me too, first trip on my boat after pickup the stb engine failed. Yes i lost the power assisted steering so my arms ached a bit but I was still able to get in.

On another occasions I couldnt get a leg to go down, kept trying it but with each try it went up a bit more, to the point where I didn't want to run it. Bobbed around a bit with both engines off while I investigated the problem. Then panicked a bit when the good side wouldnt start. Thankfully I noticed I had accidentally knocked the engine master switch off climbing down into the engine bay. I even got the leg down just after, usual dirty relay. But was thankful that I had two engines.

The only reason to stick at two? cos you don't see many with 3 /forums/images/graemlins/grin.gif
 
Nice looking boat yours, we always comment when passing.

The Raylar motor looks good.

What about a pair of Steyr's? I think Mermaid, the agent, occasionally have rebuilt ex RNLI motors for sale.

Would take a big bunch of weight out of the transom.
 
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