Single or Twin

Zippy · 23 February 2006, 22:13

Thought about putting this in the engine section but feel it may be a little bit more than just that.

I have not driven a rib (or any boat for that matter) with twin engines so what are the benefits? reliability and piece of mind??

Service costs are double, is fuel consumption double? extra weight, double the things to go wrong!......

My last boat was in the underpowered side of ok. but the idea of a twin setup appeals for the added reliability and piece of mind.

As a comparision, how would having twin 100's compare with having a single 200 on the same boat? Personally im looking at 4 stroke here but views on others would be appreciated. would i ba able to run the twin setup at lower revs for criusing speed than the single?

Does it significantly effect boat handling?

your views greatly appreciated.

Kind regards
Zippy. (really really looking for a new toy!

)

Richard B · 23 February 2006, 22:27

Quote:

Originally Posted by Zippy

As a comparision, how would having twin 100's compare with having a single 200 on the same boat?

You really ought to speak with Ben C who runs twin 115s on a 7.5 Tornado. Then compare his figures to ours. Then speak to Mike C who ran a Ribcraft 585 with twin Mariner 75 for a few seasons, and compare his experience to people who run a Ribcraft with a single 150.

The twin engine setup never compares favorably on paper.

Twin engines are a good idea if you are going to cruise offshore alone, or you just can't get enough HP on the transom with one engine. otherwise it's a single engine every time.

With most twin engine setups, you will only get on the plane on one engine if you're seriously underpropped, unless the engines are enormous.

pathalla · 24 February 2006, 02:05

My experience is that with twins the boat has a tendency to cut through waves a little better, two props holding it in better but not really a whole lot of difference, it will roll slightly less than a single in some beam seas but again not a lot of difference, at lower revs the boat will sit down in the stern quicker due to added weight, you can offset this somewhat with trim tabs but it will still be more noticeable than a single setup. A single setup will plane at lower rpm and stay on plane at lower rpm. A twin setup is going to burn more fuel (trust me) but some feel the additional cost is offset by the
benefit of quicker arrival at your destination if you are running more horses.
Outboards these days are pretty reliable so that is not much of an issue really, (add a kicker) unless you are really traveling far offshore. With twins it will come out of the hole quicker and have a higher top speed if you are adding horsepower. I personally would not put two 100 horse engines where a single 200 would do the job but hey, to each his own! I have had several single setups and loved em all. Once upon a time we had a sib with twin Threes (hp) used to get a lot of smiles from people when they saw that rig! And probably had as much fun with that boat as any of the others!

Searider · 24 February 2006, 08:45

Twin engines - good maneouverability - but RIBs aren't difficult to park anyway.

Better reliability. New engines are pretty reliable, so, if going for new engine(s) there is little to be said in favour of having two. However, old engines are less reliable. So, if going for an older rig, having two engines may be more reliable than a single - two engines are unlikely to both break down at the same time.
However, you could have an old main engine and a new auxiliary.

Expensive to run - twice the servicing costs. Not an issue if they are older engines and you are doing it yourself.

Inefficient. The drag from an outboard gearcase can typically be 25% of the total drag of the boat. Two gearcases on a twin setup = more drag = lower top speed for the same horsepower and more fuel consumed for the same boat speed.

I reckon that twin 115hp or twin 125hp could be equivalent to a single 200hp performance wise. BUT will use more fuel at ALL speeds.

Accelleration. Twin engines may give better accelleration. Have had a play with an ex RNLI Atlantic 21 and that seemed to accellerate pretty well.

Cheers

Scary Des · 24 February 2006, 10:09

In reliability terms, if you are running two engines, two separate fuel tanks and two separate batteries you could see some advantages over a single engine, but often you see a twin engine setup running of a shared tank and battery.

Two 100s will be slower than a 200 because you are dragging an extra leg.

Twin engine counter rotating prop installations on bigger boats is good but I think you would not gain much on a rib with a narrow transom.

I have often wondered about a twin set up with different pitch props on each engine. Use one engine for starting and speeds up to 25 knots and the second engine 25 to 50 there would be weight consequences etc

Des

benc · 24 February 2006, 19:49

Zippy - you are more than welcome to come out for a spin in mine at some point.

I havent had a single engine rib of my own yet (getting one very soon tho!)... but with twin engines I feel safe because I know I have two completely seperate systems so if any point fails I have a good chance of being able to still go.

I can get on the plane with my boat on single engine, and I have recieved 19.2 knots on one engine... have to bring the other out of the water and slowly get up on the plane though.

If you did go two engines, there is no point in getting single fuel tank or single batteries - imho it defeats the purpose - like others have said in this thread - new engines are very reliable - it would be more likely a whole in a fuel line or dirt/water in the fuel(had this before!) ....etc... that it would be for the actual engine to pack up.

Two engines definately uses more fuel - I went down to christchurch last weekend, and it cost me about £100 in fuel.... single engine will probably do it with £60 (have you done that trip before richard?)

At low revs two engines give better manuerveribility in ways because you can put one in revserse and the other in forward and nearly turn on a sixpence!

The worst thing about two engines I believe is the maintenance... my recent "service" cost me about £300 per engine (I had some other bits done at the same time)

Overall I think if you are going to just run around the solent normally with other boats - I would suggest just getting one single bigger engine rather than two to give the same-ish horse power.

Give me a shout sometime and we can meet up and go for a spin

The Garfish · 24 February 2006, 20:17

Quote:

Originally Posted by benc

I can get on the plane with my boat on single engine, and I have recieved 19.2 knots on one engine

igzackly de saim az mi bote wich meens dat dis..........

Quote:

Originally Posted by Richard B

With most twin engine setups, you will only get on the plane on one engine if you're seriously underpropped, unless the engines are enormous.

........mus bee reegardid az susspekt infawmashun.

gaRf

Kernow Buoy · 24 February 2006, 21:09

Quote:

I have recieved 19.2 knots on one engine

Benc, I would have thought you needed 'something faster to mess about on in summer'

benc · 24 February 2006, 21:11

thats just one engine, get 42 knots with both

im getting another boat tho, dont know what yet... might be a hard boat :| *ducks* or a cruiser *ducks further still*

Rodan · 24 February 2006, 21:38

just buy it howard - that way you will find out ands have fun!

Richard B · 27 February 2006, 12:24

Here you go Howard... nicely matched pair of engines... http://cgi.ebay.co.uk/4-5-MERCURY-BO...QQcmdZViewItem

benc · 03 March 2006, 00:33

Thought i'd update the thread.

I had to bring my boat back to warsash on one engine from Southampton (where Stuart kindly looked after my boat

for a few days while its been poorly - gearbox

)

Bringing it back on one engine was fun slowly making progress on one engine - 10 knots max before the engine start cavitating and over revving - when I realised that the engine I had off was still dragging in the water

... so I brought it up out of the water to the top of the tilt, and picked up speed....

so I played around, got the boat on the plane, and managed to get 25 knots out of it! better than my previous record of 19 on single engine

Also, I noticed is that at 25 knots it used 35 l/hr of petrol, so will have to check what the consumption is when I get the other engine sorted... I have a feeling it will be more..... ill get back with the results