Fuel economy, Twin set up

PeterL · 08 December 2003, 07:27

Does anyone have approx fuel consumption figures and top speeds expected for the following:
7 m Rib

2 x 90 hp 4 stroke v 1 x 200 hp 4 stoke.

I would like the 2 x 90 hp for the safety aspect, but am concerned about the economy, or lack of.

(1 x 200 hp with 8hp aux is cheaper, but what if you have 20 miles to go against the tide ( apart from waiting for the tide to turn) )

Diesel · 08 December 2003, 12:34

The normal way to work it out is.......

Fuel consumption is approx a third of the horse power at wide open throttle so a 90hp would use 30ltrs an hour. (2 stroke that is)
4 strokes use about a third less again so.....

a four stroke would use about 20ltrs an hour at wide open throttle.

So twin 90s will use approximatly 40ltrs an hour and a 200 will use about 44 ltrs an hour. But its not that simple as your running twin 90s you have more drag as you have two gearboxes that brings the consumption down a bit so your probably going to get about the same consumption. Althought the single 200 will be quicker than twin 90s by a fair bit. but you do have reliability with two engines.
These figures are for Mariner/ Mercury engines not sure about other engines.

PeterL · 08 December 2003, 12:49

that's the economy, anyone have a guess at top speed with Twin 90's on a Ribcraft 7m.........

Pete7 · 08 December 2003, 13:15

Quote:

anyone have a guess at top speed with Twin 90's

Hmm, are you offering a prize for the winner then ?

38 mph is my guess for twin 90s. However I recon you would be going a lot faster with the single 200 hp, probably 50 mph.

Pete

Diesel · 08 December 2003, 13:19

I reckon 32knts ish with twin 90s not sure about a single 200.

Louise · 08 December 2003, 17:28

Quote:

Originally posted by Diesel
not sure about a single 200.

OK I can help you out here: 46kts max (53mph).

Fuel burn: 1.0 to 1.1 litres per mile at cruising speeds.

PeterL · 08 December 2003, 17:31

Thanks Louise.
Is this fact or are you after the prize money.
46knts with twin 90's ? Pls confirm.

Thanks

Richard B · 08 December 2003, 17:34

Sorry that was me!

Yes - fact. Boat + engine as per profile. Not twin 90's - they'll be going backwards!

Will Pm address for prize. Thanks.

quinquarimarine · 08 December 2003, 22:22

I would concur with Diesel in approx 32 knots on a 7m Ribcraft bearing in mind that its block co. and prismatic co.( increased due to its greater under water area/displacement) would be relative to the 7.7m H.

Fuel useage will obviously be again relative to weight/loading. On a 7.7m with 1200kg payload we would achieve 25lt on a mixture of 20% WOT, 50% cruise and 30% tickover.

I would advise on a twin set up pushing the underdeck tanks further forward than the manufacturers like. (approx forad of midships by 5%). This has proved very sucessful with us and accounts for overall increase of engine mass and thrust calculations compared to a single ( seen many RIBS down by the back - on a larger RIB power the bow up rather than trim it down - far better in a big sea).

Diesel figures are based on a standard carb 2 st but with a DI they will not be far off a 4st so adain I concur with his calcs.

Peter, I am glad that you are still looking at the twin set up. Whilst many are warry of it due to the increased cost of purchase I am convinced that in your circumstance it will be worthwhile.

Best of luck, at least we know that we got you on this train of thought !

Cheers

John

fatboy500 · 08 December 2003, 23:29

Pete,
think you should definitely go for the twin 90's after reading these posts. It will then save me having to buy a new boat to give you a run for your money next summer !!

Ian · 08 December 2003, 23:38

Would maintenance and service costs be considerably more for twin 90's against one 200?

Jono Garton · 09 December 2003, 08:46

Hi

Twin 115 (normally same block as 90hp) would really be comparable to a 200hp.

My 90 four stroke on average used no more than a litre per mile, my 150 optimax also used no more than 1 litre per mile (+ oil costs). All average and cruising speeds

Things to consider-

-you can use up to double the amount of fuel when in rough seas!

-Cost of counterrotation!

-Transom Weight!

In my opinion get a Susuki 200hp v6 four stroke and 15hp aux, you will be under a litre a mile at cruising speeds. You will also save yourself about 5k.

In my opinion these days, the only reason you would go for twins is if you need more hp than 1 engine can produce.

This is why i'm going for twin 250's

PeterL · 09 December 2003, 13:02

Thanks Bren, you better buy a new boat, then we can cruise in mine and race in yours......anyway, it may be twin 115's

Pete7 · 09 December 2003, 13:47

Quote:

Would maintenance and service costs be considerably more for twin 90's against one 200?

Yep, about double in fact as the biggest cost to servicing is labour, so thats two lots of water pumps to be inspected, two lots of oil and filter changes etc.

Pete

dangers · 09 December 2003, 16:20

Maybe I've just been lucky, but I've never had a mechanical breakdown. My philosophy has been: make sure engines are well serviced, do your pre-start checks everytime and keep your fingers crossed. My new rib is going to have a single Suz 250. I haven't even thought about an auxiliary.

Does anyone have experiences of an auxiliary saving their bacon. Am I stupid going to sea with one engine and only a paddle as back up?

Ian · 09 December 2003, 17:58

depends where your going I suppose. I wont be alone more than 10nm out so a 250hp and a VHF should do the job. Any further and I'll be in company of other ribs.
No aux. on mine, not yet anyway.

Jono Garton · 10 December 2003, 08:33

Ian

Quote:

Originally posted by Ian
I wont be alone more than 10nm out so a 250hp and a VHF should do the job. Any further and I'll be in company of other ribs.

Suppose being towed home is an everage day out on the water for you

PS: Llyn Home is back Home in LLyn, will have engine (115hp) Monday

Ian · 10 December 2003, 12:47

Ha bloody ha