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Old 13 September 2008, 18:50   #1
JSP
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5m RIB best engine size for MPG

After reading through the MPG 150 HP Evinrude & 6m Rib thread. It got me to thinking, what would you say the best sized engine is for a 5m RIB for economy?
I'm looking into buying my first RIB, I went out today on a 4m flatacraft with a 40hp hanging off the back. I found 4m's a little bit small for what I want. I have 7 kids after all (only 3 of them are of an age to go out with em though) and I would like to look into doing photography from the boat as well.
So I've made my mind up on a 5m. It will have to be an old one (some good looking ones on ebay at the moment) and same for the engine. I'm experienced with diesel engines in boats, from very small to twin 6 ltr's Mermaids. But the only outboard I've ever used is a mercury 2.5 2 stroke.

So what engine size would you recommend? I like to cruise but would also like to go full out now and again. Also where I'll be using it has very strong currents that I will have to fight against. What is the min engine to battle with the elements?
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Old 13 September 2008, 20:29   #2
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Depends how you look at economy. Realistically, you need one of the 'new' generation of engines (etc,optimax etc) or a 4 stroke for economy in comparison with the old style carbed 2 strokes.

There's no definitive answer as to the economy of an engine but you won't improve much in fuel consumption for any given speed by having more HP -but you will be able to go faster if you want to and you'll be stressing the engine less. Personally I want at least 90% of the max rated HP for the boat. There's advantages with having good holeshot as well which you won't get from a smaller engine.

Incidentally, the Flatacraft F4 is a tiny boat in comparison with other 4m ribs.
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Old 14 September 2008, 03:50   #3
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JSP,

My fisr Rib was a 5.25 meter which I rigged with a 115HP E-tec.

It would have also gone pretty well with a 90 E-tec.

With the 115 I used to get .7 - .8 litres per mile at crusing speeds. This increased to between 1.1 - 1.2 litres per mile at WOT.

It was a great combo and a really managable sized boat for launching etc. If you buy a decent hull like an Osprey Viper, Ribcraft or similar you won't be dissapointed.
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Old 14 September 2008, 08:05   #4
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Quote:
Originally Posted by JSP View Post
After reading through the MPG 150 HP Evinrude & 6m Rib thread. It got me to thinking, what would you say the best sized engine is for a 5m RIB for economy?
I'm looking into buying my first RIB, I went out today on a 4m flatacraft with a 40hp hanging off the back. I found 4m's a little bit small for what I want. I have 7 kids after all (only 3 of them are of an age to go out with em though) and I would like to look into doing photography from the boat as well.
So I've made my mind up on a 5m. It will have to be an old one (some good looking ones on ebay at the moment) and same for the engine. I'm experienced with diesel engines in boats, from very small to twin 6 ltr's Mermaids. But the only outboard I've ever used is a mercury 2.5 2 stroke.

So what engine size would you recommend? I like to cruise but would also like to go full out now and again. Also where I'll be using it has very strong currents that I will have to fight against. What is the min engine to battle with the elements?
What is your budget? If you are looking at older boats, I guess you are looking at older engines, The old 2 strokes are good simple engines but use about twice as much fuel as the new opti's, etec's etc and 4 stroke. As for engine size, It will depend on your hull. I agree with Nos4r2, Go for something close to it's rated max power but the max power for a 5 meter will change a lot between different hulls. If you are looking at older boats, Stick with a searider.
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Old 14 September 2008, 17:48   #5
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Thanks for your time everyone.

My budget is small, to flaming small.
I've had a look at S/H 4 strokes and I could get one, but sadly I wouldn't be able to afford anything solid for it to attach too! So I'm stuck with 2 stroke.
I'm selling some of my photography equipment to fund the boat so I'm limited. Did think about getting finance and getting a newer boat with modern 4 stroke but my backside went abit due to the way the economy has gone since Mr Brown and his clown came in.

Nos4r2, when you say the flatacraft 4 is smaller than other 4m boats, is this just due to hull shape? I have to admit a 4m boat would be easier for me to handle/launch.
I've posted on here the other day and was pointed in the direction of a searider. And from what I can see most 4M seariders come with 25-50hp 2 strokes.
So to ask the question which is rather like askeing "what mood will she be in tonight?" What would be the best 2 stroke engine size, for economy and fun.

Thanks for your time.

P.s. Anyone interested in swapping a good RIB for a Canon 5D, Canon L glass and some studio lights? lol
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Old 14 September 2008, 21:16   #6
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Quote:
Originally Posted by JSP View Post

Nos4r2, when you say the flatacraft 4 is smaller than other 4m boats, is this just due to hull shape? I have to admit a 4m boat would be easier for me to handle/launch.
I've posted on here the other day and was pointed in the direction of a searider. And from what I can see most 4M seariders come with 25-50hp 2 strokes.
So to ask the question which is rather like askeing "what mood will she be in tonight?" What would be the best 2 stroke engine size, for economy and fun.
Well...the SR4 is still a small boat-but when I was tied alongside a Flatacraft f4 in Weymouth the f4 looked tiny. Probably the best way to gauge the siz of an Sr4 is from these-from the Avon website:-

and this:-
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Old 15 September 2008, 04:03   #7
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I would have a look at something like this, from experience the Johnson / Evinrude 70s use about 4 gallons a hour which in the grand scheme of things is pretty good for a rib.

Could it be launched across Ainsdale beach? yes with care and long ropes, but have a chat with the fishing club first to see how they do it.

Would it go to the IOM in good weather, yep

Pete

http://rib.net/forum/showthread.php?t=26218

Pete
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Old 15 September 2008, 05:00   #8
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I would have a look at something like this, from experience the Johnson / Evinrude 70s use about 4 gallons a hour which in the grand scheme of things is pretty good for a rib.

Could it be launched across Ainsdale beach? yes with care and long ropes, but have a chat with the fishing club first to see how they do it.

Would it go to the IOM in good weather, yep

Pete

http://rib.net/forum/showthread.php?t=26218

Pete
Seconded.
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Old 15 September 2008, 06:28   #9
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My 5m Humber Assault runs a 50hp/4str and is frugal beyond my expectations(c 3/gph) , yet strong enough to plane quickly and cruise at 32kts half loaded (3up).


Seen a nice one for sale....
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Old 15 September 2008, 07:03   #10
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Engine size on 5m rib.

I have a 60 hp ETEC on a 4.9m valiant. It would want to be calm and you would want to be brave to run with it flat out. A lot depends on the weight of the boat which you purchase. The boat is rated for 80 hp. I would not like to try 80 hp on the back of my boat. I would usually use 18/20 lt of petrol for 3/4 hours crusing about

TSM
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