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Old 15 March 2005, 10:42   #1
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Cruising Advice

I would like some general advice or pointers on the use of my RIB.

Okay so we all know our RIBs will go fast at Wide Open Throttle. This feels good but is sore on the engine and fuel consumption. I realise this after everytrip.

This year I want to reduce the wear and tear on my engine and also make the fuel go a bit farther.

What is the general opinion on a good cruising speed with regards engine throttle.

In a car they say 56 mph is the most efficient speed to cruise at. Is their a similar rule of thumb for a RIB, though with the RPM.

Anybodys advice would be welcome.
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Old 15 March 2005, 11:33   #2
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This would depend on engine size and boat, how many POB etc.
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Old 15 March 2005, 13:23   #3
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The 56mph for cars is NOT a rule - just a figure arrived at by bureaucrats - true some cars are more economical at that speed but others can be very different - diesels for example tend not to like high speed motorway work. Some cars can often be better at 75mph than at 56mph - down to aerodynamics - gearing - engine powerband etc etc etc.
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Old 15 March 2005, 13:36   #4
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I always thought that getting on the plane and throttle back as far as you can go without falling off the plane (ie the boat stays nice and level) would give you the best fuel economy. I could well be wrong though

Tim
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Old 15 March 2005, 14:41   #5
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i have gemini 5.5m wth 140 4stroke suzuki.... best fuel consumption at 3-4 thousad rpm... i never drive in that rpm iam usually around 5-6 because there is the speed and enjoyment for me....
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Old 15 March 2005, 15:20   #6
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There’s not going to be an easy answer we can give you. It will be a case of getting out on the water and finding out. My boat for example gains 7kts of speed in the last 1000rpm up to WOT - which makes 5200rpm most economical for me.

You are looking for the point at which increasing the throttle gives little return in speed. As a very general rule of thumb this will be about 3/4 throttle. Listen to the engine; you can hear when they begin to work really hard.
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Old 15 March 2005, 15:59   #7
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I am lucky in that I have the fuel sensor pack with my GPS and tells me what my ecconomy is.

The boat is a scraggy old searider SR5.45 with a fair hull, 90 horse 2 stroke which is fine in wing and limb with a standard ally prop which is allmost perfect.

I found that it is on the plane at 12kn and will max out at 35kn. For cruising I aim to do between 3000 and 3500 RPM which gives me 18-22kn and 1.3 NM/L. This is also a comfy speed the passengers can tolerate. Sure we can go faster but the fatigue creeps up on you, and boy will you know it tomorrow!

At WOT its doing about 0.7 NM/L. Best ecconomy is at about 1200 RPM which is about 5kn and gets about 4 NM/L. During one trip I was heading into a force 5 and at 10kn was doing 0.9 NM/L.

Sooo the upshot is dont drive at WOT. Get the boat and engine trimmed correctly. Choose a speed that is comfortable. Optimum cruising speed is about half to three quarters max speed.

ATB.
Tim'mers.
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Old 15 March 2005, 18:36   #8
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Swifty

Thanks for your reply.

It seems that overall a bit of common sense is required. Get the boat on the plane and then do a comfortable speed.

Its a shame we have to worry about fuel and its cost. Some country's don't have our problem, the americans for starters, and its also a shame that the engine manufacturers haven't found a standard to work together on.

oh well, guess I will have to save up for a Diesel engine......
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Old 16 March 2005, 03:53   #9
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Quote:
Originally Posted by Biggles
It seems that overall a bit of common sense is required. Get the boat on the plane and then do a comfortable speed...

...oh well, guess I will have to save up for a Diesel engine......
Like you said take it easy, have a blast then return to cruise mode . Diesel is cheap at the moment but when the government start taxing red diesel, which it will, then diesel will be nearer the price of petrol , then who will be better off?

I bet if a few more politicians and peers owned boats then this extra taxation would never appear.

The RYA have been lobying for changes but the chancellor has a few too many holes in his budget. Shame they're not in his boat . Anyone know if he has a boat?

Tim'mers.
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