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Old 01 November 2014, 10:59   #1
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Engine size

Is there a easy calculation for working out what engine size you need?

On a 5.8M RIB what is the minimum size engine needed to allow teenagers to wakeboard, on a flattish sea, with perhaps 3 in the boat at the same time?

Is it simply - the bigger the better, or is there some science to it?
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Old 01 November 2014, 11:15   #2
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Are you trying to work out if you have enough Hp at the moment or are you planning to re engine anyway?
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Old 01 November 2014, 13:34   #3
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The biggest the boat will take
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Old 01 November 2014, 16:46   #4
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Agree with 69cmw.

It's easy to not use everything you have; less so to use what isn't there.

jky

I should add there may be reasons for lowering the hp; money, storage, whatever. In general though go bigger if you can.
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Old 02 November 2014, 15:03   #5
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Definitely the bigger the better. I had a Honda 130 on my 6m zodiac. excellent power but was a bit scary when going WOT for more than a few seconds. Recently repowered with a new Honda 90. Im not disappointed but its a lot more strain on the smaller motor to stay at speed compared to the 130 that would cruise effortlessly at a lower rpm. I plan on getting an SR5.4 and putting the 90 on it and if I keep this 6m zodiac, Im trying the Suzuki 140.

Lesson of the day: max out the HP (keeping max weight in mind).
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Old 02 November 2014, 20:11   #6
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This is very subjective and in the main a lot of people will be happy with a 90hp on a 6m ish rib. You'll be able to water ski, tow inflatables and it should be pretty good with juice for family cruising. The comparatively "off the shelf" Ribeye's are quite under powered engine and often are 6.5m with 115hp etc and they are perfect to their customers needs.

We had a 585 Ribcraft with a 90hp 2 stroke and it went really well but for us we wanted something with a bit more that was going to last us forever and not be flogged at WOT all the time. When it came to re-powering we went for the 150hp as it's a 3ltr block and we could put a smaller prop on it to slow down users whilst ensuring longevity.

It's also a lot easier to drive in waves with a big lump on the back as it is more responsive so teaching driving in waves in a high powered rib is easier.

I have certainly found that whilst maintaining a common speed, a big lump being used sparingly is more frugal than a mid sized one which is being worked hard.

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Old 03 November 2014, 03:31   #7
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Jeremy, you will find a lot of people on here who believe Max power Max speed is best.

Its gonna depend a lot on what you want to do. Carry 6 overweight divers and gear... You want more power. Tow skis or Ringo's etc you want more power. Go out in extreme conditions you want more power. Cruising at cruising speed... ...you may not need more power.

Not sure I agree that its 'easy' to use less power. And hard to use what isn't there.

So clearly if you could power up to 100hp and fit a 25hp you will have issues. But if you put 75 on it then I'll bet it will move OK. And unless you are pushing the boundaries I bet you'd never have an accident through lack of power. On the other hand stick 100hp on a boat rated to 100hp and there will be times you should only be using 20 of those horses but you (or someone else using it) may stray into higher power than you should.

My first car had a 950cc engine and maxed out at 85mph and took 3 weeks to get there. My current car gas 2000cc and a turbo. It will do over 100. My first car I never went over 70 without realising it.
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Old 03 November 2014, 03:34   #8
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Originally Posted by ShinyShoe View Post
Its gonna depend a lot on what you want to do. Carry 6 overweight divers and gear... You want more power. Tow skis or Ringo's etc you want more power. Go out in extreme conditions you want more power. Cruising at cruising speed... ...you may not need more power.
read the OP - he stated what he wanted it for...
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Old 03 November 2014, 04:30   #9
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Originally Posted by Jeremy Fry View Post
Is there a easy calculation for working out what engine size you need?

On a 5.8M RIB what is the minimum size engine needed to allow teenagers to wakeboard, on a flattish sea, with perhaps 3 in the boat at the same time?

Is it simply - the bigger the better, or is there some science to it?
What boat is it Jeremy?

What is its max rated HP?

Some manufacturers will rate their boats for more power than others - whether it's appropriate or not. So max rated HP may not tell the complete story. For example, many Cobras seem to be powered with larger engines than other similar size RIBs.

My 6.5m Scorpion was originally supplied with a 90hp 2-stroke and apparently would do mid 30 knots lightly loaded. When I bought it it already had the 150hp. The big engine is great when heavily loaded 6-8 people and I think a smaller lump would have struggled a bit.

The hull form and construction will also have a bearing on what's appropriate. I had some friends with two similar size RIBs. One was an Avon D560, the other a Ribtec 585. Both had the same 100hp Yamaha 4 stroke.
The Avon was lightly built and had a flat hull. In all but flat water 100hp was "more than enough", possibly too much.
The heavily built, deeper V Ribtec would have benefited from more power.

For your use you'll want to have "enough" power. I don't wakeboard so don't know how much is "enough".

Perhaps someone who wakeboards from their RIB can chip in?
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Old 03 November 2014, 04:33   #10
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Quote:
Originally Posted by Jeremy Fry View Post
Is there a easy calculation for working out what engine size you need?

On a 5.8M RIB what is the minimum size engine needed to allow teenagers to wakeboard, on a flattish sea, with perhaps 3 in the boat at the same time?

Is it simply - the bigger the better, or is there some science to it?
If possible Id get an engine at the Top of the power range.IMO it is Usually a better bet,and wins out over the low power options.
....it'll do all the smaller engine can,pluss a few extras!
It won't Rev the guts out when want/or indeed NEED to get somewhere quickly (and consequently won't be much heavier on Fuel)..Will make the Boat a lot more versatile if your requirements change,which they have a habit of doing!....like more passengers/heavier loads ect.
You will be able to keep up when your Cruiseing in a group,and it will probably be easier to re-sale,and hold its money better.
For me...It's ALOT nicer to drive a Boat with the Throttle in control.....rather than the amount (or lack) of Horses!
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