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Old 23 December 2013, 00:10   #1
Country: Australia
Town: Sydney
Make: Quicksilver
Length: 3m +
Engine: Mercury 25HP
Join Date: Nov 2013
Posts: 13
RIB not planing

Hi all,
Quite new to all this RIB business and went out today in our newish 3.5m rib with 25hp mercury, 4 adults and it wouldn't get up on the plane. It would accelerate well, then at a guess, half way to plane it would just stay/bog down and engine would continue to max out as we would plough through the water?

Tried adjusting trim angle, no real difference. Could it be that we are underpowered / overloaded?

Would changing the prop make much/any difference, we have a 9.9/13 on there, standard issue I guess...

Boat flies with my wife and I, never even open up throttle to 100%, wouldn't have thought 2 extra adults would make such a drastic difference.

Bit dumbfounded and disappointed, keen for any help!!!

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Old 23 December 2013, 01:10   #2
spartacus's Avatar
Country: UK - Scotland
Town: Aberdeenshire
Boat name: Sula
Make: Ribcraft 4.8m
Length: 4m +
Engine: Tohatsu 60hp + aux
MMSI: 235087213
Join Date: Jun 2007
Posts: 3,058
It should plane with four on board. I used to run a Zodiac MKII CGT sib with a Yamaha 25hp 2-stroke. Use to fly with two on board. With four on board it would struggle initially. I used to ask everyone to move forward, then once it was on the plane they could move back a bit and it would plane okay.

Check that you're not getting prop slip (worn rubber bushing on the propeller) and that the engine is reaching WOT (wide open throttle). I'd also play about with the trim angle.

If this boat is new to you - also check the anti cavitation plate (above the prop) is in line with the base of the hull on the transom. I'd assume on a 3.5m rib it's a standard or short shaft outboard (15"). Measure from outboard saddle to top of anti-cavitation plate.

Is that with or without VAT?
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Old 23 December 2013, 02:14   #3
xs 400's Avatar
Country: UK - England
Town: yeovil
Boat name: Lilly of lyme
Make: XS
Length: 4m +
Engine: 30hp Yamaha
Join Date: Jul 2011
Posts: 631
has it a solid hull or inflatable?

any pictures?

a 13 is quite a big pitch I went down to a 12?

you could try dole fins?
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Old 23 December 2013, 05:00   #4
Jorgos's Avatar
Country: Greece
Town: Pireus
Boat name: Joanna
Make: marin stiletto 737
Length: 7m +
Engine: Evinrude ETEC 250
Join Date: Jan 2009
Posts: 157
I think you are overloaded and the boat is small for this load. if the boat was bigger it could plan with the same load.
what kind of four people ? 70 kilos each? or 100 kilos each?
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Old 23 December 2013, 05:46   #5
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willk's Avatar
Country: Ireland
Make: Redbay Boats
Length: 9m +
Engine: 370hp
Join Date: Feb 2008
Posts: 12,143
If the SIB is an "airdeck" then you may be running under inflated. If it is an airdeck and you haven't inflated to required pressures per a gauge and retested in the water, this is the first thing to try.

Moving the load forward will help in most cases.

Doel fins on the leg might help, but I haven't had much luck with mine for this type of situation.

The prop MIGHT be slipping, but I doubt it. A lower pitch prop might help deliver more power which might get you over the bump.
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Old 23 December 2013, 06:13   #6
Country: UK - England
Town: Sussex
Boat name: Bombard, Y-162
Make: Aerotec 380, Y-Class
Length: 3m +
Engine: Mercury Mariner 15hp
Join Date: Jul 2010
Posts: 2,500
Sounds like too much weight, too little power - often it's smaller the boat more critical the power to weight ratio and the 'straw that broke the camel's back' and all might fiddle around and get it planing but for general use four up you're going to need a bigger engine...

Doubling the load by adding two more adults makes a huge difference on a small boat - even a child will make a big difference.
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Old 23 December 2013, 11:01   #7
Exe treme's Avatar
Country: UK - England
Town: Exeter
Make: Highfield
Length: 5m +
Engine: Honda BF90
Join Date: Mar 2013
Posts: 272

Sounds like a weight/weight distribution issue as you say things are fine with 2 up, so it doesn't sound like a set up issue.

When you say it ploughed, was the bow up or down?
Happy when wet!
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Old 23 December 2013, 11:06   #8
Country: USA
Town: Seattle
Boat name: Water Dog
Make: Polaris
Length: 4m +
Engine: Yamaha 60hp
Join Date: Apr 2007
Posts: 1,152
Hard to know without seeing the boat & passengers in the water.

Could be: weight distribution, over propped, trim settings, engine height.
My money's on weight distibution but possibly the prop needs to go down a couple inches too.

I have planned 3 divers plus dive gear in a 12.5ft SIB with a Yam 25 4 stroke so I would expect with proper configuration you could plan 4 adults in a RIB.
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Old 23 December 2013, 11:19   #9
Country: UK - England
Town: N. Devon
Boat name: Nutkin Too
Make: X-Pro Defender
Length: 5m +
Engine: Outboard, Suzuki 90
MMSI: 235095884
Join Date: Dec 2008
Posts: 1,263
Sounds like weight to me, too much at the back, you'll need to move people around in a boat that size.

As the nose rides up on initial acceleration move weight forward to get the nose down and the boat onto the planning pad, but remember to bring them back!!!

Bude Dive Club -
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Old 06 January 2014, 04:05   #10
Country: Australia
Town: Sydney
Make: Quicksilver
Length: 3m +
Engine: Mercury 25HP
Join Date: Nov 2013
Posts: 13
Thanks all for your replies, sorry about the lack of response, have been away...

To try and answer all, it wasn't a weight distribution, nose wasn't overly skyward etc, on reflecting I really think we were just too heavy to get up and on the plane. Outboard shaft length/set up is good, fibreglass floor and all tubed inflated correctly. Also a SE 200 hydrofoil on the outboard and believe it would have reached WOT if not overloaded.

I believe that with a lower pitch prop will get up on the plane, at sacrifice of a little top end (which I am fine with).

I have investigated the outboard and Mercury recommendations are from 8-14" pitch.

I have ordered a 4 blade 10.2" x 11 pitch, to replace the 3 blade 9.9" x 13 pitch. I expect this should give me loads more punch/torque to get up on the plane, bus generally hold speed better etc. used to change out 3 blade props for 4 blade on wake boarding boats so worked on same principals.

Prop should be here in about a week, will keep you all posted and will send picks when installed etc.

Pick of current set up...
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