RIB not planing

Spaco77 · 23 December 2013, 06:10

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!!!

spartacus · 23 December 2013, 07:10

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.

xs 400 · 23 December 2013, 08:14

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?

Jorgos · 23 December 2013, 11:00

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?

willk · 23 December 2013, 11:46

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.

Max... · 23 December 2013, 12:13

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 that...you 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.

Exe treme · 23 December 2013, 17:01

Hi

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?

captnjack · 23 December 2013, 17:06

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.

treerat · 23 December 2013, 17:19

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!!!

Spaco77 · 06 January 2014, 10:05

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...

simsy · 06 January 2014, 11:38

The 13 pitch prop is the main issue.... Get a 9 or 10 for 4 people....

I have a 4.1m rib with 30hp 4 stroke. All set up perfectly, but I have to move people around the boat a lot to get it to plane with a 13 pitch prop. I now have 12 pitch prop, and with care it will plane with 4 people on board.... Not all props are equal also....

Borrow a 10 inch pitch prop would be my advice, then buy the best.

Use the 13 when out with a light load, if you miss the top speeds....

Spaco77 · 06 January 2014, 11:53

Thanks Simsy. I'm thinking the 11" pitch plus the 4 blade over 3 blade should definitely do the job. Will keep you posted!

bosun · 08 January 2014, 18:46

And get the weight forward until she's up on the plane !
I did a PB2 course in a 4m SR4 with a 25 mariner fourstroke on it once with two seriously obese people , I'm driving and they are sitting one either side on the tubes, the throttle was wide open and the engine was bogging away and the bow is in the sky, trimmed in all the way 12'' pitch prop.
Me '' err can you both move forward a bit, err a bit further, mmm, further,''
And when they were both ,practically sitting on the bow she popped up on to the plane, at which point they had to move back before the prop came out of the water !!

derdle · 09 January 2014, 11:36

I had a zodiac furura with a 30hp, i fitted a set of doel fins to cavitation plate (abouut £25) would plane with 2 adults towing a ringo with 2 teenagers no problem. without the fins no chance so they are well worth investing in for very little cost.