RIB Pitching to left... tim tabs ?

1978SeaRider · 05 May 2009, 13:44

Hi,

As some of you may remember I purchased a 5.8M Osprey Viper Max last month.

Can't say enough good things about it...however... I have one minor niggle (which i was warned about before I brought it)... basically when I'm going fast in large waves, the boat always wants to pitch to the left... not a major problem but it can become uncomfortable for the passenger after a time.

I think the engine was probably slightly miss fitted and maybe it wasnt 'off set' sufficiently.... I dont really fancy have the transome messed with, so what can I do ?

Its worth noting that the outboard doesnt have a little trim anode (not sure if thats exactly what they are called) I've been told this model (2005 Mercury Saltwater Optimax 150hp) Didn't come with them.

Could this be the problem ?

Also, would these automatic trim tabs eliminate the problem ? http://www.imsupplies.co.uk/product.php?productid=1426

Thanks for any help !

ollyit · 05 May 2009, 13:49

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1978SeaRider · 05 May 2009, 13:56

Cheers Ollyit,

Did you have a similar problem to me ? Will it stop it jumping and landing to one side ?

ollyit · 05 May 2009, 14:25

yes I was over propped and had dolephins attached to the leg, dropped down to a 15" pitch, removied fins and added tabs - changed it massively, much better

PeterM · 05 May 2009, 15:31

Dont think think the trim anode on the opti is a problem - agree with waht you have been told - they never had them .

Chris · 05 May 2009, 18:12

I would try everything you can before fitting tabs. I dont think using tabs to correct a lean is ideal.

Why don't you measure your offset to see if there is any? If not then you may want to look into getting the enging moved across. Its not as scary as it may seem as you could move the bolts down a couple of holes so they are not too close the originals and if you fill the original holes using some dowl and epoxy resin it should not weaken the transom.

Ideally you are looking for 1.5" but with that size engine on a 5.8 you may want to go to 2"

Downhilldai · 05 May 2009, 18:24

Quote:

Originally Posted by 1978SeaRider

..I think the engine was probably slightly miss fitted ..

Didn't Manning rig that boat?

Downhilldai · 05 May 2009, 18:29

What offset and cav plate height has it been rigged at and how is the engine trimmed when you're running in big waves?

Mollers · 05 May 2009, 19:01

If it runs true in flat water it's not a trim issue. If it's bunging you to the left when the hull leaves the water it's a torque reaction off of the prop. I've experienced this big time with Phantom powerboats, but not with either of my ribs.

Maybe she's a bit over propped.

The Phantoms were running big 27"P cleavers which grabbed great dollops of water with every blade. What prop are you running?

I agree, it is a bit tiresome when the boat lands cockeyed everytime it leaves the water.

Smiklos · 05 May 2009, 21:01

Quote:

Originally Posted by Mollers

If it runs true in flat water it's not a trim issue. If it's bunging you to the left when the hull leaves the water it's a torque reaction off of the prop. I've experienced this big time with Phantom powerboats, but not with either of my ribs.

Maybe she's a bit over propped.

The Phantoms were running big 27"P cleavers which grabbed great dollops of water with every blade. What prop are you running?

I agree, it is a bit tiresome when the boat lands cockeyed everytime it leaves the water.

Exactly a RH prop will lean left as all the work is done on the down stroke. As you trim out the effect should be less. On my SACS 680 I just purchased a placed all the batteries etc on the right side of the boat to dampen this. This is common on single engine Offshore Race Boats.
Steve

1978SeaRider · 05 May 2009, 22:23

Thanks for all the replies...

It only pitch's on take off... when its flat or chop its fine, but once I start getting some air it happens... its sometimes quite violant. It launches left and lands left. I can correct it by flick to the right on the crest, but thats a bit tiresome. I've tried all trim levels.

I'm not sure what prop is on it (Mercury 3 blade stainless came on it, not factory).. but I will check.

Not sure of the offset either, I'll check that too. Is there any specific spot to measure from, or just off a chime ?

Who's Manning ?

By the sounds of it I'll have to offset it more... I wouldn't want to mess with the transom myself, how much do you think it might cost to get it done ? Does it effect the transom's over all performance much ?

Thanks again

1978SeaRider · 05 May 2009, 22:38

Also... is there an off set calculation or is it rule of thumb ?

Downhilldai · 05 May 2009, 22:39

I would be very doubtful of the offset being incorrect. While I don't like to admit it, old man Manning knows what he's up to and I don't expect he'd have fitted the motor (if it was he who fitted it) in the wrong place.

I have exactly the same boat & motor, which I set-up with 30mm offset and the cav plate 40mm above the planing pad. The boat runs level with none of the characteristics you're experiencing. It does however have the distinct benefit of an oversize helm, which would counteract by far, any of these finer balance issues.

Have you checked for wear in the steering shaft bushes and/or the tilt tube?

Downhilldai · 05 May 2009, 22:51

Quote:

Originally Posted by 1978SeaRider

Also... is there an off set calculation or is it rule of thumb ?

Some say 1" of offset for every 100hp on a boat of this size, but it will vary from boat to boat.
You can establish the centreline of the transom by measuring between opposite pairs of sprayrails.

You may be interested in this short thread:
http://rib.net/forum/showthread.php?t=9339

Chris · 05 May 2009, 22:51

Quote:

Originally Posted by 1978SeaRider

Also... is there an off set calculation or is it rule of thumb ?

Before you go down this route there was a valid comment a few posts back about the height of your engine specifically the anti vent plate (the plate over the prop) above the bottom of the transom. If it is below the bottom of the transom or level with it you may want to try lifting it until it is two inches above.

As for moving the engine over I would suggest you would need a couple of inches offset. If you want the job done right then call Osprey and ask ROy to put you in touch with Mike at Wotton Glass Fibre who builds all the Osprey hulls. He would have probably built the hull in the first place and would do a top job of filling the old holes. He would also be able to advise on offset.

http://www.ospreyribs.co.uk/contact_us.html

1978SeaRider · 05 May 2009, 23:27

Excellent advice and info, thank you very much.

I will start by measuring the offset, engine height & prop size. I'll also check the steering shaft bushes and/or the tilt tube this weekend and go from there.

And as for the benfit of an over sized helm... I'm working on that !

BogMonster · 06 May 2009, 00:10

Quote:

Originally Posted by Mollers

If it runs true in flat water it's not a trim issue. If it's bunging you to the left when the hull leaves the water it's a torque reaction off of the prop. I've experienced this big time with Phantom powerboats, but not with either of my ribs.

My Humber does that, I figured it was probably torque reaction but it scared the sh*& out of me the first time it happened, only happens when you jump a wave at over about 20 knots and get the hull more or less right out of the water - the rest of the time it is fine and a quick flick on the wheel keeps it flying straight but it does get a bit tedious. I figured that probably everything did it, not having much else to compare it to round here!

Mollers · 06 May 2009, 06:57

I'm fairly sure that offset and cav plate height won't be the issue as the boat runs true in flat water. It very much sounds like prop torque, as for a cure?

We didn't manage to fix the problem on the Phantom, but we didn't really try.

I'd start with the prop.

Cookee · 08 May 2009, 12:47

Another factor is the amount of negative trim - the more you trim in, the more it will land on the left.

Bigmuz7 · 08 May 2009, 20:52

Its a lot of power for that length of hull .. throw some ballast in the bow, like 3 x 25 litre drums of water (or spare fuel ) .. or the odd teenager if you can find em

.. that should cure most of it .. not ideal but it works

then trim like the bejesus, and youll land good