Pretty serious steering concern

wyaflk · 29 October 2010, 18:52

Evening all, not sure of this is a common problem, or if it is related to the wrong prop or something, however, we have just bought a second hand rib (5.9m 90 4 stroke) and it has two pretty nasty traits:

1) If you let go of or loosen your grip, then the boat veers very sharpy to the right (either just before you get on the plane or whilst on the plane, but not so badly). If you play with the trim and trip up a bit, then it either doesn't do it or does it less. Any thoughts?
2) The prop cavitates really easilly, far too easilly i think.

Any ideas? Could they be linked?

Thanks

Fletch

Anchorhandler · 29 October 2010, 20:54

Hi Fletch

Is it possible the outboard is not mounted correctly? ie, check to make sure it is firmly mounted, perfectly vertically and in a central position on the transom.

ALthough i have never experienced it, i would imagine an 'off center' outboard would put a certain amout of torque on the hull making it want to veer off.

Also, does the prop cavitate more when the outboard is trimmed up slightly?

Simon

davej · 29 October 2010, 21:48

Have you checked that the small zinc trim tab under the cavitation plate is set at an angle to offset the turn?

Whenever this is serviced it needs to be replaced in the same position.

just a thought.

davej

Cookee · 30 October 2010, 10:15

I would say that 1) is normal for cable steering, to get rid of that you will need no feedback or hydraulic steering - as mentioned by others it can be reduced but you will always get feedback at certain trim angles.

Cavitation will be a factor of engine height, prop and throttle setting, see if you can borrow a different prop off someone with a similar setup to yours.

gotchiguy · 30 October 2010, 10:20

Quote:

Originally Posted by Anchorhandler

ALthough i have never experienced it, i would imagine an 'off center' outboard would put a certain amout of torque on the hull making it want to veer off.

Jigsaw has an "off centRE" outboard

BogMonster · 30 October 2010, 10:40

Quote:

Originally Posted by Anchorhandler

Is it possible the outboard is not mounted correctly? ie, check to make sure it is firmly mounted, perfectly vertically and in a central position on the transom.

ALthough i have never experienced it, i would imagine an 'off center' outboard would put a certain amout of torque on the hull making it want to veer off.

Unless I have misunderstood I thought nearly all outboards of any size were mounted slightly off centre to counter the torque reaction? The E-Tec on my Vmax certainly is. Also gives an inch or two more space for the aux engine as an added bonus

Anchorhandler · 30 October 2010, 11:12

Quote:

Originally Posted by gotchiguy

Jigsaw has an "off centRE" outboard

Ok, where is the emoticon for 'embarassed' when you need it?

Perhaps his outboard is offset to the wrong side then?

Simon

wyaflk · 31 October 2010, 08:07

Thanks all. I have not played with the trim tab, but i would expect that it would do exactly that, i.e. trim rather than prevent pretty sharp turns. Will have a go anyway.

Thanks for the input

Fletch

Carl · 31 October 2010, 19:00

engine to high i would think

njensen · 31 October 2010, 20:55

are you talking ventilation or cavitation ? it's two different things and requires different actions.

Ventilation is air being sucked down due to bad trim or mounting of the engine.
Cavitatation has much more to do with the propeller design (vapors around the blades due to high pressure).

Cavitation can potentially damage your prop over time. Ventilation courses bad turning capabilities, acceleration and so on...

GJ0KYZ · 31 October 2010, 21:43

Quote:

Originally Posted by davej

Have you checked that the small zinc trim tab under the cavitation plate is set at an angle to offset the turn?

Whenever this is serviced it needs to be replaced in the same position.

just a thought.

davej

This is definitely worth checking. If it's offset just centre/center/centaur it up and see if that makes any difference.

9D280 · 03 November 2010, 15:45

For Problem 1

Quote:

Originally Posted by GJ0KYZ

This is definitely worth checking. If it's offset just centre/center/centaur it up and see if that makes any difference.

"just" centring it will be a bit of a difficult job if it is bolted to the transom! I know of a Ribcraft 5.3 with a 90 sat dead centre on the transom and no pull. My MErc & Yam were both dead centre, again no pull. My money is on your trim tab.

If you are trimmed too far out, the trim tab / skeg etc won't be doing what they should, and you end up with more of the opposing torque at the wheel (Newton's 3rd law - the prop shaft goes one way, so the rest of the engine wants to go the other way with equally vigour, and if there's no water stopping it at the bottom of the leg, it's all down to you on the wheel).

Problem 2)
When does it do this? If at speed in a straight line, and it happens at the slightest sign of a wave or a turn you are either trimmed too far out or may have a too small diameter prop.

There are small diameter big pitch props out there for lightweight boats, the idea being that you loose a bit of friction by reducing diameter, and load the engine back up with the relatively high "gearing" of the pitch & get more speed. Thing is if your engine was secondhand off a light boat (As my old Yam was) the prop will let go easily on a relatively heavy draggy rib. You may need to up the diameter & down the pitch if that's possible.

What RPM do yopu get at wide open throttle, and what is your current prop?

wavelength · 08 November 2010, 21:23

Quote:

If you play with the trim and trip up a bit, then it either doesn't do it or does it less. Any thoughts?

therein lies your answer I would think. Trimmed in (down) a lot of boats will be heavy to steer and pull to one side so try it trimmed up a bit from your original problem area. You may need a stop in the frame to limit the trim's downard travel.
Trim up once on the plane on flat water. Trim down a tad for turns, after taking some speed off, so that the prop doesnt ventilate.

Be careful trimming down at speed as it can induce an unexpected and violent reaction esp if you have a heavy person on one side of the boat. Failure to trim down for turns can allow the back end to lose grip and then the boat may spin down onto its bow and cause major problems onboard.

willk · 08 November 2010, 21:54

Quote:

Originally Posted by gotchiguy

Jigsaw has an "off centRE" outboard

Are you the new spell-chequer?

Cheap dig, IMV

Screaming04 · 09 November 2010, 02:21

Quote:

Originally Posted by willk

Are you the new spell-chequer?

Cheap dig, IMV

Funny, I didn't notice a spelling error... that's the way we spell it on this side of the pond. Don't worry G, in a couple of centuries you'll drop the french spelling as well.

It only makes sense to have the lettre appear where it's enunciated n'est pas? Novembre, Decembre etc.

As to the OP. Trim anode adjustment is all that's needed.

Pikey Dave · 09 November 2010, 09:50

Quote:

Originally Posted by Screaming04

Funny, I didn't notice a spelling error... that's the way we spell it on this side of the pond. Don't worry G, in a couple of centuries you'll drop the french spelling as well.

It only makes sense to have the lettre appear where it's enunciated n'est pas? Novembre, Decembre etc.

Aww leave him alone, it's just his sense of humor

wavelength · 09 November 2010, 16:17

never found the trim anode to make any worthwhile difference in all honesty.

chewy · 09 November 2010, 17:00

Have you tried moving the engine physically by hand, if you have NFB steering the wheel and engine won't move.
My mates RIB has "normal" steering and if you let go of the wheel and apply the power at standstill the wheel whips over due to the prop torque, as John said.
Not sure about cavitation, could be a whole load of things from driver error to the wrong prop?

gotchiguy · 10 November 2010, 15:04

Quote:

Originally Posted by Screaming04

in a couple of centuries you'll drop the french spelling as well.

It only makes sense to have the lettre appear where it's enunciated n'est pas? Novembre, Decembre etc.

Funny thing is, I'm quarter French.

And sorry if I caused offence, I just don't appreciate Americanisation of words.

jyasaki · 10 November 2010, 16:39

Quote:

Originally Posted by wavelength

never found the trim anode to make any worthwhile difference in all honesty.

It does, but it's one of those things that works best at a given constant speed. Should be tweaked to minimize prop torque at your normal cruising speed.

Obviously, the tab will have little to no effect until you get a decent amount of water flowing around it, so it's not going to be too effective at low speeds.

jky