First run on new RIB

sburt10 · 20 November 2011, 21:27

Hi All

So today was the first run on my friends new RIB, 5.2m with 90 merc.

Straight off the bat it is porpoising all over the place, flat calm and its like being on a bucking bronco !

Also lots of water shooting up between the shaft and transom.

When we got out of the water and put the boat on the trailer I checked where the cav plate was sitting in relation to the keel of the boat and discovered it is set up about 3 inches below the keel

I am guessing that there are 2 issues here.

1. The engine needs to be raised, I am certain this is what is causing the water shooting up the shaft and most likely causing the porpoising.

2. The tank at the moment is in a locker at the stern of the boat, I am thinking that if we move this forward it will also help.

Do you ever have to ballast a RIB, I am thinking a bit of extra weight in the bow might also be a possible solution.

Cheers
Stephen

actions · 20 November 2011, 21:35

I Sometimes put the wife up front...............

Sent from my galaxy s2

willk · 20 November 2011, 21:45

Yes, probably raise the engine. More weight up front.

Is the boat rated for 90hp - is it a four stroke merc?

sburt10 · 20 November 2011, 21:53

Hi Willk, Yes, rated for 90, its the 2 stroke merc Optimax.

Downhilldai · 20 November 2011, 21:53

Sounds like the motor is too long for the transom. What's the height of the transom at the centre and is the motor std, long, or XL shaft?

sburt10 · 20 November 2011, 21:57

Quote:

Originally Posted by Downhilldai

Sounds like the motor is too long for the transom. What's the height of the transom at the centre and is the motor std, long, or XL shaft?

Its the correct size, 20" from top to keel and its a long shaft 20" leg.

I think he has just bolted it on too low on the transom.

Downhilldai · 20 November 2011, 22:09

Quote:

Originally Posted by sburt10

Its the correct size, 20" from top to keel and its a long shaft 20" leg.

I think he has just bolted it on too low on the transom.

He wouldn't be able to create a 3" spacing from the cav plate to the keel in these circumstances.

Chris · 20 November 2011, 22:17

Sounds like an XL shaft. Wrong length for the transom... Ideally the Anti ventilation plate should be level with the bottom or possibly and inch or two above depending upon engine, prop etc.

sburt10 · 20 November 2011, 22:28

The merc shafts are a little over 20"

The transom is approx 20"

The only other options are to go for a short shaft at 15" (approx)

sburt10 · 20 November 2011, 22:30

No, its not an XL shaft guys.

Boats&Outboards · 20 November 2011, 22:30

Quote:

Originally Posted by sburt10

The merc shafts are a little over 20"

The transom is approx 20"

The only other options are to go for a short shaft at 15" (approx)

Won't get a short shaft 90hp

Long shafts are 21,

Peter ~ Boatsandoutboards4sale ~ askboatsandoutboards4sale@sky.com ~ 07930 421007

Chris · 20 November 2011, 23:30

Quote:

Originally Posted by sburt10

No, its not an XL shaft guys.

if its a 20" transom and a 20 or 21" leg then how can it be hanging 3" below?

Jizm · 21 November 2011, 08:45

sburt10 · 21 November 2011, 09:34

Guys, give me a break here, I did not have a measuring tape when we took the boat out of the water, lets say its 2" under to allow us to progress this conversation.

Wahoo! · 21 November 2011, 09:53

sburt10

I guess I may be stating the obvious here but without seeing it is hard for anyone to assess exactly what the problem is. If it was bought as a new package I would get straight back to where it came from! A dealer should know what engines suit which boats and they may have sold you an incompatible partnership! Otherwise I would suggest the easiest solution may be to get it along to an experienced marine engineer at your local boatyard to look at it and he may possibly need to take it out for a sea trial.

two stroke mick · 21 November 2011, 12:24

What make is the rib? and how is it set up wrt seating. How many on board?

clydeoutboards · 22 November 2011, 13:24

Hi

you could take the easy option and bring it down to us or depending where you are in Glasgow I could pop down as I live near the fort shopping centre.

Dave

cgoing · 22 November 2011, 13:53

When I mounted my new engine on my Zodiac I had the same issue. The boat was rated for an engine with a 20" shaft so that is what i ordered. The transom was actually a little short, around 19" and the motor a little long, actually 22" so between the two there was a bit of an issue. This is really not all that uncommon. I jacked my engine up three holes and the water spray went away and performace got better. My cavitation plate is now a hair plus or minus above the bottom of the boat.
Unfortunately there is no law on actual produced lengths at least not in the US on transoms or engines so you will find variances. I've found brand 20" shaft engines with measurements anywhere from 18.5" to just over 22".
Best of luck with it !

9D280 · 23 November 2011, 11:50

Other thing to check - the trim of the engine.

If it's PT, did you motor it all the way down & leave it there? Yopu will very likely need to wind it back up a bit once on the plane. Most Rib transoms aren't perfectly vertical. You may find it's in line if you wind the trim back out a bit. I have my "trim pin" three holes up to stop me accidentally trimming too far in (which causes spray like that).

AKBronco2 · 30 November 2011, 21:30

I would say raise it up a hole or two. Id shoot for somewhere in the 1/2" to 1 1/4" below keel.