What could be wrong here then?

Andy JC · 26 June 2006, 23:01

Ok, some advice much needed on this one please...

I've just bought a 3.4 meter Zodiac fastroller(acti-v) with a 15hp Johnson both of which are just over a year old. I took it out on Saturday for the first time only to find that I seem to be getting a problem with either Cavitation or Ventilation. While accelerating it's not long before the prop totally looses grip and gives a very similar effect to a car when wheel-spinning. It's quite severe to the point that I have to throttle right back before it gains grip again. My first thought was to play around with the trim position and found that when trimmed fully in it is slightly better but it still happens. I have checked that the shallow water drive is not engaged and the prop is in good condition, just a slight bit of paint wear. Moving people around in the boat also seems to help a little but I am at a loss as to what to try next.
I have read somewhere that ventilation can be caused by exhaust gases flowing incorrectly and wondered whether this could be the problem but thought it unlikely on a reletively new engine.
Has anybody got a similar boat/engine setup and experienced the same kind of problems?

All comments apreciated...

Cheers
Andy

Downhilldai · 26 June 2006, 23:07

When it gives the impression of cavitating/ventilating, is there any drive forward at all?
What's the bush in the propeller like? Very unlikely to be your problem, unless an older defective prop has been fitted.

Andy JC · 26 June 2006, 23:23

Hi Downhilldai

No, there's no drive at all when it starts happening, the boat almost comes to a stand still before it starts gripping again (quite frustrating as I get the impression that otherwise it would really shift!)
Not sure what the bush in the prop is like, can this easily be checked by removing the prop? Are you refering to the spline? (presuming this model of outboard has a spline and not a key setup?!?) Must admit the prop looks almost new so would guess that the bush should be alright but certainly worth checking out I suppose

neilhope · 26 June 2006, 23:42

I had the same problem with a Zodiac Grand Raid and a 40hp Johnson.
I had the prop bush replaced which made things slightly better but still didn't really solve the problem so I had the engine checked over - no problem there though.
It was a weird on/off situation where one day it would be ok and the next slip terribly.
I eventually realised that a repair to the floor had altered the shape of the keel and even the slightest reduction in the amount of air in the keel would make the problem worse.
Heading into a strong wind would help lift the keel and the boat would run really well.
The pitch of your prop can also have an effect on cavitation so it may be worth checking suitability to the boat.

Downhilldai · 27 June 2006, 00:01

Were the outboard and boat packaged together from new?
Have you spoken to the previous owner to see what they have to say about it, or did you buy from a dealer?

yeehaa · 27 June 2006, 00:22

make sure the keel of the dinghy is there and that it is inflated fully

codprawn · 27 June 2006, 01:56

Well if it is trimmed correctly and you have people sitting at the back then it must be the wrong prop or the rubber hub is slipping(if it has one).

I have a Quicksilver 310 with a suzuki 15hp and it does almost 25mph - it's great fun!!! We have had the cavitation you mention but only because the tilt lock things(or reverse lock) weren't released and the engine wasn't down properly.

Northern Tim · 27 June 2006, 08:31

You could try getting in touch with MGA Watersports in Rochdale, of all unlikely places!

http://www.ribz.co.uk 01706 379669

There's not much on the website but give them a ring. Shaun seems to know his stuff and is a Zodiac dealer - he works their stand at the boat shows.

Good luck,
Tim.

floater · 27 June 2006, 08:54

I had same problem with my fast roller (2003) 340 with a yam 15 on,it usualy happens when there is not enough weight in the rear, but some times this is unavoidible in such a small craft, i fitted doel fins to the engine which greatly improved the handling and general overal stability and stopped cavitation , but they did slow it down a bit

before the yam 15 i used a 15 mariner long shaft which was brilliant but kept grounding the skeg and had dificulty putting in the boot of the car

the long shaft performed better than the short although you are only supposed to use a short shaft on that SIB

Andy

Andy JC · 28 June 2006, 00:57

The boat and engine were packaged together from new so pretty sure it's the correct prop.
Thinking back the inflatable keel might have been a little off centre, didn't think that would have a detrimental effect but after reading these posts maybe that could have been to blame.
As for weight distribution, we had 2 blokes near the back and 2 women on the front bench so I think it should have been fairly even.
The rubber hub slipping could be a posibility, definately worth wipping off the prop to have a look me thinks.
The doel fins could be worth considering if I don't manage to sort it, I'd rather not drill the anti-ventilation plate on a nearly new engine though, bit of a last resort really.

Thanks for all the input guys

ian parkes · 28 June 2006, 01:14

My honda did that the other day , cavitating like mad and came almost to a halt . It was small strand of weed just folded around the back of the motor , no longer than 6 inches

Andy JC · 28 June 2006, 09:55

Ah thats interesting, there was quite a bit of weed around (not the illegal kind

). At one point I was trailing about 7 ft of the stuff - maybe something else to look out for, cheers

matt h · 28 June 2006, 15:35

i had a quicksilver 430 hd and a new tohatsu 40c short shaft, and i could not stop it from doing iteverytime i turned reasonabley quickly it just cavitated and stopped!! sold it after 5 hours of useage! ashtray on a motorcycle springs to mind!!

jyasaki · 28 June 2006, 19:34

Quote:

Originally Posted by Andyboy

The boat and engine were packaged together from new so pretty sure it's the correct prop.

That doesn't mean all that much. I doubt whether much experimentation was done to find the "correct" prop; more likely they stuck something on and figured it would be good enough. That was the way my dealer treated my 14' Achilles.

Quote:

As for weight distribution, we had 2 blokes near the back and 2 women on the front bench so I think it should have been fairly even.

4 people on an 11 foot boat with a 15hp? Seems to me that that is a pretty substantial load. Not overloaded, but substantial.

How does the boat go with just you on board?

jky

Andy JC · 29 June 2006, 15:12

Haven't tried the boat on my own yet but when I eased the throttle in gently I was able to get it on the plane with the 4 of us in without too much problem, felt like it may possibly have gone up with another adult provided the prop would grip so wasn't particularly overloaded (boat is rated to take 5+1)

After inflating it in the garden last night one thing I have noticed is that the inflatable keel (act-v, box type ) was not sitting straight either front to rear or top to bottom

I'm guessing that this could well be the reason for the ventilation but after deflating/inflating a couple of times and playing around with the toggles that hold it in place I still couldn't get it to inflate in a straight line

Does anybody have an idea if there's a knack to this, ie. should the main tubes be inflated first then the floor ot vice versa??

codprawn · 29 June 2006, 15:19

On my quicksilver you have to get the floor in first(wooden) - then inflate the tubes slightly - then the keel and then finish off the tubes.

Lift the boat off the ground when you are doing the air keel - on the Quicksilver there is a big hole in the floor for the valve for the keel - you can move it around a bit before you inflate it.

Keel doesn't always go dead straight but never been a problem. My boat is smaller than yours but I have managed to get 20mph with 4 people on board. It is important to get people to move around when that loaded though - weight forward to get it on the plane - then move aft to keep the nose up.

Mollers · 29 June 2006, 16:03

I reckon the engine is sitting too high. Is the cav plate above the keel when in the fully down position? If so, it's either go for a longer shaft of take a notch out of the transom.

ian parkes · 29 June 2006, 18:21

surely if the keep wasn't fitted strait then thats the problem . It would be creating a wierd and inconsistant flow past the prop .
What you want is nice little Honda Air floor none of that hassle with floor boards and keels , as it happens I just

Andy JC · 29 June 2006, 20:54

HHmmm.... new engine or cutting in to the transom seems a bit extreme, especially as I've only just bought them both

I'm inclined to go with your line of thinking Mr Parkes (nice boat by the way

) - I'm going to focus on sorting the keel out before anything else.
I did look at the Honda 3.2 but the thing that put me off was the permanent flaps at the back preventing the fitting of launch wheels, essential when lifting up/down a pebble beach with just my girlfriend for help.
Incidentally it doesn't have floorboards, it's the fastroller version so should be just as easy to set up

floater · 29 June 2006, 22:22

After inflating it in the garden last night one thing I have noticed is that the inflatable keel (act-v, box type ) was not sitting straight either front to rear or top to bottom

I'm guessing that this could well be the reason for the ventilation but after deflating/inflating a couple of times and playing around with the toggles that hold it in place I still couldn't get it to inflate in a straight line

Sorry but had same problem again ! ended up sending back to dealer to have toggles moved which they did under warranty and sent back to me direct via parcel line good service but sold and bought Avon shortly afterwards

see attached