What could be wrong here then?

[Andy JC]

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]

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]

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]

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]

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]

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

[codprawn]

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]

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]

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]

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