Cavitation/Ventilation problem..........

[dog]

Well, the Carrera was launched today with its spangly new D3's. Everything fantastic except- if I apply any downward trim tab (ie bring bows down) or trim legs out at all (again bringing bows down) the props lose grip. The indicators for the legs are set at -8 for full trim in, any more trim up than -6 produces the effect. Volvo have suggested spacers in the lower leg to get the props lower down which I hope cures the glitch. The new legs have been installed as per the norm, with the cavitation plates in line with the bottom of the hull. I was just wondering has anyone else encountered similar problems and how they cured it.

I am hoping on some collective knowledge on the subject.....

Thanks

Brian

 

[Roy]

Are you really sure that the tabs are up when you say that legs trimmed out brings the bows down? Doesn't make sense to me...........

 

[dog]

Roy, two separate issues- leave legs in fully trimmed in position ie, props at negative angle to horizontal and put tabs down- this brings the bows down right? Now imagine leaving tabs fully up ie, bows up and bring the legs up slightly- this has a similar effect to putting tabs down- it brings the bows down.

 

[BarryH]

If your bringing the legs up then the bow will rise! Have the right props been fitted. Not unkown for the wrong sized props to be fitted. I know of a person who had the wrong prop supplied from new!

 

[Roy]

Tabs up and legs out should plane the boat or at worst, sit her up on her tail.

 

[JKay]

Good luck wiv your new baby Brian

legs in tabs down till she planes then ease the tabs up both hands on the wheel

enjoy /forums/images/graemlins/cool.gifcheers Joe

 

[dog]

Thanks to all who have posted, must be going doolally- agree legs out means bows up- not as I originally said!! Just been on the steeldevelopment site and mentions cavitation on a prop with too high a pitch. The engine was under revving by about 200-300rpm today in sea trials- which means too highly pitched and Volvo are due to change them out. So is it possible that my problem is just the props slipping as they are overloaded?

 

[stelican]

I am aware with this problem with the engine installation in Carreras and know of it being cured with leg extensions all should be well

 

[gcwhite]

The problem seems to have been addressed. Hope all now goes well, it must be an exciting time. Would be interested in how you find the D 3s . I think the D series are a great range of engines.

What was your view re delaying for IPS?

/forums/images/graemlins/laugh.gif

 

[dog]

Hi, with regard to IPS the engine bay is not big enough to take the larger engines and the boat was already a sterndrive model, so it made sense to go sterndrive again. The main reason was that I just couldnt wait to get back on the water again and the D3 is the best option for the boat.

My impressions so far are very good- the engines are turbine smooth and two lots of 5 cylinders sound the nuts when under load!!! Im sure when the props and ventilation issues are sorted then she will be a very nice boat.

I will keep you posted........

Regards

Brian

 

[davedpc]

When I had a volvo I was always confused with the markings on the leg trim control. When the switch is moved I would like it to indicate what the leg is doing ,not tte result of the bow. Therefore I think it was possible to turn the switchplate over to give the opposite diagram. Dave.

 

[Dave_Snelson]

Which is it - cav or vent?

Hi Brian - mebbe you got this sorted by now? But cavitation is vastly different to ventilation (I'll apologise now if you already know this).

Cavitation is caused by the formation of water vapour bubbles around the prop as the pressure reduces around the rear of the blade to the point where water momentarily and temporaily "Boils". This is not only bad for slip / grip problems but it is very bad for the propellor. The collapsing water vapour bubbles release the stored energy as vapour-to-liquid in such a manner that the temperature in the tiny collapsing bubble (micro-seconds) rises to 1000's C. This effect can very quickly eat away your propellors to nothing - like perpetual high pressure grit blasting!. So there is a greater urgency to sort cavitation.

Ventilation is simpler and less dangerous to props. All that happens is that you are sucking in air below your anti-cav plate, and props are useless in an air-water mix.

Both cav and vent give rise to similar symptoms.

Good luck in getting it sorted.

 

[duncan]

number of related issues here Dog; not least of which could be weight distribution.
Is your new installation substantially lighter than the previous one? My guess is yes. In this case your optimum is to shift anything heavy back on the boat to rebalance! iN an extreme case you may even need to add weight to bring the relationship between the hull/legs/water back to it's design.
On the trial look out for the following -
Once fully up on the plane does the the nose actually start to drop a bit as you approach WOT? This can happen when the hydrodynamic lift from the stern isn't matched but, unlike a small sports boat for example, there is more weight at the bow than lift being generated there - so it dips until the hull is balanced along it's length (lifting yor props/legs etc).
Finally it's not unusual for DP legs to require little, and even zero, trimming out in normal operation - I believe the Leader805 uses hardly any for example.