Cavitate problems: Humber 8m w/Volvo D3-160 - Page 2

jwalker · 27 May 2004, 23:40

If it was a performance boat with a big outboard on the back, that height would be just fine so it's not going to be that far away. For load carrying, you may need it to be about level with the keel as Pete Fowler said. If that is necessary, you are going to have difficulty without some form of modification, either to the transom or by installing 2 spacers. But before you do any of this, check the following.

Along with your Volvo will have come a diagram of the cut-out hole. On that diagram there will be a table of recommended X measurements. This is the distance from the bottom of the keel to the centre of the engine output shaft/drive input shaft. You need to read this carefully and work out what is recommended for your installation. It will be a bit difficult to check now that the leg is installed but you can probably measure from the bottom of the transom shield and add the distance on the diagram to the centre of the shaft. You will also need to know the transom angle for this calculation. Note that it is the measurement AGAINST the transom NOT a direct vertical measurement up the leg.

When you've done this (accurately) you will (and we will) be in a better position to decide the best course of action.

seafariskye · 28 May 2004, 18:31

Pete!
The "bending on the trailer malarkey" you are referring to may not be a problem you experienced, however that does not make it "malarkey". I was asking about the bending of the hull with a view to broadening my own horizons, not saying it was the problem. I think everyone is trying to help and probably could do quite well without your negative input. If you don't have something good to say, don't say anything at all.
27 May 2004 21:49
seafariskye Hello
Bajan

Sorry if you took offence at the smallest bit of my reply-- but the main bit of it was from experience and i thought i was giving something of a helpful hand to what was going on - i would say that, if i was giving something negative, then i am sorry, but sounds like you got out your bed on the wrong side -- and if you think i have nothing positive to say on the subject -- fine, but rather you PM me on this rather than clog this thread with it?? cos either way i would tell you to think about what you are saying --

Pete

Michaels · 01 June 2004, 17:44

We have been in close contact with both the Local Volvo dealer and Humber regarding the cavitation problem. Both Volvo, Humber, me and you guys had the same theory about the cause of the problem. Due to this we are convinced that installing a 1-inch add-on kit will solve the case. After the installation we will be very near Volvo's recommodations for the installation.

The kit has just arrived today and we are getting it installed by Volvo tomorrow. We are very excited about seeing the results. This said Humber and Volvo locally has been very forthcoming and helpfull in solving the problem. It is very reliefing that Humber as a manufacturer commits till the craft is actually working as expected.

More to come after the craft has been tested with the new installation.

jwalker · 01 June 2004, 18:16

I await the outcome with interest.

Michaels, what cost was involved in the extension kit?

Michaels · 01 June 2004, 18:38

The cost is aprox 10.000 NOK including work and parts (aprox £830). I have come to an agreement regarding this.

Michaels · 18 June 2004, 00:57

The drive Add-on kit has sucessfully been installed and the X-measure from the installation instructions are now very close to Volvo's recommodations.

What we achieved:
- There are less cavitation problems overall.
- We are now able to put load midship and in front without instant cavitation.
- We can do quicker turns without instantly go into cavitations, both with and without load in the boat.
- There was no speed improvements on equal propellers, but we were able to test other props that cavitated earlier. This gained some few knots extra with light load.

Result: The cavitation problem we had is now more or less gone. It gave us everything we were hoping for, except for the extra few knots we were hoping for.

Humber has been very helpfull and service minded in this matter. The local Norwegian Volvo dealer has also been doing their jobs well. Thanks.

PROPELLER:
We have tested a 3-blade 21" SS propeller that gives us 35 knots with light load and aprox 28-30knots with 4-5 divers. We can not gain the full 4000 rpm with this propeller when loaded with divers, but it seems like the engine is unstopable from 2000-3500rpms. The last 500rps are harder to get.
We actually would need some professional advice on which propellers to choose.

jwalker · 18 June 2004, 08:56

Michaels, thanks for letting us know the outcome. How far away from the X measurement are you still? Above or below?

That's a good performance from such little HP on an 8mtr boat.

Michaels · 21 June 2004, 22:48

We are talking about 2mm below the recommended X-measure. In this matter that is giving more Trim possibillities and can as far as I can understand be recommended compared to lifting it.
The craft gives best performance when loading all weight in rear of the boat. I do think we have done a good choice when it comes to seating and consolle arangemet. See attached pictures.

jwalker · 22 June 2004, 01:12

A nice set of pictures. It's interesting to see such a long narrow vessel. I wish you good boating with it.

Jono · 22 June 2004, 06:36

Nice choice of seats and console, if I may say so?

Michaels · 01 July 2004, 00:58

Thank you for the feedback.

It has been on the water for apox one month now and we have already run it for 84 hours now. Except for the problems mentioned above the engine and craft has worked out very well.

Just uploaded some new pictures of it today:
http://www.rib.net/forum/showthread.php?t=5391