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

[Michaels]

Hi.
Just received my brand new Humber Ocean Pro 8,0 meter a week ago. I have been installing the new Volvo D3-160 engine in the weekend and done some testdrives. The boat feels very stable and it performs very well especially in rough seas. It actually eats the waves and it makes it a very pleasant ride.


CAVITATION PROBLEM:
The drive has to be in the maximum "down" possition to be usefull. If I use the trim for 3 degress up or more it starts to cavitate and it looses grip of the water. The same happens when doing very sharp turns.
Since this setup never has been delivered from Humber and I am living in Norway they are not able to testdrive it and come up with a solution.

The cavitation plate are some few centimeters above the keel and I do have som few theories. Maybe some of you guys out there have experiences or other ideeas about how solving my problem.


TESTS:
- LOAD ON SEATING: 2 passengers in jockey and 1 on the engines housing. This solution gives us some very few steps up to play with on the trim before it loose grip of water. (working area from -9 to – 6 degrees on drive). It feels like we have lots of power and it is easy to enter max rev at 4000rpm

- LOAD IN FRONT: 2 passengers in the bow and driver seated in jockey. This gave us nothing extra and it felt even worse than when everybody was seated in jockey and on engine housing. There was problems with using the trim in other positions than the max down/in position.. (working area from -9 to – 8 degrees on drive) It feels like enough power and reach 4000 easily.

- LOAD REAR: We moved the 2 passengers back to the transom and it gave us extra trim possibillities and it felt like more speed.
(working area from -9 to – 1 degrees on drive). It feels like enough power and reach 4000 easily.

- MORE LOAD: We was trying with 5 persons aboard and it felt like we were more heavy loaded. The RPM stopped at around 3800-3900 RPM with this load. It was getting late and we were not able to do more tests with 5 persons. Hopefully more testing tomorrow. It feels like it was a bit more heavy and we was not able to get 4000rpms.


SOLUTIONS ???:
We do understand that there can be several reasons for these problems and we have listet some of our teories here:



o Problem: The Sterndrive is installed to high.
Solution: Buy a “add-on ” kit that makes the drive go 2-4 cm deeper in the water. This is some kind of spacer to put on.



o Problem: Wrong propeller
Solution: Find a propeller that fits the setup better. Do you have any experiences about a bigger or smaller propeller will give less “spinning” (loose grip) in the water???? A thought might be that a propeller wit smaller diameter will stay further down into the water. On the other side will a bigger propeller have more grip in the part staying under water. I suppose that there are some propeller experts out there.



o Problem: Something disturbs the waterstream
Solution: Move the item that makes problems. The Water inlet are installed as Volvo described it. Aprox 70cm from transom and around 27 from the keel on the right (starboard) side (Volvo recommended min 60cm and min 20cm)



o Problem: Hull disturbs the waterstream (Not 100% straigth keel):
Solution: When we saw the boat on land we noticed that the keel was not 100% straight in the first meter from transom and forward. From the keel as the lowest point it bent a bit (some few mm) upwards further forward. This might make a stream pushing the water down under transom and resulting in the prop loosing grip of the water. We did not measure this, but just did a visual estimate.


We are willing to buy more than one propeller to fit our needs:
Prop A) Heavy load: totals 2550 kg (1000+200+15x75)
Prop B) Light Load: totals 1500 kg (1000+200+4x75)
Prop C) All round: Optimal and good between 1700-1900kg (1000+200+8x75)


Any advice and suggestion will be very apreciated.....

Regards Steve Michael, Norway

[Lester McCarthy]

Hi Michaels,

Sounds like you have a problem. I had a boat built a couple of years ago by a company who had had no experience of fitting twin outboards and it was a complete disaster and I could not accept it. The engines were fitted too high and had similar problems to the ones you have. Changing the props did not help the boat was dismantled and taken apart.

I am out here in Norway at the moment -Arundal where I test drove the prototype Goldfish 25 at 65 knots. Great boat.

Lester

[DJL]

Originally Posted by Michaels

The cavitation plate are some few centimeters above the keel and I do have som few theories. Maybe some of you guys out there have experiences or other ideeas about how solving my problem.

The cavitation plate (technically the ventilation plate) should be below the keel not above it; otherwise when going fast it will be out the water and hence not doing much.

[John Kennett]

Steve

From what you have said I would guess that the sterndrive is too high. The improvement by loading the stern of the boat supports this theory.

If it was something disturbing the waterstream then I would expect it to happen whatever the trim. It is unlikely that changing the prop will help.

In high speed outboard applications it is not uncommon to have the ventilation plate a couple of centimetres above the keel, but in your case I would certainly not expect to see this.

Humber have installed D3s before, so I would have expected them to be able to advise.

At least there is a kit that may easily solve the problem though. I had an outboard powered RIB with similar symptoms and the only solution was to cut about 50mm off the top of the transom!

Let us know how you get on. I would be very interested to know what sort of speeds you are getting with this boat/engine combination.

John

[jwalker]

Originally Posted by Michaels

Hi.
o Problem: Hull disturbs the waterstream (Not 100% straigth keel):
Solution: When we saw the boat on land we noticed that the keel was not 100% straight in the first meter from transom and forward. From the keel as the lowest point it bent a bit (some few mm) upwards further forward.

This is normal for the boat.

I agree, you probably need to fit the spacer kit.

---------------------------------

The cavitation plate (technically the ventilation plate)

Technically, the anti-ventilation plate.

[Bajan]

When we saw the boat on land we noticed that the keel was not 100% straight in the first meter from transom and forward. From the keel as the lowest point it bent a bit (some few mm) upwards further forward.

Originally Posted by jwalker

This is normal for the boat.

I agree, you probably need to fit the spacer kit.

---------------------------------



Technically, the anti-ventilation plate.

Hi there!
Quick question re the keel not being straight. I read in one of the threads previously about a problem developing when the hull extended too far over the end of the trailer and resulted in a concave shape forming. How is this different to the characteristics of the keel above? I'm probably confusing myself, but hopefully an explanation will put me right.

[jwalker]

I don't know why Humber have chosen to do it, but they do. It simply creates a bit of stern lift. They had a bit of criticism of the early boats travelling bow high. My guesses are; 1) the very high bow needed a bit of control or, 2) they bu**ered up the mould.

It is slightly different to the sagging caused by bending on a trailer because that makes the flats of the hull hollow.

[Fjordrafting]

Hei! Jeg har en 8.75m rib med 2 x250hk og har kavitert masse. Jeg har løftet og senket motorne og prøvd store og små propeller.
Det er ikke størrelsen på propellene som avgjør om de kaviterer, men formen. I tillegg er jo noen propeller laget som overfflatepropeller. Ta kontakt med noen som er dyktige på propeller. En mindre propell med flere blader, gjerne 4 kan være en løsning. Ta kontakt med Matt på Throttle-up.com. Han sitter i USA, og svært dyktig. Han trenger alle opplysninger du kan komme på.
Har du hjemmeside med bilde av båten?
Ha en fortsatt fin dag! (ps. hvis du skriver .no bak mit alias, så kommer du på hjemmesiden min.)
Martin

[DJL]

Originally Posted by Fjordrafting

Hei! Jeg har en 8.75m rib med 2 x250hk og har kavitert masse. Jeg har løftet og senket motorne og prøvd store og små propeller.
Det er ikke størrelsen på propellene som avgjør om de kaviterer, men formen. I tillegg er jo noen propeller laget som overfflatepropeller. Ta kontakt med noen som er dyktige på propeller. En mindre propell med flere blader, gjerne 4 kan være en løsning. Ta kontakt med Matt på Throttle-up.com. Han sitter i USA, og svært dyktig. Han trenger alle opplysninger du kan komme på.
Har du hjemmeside med bilde av båten?
Ha en fortsatt fin dag! (ps. hvis du skriver .no bak mit alias, så kommer du på hjemmesiden min.)
Martin

Translation: (Not sure its that much clearer!)

hello I've a rib along with 2 x250hk and has kavitert a lot of. I've lifted and was lowering motorne and attempted big and a little propeller. That is not bulge at propellene as decides about they kaviterer , but fitness. Additional am certainly a few propeller layer as overfflatepropeller. Get contact along with a few as am capable at propeller. A less propell along with more leaves , certainly 4 can be a answer. Get contact along with Matt at Throttle -up.com. He sit in USA , and mighty able. He need full particulars you come about. Have you hjemmeside along with picture at the boat? Have a continued fine day! (ps. of which you draw up.no at the back mit alias , saw comes you at hjemmesiden min. ) Martin

[Matt]

So basically he's saying a prop change should be able to sort it and to
speak to matt at http://www.throttle-up.com?
I've spoken to julie (matt's wife) before and she was very friendly and helpful. They are sponsors of offshoreonly.com.

I think basically Mr K is right, for the prop you're running it's not deep enough. What prop is currently fitted?

[Matt]

I've been debating what to write here or not since I'd rather not post than to talk shite. (Especially with the prospect of a red blob if I get any facts wrong )

Cavitation, in it's truest sense is caused by the pressure dropping below the vapourisation pressure of water at that temperature.
Now since it's the pressure differential between the front and back of the blades that drives the boat forward, you HAVE to have a pressure differential.
2nd, almost all props cavitate. On all the underwater shots of boats and you see the twirling bubbles behind the prop - That's the tips cavitating.

Where it goes wrong is when effectively the pressure differential is too high, or when the change in direction is to sharp. These are 2 different modes of cavitation, I'm afraid I can't remember the correct names.

The 3rd factor is running the prop higher as your boat can mean that air is being drawn into the prop by the low pressure areas on the blades. (This is like air being sucked down a plug hole in the bath even though there's still some depth of water left). This isn't true cavitation though, even though people usually call it that. Still following?

So, there's a number of ways of correcting it. At the design stage, mounting the drive lower would have been a good way of doing it. But, neglecting temporarily the option of installing a drive spacer, what you might want to do is reduce the low pressure areas on the prop, or at least make sure they stay under control. It's possible even one of those add-on doel fin things might help simply to prevent air being drawn in, although IMHO this would be a real band aid solution and I think they must be a liability in offshore conditions too. Might be worth an experiment though as they are cheap.

So, as a starting point, what prop do you current have fitted and what are it's characteristics.

If it's a 3 blade aluminium prop then there's a fair chance changing prop will be able to fix it. Increasing the blade area, either through bigger diameter, increased blade area ratio, or number of blades might all help. Thinner prop blades might help. Staying with the same pitch but increasing the amount of cup and rake may also help.

I've probably got some of my points a bit mixed up cos I didn't really pay attention in those lectures, in fact I think I just copied someone elses notes. More fool me.

Mercury's pages are pretty good at explaining all the theory. You can get to them somehow through mercury's site, or go to my site (http://www.mattyorke.info), choose Reference|Propeller theory and that takes you straight to the 4 articles by mercury.

HTH.
M

[Michaels]

Here is the suggestion I reveiced from Humber:

"As the engine in a new product we have not actually tested this combination yet, however we have recently undertaken extensive tests with the 130hp Volvo in the Ocean Pro 6.3m and based on this we would anticipate that for full dive load a 3 blade 15 x 17” should be OK and a lighter load for all round performance 14.25 x 19” 4 blade. We must point out that this is not guaranteed and is offered as a guide line only."

We now use the 14,25 x19" 14blade propeller provided by Volvo....

[jwalker]

This test, which they are referring to, is documented in the latest RIB International magazine. It is quite interesting.

Michaels, is your boat an Ocean Pro or an Ocean Offshore. I just ask because an 8mtr Ocean Pro would be a long, narrow boat. What are you using it for?

I understand you fitted the engine yourself, is this correct?

[Michaels]

The boat is a 8m Ocean Pro with 2,60m beam.
Humber took £680 for making doing the Hull modification for this engine. Since they had just done the 6,3 Volvo D3 installation we thought it was a good idea to let them make everything ready for putting in the engine. So the craft came ready with a engine well, engine supports and a finished hole in the transom. Everything sealed up with Fiberglass. We did the installation supervised by the local Volvo dealer that also checked the the installation during and before starting it up. They had no complaints at all.

We will try to measure the cavitation plate tomorrow and se where it is located compared to the keel. I do believe it is some few centimeters above.

The boat is rigged for both leisure (light load - totals 1500kg) and diving (heavy load - totals up to 2500kg). The consolle are located far back with a row of jockey seats behind it.

[Rene]

wow, very nice looking boat.

Hope you solve the cavitation problem. (I'm no pro at this but I would try a spacer to lower the drive)

Rene

[seafariskye]

Hello

The cavitation plate on the leg, should always be at the bottom hull level - at the keel i mean -- the only place it 'should' be otherwise it is like running a short shaft outboard on a long shaft transom -- it goes ok, for a while then cavitates --
Question for you -- all outdrives, with the diesel inboard as far as i know - when trimmed right up, to the beaching, trailer position, whatever, never quite come above the level of the keel, as inboard owners should know, thats why i never anyway will beach the boats unless on sand/mud, as the leg will always be slightly lower, not a problem you get with an outboard, as you can raise it above the keel level.............

I would lift your leg right up, and if it comes completely clear of the keel level, then it is rightly to high, i cannot see that volvo have suddenly fixed this glitch, as with all outdrives, the other thing is of course is to put it on the trailer, and put the leg right down, the see the height of the cavitation plate?
i have seen a home job, with a new boat, and a mercruiser (fitted at home) and the smart owner decided he would raise it i bit, so he could beach as per an outboard, and not hit the leg -- all it did was cavitate --

prob no use - but there you go -- do not believe in all this 'bending' on the trailer mallarchy -- unless you are trailering a banana.

cheers
pete

[Bajan]

Originally Posted by seafariskye

prob no use - but there you go -- do not believe in all this 'bending' on the trailer mallarchy -- unless you are trailering a banana.

cheers
pete

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.

[Michaels]

Here are the pictures showing that there is a great distance from the keel to the cavitation plate. Aprox 43 millimeters. The extension kit are about one inch (25,4mm) There is also pictures from how the measuring was done and one picture showing the "bend" on the keel I was speaking of.

Any comments ????

[DJL]

Nice boat!

The leg is far too high - It looks like from the second picture that the tip of the propeller is almost at the surface.

The extension kit should make a fair bit of difference although it may not solve it completely. What are the choices if it needs to be lowered further? Can you stick two extension pieces on?

[Searider]

Time to have a look at the VP installation manual and then a discussion with whoever cut the hole for the sterndrive I think would be next.