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Old 16 November 2002, 17:48   #1
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Prop Problems

Went for a brief blast today just out of Hamble river, noticed when turning at speed a lot of either slipping or cavitation ?

Given that the present prop I am using is a little chipped at edges a replacement prop should hopefuly cure this

What is most likely cavitation or possibly hub slip ?

I plan to try a different prop anyway as at 5000 rpm GPS is indicating 25 MPH, which on flat water I think I would like to increase

Any thoughts on suggested new pitch: boat is an Arimar 5 metre rib with Johnson 50 two stroke (2.41:1) gearbox

( cant quite read existing pitch before you all ask )

Seems to get up on the plane OK, despite its appaling mediteranian style passenger weight distribution

if I get the chance I'll call steel develpments, but thoughts apreciated please with owners of similar sized boats
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Old 17 November 2002, 04:03   #2
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Whilst it could be cavitation it is more likely to be ventilation, where the prop is pulling air down from the surface. The problem is worse going around sharp corners because the prop comes up nearer the surface. You could try trimming the engine in, however the real solution is to have the prop repaired or switch to Stainless Steel. There is a big difference as the blade of a S/S prop is thinner and can be run with the engine mounted stlightly higher for speed or in its current position giving excellent grip on the water.

Pete
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Old 17 November 2002, 05:28   #3
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When running with my ali prop (engine makers issue) I have exactly the same problem. With the SS prop on she goes round tight corners like on rails. I am also going to try a set of hydrofoils to see if they improve performance (see seperate thread on this subject)
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Old 17 November 2002, 05:48   #4
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foils

I am also going to fit a pair of wings, (because the weight distribution is so poor with the bench seat style of seating and everything else heavy seems to be at the rear)

see pic

This should help the fact that the boat wants to go skywards when planing off and might help problems discussed above

Do you think that the design of a ribs tubes sitting in the water can cause ventilation when turning, as when you turn one tube will dig in deeper perhaps upsetting the water flow ?

I havnt experienced this slipping effect when turning in any other boats, its interesting that Ribald has experienced the same problems

Anyone else come across this ?
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Old 17 November 2002, 06:25   #5
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Guys, before you go starting drilling holes in your cavitation plate etc, try having the engine raised or lowered a notch. Then ring these chaps , and discuss your problems with them. Props are a scientific art form and as such, discuss it with the people who seriously know what they are talking about.
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Old 17 November 2002, 06:45   #6
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...also have a look at this (from the FAQ section on Mercury's web site)
http://www.mercurymarine.com/cgi-bin...searchAnswer=1
- yes, I know you've got a Johnson outboard, but I guess the principle's the same
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Old 17 November 2002, 17:37   #7
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similar probs

I experienced similar problems on a 4 m avon with a 50hp merc two stroke in particular while turning the rib at speed, the engine had fins fitted when I bought her, however I was able to elimanate the problem if I manually changed the trim angle of the engine, it cavitated the higher up the engine angle was, and gripped better if I lowered the trim angle.I t was also possible to make the boat cavitated on the spot and not go foreward like a slipping clutch, I never changed the prop and learned to avoid sharp turns under power. cheers gavin
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Old 18 November 2002, 04:56   #8
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Repairing your prop and / or going for SSteel is probably a good idea but just how tight are you turning and at what speed and do you really need too?! The pic of your RIB suggests big tubes that probably are close to the water or even skimming the surface whilst on the plane? So when you turn you are going to be turning very "flat" and not banking as you would in a deeper V RIB with tubes higher up. Now not sure whether this scientifically makes any difference but I used to have a Deep Sea 21 which also had the tubes set low and cornered very flat. If I hurled it into a tight turn without throttling back I would get tremendous cavitation/ventillation. (And the centrifugal force would do its best to chuck me overboard!) This phenomenen happened with two different engines and three different props - alloy and stainless. The solution. Slowing down if I wanted to make a very tight turn and powering out of it.

I guess my point if that other than for the hell of it one doesn't normally go around doing very tight turns all over the place so if thats the only time you have a cavitation problem then maybe the easiest thing to do is to live with it! If the problem manifests itself in "normal" cruising use or say in rough water you can't get a grip with your prop then thats a bigger issue and one that you need to address!

Cheers,
Alan
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Old 18 November 2002, 13:50   #9
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steel dev

steel dev suggest repairing prop first and then seeing if this eliminates problem, they suspect that it is cavitation during turning because of poor condition pro

another reason is repair= 33 quid
new ally prop=85
stainless=175

they say cavitation can occur when turning because the prop is having to work harder because of the extra resistance the boat makes during a turn, similarly cavitation likely to happen when pulling away (makes sense)

they do agree with a boat such as mine which is arse heavy and lets face it not massively powered that a set of hydrofoils can only benefit performance, especially getting on the plane

so gotta get my prop to wandsworth sometime

would recommend this company to anyone looking for props / advice, I have used them many times and they can work magic to a prop

Alan
Problems I was experiencing were not in particularly tight turns, so time will tell, lets hope weather stays mild

www.steeldevelopments.co.uk
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Old 20 November 2002, 22:55   #10
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try this link for your prop repair.

www.streamlinedpropellers.com

He sells props too and the guy knows his stuff, is very helpful and reasonably priced . He is quite local to you in the hamble (Aldershot) and is always at the boat jumbles. He repaired my prop last year. Excellent!
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