Thread Tools Search this Thread
Old 16 November 2002, 16:48   #1
Country: UK - England
Town: Salisbury
Length: 5m +
Join Date: Dec 2001
Posts: 459
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

matiboy is offline   Reply With Quote
Old 17 November 2002, 03:03   #2
Pete7's Avatar
Country: UK - England
Town: Gosport
Boat name: April Lass
Make: Moody 31
Length: 9m +
Join Date: Aug 2001
Posts: 4,837
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.


Pete7 is offline   Reply With Quote
Old 17 November 2002, 04:28   #3
Country: UK - England
Town: Leatherhead
Length: no boat
Join Date: Aug 2001
Posts: 907
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)
Peter (nick, nick) T

Age and treachery will always overcome youth and skill! Bullshit and brilliance only come with age and experience.
Ribald is offline   Reply With Quote
Old 17 November 2002, 04:48   #4
Country: UK - England
Town: Salisbury
Length: 5m +
Join Date: Dec 2001
Posts: 459

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 ?
Attached Thumbnails
Click image for larger version

Name:	dscf0405b.jpg
Views:	359
Size:	63.2 KB
ID:	1374  
matiboy is offline   Reply With Quote
Old 17 November 2002, 05:25   #5
Country: Other
Make: FB 55
Length: 10m +
Join Date: May 2001
Posts: 1,711
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.
Charles is offline   Reply With Quote
Old 17 November 2002, 05:45   #6
Richard B's Avatar
Country: UK - England
Town: Berkshire and Devon
Make: Lodestar
Length: under 3m
Engine: Mercury 5hp
Join Date: Jul 2002
Posts: 5,015
...also have a look at this (from the FAQ section on Mercury's web site)
- yes, I know you've got a Johnson outboard, but I guess the principle's the same
Richard B is offline   Reply With Quote
Old 17 November 2002, 16:37   #7
Country: Ireland
Town: Dublin
Boat name: wizzard
Length: 7m +
Engine: 225 optimax
Join Date: Sep 2002
Posts: 835
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
gavin is offline   Reply With Quote
Old 18 November 2002, 03:56   #8
Country: Canada
Town: Newfoundland
Length: no boat
Join Date: May 2000
Posts: 2,097
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!

Alan is offline   Reply With Quote
Old 18 November 2002, 12:50   #9
Country: UK - England
Town: Salisbury
Length: 5m +
Join Date: Dec 2001
Posts: 459
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

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

Problems I was experiencing were not in particularly tight turns, so time will tell, lets hope weather stays mild
matiboy is offline   Reply With Quote
Old 20 November 2002, 21:55   #10
RIBnet supporter
Jizm's Avatar
Country: UK - England
Town: Southampton
Boat name: Hissing Sid
Make: Ross Smith Cobra
Length: 6m +
Engine: 200HP Optimax
MMSI: 235038046
Join Date: Jun 2002
Posts: 3,803
try this link for your prop repair.

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!
Jizm is offline   Reply With Quote

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Our Communities

Our communities encompass many different hobbies and interests, but each one is built on friendly, intelligent membership.

» More about our Communities

Automotive Communities

Our Automotive communities encompass many different makes and models. From U.S. domestics to European Saloons.

» More about our Automotive Communities

Marine Communities

Our Marine websites focus on Cruising and Sailing Vessels, including forums and the largest cruising Wiki project on the web today.

» More about our Marine Communities

Copyright 2002-2015 Social Knowledge, LLC All Rights Reserved.

All times are GMT -5. The time now is 01:46.

Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2016, vBulletin Solutions, Inc.