Model Boat Mayhem - Forum

Mess Deck: General Section => Chit-Chat => Topic started by: frogman3 on November 08, 2019, 06:39:31 PM

Title: props vacuum
Post by: frogman3 on November 08, 2019, 06:39:31 PM

HI Guys now im gonna ask a silly question an hope I don't get laughed off the forum  %) Now i'll start at the beginning It all happened many yrs ago when I built my first large warship HMS AMAZON type 21 frigate in 1/64 scale an you might ask why the scale well at the time it was the largest F/G hull avalible an the hull was used for the MOD .Anyway I built the ship an sailed her quite a lot but one time  when I had finished sailin as I was cleaning around the hull nr the props I noticed a black line in the paint work an on closer inspection it seemed to me like a burn mark an even smelt like it an I thought the props had been hitting the hull for a moment  as I think I fitted the props to close to the hull. But I had left about a  1/8th of an inch gap between the prop tip an the hull so I thought it was ok. But turned them both an they were missing ok now as I was lookin at this an elderly gent came over an told me it was the fault of a vacuum build up on the prop tips an its gets hot so burned the paint off only a thin line but it happened an in water so can anyone exsplain if he was right or he had to much wacky bacca 
chris
Title: Re: props vacuum
Post by: Colin Bishop on November 08, 2019, 07:06:00 PM
Quite an interesting post, has anyone else experienced anything similar? Obviously high revving props must generate low pressures at their tips.

Colin
Title: Re: props vacuum
Post by: DaveM on November 08, 2019, 07:23:06 PM
The word 'cavitation' springs to mind. Google it for further info?
DM
Title: Re: props vacuum
Post by: Colin Bishop on November 08, 2019, 07:24:32 PM
Yes, but the effects of cavitation in full size usually manifest themselves on pitting on the propeller blades.

Colin
Title: Re: props vacuum
Post by: Andyn on November 08, 2019, 08:45:38 PM
Not a cat in hell's chance... %)
Title: Re: props vacuum
Post by: Colin Bishop on November 08, 2019, 09:11:53 PM
Quote
Not a cat in hell's chance...

Not on a model obviously. I think the point was whether high speed rotation close to the hull could have some sort of impact on the paint. Or are there any other reasons for what was observed?

Colin
Title: Re: props vacuum
Post by: grasshopper on November 08, 2019, 09:21:18 PM
With only an 1/8th clearance could you have caught something on a blade and abraded the hull surface?
Title: Re: props vacuum
Post by: frogman3 on November 08, 2019, 09:28:54 PM
With only an 1/8th clearance could you have caught something on a blade and abraded the hull surface?

HI Grasshopper if I remember rightly both props were clear an the line was there on the hull  an in nowich pond its pretty free of objects as its a concrete pond
chris
Title: Re: props vacuum
Post by: roycv on November 09, 2019, 05:54:10 AM
Hi, was the black mark on top of the paint or in the paint, i.e. gone through the paint?  Any photos?  Was it there for both of the props and how long was the black line?

Is it possibly spun off dirty lubricating oil?  Which may have gone hard.


regards
Roy
Title: Re: props vacuum
Post by: frogman3 on November 09, 2019, 08:58:05 AM
Hi, was the black mark on top of the paint or in the paint, i.e. gone through the paint?  Any photos?  Was it there for both of the props and how long was the black line?

Is it possibly spun off dirty lubricating oil?  Which may have gone hard.


regards
Roy

HI Roy  well no photo's as it was yrs ago an the line if I remember rightly was about  half inch long an when I ran my finger over it seems like a indentation  but thats when the eldly gent came over an stared talkin about it an he was havin a good old chat for a fair while an I dint think about this anymore until a little while ago as im now fitting my props on my tiger an it jogged my memory about it but thinkin now you may be right about oil gone hard  on the hull   so it may have been that many thanks for your reply
chris
Title: Re: props vacuum
Post by: malcolmfrary on November 09, 2019, 09:06:27 PM
Fine silt thrown out over time abraiding the paint?
Title: Re: props vacuum
Post by: Umi_Ryuzuki on November 09, 2019, 09:26:19 PM
Seems more likely that the stuffing tube grease is seeping out onto the shaft and
propeller hub and being thrown onto the hull after the run.
Title: Re: props vacuum
Post by: frogman3 on November 09, 2019, 10:11:58 PM

Many thanks for your reply's guys but a mystery but im now concentrateing on my props fittin's of tiger an lay this to rest
chris
Title: Re: props vacuum
Post by: dodes on November 18, 2019, 09:02:09 PM
Think you will find it is capitation, where there is insufficient flow of water to the prop. The blade starts thrashing the water and it breaks down to its component parts and makes steam. The 42,s on test trials as new build went full astern from full ahead and destroyed the blades, never done again. At Haslar they tried to redesign torpedo props in sealed tanks and you could sea bubbles being formed.
Title: Re: props vacuum
Post by: frogman3 on November 19, 2019, 09:41:33 AM
Think you will find it is capitation, where there is insufficient flow of water to the prop. The blade starts thrashing the water and it breaks down to its component parts and makes steam. The 42,s on test trials as new build went full astern from full ahead and destroyed the blades, never done again. At Haslar they tried to redesign torpedo props in sealed tanks and you could sea bubbles being formed.



HI Dodes so there is heat  at the prop tips to make steam   an don't you mean cavitaion ? so this vacuam  an heat as I found on my ship might not just be hot air after all then ?
chris
Title: Re: props vacuum
Post by: NickelBelter on November 20, 2019, 01:11:57 AM
I doubt that a model, especially a model of this type, is going to produce enough energy through cavitation to damage the hull paint like that.  I have gone icebreaking with one of my models as well as beached it for recovery several times and haven't scuffed the spraypaint on the hull. 
Title: Re: props vacuum
Post by: Andyn on November 20, 2019, 01:37:44 AM
Not a cat in hell's chance... %)
Title: Re: props vacuum
Post by: frogman3 on November 20, 2019, 07:09:05 AM

Well guys it looks like there is a differecnt of opinion an lets leave it at that  ;) :-)) <*<
chris
Title: Re: props vacuum
Post by: rnli12 on November 20, 2019, 07:14:50 AM
Always worthy of a different peoples views, may I suggest you may have hydrodynamic imbalance on your shafts due to the potential clearance issues making your props ineffective with prop wash and cavitation.

I would try a smaller prop and blade combination 3/4 first saves ripping the model apart

 
Title: Re: props vacuum
Post by: frogman3 on November 20, 2019, 07:24:46 AM
Always worthy of a different peoples views, may I suggest you may have hydrodynamic imbalance on your shafts due to the potential clearance issues making your props ineffective with prop wash and cavitation.

I would try a smaller prop and blade combination 3/4 first saves ripping the model apart



HI Rich not a problem now as this was many yrs ago
chris