propeller to skeg clearance

[Kieron G]

Hi, I have a 17 inch prop fitted to my 27hp 24ft boat, i'm fairly sure it doesn't have the pitch stamped on it ( hasn't been out of the water for over a year ). The thing is on river sections i feel like i'm stuck in 3rd gear and to get any speed i have to work the engine. I've noticed an 18x13 prop on ebay which i think will fit my 25mm shaft ( not sure about the 1 in 20 taper until i get the boat out ). I've measured up for 18 inches before and i know it will fit , But with only maybe 1/4 inch clearance to the skeg. Could those with ' the knowledge ' advise as to whether this would / could cause problems. Many thanks, Kieron.

[Heffalump]

IIRC you need at least an inch or so clearance to remove the chance of nasty cavitation.

 

Could be wrong though, will try to find a citation

"There should be between 1" & 2" clearance between the tip of any propeller blade and the nearest obstruction. Much less than this is likely to cause cavitation.

1" on the pitch is worth about 2" on the diameter – only any use if you are trying to solve propeller matching or cavitation problems."

 

From here http://www.tb-training.co.uk/props.htm#BMn8 a member of this forum

[Kieron G]

Ok, thanks. So the alternative would be to find out what the pitch is now and get it increased. I think i've read on this forum that there is a company in Hull who can do this called 'Crowthers'? . Have now had a proper read of the link posted and interpret that it means i should be able to reach the designed hull speed without over stressing the engine ( i think ).

Edited August 24, 2015 by Fitter kieron

[Dalslandia]

17" should have 0.7" min clearance to skeg

18" min 0.72"

so if you have 0.25" on 18" you will have 0,75" on 17"

even an od 17.5" D will be to big

 

Here is some evening reading

http://www.fao.org/docrep/x0487e/x0487e04.htm

Edited August 24, 2015 by Dalslandia

[Kieron G]

Thanks , kettle's on i can see this is going to take me some time .

[Flyboy]

Hi, I have a 17 inch prop fitted to my 27hp 24ft boat, i'm fairly sure it doesn't have the pitch stamped on it ( hasn't been out of the water for over a year ). The thing is on river sections i feel like i'm stuck in 3rd gear and to get any speed i have to work the engine. I've noticed an 18x13 prop on ebay which i think will fit my 25mm shaft ( not sure about the 1 in 20 taper until i get the boat out ). I've measured up for 18 inches before and i know it will fit , But with only maybe 1/4 inch clearance to the skeg. Could those with ' the knowledge ' advise as to whether this would / could cause problems. Many thanks, Kieron.

It sounds like you have a 3:1 ratio gearbox. I have a 58Ft. Nb. with 17"x 13" prop. & 2:1 gearbox which performs really well. 1/4" skeg clearance is not enough, you need about 1" min.

[Kieron G]

The gearbox is 2:1 ratio and with the shape of the hull and boat 3mm steel weighs i guess 3 tonnes . I get good response from the engine and can stop on the spot. I think i could just do with a bit more push from the propeller.

[Kieron G]

Thanks again for links posted Heffalump & Dalslandia, have learnt a lot from reading them.

[Murflynn]

The gearbox is 2:1 ratio and with the shape of the hull and boat 3mm steel weighs i guess 3 tonnes . I get good response from the engine and can stop on the spot. I think i could just do with a bit more push from the propeller.

if your boat will 'stop on the spot' then you have plenty of 'push'. But clearly the push runs out because you can't turn the prop fast enough at speed. Definitely need to increase the pitch, not the diameter.

[Kieron G]

Right ok , i'm due to slip the boat out next month. Once i know what i've got now i'll take it from there. Does anyone know if 13 inch is the maximum pitch for a 17 inch dia prop of this type ?

[Murflynn]

Right ok , i'm due to slip the boat out next month. Once i know what i've got now i'll take it from there. Does anyone know if 13 inch is the maximum pitch for a 17 inch dia prop of this type ?

you can get any pitch you want - a racing boat might have 30" pitch.

ask http://www.tnorrismarine.co.uk/product/standard-range-propellers/

these people can increase the pitch of your prop by 2" http://www.castlemarine.co.uk/propeller-repair.php

[zenataomm]

Messing about with props can be recipe for disaster, or at least nightmares.

Even the builders and designers of QEII took several runs at it before getting it right'ish.

 

I was told if you're cavitating on the cut the whole set up is other wrong or you're trying to do something wrong in your style of boating.

The biggest issue I understood regarding clearance was getting a discarded bit of wood jammed between blade tips and skeg or uxter plate. A well respected engineer from the Uxbridge area told me 2" was the most common thickness of wood. His suggestion was any clearance less than that could result in a lot of bearings and cast casings being traumatised by the sudden jamming of the drive train.

 

I once over pitched by an inch after the experts said it'd be fine. HA!

[Murflynn]

 

The biggest issue I understood regarding clearance was getting a discarded bit of wood jammed between blade tips and skeg or uxter plate. A well respected engineer from the Uxbridge area told me 2" was the most common thickness of wood. His suggestion was any clearance less than that could result in a lot of bearings and cast casings being traumatised by the sudden jamming of the drive train.

 

 

agree. I don't understand how a clearance of 0.7" could possibly be recommended for any boat, let alone an inland waterways boat.

[Dalslandia]

0.7" is 4% of the diameter, so teoretical the minimum, to keel or skeg, but 17% at top to hull/ uxterplate, thats 2,89" it is also recommendet not to be less then 2", so with small diamters it is obviusly not practical.

Edited August 25, 2015 by Dalslandia

[DHutch]

I also agree the 1/4 seems tight, 2-3 would be my aim although less appears to work.

 

If you feel the pitch is too small but the prop is sound then getting it re pitched could well be your best option.

 

Daniel

[magnetman]

I thought recommended clearance was 10% of prop diameter.

Edited August 25, 2015 by magnetman

[Sea Dog]

I thought recommended clearance was 10% of prop diameter.

Not sure how it works out as a percentage, but I thought the recommended clearance was a mattress plus half an inch.

[Dalslandia]

I thought recommended clearance was 10% of prop diameter.

seen, 17% on 3 blade, 15% on 4 blade and 13% on 5 blade

^{1 These clearances are closely associated with the number of blades and can be estimated by ¬ = 0.23 - (0.02 x n), (Vertical)}

^{and ® = 0.33 - (0.02 x n) where n = the number of blades on the propeller. (horizontal to deadwood)}

Also seen 20% 15% 10%

if it make noise it is to little

http://www.fao.org/docrep/x0487e/x0487e04.htm

Interesting that a 50% bigger propeller made a 30% improvement in efficiency, (well obviously not a canal boat) on a boat where it was possible to install a bigger prop and different gear ratio.

Edited August 25, 2015 by Dalslandia

[Kieron G]

Ok, getting the existing prop repitched it is . Thanks for input everyone & links, has saved me wasting £230 on something that wouldn't have worked. Re the clearances on Dalslandia's link , i think that it has been written with sea going fishing boats in mind ( if that makes a difference ).

[by'eck]

IIRC you need at least an inch or so clearance to remove the chance of nasty cavitation.

 

Could be wrong though, will try to find a citation

"There should be between 1" & 2" clearance between the tip of any propeller blade and the nearest obstruction. Much less than this is likely to cause cavitation.

1" on the pitch is worth about 2" on the diameter – only any use if you are trying to solve propeller matching or cavitation problems."

 

From here http://www.tb-training.co.uk/props.htm#BMn8 a member of this forum

 

I understood log jam was an issue as well with clearance below what you suggest.

[Dalslandia]

Ok, getting the existing prop repitched it is . Thanks for input everyone & links, has saved me wasting £230 on something that wouldn't have worked. Re the clearances on Dalslandia's link , i think that it has been written with sea going fishing boats in mind ( if that makes a difference ).

 

probably so, the minimum 2 and 1 inch always apply. nothing say it can't me more. except the propeller get to small.

 

Had a 20 feet long, 1-2 feet diameter log jammed above prop shaft, between propeller and deadwood (dead steel?) the 27% didn't take that log in count.

 

had to feed it out with propeller, it halved the diameter on one side.

[blackrose]

Messing about with props can be recipe for disaster, or at least nightmares.

Even the builders and designers of QEII took several runs at it before getting it right'ish.

 

I was told if you're cavitating on the cut the whole set up is other wrong or you're trying to do something wrong in your style of boating.

The biggest issue I understood regarding clearance was getting a discarded bit of wood jammed between blade tips and skeg or uxter plate. A well respected engineer from the Uxbridge area told me 2" was the most common thickness of wood. His suggestion was any clearance less than that could result in a lot of bearings and cast casings being traumatised by the sudden jamming of the drive train.

 

 

Surely the most common thickness of wood is whatever thickness of tree branch that happens to have fallen into the water? I've seen huge floating tree trunks on the Thames.

 

As for messing about with props, if you do your homework then repitching or matching a prop with an engine and gearbox will only improve things. It did in my case anyway.

Edited August 28, 2015 by blackrose

[Murflynn]

well I suppose if you have slightly less than 2" clearance, when something gets stuck it is likely to be about 2" thick, and many narrowboats have about that amount of clearance.

 

so what the 'well respected engineer' should have said is that 2" is the most common thickness of wood that we find stuck in boats' propeller apertures.

 

 

 

 

 

 

 

 

............. I could go on about the definition of 'engineer' - I don't think any professionally registered engineers have ever done a study on this matter, so I will assume that zena's advisor was an experienced boat mechanic.

 

 

 

Murflynn BSc., CEng, MICE.

Edited August 29, 2015 by Murflynn

[zenataomm]

A name I imagine most of us have heard of and I imagine RLWP is having great fun opening all of the packages he's bought from him. (If I've understood his situation) ..... apologies if I haven't.

I couldn't imagine the value of describing the average branch diameter, I've always found nature to be wonderfully irregular and unpredictable in such matters.

 

His actual quote (if it really matters) was that in industrial areas the most common piece of floating wood tends to be 2X3" and as it's unlikley to float edge down his suggestion was 2" should be considered the minimum, however, probably bigger the gap the better.

[Tony Dunkley]

Another way of looking at the blade tip to skeg clearance question, leaving aside considerations about noise and vibration, is that a gap as small as possible, down to a quarter or half an inch is the least likely to trap bits of wood with any real strength in them, and therefore less chance of doing any damage.

Before anyone dismisses this as just a theory, I'll say right now it isn't, and it was successfully put into practice over a great many years on working boats of different types.

It wasn't done to reduce the chances of damage, but merely in order to swing the biggest possible propeller diameter, which is one of the reasons why working boats out performed most of today's pleasure craft, with around half of the horsepower that is now thought necessary, propelling a small barge or a pair of narrowboats of around three times the tonnage (displacement).

Edited August 29, 2015 by Tony Dunkley