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Four over three, and "Braced for ramming"

MauriceMynah

By MauriceMynah
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MauriceMynah

MauriceMynah

Posted
Posted

I'm curious,. firstly...

What are the advantages of having a four bladed prop over a three bladed one?

How much will one cost and how much to fit?

And secondly...

I've noticed many hire craft have a metal reinforcement on the bow.

How much to have one of those fitted to my Elysian Bounty, and where could I get one? (I make mistakes too you know)

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ExMemberBobdog

ExMemberBobdog

Posted
Posted

Some years ago I hired one of Mr. Buttifant's beautiful yachts for a week.  I was moved to point out to the skipper of a small dayboat, who cut across my course while I was under full sail on Hickling Broad, that the bowsprit was a mighty weapon that would probably go right through his craft. He changed course PDQ.  A friend, sailing beside me, took the photo.

post-14321-0-25077900-1411057513_thumb.j

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650xs

650xs

Posted
Posted

three blades catch three lots orf water .......!!!!

 

 

four blades catch four lot s orf water .......!!!!!!!!!

 

 

but be warned not as simple as thta talk to a prop expert not an ex ...pert 

 

 

but be carfull 

 

 

u can buy of the shelf .....................................

 

 

check yer taper on yer shaft etc  clearence from skeg and bottom orf the ole boot etc etc 

 

 

and of coarse get the correct " hand" ...........................l/h................................r/h..............

 

 

not that  dear .............................

 

 

 

search the net ........................???? there are lots of prop manufactures etc 

 

 

 

talk to ...............................markwell ..peachmants etc  local ............go to sillet sonic ...etc etc 

 

 

 

ask some people who have same hull etc as yer boot and see what they got on her with asame type of engine etc 

 

 

or try a decent correct pitch prop for yer boat secound hand good one with good "ring to it "

 

 

 

talk to me i have some props to try ......migth fitt 

 

 

 

 

but there is more technical and indepth for whole prop thing ..........................look at turbina ,s props all that time agoo............ummmmmmmmmmmm

 

 

horse for coarse,s

 

 

 

i n reckon a nice five blade surface peirce/ cleaver  job for me ........................with 850 hp ................!!!!!!!

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Jonathan

Jonathan

Posted
Posted

Maurice,

I know nothing about bow protectors, but propeller blades ….

More blades is not necessarily a good thing!  Propellers are designed to be efficient in transferring engine thrust to the water and so moving the craft forward (and therefore the water backwards).  A particular design will have a pitch and curvature optimised for a particular speed, this is similar to gears in a car, some are better for dragging a heavy weight up a hill at low speed, others for cruising fast on a motorway, in both cases the engine rpm is within a similar range.  But you are asking about the number of blades, not its’ pitch; obviously, the more surface area a propeller has the more water it can “push against”, and so the more power it can transfer.  You can gain area by increasing the diameter or adding blades.  But consider the following:

You are travelling at 4kts using 1000 rpm.  4kts is 4 x 6080 feet per hour or (4 x 6080)/60 feet per minute or (4 x 6080)/( 60 x 60) feet per second which is about 6.75 feet, or 80 inches per second.

1000 rpm is 1000/60 rev’s per second which is about 17.  That means that in one revolution of the drive shaft the propeller had moved about 5 inches forwards (80/17).  I can tell that you are already bored and wondering where this is going, so the point is:  If your propeller had just one blade then each time it reaches the same point in the rotation (let’s say vertically up (or down)), then it is meeting water that is 5 inches away from where it last disturbed, and pushed-on, the water.  If you have a 2-bladed propeller each blade will cut through the water every 2.5 inches, 4-blades every 1.25 inches etc.  The problem is that the passage of the first blade has “disturbed” the water, so a following blade is not so effective, and the closer it follows the less effective it is.  From the efficiency point of view a single blade would be best … but you might notice the vibration!

Consider how the spitfire (wartime aeroplane) design evolved from 2 to 3, 4 and 5-bladed propellers as power and speeds increased over a few years.

Sorry if this has been a bit of a lecture, but I think the bottom line is that for cruising gently on the broads it probably makes no difference!

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Jonathan

Jonathan

Posted
Posted

Hello Alan, Tan, Iain, Maurice and everyone else!  I've been reading this forum for a few years, "lurking" in the background, and we have been on several holidays on the broads in that time, so I have found several comments and suggestions helpful.  I don't consider myself to be an expert on anything ... particularly things that float ... but having recently joined the forum I thought I might be able to help someone else; hence the above.

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BroadAmbition

BroadAmbition

Posted
Posted

We had the similar situation with 'B.A' she originally had 3 x blades which was fine with 34bHP from the trusty perky 4108.  Installing a 50HP beta meant we would not be transmitting that power efficiently to the water.  We could not get a bigger diameter prop in as there just isn't room, increasing pitch - we would have gone way past the sweet spot and had to much pitch or far too coarse.  The Beta will rev to 2'800rpm max  sticking with the three blade prop would have meant getting to 2'800rpm very quickly and being 'Under Propped'  Too many blades and/or pitch would have meant never reaching anywhere near 2'800rpm and having the Beta labouring constantly and being 'Over Propped'

 

The solution was to enlist one that knows and is well versed in the black art of prop/size/pitch in relation to engine/gearbox type, hull size/profile.

 

Jason at LBBy was our chosen expert.  He opted for the four blades and pitch to suit.  This resulted in our Beta reaching 2'600rpm and being just slightly 'Over Propped'. He got it absolutely spot on first time - He likes to put this down to his expertise / knowledge - I just think he's a jammy git

 

Since the Beta is now fully run in (Nearly 2000 x Hrs) and since then I have treated the engine and gearbox with 'ZX1' we are now achieving 2'700rpm, burning less diesel and being even quieter/smoother (I did not think that was possible but was happily proved wrong - that's not a first I can tell you)

 

Hope this helps,

 

 

Griff

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BuffaloBill

BuffaloBill

Posted
Posted

Hi Griff.

What's with this ZX1?

When we bought our car new, it used to slow so quickly

when you took your foot off the accelerator that it felt like

you'd put the brakes on, but after about 7000mls and

was run-in, we got more miles per gallon than when the

engine was still tight from new, so Griff's Beta is just a

good example of that running process.

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Timbo

Timbo

Posted
Posted

Jonathan, Welcome and thanks for the info, I'll have a word with Jason when I'm up for a while re a bigger/better screw, and a word at Ricko's for the bow. Get some numbers that sort of thing.

 

Thanks all for the input.

MM....now that one is wayyyy to easy! :naughty:

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