Two Bladed or three bladed prop on Long Keeler?

[Roach1948]

Having replaced the old Stuart 4hp with a new Beta 10, I am having doubts over 3 bladed the prop I ordered with all the new engine ancillaries. Basically ordered without thought as I previously had a three bladed Stuart prop. Its 11 inches across to fit a cavity around 13 inches in diameter. The cavity is quite small - but she is a small yacht.

The question is whether I should consider a two-bladed in such a small cavity and will this markedly improve my sailing performance (by marking the shaft so that I can put the engine in gear when the prop is vertical thereby "shading" the prop in the cavity) ?

As I have not used my new three bladed I am sure I can exchange it, but am worried that maybe I will loose out on cruising speed and manoeuvrability in astern. Any of you gurus have had experience of a two bladed prop performance compared against a three bladed on a longe keeler?

 

[cliffordpope]

There's another thread somewhere here on the subject. If I knew how I'd add a link.
This is a controversial subject with conflicting theories.
The conventional one says two aligned blades will reduce drag when sailing. Another study says it's not the prop that causes the drag it's the cavity it turns in. So it doesn't matter how many blades you have. Even removing the prop altogether wouldn't improve matters.
Another theory says locking the shaft itself introduces drag, and it's better to let it turn, so again, the number of blades isn't a factor.
So there you have it. When I fitted a new Yanmar recently the engineer at the yard did his calculations and recommended an appropriate three-bladed screw. He said two-bladed ones were less efficient and the drag effect was negligible. That's with a longer keel than yours looks - plumb stem and keel running straight back from forefoot to rudder support bracket.

 

[Salty John]

I went from two to three bladed on a long keel boat in order to better utilise the engine power available. I had expected a 5 - 10% reduction in sailing performance but actually didn't notice any effect. Mind you we motored in very light winds anyway. Two blades theoretically gives you less vibration.

 

[Roach1948]

I have done a huge archive search and found it hard finding anything that did not relate to Mobo props coming up - I always try and be specific on my thread titles for that reason.

As I have a good cut-away forefoot she is not a slow yacht for a classic - and sailing performance is the biggest priority for me as in the end she will be on a River mooring and I wont need to get into Marina's too often.

I suppose the best thing would be to experiment with the two types of props, but I was flabergasted at the price of a new props and sadly this is not an option - so looking to get it right before I launch.

 

[Salty John]

I meant to say, of course, that three blades is theoretically less likely to vibrate than two blades. A slip of the digits.

 

[Mirelle]

I swapped a three bladed prop for a two bladed one on "Mytica", retaining the Stuart P5M. Frankly, though I did my best to convince myself that she sailed better, I am not sure that she did. Two years later I took the engine out and closed up the aperture and she sailed very much better - but that is a drastic remedy!

 

[Roach1948]

Interesting - maybe I should have done that and used an outboard as opposed to re-engining as it has taken most of my time/cost on this restoration. Too late now......I have a hole in my rudder which needs filling with a prop!

Could this be why it is often advised not to centralise offset proped yachts - opening an aperture must play significantly in downgrading sailing performance?

Anybody heard of a temporary aperture plug for more sporty days?

 

[kds]

I am afraid that the real answer is even more expensive - a self-feathering prop. My 3 bladed Darglow is about the same size as yours at about £650, but it is effective under power and sail.
Ken

 

[Transcur]

I have little doubt that a three blade prop has significantly more drag than a two bladed prop. Keeping it in line with the deadwood would help . As kds said a folding /feathering prop would be even better. Before you commit yourself to much, try to get a cheap old 2 blade prop to try out. Of course the 2 bladed prop will not have the same bite as one with three blades, but then its a sailing boat isnt it. What do you realy want sailing performance or motoring?

I know of several people who have gone racing and removed their 3 blade prop and 'filled in the hole ' with ply or similar filler. Seem to make a huge difference.

When we got rid our two two bladed fixed props and fitted folding props the difference was dramatic. At least 1/2 to 3/4 knot faster.

 

[DanTribe]

I upgraded the engine on my Stella from a Stuart 5hp to a Yanmar 9hp. The engineer recomended a 3 blade prop, but as we wanted to race we opted for a 2 blader.
The engineer got huffy and said it wouldnt' work properly, would vibrate horribly, would give no drive astern and the chickens would stop laying etc.
In fact it worked excellently and we could set it vertically behind the dead wood to reduce drag.
A sailing boat is for SAILING, that should be your priority I reckon.

 

[oldharry]

[ QUOTE ]
A sailing boat is for SAILING, that should be your priority I reckon.

[/ QUOTE ] ... and the engine is for when you can not sail - like locking in and out of the Marina, having to get back ashore for work commitments, have to cross shipping lanes safely with adverse or no wind, etc etc etc. So should be given the same level of priority as the rest of the gear.

Engineless sailing is simply not a realistic option for most of us - not because we cant handle the boat undr sail, but simply there are too many situations in todays crowded waters where an engine is the only safe option.

It would be interesting to know whether the 3 blader your engineer recommended would have made any difference either way!

And the chickens would have stopped laying anyway..... if I know anythng about them/forums/images/graemlins/smirk.gif

 

[DanTribe]

Quite right, an engine is a necessity these days for the uses you mention, I'm not advocating removing the engine completely. However, the possibility of a slight loss of performance under power should not take precedence over the greater loss of sailing ability.
Stellas are raced very competitively as one designs and class rules stated minimum engine weights and propellor sizes.
I can remember at least two which fitted 3 blade propellors as recomended by an East coast engineer. Their performance suffered horribly. From being in with the leaders, they always trailed in miles behind. One changed back to a two bladed prop and could not detect any adverse effect under power, but sailing performance was back to Stella magic.

 

[Roach1948]

I presume this folding prop is in the aperture in the keel? I am not sure a folding prop can even fit in my hole, as it is slightly eliptical so the baldes might hit the rudder when folded. Anyway, the cost seems massive and I really can justify it with all the other priorities I need to get (new mainsail).

I am getting the feeling that maybe I should exchange my present 3 blader prop for a two blader (at no cost). After all I have more than double the horsepower than before and to be frank anything would be an improvement on the Stuart.

As for manouverability, I dont think she will ever be an angel in a marina as she is a long keeler after all. I will be river moored so that helps.

 

[Transcur]

I have two props one on each quarter in open water. They are not particularly large considering the weight of the old girl, 11 Tons+ deadweight. 16.6x 13 if I remember correctly. Not really deep enough but that is a problem to do with hull form. We only really use the engine when we have to. Pushing into a force 6 under motor is unacceptable to us, we would rather sail for comfort. Brilliant when the wind dies and we need to get home from wherever.

 

[Roach1948]

The good thing is that I can return the prop and get a two bladed which is what I will do.

Whilst I was at it I asked about a feathering prop and the engineer said he knew of no feathering props under 11 inches diameter - so it looks like that was never going to an option for me unless I opened up my aperture some more which I am reluctant to do.

 

[Mirelle]

On the other hand, in Mirelle we are seriously under-engined with 15 ponypower and a single 15 x 9 on 9 tons displacement. Pushing into an F2-3 is about the practical limit and marinas are seriously very scary places, which are never entered if the wind is blowing!

We plan to pick Transcur's brains over the winter and work out how to copy her system!

 

[Transcur]

Give me a ring some time Andrew, we can talk about the set up in detail. I am sure that I could cut the machinery cost quite considerably. Sorry that I didn't get to go for the sail at the end of August when you were at Stone Point, it was a bit breezy that day.

 

[Mirelle]

Thanks very much, Peter

I was a bit pre-occupied, that day, as I had just spotted two 11 year olds, our own Alex and his best friend Josh ("Clytie") rowing our tender (they had tried to sail round Horsey Island, but the rudder broke) down Hamford Water without appreciating the force of the ebb and the strength of the offshore wind! Ruby and I tore off after them in the Seagull-powered Avon but to their credit they just managed to get ashore on the tip of Stone Point (somewhere a little to seaward of the "No Sailing Beyond this Line" line!) under their own steam, so they were not actually rescued, but were very glad of the tow back!

Needless to say it only occurred to me afterwards that I had gone tearing off without a VHF or flares and wholly dependent on a 34 year old outboard (but it is a Seagull!). /forums/images/graemlins/wink.gif

 

[Roach1948]

OK, there is more to propellors than I had initially thought.

Has anybody had experience of two bladers that automatically lock in the vertical position? I would be intrigued to see how well this these props actually perform both when being used and being auto-vertical

 

[Roach1948]

Sorry Tanscur - I did not realise you had an unusual system hence assuming the x2 prop was an error until Mirelle enlightened. I presume you don't have a hole in the rudder then and the 1/2 - 3/4 knot increase is without a hole?