Prop resize

[cindersailor]

A success story I thought I would share for those dealing with prop selection issues.

Despite the availability of various prop calculators and advise from manufacturers matching your prop to the engine/boat/gearbox is still a tricky business, especially if, like me you are trying to do everything on a shoestring which means making do with 2nd hand, and you cannot therefore hold the manufacturer responsible for recommending an incorrect size. When I installed a Yanmar 1GM (6.5 hp, 2.21:1 gearbox) into my Cinder 22 (2 tons, long keel, 17 ft waterline) last winter Castle Marine's calculator suggested a 10x8 prop. After some searching I found a good 3-blade one for £15, although I was a little concerned that the the balde area was rather small (quite narrow blades). Sure enough, the engine would rev easily to 3400 (max for this engine) even at the pontoon, and only reach 5 knots in flat water. And sea or headwind gave real problems with cavitation seriously reducing speed. Having had the shaft machiened to suit the taper of the prop I was now prepared for a long search for a bigger prop with the same taper. Eventually I found a 10x10 (3-blade) on e-bay and again paid £15, this after having to reject several LH ones which would have been otherwise ideal. I bacame quite expert in explaining to bric-a-brack dealers how to determine whether they were selling a left or right handed prop! This one had more conventionally shaped, broader blades. Having just launched this weekend the difference is amazing and I am a happy boy! Tied up to the pontoon the engine will rev to 3000 and further throtle just gives black smoke. On the water it will just reach 3400 without black smoke at which point the log is showing 5.8 kts, which is just about hull speed. But more importantly the prop has a lot more "bite" in the water and deals with waves in a much better style. Amazing what a 22 yr old 6.5 hp can do with the right prop.
Looking forward to a less frustrating season, although I will be sailing everywhere of course! /forums/images/graemlins/smile.gif

 

[steve28]

good to hear you got it sorted, a friend of mine has a triple keel boat with a 19 foot waterline and the exact same engine/gearbox but he is running a 15x10 prop, he can acheive 6.3 knots sog.

 

[Das_Boot]

I am putting in a new engine going from 36 to 50hp I have also done the calculation on Castle marines prop calculator. If I could find a second hand one I might settle for trial and error however if I cant I will have to spend more than £300 for a new one. Is is realy that difficult to get the right prop first time.
Does anyone know of how best to get it right first time taking into account cindersailors experience with the prop calculator.

 

[cindersailor]

I did not mean to suggest that the prop calculator was misleading, my problem was mainly that the blade area on the 1st prop was too low, a factor I had not taken sufficiently seriously. A 10x8 as suggested by the calculator with the correct blade area as for my 10x10 would probably also be fine.

 

[cliff]

[ QUOTE ]
Does anyone know of how best to get it right first time taking into account cindersailors experience with the prop calculator.

[/ QUOTE ]Find someone with the same boat, engine and gearbox/reduction ratio and ask them what size and type of prop they have and if they have any problems with revs or speed.
----------

 

[snowleopard]

not the answer fro those on a tight budget but i went to CJR props and took their recommendation. because of a communication problem i got the wrong one but they changed it without query and now it's fine.

 

[Das_Boot]

Thanks guys I can see this is a very difficult area. When you do the calc on Castle it does not mention blade size I thought this was a function of pitch. I also dont think the prop calc is misleading just doing the best of a difficult job.
If you look at the post of steve28 he says with exact parameters as you(cindersailor) his mate uses a 15 X 10 and achieves a better outcome. Yet you say the original calc was for a 10 X 8 prop.
Can you see why the mind boggles.
How am I going to achieve the correct prop first time I wonder if it is even possible.

 

[cindersailor]

Given my experience this is interesting, unless it is a much lighter/more easily driven hull than mine I would be surprised if he is getting anywhere near to max revs which means that he is not getting using the full 6.5 hp.

 

[reeac]

I seem to recall that the prop. section of the Vetus catalogue mentions a parameter which refers to total blade area but I can't remember the details. Nevertheless, Cindersailor has hit upon an interesting phenomenon as blade area doesn't feature any prop. calculation software that I've seen. Maybe there is more serious software out there for professional designers to use.

 

[cindersailor]

As I recall the calculator assumes a sandard DAR (disc area ratio) of about 50% I think. If this parameter could be varied a more accurate prediction should be possible. However, for 2nd hand props the DAR is an unknown quantity.

 

[Das_Boot]

I would hope so (software for pro's) you could not use trial and error on the QE2.
I think I will go with the standard vetus size that the prop calc throws up. Perhaps their experience allows for blade size in the prop. I might not achieve the best possible outcome but would probably spend £1000000 getting exactly the right one.
I just dont want to be that far out that it bugs me everytime I switch the engine on.
I was just going to trust the prop calc but now I will have to run it through a couple of different ones.
Nothing is easy.

 

[supermalc]

I too have had an interesting 'learning curve' regarding props. Story is elsewhere but I'll repeat.

Just over a year ago I got a 20ft Mahogany clinker for use on the river with maybe an occasional fishing trip into the Wash.

It came with a RH prop on a Yanmar YSE12, which was wrong handed. It was underpropped only reaching displacement speed when flat out, causing black smoke. It had hardly been run since the engine had been fitted because after only 10 hours the gearbox whine had become a scream, and it would soon have wrecked it.

The original prop was a 14 x 8, and the reduction in reverse meant a slight gain would probably enable the same in forward, but maybe not enough.

I eventually bought a 15 x 14 prop, which had thin blades when it arrived, but after fitting the boat did displacement speed at tickover, with almost no rev gain on the engine.

My luck changed and I got a 15 x 10 newly referbished from a boat jumble for £30, although I now had quite a stock, having bought one with the wrong taper, and one too small.

In my experience the size of the blade made no difference to the

This prop was still a little large yet enabled me to do 600 odd river miles last year. The engine laboured a little, and had to be run almost as fast as possibe to achieve some smoothness....approx 1500 of it's 3000 revs.

I've cut the prop to approx 14 inches, and made a new shaft, balancing the whole assembly. I'm very happy to say the engine now revs to plus 2000 revs, and will happily propel the boat along at displacement speed without the engine racing. A little more speed is available, although this does cause the engine to smoke. It also has very good stopping power in reverse.....something far more necessary on rivers than seas.

These were all 3 blade bronse props btw.

I've sold my spare props, with just a small loss due to postage, but have ended up with the correct prop, and a spare for around £30.

 

[Superflid]

[ QUOTE ]
I would hope so (software for pro's) you could not use trial and error on the QE2.

[/ QUOTE ]

I remember reading somewhere (maybe wrongly...) that they got the QE2 props wrong first time!

 

[Das_Boot]

If that is the case I am buggered where can I go to if the pro's get it wrong. I will just order the prop not be happy try another one and another untill I get the right one. Hope there is a market out there for slightly used props.

 

[davidbains]

It is a black art . I was told to fit a 13 by 10 full two blader to a Yanmar 2GM20 on
a 39ft multi which should be easily driven. However it was way too big and the
engine laboured. I reduced to 12by8 skinny two blader which allowed the enginer to
rev but then we had not enough power against wind and wave. We do a lot of motorsailing!
I am now seriously considering changing the gearbox from 2.6/1 to 3.2/1 so
I can fit the larger prop again!!
I realise now I should have fitted a 3GM30 in the first place but did not have the
experience to overide advice.

 

[Das_Boot]

It seems every decision that needs to be made is fraught with crap. I am going to have to stop beeing lazy and do some serious research.

 

[Shantyman]

There is a prop calculator on boatdiesel.com which I think is a bit more refined than the Propcalc one.

It's an American website for Members and you have to pay about12 quid to join but I reckon it's worth it.

They also have calcs for engine power and shaft size.

Regards,

Shantyman.