Props again but a different question maybe

[ShipsWoofy]

I need to re-prop. I have calculated they should be 13" x 10" 3 blade.

But what if I was to choose 2 blades in an effort to reduce drag?

Would the sizes remain the same?

Would I 'really' notice any improvement?

Background info: Heavenly Twins catamaran, generally overweight. Two people with a probability of long term live aboard cruising in the Med, so engines might be used more in future. 1GM10 x 2. Would like not to loose any thrust, using reverse essential as high windage working of both engines when manoeuvring in tight spots.

All answers appreciated, I have no experience of 2 blade props. I also cannot afford folding props, though they would be nice!

 

[jleaworthy]

Went through this exercise. Slight improvement in sailing performance BUT a considerable increase in vibration particularly through rudder. I changed back. Expensive lesson.

 

[Rohorn]

Hi.....I have a Dean cat which I keep in the Aegean. Bit bigger than H.Twins, 10 metres by 5.5 metres, weight around 5.5 tons. Had 1GM10/sd20 saildrive units for first 12 years, two bladed alli props from sillette. Seawater cooling. I think the diametre was 13 and pitch 11 but really not sure. Ran like little sewing machines, soporific even, but always underpowered into wind. Everyone said how quiet they were compared to other boats. I think the very flexible suspension was the reason. Consumption about one to one and a half litres per hour each. Had the usual impeller and exhaust elbow problems. Max 5.5 knots flat sea. Last year one hull hit a rock and water came in the starboard diesel air intake at cruising revs, ....in brief, this year (2004) I put in new 2gm20f/sd20s but took the opportunity to open up the old 1gms to see how they had faired, after about 2500 hours. The cooling water ways were almost completely silted up, but otherwise the motors were still functioning well as ever, but apparently major work would have soon been required. Greek Yanmar agents don't sell 1GMs because of this and low power in local conditions, though they get spares in 24 hours.
I'd love to have folding props on my 2gms, 'cos now the bigger two bladed bog-standard props have taken about 1.5 knots off my best sailing speed. It's not the extra weight...the Dean is a fantastic load carrier and it's only 50 kg or so more anyway. Now I've all the power I need against the weather, and indirect cooling. Again very smooth.
On your 1GMs, I think the three blade option will produce a lot more drag under sail. I don't think you'll notice much difference in vibration...they're already very smooth. The motors aren't really powerful enough to profit from the expense of fitting three bladed props in my opinion, especially if your sailing suffers.
I looked up that Kiwi feathering prop outfit mentioned a few posts back....looks very competent....but still too expensive for me to justify!
Hope these thoughts help......cheers....R

 

[Ships_Cat]

Hi Woofy One

My personal view is that for cruising where one may need to power into wind to meet tide gates, power against tides in passes, etc or just get to a destination within a set time frame, then 3 blades, not two, is the way to go.

Obviously if just want to power in and out of marinas anything goes but as far as I am aware, the best regarded propellor manufacturer out here does not even build 2 bladed sail boat props and maybe there is a message in that.

We went for a narrow bladed fixed 3 bladed sail boat prop, on a monohull, rather than a folding/variable pitch/etc one, as we wanted to keep the boat reliable underwater as we only lift out for a couple of days every second year. I assume something similar is available in UK but the one we used and is well regarded here is on http://www.henleyspropellers.com/propeller_range.htm, look for "Skip Jack". Also a very experienced sailing friend has just repropped to one of these from a simple 2 bladed folding one on a long keeled boat and is happy (he is going extended cruising in remote areas).

The subject of drag is a thorny one, but I suspect from a cruising boat's point of view it is small when weighed against other things, if a prop such as that in the link is used. But there may be drag and other issues with sailing cats that I don't know about (I only deal with powered cats) especially seeing you have to tow around two props, although I assume that might only become of increased importance in faster cats.

I think I remember you mentioning that your shafts are not of opposite rotation, but I may recollect incorrectly - however, things like that might be worth raising with a good prop manufacturer, if you had not already intended doing so. A respected prop maker will also size your prop and should reliably get it right - much better than making a mistake in the sizing oneself, in my opinion.

Regards

Catty One

 

[Sheerline]

I have a 25ft keel boat, which we've tinkered with over the years to make it go faster. It has a taller mast, longer boom, deeper keel, better rudder...and a folding prop. The folding prop made the biggest immediate speed difference - amazing. It's a two-blade geared prop from Austral (Sydney) that I've had now for more than 15 years. It has never failed to open - and works well in reverse etc.

A friend has the identical hull to mine and a three blade fixed prop with more pitch - he is slightly quicker than I am at cruising speed, but nothing in it really. I reckon if I repitch my old prop to match my new grunty engine, it'll work perfectly. I race the boat, so speed under sail in very important - but after 15 years with no problems I cannot envisage getting a fixed prop again.

 

[Ships_Cat]

...two-blade geared prop from Austral

Which is, perhaps, actually the Slipstream folding propellor from Martec in USA?

John

 

[Sheerline]

They've recently replaced their two-blade with an imported job - I'm sure you're right about its origin. But at more than 15 years old, the true heritage of my steam-driven version is unknown.

web page

 

[ShipsWoofy]

what did you do with your old props?

Thank you for your reply by the way.

 

[ShipsWoofy]

Thank you all for your replies, a real mixed bag /forums/images/graemlins/grin.gif

I do want to look towards long term cruising, esp. in the med where from reading more motoring is inevitable.

Would like to maintain the power I have now, but like to reduce drag.

If I read correctly, I probably wont see any major differences unless I invest in folding props, which I just cannot afford right now, maybe I should stick with what I have and start saving up for some folders.

Can someone explain how they open in reverse, I have never got that bit?

 

[Sheerline]

Centrifugal force seems to open them with a very solid clunk, which ever way the shaft is turning.

 

[BrendanS]

Re: A pedant replies

Sorry about this, but my Phyics teacher would turn in his grave after drumming it into me at great length, if I didn't respond

<font color="blue">Centripetal</font> , not centrifugal

centrifugal force doesn't actually exist at all, it's a 'virtual' force. The correct term is centripetal, but not many people understand that, so centrifugal tends to get used instead, even when incorrect. I await a barrage of messages telling me I'm wrong /forums/images/graemlins/grin.gif

 

[halcyon]

Re: A pedant replies

Ok centripetal is a force towards the centre, ie gravity, it tries to compress a revolving body, cetrifugal tries to expand a revolving body.

Brian

 

[BrendanS]

Re: A pedant replies

I knew this was going to be difficult. Centrifugal 'force' is not a force. Centripetal force is. Centrifugal force is a virtual force, and also something of a myth. The force acting on the propellor is centripetal, not centrifugal.

Have a look here, it may help
http://www.mcasco.com/qa_cacd.html

 

[ShipsWoofy]

Ok, sort of....

I have no problem with Centripetal force or what ever opening up the blades when the shaft is spinning. I just realised my conundrum is the opposite way around. In reverse the thrust is leaving the back of the prop, therefor holding the blades open.

In forward the thrust is out from the front, which in my quick head maths should try to close the prop? I think that is right.


Another question, do you have to apply a large burst of power to open them, and then slow them down? Or will they open the moment you touch the engine into gear.

I am really struggling try to fathom how these things work. Why am I struggling so much with this obviously proven concept.

 

[macd]

Re: A pedant replies

And another pedant writes...I was always taught that the force is neither centrifugal nor centripetal, but that both were merely a simple way of grasping the effect. The force is plain old Newton: a body continues on its course unless acted on by an outside influence, in this case by the fact that the prop blades are fixed to a hub on shaft. What they actually 'want' to do at any time is continue in a straight line, ie tangential to the circumference of their arc. The result is that they are constantly accelerating towards the centre, the prop shaft.

Can't imagine that this illuminates things very much, but please consider it a small tribute to Mr Hall, my cantankerous old physics master.

 

[Aeolus_IV]

Re: Ok, sort of....

In both forward and astern the blades on a folding prop are "spun" out from begin in-line with the shaft by centripedal force. This is the same force that keeps a rope straight and flying if you spin it round and round (poor example I know). When going ahead the thrust the blades generate finishes the job of opening the propeller, and the blades sit fully open against their stops. In astern however, once the blades start openning, the thrust tries to close them as it pulls the blades backwards away from the shaft. The drive generated in astern is a balance between the mass of the blades spinning out, and the generated thrust pushing them back in again. It sound like it shouldn't work - but it does.

Regards, Jeff.

Have I just bitten a post I should have ignored? /forums/images/graemlins/confused.gif ah well.

 

[Sheerline]

Re: Ok, sort of....

No need for a burst of power, the moment the throttle is put into gear you hear the "clunk" as the blades open.