Prop talk

[Terry B 71]

My old folding prop is beggared. Not repairable.

 

So I have a choice of buying another folding prop OR putting on a 3 blade standard prop (which I already own, came as a spare with the boat) on my Whiting 29. I don't race the boat, I cruise her.

 

Aside from the obvious advantage of not spending on a new prop, what are the disadvantages (or advantages?) of a fixed vs folding?

I understand that performance wise it's about a 10% loss in speed when sailing????? Is that right?

Is there an advantage in speed when motoring?

 

Also, what about other factors - like prop walk (i reverse into the marina)?

 

Anything else I should know?

 

 

ta in advance.

 

[Island Time 1,235]

A fixed prop is like towing a bucket! Don't do it...

What is the prop that's on there? Likely someone here might have a good spare.

If it's a saildrive, which one?

[waikiore 399]

Might be slightly more than 10% on a Whiting 29 at say 6 knots, if the fixed prop is sized correctly you should have good forward drive and moderately quick stopping power but with a little prop walk.

What size diesel ? and do you know your gearbox reduction ratio?

[Kevin McCready 83]

Like towing a bucket of small? size versus towing a bucket of smaller size? I'd love to see some actual measurements from some actual yachts and in various conditions and weights of yachts etc. Once you've reached hull speed it doesn't matter a lot does it? Only matter under certain conditions? So question might be, at what windspeed and what point of sail for what yacht and by what amount will you be slowed dow. My gut feeling is that it would only matter at low windspeeds and the "slowdown curve" in yacht speed prop versus fixed might not be significant.

[Fish 0]

We recently put the old 3 blade fixed prop on, in order to take our kiwi prop for a full service.

Substantial improvement in motoring performance. That is most likely because the kiwi prop needed a service though.

 

Lesson from that is there is less maintenance required on a fixed prop. Motoring performance should be basically equivalent (if maintained properly). There is a difference in sailing performance, extent depends on conditions. 

 

The big thing for us is the constant whine from the prop shaft when sailing, with a fixed blade.

 

It probably comes down to personal preference. I'll bet a bottle of rum, that when you get a quote for a new folding, you will all of a sudden be happy with the fixed prop you already have....

[wheels 543]

It does depend, but pretty much what IT said. It's like towing a bucket. 3 Blades are more efficient than 2, because they have more blade surface in the water, so more grip.  But there are Sail boat props, which have smaller blades, thus surface area and then there are power props that have greater surface area. So it depends on what you have in that regards.

One really important thing to check though. The gearbox. You need to know if you can free wheel, or if you need to leave it in gear, or if you need to lock the shaft.

[Steve Pope 243]

Have a mate with a Whiting 29, he has put a Kiwi prop on it and loves it. Reverse pitch stops you considerably faster than a conventional prop. Forward pitch is adjustable even in the water. I have one on our boat, I have had damage to the tip of one of the blades, the blade being so light the effect wasn't really all that noticeable in comparison to an out of balance fixed 3 blade prop. and the 1 blade was easily replaced.

Up in Fiji earlier this year a cat caught a rope around the one of its Kiwi props, it broke off 2 of the blades. The owner dived, removed the prop, replaced the broken blades and refitted the repaired prop in a couple of hours + - 

I found Kiwi Prop great to deal with. Around $115.00 per inch of diameter. Lots of info on their website.

[harrytom 643]

kiwi prop on the cav26 6 knt forward 6 knt backwardsalways good for a laugh going in reverse as long as you reduce speed to turn.

[Guest 000]

Yachting Monthly's site has a definitive article on sailing with a locked vs a spinning prop. The article states a spinning prop has approximately 10% more drag than a feathering prop, but hugely less drag than a locked prop.

So, if your gearbox is of a type that can stand freewheeling under sail then I would go with that.

[Island Time 1,235]

+ 1 for the kiwi prop. I use one. I also have a spare prop - a fancy 3 bladed folder from Volvo Penta. I prefer the kiwi prop...

[Steve Pope 243]

An interesting analogy re spinning versus fixed props, A helicopter when the blades are in auto rotation (no power) comes down with the blades spinning from their passage through the air and acting as a brake, it will land hard but generally in one piece.. A Helicopter with the blades stopped drops like a brick!

[Island Time 1,235]

Yeah, but look at the blade type and speeds, completely different. One is low area high speed, one is high area low speed.

[wheels 543]

 

An interesting analogy re spinning versus fixed props, A helicopter when the blades are in auto rotation (no power) comes down with the blades spinning from their passage through the air and acting as a brake, it will land hard but generally in one piece.. A Helicopter with the blades stopped drops like a brick!

You are correct to a point. But what the Helicopter does is allow the blade to free wheel to build up speed. At this point, the Boats prop would be free wheeling, little drag. Then at the last moment, the chopper Pilot pulls back on the stick, changing angle of attack of Blade and the momentum of the blades causes a lifting and braking affect of the choppers decent and the Pilot can actually land the chopper quite safely and in fact softly. On the boat, this would be like engaging the gearbox while boat is at speed and the prop has enough power to spin the box and motor. While the blade is acting like a Wing, it can generate a lot of drag. Once the prop slows to a point of stall, then the drag is less, but is still there. That same drag is what gets the Prop spinning in the first plae and is what gets the chopper blades up to speed in the first place. So there is Drag, but just not as much as a spinning prop. The difference in the spinning prop is that the drag is converted to rotation and the effort is proportional to resistance of rotation caused by Gearbox resistance and motor if engaged. Thus Free wheeling would be less drag, hopefully.

[Steve Pope 243]

a brick still drops like a brick.

[Terry B 71]

Hi all, thanks for the great feedback! Sorry I didn't get back to you lot earlier.

 

Waikiore - engine is volvo 18hp series 2000, 2 cylinder drive shaft. As to gearbox reduction ratios - thats just gobblydgoop to me  I'll see if I can find out.

 

And I understand the whine of freewheeling versus locked and that some gearboxes require freewheeling and others don't. 

 

And ChrisC - ta for that info.

 

and all the other info from you guys.

 

My inclination is to go folding (I own a yacht to sail it, not motor, so I'd like to get as good a performance out of her as my limited ability will allow).

But, and it's a big but, cost is a serious, awful, horrible, tragic, nasty, sickening, unavoidable consideration!

[wheels 543]

My old folding prop is beggared. Not repairable.

 

OK, going back to the beginning then, what is wrong with it??

[wheels 543]

Here are a few on Tradme. May or maynot be in the size range you need of course. Pitch can be adjusted and if you are within and inch over with diameter, it should be able to be cut down to suit.

The last one is dreaming with price however.

http://www.trademe.co.nz/motors/boats-marine/parts-accessories/propellers/auction-1137979992.htm

http://www.trademe.co.nz/motors/boats-marine/parts-accessories/propellers/auction-1137982728.htm

http://www.trademe.co.nz/motors/boats-marine/parts-accessories/propellers/auction-1139668701.htm

http://www.trademe.co.nz/motors/boats-marine/parts-accessories/propellers/auction-1137970792.htm

http://www.trademe.co.nz/motors/boats-marine/parts-accessories/propellers/auction-1136203793.htm

[Romany 162]

The 1st bri ski seems like a bargain if it is in good order, in fact I'm quite tempted...

 

And wheels - why is the last one is dreaming re pricing? That seems like its pretty cheap price - aren't these things normally around the $1700 new

for that size?

[John B 106]

I had one , the price seems good to me.  The problem that seller has is its shaft size and maybe its spin.  I actually thought the things could be changed to left or right, perhaps I'm wrong on that( ring em and ask), but 1.25  which I interpret as 1 1/4 in shaft is not exactly the most common size.

 That  pitch info is irrelevant anyway, one of the main reasons you get a feathering prop is that its adjustable and you can tune it to your hull and engine/ gearbox.

[Terry B 71]

Wheels, its jammed on the shaft, bolt busted, only way to get it off is cut it off. 

It was marginal whether it was repairable if we got it off anyway, so I was kinda resigned to getting a new one.

 

And ta Wheels for the Trade Me link. Hadn't even thought of second hand!