free spinning shaft makes whirring noise

[wingssail]

Originally Posted by bradandnatalie

Hello all.
I have recently fitted new 3ym30ae Yanmars to my catamaran. These replaced the 3hm35f Yanmars that I bought it with. The older engines has the KBW10 gear boxes and when sailing you could put the gear in reverse and stop the shafts from spinning. The new gear boxes don't allow for this and the manual clearly states that the warranty will be void if the gear is not left in neutral when sailing. The problem is around 5 to 6 knots it's quite noisy with the spinning whirring sound. Is this normal or does anyone have any suggestions that would not void my warranty. The props are 3 bladed fixed and I'll not be replacing them. Thanks very much.

Whether or not the drag is worse with locked or freely spinning props, the drag is significant either way. The drag is more of a hinderance in a low drag hull, such as a light catamaran. Less so if the boat itself is already high drag.

Re think your parameters and consider folding propellers. You will see a difference and the rotation sound will be gone.

I have used Martec folders for about 40 years, and have gone through three of them, but I don't motor much. They are simple, cheap, and low drag and they will work in reverse, but just don't count on them stopping you from 4 knots as you approach a berth.

[bradandnatalie]

Thank you everybody for your replies and yes GILow I'm still here enjoying the banter of fixed versus free spin drag, very interesting really. The props I replaced were 3 bladed Maxi Props that feather. They are a little old and worn and the cost of rebuilding them would be prohibitive. That together with the maintenance requirements and the every present possibility of them not opening properly has driven me to fixed. The drag issue isn't really the issue I had any concern with it was mainly the whirring noise which has been addressed with some terrific ideas. Thank you all again, especially for the off topic banter.

[Wotname]

^^ thanks for the update; good to see you weren't dragged down by the drag issue

[Wotname]

Originally Posted by bradandnatalie

.......... That together with the maintenance requirements and the every present possibility of them not opening properly has driven me to fixed. The drag issue isn't really the issue I had any concern with it was mainly the whirring noise which has been addressed with some terrific ideas. Thank you all again, especially for the off topic banter.

Yes, I have to agree. IMO, the increased drag is simply not an issue unless you regularly sail in very light airs. Once the breeze is up and I'm are getting towards hull speed or as soon as I need to reef, prop drag is not at the forefront of my mind - it isn't slowing me down. There is enough wind to overcome the additional drag and the wind free. But the noise - that needs stopping!

[Seadaddler]

I have Flex o Fold folding prop and just put in reverse and nothing needs to be done to them except a little keeping clean
Nick

[Thumbs Up]

With saildrives the noise can be pretty loud over 8 knots. I used to put them in reverse but one side had a problem unlocking and one time I noticed that at 11 knots it was trying to rotate the motor in reverse. After that I never locked them again. I too have 3 blade fixed props and don't plan to upgrade. Just have to live with the noise or go slow.

[auruguay]

Originally Posted by transmitterdan

Water is not air. A propeller in water free spinning does not act like a solid disk.

One cannot extrapolate behavior of an air foil propeller to a three bladed water propeller. The only thing they have in common is the name “propeller”. The way they work is completely different. An airplane propeller is shaped like an aircraft wing but a boat propeller is not. There are several other differences. Many studies prove that a locked boat propeller slows the boat more than free spinning.

Reynold's Number is another difference. That's why a bee doesn't actually "fly". Because a bee "feels" the air the same way an helicopter would feel it if trying to "fly" in a pool of honey.

https://study.com/academy/lesson/rey...equation.html#

Another HUGE difference is that air is compressible, water (in its liquid state) isn't.

[Cheechako]

Put the Max Props back on. Mine worked fine. If they need cleaning.... then so do your fixed props.

[PeterS]

Oh thank god that's over, now on to 2019,
Happy New Year Everyone