To prop or not to prop, that is the question...

[David Underwood]

I recently discovered that my old 2 blade prop is prob. bent and out of balance. I got on the web and started searching and found a place in Florida and called, and found myself ordering a 3 blade prop. When it arrived and I opened it.. WOW... large! I had given the prop shop the numbers off my original, a 14x12 two blade, and they suggested the 3 blade with a 14x10. Ok, sounds good. But, I did not expect to see such a large difference. So, its been worring me ever since, can't decide if this new prop is going to slow me down too much while under sail. Soooo..... thought I would take a pic to show the two together and get some input from my new best friends... all of you. Help me make the decision.. please.. all opinions welcomed. Thanks, David

 

[Guest]

To Some Extent .............................

...... it's a bit of hit and miss. We recently went through exactly the same thing with our H310 with a Yanmar 2GM20F (18 HP).The original prop was a two bladed 15 X 12 and the custom replacement was a three bladed 14 X 10. Now, looking at you photo, the original prop was almost identical to yours. However, our repalcement prop has a DAR (can't remember what the acronym stands for) of 0.45. This means that each blade is 45% of the area of the 14" circle. This is not immediately apparent as the blades have a curved surface which has a larger surface area than if the blade were flat. The three blades have an area of 135% of the 14" circle. Your blades seem to have somewhat less area. Offsetting this is the fact that you have retained your 14" diameter. It seems that your prop is certainly in the ball park.So that's what I paid for and that's what I got.After a summer of running and talking to the prop manufacturer, I will be adding one inch of pitch to the prop at the next spring haul out. What I have noticed is:1. It runs as smooth as silk all the way to 3600 whereas it used to vibrate badly before. The same shop had balanced the two bladed prop the year before.2. It has basically the same speed for the same revs.3. It will pick up to 3600 revs immediately while the boat is still dead in the water.This last item tells the shop that it could still use a bit more pitch. So it's back to the shop for 1" of pitch increase for a small charge.Every shop seems to have a slightly different twist as props seem to be an art form combined with underlying hint of engineering. My suggestion is that you install the prop and give it a years try and if it needs tweaking, do so at next years haul out.Hope this is some reassurance.Regards,s/v Island Hunter

 

[chet p]

Campbell Sailor

that is aboutr all i can tell you. Check the www.westbynorth.com site and see the difference. on my 37 i went from a 14 dia to 15 and from a power boat prop (like the one you show in the picture) to their high tech designed prop. if you can send it back you might consider after checking out the above site and listening to what they have to say. can't tell you the before/after as i also change the engine and needed a prop anyway. the new engine with the old prop was a joke. nwe prop is sweet. paid less the $400 for the 3 blade 15 X 13 X1 and don't see a noticable drag factor.....FWIW

 

[jcherubini]

Prop it up

David, after reading the informative response by the owner in Vancouver I'd say you have nothing major to worry about. Hearing about his going down one inch in diameter whilst yours stays the same suggests that you could have a SLIGHT problem getting it to spin quickly (more black smoke in harbour) but other than that it should be fine.I've said a lot about the effects of changes in prop pitch, but if you don't mind that black-smoke issue a little more pitch is always better than a little less. I'd rather have the mill turning over fewer revs once it is up to hull speed than having it turn over too much. The only way you can err here is with FAR too much pitch which will actually keep the motor from reaching full cruise RPM. But one inch from 10 to 11 is hardly going to be that much of a problem.Put it on the boat and go sailing.JC 2

 

[jcherubini]

Prop it up

David, after reading the informative response by the owner in Vancouver I'd say you have nothing major to worry about. Hearing about his going down one inch in diameter whilst yours stays the same suggests that you could have a SLIGHT problem getting it to spin quickly (more black smoke in harbour) but other than that it should be fine.I've said a lot about the effects of changes in prop pitch, but if you don't mind that black-smoke issue a little more pitch is always better than a little less. I'd rather have the mill turning over fewer revs once it is up to hull speed than having it turn over too much. The only way you can err here is with FAR too much pitch which will actually keep the motor from reaching full cruise RPM. But one inch from 10 to 11 is hardly going to be that much of a problem.Put it on the boat and go sailing.JC 2

 

[Kevin]

More drag!!!

Hello David,A test was once done that used a 16” three blade on a 1”shaft, where drag comparisons were made on various propellers. If I recall correctly, the two blade fixed, like yours, produced apx. 150 (ish) pounds of drag at six knots. The three blade however exceeded 220! I would venture to say that your current two blade slows the boat apx. .35 to .45 knots in medium air. Your new three blade could easily create .6 to .75 knots in same air. Besides the speed loss you will see a reduction in pointing ability and the speed you carry into a tack. The vessel will be much slower to react when coming about. This will force you to bare off further in order to gain speed to start to point again. Hope this helps. Kevin

 

[John Nantz]

Interesting Drag Figures

Those are interesting drag figures. Kevin - question: When there is more drag, for example dirty bottom, the boat makes less distance to weather when beating. With a fixed-blade prop the drag is tremendous so do the speed loss figures take this into account? If not, then the speed lost in going to weather in terms of distance made good is much worse that the numbers would indicate.There needs to be a bumper sticker: "Sailors don't let Sailors use fixed-blade props"