Originally Posted by rossad
I think a 2 bladed prop is everything as good as a 3 bladed prob going forward but if there is serious resistance 50 plus knots a 3 bladed would have more grunt once on full rev's. The most important aspect is getting the prop size and pitch
to your boat and engine
Let me also point out that with a 2 blade
, at some point, both blades are hidden behind the aperture. This is why a 2 blade produces more of a thumping vibration through the hull
. A 3 blade
always has 2 of the 3 blades exposed to the water
Sizing is important. Depending on the actual final drive ratio, it needs to be noted to keep the blade a minimum of 1 1/2" away from the hull
. More if you can afford it. Also, a note to the wise. Make damn sure that when the rudder
is hard over either side that the blades don't whack the rudder
. Don't ask me how I know this
Originally Posted by osirissail
Generally the choice between 2 blade and 3 blade props has to do with hull shape and aperture where the prop resides.
Based on engine
power and transmission
you select a 2 blade prop to fit. However, if the planned 2 blade is too large (diameter) for the hull shape and/or if you have a keel
hung rudder with aperture, you switch to a 3 blade that will have shorter (smaller diameter) blades. Also your prop selection need to be about 25% of radius away from any hull surface above and below to reduce cavitation.
Another consideration is to chose a prop with enough blade diameter to "reach" out from the shadow of the keel
in front of it to get clear flow water
to bite into. All in all, there are a lot of compromises in the prop size / type selection.
Actually diameter is not a huge factor as to having to switch to a 3 bade. Pitch
is changed after the maximum diameter of the wheel
is established. Also keep in mind that the more the pitch the less drag it has. this is why I scoff at all the numbers of how slow a non- folding prop is. If the angle of the blade is 10 degrees lets say (they're usually measured by inch/screw through water) and compared to one that was 45 degrees. The 10 degree blade would be more of a barn door through the water than the 45. It is folding prop companies that usually come up with these numbers.
Originally Posted by skipmac
Damn expensive is right. Not yet on the top of my list for where to put more of my vast quantities of disposable income
Performance. Say you get an extra 1/4 kt which I think is in the ball park (real world experience from others very welcome on this subject). That would be an extra 6 miles/day. In a week long passage
that's an extra 42 miles which is starting to sound almost worth while. If you got an extra 1/2 kt thats 84 miles/week.
I think at slower boat speeds the savings is more significant than when close to hull speed
. This is when I would care most about the savings. Again, real world experience would be interesting.
However I have another motive for considering a folder; no prop spinning under sail. Transmission
manual says its not a problem but there will be wear on the cutlass, the stuffing box and my nerves. The boat came with a shaft lock which is not working but could be repaired but it adds more junk it a very tight spot on my drive train due to the V-drive configuration.
Now Skip...stop that
. If you go with a folding then never troll for fish
, it will cause drag. Never pull into a beautiful anchorage while on your way to a destination
. You can't afford the time. And please, please, no extras like beer
, wine, and unnecessary food
. It adds weight and slows the boat down.