Fixed prop: 2 or 3 blades?

[jb267]

Can't afford a fancy folding or feathering prop and I would worry about damaging it if I ran over a log in a river.
So, how much extra drag am I talking about if I have 3 blades instead of my current 2 blades. Boat is bilge keeled so my 2 blades are dangling in clean water (locked shaft when sailing) at the moment. Is one extra blade really going to feel like dragging another bucket around?

Current 2 blade prop is under-pitched so I need to change it and am interested in a bit better motor and motor-sailing performance. I guess that a 3 blade prop will score here, but how much drag will be the compromise and is the extra 'grunt' worth it?

 

[cmedsailor]

I changed from 2 blades to 3 blades (fixed). Sailing at high speed (6 knots plus for a 36ft fin keel) the difference is around 0.3-0.4 knots less. In slower speeds is more (0.5-0.7). Motoring against head seas, motor sailing and especially manouvring in the marina is like having a bigger engine!

 

[Houleaux]

Years ago a friend and I had the same make and model of boats - bilge keel Konsorts. I had a 2 blade prop that my friend described as an egg whisk. He had a 3 blade prop which had a huge blade area by comparison. The difference in motoring performance was really quite marked, with his boat easily achieving 5 knots whilst mine was probably about a knot behind at the same revs. We never sailed the two side by side, so I'm afraid I can;t tell you what effect the larger prop had on sailing performance.

If you haven't already come across it, you might find some of the information in this article illuminating http://www.flexofold.com/upload_dir/docs/Test_YachtingMonthly_low.pdf . I swapped the fixed 2 blade prop on my existing boat to a folding 2 blade and have never regretted it.

 

[Tranona]

No quick an easy answer except that in general 3 blade will give better motoring performance and cause more drag under sail. The latter is very difficult to quantify. 2 blade props vary in performance as there is a wide range of blade shape and area so some are better than others. Usually a 3 blade is a different diameter or pitch than a 2 blade in the same application.

Suggest initially you calculate prop size using the Propcalc programme on www.castlemarine.co.uk with both a 2 blade and a 3 blade to see how far out your current prop size is. Also talk to a supplier such as Sillette for a recommendation, explaining your requirement for improved motoring performance.

BTW can appreciate not wanting to spend to get a folding or feathering prop, although if you have to change anyway the cost differential is not so daunting, but not sure why you would worry about damage hitting a log. The chance of that are very very small and would damage any prop - and that is what insurance is for!

 

[jwilson]

Can't afford a fancy folding or feathering prop and I would worry about damaging it if I ran over a log in a river.
So, how much extra drag am I talking about if I have 3 blades instead of my current 2 blades. Boat is bilge keeled so my 2 blades are dangling in clean water (locked shaft when sailing) at the moment. Is one extra blade really going to feel like dragging another bucket around?

Current 2 blade prop is under-pitched so I need to change it and am interested in a bit better motor and motor-sailing performance. I guess that a 3 blade prop will score here, but how much drag will be the compromise and is the extra 'grunt' worth it?

When I bought my present AWB I was a bit suspicious about it having a two-bladed prop, but in fact it seems to deliver the thrust very well, and is certainly less drag than a three-blader. It is probably pitched just about right as the engine delivers steadily increasing thrust right up to max revs, which gives 7.8 knots on a clean prop clean bottom 35 ft AWB.