Notice:
The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.
I needed to replace the three blade prop on my 10 year old Mercury 8hp OB, due to a run-in with some debris that I did not see...
The yard could not get a three blade replacement prop for 7 - 10 days, and offered me a 4 blade prop that they could install the next day...so being the impatient sod that I am, I went for the 1 day and four blades.
I've trolled the net but can't find a straight answer as to the difference (to me as the user) between three or four blades. Speed seems the same, mileage is pretty hard to tell, no difference in vibration, but at lower RPMs it does a bit of a whining sound.
It's not quite that simple... It's the combination of diameter, pitch, and then the number of blades. The first two might be more significant than the last, since 3-blade props tend to have greater surface area on each blade. The best source for advice is the engine manufacturer (or a knowledgeable dealer, which can be hard to find for small motors).
I discussed this with a Honda manufacturer's rep and another from Yamaha at a boat show--the Honda high-thrust 8 (and 9.9) had 4-blade props and the Yamaha 8 had 3 larger blades. Both had lower pitch than on "ordinary" outboards meant for lightweight boats. I switched from an older Honda 8 with a "standard" prop to high-thrust Honda with with a larger, lower-pitch, 4-blade... The difference in maneuvering the boat was striking. (As I've said in the past, it would stop Passage as if she'd run into a pillow.)
Just about any prop will push a boat... For optimal acceleration, stopping, and economy at the desired speed, a planing inflatable dinghy wants a different prop from a 5,000 lb. sailboat.
BTW, I believe prop-walk increases with pitch more than anything else. It's the tendency of the prop, at lower or no boat speed, to throw water outward (waste) in addition to aftward (thrust).
Dave Bristle Association "Port Captain" for Mystic/Stonington CT PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired), Now on Eastern 27 $+!nkp*+ Sarge
You dont say what the dimensions are on the new prop compared to the old so hard to say about any differences. Most motors will have three props to choose from if you look up parts for them. Gas mileage is irrelevant with to many variables affecting it. Wind, Waves, Current etc. If your happy with the speed and maneuverability around the dock then your good to go. Since we are pushing @5000lbs with only 8hp most of the differences between the two props like planning, hole shots etc are lost to us.Might matter more to someone putting a 8-10hp on a bass boat. If it matters I found this.
quote:The most asked question at the boat shows is almost always” what is the difference between a 3 blade and a 4 blade boat propeller” The difference between 3 blades over 4 blades is that the 3 blade (smaller blade ratio) usually goes faster and the 4 blade (higher Blade ratio) is better hole shot and better stern lift as well as better handling and fuel economy because it will keep you on plane at a lower speed
If you look at a propeller from behind, and visualize a circle that the blade tips would draw, you will see that the blade of a 3 blade prop covers about 50 -55% of the circle. (This is called diameter area ratio, or DAR). You can add thrust within this drive circle by increasing that percent (to say 60 – 65%) by adding a 4th blade. When you look at a four blade prop you will notice that more of this area is covered because of the extra blade. That means more push contact with the water
Scott-"IMPULSE"87'C25/SR/WK/Din.#5688 Sailing out of Glen Cove,L.I Sound
Notice: The advice given on this site is based upon individual or quoted experience, yours may differ. The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.