Prop sizing

Hello all, I recently pulled my boat for some bottom and routine maintenance and realized the PO had not put on a zinc on the out drive shaft and had painted the prop. I've reordered a new prop but the PO had ordered a 12 x 10 whereas the Internet recommends a 14 x 8 or a 13 x 10. I've already ordered the 12 x 10 but am not afraid to send it back if someone has any other reason to keep and install it. I'd appreciate any help you can give. I have a 1gm 25+

Propping up my Assumptions

I have always understood that the limiting factor, within limits, is the tip clearance to the hull. One company web site calls for 15% of the prop diameter. Most of us strive for about 1.5" of clearance. This is said to reduce the vibration from water hammer against the hull, each passage of a blade.

When spec-ing out a prop, I would then try to balance the increasing blade area as the diameter increases with a proportional decrease in pitch. That's my intuition, and it might be wrong.
After owning our boat for over 2 decades, and using several different props on it, we have "evolved" to the present sizing and we are the fastest under power that we have ever been. There seems to be some gray area in selecting the right prop, although most of the factors are indeed science-based.
I suspect that the shape of the nearby hull and the aft shape are important too.

(The best prop engineers and installers may have a Dark Wizard working in their technical department...)

If you (using google) search this site for prop information, there is a lot of discussion of what works. While it's anecdotal, it is sincere and first hand.

Best,
Loren

When I bought my used 1985 E26, the prop did not seem to be propelling the boat very well at a given RPM, so after awhile I looked into getting a bigger prop. I found that the two-blade prop I had was the a biggest that would fit and still allow sufficient clearance. The original prop was a 12x10 two-blade prop. I ended up buying a new 12x12 two-blade prop. The increased pitch made a lot of difference and I'm satisfied.


Captain Don
Gitana, E26

Thanks for the help. I may do a little more digging and measuring before I just go with what we had before.

On my E-26 I have a "sailor" type three blade that is 12inches diameter and I have had it re-pitched to about 10" Its perfect.

Here is another discussion on props in your size range.

http://www.ericsonyachts.org/infoexchange/showthread.php?14055-propellor-selection

Shelman said:

On my E-26 I have a "sailor" type three blade that is 12inches diameter and I have had it re-pitched to about 10" Its perfect.

Here is another discussion on props in your size range.

http://www.ericsonyachts.org/infoexchange/showthread.php?14055-propellor-selection

Click to expand...

Thanks for that link! I have ordered the 12 x 10 like you have transitioned to, but I ordered the two blade instead of the three blade. I don't have anything to compare it to because the original prop was pretty worn and bent. As a measurement of power, I was getting about 5.5 knots at around 3200-3400 RPM (which is the recommended operating rpm for my 1GM). Do you think I'll get any better preformance from a three blade? Also, is that a zinc screwed into your cutlass bearing (from your link and picture)? I have opted to go with a shaft zinc.

As always, thank you for any help and assistance

907Juice said:

Thanks for that link! I have ordered the 12 x 10 like you have transitioned to, but I ordered the two blade instead of the three blade. I don't have anything to compare it to because the original prop was pretty worn and bent. As a measurement of power, I was getting about 5.5 knots at around 3200-3400 RPM (which is the recommended operating rpm for my 1GM). Do you think I'll get any better performance from a three blade? Also, is that a zinc screwed into your cutlass bearing (from your link and picture)? I have opted to go with a shaft zinc.

As always, thank you for any help and assistance

Click to expand...

5.5 kts. is reasonable for your E26 with that little 1GM (not 1GM10). I have the same engine in my boat. I have a Campbell Sailer 3-blade, which is highly efficient for a fixed 3-blade under sail and still gives good motoring performance. Though you can run that engine at 3200-3400 you are pushing it a bit to run it at that all the time--especially the 3400 number. Cruising RPM for that engine should be around 2900, maybe just a tad less. But then you should run it at higher RPM occasionally to blow the carbon out.

Anyway, back to the prop: Mine is 12" dia., 3-blade, 6.5" pitch. Due to the blade shape, Campbell-Sailers pitch a little lower than typical. They also have far less drag than your typical 3-blade. (See MaineSail's tests here: http://forums.sbo.sailboatowners.com/showthread.php?t=112587) West by North's tech support is superb. You can contact them at info@westbyhnorth.com or phone at 778-802-4202 if you are interested in knowing more.

Good luck to you. There is a certain amount of voodoo when it comes to prop sizing, but it's worth trying to get it right, especially on a little engine like ours that needs all the help it can get.