Fixed propeller choices

[wipe_out]

I am going to replace my fixed 3 blade prop because it's clearly not the correct size (3 Blade 15x9).. Original spec from boat builder was a 2 blade 15x12.. After looking online, using calculators, reading what I can find and speaking to a couple of propeller suppliers it would seem I have a few options but ultimately they all say it's my choice..

Options I seem to have so far are (all 3 blade)..
15x10
15x11
16x9
16x10

Boat is a Hunter 326 which has a 28ft water line and 8550lbs displacement.. Engine is a Yanmar 2GM20F with a 2.62 reduction ratio..

So if the original Hunter spec was 15x12 2 blade then I am told the 3 blade equivalent would be a 15x11..

This page suggests 16x10 for both 2 and 3 blade configurations.. Propeller Recommendations from Yanmar at Marine Diesel Engine from Torresen Marine's Marine Diesel Direct

This site suggests 15x11 or 16x10 - Prop-It-Now // Michigan Wheel

This site suggests 15.2x10.4 so in the middle- Vicprop - Prop calculator for Displacement and semi-displacement hulls

One engineer I spoke to said the "book" called for 16x9 so if I wanted less drag I could use 15x10 or if I wanted a slight over prop for more comfortable cruising I could go 15x11..

Does anyone have any suggestions or real world experience changing from 2 - 3 blades?
Should I go 15 or 16 inch diameter considering additional drag, if there is much more for the additional inch?
Is it better to have bigger diameter and less pitch for things like prop walk or does it not really make any difference?
Is a slight over prop a good thing, from what i have found online it seems quite common?

Would have been really good if all sources said the same thing but obviously that would be too easy..

 

[JumbleDuck]

Before replacing it, why does it have a three-blade 15x9? Is it possible that a previous owner has been through all this and concluded that that worked best? Is something dissatisfying about the performance?

 

[wipe_out]

Before replacing it, why does it have a three-blade 15x9? Is it possible that a previous owner has been through all this and concluded that that worked best? Is something dissatisfying about the performance?

Yes, motoring performance is awful with the current 15x9 three blade.. Like trying to move the boat with a egg whisk.. Only get about 5kn at 3000 rpm when I expect it should be closer to 6.5kn.. Considering the original was a 15x12 it stands to reason I am seeing theses results since it's gone down 3 inches of pitch..

 

[LONG_KEELER]

From what I understand , it is the blade diameter that will provide the most power. In theory that is. In displacement mode, a helicopter sized prop
would be just fine if the engine could turn it and you could suffer the depth loss.

Also, tip clearance of the blades is critical with 20% as a rule of thumb of the prop width, otherwise you don't get the benefit.

I've had two blades mainly over the years which have worked out well with the minimum of 20% clearance.

 

[Catalina36]

To add more confusion, if you're going to the expense of a new fixed prop have you considered a feathering one such as Featherstream or a folding one? They can make a noticeable difference to speed under sail.

 

[wipe_out]

To add more confusion, if you're going to the expense of a new fixed prop have you considered a feathering one such as Featherstream or a folding one? They can make a noticeable difference to speed under sail.

Yes, I looked into them.. Don't like the folding options with not having good reversing ability because we are in a pretty tight marina.. Feathering are too expensive considering how much sailing we get to do through the year, would rather just motor sail in light wind.. So the fixed makes the most sense for us..

 

[peteK]

If you like to motorsail go for a slightly coarser pitch,go for the largest diameter that will give correct clearance.

 

[dunedin]

Yes, I looked into them.. Don't like the folding options with not having good reversing ability because we are in a pretty tight marina.. Feathering are too expensive considering how much sailing we get to do through the year, would rather just motor sail in light wind.. So the fixed makes the most sense for us..

Absolutely no issue with reversing performance on a decent folding prop. Done hundreds of reverses with our past two boats. Flexofold 3vblade is brilliant.
Must be huge drag with a three blade fixed prop.