One More Prop question.

[DC45]

In short, my CD19' Angler 90HP Honda came with a 13.5 X 15 - 3 Blade.
Its seems to be a good combo, however in "perfect" sea conditions (pure glass/low current) I can trim the boat out with the motor and trim-tabs and get her moving wide-open... to a full 6000 RPM. Its not everyday that the water conditions allow this. With that knowledge, I needed/wanted to invest in a 2nd Prop to carry along for a spare (is case a migrating "wood-salmon" decided to attack my boat) so I bought a 13.75 X 17 -3 Blade, with the thought that during ideal conditions, she didn't need to wind up to 6000 RPM (I just cant find the sense sometimes to pull the handle back). I of course ran this prop a few times and it worked as the theory claims (increase pitch/lower top RPM by about 200). However, that combination took away my cruising "sweet spot" of about 4400 RPM and changed the power-band for rough water.

I am going to buy a 4 Bladed Prop now. The Big Question is, with a 4-Blade do I need to increase the pitch to get similar results as the 3 Blade ?
(example: If a 15 pitch is perfect for a 3 - Blade, will a 15 pitch equal the same on a 4 - Blade) or is there a "general rule of thumb" that going from 3 to 4 blade you make a pitch change ? I have narrowed it down to either a 13 X 15 - 4 Blade or a 12.75 X 17 - 4 Blade. I really do not want to buy both. (wife says: a man don't need to have 1 motor and 4 props..hahaha)

I know somebody out there has changed from a 3 to a 4 Bladed prop and has some insight on the pitch ratio between the two.

Thanks in advance

 

[C-Green]

DC45,
I have a CD-22 with twin 40 Hondas. Originally had 11.1x13 three blade al props (original Honda, made by Solas I believe) and "upgraded" to 10.8x12-4 bladed al props by Solas. The 3 blade had a wot of about 5700 and the 4 blade of about 5400-5500. The 5700 was on a very light boat and the 4 blade on a load on the medium side. Used the 3 blades for a year and the 4 blades for the last 4; since I was a novice the first year, I couldn't make a judgement at the switch over time especially since it was at the start of a season before the boat was in the water. however, I have averaged close to 1000 miles/yr with the 4 blade and am happy with them. I've thought of going to a smaller pitch but that would probably increase my cruising rpm higher than I want. Now I cruise at about 4000 rpm and like the noise level. I might get the smaller pitch as spares (I now keep the 3's for that) for when I'm really loaded where wot is below 5000 rpm. Hope this helps.
Jay