Re Proping from 3 to 4 blade

Tuna Pursuit

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I just switched on my twin cummins & there's a world of difference. Much smoother. Capt Larry helped my size them and he knows his stuff. Def go with nibral. I went with Michigan DQX props. PM me if you want to know where I got them.
 

Brooksie

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Conventional wisdom is that 3 blades are more efficient than 4, if you have room for diameter enough to absorb your HP with 3. And that 4 blades are smoother than 3.

To go from 3 to 4 blades, assuming you have the ideal 3 blade, multiply diameter by .94 & pitch by .98 so you would become 18 x 19.6 . Not much of a change really

But behind the keel of a DE conventional wisdom says always use 4 blades.
 
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Brooksie

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So based on 20 X 19, x.94 & .98 would become 19 X 18.6 or 19 X 19.
BUT I would not give up the 20" diameter if you have the proper clearance for it. So, depending on your gear ratio 20 X 18 4 blade may be right.
Remember the multiplying by .94 & .98 works ONLY if you are sure you already have the ideal P & D on your 3 blade now. And of course I'm not at all sure because all I know is Nauset 28 and a 20 X 19 3 blade

So, and yours is a good example, to go to 4 blades you may be giving up an inch of pitch and unless that added blade decreases your slip, you will loose top speed (18" x your shaft RPM vs: 19" X shaft RPM) That extra blade takes power to turn so that is why you must drop a little pitch, RPM, or both.
 
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Brooksie

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The boat weighs 11500, 220 [email protected] 2600 rpm ( I can make 2675) 1.53 trans, 1 1/2" shaft with the current prop I have 1 7/8" clearance to the hull.

Almost, what speeds do you make now and what are you trying to do, cure the vibration or get more speed?

If vibration, and everything else is alright (cutlass, shaft, alignment, prop) then the 4 blade may help you cure it and I would do 20 X 19 4 blade and use up that extra 75 RPMs.

If it is more speed, then I'd have an inch of pitch added to the 3 blade, the vibration will be no worse (maybe better after prop check), you will get a little speed, and save your engine.
 

Brooksie

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The boat currently makes 19.5 kts, I prefer to cruise at 13-14. I am looking to reduce the vibration, comfort is more important than speed in my case. I still want to be able to exceed the rated rpm.

Then I think the 4 blade will do that for you and the 20 X 19 should still leave you 25 RPM over rated RPM. You could have it "clipped" next year if you want to turn more over rated RPM and get it the full recommended 15% away from the hull.

Get a second opinion from a shop like Scandia in your area b/4 your invest and maybe wait for Capt. Larry to jump in to confirm what you are doing.
 

captainlarry84

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$ vs 3

I think this photo tells it all. The Titanic a triple screw wonder. She carries a 4 bladed wheel behind the keel for smoothness., no matter where the blades are in rotation you always have the same amount of blades grabbing water on both sides. The wing motors are both in clear water flow so the better efficiency of the 3 blader rules and gives her it speed. So case closed the 4 blader is the most efficient propeller for a keel DE boat.
However I am sure that there are some but not many that run better with 3 blades.
When going from 3 to 4 the rule of thumb is to reduce pitch by 1". The reason is because of the extra blade. If you are going from a Dyna Jet 3 blader to a DQX 4 blader the 1" pitch rule is out due to the fact that DQXs have great blade area.
Blade area vs pitch is always a hard call. For a DE boat I am still a big fan of the Dyna Quad.
On twin screw boats the DQX rules.
Also some timeds a little cup always helps.

images[4].jpg
 


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