HT Prop Sizing

[FourPoints]

I currently have a 7.5" pitch 3 blade prop. going to switch to a 4 blade high thrust, my options are 5" or 7"

the 7.5 I have currently is slightly over pitched, and a 7" would be ideal.

am I correct in my assumption that I would want a lower pitch with the 4 blade than I need with the 3 blade, or does the pitch remain the same when converting from one to the other?

 

[FourPoints]

btw, I previously had a 3 blade 6.5", it was undersized and suffered greatly in any semi rough conditions, and took much longer to accelerate the boat. however unlike the 7.5, the engine could get to full RPM.

 

[tommays]

It depends on you motor brand BUT i was very happy with the Tohatsu HT props

Even in the same size there completely different and have a large blade area BUT the shape allows the motor to reach full RPM on a displacement hull

The best way I can describe it is a like having a jet drive lower unit which is kind of rare around here

From what i have heard the HT 7" should work if you pick the right brand the 3 blade i bought for the 4/5/6 HP let the 4 HP move and 8000# boat at 4.5 knots

 

[FourPoints]

It is the Tohatsu prop, and Tohatsu 8hp engine.

 

[tkanzler]

The general rule, as I've always heard it, is to drop an inch of pitch, while keeping the same diameter, when going from 3-blade to 4-blade. I did that with the 50 hp on my Honda, and it pulls a wakeboarder up a bit quicker, while the engine still tops out at the same speed, give or take 50 rpm.

I don't know how the top speeds compare, but I would expect a slight reduction with the 4-blade. I also don't know how the 1" reduction 'rule' holds up with smaller outboards, and even how well it holds up in general, but it worked just right for mine.

Oh, and I tried a 3-blade high-thrust prop on an Evinrude 9.9 2-stroke I used to have, and it was a huge improvement for the 23 foot sailboat it was on, including and especially reverse. Since this was a displacement hull, it was slow no matter what, and tractor pull is a more accurate description of what's needed than sports car, so shallow pitch and lots of blade area seemed to be the order of the day.

The pics below are of the 3-blade high-thrust. I don't know if such a thing is available for the Tohatsu, or even desirable, but they're out there at least.

 

[FourPoints]

The general rule, as I've always heard it, is to drop an inch of pitch, while keeping the same diameter, when going from 3-blade to 4-blade.

I have heard that, and similar versions before, but my understanding is that doesn't apply to A) small engines, or B) low speed displacement hulls.

Would love to hear from anyone with experience switching their prop from a 3 blade to a 4 blade HT prop...

Tommays I did some searching, and came across the same iboats thread you copied that from, interesting setup you had with the swim ladder! Did you try that setup with a regular prop first and switch to the HT, or only with the HT prop?

 

[tommays]

When i bought the boat to refit it it was broken down on a mooring in NYC

I had to row out to the boat take some measurements go home and build something that would not fall OFF or ONE 16 mile trip



Everybody involved was somewhat surprised at the performance of the prop as I had NOT found it necessary in its standard job of moving the J24