Requesting recommendation on prop

N

Nancy and Bud

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Aug 2, 2019
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C Dory Year
2005
C Dory Model
22 Cruiser
Vessel Name
Emma Mae
We just re-powered with a 90 HP Merc. Have enough hours on it now that I can run it up to WOT. 5,000 max RPM's . Should be closer to 6,000.

Current prop is 3 blade 13.5 X 15.

What prop would allow me to have higher RPM's at WOT.

Thanks in advance.

PS: After waiting months, Bass Pro came thru with the correct motor. :D
 
Nancy and Bud":2uh3fwre said:
We just re-powered with a 90 HP Merc. Have enough hours on it now that I can run it up to WOT. 5,000 max RPM's . Should be closer to 6,000.

Current prop is 3 blade 13.5 X 15.

What prop would allow me to have higher RPM's at WOT.

Thanks in advance.

PS: After waiting months, Bass Pro came thru with the correct motor. :D
Click to expand...

probably a 11 pitch to get you to 5500rpm or so :thup
 
Most of the boats are running 13" to 15" pops. Colby had a Merc--and it may be that they have a higher gear ratio.

By calculation the 11" prop would be the one, but we tried an 11 on Lake Powell and ended up with a 13". The 15 was good for sea level.

Your boat should be light--but was it trimmed for max RPM. Put the Throttle down, Trim tabs up. and then move the motor trim up and down until you get max RPM. If you have trim tabs all of the way down, and motor trimmed down, it will not allow to achieve full RPM.
 
Don't have trim tabs installed yet.

Have Permatrim installed.
 
Everyone knows that you should prop your boat so that you can achieve WOT. The problem is that your 'boat', like almost every aspect of this dastardly prop puzzle, changes with weather and weight, among other factors.

I do not believe, that for general purpose boating, you should prop for a light load. This will require you to be a responsible operator (you may overrev at light loads at full throttle).You will be able to manage a heavier boat more precisely, and most likely, more safely. You will, when medium or heavily loaded, be able to plane more easily, and cruise with less engine stress. You may have a bit less top end, and plane a bit less effectively when you are light.
All in all, the material, diameter, pitch, cupping, permatrim, tabs, matter less than the variable weight of the vessel you are pushing. Because I push my angler 19 in very shallow water, sometimes alone with almost no extra weight, and sometimes with five aboard with a weeks worth of groceries and a couple suitcases and full fuel, my prop range requirements are huge. I use a 14 x 11 stainless 4 blade with some cup removed in the winter (I really need the hole performance with the extra shallow water, and a three blade 13 3/4 x 13 stainless in the summer.
 
On the 22 with a Merc 115, I was running a 4 blade aluminum 13 1/4 x 13. When I ran a 3 blade it was an aluminum 14 x 13. Allowed me to get close to my WOT specs. Colby
 
Max 5000 rpm is on spec if you are running with an very heavy load that will lighten up as you burn fuel & water etc. The mercury dealers want you to run at 5250 max rpms with your usual load for best fuel economy. 6000 rpms is a maximum target with a light boat for speed and water sports, but that prop will waist fuel for regular cruising rpms.
I use a 15 x 17''x 3 prop with the CT 90 Merc. [higher gearing] and hit 5500 rpms with medium cruising load. If I'm heading off shore I use a 15 x 15" for the little extra lift on the swells. My comfy cruise rpm is 4200 rpms at 20 knots!

Jim.
 
My Honda BF90D runs a 13.5x15 prop. This gets the WOT rpm to ~5700 rpm. WOT speed is just under 30 mph. Of course it depends a bit on the boat loading and the position of the trim tabs.
 
I do not believe, that for general purpose boating, you should prop for a light load. This will require you to be a responsible operator (you may overrev at light loads at full throttle).You will be able to manage a heavier boat more precisely, and most likely, more safely. You will, when medium or heavily loaded, be able to plane more easily, and cruise with less engine stress. You may have a bit less top end, and plane a bit less effectively when you are light.
All in all, the material, diameter, pitch, cupping, permatrim, tabs, matter less than the variable weight of the vessel you are pushing. Because I push my angler 19 in very shallow water, sometimes alone with almost no extra weight, and sometimes with five aboard with a weeks worth of groceries and a couple suitcases and full fuel, my prop range requirements are huge. I use a 14 x 11 stainless 4 blade with some cup removed in the winter (I really need the hole performance with the extra shallow water, and a three blade 13 3/4 x 13 stainless in the summer.
Click to expand...

Scuppers you make a very valid point. But the way I prop is for the Max at WOT to be the 6000 RPM at light load but normal gear aboard. When I am fully loaded at sea level I can still get 5000 RPM with that same prop. Since we don't run at 5000 RPM, but have a nice planing speed at about 17 mph at around 4000 RPM, I don't believe that is lugging the engine significantly, As we burn off fuel, we get into the 5500 RPM range WOT, and may bring the cruising speed up into the 20 mph range. However much of our cruising is at 6 knots. With an 11 inch prop pitch one would be running considerably higher RPM. Now if you add in altitude, then certainly the lower pitch prop is essential.

The Permatrim is not to make up for inadequate HP, or prop slip, but is to get the bow down when going into chop. (As are trim tabs for side to side trim as also being an aid to get bow down in chop.)
 
Thataway posted
The Permatrim is not to make up for inadequate HP, or prop slip, but is to get the bow down when going into chop. (As are trim tabs for side to side trim as also being an aid to get bow down in chop.)

Sounds like you got a good setup, Thataway.

I have found that with the 4 blade I get more stern lift, reducing the need for as much tabs, which in turn reduces drag and makes me a bit more efficient and speedy, while still accomplishing the main goal for me, which is to get the bow into the chop. Boating is a fantastic engineering puzzle, no?
 
DrewbirdII":1bt22o8s said:
Max 5000 rpm is on spec if you are running with an very heavy load that will lighten up as you burn fuel & water etc. The mercury dealers want you to run at 5250 max rpms with your usual load for best fuel economy. 6000 rpms is a maximum target with a light boat for speed and water sports, but that prop will waist fuel for regular cruising rpms.
I use a 15 x 17''x 3 prop with the CT 90 Merc. [higher gearing] and hit 5500 rpms with medium cruising load. If I'm heading off shore I use a 15 x 15" for the little extra lift on the swells. My comfy cruise rpm is 4200 rpms at 20 knots!

Jim.
Click to expand...

Hey Jim, Great to see you here. Thanks for the economy tip.

Harvey
SleepyC :moon

1_10_2012_from_Canon_961.thumb.jpg
 
Thataway posted
The Permatrim is not to make up for inadequate HP, or prop slip, but is to get the bow down when going into chop. (As are trim tabs for side to side trim as also being an aid to get bow down in chop.)

Sounds like you got a good setup, Thataway.

I have found that with the 4 blade I get more stern lift, reducing the need for as much tabs, which in turn reduces drag and makes me a bit more efficient and speedy, while still accomplishing the main goal for me, which is to get the bow into the chop. Boating is a fantastic engineering puzzle, no?
 
Have a pretty new Honda 100 with a 3-blade. Would like to get a 4-blade (alum), but what size should I get? (I'm more concerned with getting out of the hole, and control, than top speed.)
 
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