Help with prop selection...

Fish On Fabrications
Fiberglass Supply Depot
TinBoats.net

Help Support TinBoats.net:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
D

dkuster

Well-known member
Joined
Jul 15, 2011
Messages
84
Reaction score
0
Hi All,

I have a Grumman 14' Fisherman with '89 2-stroke Mercury 9.9.

I've framed and floored the boat using aluminum, and added 1/2" plywood decking, storage lockers, livewell, etc..

I also added a 55lb. thrust trolling motor + batteries. I figure just my mods have added somewhere around 200lbs to the boat (maybe more).

With myself, my wife, and son plus extra gear we're probably at ~650lbs. total (boat capacity is listed at 865lbs.)

I've raised the motor so the anti-ventilation plate is about an inch below the bottom of the boat. I've also adjusted the trim so that the top speed is maximized.

The motor came with 9 1/4" 9-pitch prop. I recently added a Hardline Products hour meter/tachometer so I could monitor RPMs and change the prop if necessary.

I have the tach configured for 1 pulse per revolution, which should be correct for a 2-cylinder 2-stroke motor. It _seems_ to be correct, because I get an idle RPM reading of around 550 RPM. What I was shocked to find was that at wide-open throttle my RPMs only read ~3900 RPM (!?) I realize the boat is heavily loaded. It obviously doesn't plane and my top speed on a calm day is about 9mph give or take.

But if I go by the rule-of-thumb that reducing the prop pitch by 1 increases motor RPM by 200, then I would need a 3-pitch prop (!!) to get my motor RPMs into the recommended range of 5,000 - 6,000.

Any thoughts?

Thanks!

Dan
 
Does it currently have a 2 blade? If you do, and are going to a 3 blade I have heard that you should drop another degree of pitch. And since you obviosusly need to drob at least 1 degree you would need to drop 2 total. Thats what I was told when debating going from a 12pitch 3 blade to a 4 blade.. It might not be as drastic on a smaller motor though. Hopefully some of the guys with more prop knowledge will chime in.
 
I thought dropping pitch by 1 increases RPM by 200 (not 2000) ?

So to go from 3900 RPM to 5100 RPM (just barely in the recommended range for my motor) I would need to drop the pitch by 6, which isn't possible.

My current prop is a 3-blade, 9.25" 9-pitch.

The best I could find was this 4-blade, 10" 5-pitch prop:

https://boatpropellers.iboats.com/Mercury-Outboard-Propellers/?pitch=5.00&chart=43&engine_id=222&cart_id=652093117

Even disregarding the effects of the increased diameter and number of blades, the best I could hope for is 4700 RPM(!)


[url=https://www.tinboats.net/forum/viewtopic.php?p=331484#p331484 said:
reedjj » 35 minutes ago[/url]"]Does it currently have a 2 blade? If you do, and are going to a 3 blade I have heard that you should drop another degree of pitch. And since you obviosusly need to drob at least 1 degree you would need to drop 2 total. Thats what I was told when debating going from a 12pitch 3 blade to a 4 blade.. It might not be as drastic on a smaller motor though. Hopefully some of the guys with more prop knowledge will chime in.
Click to expand...
 
I understand your problem. Make sure your tach is correct. Even if it's right at idle doesn't mean it's right at full throttle. My engines won't idle at 550 rpm so maybe the tach is off.
 
[url=https://www.tinboats.net/forum/viewtopic.php?p=331480#p331480 said:
dkuster » Today, 10:14[/url]"]Hi All,

I have a Grumman 14' Fisherman with '89 2-stroke Mercury 9.9.

I've framed and floored the boat using aluminum, and added 1/2" plywood decking, storage lockers, livewell, etc..

I also added a 55lb. thrust trolling motor + batteries. I figure just my mods have added somewhere around 200lbs to the boat (maybe more).

With myself, my wife, and son plus extra gear we're probably at ~650lbs. total (boat capacity is listed at 865lbs.)

I've raised the motor so the anti-ventilation plate is about an inch below the bottom of the boat. I've also adjusted the trim so that the top speed is maximized.

The motor came with 9 1/4" 9-pitch prop. I recently added a Hardline Products hour meter/tachometer so I could monitor RPMs and change the prop if necessary.

I have the tach configured for 1 pulse per revolution, which should be correct for a 2-cylinder 2-stroke motor. It _seems_ to be correct, because I get an idle RPM reading of around 550 RPM. What I was shocked to find was that at wide-open throttle my RPMs only read ~3900 RPM (!?) I realize the boat is heavily loaded. It obviously doesn't plane and my top speed on a calm day is about 9mph give or take.

But if I go by the rule-of-thumb that reducing the prop pitch by 1 increases motor RPM by 200, then I would need a 3-pitch prop (!!) to get my motor RPMs into the recommended range of 5,000 - 6,000.

Any thoughts?

Thanks!

Dan
Click to expand...


you gain about 150 -200 rpms for drop in pitch. you say you raised you motor so the cav plate is 1 inch below the hull??? you want your cav plate level with your hull for best performance and some boat/motor combos can be raised a little bit higher. you may never get your motor to max rpm operating range with the 9.9 and your load but I don't know it just sounds like a lot of weight for 9.9hp.. is your motor running right?? maybe its firing on one cylinder????
 
Yeah, I'm stuck with the 9.9 due to HP limitations on the lakes in my area.

The motor was in the shop earlier this year to be checked out, cleaned/tuned, and to have the water pump impeller replaced, so I'm sure it's running well.

I spent a lot of time on the tach installation. I followed the instructions and wrapped the pick-up wire around the spark plug wire, and the results were not good. The RPMs were reading too high, and they constantly jumped around +/- 500.

After some googling, I modified it so about 1" of pick-up wire was laid parallel to spark plug wire as close to the plug as possible.

After that, I got very steady readings that seemed reasonable.

I guess if there are no other suggestions I'll try the 4-blade, 5-pitch prop I linked above. Maybe it's just not possible to hit 5000 RPM with the weight of my boat + 3 passengers...
 
Sorry . . . there is no replacement for displacement . . . your motor size is what it is . . . adding pitch may sound good on paper but you don't have enough umf to swing it.
 
Understood.

But just to be clear, I'm not looking to go faster, I'm just looking to avoid damage to my motor from operating outside the recommended RPM range. So in this case I'm reducing pitch.
 

Similar threads

LDUBS
Impact of Prop Pitch on Trolling Speed
Replies
15
Views
2K
LDUBS
LDUBS
Stephen Fournier
Mercury Force 1998 50 HP Propeller Recommendation
Replies
15
Views
778
thill
thill
Makaw
Gear Ratio for Older Johnson 20?
Replies
7
Views
929
Makaw
Makaw
Catfish73
All propped out
Replies
8
Views
823
Zum
Zum
LaqueRatt
Best Prop?
2
Replies
22
Views
1K
LaqueRatt
LaqueRatt

Latest posts

Top