help with my backup motor

[coolbri70]

i got to try out my motor today, the guy i got it from had it on a jon boat, i have it on my 68 larson 18' runabout, i made a video, its a 9.9 with 15hp carb and exhaust done 94 evinrude, i think this prop is wrong it loses bite after half throttle,

help would be appreciated

 

[coolbri70]

Re: help with my backup motor

i should add my speed gps 7 mph

 

[steelespike]

Re: help with my backup motor

Without your present prop size there is no way to tell what size you need to go to.
Prop size should be under the prop nut or on the barrel of the prop.I do wonder if the prop may be too close to the surface.
Depending on the present prop I think a 10" X 7" 4 blade should work well.
I can't speak to the 94 motor but as far as I know the 15 is rated at 7,000 rpm.
A prop change isn't likely to change your speed much but should let the motor rev easier and the 4 blade will probably have a little better control.

 

[Georgesalmon]

Re: help with my backup motor

Sounds like it might be ventilating. How far is the prop in the water? Or, more important where does the cavitation plate sit in relation to the hull? You need clean water for the prop to bite into, even a transducer in front of the motor could mess that up. It needs to be just even or better a little below the hull at the point where it sits behind the hull for a kicker. If its a 15" shaft engine (might be coming from a jon boat) on a 20" transom even with the deadrise going up where its mounted it could still be a little high. Probably won't get much more speed since it won't plane the hull and your pushing at displacement speeds. Worst case you'll need a trolling motor bracket that goes up and down. With any prop size, as steele said your only interested in being able to get rpm's in the WOT range for your engine.

 

[WIMUSKY]

Re: help with my backup motor

Given the size of the boat and kicker motor, I'm not sure you're going to get much better performance.....

 

[jimbo_jwc]

Re: help with my backup motor

Motor mounted to low cavatation plate on motor should be at bottom of boat or your weight or trim of motor trying to lift bow .

 

[coolbri70]

Re: help with my backup motor

View attachment 183021 i will have to remove the prop to find the size, i like the depth for exploring shallow places, with the big motor tilted up i was able to go upriver alot further, just not very fast, and i feel more secure out on the water with a backup, just doesn't grip after half throttle

 

[coolbri70]

Re: help with my backup motor

i'm not having issues reaching rpms, it will wind out fine, it keeps loseing bite and i have to let off so it will catch, then i try to give it gas and starts slipping again. the cavitation plate is a few inches below water and i can trim it out to the next to last hole and theprop is still under water, that makes it blow water in the air behind, had it in the middle in the video, i may try going down 1 hole

 

[Georgesalmon]

Re: help with my backup motor

from your photo it looks deep enough. might try a four blade high thrust prop on there like they use on sailboats and purpose built kicker motors.

Your prop might be way to high a pitch and is losing traction (actual cavitation in this case not ventilation) so to speak trying to push your heavy boat as opposed to its old job pushing a light jon boat much faster.

 

[MH Hawker]

Re: help with my backup motor

I wish you were a bit closer you could use my hand held tach to see your RPM`s. I am by Huntington.

 

[coolbri70]

Re: help with my backup motor

i took the prop off today, its 9 1/2 X 10. i'm not expecting major performance, just trying to better suit it to it's purpose

 

[steelespike]

Re: help with my backup motor

A 4 blade 7" should do better probably not much more speed but better control.

 

[coolbri70]

Re: help with my backup motor

A 4 blade 7" should do better probably not much more speed but better control.

is that 4 blades with less pitch?

 

[coolbri70]

Re: help with my backup motor

i don't understand i would think that with less hp to weight, i would want less blades for lower gearing. please help me understand

 

[Part-time]

Re: help with my backup motor

more blade = more prop surface to push water
the pitch is your gear ratio, the lower the pitch, the lower the gear ratio
a high thrust prop will also have a different shape to the blades making more efficient in pushing a heavier load.

 

[coolbri70]

Re: help with my backup motor

more blade = more prop surface to push water
the pitch is your gear ratio, the lower the pitch, the lower the gear ratio
a high thrust prop will also have a different shape to the blades making more efficient in pushing a heavier load.

ok, i was just trying to understand why 4 blades rather than 3 larger blades as i'm working with a limited diameter, isn't it about thrust surface? why not 2 wide blades? harder to balance?

 

[Part-time]

Re: help with my backup motor

there are 3 blade high thrust props as well...However I never seen two blade.
I guess it's as much in the shape of the blades as the size.

 

[coolbri70]

Re: help with my backup motor

ok, i think i will try the 4 blade 7"