What is desired trolling hp?

[tugboat80]

I have a '84 Marlin FG weighs about 1000 lbs powered with a 90 hp two stroke. It runs the boat wot at 35mph. It has a trolling plate on it. What hp two stroke will I need to troll leasurly at 3/4 to 2 mph. I have now a 4.5 Erude, but it is not enough. Will 6hp be better? Will 7.5 hp be too much? I had a 4 stroke 8 hp which could not go slow enough.

 

[roscoe]

Enough hp to push boat, to control boat, to maneuver boat, but slow enough for fishing.

I have similar boat in weight and size, any motor smaller than 8 hp was not enough to control boat in wind.

I am using a 9.8 hp now.

If it won't go slow enough, you need to change the prop to one with less pitch.

I did this with mine, switching from an 11 pitch, to a 5 pitch.

I can get down below 1/2 mph, yet have the torque to maneuver the boat.

Its kind of like driving a tugboat.

 

[tugboat80]

Thanks for the reply, Roscoe. I am trying to use the least weight on the transom for two reasons. One, I will remove trolling motor for transit, I have an aged back; And two, I must sit at the rear to steer and control speed, adding weight and rear ballast. I have a 15 Johnson which may run hot at minimum speed even after 500$ work on water pump and power head grommet. Plus the 15 is an exertion to remove and replace at the ramp.

 

[KC8QVO]

tugboat - what are your boat dimensions? length/beam? That sounds similar to what I am looking at getting so I'd be curious. I want to add a trolling motor also, for trolling primarily but it has to be capable of getting me home from 100 miles out if the main goes out too.

 

[roscoe]

Mine's a 18.5 aluminum Starcraft center console, 900# hull, plus 2 motors, 18 gallons of fuel, 2 batteries, tackle, gear cooler, 3 people, so about 2000# in the water.

A 9.9 or 15 won't plane my boat even when empty, but will move it along anywhere between .3 mph and 7 mph ( hull displacement speed).
The motor has a lot of pushing power with the 5 pitch 4 blade prop I put on it.

And the motor will get me home (slowly) from 15 miles out.


TUGBOAT80, I would think your 8 hp 4s motor would have been perfect, with a lower pitch prop.

 

[tugboat80]

KC8, my length is 16' and beam is 6' 3".

 

[ondarvr]

A 6, 8, 9.9 or 15 will all do it, depending the make and model there may not be a much of a weight difference between these sizes. Your 15 is the same weight as a 9.9 and would be a very good option, but the $500 you put into it could have purchased a good running motor. I had one and the problem was the water passages were plugged with corrosion, had to pull the head off and clean it out. If you buy another motor make sure you get a long shaft, it will work much better, the 4.5 may work if it was a long shaft.

Like roscoe said, get the right prop and it will perform much better.

 

[Scott Danforth]

the HP will depend on wind, current, etc.

no wind, no current, you could probably do it with a 2.5hp motor
heavy wind and current, and you may need more than 15hp.

you have a 4.5 now, what exactly is the short commings with it? does it just not have enough thrust to get you moving, yet once moving its fine. Or are you in an area with high winds and current?

 

[airshot]

I can't see where your 4.5 would not work, perhaps a prop change is in order. If the motor is in good running order you should achieve that trolling speed without any difficulty. I have a 22' with a 9.9 and it pushes without a problem. I have had it up to 10mph as a test to see how it would work in an emergency, even hit 12 mph on a really calm day, your 4.5 should work fine.

 

[tugboat80]

Thanks guys for the info. I thought I might sell the 4.5 as a package with a 12 alum and trailer. I don't need three boats. The 15 goes very well on a 14 Hewescraft up to 20 mph, and is light enough to pull off the transom for transporting over rough roads. I like short shaft for the trolling mount on my 16 FG because they will lock up higher out of water when using the main 90hp motor. The 4.5 can push up to 5 mph but working very hard. Oh well, what to do.

 

[Scott Danforth]

change props on the 4.5 since you dont need to troll at 5mph. drop down in pitch.

 

[ondarvr]

change props on the 4.5 since you dont need to troll at 5mph. drop down in pitch.

I thought the same thing, if all you need is 2 mph, the 4.5 with a different prop will do it easily. Plus it's light and they're good motors.

The reason for the long shaft suggestion is on many boats a short shaft kicker will have the prop above the bottom of the transom, or very close to inline with it. It's not in clean water in that location, this results in poor thrust, less control and lower speeds. Going to a long shaft drops the prop down into clean water below the hull and it works better. Depending on the exact boat and kicker combination some times the skeg on the kicker will drag in the water at speed when using a long shaft though.

 

[joetheis]

Last season I had a 9.9 Mariner on a Silver Line cuddy, going for @ #3200.
The 9.9. had the prop it came with, can say if pitch was smaller of larger.
The prop wasn't one of those HUGE looking trolling props I've seem.
This motor would push the boat with 4 adults on the St. Lawrence with and against the current with some good winds.
I bought this because of the forward controls, made a steering link to hook to the outdrive.
I could get 1/2 mph to 5-6 WOT.
Worked great all season. I think I ran that more than the 228 V-8 in the boat!
I sold THAT boat and now have a Grumman 19' SF Almn fishing boat that goes @ #1200.
It has a 70 HP "Rude and I just installed a British Seagull 4 HP troller.
It's lightweight AND can push this boat @ well.
I made a steering linkage for it and throttle, so I can sit up front and steer with the wheel.
I can't believe your 4.5 won't push that boat @, when you think a E-lectric trolling motor will pull it @?
Joe
Take any advice I give with a beer er 2!