Prop size?

D

David Allen

Active member
I have a relatively new to me G3 16 foot side console with a 60 2 cycle.

It does not seem to rev high enough or give me as much speed as I think it should. Specifically it only revs to a hair over 4k and top out at 26-27 mph.

It should rev to 4500 to 5500 at full throttle.

When I bought the boat it had a bad prop so i replaced it with the same size and pitch, but i think i need a lower pitch prop. However be for i invest in a new prop i was hoping someone had a similar size boat and motor and could share what size and pitch that they are running.
 
If I remember correctly, and lately that can vary, lol, you pick up about 500 rpm or a little more, per inch of pitch, so if you have a 19 pitch, a 17 should get you where you want to be.
 
you don't say what prop you currently have. also we need to see pictures of the motor cavitation plate location versus the bottom of the boat. and what pin hole is the trim plate set at?

there are lots of variables to consider when sizing a prop/motor to a boat. also what load is in the boat?

are you plowing the water or does the boat jump right up on pad ?

give us more detail and we will try and help
 
gcs said:
If I remember correctly, and lately that can vary, lol, you pick up about 500 rpm or a little more, per inch of pitch, so if you have a 19 pitch, a 17 should get you where you want to be.
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I think it's 200 RPM per inch
 
I have a 15 inch pitch prop now. There is no "pin" as this is a power trim motor.

Given the 400 rpm per 2 inch pitch step it seems I should have an 11 inch.

Reviewing the Yamaha website it seems like similar boats with 4 cycle motors are running 13 to 17 inch pitch.

As 11 inch pitch is below that range I was hoping someone had a similar set up and could share there prop information.

I hate to drop a hundred bucks on a prop and it not be right.

Thanks
 
with that boat you shouldn't need to go to an 11 pitch. I am running an 1860 with a 60 hp two stroke. my boat has a full alum duck blind, 8 doz decoys, LP tank, heaters, ect and I am running a 13P at WOT I am getting 4800 RPM without trimming up and getting 28 MPH. I can start trimming up and get to 5100 RPM and 30 MPH. I don't push the RPM's any higher than that. I am not trying to win a race.

you need to double check the motor mounting. is the cavatation plate even with or slightly above the bottom of the boat? and even with power T&T; isn't there a cross pin that stops the motor position on the trim down ?

also, if you trim the motor up while running WOT, you will pick up RPM's

I wold say that you should try a 13P and trim up
 
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David I have a 16ft Smoker Craft side console with a 40hp Johnson 2stroke. it is a heavy boat with the minimal motor.
when I bought it it had a 17 prop and would only pull 4500rpm and was sluggish. I put a 13 prop on and it now runs 5500
at WOT and gets on plane much faster and goes 3mph faster.(it still only goes 23mph) It will also troll at idle (@2mph) for hours.
John
 
Okay I put an 13 inch pitch prop on and the RPMs are now at up to 4500 by it trimming up. I also gained 1 mph. I also had a better "hole shot', or got on plane quicker.

However, this set up seems to be more prone to cavitating or ventilating.

The motor does have hydrofoil and a trolling plate. I thought the hydrofoil was supposed to help and with the trolling plate was supposed to not be a factor but I wonder if that could be contributing to the cavitation.
 
I actually had more cavitation with the 17 pitch prop. Your RPM should be higher!
My boat is slow but my top speed is nearly constant with 1 or 4 people on board
Don't know what to say! John
 
can you post a picture of the back of the boat with the motor trimmed all of the way down. also we have not asked about weight distribution. how many batteries on the boat? are they all at the back? is your gas tank in the back? how much do you weigh? other people in the boat and where are they sitting?

that trolling plate is hurting your top end. I have mixed feelings about hydrofoils I think they are a band aid that masks other problems.
 
David Allen said:
Okay I put an 13 inch pitch prop on and the RPMs are now at up to 4500 by it trimming up. I also gained 1 mph. I also had a better "hole shot', or got on plane quicker.

However, this set up seems to be more prone to cavitating or ventilating.

The motor does have hydrofoil and a trolling plate. I thought the hydrofoil was supposed to help and with the trolling plate was supposed to not be a factor but I wonder if that could be contributing to the cavitation.
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I've been out of the country for a few days and am now late to this discussion, however I'll offer an opinion.

You removed the trolling plate and hydrofoil = IMHO that is a move in the right direction.

The 13 inch prop should be very close. Before trying different props If it were mine, I'd raise the entire motor up one hole on the transom and take the boat for a spin. Raising the motor height will reduce motor drag and alter the thrust line in relation to the hull. This change should increase top end and have a slight positive effect on hole shot. Raising the engine height may possibly have a negative affect, in regards to "loosing bite" when trimming the motor up, however, with the engine now higher in the water column, less trimming may be needed to achieve top speed. A prop with more "cupping" built in to it will help keep the prop from breaking loose when trimming up.

Getting the right combination of motor height, prop diameter, prop pitch, prop style, prop rake, cupping etc., can be challenging or one can get lucky and hit it right off the first time.
 
Okay. I had not considered that. I did some searching and sure on line and see there is a lot of different options on motor height but the lowest recommended seem to be the cavitation plate even with the bottom of the hull. Moving up 1 hole should give me that. THANKS!!!

Is this something I can do myself or do I need a shop? I have a loader on my tractor that can lift the outboard. It looks like something I could handle.
1. Remove the outboard.
2. Clean the surfaces.
3. 3m sealant in bolt holes and outboard mounting plate.
4. Reinstall outboard. Torque bolts to 55 ft/lbs.

The trolling plate will need to be reinstalled when trolling for trout and togue but can be off for land lock salmon and mackerel.
 
David Allen said:
Okay. I had not considered that. I did some searching and sure on line and see there is a lot of different options on motor height but the lowest recommended seem to be the cavitation plate even with the bottom of the hull. Moving up 1 hole should give me that. THANKS!!!

Is this something I can do myself or do I need a shop? I have a loader on my tractor that can lift the outboard. It looks like something I could handle.
1. Remove the outboard.
2. Clean the surfaces.
3. 3m sealant in bolt holes and outboard mounting plate.
4. Reinstall outboard. Torque bolts to 55 ft/lbs.

The trolling plate will need to be reinstalled when trolling for trout and togue but can be off for land lock salmon and mackerel.
Click to expand...

Sounds like you can handle this job, provided you either have good control of the loader or simply use it as an anchor point for a chain hoist. Be sure to use a proper lifting ring if your engine does not have a lift point(s) already in place. Many engines will use a lift eye which is attached to the crank or flywheel.

If you find moving up one hole was an improvement, you might try going up one more. I'm guessing a second move up will be too much but it up to you if you want to spend the time involved to verify that. Worse thing that can happen is you have to move the engine back down.
 
If your bottom holes on the outboard are slotted no need to remove them. My 75 has slotted holes i just remove top bolts loosen the bottom and use a floor jack on the foot or lifting eye on motor reinstall top bolts then reseal and torque bottom.
 
do as Dave has suggested. move up two holes I have my 60 mounted with the cavitation plate at 1" above the bottom and it works just fine

one other question; when you said that the motor was cavitating a lot and blowing out; was that when you had it trimmed up and running WOT? if so; you might have been trimmed too high.

I just saw the picture of your rig. definitely move up two holes. also try moving the trim pin out one hole.
 
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