I'm under propped. What should I move up to?

[Chotu]

I have Evinrude etec 30hp engines on my 7 ton cat. I'm considering brining her up north to work on. It's a 1400nm trip. To maximize fuel economy I'd like to get the most speed possible out of the boat at the 3 gallon per hour burn rate. I have no income currently and every dollar counts.

I currently have the Evinrude 4 blade, 11 inch x 9 inch pitch prop on these motors. They hit the RPM limiter very easily at 8 knots and that's it. 3gph at 8 knots. Expensive trip.

I should be able to change the props and get a better top end out of this for the trip up.

How do I go about finding the right prop? I know aluminum should be fine. I want ones that don't ventilate easily. I know I need to pitch up.

Do I need 4 blade or 3 blade?

Looking at solas props. I just need to go 10+ knots under power at full tilt to get the expense of motoring up down to something more manageable for Covid-19 no income times. I'm looking at $2500 to move it otherwise. Trying to bring that down by getting the right props.

[boatpoker]

Originally Posted by Chotu

I have Evinrude etec 30hp engines on my 7 ton cat. I'm considering brining her up north to work on. It's a 1400nm trip. To maximize fuel economy I'd like to get the most speed possible out of the boat at the 3 gallon per hour burn rate. I have no income currently and every dollar counts.

I currently have the Evinrude 4 blade, 11 inch x 9 inch pitch prop on these motors. They hit the RPM limiter very easily at 8 knots and that's it. 3gph at 8 knots. Expensive trip.

I should be able to change the props and get a better top end out of this for the trip up.

How do I go about finding the right prop? I know aluminum should be fine. I want ones that don't ventilate easily. I know I need to pitch up.

Do I need 4 blade or 3 blade?

Looking at solas props. I just need to go 10+ knots under power at full tilt to get the expense of motoring up down to something more manageable for Covid-19 no income times. I'm looking at $2500 to move it otherwise. Trying to bring that down by getting the right props.

LOA ?

[Chotu]

Originally Posted by boatpoker

LOA ?

50' LOA, 25' beam 7 tons. Very narrow hulls.

Think early Gunboat 48. That's the closest production boat.

[Rucksta]

Have you investigated cupping the props?

It involves bending the tips to increase thrust.

It's cheaper than new props and whilst it does not deliver top end it does increase punch into the upper mid range e.g. 8 kt cruise at lower throttle opening and less knock to speed on pitching moment.

Combined prop mod cost and moderate fuel saving may add up better than prop replacement and greater fuel saving.

A prop shop set up to do the mods will have the calculations.

[LittleWing77]

Chotu,

You can't sail, her, right? She still doesn't have a mast or sails yet?

I'm just trying to clarify because it's going to be a different use - primary rather than auxillary.

As an aside - wow. What a trip that'll be. You're gonna tell us all about it, yes?

Good luck! I'm sort of excited for you,
Warmly,
Little Wing77

[Chotu]

Originally Posted by Rucksta

Have you investigated cupping the props?

It involves bending the tips to increase thrust.

It's cheaper than new props and whilst it does not deliver top end it does increase punch into the upper mid range e.g. 8 kt cruise at lower throttle opening and less knock to speed on pitching moment.

Combined prop mod cost and moderate fuel saving may add up better than prop replacement and greater fuel saving.

A prop shop set up to do the mods will have the calculations.

Getting these props modified is a fine idea but with the virus and all, it's not possible. All the businesses are closed.

Props are $65-$150 each. These are just aluminum outboard props. I'm hoping to save a lot more than that on fuel.

[Chotu]

Originally Posted by LittleWing77

Chotu,

You can't sail, her, right? She still doesn't have a mast or sails yet?

I'm just trying to clarify because it's going to be a different use - primary rather than auxillary.

As an aside - wow. What a trip that'll be. You're gonna tell us all about it, yes?

Good luck! I'm sort of excited for you,
Warmly,
Little Wing77

Hi.

Yes. The boat has no rig or sails. It's just a lightweight displacement powercat with Long, narrow hulls. Think old Gunboat 48 and take the rig off in your mind. That's the boat.

And sure. I'd be happy to fill you in on the trip. It's the reverse of a trip you'd take to Florida from Toronto. Minus the inland waterway part. Lots of icw though since without a rig it'll be fast in the flat Water.

[Rucksta]

Originally Posted by Chotu

Getting these props modified is a fine idea but with the virus and all, it's not possible. All the businesses are closed.

Props are $65-$150 each. These are just aluminum outboard props. I'm hoping to save a lot more than that on fuel.

OK mods no, new props by mail order? yes?

I think you should be looking at increasing the diameter rather than pitch.

A 9" pitch prop spinning at 4000rpm is trying to proceed at about 30mph.
If you are not going 30mph at 4000rpm you have excessive prop slippage

More thrust is required.

A larger diameter will achieve more thrust.
As power is finite (30hp) reducing the pitch is required to balance the equation.

Another effect of outboard props is they are designed to overcome some of the inefficiencies of moving fast through the water. Go slow (boat speed) and you loose that benefit.

Round tips rather than pointy tips are a choice for slower speed hulls

I don't know how large a diameter prop will physical fit your engine.
I'm thinking a 16" diameter with a 6" pitch (round tips) would be closer to a solution.

Get hold of a free online prop calculator and work through the options compared to what is physically available.


Hope this helps - good luck with the voyage.

[Chotu]

These are outboards so the diameter is fixed by the geometry of the outboard leg. 11" is nearly scraping the anti cavitation plate.

All the rest of the props that fit are 10.5 or 10".

It's running out of top end. Hitting the Rev limit at 5500 Rpm. 9" is the smallest prop pitch available for this engine. It's under propped, not over propped. The prop isn't turning at 5500 RPM right? owing to the drive leg gearing?

These are the available props.


https://www.deepblueyachtsupply.com/...011-2014/32330

Originally Posted by Rucksta

OK mods no, new props by mail order? yes?

I think you should be looking at increasing the diameter rather than pitch.

A 9" pitch prop spinning at 4000rpm is trying to proceed at about 30mph.
If you are not going 30mph at 4000rpm you have excessive prop slippage

More thrust is required.

A larger diameter will achieve more thrust.
As power is finite (30hp) reducing the pitch is required to balance the equation.

Another effect of outboard props is they are designed to overcome some of the inefficiencies of moving fast through the water. Go slow (boat speed) and you loose that benefit.

Round tips rather than pointy tips are a choice for slower speed hulls

I don't know how large a diameter prop will physical fit your engine.
I'm thinking a 16" diameter with a 6" pitch (round tips) would be closer to a solution.

Get hold of a free online prop calculator and work through the options compared to what is physically available.


Hope this helps - good luck with the voyage.

[Chotu]

A more straightforward Listing of the props available for my outboards:

[VBsail]

I tried 4 solas props on my DF20A outboard to get my boat moving more efficiently. Mine were 9.25" diameter since thats all that motor would fit.



(I also used an adapter and the 10" dia 7 pitch 4 blade high thrust with big flat fins, it was too low a pitch)

The 4 blade solas amita did not work for me. In that small diameter had skinny fins compared to a standard 3 blade. I could feel it slip at about 3/4 throttle and rpms would jump from about 4k to redline with no increase in speed.

I tried the amita 3 blade in 7 and 10 pitch. The 7 pitch gave a hair more speed and would rev higher, but was noisy at all speeds since revs were higher and used more fuel (I had the motor connected via NMEA to my garmin) The 10 pitch on it was overpropped and would only get to about 4500 rpm, but cruise 6mph at 3000rpm and used .4 gph.

From my experience if you want flatwater cruising efficiency error a bit on overpitched and 3 blade. It will suck in reverse and acceleration/changing boat direction though.

I would probably try the 10.5 diameter 11 pitch. Higher pitch than that and the diameter is getting small.

Your motors have a 2.15 gear ratio, so 9mph at 5500rpm and 9 pitch is 60% slip.

[Chotu]

Originally Posted by VBsail

I tried 4 solas props on my DF20A outboard to get my boat moving more efficiently. Mine were 9.25" diameter since thats all that motor would fit.



(I also used an adapter and the 10" dia 7 pitch 4 blade high thrust with big flat fins, it was too low a pitch)

The 4 blade solas amita did not work for me. In that small diameter had skinny fins compared to a standard 3 blade. I could feel it slip at about 3/4 throttle and rpms would jump from about 4k to redline with no increase in speed.

I tried the amita 3 blade in 7 and 10 pitch. The 7 pitch gave a hair more speed and would rev higher, but was noisy at all speeds since revs were higher and used more fuel (I had the motor connected via NMEA to my garmin) The 10 pitch on it was overpropped and would only get to about 4500 rpm, but cruise 6mph at 3000rpm and used .4 gph.

From my experience if you want flatwater cruising efficiency error a bit on overpitched and 3 blade. It will suck in reverse and acceleration/changing boat direction though.

I would probably try the 10.5 diameter 11 pitch. Higher pitch than that and the diameter is getting small.

Your motors have a 2.15 gear ratio, so 9mph at 5500rpm and 9 pitch is 60% slip.

Wow!!

Thank you for this information. I couldn't have learned this in 3 weeks of Googling full time.

I suppose I can change the props when I know I'm going to need good handling for close quarters vs open water efficiency.

I can keep the high thrust 4 blade prop for those situations.

NOTE:. My 11" prop is the 4 blade, Hydrus high thrust, 7" pitch. Not 9". I had it wrong in the initial post. Sorry!

[Rucksta]

Checked the list and ran a prop calculator based on a 2.15:1 gear ratio.
Don't like your choices from the available options.

A higher pitch may give you more speed at the cost of even higher fuel consumption.

You could try shrouding the prop (Kort Nozzle) to increase thrust.

There are aftermarket items available as bolt ons.

West Marine would be a starting point.
Under $150.00.
Should improve low speed turning and backing up as well.

Sorry I don't have an easy answer.

[MicHughV]

this is not as easy as it seems...
David Gerr wrote a good book about props, engines, etc..called the " the propellor handbook"
there is much to consider here, far too much to explain on this forum....but one can't arbitrarily switch props without also considering engine horse power....shaft rpm...blade tip clearance to hull..fuel economy....and a myriad of other things..

[CarlF]

I've gotten props here.

Boat Propellers for Sale, Stainless Steel Boat Props - iboats.com

You probably don't have a lot of choice on diameter without hitting some part of the lower leg so pitch is what you can play with.

As others have suggested, you may be hitting the governor because of cavitation. Usually you want a lower than standard pitch for a heavy, displacement boat. I found that reducing pitch 2" on my 20HP Honda dinghy engine made it come on plane with a heavy load much better. I didn't mind losing a bit off the top speed.

But if you think a higher pitch would give you more speed I'd just try 1" higher pitch. The props are relatively cheap and if it doesn't make a difference it's always good to have a spare prop onboard.