1982 GB 36 single prop

[Caballero II]

Who knows what the specified prop should be, and what might be an upgrade? I have a single screw, 1982 GB 36 with a Lehman 120 hp and a Borg Warner transmission. Standard equipment.

I'd like to know what was the specified propeller for this model, and has anyone found an upgrade to buy an extra 1/2 knot, or some other efficiency. We cruise at about 1700 rpms at close to 8 knots(which feels right to us) and burn about 1.7 gph. The boat is not out of the water now, so I cannot even swear I have 3 or 4 blades (I'm embarrased), but I will find out. I am due to pull out of the water this winter and bottom paint, so I would like to do some due diligence to see if there might be a better prop prescription to get the most out of the boat.

Also, I'm looking for a legitimate and bona fide prop shop on the southern California coast to consult with on this.

 

[Cigatoo]

Your prop seems to be nicely pitched. You are at hull speed turning 1700 RPM. To get that extra half knot you either need to bump up your RPM or increase your propellers pitch. Trying to push the boat past hull speed is going to burn a lot more fuel and / or tax your engine. Leave well enough alone. (You probably have a 3 bladed prop by the way.). Find something else to tweak

 

[EVERFOR]

GBs props

This list should prove helpful. I agree with Cigatoo. You are probably fine. If you are due for a hull out your speed is most likely being determined by bottom and prop growth rather than your prop specs. It doesn’t take much to reduce efficiency. While you’re out have the prop shop do a scan and make sure the blades were never damaged or modified by a previous owner. Also check with Brian (he is the Lehman guru) at American Diesel on the optimum cruise rpm for the 120 Lehman.

https://americandieselcorp.com/

 

[Cigatoo]

This list should prove helpful. I agree with Cigatoo. You are probably fine. If you are due for a hull out your speed is most likely being determined by bottom and prop growth rather than your prop specs. It doesn’t take much to reduce efficiency. While you’re out have the prop shop do a scan and make sure the blades were never damaged or modified by a previous owner. Also check with Brian (he is the Lehman guru) at American Diesel on the optimum cruise rpm for the 120 Lehman.

https://americandieselcorp.com/

Thank you Everfor! That’s great information. Will keep for my records. Maybe I do have four blades?

 

[Caballero II]

This list should prove helpful. I agree with Cigatoo. You are probably fine. If you are due for a hull out your speed is most likely being determined by bottom and prop growth rather than your prop specs. It doesn’t take much to reduce efficiency. While you’re out have the prop shop do a scan and make sure the blades were never damaged or modified by a previous owner. Also check with Brian (he is the Lehman guru) at American Diesel on the optimum cruise rpm for the 120 Lehman.

https://americandieselcorp.com/

Thanks Cigatoo and EVERFOR. The chart is indeed great information and just what I was looking for. I mostly wanted to be sure I was getting the most out of my boat.

Where did you get the Prop Specifications chart? It looks like a great source of information that I have not seen before.

I have known Brian for years, and his father before him. Great Stuff.

 

[EVERFOR]

A friend with a 36 gave it to me. I believe it came from the GB Facebook site.

Enjoy your boat. I admit that I am a Grand Banks snob and I think the 36 Classic was the best one they ever made.

 

[Cigatoo]

Yep. 4 bladed prop. Oh well. Lol

 

[mbdana]

GB Prop

Yup, 4 blades on our prop. 2" shaft running from CAT 3208 NA. Your data sounds right on for your hull and engine RPMs....you know, like Cigatoo implied, don't mess with mother nature or GB Classic specs.....1/2 knot is nothing in the big picture. Enjoy your GB the way it was designed.
Good luck
Mike Dana
Third Reef
36' Grand Banks Classic #819
Potts Harbor, Maine. (South Harpswell)

 

[KatyLeigh]

My 1981 36 GB Classic (same engine and transmission) has a 4 bladed prop, 24" diameter x 14" pitch LH

 

[High Wire]

If those performance numbers (8 knots) are accurate, you are already way above average in performance. Are you sure that it’s not 8 MPH?

 

[Old Sea Dog]

'79 GB36 with same. 120 single with 24×14×4. As has been said. The cost ain't worth the gain.

 

[Caballero II]

If those performance numbers (8 knots) are accurate, you are already way above average in performance. Are you sure that it’s not 8 MPH?

Yes, it's knots measured by GPS. I don't mean to imply I always get that speed, but that's best case scenario, as when running "downhill". I plan on 7.5 knots.

 

[Jeff F]

If you're running at 1700 rpm and want to go a half knot faster the obvious effective way to do that is to bump up the throttle.

Unless your existing prop is damaged or deficient there is likely to be little efficiency to be gained in prop swapping.

Maybe I misunderstand the goals...

 

[High Wire]

Yes, it's knots measured by GPS. I don't mean to imply I always get that speed, but that's best case scenario, as when running "downhill". I plan on 7.5 knots.

As long as you’re not always counting on that extra push. On the Loop you get a favorable push from an inlet behind you then 5 miles later you’ll have that same tide pushing against you from the next inlet. Even a canal between locks will have some current. You want to make good decisions based on correct data.
Btw don’t mess with anything until you know exactly how accurate your tachometer is. Most tachs are fed by your alternator which can be significantly wrong due to belt wear, pulley size, slippage or the previous owner tweaked the adjustment on the back. Use a photo tach accurate to +/- 1 rpm to make a RPM table for each helm tach.
Lastly, 7.5 knots no-current at post 1’s RPM and fuel consumption is still excellent! I would not mess with that.

 

[Caballero II]

If you're running at 1700 rpm and want to go a half knot faster the obvious effective way to do that is to bump up the throttle.

Unless your existing prop is damaged or deficient there is likely to be little efficiency to be gained in prop swapping.

Maybe I misunderstand the goals...

I guess I didn't articulate well enough. Obviously I could bump up the throttle. The point of this thread was trying to confirm the correct Propeller prescription, and how might I optimize my efficiency if a different specification might do that. I am not interested in burning more fuel.

To my knowledge my propeller is in perfectly good shape. I just want to be ready to inspect it with foreknowledge when I next pull the boat out.

 

[Jeff F]

I guess I didn't articulate well enough. Obviously I could bump up the throttle. The point of this thread was trying to confirm the correct Propeller prescription, and how might I optimize my efficiency if a different specification might do that. I am not interested in burning more fuel.



To my knowledge my propeller is in perfectly good shape. I just want to be ready to inspect it with foreknowledge when I next pull the boat out.

Thanks. That makes sense.

If optimizing efficiency means going faster with the same fuel burn, you're not going to see much if any improvement swapping props.

Your current numbers look great. I wouldn't mess with a thing.

 

[Choices]

I have a 36 Europa and run a prop with 1" extra pitch.

At 1800 rpm I get a hydraulic vibration through the hull, not there with the spec prop.

The half mph speed increase is not worth the vibration.

 

[Max1]

This list should prove helpful. I agree with Cigatoo. You are probably fine. If you are due for a hull out your speed is most likely being determined by bottom and prop growth rather than your prop specs. It doesn’t take much to reduce efficiency. While you’re out have the prop shop do a scan and make sure the blades were never damaged or modified by a previous owner. Also check with Brian (he is the Lehman guru) at American Diesel on the optimum cruise rpm for the 120 Lehman.

https://americandieselcorp.com/

Well, this is interesting. Thanks for the table.
I have an MT36 Europa (Similar hull to the GB36) and according to the table, I should be using a 4 blade 24x14. However, when I got the boat it came with a 24x20 3 blade. We like to cruise at a leisurely 1300rpm and we get 6 knots. By my estimation, I burn approx 1.5 gal/hr. Any need to be concerned about my current configuration?

 

[ronobrien]

This may not be relevant, but my GB36 (1973) has twin FL 108HP (according to Brian at AD) and it has been fitted with 22 X 17 X 4. I have limited experience running the boat as I bought and moved it from CT to RI and pulled it out for a multi year refit. My recollection is that we were making 8kt through the water at around 1600-1700 RPM. My recollection of the RPM may be off a bit but the 8KT is correct. I have no idea yet what it burns/hour.

 

[petehdgs]

I know this is an old thread, but have you thought about the hull power demand curve as it applies to adding speed to the boat? >>" We cruise at about 1700 rpms at close to 8 knots(which feels right to us) and burn about 1.7 gph.">>
1.7 gph means the engine is producing about 32.5 brake HP depending on the engine efficiency. A diesel engine usually is most efficient at or near peak torque RPM. That is the preferred RPM for cruise if possible. As you move higher towards peak HP RPM it takes more fuel to make the same HP, not to mention louder and noisier.

At 97% marine gear efficiency your hull and prop are pulling about 31.5 HP at 8 knots. (Most propellers are about 60-65% efficient) That same hull will pull 37.8 HP at 8.5 knots and 44.85 HP at 9 knots. It varies directly with hull speed cubed. That means adding speed makes the HP demand and fuel consumption go up fast. At 8.5 knots you should burn 2.0+ gph and at 9 knots you will burn about 2.4+.

I don't think you can add speed without some trade off, although if you were able to put the engine in its most efficient BSFC power band, AND changing the prop would do that while gaining propeller efficiency, then you might gain a little, but the experiment might end up losing it too.