Gear and ratio question

[Blackfindan]

I am a Newbie with an interest in moving up to a downeast boat. Now my ignorant question...

What is the importance of the gear and ratio information that is typically provided in a boat listing? What is a gear and ratio and what combination is good?

Told you I was a Newbie.. Thanks

 

[Toolate]

I am a relative newbie but its all a part of a big equation really.

Basically it allows you to have a greater understanding of the drivetrain and boats performance- there is no "best".

A slow turning diesel will usually be combined with a lower gear ration where a high revving gas engine with usually be paired with a higher gear ratio to get the prop speed down. Its just one more bit of info that helps the buyer understand whats under the hood and there is no better or worse. Each boat has a different prop size and hull and the combination of engine/prop/gear is not the "best" for all DE boats but individual to each.

For a real working, fishing boat for sale, I think the advertiser is smart to include this info as the perspective buyer will definitely want to know that info. These guys know their stuff or at least what they are used to and what they want based on that.

 

[Blackfindan]

Are you considering the Nauset 28 in New York?

I have seen it on Yachtworld. Looks nice.

I am still trying to figure out what models will meet my needs. I have always admired the Nauset lines, but I really am not familiar with many downeast models.

 

[Keelboater]

"What is a gear and ratio and what combination is good?"

The gear generally refers to the transmission. The ratio refers to the gear ratio inside the transmission. This is usually a reduction ratio, meaning that the output RPM of the prop shaft will be less than that of the input RPM of the motor. There is no combination of gear and ratio per say, there is only a transmission that has a gear ratio. The best gear ratio for a given vessel is based on multiple factors such as vessel's intended use, hull speed, prop size, and motor HP. A gear ratio of 2:1 will reduce the input rpm by half; a 3000 rpm motor would turn the prop shaft at 1500 rpm. A ratio of 1.5:1 would reduce it less to only 2000 rpm. Just divide the motor rpm by the ratio to get prop shaft rpm. Toolate might have been putting the cart before the horse. I say that only because of the way you worded the question.

 

[Toolate]

Toolate might have been putting the cart before the horse. I say that only because of the way you worded the question.

You are right- I didnt really answer the question as much as address the theory.

I leared a lot about this with my boat. I looked at 4 or 5 boats with different engines and gear ratios but similar props and the performance was at least similar at similar prop speeds so I think understanding the boat you want and how different ones run is the key to sorting out what is best for that boat.

Best to put the boats you are looking at up here and see what people have to say or gather performance info on boats you like and save it so you can reference it later.

 

[Blackfindan]

"What is a gear and ratio and what combination is good?"

The gear generally refers to the transmission. The ratio refers to the gear ratio inside the transmission. This is usually a reduction ratio, meaning that the output RPM of the prop shaft will be less than that of the input RPM of the motor. There is no combination of gear and ratio per say, there is only a transmission that has a gear ratio. The best gear ratio for a given vessel is based on multiple factors such as vessel's intended use, hull speed, prop size, and motor HP. A gear ratio of 2:1 will reduce the input rpm by half; a 3000 rpm motor would turn the prop shaft at 1500 rpm. A ratio of 1.5:1 would reduce it less to only 2000 rpm. Just divide the motor rpm by the ratio to get prop shaft rpm. Toolate might have been putting the cart before the horse. I say that only because of the way you worded the question.

Thank you, Great explanation. So I guess there is a third item that fits into the equation...Prop size. Depending on the gear ratio, one would want an appropriately sized prop to make sure the motor is not stressed. So if I understand correctly, if a boat has a small gear ratio such that the shaft spins nearly at the same rpm as the input rpm, then you would want a prop that is not oversized or it will stress the engine.

Next question. How does one know what a correctly sized prop is for a given gear ratio.

 

[Keelboater]

"How does one know what a correctly sized prop is for a given gear ratio."

To keep it simple, the answer is maximum motor RPM at wide open throttle. This is specified by the motor manufacturer. Higher RPM means the prop does not have enough pitch. Lower RPM means the prop has too much pitch. Prop diameter also plays a role here.

 

[F/V First Team]

Trial and a whole lot of errors

 

[captainlarry84]

I am a Newbie with an interest in moving up to a downeast boat. Now my ignorant question...

What is the importance of the gear and ratio information that is typically provided in a boat listing? What is a gear and ratio and what combination is good?

Told you I was a Newbie.. Thanks

A good combination on a DE boat means that you swing the largest wheel possible as slow as possible. Slow turning large wheel are better able to grab the good water as it flows around the keel which blocks some of the water flow.

Example from 1st hand Trial & Error.

On a 31 JC we repowered with a 370 Yanmar @ 3300 RPMs & a 1.77 gear. The largest wheel the boat could turn was a 23 X 18 & a shaft speed of 1864 RPMs.
We slowed the wheel down by changing the gear to a 2:0 with a shaft speed of 1650 and a wheel size of 23 X 21. A big performance boost.
If I would have used a 2.5:1 gear the wheel would have been a 23 X 25 with a shaft speed of 1320. I never tried the 2.5 but I bet it would have out performed the 2:0 gear set up.

Slow shaft speed saves a lot on the cutlass bearing & the larger wheel holds a lot more water to push the boat with a full load & in heavy seas.

 

[petrel]

Fortunately a lot of the down east hulls today are built to accommodate a fairly big prop. 30 or 40 years ago that wasn't always the case. Some of those boats (like Larry's JC) can be modified to take a bigger prop and it can help a lot. A friend of mine had a 3 to 1 reduction in his 31' BHM. When he started the gear was behind a 3208 natural and he had maybe a 24 square wheel. Later on he went to a 320 hp 3208 T and kept the same reduction, but the wheel was changed to 28 square. Turned out to be to be a good set up for a commercial boat w/ an economical 15 knot cruise w/ the 320hp. Keith Otis built that boat and it was a successful trial I'd say. That's winterking's boat now. 3208's turn 2800 rpms, so you can do the math. My BHM goes good w/ a 2.5 to 1 and 315 hp. also 2800 rpms. If I had that hp at 2200 rpms I'd be better off w/ 2 to 1.

 

[Blackfindan]

A good combination on a DE boat means that you swing the largest wheel possible as slow as possible. Slow turning large wheel are better able to grab the good water as it flows around the keel which blocks some of the water.

Capt. Larry, thank you for the example. Two questions:

1) "370 Yanmar @ 3300 RPMs" is 3300 Rpms at cruise or max Rpms of the engine?

2) "The largest wheel the boat could turn was a 23 X 18" What are the dimensions 23 x 18? One must be diameter of the prop, what is the other?

 

[F/V First Team]

23" of diameter
18" of pitch

So in one rotation the propeller should go ahead 18 inches with no slip

 

[gary49]

Most downeasters have a 4 blade prop, to grab more water coming off the keel that's in front of it. Also, size matters: having a big prop will help the boat handle better around the dock, i.e. "propwalk" is effective in moving the stern, as well as the big wheel providing more thrust against the rudder the better to manuver the boat.

 

[Blackfindan]

Thanks for the responses everyone, I am really learning a lot here.

Next question...

What is a trolling gear?

I get it must slow down the RPM of the shaft,but how does it work? Does it tax the transmission and cause any increased maintenance issues?

 

[F/V First Team]

The trolling valve limits the amount of pressure used to lock up the clutches within your transmission allowing them to slip and thereby turn the propeller shaft slower. Over a period of time you can glaze the clutches and they will just slip all the time. Additionally you can leave the valve open just a bit and on a long run the build up of heat will cause the clutches to fuse together so that when you get back to port and you put the engine in reverse as you're coming into dock the transmission will shell out and you'll make a glorious arrival.

 

[F/V First Team]

The electronically controlled transmissions have a safety built in, so if the engine rpms go above a certain amount it automatically kicks out of trolling mode. So that's an improvement.

 

[captainlarry84]

A good combination on a DE boat means that you swing the largest wheel possible as slow as possible. Slow turning large wheel are better able to grab the good water as it flows around the keel which blocks some of the water.

Capt. Larry, thank you for the example. Two questions:

1) "370 Yanmar @ 3300 RPMs" is 3300 Rpms at cruise or max Rpms of the engine?

2) "The largest wheel the boat could turn was a 23 X 18" What are the dimensions 23 x 18? One must be diameter of the prop, what is the other?

3300 RPMs is WOT the continues running speed is 3100 RPMs

On all wheel the 1st # is the diameter the 2nd # is the pitch.