There are no dumb questions, right?

backman

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Here are 3 that are pretty close.

1. can a gear be rebuilt from one ratio to another?
2. Can a gear get pulled out of an engine room without removing the engine?
3. What si the rough cost of a new ZF/311 1.75 gear ratio and what is the rough price of a quality rebuild?


Background is that I am finding my boat/engine/gear/prop is not giving me what I want when fully loaded or in moderate sea conditions.

On one end the God of Cummins says "reduce pitch to decrease load" and on the other end the God of Props says "all that do will slow you down - your not getting what the engine can give w. your current 1.5 gear ratio"

This is a winter project (if at all) and I am invetigating my alternatives"

NB 36/ QSM 670/IRM10 w/ a 1.5:1 ratio, DQX 26 X 26 prop. I was hitting WOT correctly in perfect conditions, lightly loaded with a fresh bottom, off by 60-80 RPM now...
 

F/V First Team

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1: Yes, however it would be more cost effective to just buy a gear in that ratio

2: Yes, depending on how things are set up (I've done mine a few times, but there is some engine disassembly that is required)

3: Similar, very similar
 

Powderpro

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In an ideal world, a 26" diameter prop is small for a 670hp engine, a 28" or 30" would be a much more efficient transfer of that much power to the water. But your boat is obviously limited to a 26" prop so they had to install the 1.5:1 ratio to get to a square prop. I don't see how switching to a slightly lower ratio (1.75:1) would be any better than your current ratio of 1.5:1, unless you could switch to a 28" prop, but you cannot, so I think reducing your pitch 1" will allow you to make full RPMs, and you won't loose top speed by reducing the pitch. The higher shaft RPMs you will gain by reducing the pitch will make up for the reduced pitch. And then you will not be overloading the engine, and you will be burning less fuel because your engine isn't overloaded. If someone tells you your boat will get slower by reducing pitch when your current prop will not allow the engine full RPMs, then they don't have real world experience and I would seek advice from a different prop shop.

If you went to a lower ratio of 1.75:1, then you would have to increase pitch to make up for the slower shaft speed. The boat is still going to go the same speed whether you have the 1.5:1 or the 1.75:1. Just pitch the prop correctly to obtain full RPMs, and if the performance doesn't satisfy, start reducing the weight of the boat. Taking weight off a boat is the cheapest performance enhancement.
 
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tunaorlater

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I agree why not take a little pitch rather then spending the time and money replacing the gear? Btw you can swing a 28 in that boat. My old nb was 610hp. 1.53 to 1. 28x27 3 blade med cup. 2200rpm. Spencer Lincoln told me optimum shaft speed at cruise is 1200rpm for this hull fwiw.
 

BillD

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In an ideal world, a 26" diameter prop is small for a 670hp engine, a 28" or 30" would be a much more efficient transfer of that much power to the water. But your boat is obviously limited to a 26" prop so they had to install the 1.5:1 ratio to get to a square prop. I don't see how switching to a slightly lower ratio (1.75:1) would be any better than your current ratio of 1.5:1, unless you could switch to a 28" prop, but you cannot, so I think reducing your pitch 1" will allow you to make full RPMs, and you won't loose top speed by reducing the pitch. The higher shaft RPMs you will gain by reducing the pitch will make up for the reduced pitch. And then you will not be overloading the engine, and you will be burning less fuel because your engine isn't overloaded. If someone tells you your boat will get slower by reducing pitch when your current prop will not allow the engine full RPMs, then they don't have real world experience and I would seek advice from a different prop shop.

If you went to a lower ratio of 1.75:1, then you would have to increase pitch to make up for the slower shaft speed. The boat is still going to go the same speed whether you have the 1.5:1 or the 1.75:1. Just pitch the prop correctly to obtain full RPMs, and if the performance doesn't satisfy, start reducing the weight of the boat. Taking weight off a boat is the cheapest performance enhancement.

I agree why not take a little pitch rather then spending the time and money replacing the gear? Btw you can swing a 28 in that boat. My old nb was 610hp. 1.53 to 1. 28x27 3 blade med cup. 2200rpm. Spencer Lincoln told me optimum shaft speed at cruise is 1200rpm for this hull fwiw.

Bingo, I'd take an inch of pitch out of the present prop.
And you won't lose any speed.
 

backman

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I was fully intending to take an inch of pitch out when I called the God of Props today and got his view that I was just moving the problem so to speak and the real solution was a better gear ratio and a bigger slower prop.

It should be noted I took 1 cup out this winter which gave me 30 RPM's but increased noise and cavitation. A bunch of thinking and asking led me down the path of going from 26 X 26 X #2 to 26 X 25 X #2 which from what I had understood would knock a bit of my emaningless WOT speed of 29 knots and not significantly affect my cruise speed of 20-21 knots.

Need help here - as I was just told that decreasing pitch will *lineraly* decrease speed across the entire RPM range with a corresponding decrease in fuel burn.
 

djmarchand

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Need help here - as I was just told that decreasing pitch will *lineraly* decrease speed across the entire RPM range with a corresponding decrease in fuel burn.

That is true. But increase your rpms to regain the lost cruise speed. Your engine will love you for it because you will be putting less stress and heat on internal engine components. More rpm isn't the curse of diesel engines; stress and heat is.

David
 

Powderpro

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Backman- the God of props is right in that a bigger (like 28" diameter), slower turning prop would give you better performance, especially with 670hp; that's a lot of power for a 26" prop. But unless you can go up in prop size to a 28" diameter that will more efficiently transfer the power, changing gear ratios would be a waste of time and money.

Also, reducing your pitch by 1" in order to obtain full RPMs will hardly effect your cruise speed, don't let the prop God scare you. So let's say you cruise at 2,000 RPMs; by reducing your pitch by 1", your speed at that RPM will drop by maybe 0.2 -0.4 knots, but your fuel burn will be in line with what it should be and your load will be reduced to where it should be. I would rather be running my engine the way it was designed to run and lose at most 0.4 knots at 2,000 RPMs, then to be overloading the engine. If you want to regain the insignificant loss of cruise speed, increase the RPMs to 2,050 and you will have regained the speed and your engine will be much happier. Best of luck.
 

backman

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I don't need or want 2000 RPMs. 1850 will get me 21 knots. If I can get my fuel burn down from 19+ GPH at 1850 to 18GPH with a loaded boat I'll be very happy.


I hit this in March with full fuel, clean bottom and 4 guys on board but haven't seen it since. I could care less about WOT speed, just want a safe and steady cruise speed and fuel burn under any conditions with full canyon load.

Granted I have a Slimy bottom and when it's cleaned I'll get some efficiency back but I now have 2 seasons data telling me to prop down.
 

Powderpro

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Backman- I used 2,000 rpms as an arbitrary number, or as an example. That's why I said "so lets say". I don't care what RPM you cruise at, my point was reducing pitch by 1" will not effect your cruise speed by any noticeable amount. Good luck to ya.
 

backman

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The Prop is coming off Monday and going to the other Prop Shop who agreed that while gearing could be improved; removing 1" pitch before canyon starts is the right thing to do. IF all goes well its abck on the boat a week from tomorrow and I will be able to report accurate results next weekend.

Most important - I enter into canyon season with the mental comfort I am not overpropped on those 6 hour runs...
 

backman

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The prop is back on, with an inch of pitch removed and preliminary results indicate the operation was a success. Takibg 1" or 4% pitch out seems to have taken ~1 knot off my speed across the entire power curve and decreased fuel burn by that same 4%. End result is to go 20+ knots I need 40 more RPM than before and burn the same fuel as before; at a higher RPM. EGT is down 25 degrees; load is down 4% and most important I see full WOT for the 1st time since I owned the boat! Bring on the weather please!
 
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