Walter V Drive gear ratio question.

[hoplite808]

Hello all,

On my boat I have a Perkins T6.354 engine with the Borg Warner velvet drive transmission and a Walter RV-26 V drive that has 2.05:1 gear ratio. Originally I think it was equipped with a Crusader 270 engine but was later refitted with the diesel. It’s getting to be time to look into changing out the v drives due to corrosion concerns in the water jackets. My question is since the crusader engine had higher RPM and lower torque than the Perkins should I be replacing the v drives with a closer to 1:1 gear ratio or would that just not work at all?

 

[O C Diver]

The other component of your equation is the pitch of the propeller. I would expect that the propeller was changed when the engine was changed.

In my own situation, I swapped a 450 HP engine for a 132 HP engine. I used the same transmission in part because a taller ratio with a down angle isn't available. As a result, I flattened the propeller from 28" to 20".

I would consider contacting Walter and consider rebuilding the cooling system.

Ted

 

[vacser]

The main reason for the ratio change was rpm's. The gas engines turned approx. 4400 rpm. Your diesel turns around 2200. Going to 1 to 1 on your diesel would require a larger diameter and pitch prop.

 

[hoplite808]

I believe the prop I have is an 18x19 three blade bronze one. I would have to double check them physically to be certain. These would have been the same size props for the original gas engines.

 

[High Wire]

I had a boat with Walter V Drives and I can remember the cooling system was very simple. There was a rectangular cover over the space with a sinusoidal piece of tubing inside. Super easy to maintain. You might want to get a marine mechanics opinion before laying out tons of dough.

 

[hoplite808]

The props seem good enough to me but they just don’t spin fast enough to go more than 9 knots at full throttle. I figured if I just changed the gear ratio of the v drive the. They’d spin faster maybe get 12 knots out of the old girl. The v drive is going to need replacing within the next couple of years due to corrosion in that water jacket. I know that for a fact. I just need to know if anyone has a similar setup what gear ratio they have to see if this would work.

 

[O C Diver]

I would start with comparing the HPs of the 2 engines.

Next, it would help to identify the boat manufacturer, hull length, and hull style (planing, semi planing, or full displacement).

Ted

 

[STB]

My current 42' twin screw displacement trawler has Perkins 6.354(M) engines, 26" diameter x 20" pitch 3-blade props, and BW CR2s with a 2.47:1 ratio. The injector pump shows a WOT no-load rotational speed of 3130 rpm. One would expect a nominal max of about 2600-2800 rpm under load. I see about 2600rpm and have a little bit of a control set back. I choose to cruised at 2000-2200rpm, beyond that and there is just more noise and fuel burn, but little to show for it


My prior 42' twin-screw trawler had Perkins 6.354MGTs and 23"x 19" 4-blade props. I think it was marked for 2800rpm WOT no-load, developed 2400 WOT under load, and cruised at ~2000rpm. But don't hold me to those numbers, it has been a while. I just remember my cruise speed and prop specs confidently.

So, I guess what I'd like to know...

1) what is the max rpm according to the injector pump stamp. It'll be on the same line where the idle rpm is stamped on the model plate.

2) Out-of gear at the slip, WOT at the slip during a /brief/ test what rpm do you read?

3) With your controls set to allow for full throttle, what rpm do you read at full throttle underway?

4) Underway, if you slowly advance your throttle from idle allowing plenty of time for the turbos to catch up and things to settle in, at what rpm does the smoke get noticeably blacker?

I'd look to be really close to the no-load rated rpm, about 85% of that as your WOT underway rpm, and a cruise speed of about 80-85% of that. What I am here calling the cruise speed is the highest rpm before it starts to noticeably run with darker smoke.

If the smoke gets dark before 80-85% rpm the props need less pitch or a transmission ratio closer to 1:1. The engines are getting loaded too muchnfor the amount of air that they can get at that rpm.

If the smoke never gets darker, I'd look for.more pitch on the props or a corresponding adjustment on the transmission. The engines aren't getting to the point where air is limiting, so they can burn more fuel, so you can load them more.

Figure out where you are right now and use that to guide what type of change you'd like to make. Obviously changing the transmission ratio directly impacts rpms. As a rule of thumb, eelqch inch of pitch is worth very approximately 200rpm, maybe a hair less.


I'd want all rpms measured with a phototach until gauges were calibrated to it and were proven accurate and precise enough, maybe +/-50rpm.

 

[hoplite808]

Okay, does anyone have a Perkins T6.354 paired with a Borg-Warner velvet drive that uses a Walter v-drive on their boat? If so, what is the gear ratio of your v-drive? I'm curious what it's usually paired with.