[Clansea]

My 35 footer has a pair of 350's with velvet drives both engine rotate the same, the 2.91 to 1 changes the rotation of one engine and it is matched with a 2.10 to 1 that rotates the same as the other engine.

[captdirk]

Glad to hear 6.2s are working in a Roamer!
What ratio are the Velvet drives, and what prop size are you using? How noisy would you say they are? How close in size to a 350 V-8 are they? (I'll probably build a mock up to see if they'll fit). Also, how is the performance? Would you do anything differently if you had the chance? I have a 35, what length is yours?
I know - a lot of questions, but better dumb questions than dumb mistakes.

[K1W1]

Originally Posted by Clansea

My 35 footer has a pair of 350's with velvet drives both engine rotate the same, the 2.91 to 1 changes the rotation of one engine and it is matched with a 2.10 to 1 that rotates the same as the other engine.

Hi,

Do both your props turn the same way?

If you have the same input speed from the engine and different gear ratios you should if there is no clutch slip get different shaft and prop speeds.

[artwork]

I'm with K1W1 on that. Those velvet-drive ratios sound like the difference between fwd and rvs. Clansea, are you SURE your 350's are not counter- rotating? They were typically installed as R and L turners. I owned a marina in the 80's and had a brand new Trojan deliverd with same rotation 350's. Due to the cost of pulling the eng, we did the conversion in house. That's a long time ago, but memory says it was just a cam and distributor change.

K1W1 - tell us more about 'outward turning' props. What is the advantage - and does it outweigh the 'denser water' theory?

[Capt J]

Originally Posted by artwork

I'm with K1W1 on that. Those velvet-drive ratios sound like the difference between fwd and rvs. Clansea, are you SURE your 350's are not counter- rotating? They were typically installed as R and L turners. I owned a marina in the 80's and had a brand new Trojan deliverd with same rotation 350's. Due to the cost of pulling the eng, we did the conversion in house. That's a long time ago, but memory says it was just a cam and distributor change.

K1W1 - tell us more about 'outward turning' props. What is the advantage - and does it outweigh the 'denser water' theory?

Yeah, it is pretty simple to convert a gas motor to turn the other way. It is simply a cam change, distributor change, and a reverse rotation starter, and I think freshwater pump (although that might not need changing).

The traditional counter rotation setup is that both engines turn outward. The stbd propellor turns clockwise and the port turns counter-clockwise. This is more efficient then both having clockwise turning propellors, because the power pushes the boat foward instead of slightly off to the side.

I remember reggie fountain experimented with having both props turn inwards instead of outwards and I think on race boats it picked up a couple of knots. However, it never spread to anything else due to less manueverability (slow speed), and I am not sure if they still practice that.

[John_C]

The 5000V and the 5000A Velvet drives both come designed for use in boats using twins, while still using automotive rotation on the engines. My 6.2's are both running just as they would for automotive, the Velvet drives take care of the rotation. My Roamer also uses a closed cooling system that allows me leave the engines virtually identical to automotive use, even the exhaust manifolds.

[John_C]

Captdirk....we are still working out the bugs, I picked the boat up with most of the conversion done, so a lot of the fabrication that went on to make them fit I am still getting familiar with. The mounts for the chevy small blocks are in a bucket on my floor though,so I know they did not work...lol
My boat is a 1963 Rivera Express,32 feet,steel. The drives are 1.1(Velvet),and allow both engines to run the same, one of the trannies reverses the rotation.
I am using the same props it had on it for the gas engines right now, and thats not working out so well, so I found a company in the states that can calculate what I need in props to make it perform better. If you want to see the boat running on the water,just search 1963 chris craft on youtube. The video on there is from test runs earlier, its running much better now,and huffing far less black smoke....lol

[Capt J]

Originally Posted by John_C

The 5000V and the 5000A Velvet drives both come designed for use in boats using twins, while still using automotive rotation on the engines. My 6.2's are both running just as they would for automotive, the Velvet drives take care of the rotation. My Roamer also uses a closed cooling system that allows me leave the engines virtually identical to automotive use, even the exhaust manifolds.

You should be using water cooled exhaust manifolds (marine) and not automotive exhaust manifolds and risers as they throw way too much heat in a confined engine room and are a fire hazard.

[artwork]

We're dealing with symantics - ie. 'top of the prop or bottom of the prop' when saying turning inboard or outboard . No question that 'traditional counter-rotation' is outboard at the top of the prop. But the greater thrust is at the bottom, where the water is more dense (providing the advantage of 'inboard turning' at the bottom). My question to K1W1 was "what is the advantage of reversing the 'traditional counter rotation'?"
I guess "the build Captains preference" could be the answer.

Previously K1W1 said:
"both shafts turned inboard when viewed from astern when going ahead.
. . . . . . .
This time we are going for outboard turning when going ahead- the build Captains preference."

BTW - I agree with Capt J - get water cooled ex manifolds unless you have a keel-cooler and dry exhaust. But that's another story.

[K1W1]

Hi,

The use of Inboard turning Props when going ahead has shown in powering and propeller tank testing to be slightly more efficient than outboard turning and supposedly less prone to noise.

The efficiency bit corresponded with my own experiences being on a 51m vessel with inboard turning and using 52 to 55,000 Lts of fuel to get from Antigua to Gib, a sistership with outboard turning wheels supposedly used 54-58,000 for the same journey.

The big dis advantage of inboard turners for the average Yacht Captain comes in maneuvering in close quarters especially when against a face dock. The Captain who was involved in the project I mentioned previously had also driven an earlier one in the series with inboard turners and said he didn't like it so we have them the other way around now and a new Captain who says he doesn't care which way they go.

[Capt J]

Originally Posted by K1W1

Hi,

The use of Inboard turning Props when going ahead has shown in powering and propeller tank testing to be slightly more efficient than outboard turning and supposedly less prone to noise.

The efficiency bit corresponded with my own experiences being on a 51m vessel with inboard turning and using 52 to 55,000 Lts of fuel to get from Antigua to Gib, a sistership with outboard turning wheels supposedly used 54-58,000 for the same journey.

The big dis advantage of inboard turners for the average Yacht Captain comes in maneuvering in close quarters especially when against a face dock. The Captain who was involved in the project I mentioned previously had also driven an earlier one in the series with inboard turners and said he didn't like it so we have them the other way around now and a new Captain who says he doesn't care which way they go.

I agree that they're more efficient.

The big disadvantage is maneuverability in close quarters. Since they turn in opposite directions as what most Captains are used to. But the problem lies when one propellor is in foward and one is in reverse it washes the wash more into each propellor and the vessel does not respond nearly as quickly or as fast. The propellors will cavitate a bit from what I hear. I guess it's the difference of maneuvering a 35' outboard boat with both motors mounted fairly close together, or maneuvering an inboard boat. The inboard boat is easier to spin and such.

[John_C]

Originally Posted by artwork

We're dealing with symantics - ie. 'top of the prop or bottom of the prop' when saying turning inboard or outboard . No question that 'traditional counter-rotation' is outboard at the top of the prop. But the greater thrust is at the bottom, where the water is more dense (providing the advantage of 'inboard turning' at the bottom). My question to K1W1 was "what is the advantage of reversing the 'traditional counter rotation'?"
I guess "the build Captains preference" could be the answer.

Previously K1W1 said:
"both shafts turned inboard when viewed from astern when going ahead.
. . . . . . .
This time we are going for outboard turning when going ahead- the build Captains preference."

BTW - I agree with Capt J - get water cooled ex manifolds unless you have a keel-cooler and dry exhaust. But that's another story.

I do have a keel-cooler and dry exhaust,the whole system runs through pipes along the keel,and back up through the engines using the automotive pumps.

[Charlie D]

The 6.2 Diesels quickly developed a bad reputation when installed in Pickups, Suburbans, etc. An engine in a road vehicle has a much easier life than one in a boat. So, with the 6.2's being bad in a road vehicle, I wonder why you would want to go to the expense & work of converting and installing lousy engines into your boat. Look for better engines.

[K1W1]

Originally Posted by Charlie D

An engine in a road vehicle has a much easier life than one in a boat.

Hi,

Would you care to elaborate on this rather wide sweeping statement?

[Marmot]

Originally Posted by K1W1

Hi,

Would you care to elaborate on this rather wide sweeping statement?

That sounds like a reasonable debate, that might go either way depending on the road and the vehicle the poster had in mind.

I would love to see "artwork" elaborate on this claim though:

"... the greater thrust is at the bottom, where the water is more dense ..."

The figures for density are very easy to obtain and it would be nice if the poster would tell us the difference in density of seawater between the top and bottom of a 1 meter diameter prop rather than expect readers to just accept this as fact.