GM 6.2 diesels for Roamers

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.

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

It's all in the way you look at it

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.

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 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.

6.2 Diesels in a Roamer?

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.

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.

The analogy I like best is that every engine has an amount of power it can produce. The faster you extract that power, the sooner you'll be rebuilding the engine. This is why drag racing engines expire after one 1/4-mile run while F1 engines might last between 6-24 hours--the F1 engines are far less stressed than the nitro methane-burners. Less exotic applications, like passenger cars and OTR truck engines, last for years or even decades because the power extraction is less per unit time.

In addition, on-road vehicles are basically only really working when accelerating and climbing hills. Their gearboxes allow them to operate at optimal RPMs in normal usage through a relatively large speed range. Get a car or large truck up to cruise speed, throw it into overdrive, and you continue at cruising speed with relatively low throttle input. Compare that to a boat, where there is no such thing as downhill (in any practical sense), most propellers are fixed, and cruising along requires somewhere around 3/4 throttle for every hour of operation (to say nothing of the salty, wet environment and long spans of disuse in most areas), and it seems clear that on-road engines have an easier life than marine ones.

If I had to chose between engines of a given power rating, longevity and reliability would absolutely factor into the equation. If, in fact, the 6.2 had known issues on the road, it's unlikely that it would have fewer issues on the water.

As to Artwork's density statement, I suspect near-hull turbulence and perhaps air bubbles have something to do with it. Let's see what he has to say.

out-turning propellors

Geez, guys - I said I learned it a long time ago - like 40 years ago.................. But, I looked it up - in Dave's Gerr's 'Propellor Handbook' p.20 quote:
"The water at the bottom of the propellor is a bit denser and freer to flow (there's no hull above it) than at the top of the propellor. This makes the lower blades a bit more effective, so the propellor and stern "walk" sideways in the direction of rotation."
" On a twin screw craft, the propellors should be out-turning. The starboard or right propellor should be right-handed, and the port or left propellor should be left handed. This gives the best efficiency. Twin-screw vessels with propellors of the same hand can experience serious handling problems." Unquote
Dave Gerr is a naval archetect (and author of 3 or 4 books) and now a director of Westlawn Institute of Marine Technology, the education arm of ABYC - really, I didn't make this up. I also didn't calculate the difference in density. The point is that it is a 'bit' more, causing 'walk'. And I can attest to it, having owned several single screw boats that I could turn around in little over a boat length using that 'walk'.

Now, I was open minded asking why the reverse turning was done, looking for a scientific answer. 'Customer's preference' is an acceptable answer, just not scientific enough for me to build mine that way. So I can accept differences of opinion, I learned this stuff years ago and never questioned it - til now??. should I ??

C'mon, Art...it's not like you're working on your Roamer. What else have you got to do with your time?!? lol

reverse turning props

Capt J

OK, I can see the theory in that - Deep V particularly, and at high planing speeds - Makes sense. Shows there is something new to learn every day.

q, right-o. gotta take one day off a week.

I want to make sure I am getting this.

When repowering with enignes that both turn in the same direction, you can change the valve in one of the Velvet Drives (except the 1.9 ratio) so that it will work with the new engine. This just means the transmission operates in that direction, it does not reverse the output shaft direction relative to the engine. With no other adjustment, you will have both prop shafts turning in the same direction.

To get the props counter-rotating, you actually have to run one transmission in reverse. This means the sun and planetray gears are always in motion in that transmission, in contrast to the forward gear which simply locks the input and output shafts together. This would seem to cause more moving parts, more friction, heat and wear in the transmission that is always running in reverse. It sounds like this may be an acceptable compromise providing the transmission can handle the stress? Is this fairly common practice?

Charlie I did not notice this post at first,so sorry for the late reply. The engine that developed such a bad reputation was the 5.7, often confused with the 6.2. The 6.2 is a true diesel, not a converted gas engine, and is much tougher than the 5.7. I did a lot of research on these engines,and although they are not great pullers,compared to a 6.5,but they are reliable under continuous use.

I'm deducing that instead of "2.91 to 1", this should read 1.91 to 1?

In reading the Velvet Drive literature, it appears the 1.91 ratio is the reduction unit with output rotation opposite of the input rotation. All others spin the output the same direction as the engine.

That's actually untrue in practice though. It's really an enhancement to handling, especially with no bow thruster. With inboards, you can give it 20* Port helm, put port in forward and starboard in revers and make the boat walk straight to starboard. If you give her full port helm, she'll actually turn to port in that configuration while walking to starboard. The thing is though that you have to use the rudder when backing. You can't just use starboard prop to pull the stern to port and port to pull it starboard because the prop walk counter acts the offset thrust, so you have to use rudder at speed and prop wash from the outside in Fwd at low speed. Once you understand it though, inboard turning screws (when viewed from the aft) is a much more precise handling setup. Lot's of the bigger oilfield boats in the days before bow thrusters were set up like this, many still are.