[q240z]

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.

[Capt J]

Originally Posted by K1W1

Hi,

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

In general I would have to agree for a non commercial vehicle and a non commercial boat. IN GENERAL, Engines in most road situations are geared so that they run a lighter load factor most of the time instead of 80% load for hours on end. They're not injesting salty air. They're used much more often then a recreational boat. The fuel is generally fresher because of the use, algae in the fuel of a diesel road vehicle is almost unheard of. There generally isn't as much condensation on land as there is in an engine room on a boat. The exhaust on a road vehicle is dry and you don't get condensation when it sits from water in the risers.

[artwork]

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 ??

[Capt J]

Originally Posted by artwork

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 ??

Reverse turning causes the wash of both propellors to combine, you're pulling water from the exposed side of the hull (not the center where there could be a keel and the v is deeper) and used to pull the boat foward and therefore a little less thrust is lost to the sides of the boat and creates a little more lift from what I've been told. Increased speed/efficiency was a small %. I think back when Reggie Fountain was messing with it....They gained something like 3-4mph on a 140mph boat, at the expense of turning radius and such, and he deemed it only worthwhile for setting speed records and not actual racing with turns.

[q240z]

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

[artwork]

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.

[biodon]

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?

[John_C]

Originally Posted by 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.

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.

[biodon]

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.

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.

[Henning]

Originally Posted by Capt J

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.

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.

[captdirk]

Originally Posted by 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

How are the engines working out for you, and did you ever work out the props? I'm going to try 17x16 props and 1.91 / 1.6 Velvet drives.
Especially - how many gallons per hour do they burn? I got a couple 6.2s and am trying to size the fuel tanks. Are two 50 gals too small? I'm guessing about 5 GPH at full throttle, but can't get any real boating figures.

[John_C]

So far the engines are running great. I have not gone enough distance to give you an answer on consumption,but will let you know as I figure it out. My boat has two new 75 gallon tanks in it. I have made changes to the fuel filters etc to make sure I am getting any moisture out, and a good flow. I do believe I am going to have to change the props though, the boat is lagging, like a boat that does not want to come up on plane, and if I push the issue,the engines start to rev out, and the boat does not move. At first I was worried I had a transmission problem, but I have had both engines do it, so I believe its a prop problem. Can anyone recommend a good company to contact, who could advise me on size and pitch?

[chesapeake46]

Originally Posted by John_C

Can anyone recommend a good company to contact, who could advise me on size and pitch?

I got this information off of a great forum.... err ah, here in fact.

http://www.boatfix.com/how/props.html

[John_C]

Great link chesapeake46,that is some good basic knowledge I really needed to read. I am having a slipping problem,and I have a couple things I need to try. Thanks for the prompt and good reply.

[chesapeake46]

Originally Posted by John_C

Great link chesapeake46,that is some good basic knowledge I really needed to read. I am having a slipping problem,and I have a couple things I need to try. Thanks for the prompt and good reply.

Your welcome but the credit goes to either Marmot or K1W1 who passed it on here a while ago.
I just pasted it into my favorites.......