props direction?

[gttdnbman]

I know you all hate or love to laugh at us newbies but I appreciate the info, That being said I've got a project boat twin engines, could I put props going the same directions instead of trying to reveres the other drive. I've heard that if i had trim tabs it would just need to be adjusted?
Sorry the boat is a 27' fountain 220hp engines and I'm not trying to set speed records but hope to hit 50 maybe, more for crusing also the boat has velvet drives with props thru the hull
One more question what do most twin i/o's do are they lh@rh, not the race boats but average Thanks Bman

[marshmat]

Yes, it's possible to fit two identical (as opposed to mirror-image) props on a twin-engine boat.

That doesn't mean it's a good idea, though. Usually, the two props spin opposite directions. Part of what makes twin screws appealing is the cancellation of prop torque effects that you get when you have one LH and one RH prop. Put two LH props on her, and you'll probably find that backing up to starboard will be very, very different from backing up to port.

Is there something about the gearboxes you have (or can buy in your area) that prohibits you from having opposite rotations?

[gttdnbman]

Okay here is a little bit more information the project was started by some one else and then I puchased the boat and I'm trying to right as much as possible. When i got the boat it had two engines out of a taurus sho, I've changed the starters and altenators to marine grade and jacketed the headers and would like to keep the engines. the trannys were power glides (auto tranny) and the one has a gear box mated to the end reveresing it like it should be, although it works I have a problem with changing from forward to reverse (there's no pavement to stop it) so i was going to add disc brakes to the shafts and thats when I realized lets do it right so I found a pair of velvet drives but the only way to use them is to reverse the direction of the engine, and I'm not sure on that with the ecm and all so thats what I have and am trying to make right without going to far down the rabbit hole. This is my first boat, I'm very new to boating and i feel like a sponge wiht all of the information I've learned and want more. I could go on with questions but I don't want to wear out my welcome so thanks for answering and any ideas that you have would be grand tks Bman

[marshmat]

No, no, no. The engines aren't counter-rotating, that would destroy them- they always spin the way their designer intended. (Ignoring, for the moment, a couple of weird 2-stroke designs.) Counter-rotation is done by the gearbox. If your gearboxes are rated the same in forward and reverse, and have roughly the same ratio in forward and reverse, it's a simple matter of hooking one shift cable up backwards. There are a few gearboxes that aren't designed to operate this way, though. The installation manual for yours should say what's OK and what's not. If you don't have the manual, maybe we can scrounge it up on the 'net somewhere....

[gttdnbman]

ok thats good to know, from what I understand in the manual for my 71c velvet drives the pump can be flipped to change direction to the engine it is mated to but the input and out put will always be the same direction of each other but I could not find anything on just running it in reverse so I'll have to dig a little deeper.
I understand the rotation issue's but I'm just wondering on a race boat with 3 I/O's how does that work L/R/L?

[Jeff]

I'm just wondering on a race boat with 3 I/O's how does that work L/R/L?

I've never had a boat with triples; this seemed to be the rage a while back; now it seems there are more higher powered twins outpacing them; I think typically you'll see counter, standard, standard. For example I used to love the aronow alpha 45 http://www.boatdesign.net/gallery/sh...o.php?photo=71 (speaking of which there is one of these hulls for sale less power for $7500 in Freeport NY now! But I couldn't afford the $$$,000 to power and rig her properly...)

[Jeff]

One more question what do most twin i/o's do are they lh@rh, not the race boats but average

In the "do as I say not as I do" department, Matt is definitely right in theory that counter rotation is the only way to go, and if you have the money or time to do it right, I'd definitely recommend working to make it optimum. Of course, when it was my own money and I was fixing up an older 1990 boat for my own general and occasional use this summer which had twin standard-rotation alpha drives, I stuck with it. Counter would have been better, but then I would have needed to spend an additional $200 for the lower and $600 for a new set of props as no matching counter rotation prop was available, plus a few hours modifying the helm shifts and the way things go, likely $ there for parts as well. I would have gained some handling "grace" and maybe 2 mph on the top end, but I had other expenses so "fine" rather than "perfect" was good enough for me. It handles fine under power and I decided it would be more fun to be on the water and click the trim button a couple times than spend the time parts shopping this year. After 1991 I would say all twin IOs had either counter rotation or duoprops. But in the 80s there were hundreds or thousands of boats that did "just fine" with twin standard rotation outdrives with <$2500 outdrives. Not ideal or great from a theoretical sense, but in a practical sense, I'm not going to lose sleep over it with my own project. When I strike it rich, or have lots more time for parts swapping, I'll upgrade again.