Dual prop drives, does their extra bite need more torque

[Maclin]

My question is:

Can a Dual prop drive help a smaller engine with torque multiplication?
Or does their extra bite actually need more low end torque to move out from a standstill?

I have a different view than QC (and maybe others) on the low end torque "required" with a dual prop drive. I may need an adjustment in my understanding of the actual dynamics, but In my mind the dual prop setup digs in better and thus will act more like a tall gear in a car or like really big tires that will not slip. In that scenario the more torque available at just above idle the better to get going from a standstill in my view. That is why I always feel a 5.7 is better than a 5.0 for those drives in the 22 foot and up category.

I know the Volvo DP drives and maybe the Bravos as well have a lower final ratio than the single prop drives in the same applications. For instance on my duoprop drive with a 5.7 the ratio is 1.95 where on a single prop drive with a 5.7 it would be more like 1.6. Does the different ratio make up for the extra torque that I am thinking would be required? and the end result is actually a multiplication of torque even on smaller engines?

Maybe it is just that the dual props allow for more torque to get translated into motion via the dual screws, and a 5.0 is good enough but a 5.7 is better and a 6.0+ is better still.

I appreciate all replies with your thoughts (and even would entertain supportable facts ).

 

[Maclin]

Re: Dual prop drives, does their extra bite need more torque

To start things off,

It seems to me that a single prop will slip more on takeoff and thus act like a stalling torque converter and let the engine get up into the powerband and stay there for a bit until the boat catches up to the prop, like when a converter flashes over and starts to lock up or a clutch is slipped until the engine can take more load.

With a dual prop it seems to me that it digs in more and acts like a really low stall torque converter or even letting the clutch out at 1500 rpm with no slip. In a car if the tires grab hard then the car may or may not move out well depending on the torque available.

That is where my view is coming from.

 

[tommays]

Re: Dual prop drives, does their extra bite need more torque

1.81, 2.00, 2.20, and 2.43 to 1.


Just like and alpha the ratio must suit the motor

 

[HT32BSX115]

Re: Dual prop drives, does their extra bite need more torque

With a dual prop it seems to me that it digs in more and acts like a really low stall torque converter or even letting the clutch out at 1500 rpm with no slip. In a car if the tires grab hard then the car may or may not move out well depending on the torque available.​

(All the prop slip calculators indicate about 1/2 the slip of a single prop)​

Both are true more or less except that with a dual prop you have more drag due to more blade area in the water and the friction from more gears.​

It's not very significant though.​

My 21' Liberator was VERY slow out of the hole and went about 60 mph at 5000 rpm with the previous 340hp 460/King Cobra (about 300 est prop hp)​

With the 330hp (300 propshaft) 454/Bravo III, it does about 57mph (gps) and just ROCKETS out of the hole now.​

My brother (who bought the boat new) said the boat hole shots and accelerates more than twice as fast at ANY speed as it ever did (even when new)​

Regards,​

Rick​

 

[Uraijit]

Re: Dual prop drives, does their extra bite need more torque

It depends on where your torque is in the power band to begin with.

If you have an engine that's just gutless at low RPM, but gains torque at higher RPM, maybe a dual prop would cause problems.

Otherwise, all you're doing is giving the engine better "Bite" into the water, which will translate into less wasted power.

Are you more likely to win a drag race by stalling the engine a little more, or by sitting on the line spinning your tires?

A dual prop drive is not so much like "gearing the car taller" as in your analogy, it's more like putting stickier tires on that car.

 

[Maclin]

Re: Dual prop drives, does their extra bite need more torque

Thanks for the viewpoints! I know where my hang up was. In the Volvo prop guide it shows the same ratio for 5.0 thru 5.7 size engines for the duoprop drives, 1.95 to 1. I was focusing on that. I went back and looked up the smaller engines and the ratio does change for the smaller engines to a 2.3 to 1. I was hung up on the ratios shown for the 5.0 and 5.7 being the same. In those cases (same 1.95 ratio) the 5.7 would have more oomph over the 5.0 if the propset is the same pitch. Have to have the 5.0 go down in pitch to get the same pull out of the hole as the 5.7 (at the expense of top speed of course).

The ratio changes to 1.78 to 1 for the 7 liter engines.

It appears the Bravo may have more finely tuned ratios available. This is based on the only reference I have used for Volvo so that may not be the exact case.

The duoprop is definitely like using slicks with a sure-grip (posi-track to the GM contingent) compared to a pegleg with skinny tires. It is hard to describe the look on a new person's face when I hit the throttle, it just goes like it is being pulled along on a track under the boat. They look even more puzzled if they have been boating a lot and thought they knew what to expect. If you are used to the engine spooling up some as the prop searches for "traction" then there is something off-putting about the sound of an engine that is not revving like crazy yet the boat is accelerating briskly out of the hole. The engine just has a nice linear increase in rpm as the speedo and tach appear to be synchronized.

So, note to Maclin, just like anything else, pick the proper ratio for the engine and boat combo and the torque is multiplied via the gearset and the props dig in and efficiently transform whatever torque comes out into forward propulsion.

My brother shot a video of me heading back to the ramp, it is in the post link below if you are interested. I was just putting to turn around then went from 1/4 to full throttle at about 7 seconds in.....:
http://forums.iboats.com/showpost.php?p=1830246&postcount=1

 

[QC]

Re: Dual prop drives, does their extra bite need more torque

The reason I claimed on that other thread that a Duo-prop or B3 is less dependent on low end torque is because they have better time to plane speeds than single prop drives with the same power plant. The point being is if they required more torque their planing times would be slower than a single prop . . . This is confusing, but if you accept that planing times are lowered with the B3, then you could argue that you could achieve the same planing times with less power, which would of course also be less torque . . .

 

[mtnrat]

Re: Dual prop drives, does their extra bite need more torque

I must get a video of my Volvo DP coming out of the hole. At 1/2 to 3/4 throttle the whole hull is out of the water, it touches down twice, settles and is going 50mph before you can say Jack Rabbit Johanson.

 

[Maclin]

Re: Dual prop drives, does their extra bite need more torque

The reason I claimed on that other thread that a Duo-prop or B3 is less dependent on low end torque is because they have better time to plane speeds than single prop drives with the same power plant. The point being is if they required more torque their planing times would be slower than a single prop . . . This is confusing, but if you accept that planing times are lowered with the B3, then you could argue that you could achieve the same planing times with less power, which would of course also be less torque . . .

Makes sense, the quicker you get on plane the easier it is to keep it going, that's for sure.

 

[Maclin]

Re: Dual prop drives, does their extra bite need more torque

Mtnrat,

DP is the "stuff" for sure. When I give mine 1/2 to 3/4 or even full from a stand still it hops up on plane almost immediately and scats to 45 like it is being chased, pretty cool. My Hydroswift hull does not bounce much though...! I may try to get a full throttle blast captured next time just for my own reference.

 

[billbayliner]

Re: Dual prop drives, does their extra bite need more torque

I have a 1995 Crownline 250CR... Standard engine drive setup on the boat was a 454 with a Bravo drive. Read many post in regards to the boat having issues getting plane.

In my boat I have a 350 with a Volvo Penta C1 DP. Well I can tell you with my heavy boat the 350 pushes it well and no problems getting on plane. This is only because of the Volvo DuoProp.

Man I love that drive and it handling.

Do some searches here and Google about people upgrading the drive to a Duo and hear the amazing result even with out a engine upgrade.

 

[Don S]

Re: Dual prop drives, does their extra bite need more torque

There is also a big difference in the props between a Merc B3 and a Volvo DP.
The B3's are just a LH and a RH prop. Both props are exactly the same pitch and dia. Not so with Volvos DP. The props are designed to work together the best, that is why the aft prop is smaller and different pitch.

 

[Maclin]

Re: Dual prop drives, does their extra bite need more torque

Good stuff Don, that could be one of the reasons VP does not seem to need so many ratios. The propsets available fill in the gaps.

 

[mtnrat]

Re: Dual prop drives, does their extra bite need more torque

Mtnrat,

DP is the "stuff" for sure. When I give mine 1/2 to 3/4 or even full from a stand still it hops up on plane almost immediately and scats to 45 like it is being chased, pretty cool. My Hydroswift hull does not bounce much though...! I may try to get a full throttle blast captured next time just for my own reference.

It is a lot of fun, just make sure everything is off the dash, the glovebox is closed, and any passengers are fully ready.

 

[Maclin]

Re: Dual prop drives, does their extra bite need more torque

Totally gotcha mtnrat. I don't "floor" it like I used to, no need, 1/4 to 1/3 will plane it out in less than 2 boat lengths even towing. On this one trip I had new-to-boating people along and harped on the young ones to not switch seats while we are moving, especially during take-off and stopping. One little neighbor girl was all the way up front in the Vee of the cuddy facing the cockpit and leaning back on the cushions, and when I took off with a lot of throttle it looked to me like she started walking awkwardly towards the cockpit and I said "Honey, you can't switch seats now" and her mom said "She isn't, she was trying to stay seated until you took off like that and she couldn't hold it...." "Oh, sorry...". After that is when I realized what it did to some of the passengers and take off easing into it now most of the time....Good times!