Dual Prop pros and cons?

[thegrizzly1]

Let's give myself and hopefully a lot of others an education on dual prop drives.

In a different post I had questions on Bravo interchangeability, but I want to know from those with first hand experience, what are the pro's and con's of dual prop drives in general.

I'll start with what I have been led to believe. All comparisons I am about to list are vs. a single prop setup.

I am under the impression that with a DP setup you will generally have a better hole shot, less cavitation/ventilation in tight corners, better low speed control, especially in reverse, better efficiency, slightly lower top speed, and generally more expensive to repair/replace. Again, these are just the impressions that have been made on me, and I am asking for an education as to what others opinions are.

I've heard claims that a DP can be up to 30% more efficient. Does this mean that one should get 30% better fuel economy at cruising or other speeds? Can you in fact plane out at a lower RPM? I can understand the less slip/better hole shot easy enough. How about boat weight? Is a DP more advantageous on a lighter boat or heavier boat? Is a dual prop setup really slower than a single prop on top end? If so, why? Is there some rule of thumb to use to use when selecting prop sizes/pitch if one were to convert from a SP to a DP?

Any education or references to DP advantages or applications where they should or should not be used is greatly appreciated.

 

[HT32BSX115]

Re: Dual Prop pros and cons?

I am under the impression that with a DP setup you will generally have a better hole shot, less cavitation/ventilation in tight corners, better low speed control, especially in reverse, better efficiency, slightly lower top speed, and generally more expensive to repair/replace. Again, these are just the impressions that have been made on me, and I am asking for an education as to what others opinions are.

Howdy,

The above "impressions" are what I have observed with my Bravo III.

I had a 460 King Cobra engine/drive in my Four Winns Liberator that was similarly powered. (the 460 was rated at 340 crankshaft hp, The 454 is rated at 330 Crankshaft HP)


The boat now is about 3 mph slower. (My brother bought this boat new and according to him it topped at about 60 mph (checked with Radar) with the original 1.43:1 drive, 5000 RPM, & 20p SS prop.

Now with the 1.81:1 Bravo III drive, 26p props and 4600 RPM, lightly loaded, I can get 57 (GPS) MPH at WOT.



I don't believe the boat will plane at a different (higher or lower) speed than before. Planing speed is a function of thrust, weight and CG (center of gravity) It has nothing to do with the way the thrust is produced......"Thrust" is Thrust......

How efficient the thrust is produced has nothing to do with how the boat planes.....except with the 'same' hp, you would plane quicker.....not at a different or lower hull speed.

Fuel efficiency is another matter. you get less prop slip (about 1/2) with twin, counter-rotating propellers. It HAS to be more efficient (it's sort-of like driving a car where the rear tires are slipping all the time vs NOT.......


A much better analogy would be 2 cars at the drags...

The left lane is hard packed gravel......and the right lane is smooth concrete with a thick layer of fine sand on it.

Both cars will spin (slip) their tires but clearly the car on the "sand" side will "slip" a lot more than the one on the hard packed gravel during hard acceleration.


The difference for a boat is that the cars, once at cruising speed, have very little drag, so once they stop accelerating, both cars will stop slipping the tires......or slip very little....

With a boat the drag (of the water) is still significant and at higher speeds the drag is very large. So the boats STILL slip their "tires" at higher speeds (even on plane).....LESS slip will produce better fuel economy.

Also, below planing speeds the drag is very high so there's a lot of "slip" when getting up on plane.....so less slip due to more efficient props, will get you up on plane quicker with the same HP.



Now........more expensive? yes. you have 2 SS props instead of 1. If you bash them, it's costs twice as much to replace them.

In all the years of boating I have not seriously damaged a prop. I also don't EVER loan my boat. I am always cognizant of the drive back there......AND I've been lucky.


I would never go back to a single prop drive,



Regards,


Rick

 

[thegrizzly1]

Re: Dual Prop pros and cons?

Thanks for the input Rick. This is just what I was looking for. I have also spent some time using the "search" function and have been reading up on a lot of great information. I'd like to keep this thread alive and get some more feedback though.

One thought on the lower planing speed (please excuse me as I am an engineer and tend to analyze everything). On my current Sea Ray, I was able to keep the boat on plane at a lower RPM and hull speed with a 4 blade prop vs. the same pitch same diameter 3 blade prop. I am assuming the 4 blade had less slip or more bite at that given rpm. I agree with you about hull design and weight and thrust and that it takes a certain amount of thrust to plane a given boat.

That amount of thrust required is dependent on hull design, weight... Lets take a single prop of a given pitch and diameter. We know there is a certain percentage of slip so for our SP to plane the boat, lets say we need 3700 rpm to develop enough thrust to plane the boat. At that point the prop "grabs" and slips much less. We are going at a certain speed at this point. (prop pitch x RPM x % slip) to overcome the drag of the boat. As we back off the throttle, the drag increases slightly, our prop begins to slip more and the boat falls off plane, till we speed it up again.

With a DP, I am thinking the principals are the same. However we can develop the required thrust to plane the boat at a lower RPM (say roughly 2800) because we have less slip or a more efficient system. Lower RPM and I believe a lower planing speed. What is not constant, is the "drag". There would be more drag at a lower hull speed, but we can still develop enough thrust to overcome this drag because of the better efficiency DP system and keep the boat on plane.

You said thrust is thrust. I agree, and to summarize, we have to spin a single prop faster to develop enough thrust to overcome the drag created by the hull and plane the boat. This faster RPM results in a higher hull speed. With the DP we can develop the same thrust at a lower RPM due to better efficiency. As a result of the lower RPM, there is also less hull speed.

Does this make sense? Again, I fall back on my current boat. Same weight, hull design etc... I switch from a 19P 3 blade prop to a 19P 4 blade prop and I can drop the rpm required to plane by about 800 and hull speed by a few MPH. I am assuming the DP to be even more efficient than the 4 blade single prop setup.

Thanks for challenging me to think about this for a while.

All other opinions are appreciated.

 

[HT32BSX115]

Re: Dual Prop pros and cons?

I think you have it pretty much right on. (We engrs think alike I suppose....although I'm an EE.....so I'm brain damaged)

What is not constant, is the "drag".

Don't forget that prior to planing, there's quite amount of drag that goes away when the boat planes. Then the only drag from that point is hydrodynamic drag due to velocity (which increases as the square of the velocity like drag in any fluid)

I suspect that the slightly lower speed with constant HP is probably the additional (2x) friction on the "propeller" ( because there's 2 of them)

The drag on the hull is something like the following,

I must really have it bad today!

Don is sure to chide me for this one!!!

 

[Maclin]

Re: Dual Prop pros and cons?

It will stay on plane at a lower speed, the actual planing speed will probably not change.

And at the drags, it is more like one side gets to connect their frame to a powerful chain drive installed down the middle of the track and the other side has to make do with tires on sandy pavement!

I was going to use a steam cannon drive like on an aircraft carrier as an analogy but it is not QUITE that dramatic.

The cost of maintaining the drive is the same, but prop trouble is double if you happen to booger up both of them.

The Volvo DP has differing props on their setup, quite a bit of engineering behind it.

 

[HT32BSX115]

Re: Dual Prop pros and cons?

Yeah.....those VP's are pretty good units......(Not bad for an originally Keikhaefer Mercury design!)


The VP has more choices in props too. You can get SS, aluminum ("aluminium" for you Brits), and I think there's even composite props available too?

SS only for a Merc Bravo III......

But if you look on ebay, you can buy multiple Merc drives, props, parts, etc all day every day. Not so much VP stuff on there.... (3 to 1 ratio difference)


I'll stick with the Merc....

 

[gadget73]

Re: Dual Prop pros and cons?

Our Rinker has just been changed from Bravo II drives to Bravo III drives (lower units actually, our uppers are original) and what I notice so far is this: (all speeds from the GPS)

top speed about 5 mph lower from absolute best-ever. 42 mph with 3 people and a bunch of tools, but nothing else onboard, vs 47 mph with 2 people, no gear onboard. Normal top speed was more like 45 with all our junk onboard.

Cruise RPM much lower, used to be 26 mph at 3600 rpm, now 26 mph at 3200 rpm. 3600 puts us into the low 30s. I can maintain the boat on plane at about 20 mph now, before if I got below 23 it would drop and wallow.

planing is faster, and there is less bowrise. Also does not require any assistance from the trim tabs to maintain a fairly level bow when planing, which is very nice.

Turn radius seems slightly improved, but most noticeable is the fact it doesn't need more throttle to maintain speed in a tight turn. It used to bog if I was at a low cruise speed, sometimes to the point it would drop off plane. Thats no longer an issue.

All in all, I have to say I'm pleased with it. I was sort of hoping to see a little more top end speed, but this is not a speed boat by any stretch of the imagination. It gets hammered maybe once a season so its not a big concern. The biggest reason for doing this was we got a screaming deal on it, only slightly more than re-propping and refinishing the BII's would have cost, but the push for more fuel efficiency was not insignificant. This boat last year with the tired motor and one used motor was eating 25 gph at 26 mph. We haven't filled up since the maiden voyage, but just watching how the needles have moved, I'm guesstimating its more around 18-20 gph now, and thats where I'd expect it to be given the weight and power.

 

[Maclin]

Re: Dual Prop pros and cons?

I am not sure ol' Kiek was around for the DP innovation was he? I have no loyalty one way or the other, just the VP DP is a really good performer and their prop design is way different than Merc's regarding duals.

 

[bruceb58]

Re: Dual Prop pros and cons?

Way easier to steer when slow especially in reverse.

 

[TilliamWe]

Re: Dual Prop pros and cons?

the pros:

Better out of the hole acceleration.
Less time to plane
Lower planing speed
Ability to hold plane longer when slowing down
Less prop blowout in tight turns
Less wander at idle speeds
Better control in reverse (more immediate direction change, and turns better in both directions.)

The cons:

Propsets are expensive
Less propsets available (especially true for Mercruiser) no aftermarket props
Much higher initial cost
Higher repair bill if damaged
Bravo 3s are more suseptible to corrosion. So are VPs with the stainless props
(Mercruiser's trouble with corrosion has been WELL documented... VP has managed to cover theirs up, but they are having paint and corrosion issues something fierce since 2008)
Better prop bite means that at idle speeds (especially when docking) the boat is faster than a single prop drive. You can get close to stuff very quickly and can bump into things harder than you would like.

 

[26aftcab454]

Re: Dual Prop pros and cons?

the pros:

Better out of the hole acceleration.
Less time to plane
Lower planing speed
Ability to hold plane longer when slowing down
Less prop blowout in tight turns
Less wander at idle speeds
Better control in reverse (more immediate direction change, and turns better in both directions.)

The cons:

Propsets are expensive
Less propsets available (especially true for Mercruiser) no aftermarket props
Much higher initial cost
Higher repair bill if damaged
Bravo 3s are more suseptible to corrosion. So are VPs with the stainless props
(Mercruiser's trouble with corrosion has been WELL documented... VP has managed to cover theirs up, but they are having paint and corrosion issues something fierce since 2008)
Better prop bite means that at idle speeds (especially when docking) the boat is faster than a single prop drive. You can get close to stuff very quickly and can bump into things harder than you would like.

yep- exactly right.
BTW- my diploma is from the school of hard knocks!

 

[thegrizzly1]

Re: Dual Prop pros and cons?

Great information everyone. Thank you! This is just the discussion I was looking for.