Single prop VS Duoprop questions.

[ron-bc]

Hello all.
I have an 18.6 ft Malibu, built in Kelona. It has a AQ171 Volvo and a single prop. I have tried running a couple different pitched props, 1 for power and 1 for speed. Both are fine, but with a couple people on board, its a dog to get out of the water. We move up forward and utilize the trim tabs, but it just seems to be too slow, not enough power.
One person I know in the boat repair industry told me that a duoprop would be the way to go. Another told me I would need more engine to make the duoprop work properly. Who is right? Should I be looking to get more jam out of my engine?
I have seen these boats come out with 4.3 engines...
Thanks for your input...

 

[180shabah]

Re: Single prop VS Duoprop questions.

I think DP is probably overkill on an 18.5' boat. Your are running two different props, what pitches? and what is your WOT with each prop?

If your engine is running properly, perhaps you should investigate a four blade prop at an even lower pitch.

 

[ron-bc]

Re: Single prop VS Duoprop questions.

I think DP is probably overkill on an 18.5' boat. Your are running two different props, what pitches? and what is your WOT with each prop?

I will have to look at my notes at work for those pitches...

If your engine is running properly, perhaps you should investigate a four blade prop at an even lower pitch.

What are the benefits of a 4 bladed prop?
Thanks for your reply and info..
Ron

 

[180shabah]

Re: Single prop VS Duoprop questions.

Typicaly, better hole shot, slightly better cruise economy and less vibration, all with a slight sacrifice in topend speed.
Same thing you were looking for in a six blade (DP) setup.

But, none of it matters if your engine is running poorly, or the wrong pitch is chosen.

 

[ron-bc]

Re: Single prop VS Duoprop questions.

Typicaly, better hole shot, slightly better cruise economy and less vibration, all with a slight sacrifice in topend speed.
Same thing you were looking for in a six blade (DP) setup.

But, none of it matters if your engine is running poorly, or the wrong pitch is chosen.

My engine is running great. Once this thing is out of the water, it performs well. Excelleration is fine, power is good. Just getting it out of the water is a problem. The actual prop pitch is not something I remeber right now, but I do recall that it is a recomended and ideal pitch for my setup. It could be that I am just too heavy for my engine torque. I have to wonder why the boat now comes with the 4.3 now, and mine is 4 cyl powered... It may be worth looking at re-powering the boat, but It could be worth replacing the boat with a better full setup as well... It comes down to $$ I guess.
Thanks
Ron

 

[180shabah]

Re: Single prop VS Duoprop questions.

but I do recall that it is a recomended and ideal pitch for my setup.

What is your WOT RPM?

 

[andy1canada]

Re: Single prop VS Duoprop questions.

Howdy Ron!

Welcome to iboats!

I live in Victoria. You'll find a TON of helpful people on this forum; nobody is tryin' to SELL you something.

I thought you might find my post 'Merc vs. Volvo..." a half page or so below this one, useful.

I'm in the midst of repowering an old (1980) Zeta/cuddy with a 4.3/injected. It currently has the original 305/Chevy/Merc/sterndrive in her.

My boat will weigh-in at about two-tons loaded, so I'm not sure if it would benefit from a Volvo DP or not. Plus, they are quite a bit more $$$ than a straight Alpha set-up.

I've got a mechanic here in Victoria whose willing to install a new Merc 4.3/MPI/Alpha/FWC for just over $14/k taxes-in.

It's looking like I'll need to cough another one or two large-ones to get into Volvo anything. But I'm not discounting that either. A guy's got to think about things like service-life, reliability, economy, and re-sale, to name but a few.

One thing seems clear to me considering a boat in my class: the 220/hp 4.3 is the way to go. If you don't get airbourne first, your boat would likely broach 60/mph with one of them babies pushin' her.

Good luck and happy boatin'.
Cheers,
Terry

 

[ron-bc]

Re: Single prop VS Duoprop questions.

Sorry, sleep got to me...
My WOT is about 4000 -4400 rpm.
My prop sizes I carry on board I believe are 14x17 and 14x19
One of them is my spare. I do get out better with the 17
I will have to re-test the setup with both props and write the info down this weekend, if the weather and work permits.

Andy1...
I only paid 12K for the boat, and its worth less than 20K with the improvments I've done, so to spend that kinda money on it would be against my better judgment. I would probably upgrade the whole craft, and maybe get into a Trophy, as most of my time out there is fishing and crabbing anyways.
Ron

 

[QC]

Re: Single prop VS Duoprop questions.

Welcome Ron,

I am confused about a couple of comments on Terry's and this thread. A Duoprop will have better hole shot period. Doesn't matter what's pushing it. Better than a 4 blade and better than a 5 blade. The benefits of no prop torque (not torque steer) are more obvious on a small boat. Also, they have much better performance in reverse. Single prop drives are usually good backing to starboard, but poor to port with right hand props (yours is probably left hand I think). I am not suggesting that it makes economic sense to switch drives on an older boat though, just pointing out benefits of twin screw drives vs. single.

With that said, the question is whether or not she is right as she is now. 180shabah mentioned WOT RPM.

WOT is Wide Open Throttle. To determine if she is propped right as is, we need to know what the RPM is with the engine at WOT, and trimmed as fast as you can make her run, with an average load. Your tach must be accurate! We don't really care about the actual speed, just the RPM. Then we can compare that to the recommended range for your engine and determine if we can get better hole shot with a pitch change. I just looked around and apparently that is a double overhead cam (DOHC) 2.5 liter and it's recommended WOT RPM is 5000 - 5700. If your boat won't turn toward the top of that RPM, then I would start with prop selection first. Top speed may be helpful in guessing as to whether she is running right. So, as always, the most info you can give us the better.

Here is a great doc from Volvo I just found. Check out page # 17 : Volvo Prop Selector thing

I am also curious about your trim tabs. Can you describe them and how you use them?

 

[QC]

Re: Single prop VS Duoprop questions.

Ron,

Just saw your post. I was typing as you posted. Need to verify that the tach is correct. If it is . . . I think we have identified an issue. Not sure which issue, but an issue. That thing is either not running right or she is way over pitched. Check out the doc I posted. If the props seem with in their recommended range then the engine may not be right. Also, the hull could be waterlogged . . .

 

[ron-bc]

Re: Single prop VS Duoprop questions.

Interestingly, I just changed my tach last week. I was pretty sure it was close, but it changed when I tapped it while running, so I replaced it with new.
A few other things to note, I had my props re-done and re vulcanized last year at a local Nanaimo shop. At Christmas, I was out and the prop vulcanizing let go. I changed my prop to my spare then. It has also gone through the same repair and re vulcanizing. Perhaps he did a crappy job of the props and this is a reason for poor performance? Possible, I think.
This weekend, hopefully, I will be installing the new props and testing them with the new tach. I will look at the performance of that and note the WOT, also need to check the red line spec for my engine too. Next time I have it on a trailer I will have it weighed somewhere and compare that spec. I do leave it moored in the channel here, but the engine bay is always dry, there are no holes, and the hull is in nice shape.
In your other post you said that if I did have the duoprop, it would get out of the water better? That would mean I wouldn't need a bigger engine or more torque to drive the duoprop.
Ron

 

[ron-bc]

Re: Single prop VS Duoprop questions.

Just got this info from another website...

Min WOT RPM: 5000
Max WOT RPM: 5700

from http://www.piranha.com/motor_specs.php?id=5484
And this link shows my package. (Very sweet layout from Volvo)
http://www2.volvo.com/NR/rdonlyres/A23CE43B-775C-4A41-A67B-216F333FB010/0/251ADOHC_SP_1990_en.pdf
Ron

 

[QC]

Re: Single prop VS Duoprop questions.

, also need to check the red line spec for my engine too.

Well you got it right too, but I had it ^^^ there for you . . .

In your other post you said that if I did have the duoprop, it would get out of the water better? That would mean I wouldn't need a bigger engine or more torque to drive the duoprop.
Ron

No, but let's not worry about that yet. I am not suggesting you go with a Duoprop, but every test I have seen with the same power package favors twin screw drives for hole shot . . .

 

[salty87]

Re: Single prop VS Duoprop questions.

you said you keep her in the water, what condition is the bottom in?

 

[car5car]

Re: Single prop VS Duoprop questions.

Ron, could you look please if you have wire going to carburator? I have the same engine and I don't know where this wire should go to. My carb is twin Solex

 

[ron-bc]

Re: Single prop VS Duoprop questions.

I pull this boat out every few months and clean it right up. Its not like thos boats you see in the marina that are covered with barnicales and mussels...
Car5car, I will have a look at that wire for you on the weekend and report back to you. What color is the wire, and where does it originate? I will have a look at my wiring iagram also regarding that.

 

[ron-bc]

Re: Single prop VS Duoprop questions.

Finally got the props I needed to try out. Tide too low to get out of the slip. Will try to play hooky from work awhile tomorrow to test the 1st prop.
The 1st prop actually is a 14x19 prop. The second to test is a 15x17 prop.
CAR5...
The wire in question controls a magnetic valve at the underside of the top area of the carb where the air breather sits.
It seems to be part of the cold start carb function. There is a vacume line running from the same valve. This is shown in the workshop manual. The wire color is a solid light brown, and connects at the back of the alternator.
If you need the page showing the setup and a wiring diagram, get me a fax # and I would be happy to send a copy to you.
Ron

 

[ron-bc]

Re: Single prop VS Duoprop questions.

Testing results have been interesting. I will get to the best part of it all first. With either of these props, this boat launches out of the water so fast, its incredible. No trim tabs used. Its like a new boat.
That being said, I can only assume that the job of repairing and re-vulcanizing my old props was a waste of time. It must have been a lousy or improper repair done.
14x19 prop WOT was 5800 RPM, basically burrying the needle. Top speed was only 26 mph at WOT. Again, it was out of the water in a heartbeat.
15x17 prop WOT was also 5800 RPM at a top speed of 32 MPH This prop launched the boat out in 2 heartbeats, without the use of trim tabs.
These are the same pitch as the old props I had used. I have ordered a 14x21 to test out for next week. Hopefully bringing the WOT down a bit.
Thanks for your help! It seems like I will be falling in love with my boat again.
Ron

 

[tommays]

Re: Single prop VS Duoprop questions.

good to here your moveing forward

BUT the numbers do not add up unless you mixed them up the 19" should be the faster prop

i do not no the final gear ratio but a 19" prop should be going faster than than 32 MPH at 5800 RPM unless the spedo or tack is giveing a bad number


Tommays

 

[ron-bc]

Re: Single prop VS Duoprop questions.

I just installed a brand new tach, same brand as the old one, same settings in the rear. I am pretty confident on the tach.
I used my Standard Horizion GPS to give the MPH readings. Those numbers also matched my dash spedo numbers.
I just took the lower speed prop out of the box and looked at it. Definitely a 14x19.... And according to the chart from Volvo, it SHOULD be faster speed. I tested both props in the same water, withing 30 minutes of each other. Same current, etc.
I will take it out for another ride tomorrow and see again, with the 15x17...
Ron