Going from Volvo duo prop to single prop for the winter

[Magnus W]

I have an aluminum boat, approx 26 by 9 feet with a vee of about 20 degrees and 7500 pounds.

Engine's a Volvo AD41P diesel (200 hp) and the regular leg is a DP-E (290) with 1,95:1. Normally I cruise at 24-25 knots at 3200 rpm with a 12 psi boost with A7 props. Top is 32 knots.

Now I'm swapping for a SP outdrive for the winter (due to ice) with the correct 1,61:1 gearing. Prop calculators state that I need about prop that's about 15,5-16 x 19 to keep my speed. I tried a 14,3 x 19 Solas and it was useless (17 knots at 3500 rpm) , now I have a Volvo 16 x 19 which is better but still a long ways from the DP. Now I'm doing 21 knots at 3300 rpm so 16 x 19 seems a tad small still.

From trying out several different props on my DP it seems that my boat likes big props so I guess that might also be applicable to the SP drive.

I've found an adapter that lets me use flo torq hubbed props on a Volvo leg. Now I'd like opinions on what prop to get.

I've looked at Suzuki and they turn 16 inch dia props on their larger outboards. So maybe a 16 x 20 or perhaps even a 21 stainless (I need stainless because of the ice). But the Mercury has loads of props but I know nothing about them.

Suggestions?

 

[Scott Danforth]

Why would you degrade from a duoprop to a single prop?
The diesels were never in front of an SP

 

[Magnus W]

Yes, you could have the diesel with single prop drive although the most common choice was the DP. Maybe though it was different in the US and you couldn't whereas we could.

But as I said, this is only for the winter because the DP doesn't agree with ice. Chunks of ice gets caught between the trailing and leading edges of the front and aft prop, damaging them and wreaking havoc on the inner (and relatively weak) inner axle. The SP drive is much more sturdy (for an outdrive that is).

 

[Scott Danforth]

the ice thing kind of makes sence

however your currently running A7 props. that would be approximately a 24" pitch

the 1.6:1 ratio would load it a bit more than the 1.95, and the single prop would load it a bit less. my guess off the top of my head would be a 22p, however a VP dealer in your neck of the woods would have experience with the SP drive behind the AD41P with a 1.6:1 drive

 

[QBhoy]

Are A7 props not very lightly pitched (for diesels to be able to spool up the turbo and make best use of the low revving diesel) ? I'd be surprised if they were as much as 24"pitch. Haven't looked it up, may be right.
I used to have B5 props coupled to a petrol GM engine. These looked pretty fiercely pitched, understandably.
Just a thought. I'm sure google has the answer.

 

[Scott Danforth]

the difference between the A series and the B series is the A series has one prop with 4 blades (rear prop). the pitches are the same.

A7 props have much more pitch than your B5's (about 4" more) as they jump up in pitch approximately 2" per series number The A5 and B5 is about a 22/23 pitch, the A6/B6 about a 24/25 pitch, and the A7/B7 about a 26/27 pitch. Diesels make about twice as much torque per RPM as spark ignited (Petrol) for example I had been running B6's on mine with the small block, now looking to run B8's or A8's with the Big Block when its finished

 

[QBhoy]

Scott
I see...that makes sense. I just presumed that a turbo diesel wouldn't have the low down power to get up and on plane with boost without a lesser pitch to help. I had noticed the petrol had 3 blades as opposed to the 4 on the diesel.
Thinking about it now, my friend and I had similar weighted boats, mine with the 4.3 petrol and b5's and his with the tamd31 150hp and A7 (I think) Mine would cruise at 3000rpm and 22 mph and his would be almost flat out at 3000rpm and 25mph. This would suggest the pitch is higher on the diesel right enough. Every day is a school day on here ! His took an eternity to plane though. Haha. Scott, not sure is you have seen my post on the 4.3 revving up. Loose track of names and who has replied (and doing this on phone so hard to flick back and forward). Needing some sound advice !
Ta

 

[Magnus W]

I have tried multiple sets on my boat: A5 to A8, C4 to C5 and B5 to B6

All boats are different but in my case a B6 is between A6 and A7. A C6 is the equivalent of A7. Only talking speed vs rpm here.

Side to side comparisons between the three models: Aside from the fact that the rear prop on the A has four blades the A:s have thinner blades than B:s. Although I haven't measured them, judging by the looks the C:s are basically B:s but stainless and with thinner blades.

Penta suggests that A-props are for diesels and B:s are for gassers and more powerful diesels like KAD44 and KAD300. But one never knows what works on a particular boat, a friend has a KAD44 and 1,68:1 with and it runs best with A6.

On a side note I find that I build boost earlier with higher pitched props. It makes sense since boost is related to load. So if your boost is coming in late it may well be that you to small props (assuming you can reach max rpm and all else is well).

 

[Scott Danforth]

The B series is exact as C series, except B is aluminum and C is stainless

The A series has a slight blade shape trim change on the inner compared to the B, and the aft is a 4 blade

 

[Magnus W]

From my experience a C doesn't behave the same as a B, despite having the same number. I guess it has to do with the stiffer and thinner blades and smoother surface.

 

[HT32BSX115]

Since you're intentionally running your drive in "contaminated" water, stainless steel blades might fare better than aluminum blades.

The single prop drive will never perform as good as a dual contra-rotating prop drive.

No drive will be unscathed running in 'rocks" though depending on how big those rocks are.

 

[Magnus W]

My intention is to swap to stainless as soon as I've figured out the approx. correct size. I should be getting a 16 x 21 tomorrow (Volvo aluminum) and if that one works out I see if I can find a stainless one. Stainless props for SP aren't too common so I might have to go the adapter route. Solas has 15,5 x 19 four blade but after trying their Amita 4 in 14,3 x 19 I very much doubt it will work and there are no larger ones.

 

[Magnus W]

my guess off the top of my head would be a 22p

Not a bad guess!

I swapped the 16 x 19 for a 16 x 21 yesterday, still Volvo oem. I don't know which version the 19P was bet the 21P is the high speed version (the only one that Volvo sill makes apparently).

The boat performs much better in speed/rpm/boost figures (it obviously still lack the maneuverability and smoothness of the DP). I'm basically down 1 - 1,5 knots in cruise for a given rpm, same for top speed, and I see about a 2 psi boost increase which suggest a slightly higher load and lower efficiency compared to the DP.