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Volvo Duo-Prop, what X Dimension?

S

Scarron

New member
Having suffered a terminal cambelt breakage on one of my Volvo 171's [not unexpected given their appalling reputation] I have disposed of the things and am now in the middle of re-engineing the boat from twins to a single.

The boat is a 1969 Sonny Levi designed Triana 25' and I am going from 2 x 167hp [on a very good day] Penta 171 4 pots to a single Mercruiser 454 Mag MPI [385hp at the prop] with the correct number of cylinders. This setup should increase the horsepower at the prop by about 50% and hopefully put the boat speed somewhere in the mid to upper 40 knot range at WOT.

Because it is a heavy hull and I want to have the best of all worlds in terms of prop bite, easy mid speed cruising, and good top speed as well as good handling in the marina I have decided to stick with Volvo Penta for the Outdrive and have chosen to go with a DP-D1 Duo Prop.


So the big question is how high do I cut the hole in the newly rebuilt transom to give me the best combination of efficiency and speed versus good handling characteristics?

I can't find any installation information for the older AQ series Aquamatic drives only the later versions and have so far managed to glean the following recommendations [all dimensions are using the bottom of the keel as a datum and show the height of the anti-ventilation plate above or below that].

DPR Drive [for fast vee bottom boats capable of more than 45 knots] +36.5mm above the bottom
DPS Drive [general purpose drive] +6mm above the bottom
DPH Drive [heavy duty drive for diesel powered cruisers and flybridge boats] -12mm below the bottom

To contrast with those numbers single prop outdrives like the Bravo One on medium to fast boats seem to have their plate placed about 50mm above the bottom. however they also have a choice of prop designs that can use a degree of surface piercing effect so I am not sure that is a fair comparison with the Volvo duo prop technology.

These are quite big differences and I am thinking of going fairly high i.e. at the DPR end of the spectrum, but will it struggle to plane, lead to more prop cavitation or be a pig at slow speeds?

Any thoughts?
 
Yes.... a few thoughts if I may.

With a properly Q/E built 377/383 SBC, you can shave approximately 250 lb from the package.
The key is quench/squish effect when building one.
Say NO to the full dished pistons! :mad:

Torque is nearly that of the BBC.... and she's now 250 lb lighter in the arse end!

The Q/E built 377/383 can also use the 1.78:1 ratio as does the BBC.
(steer clear of the 1.95:1 for either of these two more torque producing engines, IMO.)

NOTE: not all flywheel covers allow for the larger 168 tooth flywheel.
Most early ones require the 153 tooth, and the straight accross bolt pattern starter motor.

For the X dimension, I'd not compare Merc to Volvo Penta.
If you are having trouble, I'd suggest visiting a boat salvage yard.
No doubt there'll be a similar OEM installation that you can take dimensions from.
If need be, Volvo Penta offers a drive extension that can be used.

BTW, great choice in staying with the AQ series drive..... the DP-C1 or the D will be just fine. :cool: Much less troublesome than the later Gimbal System Duo Prop drives.
In your hull speed range, the Duo Prop will outperform the BIII drive due to pattented Duo Prop propeller design.

If you need a Flywheel Cover, I happen to have a brand new late model single bearing one here.
This allows PDS bearing replacement w/o pulling the engine.
I also have the 1" drive extension.

Good luck, and have fun with the project! :D

.
 
Thanks for coming back Rick

I have already bought the 454 Mag MPI so a small block is no longer an option, it's done around 350 hours so we will take the heads off for a look as it seems that they need a head job at about that age. It depends what else needs doing but if budget allows I may skim the heads to up the compression a bit [I am UK based so the octane of regular fuel is a little higher] and maybe fit a slightly bigger cam while we are in there.

I picked the DP-D1 based on your recommendations on this forum, hopefully it should take the torque OK. I found one that had been fitted to a 7.4GL so I also picked up the flywheel cover to suit the larger flywheel and also will be following your advice will be changing the PDS bearing.

I am thinking go with the DPR numbers on drive height and if need be I can always fit the 1" extension.

What I am not clear on is engine to drive alignment - unlike the Mercruisers which have an alignement tool it seems to me that the two big O-Rings on the Penta flywheel cover nose would allow quite a bit of misalignment without leaking, is that the case? so do I align it by eye or is there a correct procedure?

Thanks

Paul
 
Scarron said:
I picked the DP-D1 based on your recommendations on this forum, hopefully it should take the torque OK. I found one that had been fitted to a 7.4GL so I also picked up the flywheel cover to suit the larger flywheel and also will be following your advice will be changing the PDS bearing.

I am thinking go with the DPR numbers on drive height and if need be I can always fit the 1" extension.

What I am not clear on is engine to drive alignment - unlike the Mercruisers which have an alignement tool it seems to me that the two big O-Rings on the Penta flywheel cover nose would allow quite a bit of misalignment without leaking, is that the case?
so do I align it by eye or is there a correct procedure?
Click to expand...
Paul, I'll take the first comment as a huge compliment... thank you.

As for engine alignment, this is a one-time deal. There will be six flats that triangulate (3 on the inner side of the shield, and 3 on the F/C)
When all six are of equal distance or separation, the engine/PDS/shield will be aligned.
Use the engine mounts for height and side-to-side adjustment. Lock her down once aligned, and ONLY then will the six F/C "clamping ring" bolts be finally tightened.

No further annual alignment is required.


Make sure that the F/C side rubber cushion ring is NEW..... this one does the water sealing (the exterior ring does not).
Used rubber cushions will have taken a "set" and may not seal second time around.

NOTE: transmissions that have been subject to high HP or Torque, may be in need of main drive gear tapered roller bearing replacement.
You'd need to pull the main drive gear "bearing box" from the transmission, and then press the gear out, in order to examine them.
These will be a high quality 30207 and a 31307..... and must have the correct rolling torque value re-set.
This is not all that complicated to do..... and I would sure recommend it.

The "driven" gear angular contact bearings seem to survive very well.

You may also want to pull the prop shaft bearing carrier, and expose several of the lower unit bearings/gears.
There is a demonstration of the home built puller tool here.
Cost to do so is two O-rings only.
No change to shim control occurs while doing this.

Have fun.

.
 
Many thanks for that Rick, excellent comprehensive advice as always.

Fitting sounds pretty straightforward, but I think it'll be cheap insurance to go through the leg as you suggest and make sure the bearings are top notch.

I have pretty much made my mind up to go with the DPR X Dimension as I can always take that 1" extension off your hands if the handling is'nt how I hope :-]
 
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