Volvo Duo-Prop, what X Dimension?
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?