I need to re-power my Newport 27 sailboat. The boat weighs about 7500 lbs loaded and has a waterline
length of 23 feet. With the Atomic 4 engine direct drive and a 12 x 9 3-bladed prop, I make about 2100rpm's. According to the Atomic 4 manual the engine is making 16hp at that RPM. The current
engine prop arrangement can push the boat at hull speed which is around 6.5 knots. I'm considering the Volvo MD2020 because of it's small footprint and 3 cylinder vs. other engines which
are 2 cylinder in that size. Furthermore, to reduce installation height, I want to install a ZF 15 MA
8 degree downangle drive with a 1.88:1 ratio.
I believe I can purchase the engine without gear and install the ZF tranny using a #5 bell housing,
a Borg Warner adapter, and a damper plate.
I'm hoping that this engine will have enough power to do the job. But after calling Michigan Wheel for
a prop analysis, they said that the nominal 16hp that the MD2020 makes will not be matched to one of
their props. Because I am limited to a 12" prop, they say that this won't work. I was figuring my 12 x 9 3-blade would be close and if not I could guesstimate the correct prop from there.
I used the prop calculator on this site using the 1.88 reduction and come up with: 12 x 7.6 3-blade prop
Alternatively, I could get a 2.14 ratio and the same size prop would work also.
The people at Michigan Wheel said I should go with the lower reduction 1.88 and a 11 x 10" 3-blade prop on a 30hp engine which makes no sense to me.
I think the 30hp is probably more engine than I need for this size boat. I realize that swinging a 13 or 14" prop with more reduction is probably more efficient but I don't really have the room to do it unless I extend the shaft 3 or 4" beyond the cutlass bearing. But I think that causes problems. Does anyone have any input on this? Also, is my parts list correct?
I've talked to the 3 Volvo Penta dealers in my area and they don't have any experience with the small MD2020 engines and don't seem eager to get involved unless I buy a Universal M3 20B, Westerbeke 20B Two or a Yanmar 2GM20F. These engines are all much taller and have an 2.5" downward offset gear which would make them mount even higher necessitating modifying the cockpit floor.
Atomic 4 Dimensions
Width to center of motor mounts: 11 ~ 11.5"
Width of engine pan cavity: 10" top; 9" bottom
Height top of engine mount flange to overhead: 17"
Distance from engine mount flange to hull: 9"
Height from top of engine mount flange to highest point on engine: 14"
Length from shaft coupling to front of engine: 28"
Existing engine width: 17"
Existing engine opening: 17"
Maximum engine opening can be enlarged to: 22"