Re-powering from Chevy V8 to Volvo Penta G

[rsn48]

So I know I am asking for speculation but please speculate. I am going to need to re-power. Currently I have the marine Chevy 350 block engine with 270 HP and it the new recommendation is The Volvo Penta 200G mated to Volvo’s Duoprop system.

With my current system of 270 HP I can get roughly 11.5 top speed and cruising at 7 knots about 14 liter burn, this with an over propped leg, massive torque (I bought the boat this way). The Volvo has 200 HP and the duoprop for better tracking performance. So I'm losing 70 HP, but less weight in the boat and a better prop system.

So the speculation is, do I lose some top end speed and do I get better gas performance - logic says yes to this - when cruising at 7 knots. The boat is exceptionally heavy with a plywood/glass hull (original construction) and is semi-displacement, never seen a plane.

Thanks in advance.

[flyingfin]

If you only get 11.5 knots WOT, then you may be overpowered for a hull that wont plane. Accordingly, why not go with a smaller V-6 Mercruiser that will be lighter and will get you better fuel economy while using your existing outdrive. Is your existing outdrive in decent shape? Going from V-8 to V-6 Volvo with new outdrive may not improve your fuel mileage to pay for all that upgrade cost. Save money and invest in a secondary fuel tank.

[rsn48]

The engine has been inspected but not the leg, but the speculation is that the leg could be in rough shape, they don't know. But there are issues with the engine, one being the heat exchanger, what may possibly be water in the transmission fluid, and a possible oil leak.

I don't mine re-doing the engine and leg although it won't be cheap in particularly since my son will inherit the boat and I want to pass on a working viable vessel. I'm 71 and when I get substantially older he can pay for the maintenance and drive me around... lol. And whatever it would cost in the States, you can add on 25% exchange on the dollar, and god knows what tariffs created by "he who shall not be named."

If I don't re-power now, it will be required in the next few years I'm sure. One advantage is the dual prop which will give me greater control, especially going astern, this I look forward too, trust me. My marina is exceptionally tight. When you are out on a 50 year old boat with a 50 year old engine, you wonder when the thing might conk out; this fear makes C-Tow a viable option... lol.

[kapnd]

If you can get a few more hours out of the motor, it would be worthwhile to experiment with a smaller propeller.
When properly propped, you might realize better top speed, control, and fuel economy.

[Bill Seal]

I would guess the WOT RPM should tell you if it's over-propped or not.
Just my wildassguess.

[rsn48]

The scenario has changed, there were things wrong with the engine, some items hard to get but there were issues with the leg. I decided to cut my looses and re-power with the V6 280 HP gas Volvo Penta Duoprop. Because the boat was so over propped by the previous owner going in a straight line was a challenge. It took experienced boating guys a half hour to adapt to the crazy steering. The duoprop should solve that problem.

At first the refit guys were looking at replacing a V8 with a V8 and we decided to go with a V6 200 HP. But I discovered instead of replacing cylinders I could replace horsepower instead. The old motor was 270 HP and I can get the V6 in a 280 HP version which is what I've decided to go with.

[kapnd]

Have you actually calculated the horsepower requirements for your vessel?
Do you know how much it weighs?
I would not spend a dime on it until I knew some hard facts about the situation.
Even for a slow moving vessel to operate efficiently and effectively, some pre-engineering would be a very wise prerequisite to purchasing motors and drives.
Simple horsepower numbers are only part of the equation!