BoatBanter.com - View Single Post - The truth about Volvo IPS drive technology

Mr. Luddite

On 9/27/2015 2:21 PM, wrote:
On Sun, 27 Sep 2015 11:47:27 -0400, "Mr. Luddite"
wrote:

To me (and obviously to the guy who wrote the above) the IPS system
re-introduces all the drawbacks and problems of outdrives.

I understand real big boats use inboards but outboards offer an
elegant simplicity that is hard to beat.
I see boats approaching 40 feet with outboards. Granted they use 2, 3
or even 4 but that is great redundancy for guys running offshore.

It is like the B-52 pilot who radioed in that he had lost an engine.
The tower asked if he was declaring an emergency and he said, "no I
think we will make it back on seven)

I agree with multiple outboards on large, go-fast, center console type
fishing boats. See a lot of them in the 30'-40' range. They are
designed to get out to fishing spots quickly to get more actual fishing
time.

But for a cruising boat of that size nothing beats a couple of diesels
spinning big props. Well maintained they will go for several thousand
hours of hard use. I think the rule of thumb for a gas engine I/O is
about 1,000 to 1,500 hours before you have to start thinking of
re-powering or a rebuild. Don't know what a modern, 4 stroke outboard
is good for. The 1984 Grand Banks we had with the little 120 hp diesel
had almost 7,000 hours on it when we were first looking at it. We had
a survey done and a mechanical inspection and I expressed my concern
about the number of engine hours. Both the surveyor and the mechanic
said, "it's just about broken in".

A gas engine has to run at a much higher RPM to develop it's rated
horsepower. A diesel, with it's superior torque runs at almost
half the RPM. So, everything else being equal, the gas engine is
wearing out about twice as fast.

I think the Volvo diesels I had in the Navigator were full throttle
rated for about 2600 RPM which would push the boat at 24 knots.
Cruising at 19-20 kts they were turning about 2200-2300 RPM. That's
the other big difference between gas and diesel. The recommended,
continuous "cruising" RPM is much closer to the max RPM with a diesel
when compared to gas.