I am hoping to cruise around 2200-2500. Normally aspirated, the Gm motor
(actually designed by Detroit Diesel) achieves peak torque at 1800 rpm and
at peak power (best combination of torque and hp) at 2800rpm. The motor is
governed by the fuel pump to 3600-3800. Its best to keep the rpms under 3000
since power drops off rapidly above that.
With the addition of a good sized detroit diesel allison turbo from 6V71
which is about 20ci larger than my motor my goal is 18-20lbs of boost at
cruise. I will be monitoring an exhaust gas temp gauge to make sure I dont
overheat the motor from too much boost.
Some research leads me to believe the design of the suface drive does have
more slip than a submerged prop. It is designed so that at planning speed
only half the prop is in the water. Thus the "surface piercing" prop is
likely slipping more, but this can be varied by the trim.
Interesting paper on the topic:
http://www.well.com/user/pk/SPAprofboat.html
The main advantage of this is less drag, only the rudder and half the prop
are in the water. The disadvantages I see are that at hull speed I have too
much prop in the water and may get too much bit for the engine\trans. and
reversing can cause spray over the transom if the drive is not trimmed down
first.
I still dont have a good feel for the prop size yet. In the past I have
relied more on a trial and error process for prop selection.
How big do you think my prop size should be????
Figure my engine, trans and drive weigh approximately half of the two old
302s , omc drives. The weight is now farther forward as well due to the way
the engine and trans is mounted inline. I am estimating my empty boat weight
to be 5500lbs with another 1000-1500 for fuel and gear.
More as the story develops.
Paul
"Terry King" wrote in message
.. .
Paul, What RPM do you plan to run that engine at for 'cruise'?? The prop
will be 1/2 that, I understand. Does the surface drive have more 'slip'
than a non-cavitating prop? And how much does that change the RPM
calculations??
I have OSCO manifolds on the 350 GM I'm running and they've been fine,
and a decent price. Wish they had one for the Rabbit/Golf diesels!
Although there is an British supplier... I built welded copper manifolds
years ago, but had trouble keeping flanges intact. They were real
effective cooling the exhaust, with a .062 copper wall, though. Maybe
I'll think about doing that for a VW Diesel. I think I'd start with a
1/2" steel plate bolted to the block as a 'manifold starter plate' and
braze the copper pipes to that. I have to look at the VW geometry. Maybe
I can pick up a blown engine to play with and measure before making any
committments :-)
Thanks for all the info; I'll be waiting to hear about your launch!
In article SD83c.210320$jk2.722631@attbi_s53, you say...
I am using a standard GM marine bellhousing and shaft coupler mounted to
a
manual transmission flywheel. Attached to that is a volvo transmission
to
give me a 2:1 reduction and forword-nuetral-reverse gears. I had a
constant
velolcity joint made from 2 universal joints that connects between the
trans
and drive to take up the engine vibration and slight angle change.The
exhaust manifolds were right out of the OSCO catalog and I am having
custom
risers made to connect them to the turbo.
My prop is 17X18" so I may need to get it recupped after I see how it
performs.
--
Regards, Terry King ...In The Woods In Vermont
The one who Dies With The Most Parts LOSES!! What do you need?