Re: [PCW] Motorcat 30 - An Owner's Impressions
Drew wrote:
The first thing I did, and I wouldn't do it quite this way again, was to
repower. Since we knew she "wanted" to go faster, I replaced the Suzuki 50HP
4 stroke engines with Evinrude 90HP ETEC engines. I figured 80% more
horsepower for only 180 pounds more weight was a fair trade. Since the
repower I have spent many hours researching and testing propellers and engine
setup to achieve what I felt, and still feel is appropriate speeds for this
vessel with these engines. In short, we have a WOT speed of 23.6 knots,
13-3/4" x 11" 3 blade aluminum props @ 5600 RPM. That's not a lot of gain for
that extra 80 horse power, is it? This is an ongoing effort, and I have
several options yet to pursue before I call a halt to this effort, but 30
knots does not seem to be an attainable goal now, IMHO.
Brian replied:
At just a quick glance I would say you are in a speed range to consider
Hysucat foils for really increasing your performance.
and they might well be fitted to your relative narrow beam.
Next I might suggest that your engines are too far submerged. It surprising
the amount of drag that exist from these lower units too far below the water
surface. Just stick you fist overboard at speed and see what I mean. Jack
plates might be on order?
Brian Eiland
beiland@usa.net
www.RunningTideYachts.com
distinctive expedition yachts
We started the PT-32 with 40HP Suzuki and had a 10 cruise and 14 wide open. This
was really a low wavemaking and not a planing speed. Great for the original intent
doing of the Great Loop.
Calculations showed that the craft should do 25 with 100 HP and we put a DT-100 on
it and actually lost speed.
After a lot of Prop work with a custom prop maker we got a 23 WOT and about 17
cruise.
So 2 1/2 time HP gave 50% more speed. We went from 140 to 340 lbs of motor.
After some time the conclusions on the hull is it is not getting up to plane and
is stuck in the transition zone. We have too much weight on too small aft surface
and buoyancy.
If we continue with the experiment we plan to either shorten the main hull to get
more surface and buoyancy or extend the OR's aft for the same purpose.
Extending means adding weight to the hull while shortening will lighten it so in
this case we may be smaller being better.
Will lose some cockpit area but maybe not that big a deterrent in a day boat with
the small cabin we have.
Engine height is important and so is the angle of attack (trim) If you have room
Trim tabs to get on plan may help.
Part of it is that the props and motors are just not designed for this speed
range. You may get the rpm but your not transmitting the HP to the water.
So your not alone and I doubt you can get 30 K either
Drew wrote:
The first thing I did, and I wouldn't do it quite this way again, was to
repower. Since we knew she "wanted" to go faster, I replaced the Suzuki 50HP
4 stroke engines with Evinrude 90HP ETEC engines. I figured 80% more
horsepower for only 180 pounds more weight was a fair trade. Since the
repower I have spent many hours researching and testing propellers and engine
setup to achieve what I felt, and still feel is appropriate speeds for this
vessel with these engines. In short, we have a WOT speed of 23.6 knots,
13-3/4" x 11" 3 blade aluminum props @ 5600 RPM. That's not a lot of gain for
that extra 80 horse power, is it? This is an ongoing effort, and I have
several options yet to pursue before I call a halt to this effort, but 30
knots does not seem to be an attainable goal now, IMHO.
--
Capt. Len Susman
trikini23@trikini.com
WWW TRIKINI Project
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