T&T: Powering a 50' trawler

[Rock Bradford]

I have a little different slant on this. My 44' Cygnus weighs in at 86,000
(half fuel) and we have a Gardner 8lxb, 170 hp @ 1150 rpm w/ an MG-509 2.91:1
gear, turning a 42" 4-bladed prop.  The Cygnus is a true trawler, full
bilges, 7.5' draft and a 17' beam.  On our trip from South Carolina to
Bermuda and back this past May, we averaged 7.6 kts and burned 3.2 gallons
per hour for 260 hrs per my floscan 7500.  If I want to increase speed to 9
kts, my fuel burn increases to 6 gph (hardly worth the expense).  Thirty
years ago, I delivered a 61' Cammenga trawler from Europe to Florida.  It
was in the 80K# range and had a Gardner 6LX (127 hp) and we averaged over 8
kts  @  3 gph.    In other words, I believe 145hp will propel the boat in
question quite well.

Rock Bradford
44' Cygnus "Magpie"
Georgetown, SC




On Feb 9, 2008 9:21 PM, Kim Boyce & Eric Thoman <kimeric at gocarefully.com>
wrote:

> It was posted:
>
>
> > Need help please. If one is to power a 50' full displacement steel
> > trawler that has a displacement of about 72000 pounds with a single
> > engine, I understand that the rule of thumb is 2-3 HP per 1000 pounds.
> >
>
> And also posted:
>
> Hull speed on your boat is about 9.5 knots. It takes 106 hp to power it at
> hull speed. Add 20% for a safety factor and you need 122 hp. Go with the
> 135
> hp continuous duty John Deere.
>
>
> Reply:
>
> I mentioned this a couple of weeks ago - there seems to me that there is
> an
> 8 knot hurdle that you will not get past with this hp.  I am not
> questioning
> the recommendation as I think that this will be an efficient 7.5 to 8 knot
> boat.  But, if this 50' steel trawler will do 9.5 knots with only 106 hp
> (and
> burn approximately 5 gph) I will eat my keyboard <g>  I do not believe the
> common formulas.
>
> My seat of the pants, unscientific, non-calculated guestimate is that it
> would take 250 to 300 hp to get this thing up to 9.5 knots.  Then, of
> course
> the owner would back it off to 7.5 to 8 knots for efficiency and probably
> burn only 3 to 4 gph.
>
> What experience has anyone else had with a boat of this size and weight?
>
> Happy to be proven wrong,
>
> Eric Thoman
> Abyssinia
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