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(no email)
Date: Tue May 22 2007 - 21:13:31 EDT
Interesting topic, from my original post I have received very positive
information regarding repowering a 20 ton boat and whether the 3160 Cats at
210 HP are necessary.
Considering the boat in question is a 48 Ft Gulf Commander with a semi-
displacement hull (won't be as efficient as a full displacement) it will
have a max hull speed before extra power is required to take it past the
theoretical hull speed value. Using the formula for Hull Speed = 1.34 *
(LWL)1/2 (I know this is for full displacement hulls, but it is a good
approximation for semi-displacement as well) and asuming an LWL of between
43 to 45 feet, the hull speed is approximately 8.8 knots.
Using the power formula below 40000/500 = 80+(80*.2) = 80+19 = 96
Horsepower.
Considering the number of boats out there with twin 120 HP diesels, I
submit that this boat could easily use a pair of 120 HP diesels (maybe even
100HP diesels considering two shafts) quite nicely, and as Pascal has
stated, getting rid of a significant amount of weight in the process is a
positive aspect.
I will do more research on this of course. I will also be going to
Transport Canada to find out what type of information is required when a
pleasure boat is brought into Canada or designed and marketed here. I have
been told that they do require some stability and power calculations from
manufacturers. The other interesting aspect of the Gulf Commander that I
was told is that they were designed for commercial fishing in Alaska before
being used as pleasure craft.
Thanks to all who sent me information.
Ernest
On Tue, 22 May 2007 20:10:03 -0400, "Bob Clinkenbeard"
<> wrote :
> > wrote:
> >
> >> The usual NA rule of thumb figure for propelling a "displacement"
yacht
> >> at hull
> >> speed is 1 hp. per 500 lbs. of displacement.
> __________________________________________________
> Let's see...using Larry's formula...5800 lbs divided by 500 = 11.6 hp +
> 20% - 2.32 hp = 13.92 hp
>
> The architect for my "semi displacement" boat recommends "Power: Single
> centrally mounted diesel or gasoline inboard; 10 shp for 7 knot cruising
> speed, 25 shp for maximum 9 knots intermittently. Reduction gear may be
> required. Engines rated over 35 shp not recommended."
>
> I guess it is in the ballpark. The PO and builder installed a Perkins M-
30
> with 29 hp and it does the job with plenty of power to spare.
> As far as fuel economy with my excessive 10 extra hp, I'm not sure I can
> beat the 1/2 gallon per hour at 7 knots that I'm getting now. :>)
>
> Using the BoatUs calculator for hull speed
> http://www.boatus.com/boattech/nautcal.htm with my waterline length of
22',
> I get 6.285 knots. I know that can vary with hull shape so I'm pretty
much
> right on with the calculations.
>
> Good exercise...thanks Larry.
>
> Bob Clinkenbeard
> 24' custom trailer trawler
> http://home.bellsouth.net/p/PWP-aboardbobbinalong
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