I have experienced 4 Yamaha 9.9 hp high thrust engines on 32 and 36 foot
sailing catamarans. In both cases, one engine would move the 8,000 to
10,000 pound boats at six knots with 3/4 throttle and .8 gallons per hour.
I would run both engines at full throttle for short periods of time to deal
with adverse tide or weather problems, or to make the next bridge opening.
They would give me 8 knots. My 32 was at least a knot faster than a similar
boat equipped with 9.9 Hondas, and just as fast as a longer waterline Gemini
cat running a Tohatsu 40 2-stroke. If you could live with something between
6 and seven knots, I would strongly reccommend the High thrust Yamaha
because of its bigger prop, MUCH higher gear ratio, reliability, low noise,
and easy installation. The full pop electric start, remote control
longshaft is a bit heavy at something like 111 pounds, but it will recharge
your batteries or run your lights and GPS.

Its possible that your builder was working with a theoretical extrapolation
of speed vs. horsepower based on tests run with 2-cycle outboards. If that
is so, I suspect you could get your 8 knots with this engine
"Brian Whatcott" wrote in message
...
Any motor that couples efficiently to the water, gives higher thrust
at slower speed. So a "High-thrust" motor is one that is intended to
push water efficiently at slow speeds. To do this, you need a slow
moving, big prop geared down from a reasonably fast moving engine.
These are the target pitch times RPM( in thousands) products for the
corresponding desired vessel speed:

2 kt 2.2 inch pitch X RPM (1000s)
4 4.4 inch pitch X RPM
6 6.6 inch pitch X RPM
8 kt 8.8 inch pitch X RPM (1000s)

The smaller the pitch, the bigger the diameter prop needs to be for a
given HP dissipation.

Brian W


On 26 May 2004 08:52:32 -0700, (Sylvester
Sullivan) wrote:


Here is a horsepower to speed chart Glen-l supplies for the eagle:

6 knots 10 SHP
7 knots 12 SHP
8 knots 16 SHP
9 knots 21 SHP
11 knots 27 SHP
13 knots 32 SHP
15 knots 43 SHP
18 knots 56 SHP
20 knots 70 SHP
22 knots 88 SHP
25 knots 105 SHP
27 knots 125 SHP

The question I have is would these same values apply to the Bigfoot or
"high thrust" motors? Would there be an advantage to going to a high
thrust motor for long range, low speed cruising? I'm just not
understanding these high-thrust, big prop motors, so please bear with
me.

Thanks,
Syl