I have a boat very similar to Glen-l's Eagle 22 ft. semi-displacement
lobster boat. The main motor is a 110hp 2 stroke which gives me all
the power I need. I also have a Yamaha 30hp 2 stroke kicker which I
use for low speed cruising at about 8-10 knots. The kicker is mounted
on a Garelick heavy duty (up to 40hp) auxiliary motor bracket.

The Yamaha is old and tired and gets very disappointing gas mileage.
I would like to switch over to a 4 stroke kicker, which means probably
having to change the mounting bracket. I would like the smallest
motor possible that will give me about 8 knots cruise at 3/4 power.
I'm looking for good fuel mileage and long range. I don't care about
speed and hole shots, and planing isn't an issue.

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

Sylvester,
Nobody can say with certainty what your boat requires. At best, you will
have an estimate. I suspect that an engine between 15 & 25 HP would be just
fine. Your other question is easier. A high thrust engine would allow a
larger diameter prop and for the speed you wish to run, would be better than
a standard outboard. There are many factors, as I am sure you know, that
contribute to the decision of what prop to use, but here are just a few. A
high thrust motor will use more gear reduction at the prop so the prop will
go slower and have more prop torque. This is ideal for a displacement hull.
When you use your boat at 8 kts, you are in displacement mode. In order to
take advantage of the extra prop torque, you will need a larger prop to
prevent slippage (cavitation). The next question is what pitch to use, what
Blade Area Ratio (BAR) and the number of blades. This can be calculated if
you know two things, the engine speed that developes the highest torque at
the most economical RPM and the reduction ratio of the lower end driving the
prop. They both come from the engine manufacturer's catalog or brochure.
You must divide the engine speed by the reduction say 3,000 rpm and 4 to1 =
750 prop rpm.. Then calculate in inches the distance you need to travel in
one minute for an 8 knt speed, less 10%.(Max allowable slippage). That is
8756 inches in one minute. Then divide 8756 by 750 = 11.6 inches, which
equals the desired prop pitch. The BAR is like tire width on a car.
Effectively it is the calculation of traction in the water verses prop drag.
It is expressed in % of the circle traversed by the rotating prop occupied
by the prop surface. This is estimated by your boat's resistance through the
water at 8 knts. This is where there is uncertainty. I would guess in this
case that a 3 bladed prop with a 70% BAR is as good a guess as any. If you
think that a BAR of 50% or less would do, then a 2 bladed prop would work.
Good luck.
Steve

"Sylvester Sullivan" wrote in message
om...
I have a boat very similar to Glen-l's Eagle 22 ft. semi-displacement
lobster boat. The main motor is a 110hp 2 stroke which gives me all
the power I need. I also have a Yamaha 30hp 2 stroke kicker which I
use for low speed cruising at about 8-10 knots. The kicker is mounted
on a Garelick heavy duty (up to 40hp) auxiliary motor bracket.

The Yamaha is old and tired and gets very disappointing gas mileage.
I would like to switch over to a 4 stroke kicker, which means probably
having to change the mounting bracket. I would like the smallest
motor possible that will give me about 8 knots cruise at 3/4 power.
I'm looking for good fuel mileage and long range. I don't care about
speed and hole shots, and planing isn't an issue.

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

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

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