Trojan Boat Forum

Im running 20X22 props on a 36 Sedan, anyone else?

I own a 1972 36' sedan with twin 454's with 20 x 21 props. I have not owned the boat long enough to have a good feeling on performance and or fuel economy

it certainly handles well at low speed.

on my 36 is 20x21 - 3blade michigan props- My mech believes they are too big??????? max rpm I get is 3900 ,,,,,,,should be 42 to 4400............boat handles great at low speeds, cruises great near 2800 rpm.......18-20 mph.....1000 hours twin 454's.......

What transmissions and ratio?

Mine are 22 x 19 four blade props with 2.5 to 1 trannys. I cruise at 3100 rpm and get just about 19 mph. When tested, the engines (both less than 200 hours 454s) got just over 4250 max rpm. It sure is a trip going over 30 mph while up on the bridge!
I was told the four blade props are a little more efficient at slower speeds and a little less efficient at higher speeds. So, I got the 4 blades since I tend to try to poke along and save gas these days.

Tony

Im running with some old nasty tachs, but at 2800 we are cruising along at about 18 knots. 1.81 ratio 4 blades,. Engine builder wants to see more RPM, so we are having a mech put a True Tach on and see exactly what we are turning,

I ran this through my Prop Calculator:... looks like your mechanic is right ...

18,000 lbs, Avg Planing hull
34' Waterline Length
2x300 HP, 1.81:1 straight drives
4-blade, Cupped Props

=________________________________________________________
MAX PERFORMANCE
Available Power: 601.4 SAE hp ( 448.5 kW) @ 4250 RPM
Max Speed: 28.3 kts
Fuel: 36 US Gall/Hr (typical)
CRUISING PERFORMANCE
Cruising Power: 195 SAE hp ( 145.3 kW) @ 2800 RPM
Cruising Speed: 16.1 kts
Fuel: 11 US Gall/Hr (typical)
RECOMMENDED PROPELLER
Diameter: 17.6 in. Pitch: 17.5 in.
Blades: 4 Cupped: Yes
Transmission Ratio Acceptable
PROPELLER BASED ON INPUT DIAMETER
Diameter: 20.0
Pitch: 14.6
Blades: 4
ALTERNATIVE GEAR/PROP COMBINATIONS
Gear Ratio Shaft RPM Dia. Pitch
1 4250 12 10
1.2 3542 13 12
1.4 3036 14 14
1.6 2656 16 15
1.8 2361 17 17
2 2125 18 19
2.2 1932 19 21
2.4 1771 20 23
2.6 1635 21 25
_______________________________________________________
(c) boatdiesel.com

Now THAT is interesting, thanks mucho allthough we are rated at 350 HP, the 300 idea is on a hot summer day. Now talk about weight, this thing is loaded with 300 gallons of fuel, etc. etc.

Stock Merc Crusaders aren't 350 HP w/o mods - not sure what you'e got there ... But even if they are 350HP rated, the age and heat will bring them down to a more realistic 310 or so.

Just for fun - assuming 350HP...............................................
MAX PERFORMANCE
Available Power: 698.4 SAE hp ( 520.8 kW) @ 4250 RPM
Max Speed: 30.5 kts
Fuel: 42 US Gall/Hr (typical)
CRUISING PERFORMANCE
Cruising Power: 226 SAE hp ( 168.8 kW) @ 2800 RPM
Cruising Speed: 17.4 kts
Fuel: 13 US Gall/Hr (typical)
RECOMMENDED PROPELLER
Diameter: 18.1 in. Pitch: 18.7 in.
Blades: 4 Cupped: Yes
Transmission Ratio Acceptable
PROPELLER BASED ON INPUT DIAMETER
Diameter: 20.0
Pitch: 16.5
Blades: 4
ALTERNATIVE GEAR/PROP COMBINATIONS
Gear Ratio Shaft RPM Dia. Pitch
1 4250 12 10
1.2 3542 14 12
1.4 3036 15 14
1.6 2656 16 17
1.8 2361 17 19
2 2125 18 21
2.2 1932 19 23
2.4 1771 20 25
2.6 1635 21 27

like I said on a hot summer day 300 is more like it. I have Crusader xl's low hours. One rebuilt at 100 hours by a Merc Racing shop that I have used for a long time . I'll see if they can remember the dyno numbers.[/img]