Well, Paisan's new Universal 35 B is in place and we sea trialed it Friday. The install is excellent. I have taken many photos and will post them soon, for whomever may be interested. My question is this. When full throttle, engine RPM's would not meet spec, however boat was traveling in excess of hull speed. For the Universal 35 B , the yard feels like 2,800 RPMs is the efficient engine
speed. We were getting around 2350. I know we were traveling in excess of the 6.5-6.7 hull speed for CD 33 82 Model. the prop used for the trial is RH 14-10. Am I to understand we are over pitched?
Paisan was actually trying to plane, and consequently making rough
noises under the hull.
Propeller Size
Moderator: Jim Walsh
-
adevans
- Posts: 14
- Joined: Feb 7th, '05, 11:36
- Location: Whisper, Bristol 45.5, Handy Boat Services, Falmouth, Maine
Pitch
Bob,
10" is probably too much pitch. You can calculate the right pitch yourself using Dave Gerr's Propeller Handbook, but that's pretty tough sledding. When I changed to a MaxProp on my CD 36, I struggled through the Gerr methodology, and the boatyard (Zahniser's, Solomons Island, MD) was able to confirm the calculations independently with a computer program they had. Ask your yard if they have such a program. It's worth the candle to get a match between engine output and propeller requirements. On the 36, we went from a 15" pitch to 10". Good luck.
10" is probably too much pitch. You can calculate the right pitch yourself using Dave Gerr's Propeller Handbook, but that's pretty tough sledding. When I changed to a MaxProp on my CD 36, I struggled through the Gerr methodology, and the boatyard (Zahniser's, Solomons Island, MD) was able to confirm the calculations independently with a computer program they had. Ask your yard if they have such a program. It's worth the candle to get a match between engine output and propeller requirements. On the 36, we went from a 15" pitch to 10". Good luck.
Allen Evans
-
fenixrises
prop pitch calc
Hi Bob,
Generally speaking cruise speed under power should equal square root water line. For your boat say 5 knots.
5kts = 5 X 6020(length nautical mile) X 12" = 361,000 inches per hour
Generally speaking the ideal maximum shaft RPM for a displacement hull is 1,000 RPM. The reduction gear should be matched to the engine HP curve. So your engine max recommended speed 2,800 RPM / 1,000 RPM for shaft = 2.8:1 reduction gear. Unfortunately most boats do not have the proper reduction. It is usual to have only a 2:1 reduction.
@ 1,400 RPM for the shaft = 1,400 X 10" pitch = 14,000"/minute minus 50% for slip factor, therefore 7,000"/minute.
7,000" X 60 minutes = 420,000"/ hour
You are trying to go 420,000" - 361,000" = 59,000" too far per hour.
Reducing pitch to 8" would be 1,400 X 8" = 11,200"/minute minus 50% slip factor, therefore 5,600"/minute.
5,600" X 60 minutes = 336,000"/ hour
This should be about right for your engine/reduction gear combination.
Diameter calcs can be found in Skeenes
Happy sails,
Fred B.
Generally speaking cruise speed under power should equal square root water line. For your boat say 5 knots.
5kts = 5 X 6020(length nautical mile) X 12" = 361,000 inches per hour
Generally speaking the ideal maximum shaft RPM for a displacement hull is 1,000 RPM. The reduction gear should be matched to the engine HP curve. So your engine max recommended speed 2,800 RPM / 1,000 RPM for shaft = 2.8:1 reduction gear. Unfortunately most boats do not have the proper reduction. It is usual to have only a 2:1 reduction.
@ 1,400 RPM for the shaft = 1,400 X 10" pitch = 14,000"/minute minus 50% for slip factor, therefore 7,000"/minute.
7,000" X 60 minutes = 420,000"/ hour
You are trying to go 420,000" - 361,000" = 59,000" too far per hour.
Reducing pitch to 8" would be 1,400 X 8" = 11,200"/minute minus 50% slip factor, therefore 5,600"/minute.
5,600" X 60 minutes = 336,000"/ hour
This should be about right for your engine/reduction gear combination.
Diameter calcs can be found in Skeenes
Happy sails,
Fred B.
-
fenixrises
prop pitch calc
Hi Bob,
Generally speaking cruise speed under power should equal square root water line. For your boat say 5 knots.
5kts = 5 X 6020(length nautical mile) X 12" = 361,000 inches per hour
Generally speaking the ideal maximum shaft RPM for a displacement hull is 1,000 RPM. The reduction gear should be matched to the engine HP curve. So your engine max recommended speed 2,800 RPM / 1,000 RPM for shaft = 2.8:1 reduction gear. Unfortunately most boats do not have the proper reduction. It is usual to have only a 2:1 reduction.
@ 1,400 RPM for the shaft = 1,400 X 10" pitch = 14,000"/minute minus 50% for slip factor, therefore 7,000"/minute.
7,000" X 60 minutes = 420,000"/ hour
You are trying to go 420,000" - 361,000" = 59,000" too far per hour.
Reducing pitch to 8" would be 1,400 X 8" = 11,200"/minute minus 50% slip factor, therefore 5,600"/minute.
5,600" X 60 minutes = 336,000"/ hour
This should be about right for your engine/reduction gear combination.
Diameter calcs can be found in Skeenes
Happy sails,
Fred B.
Generally speaking cruise speed under power should equal square root water line. For your boat say 5 knots.
5kts = 5 X 6020(length nautical mile) X 12" = 361,000 inches per hour
Generally speaking the ideal maximum shaft RPM for a displacement hull is 1,000 RPM. The reduction gear should be matched to the engine HP curve. So your engine max recommended speed 2,800 RPM / 1,000 RPM for shaft = 2.8:1 reduction gear. Unfortunately most boats do not have the proper reduction. It is usual to have only a 2:1 reduction.
@ 1,400 RPM for the shaft = 1,400 X 10" pitch = 14,000"/minute minus 50% for slip factor, therefore 7,000"/minute.
7,000" X 60 minutes = 420,000"/ hour
You are trying to go 420,000" - 361,000" = 59,000" too far per hour.
Reducing pitch to 8" would be 1,400 X 8" = 11,200"/minute minus 50% slip factor, therefore 5,600"/minute.
5,600" X 60 minutes = 336,000"/ hour
This should be about right for your engine/reduction gear combination.
Diameter calcs can be found in Skeenes
Happy sails,
Fred B.
-
fenixrises
prop pitch calc
Hi Bob,
Generally speaking cruise speed under power should equal square root water line. For your boat say 5 knots.
5kts = 5 X 6020(length nautical mile) X 12" = 361,000 inches per hour
Generally speaking the ideal maximum shaft RPM for a displacement hull is 1,000 RPM. The reduction gear should be matched to the engine HP curve. So your engine max recommended speed 2,800 RPM / 1,000 RPM for shaft = 2.8:1 reduction gear. Unfortunately most boats do not have the proper reduction. It is usual to have only a 2:1 reduction.
@ 1,400 RPM for the shaft = 1,400 X 10" pitch = 14,000"/minute minus 50% for slip factor, therefore 7,000"/minute.
7,000" X 60 minutes = 420,000"/ hour
You are trying to go 420,000" - 361,000" = 59,000" too far per hour.
Reducing pitch to 8" would be 1,400 X 8" = 11,200"/minute minus 50% slip factor, therefore 5,600"/minute.
5,600" X 60 minutes = 336,000"/ hour
This should be about right for your engine/reduction gear combination.
Diameter calcs can be found in Skeenes
Happy sails,
Fred B.
Generally speaking cruise speed under power should equal square root water line. For your boat say 5 knots.
5kts = 5 X 6020(length nautical mile) X 12" = 361,000 inches per hour
Generally speaking the ideal maximum shaft RPM for a displacement hull is 1,000 RPM. The reduction gear should be matched to the engine HP curve. So your engine max recommended speed 2,800 RPM / 1,000 RPM for shaft = 2.8:1 reduction gear. Unfortunately most boats do not have the proper reduction. It is usual to have only a 2:1 reduction.
@ 1,400 RPM for the shaft = 1,400 X 10" pitch = 14,000"/minute minus 50% for slip factor, therefore 7,000"/minute.
7,000" X 60 minutes = 420,000"/ hour
You are trying to go 420,000" - 361,000" = 59,000" too far per hour.
Reducing pitch to 8" would be 1,400 X 8" = 11,200"/minute minus 50% slip factor, therefore 5,600"/minute.
5,600" X 60 minutes = 336,000"/ hour
This should be about right for your engine/reduction gear combination.
Diameter calcs can be found in Skeenes
Happy sails,
Fred B.
-
fenixrises
prop pitch calc
Hi Bob,
Generally speaking cruise speed under power should equal square root water line. For your boat say 5 knots.
5kts = 5 X 6020(length nautical mile) X 12" = 361,000 inches per hour
Generally speaking the ideal maximum shaft RPM for a displacement hull is 1,000 RPM. The reduction gear should be matched to the engine HP curve. So your engine max recommended speed 2,800 RPM / 1,000 RPM for shaft = 2.8:1 reduction gear. Unfortunately most boats do not have the proper reduction. It is usual to have only a 2:1 reduction.
@ 1,400 RPM for the shaft = 1,400 X 10" pitch = 14,000"/minute minus 50% for slip factor, therefore 7,000"/minute.
7,000" X 60 minutes = 420,000"/ hour
You are trying to go 420,000" - 361,000" = 59,000" too far per hour.
Reducing pitch to 8" would be 1,400 X 8" = 11,200"/minute minus 50% slip factor, therefore 5,600"/minute.
5,600" X 60 minutes = 336,000"/ hour
This should be about right for your engine/reduction gear combination.
Diameter calcs can be found in Skeenes
Happy sails,
Fred B.
Generally speaking cruise speed under power should equal square root water line. For your boat say 5 knots.
5kts = 5 X 6020(length nautical mile) X 12" = 361,000 inches per hour
Generally speaking the ideal maximum shaft RPM for a displacement hull is 1,000 RPM. The reduction gear should be matched to the engine HP curve. So your engine max recommended speed 2,800 RPM / 1,000 RPM for shaft = 2.8:1 reduction gear. Unfortunately most boats do not have the proper reduction. It is usual to have only a 2:1 reduction.
@ 1,400 RPM for the shaft = 1,400 X 10" pitch = 14,000"/minute minus 50% for slip factor, therefore 7,000"/minute.
7,000" X 60 minutes = 420,000"/ hour
You are trying to go 420,000" - 361,000" = 59,000" too far per hour.
Reducing pitch to 8" would be 1,400 X 8" = 11,200"/minute minus 50% slip factor, therefore 5,600"/minute.
5,600" X 60 minutes = 336,000"/ hour
This should be about right for your engine/reduction gear combination.
Diameter calcs can be found in Skeenes
Happy sails,
Fred B.