What causes the boat to squat under power? My 1986 Catalina 27 with the M-18 does nicely under power. It pushes the boat with little resistance up to about 5.5 knots {hull speed is 6.3} after 5.5 knots it takes a lot more power to reach hull speed. The boat seems to want to squat more than it does go forward and the exhaust port submerges. What force makes this happen? Would changing to different type of prop help? I know I am only talking about a single knot of speed but a 15% improvement is a 15% improvement. Thanks in advance, Jack
- Status
Hull speed and drag
Powerboats make use of that force to sit up on top of the water and reduce drag. Sailboats with motors however don't have planing hulls. The hull speed is that point where drag, specifically that associated with wave making, increases by large amounts. So as you near hull speed and the bow is pushed up in the process of displacing water, instead of riding atop the water you experience what you call squat, as well as the noticably smaller increase in speed for a given increase in power. The bottom line is that at and above hull speed, you need much more power from whatever source in order to increase speed.
Powerboats make use of that force to sit up on top of the water and reduce drag. Sailboats with motors however don't have planing hulls. The hull speed is that point where drag, specifically that associated with wave making, increases by large amounts. So as you near hull speed and the bow is pushed up in the process of displacing water, instead of riding atop the water you experience what you call squat, as well as the noticably smaller increase in speed for a given increase in power. The bottom line is that at and above hull speed, you need much more power from whatever source in order to increase speed.
Drrr, sometimes the answer is right there.....
Thanks for the answer, Your saying the stern isn't squating as much as the bow is trying to climb. Correct? I guess I could have figured that out with a little thought. If I rearrange weight to the bow would that help a bit? I guess it couldn't hurt. Thanks again, Jack
Thanks for the answer, Your saying the stern isn't squating as much as the bow is trying to climb. Correct? I guess I could have figured that out with a little thought. If I rearrange weight to the bow would that help a bit? I guess it couldn't hurt. Thanks again, Jack
It all has to do with the wave
making characteristics of a displacement hull. To completely explain it would take a book but Phil's post pretty well covered the answer. You will notice the same thing under sail, you can easily reach 5.5 kts under sail but to get 6.3 you need a lot more wind.
making characteristics of a displacement hull. To completely explain it would take a book but Phil's post pretty well covered the answer. You will notice the same thing under sail, you can easily reach 5.5 kts under sail but to get 6.3 you need a lot more wind.
Jack, I've notice the same thing ...
I also wondered why the exhaust port sinks below the waterline when I am pushing for that last 15% in speed. One time, I put my wife at the helm and I climbed as far forward on the bow and sure enough the exhaust port did come out of the water (barely). I have been promising myself that I would exercise more and eat less! :{
I also wondered why the exhaust port sinks below the waterline when I am pushing for that last 15% in speed. One time, I put my wife at the helm and I climbed as far forward on the bow and sure enough the exhaust port did come out of the water (barely). I have been promising myself that I would exercise more and eat less! :{
Its the formation of the bow and stern wave .....
'Hull speed' is actually the speed of the waves (bow and stern) that a boat makes ... and if the boat is faster than especially the stern wave, the stern will begin to 'squat' into the trough. As you go beyond 'hull speed', the stern wave starts to move aft of the boat and starts to no longer support the stern ... and the aft section of the hull begins to sink into the trough in front of the stern wave. At the same time the bow begins (or rather attempts) to raise up over the bow wave and the bow starts to lift up ... and by 'balance' about the center of mass the stern sinks in a little further. ;-)
'Hull speed' is actually the speed of the waves (bow and stern) that a boat makes ... and if the boat is faster than especially the stern wave, the stern will begin to 'squat' into the trough. As you go beyond 'hull speed', the stern wave starts to move aft of the boat and starts to no longer support the stern ... and the aft section of the hull begins to sink into the trough in front of the stern wave. At the same time the bow begins (or rather attempts) to raise up over the bow wave and the bow starts to lift up ... and by 'balance' about the center of mass the stern sinks in a little further. ;-)
All displacement hulls squat
You may remember, maybe it was 10? years ago that the QE2? bumped aground somewhere near Nantucket? Some of the blame was put on the fact that even ocean liners "squat" and effectively increase their stern draft when they are moving quickly. As Phil said you will burn a lot more fuel getting that last knot. Changing your prop may or may not mean anything, the question is how fast your engine is turning and whether it is at the right speed when you are putting full load on it. And whether you want to optimize it for "full load full speed" or more for cruising speed. David Gerr has a whole "propeller handbook" written on the subject if you really want to find out more. Powerboats often use trim tabs to change their fore/aft trim to deal with some of this problem, but with a sailboat you just live with it. If you trim "bow down" you'll pull the stern up...but you may hurt other handling. Or, you may simply find too many decades worth of "stuff" and a wet hull have raised the waterline.<G> "{hull speed is 6.3}" Don't pay much attention to that, speed ratings are only approximations under best circumstances and unless you are bolting on a jet engine, on perfectly flat water, with a perfectly clean bottom...they are only a comparison point.
You may remember, maybe it was 10? years ago that the QE2? bumped aground somewhere near Nantucket? Some of the blame was put on the fact that even ocean liners "squat" and effectively increase their stern draft when they are moving quickly. As Phil said you will burn a lot more fuel getting that last knot. Changing your prop may or may not mean anything, the question is how fast your engine is turning and whether it is at the right speed when you are putting full load on it. And whether you want to optimize it for "full load full speed" or more for cruising speed. David Gerr has a whole "propeller handbook" written on the subject if you really want to find out more. Powerboats often use trim tabs to change their fore/aft trim to deal with some of this problem, but with a sailboat you just live with it. If you trim "bow down" you'll pull the stern up...but you may hurt other handling. Or, you may simply find too many decades worth of "stuff" and a wet hull have raised the waterline.<G> "{hull speed is 6.3}" Don't pay much attention to that, speed ratings are only approximations under best circumstances and unless you are bolting on a jet engine, on perfectly flat water, with a perfectly clean bottom...they are only a comparison point.
You might get to your hull speed easier
I would try and heel the boat some, with a little extra heel on a Catalina 27 you will get maybe another foot longer water line (from what I remember a C27 doesn't have a plumb bow or stern and has a nice overhang going). Like mentioned below the added waterline will create a longer standing bow wave and allow the boat to reach a faster speed. That quarter wave created that you see off your stern will move a little further back. Here is a great description on what happens http://www.sailnet.com/collections/learningtosail/index.cfm?articleid=colgat006 Now how would you get more heel while motoring you might ask. I do it by putting out the jib or main and pulling it in tight while motoring. Unless the wind is right on the nose it should create at least a little heel and maybe increase your speed 1/2 kt or so while motoring (not to mention the potential added lift from the sails) Its well known many boats sail much faster than motor. My C36 might hit ~7.2-7.3 kts under full power, but I have had the boat much faster (8+kts) with the winds just right and strong (20+kts)
I would try and heel the boat some, with a little extra heel on a Catalina 27 you will get maybe another foot longer water line (from what I remember a C27 doesn't have a plumb bow or stern and has a nice overhang going). Like mentioned below the added waterline will create a longer standing bow wave and allow the boat to reach a faster speed. That quarter wave created that you see off your stern will move a little further back. Here is a great description on what happens http://www.sailnet.com/collections/learningtosail/index.cfm?articleid=colgat006 Now how would you get more heel while motoring you might ask. I do it by putting out the jib or main and pulling it in tight while motoring. Unless the wind is right on the nose it should create at least a little heel and maybe increase your speed 1/2 kt or so while motoring (not to mention the potential added lift from the sails) Its well known many boats sail much faster than motor. My C36 might hit ~7.2-7.3 kts under full power, but I have had the boat much faster (8+kts) with the winds just right and strong (20+kts)
The actual theroy is
that longer wavelength waves move faster. So a boat that is longer and makes a wave will fit nicely into that longer and faster moving wave. What Jack is experencing is the boat trying to climb out of the "hull speed" wave the boat has made. The bow goes up and the stern goes down or squats. Jack has a 5.5 knot hull speed boat assuming his knotmeter is accurate. Just because the book says 6.3 don't mean it is true. He can get to 6.3 under sail because he has the boat heeled over and the length of the boat in the water is longer.
that longer wavelength waves move faster. So a boat that is longer and makes a wave will fit nicely into that longer and faster moving wave. What Jack is experencing is the boat trying to climb out of the "hull speed" wave the boat has made. The bow goes up and the stern goes down or squats. Jack has a 5.5 knot hull speed boat assuming his knotmeter is accurate. Just because the book says 6.3 don't mean it is true. He can get to 6.3 under sail because he has the boat heeled over and the length of the boat in the water is longer.
- Status