drz Posted October 13, 2004 Posted October 13, 2004 Ok, so my friend and I finally got a boat running the other day, old 75 hp 2 stroke mercruiser. Its a quick boat, but man is it nasty. Anyhow, getting out on the water, cruising around in the boat has got me thinking. I'd like to understand more about how boats propel themselves through the water. His is a typical outboard engine, prop, etc. One thing I'd like to know is how to determine how much prop size will effect acceleration, top speed, etc? Basically, I'm wanting to learn alot about boats, as we are now thinking of trying to build a custom boat, I need to learn some stuff. Any info would be appreciated.
Cap'n Refsmmat Posted October 13, 2004 Posted October 13, 2004 The bigger the prop size, the better the thrust (this also depends on prop pitch, etc). But there is a limit. If the prop is too big the engine will not be able to drive it as fast, and thus you lose that bonus of power. Instead it's just extra weight.
drz Posted October 13, 2004 Author Posted October 13, 2004 well, we're working on a boat that has a chevy 350 4barrel, supposedly produces 500hp at the crank. I was also curious if there is a way to calculate the amount of water a specific size prop could move?
Skye Posted October 13, 2004 Posted October 13, 2004 I'd guess a propeller would be a fairly complex situation. They are a long way from a 'standard' aerofoil (think they're called airfoils in the US). Maybe look at some basic aerodynimcs first. If you're really interested though, this goes into alot more depth: http://ocw.mit.edu/OcwWeb/Ocean-Engineering/13-021Marine-HydrodynamicsFall2001/CourseHome/
coquina Posted October 20, 2004 Posted October 20, 2004 and I've messed about with them most of my life. The prop operates on the theory of a screw, so there are a number of things to consider. The overall diameter is one - another is how many blades it has. Most small boat props have 3 blades, some have four. The third consideration is the pitch of the blade. When you think of pitch, compare the prop to a screw. The "pitch" of a screw is the length of one complete revolution. For example - if you have a 1/4-20 screw that means it has diameter of 1/4" and twenty threads to the inch. The pitch is therefore .05". That is a "coarse thread". A "fine" thread of the same diameter is 1/4-28, so the pitch is .0357". So, how much each blade of the prop is bent determines how far one revolution will travel through the water. Less pitch is used for more control and power - for example - tugboats. Whether you have one engine or two also comes into play. A single engine boat has more torque, so it tends to push the boat to one side instead of straight ahead. When you have twin engines, they are counter-rotating - one rotates clockwise, the other counter clockwise. This eliminates the torque. You can also turn the boat on its length by putting one engine ahead and one astern. If you want to learn more about boats, engines, and seamanship, I recommend you contact your local United States Power Squadron or Coast Guard Auxilliary flotilla. Both offer free courses that cover basic boat knowledge, seamanship, and piloting. Boats are a lot of fun, but they can be deadly if you don't know how to handle them properly.
Mad Mardigan Posted October 20, 2004 Posted October 20, 2004 I would like to build a hovercraft, or a hoverwing.
coquina Posted October 20, 2004 Posted October 20, 2004 Lucid Dreamer - thanks ;-) well, we're working on a boat that has a chevy 350 4barrel, supposedly produces 500hp at the crank. I was also curious if there is a way to calculate the amount of water a specific size prop could move? What kind of boat is the chevy engine in? I warn you - when those 4 barrels open up, it will be like pouring gas into the carb with a firehose. With regard to "how much water a specific prop can move", it depends on how much weight it's pushing and how much drag there is. You can determine a theoretical amount by knowing the "pitch" of the prop - you can determine the distance it will travel in one revolution. Again - lots to consider here. One thing is the ratio of water-line length to weight. Consider an oil tanker, when it is loaded it is pushing more weight through the water, so it either goes slower at the same rpm, or the engine rpms must be increased to generate the same amount of speed. Small boats follow the same principle. You will see "trim tabs" on the sterns of some boats - they can add water line length with minor weight, and by adjusting them you can bring the bow down (and the stern up) with the boat hits plane. OK - fuel economy - in a planing hull, the place just after the boat breaks out of the water and goes on plane is the most economical. You can determine the precise RPM by making a graph that shows the relationship of SOG (speed over groun) to RPMs. In this day of GPS it can be done other ways, but I'm going to tell you the old way, because it explains your objective better. In order to do this, you have to remove the effect of current (horizontal movement of water) on the boat. You look for two daymarkers (marks on poles stuck in the bottom) where you can run the boat unrestricted. You don't want the marks to be more than a quarter mile apart if you can help it, and you don't want floating bouys because their position can't be placed accurately due to their movement on their chains. You want to be where the current is either running in the same direction as the boat or against it - not sideways to the direction. Set your RPM on 500 and run between the two marks. Record the time and use it to calculate speed through the water (the current either added to your speed or subtracted from it). Turn right around and run back, and again calculate your speed through the water. Average the two together and you will eliminate the effect of the current, which will give you speed over the bottom. You want to run both directions at the same RPM one immediately following the other, because the current changes over time, going from slack water (no movement), to max flood, followed by another slack water, then max ebb. If you are in a lake that has no current - you can just run each rpm once. Do the same thing for every 500 RPM up to WOT (wide open throttle) Plot the points on a graph. With RPMs on one side and speed over the bottom on the other. You will see that at lower rpms the graph has a gradual slope, but after the boat breaks out of the water, speed increases faster in relation to rpm so the slope is more vertical. There will be a third slope between two points that is an interim. Your objective is to find out the exact point at which the angle of the graph changes and eliminate the interim slope. So, you make additional runs, run at the rpm which is midway between the points, and continue to hone it down until you find the point where you have only 2 lines, which is your most economical engine speed. The boat has just broken out of the water and is riding on top rather than pushing through it. This is where you get the most "bang for the buck". If your boat is kept in the water, it is also critically important to keep barnacles and bottom growth off it - especially the running gear. Use a good grade of bottom paint move the boat frequently. Again, I have just hit the high spots. I used to navigate in contests before the days of GPS. You also need to know how to use a compass and understand the effects of magnetic variation and deviation, and how to plot currrent vectors. GPS will do all that for you, but might find yourself in a hell of a spot if you are out there, the GPS breaks, and you don't know how to navigate for real. It is not difficult, and what's more is fun to learn.
drz Posted October 20, 2004 Author Posted October 20, 2004 Wow, thanks for all the info, I'm still digesting it. As for navigating, we are currently in a lake which I basically grew up in, I know it well. My friend had a 21 ft boat with a ford v8 4barrel, and it stripped out the foot. This boat has a "Alpha1" foot and is supposed to be able to handle the power. And rumour has it that the boat used to be owned by micheal waltrip.
coquina Posted October 20, 2004 Posted October 20, 2004 You're welcome - when I saw North Carolina as your location, I figured you were on one of the sounds or the ocean. I frequent the Chesapeake Bay and its tributaries, which is often besieged by very rough water and strong tidal currents, and am familiar with cruisers and workboats. Is this the kind of boat you have? http://tradoc.monroe.army.mil/casemate/archive/aug04/081304regatta1.htm
drz Posted October 20, 2004 Author Posted October 20, 2004 no, its like a cruiser type boat, inboard (ofcourse). I haven't actually seen it yet, its my friends, I've just been diggin for info. He compared it to his other boat, which is like a standard, probably 8-10 passenger, enclosed front. I've always been a lil scared of the ocean since going on a deep sea fishing trip, but the problems we ran into came from the captain driving right into a storm, supposedly for better fishing. Needless to say, it was a wild ride, very wet, alot of sick people, and I swore we were going to tip a few times. the boat is something simliar to this, but more roomy http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&item=2493979542&category=26432#ebayphotohosting
coquina Posted October 21, 2004 Posted October 21, 2004 This boat is a sister-ship to mine. http://www.woodenboatsnj.com/Images/32ft1973Pacemaker-glass.jpg She has twin 220hp crusaders, and her name is Coquina. When you get caught offshore in a storm, often the best course of action is to head into it. The ride might have been miserable, but it would have been worse if you had been side ways to the waves (known as being in a beam sea). If the waves hit the boat from the side it is far more dangerous, because with each succeeding wave she will roll worse and eventually capsize. A stern sea ain't so hot either, because a wave can rise up from behind and flip her end for end (pitch pole). The bow is pointy 'cause it is designed to cut into the seas. If a captain sees that he is going to be caught in a storm and there's nothing he can do to avoid it, he will head for deep water. The last place you want to be is close to shore where waves are breaking. If you happen to lose power, you will immediately be thrown into the surf.
drz Posted October 21, 2004 Author Posted October 21, 2004 see, thats why I like lakes. No matter what happens, I can always swim, and make it, to shore. That seemed to be the only option, heading right into it. I can remember seeing the storm as we approached it, it was quite a sight, and I suppose much better then the alternatives you mentioned. nice boat btw, and good to know I'm not the only one who chooses his/her nick by the name of their recreational vehicle
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