High Thrust Prop

[walt]

You can calculate the HP required to pull a dingy based on how hard of a pull it puts on the painter when being towed.
Power is equal to force * velocity and a consistent set of units for this is Power (watts) = Force (newtons)* velocity (m/sec).
edit - fixed equation
So if you were pulling that dingy at 6 knots and the force you felt on the rope was 40 pounds,

1 newton = .2248 pounds
1 meter/sec = 1.9438 knots
1 hp = 754 watts

Power in watts = (40 pounds/.2248) * (6 knots/1.948) = 550.3 watts or .729 hp Ie, if you feel 40 pounds of force holding the rope to pull something at 6 knots, you are using .73 hp to pull that something.

I guessed at 40 pounds of pull if you were holding the rope, what it actually is I dont know. What is clear is that I have a little too much time on my hands if Im spending time on this LOL..

 

[Mikem]

Walt, that is too funny. I'll let you know how my dinghy tow length experiments results and if I decide to switch to a high thrust prop I'll let you know how that works too. Cheers, Mike

 

[jlp0217]

I always thought the question of prop pitch was an easy one. If you get the rated RPM from the engine you are not over propped. If you get rated RPM at less than full throttle, you are under propped. If the boat speed lost while towing was not accompanied by an RPM loss, its still not over propped It may in fact still be under pitched. But if the motor was bought off the shelf, set up for a light fishing boat it may have always been over propped on a sailboat. Get a cheap optical tach, remove the engine cowling and measure RPM at full throttle and compare to specs.

 

[Mikem]

There was no loss in RPM as measured by my ear...sure could feel the drag of the dinghy. The 9.9 is a long shaft so I have to assume it was meant for a sailboat. Thanks for the tip on the optical tach.

 

[Brian S]

I used one of these to determine that my 6hp Nissan doesn't get anywhere near RPM max, either with standard factor prop, or a larger diameter, lower pitch, 3 blade elephant ear "high thrust" prop. http://www.amazon.com/gp/product/B00ACDP6YU?keywords=tachometer&qid=1448377150

I shoulda put it on my 15' planing hull Harpoon just once to see how fast it would go, but I never did, and now I've gone and sold the boat. Kinda makes me wonder if there's some kind of problem that's keeping the motor from running up closer to max... Guess I could try and find someone's jon boat to test it on, or just pony up some boatbux to take it to a mechanic. But where's the fun in that?

 

[walt]

Im also thinking about getting one of those as Im curious where Im at with my 9.8 hp on a Mac 26S. Im definately "over propped" so not getting peak RPM but it in my case, I have more hp than I need so I like it just the way it is. I did try the lower pitch prop and it did achieve higher RPM and higher HP but it just pushed the boat higher up into the hull speed limit. I got about a 4% increase in speed but at an expense of higher vibration, noise and fuel use. Wasnt worth it to me so I went back to the original "over prop" which I like much better.

Brian, what RPM numbers did you measure vs what prop pitch (if you dont mind sharing). For both gas or electric, you have to match the prop pitch for how fast the boat will go at the rated HP. Its somewhat easy to see this as the prop needs to screw faster than the water is moving by it in order to generate thrust. Just looking at this in terms of the prop simply screwing exactly with the water speed and not generating any thrust, for zero thrust, the prop needs to rotate twice as fast at 10 knts compared to 5 knots. Add in thrust (or slip) and the prop must rotate even faster. If the boat just wont go very fast (like out sailboats), the water moving by the prop is too slow to let it get up to the rated HP. You "fix" this by going to a lower pitch prop.

 

[Gunni]

I put the high-thrust 4-blade option prop on my 9.8 hp 4-stroke Tohatsu. It is a short-shaft motor that propels my RIB, so it probably has little similarity to your concerns with a displacement hull. However I notice that that although the boat comes up on plane quicker, it has a lower top speed. It seems to stay on plane easier across a broader throttle range. In short, it is a good slow-speed power option. The boat kicks along nicely at fast idle. My expectation is that a high thrust prop would give you better punch when pushing into rough water.

 

[walt]

Also.. I sort of looked for this but didnt spend much time. Is there a plot of HP vs RPM for these small outboards typically used on sailboats? If there is, you could measure your RPM and look at the graph and see how much HP you are actually generating..

 

[Joe]

.......but it is a close enough model that we can use it in an attempt to have a reasonable starting point to work/plan/discuss from..

Just not a good idea, sir. You're trying to mix apples and oranges. And you can't keep modifying the apple until it turns into an orange. A vessel's theoretical hull speed is very limited in it's purpose and application. Let's just leave it at that.

 

[frankies56]

I would try to bring the dinghy up as far up tp the transom as poss, or even lifting the front out of the water so as just to let the tail drag in the back I had this problem but its just your creating drag. if it conserns you put on the deck or roll it up and stow if you can. good luck

 

[Mikem]

I will cinch it up to the transom and give that a try. The dinghy is way too big for the foredeck and for day sailing way too inconvenient to deflate and inflate. Thanks.

 

[centerline]

I will cinch it up to the transom and give that a try.

yep. cinch it up close... even lift the bow out of the water a bit to get as much of the hull out of the water as possible. you will see a difference, because no matter what we all want to argue amongst ourselves, its the hull drag of the dinghy that is causing you the loss in speed.
if you are cruising across the water towing the dinghy at 6kts, and you let go of the tow line to the dinghy and your boat immediately increases its speed to 7kts without changing its rpm, what else would one suppose is causing the loss of speed?

its because a little bit of extra drag causes a little higher percentage of prop slip, so it doesnt push the boat so fast... a bit more prop speed may compensate for it, or a prop with more bite, but both fixes require a more horsepower/fuel to drive it.
if you reduce the drag, or apply more "push" to the water, you will gain your speed back.

 

[walt]

a little bit of extra drag causes a little higher percentage of prop slip, so it doesnt push the boat so fast.

Take another look at the definition of prop slip. It is simply the ratio of how fast the prop is rotating compared to the flow of the water. If the prop is screwing at the same speed as the water flow, there is zero slip. And of course there is also zero thrust. As the prop screws faster than the water flow, now slip increases. But this is exactly how the prop generates thrust. You must have slip in order to generate thrust. The higher the slip, the higher the thrust.

So what you said above is that the extra drag caused higher thrust (higher percent slip is the same as higher thrust). In order to increase thrust you have to increase HP. I dont think this is the case - ie, that extra drag increases HP.

 

[centerline]

The higher the slip, the higher the thrust.

you may have meant this different, but the way it is read in the context given, its exactly the opposite...
maybe you meant the higher the rpm, the higher the thrust... which is almost always true as long as the prop does not ventilate.


prop pitch, horsepower and rpm being adequate and equal, the faster you turn the prop, the more thrust you will get... OR, the larger/wider the individual blades, the less amount of slip there will be, and in turn, create more thrust.

prop pitch, horsepower and rpm being adequate and equal, a skinny bladed high slip prop, vs a fat bladed low slip prop, the fat prop will create much more thrust than the skinny slipping blades.
prop slip is properly measure as a percentage and is independent of thrust in every way...
the amount of slip that any given prop has is NOT a constant, but it is variable with the rpm changes. and it can have different slip percentages when installed on different shapes of hulls.
like theoretical hull speed, actual prop slip is hard to calculate, but when the boat is tied to the dock and stationary, with the engine running and in gear, the prop slip in now at 100%. at this time the thrust can be easily measured.
with the information gained by measuring thrust at different rpm ranges, one can follow a formula that will lead you to the theoretical slip that the prop could have at any given rpm. its close but not exact because there are so many small things that can change the model, one of which is the shape of the hull.
and you can get the same amount of thrust from a slower turning, low slip prop that you can from a high slip, fast turning prop.

 

[walt]

You can google and find multiple sources of what the definition of slip is, I used this one but all the definitions are the same (quote below) http://www.halepropeller.com/PropellerTerminology.html

Prop slip definition:

The difference between the theoretical distance the propeller should travel in one revolution and the actual distance the vessel travels.

I said "the higher the prop slip, the higher the thrust" and I think this is correct. If the vessel speed is constant, if we increase rpm, the theoretical distance the prop should travel in one revolution INCREASED. This accelerates more water backwards and increased thrust AND by the definition above, this also increased the slip ratio.

I will add a little to what I said - at a constant vessel speed, the higher the slip (ratio), the higher the thrust.

Prop slip is is very related to thrust. Its related to hull efficiency because some level of thrust is required to propel a given hull through the water at some speed.

Hopefully the discussion is based on the actual definition of prop slip.

 

[centerline]

You can google and find multiple sources of what the definition of slip is, I used this one but all the definitions are the same (quote below) http://www.halepropeller.com/PropellerTerminology.html

Prop slip definition:

I said "the higher the prop slip, the higher the thrust" and I think this is correct.

If the vessel speed is constant, if we increase rpm, the theoretical distance the prop should travel in one revolution INCREASED. This accelerates more water backwards and increased thrust AND by the definition above, this also increased the slip ratio.

"If the vessel speed is constant, if we increase rpm"....

here is where we are having the problem.... what you are doing is, increasing the rpm to get the higher thrust, and yes, the slip is also being increased, but it is only a by-product of the action, dependent not on the amount of thrust, but on the size of the blades, the pitch of the blades, the rpm, and the restriction of the forward movement of the vessel. one needs to examine all these things in an attempt to measure the slip that a prop has. "thrust" is only measured as the amount of force the prop developes as it turns at any given rpm, and makes no difference at all how much the prop is slipping.... although it needs to be noted that if you have too high of percentage of slip, it means that the thrust is lower...

and yes, IF the boat hull is restrained in some manner, such as towing an object, the prop slip percentage will increase quicker than the thrust will, when more rpm/power is applied... the prop slip will actually increase even if the rpm remains constant, and if no gain/loss of rpm, the thrust will also remain constant, even when the slip increases.

the vessel speed will naturally want to increase when more rpm is applied, and IF the vessel is allowed to run free and not be restrained when the rpm is increased, percentage of slip will increase, but at a slower rate, while the amount of available thrust will quickly rise and be restrained only by the amount of throttle given

and then, if you were to change to a fat bladed prop with very little slip, you can get just as much or more thrust with even less rpm and less percentage of slip.... it should be clear that slip and thrust are not related, but due to the medium that the propeller operates in (water), you can have/create slip without any thrust, but you can not have thrust without at least some small amount of slip.
slip is largely an undesirable and energy wasting characteristic, but because it is always present in this usage and cant be eliminated and is hard to predictably measure, it has allowed a whole industry to grow from the science of propeller design and theoretical formulas, in an attempt to find a way to use the slip to its best advantage in an attempt to save/create more energy from it.

slip is the wild card in the equation, and without propeller slip, the formula for sizing props to a boat would be as predictable and simple as choosing gear ratios for an automobile... where the wheels dont normally slip.

or think of it this way... how would you know what is the proper gear ratio you would need in a car if you were forever driving around with the wheels spinning and slipping in the mud?... there are a lot of variables that need to be addressed before one could come up with a reasonable choice, and it would not work efficiently for all speeds and rpm ranges that one wants to use... or if one decides to tow a trailer thru the slippery stuff also...

 

[walt]

it should be clear that slip and thrust are not related

Not even remotely for me. One of us doesnt understand prop slip.. Im OK with that, hope you are also since I dont want to get into a long drawn out discussion that no one is reading anymore and possibly finding obnoxious.

 

[walt]

By the way Centerline.. Im not at all saying who is wrong.. could be me. Im just not enjoying arguing, its causing my panties to wad (and probably my BP to rise).... and you have a lot of good input in the thread..

 

[Bill Roosa]

Yea, pretty sure it is more of a wetted surface drag math problem than displacement hull speed one,
and thanks Johann for catching my math error. 36 minutes is a whole bell on sail time, the horror of arriving over a half an hour late on a sailing trip of 6 NM ..... My view is that I get to sail for an extra 1/2 hour.....I'd get up early for that....

As for the dingy towing force that is pretty easy to calculate. Get a grocers spirng scale and make a bowline on a bight in the painter. measure the force directly. adjust the painter lenght and remeasure. Probably easier to (in that 36 minutes) just play with the painter length while holding it in your hands and find the least force position.

 

[Siamese]

If you dont mind my asking, how did you come up with those numbers?
(no one is in the dingy, its 90 pounds total)

Yeah, 90 pounds, but it's not ninety pounds being pulled straight up through the air. If it were, it would compare favorably with a dinghy weighing 180 pounds being pulled straight up in the air.

It's ninety pounds being dragged through the water, and that 90 pounds is spread out over the maximum amount of surface area the designer of the dinghy could achieve and still carry a couple humans. Hardly optimal.

To the OP:
Rig a tow line for your dinghy such that you can very easily adjust it's length from the cockpit while underway. Being able to do it with just one hand would be ideal. Get your vessel up to cruising speed while towing your dinghy and adjust the length of the line so that the dinghy is sliding down the front of the wave that forms at your stern. Then, with just a couple fingers, tug on the line to create some slack and note how much effort it took to pull the dinghy forward. Then, ease out the line until it's on the backside of the wave and secure it. Do the tug test again and note how the effort to pull the dinghy forward has been greatly increased. It will be extremely obvious at that point how important dinghy position is.

Depending on your boat speed, the distance of the wave that forms at the stern of your boat will vary. Hence, the need to be able to easily adjust the length of the line to accommodate your current speed.

You'll still have a loss of speed, but you'll at least be able to minimize it. Sailboats are slow enough without crippling them.