Thrust measurements

[Fastjeff57]

Dumb question: Anyone know how to calculate the thrust in pounds of force developed by an outboard motor? (Electric OBs are rated in pounds of force, but not in hp.) I'm looking at designing a tilt system and need to know what kind of force it has to resist.

Thanks,

Jeff

[Mark75H]

As I understand it, you want to calculate how much thrust the motor is applying to the water so you can know how much force is being applied to the assembly you want to build.

If you learn how much force you are making at the prop ... you will still have to make a correction at the bracket location ... half way between the prop and the tilt pivot will have twice the force being applied to the transom.

Subtracting "slip" as has been suggested to you on another forum is incorrect. Slip is force that the prop applies but is "consumed" by the boat as friction against the water - its still part of the push against the transom

[david bryan]

other factors need to be addressed i would not expect you would have as much force on the transom on 100 pound boat as you would on 1000 pound boat unless you where tied to the dock. But look a the Mercury kick outs they have done lots of research

[Fastjeff57]

Sam:

Thanks for the come back. Calculating the force on the transom is hte easy part! What I need is the force produced by the prop. I double checked Min Kota's e-motor site for kWs as well as thrust, but no luck.

Gotta be a way to do this!

Jeff

PS to David: There's no difference in thrust on a 100 or a 1000 lb boat if the motor is at full song. One just goes a lot faster.

[MN1]

Does this help?
Mark N
http://sites.mercurymarine.com/portal/page?_pageid=126,48572,126_49291:126_49315&_dad=portal&_schema=PORTAL

[Seagull 170]

As I understood it, slip is the difference between the theoretical speed & the actual speed

i.e. if you have a 12" pitch prop running at 5000 revs, your theoretical speed would be 12" x 5000 x 60 = inches per hour which when turned in miles per hour = 56.8 miles per hour

But the speed you record at 5000 revs in your boat using a 12" pitch prop, on your GPS is 45mph, your slip is 20.7%, which is caused by a combination of hull drag & prop inefficiency.

& I think you will find that if your prop is exerting 100 lbs thrust into the water it will put the same thrust 100 lbs onto the transom. As the motor is not acting as a lever but as a fixed arm, otherwise we would all be using the longest leg motors you could find!

Your answer is only right Sam, if the pivot point is attached to a fixed point, not on the boat.

Perhaps Ron would be the best qualified to pass comment on this!

[david bryan]

Sam:

Thanks for the come back. Calculating the force on the transom is hte easy part! What I need is the force produced by the prop. I double checked Min Kota's e-motor site for kWs as well as thrust, but no luck.

Gotta be a way to do this!

Jeff

PS to David: There's no difference in thrust on a 100 or a 1000 lb boat if the motor is at full song. One just goes a lot faster.
funny transoms are torn out not pushed in

[Fastjeff57]

Mark nailed it!

Prop thrust = 375 x Hp (output)/ boat speed.

Sample calculation: 47 lbs of thrust for a 50 hp motor pushing a boat 40 mph. (Not much force.)

And I was wrong about the force on the transom of a 100 lb vs. a 1,000 lb boat, even at full throttle, for when boat speed goes down, force goes up.

Thanks all,

Jeff

[JohnsonM50]

Mark nailed it!

Prop thrust = 375 x Hp (output)/ boat speed.

Sample calculation: 47 lbs of thrust for a 50 hp motor pushing a boat 40 mph. (Not much force.)

And I was wrong about the force on the transom of a 100 lb vs. a 1,000 lb boat, even at full throttle, for when boat speed goes down, force goes up.

Thanks all,

JeffInteresting Q & A's Ive got ideas but am not so sure as to write then in stone. Consider this tho, It makes sense that it takes more thrust to get going than to hold top speed [ever push a car? lols] The bracket also has to be able to withstand max straight line force + pounding & turning. Even the smoothest boat is just absorbing the hammering well. It will need to withstand side force to turn & when halfway between the propshaft & tilt pivot point there wont be more thrust on the boat but there will be double the squeeze on the bracket. I think theres more question about the question than anyone being wrong.:D

[Fastjeff57]

No wonder I was such a bad engineer! I dropped a decimal place in my calculations: It should be 460 plus pounds of force from a 50 hp motor going 40 mph.

Sorry!

Jeff

[JohnsonM50]

No wonder I was such a bad engineer! I dropped a decimal place in my calculations: It should be 460 plus pounds of force from a 50 hp motor going 40 mph.

Sorry!

JeffIts catching it 1st that counts.:D

[david bryan]

No wonder I was such a bad engineer! I dropped a decimal place in my calculations: It should be 460 plus pounds of force from a 50 hp motor going 40 mph.

Sorry!

Jeff
its amassing to me how guys get screwed up on propeller slip I2 inches of pitch at 5200 rpm = a mile minuet less slip i think that is no way to describe how a prop works please conceder this. the queen marry at long beach ca has about a 18 inch diameter prop shaft and turns 175 rpms a min. the boat goes about 30 miles a hour. now how much pitch do we need to do that

[Fastjeff57]

My calculator doesn't go over 17 inches. Sorry!

Jeff

[david bryan]

My calculator doesn't go over 17 inches. Sorry!

Jeff
Jeff you mist the point you can not get enough pitch to go 30 mph @175 RPM but the queen marry dose luck it up on google

[bandit]

Jeff you mist the point you can not get enough pitch to go 30 mph @175 RPM but the queen marry dose luck it up on google

Last time i seen that boat in long beach it didn't move much, but i'm following.

[JohnsonM50]

Jeff you mist the point you can not get enough pitch to go 30 mph @175 RPM but the queen marry dose luck it up on google

Somethings wrong there Dave it works out to 1.9mph. Variables in the realm of possible would be a small pecentage of 1.9 so not even close. Either the RPMs are the wrong # or theres a ratio being overlooked.. [2700Rs x 12P would be potential speed.] Besides there being a mistake Im missing the point too. :confused:

[david bryan]

what i am trying to prove is that a propeller dose not screw it self through the water but is a pump that produces thrust like a jet and i have broken world records and won national championships to prove it

[JohnsonM50]

what i am trying to prove is that a propeller dose not screw it self through the water but is a pump that produces thrust like a jet and i have broken world records and won national championships to prove it
My limited experience has it that more blades -can- equal more speed per RPM at the same pitch. So does it have multiple blades? Progressive pitch? Just asking how.

[Barry Strawn]

Jeff you mist the point you can not get enough pitch to go 30 mph @175 RPM but the queen marry dose luck it up on google

At 175 rpm it takes a pitch of around 15 to 16 feet to yield 30mph. Pretty normal looking prop other than being about 20 feet in diameter.

[Seagull 170]

Looking at pictures of those props the pitch looks more than 16 foot & the stuff I read said maximum revs 200.
What was the hull speed?

[david bryan]

35 mph

[david bryan]

ps never hear of pitch in 16 foot range diameter yes

[david bryan]

getting back to pitch again i make Small propeller gages and 12 inches of pitch is a 45 degree angle

[Barry Strawn]

So what was your confusion? The Queen Mary used big props. So do all ships.

[david bryan]

two blades are the fastes. Air plain guys say that two blade are the fastes but the rate of clime is better with 3 blades. so on the race corce the 3 blades and in the kilo runs i go for two bades but have done some work with 1 blade props one side of the prop pushes and the other side pulls do you know which is which good luck guys iam done talkig props less try boats next time David Bryan

[david bryan]

So what was your confusion? The Queen Mary used big props. So do all ships.
Barry do do you have any clue as to how much pitch takes to go 35 mph at two hundred rpm per minuet if you get to much pitch you go back wards .diameter dose not move you forward it the pitch

[JohnsonM50]

This has gone pretty far from transome thrust & every engineers, dilemma strong enuff to function - light enuff to float, fly or not tip over :eek::D

[Barry Strawn]

Barry do do you have any clue as to how much pitch takes to go 35 mph at two hundred rpm per minuet if you get to much pitch you go back wards .diameter dose not move you forward it the pitch

Yes, I know exactly what effective pitch is required.

But I'm writing in English so perhaps this is a waste of time.

[Seagull 170]

Check my maths please.

A 12" x 12" prop will have the same tip angle as a 20 foot prop with 20 foot pitch.

A 12" pitch prop will only have a pitch angle of 45 degrees, 2.7" from the centre line of the prop shaft.

[zul8tr]

[QUOTE=Seagull 170;85810]As I understood it, slip is the difference between the theoretical speed & the actual speed

i.e. if you have a 12" pitch prop running at 5000 revs, your theoretical speed would be 12" x 5000 x 60 = inches per hour which when turned in miles per hour = 56.8 miles per hour

But the speed you record at 5000 revs in your boat using a 12" pitch prop, on your GPS is 45mph, your slip is 20.7%, which is caused by a combination of hull drag & prop inefficiency.

& I think you will find that if your prop is exerting 100 lbs thrust into the water it will put the same thrust 100 lbs onto the transom. As the motor is not acting as a lever but as a fixed arm, otherwise we would all be using the longest leg motors you could find!

Your answer is only right Sam, if the pivot point is attached to a fixed point, not on the boat.

--------------------------------------------------------

The prop thrust will not be 100% transferred to the transom because there are other forces on the lower unit when underway. When the motor and boat is on the beach the horizontal load transfered to the transom kick out bracket is just due to the weight of the motor acting at its center of gravity at a distance 'L' from the clamp pivot to the rear of the transom. There is a moment of Engine weight x L that is balanced by Transom force at kick out x vertical distance from kick out bracket to the clamp pivot. From this the horizontal force at the kick out can be easily calculated.

When the boat is underway there are dynamic forces that enter the calc. These are the thrust and its leverage arm to the clamp pivot and opposing the thrust is the drag on the gear case,skeg and the rotating prop airfoil. This drag force will reduce the effect of the thrust force that transferes to the transom kickout bracket. As speed increases toward terminal max on very light fast planing hulls the thrust load gets reduced quite a bit by these lower unit and prop drag forces. This is ONE reason that there are engine tie downs to the transom to prevent engine bounce up :eek:at the transom that will effect performance. Some old timer figured this out way back.:cool:

[Mark75H]

I agree, we don't "use the longest leg motors you could find". But you are not thinking of the lever correctly. The force is applied to the water at the prop, here the force is the output of the motor and does not change relative to the water. Speed comes from the force applied to the water which does not change regardless of the length of the mid section or transom height.

What I say changes is the force applied to the boat - at the contact point closest to the prop. The force is not being applied to the boat at the prop; the force is being applied to the boat closer to the pivot than the prop and is amplified by the difference in length between the prop and where the mid section contacts the boat.

I am not saying there is a lever effect against the water, there is a lever effect against the transom structure (which is the subject of this thread).

This is the reason thrust blocks are used and why motor/transom brackets are not shorter (just an inch or two from the tilt axis total length). If there was no thrust block and a shorter transom bracket, the transom bracket would twist the top of the transom right off the boat because of the distance between where the force is applied to the water (the prop), and where the boat receives it (the thrust block or bottom of midsection/transom bracket).

Think of putting a pipe wrench on the transom of a boat and pushing with a 100 pound weight. If the wrench is supported against the transom half way from the top to the bottom and half the length of the wrench handle you probably can't break the transom. Move the support closer to the wrench jaws and you become more likely to break something off the boat. Move the support more to the bottom of the boat and closer to the far end of the wrench and there is less multiplied force on the boat because the length between the 100 pound force and the boat is less.

[zul8tr]

I believe you may have missed the point of my post? I completly understand the forces envolved, their related moments and free body diagram of the loads as well as the benifit of the design of the clamp bracket used, simple physics. While instructive your analogy of the pipe wrench may help others who need that knowledge. I think without a free body sketch of the forces and moment arms I speak about in my post we will run in opposite circles here. My post was directed to the force experienced at the thrust bracket due to static engine load and dynamic forces of thrust and drag experienced at the gear case and prop while underway and their related moment distances.

[Mark75H]

Your answer is only right Sam, if the pivot point is attached to a fixed point, not on the boat.

You are going to have to re-explain this to me then. To me it seems to say I am wrong in all practical application and only correct in one abstract case.

Is not the pivot point the axis of the tilt bracket? Is the tilt bracket axis not a fixed point on the boat?

A nutcracker is just as much a lever as a seesaw.

[JohnsonM50]

All said & done its a variable set of forces that can go from smooth water steady to violent ruff water hammering. Without tie downs it would hammer all the more every time you let off or turned & then went back on the throttle it would release then smack down good. For the reason of the post you could measure & do the math all day, it sums up to the part[s] to be made need to be strong, real strong & yet not too heavy to defeat its own purpose.. performance. Ive made & run extensively 2 mechanical tilt adj. devises, the 2nd of which worked well with 1 problem. For it to be totally effective.. a variable interactive tie down I didnt make yet... From this minds eye-seat of pants experience I can tell ya.. whatever you make It will take a beating! :eek::rolleyes::D

[OUTBOARDER]

depends on prop, copy something that works and improvr if possible.