Thrust measurements

[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

[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.

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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 at the transom that will effect performance. Some old timer figured this out way back.