Strut to Prop Distance

[JReel]

What is generally accepted regarding prop to strut distance on a Planning power boat with the following:
Diesel 315 hp 3800rmp 1.25 dia.Aquatmet 22 shaft 17in prop?

[Landlubber]

40 times the shaft diameter is the maximum between bearings.
Prop shaft diameters
D=3x Square root of 321.000 x P x SF divided by St x N

D= shaft diameter (inches)
P= shaft horsepower
SF= safety factor (should be at least 5)
St= torsional yield strength (in lbs per sq inch)
N= shaft speed ( in rpm)

……………………………………………………

SF+ D3 x St x N divided by 321,000 x P

[JReel]

Thanks for your info. What I am trying to determine is how far away can the front of the prop hub be from the back of the shaft barrel on the strut? thanks.

[Landlubber]

The boss of the prop is no more than the thickness of the shaft, so if the shaft is 2 inches, the prop is no more than two inches away.

[JReel]

I am trying to reconfigure the set up on my boat as it was done incorrectly. I have to get the prop closer to the stern of the boat and the transom mounted rudder. I have about five to six inches to work with. The stainless strut is pretty much vertical. With out repositioning the strut the consideratioin is to rake in back about 2.5 inches and ad on to the barrel. I have a 16 inch prop squeezed under there now and their is some vibration and abrasion. The tip clearance in only about 1/4inch. Ideally a t 17 inch prop would be best. The drop (shaft angle) is 1.25 inches for 6 inches in shaft length.You may need more info but what do you think about this idea.

[steele m.a.]

If , as I understand your problem , prop is so close to hull as to cause
unwanted vibrations and physical damage to hull , you have to be
careful how you apply any modifications.When the shop re-fitted your
new propeller , they attached one with similar diameter / pitch than the
original , OEM propeller.The fact that it's not an identical propeller is not
unusual - aftermarket parts often have propietary differences than OEM.
However , depending on what lengths you plan on pursuing your fix ,
you don't want to stray into dangerous new geometry.The power from
the engine is transferred and kept in balance by that system of hanger
bearings.To change any angles along the way , or distances between
bearings , is to invite the possibility of a catastrophic failure when under
load.A fixed single shaft handing 315 hp is not something you want to
change as far as angle into water , or distance between bearings.
If the prop is too big , have it removed and machined down.

[Jango]

JReel,
I see no reason why you couldn't lenghten the Strut Barrel and use a longer shaft in order to move the Prop back and away from the Hull. Problem is, with a 17 dia prop, you should have at least 2" Hull clearance. With 1.25 drop per 6 in.(shaft angle) will require a total of around 12" of movement. If you stay with the 16" Prop, you still need to move more than 6" to get adequate clearance. Appears like a Deeper (higher) Strut is what is required.
Or, have you considered a 4 or 5 Blade 15" Prop?

[Landlubber]

There should be a minimum of 10% the prop diameter clearence, minimum.......10 to 15% is where they usually are, less than 10% you are going to get vibrations and undue turbulence.
Wasting your time if you only have 1/4 inch mate.

[CDK]

You could make a thick walled sleeve around the prop shaft, that neatly fits between the hub and the strut. The outer diameter should be the same as the hub diameter, the inner diameter just a fraction more than the shaft, so it can rotate freely.
This stiffens the shaft, so there is less vibration and prevents damage from ropes or fishing nets. The drawback is that you cannot use certain types of propeller pullers that grab the back of the hub, so you need the more expensive ones with 3 or 4 arms.

[Çemberci]

a lotus program is for typical placement of appendages
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