So why 10% clearance from prop tip to hull?

[Carl]

I know its what is recommended, I suggest to customers as that is what I have been told for years.


But why is this distance recommended? I had heard that not enough distance will cause wear to hull, antifoul...noise when running, vibrations, cavitation lack of performance etc etc...never an answer I understood.

What I do know...when I changed the tranny ratio, the closest wheel I had to choice from put that distance way under where it should be...and i mean "way under". But I had done so much work at the time and figured dump it in...get a few hours like that, dial in props, then pull boat to fabricate new shaft log, strut or possibly find a smaller diameter wheel.
But the boat ran great, added some cup to the second set and it ran better, hit my numbers. I figured at the end of the season I'd see issues someplace...but paint was in place, hull all nicey nice, wheels didn't show any signs of problems and never got around to changing the strut (ethanol issue for a couple years, then go that taken care of)...figuring I'd repower and have to redo anyway...but now its years I have used boat with very little tip to hull clearance and wonder why is this so important...true...I hit something and the blade is damn close to bottom...but...

So, why 10% clearance? Would performance be better?

I have asked our prop shops...I get the "you need to have 10% clearance minimum", after my " why" I get...because you have to.

Inquiring minds want to know.

Thanks.

[CaptPatrick]

Carl,

My prop guru drilled the same 10% minimum into my mind many years ago. He never gave any other reason than "If the clearance is less, there will be more of a chance of hull damage from the prop wash. I suspect that different types of hull configurations may be more or less subject to potential damage. The B31 with the most common shaft angle is 15º and the 24º deep V means that a very small area of the hull is affected, less than would be with a flatter shaft angle or lower dead rise. But suspicions aren't facts so who knows? Maybe the best answer is that some slip stick marine engineer from years gone by decided that 10% was a good safety factor and the industry just bought into it...

[Rawleigh]

Cal: You are a very good guinea pig for us! Maybe it is an old time myth that is left over from the wooden boat days??

[Yannis]

I'll follow "guinea pig's" requisition.

[bob lico]

i run a 21" prop so 10% would be 2.1 inches.i have about 1" and the ablative bottom paint is not even worn off at end of season above the prop..looks like they picked this number out of the air,how ever never exceed 1" from back of prop. hub to back of struct on high performance diesels on 31 Bertram.

[Tony Meola]

Do and online search and you will find 15% recommended.

If less works and you have no problem go at it. I run 20X23 props no problem on 270 Cummins. UV with the 250 Cummins was running 21 inch props. I bet our perfomance was almost identical. If I moved up to a 21 or even a 22 inch blade, I would have to reduce the pitch and get the same numbers for RPM, the big question becomes, what happens to speed and fuel usage.

[scot]

. If I moved up to a 21 or even a 22 inch blade, I would have to reduce the pitch and get the same numbers for RPM, the big question becomes, what happens to speed and fuel usage

Yes, you would have to reduce pitch. As a general rule, cruise goes up and top end goes down. The loss of pitch is made (over come) up with the increased efficiency, to a point. The tricky part is what is too much diameter? There is a point of dimensioning return. That's where a good prop guy comes in handy and trial and error.

If speed goes up at cruise (1-3kts), for a given rpm, you have increased your fuel economy. Which is typically the case. Only downside is top end, which commonly drops by the same amount you increased your cruise (1-3kts). But how often do you run the boat at WOT and how often do you run the boat at cruise?

Used EBAY bronze wheels come in handy for this. Don't go out and buy a new set of Nibrals if your not sure what the results will be. Bonus; if the bronze are good, the Nibrals will be better, as the flukes are thinner = even more efficient. These are common sizes, typically lots of good stuff on Boatdiesel and in Boats and Harbor.

[bob lico]

scot you know your prop. theories and i could not agree more-------wot is total nonsense on a diesel 31 Bertram .use it for two minutes as a guide to where you want to be in rpm range ,no matter what diesel must turn up to rated rpm plus 75 to 100 rpm!!!!!!!!!!!!

[Carl]

Funny how we all know the clearance rule and no absolute reason behind it...at least not yet.


Bob- I'll disagree with the never more then an inch from the prop hub to strut...that only works for our size shafts. Rule of thumb is to keep overhang between at 1 and 1.5 shaft diameters. Too far you have to be concerned about fatiguing shaft and too close you have the issue of reducing water flow thru Cutless Bearing. Then again I have seen people hang a descent size wheel 3-4" past strut on 1" shafts...I made mention, they said its good...I agreed and went back to minding my own business.

[bob lico]

carl i did narrow that info. down to a high performance diesel in a 31 Bertram that would indicates a 1 1/2" shaft unless you are the one and only one 31 that has 420hp cats.all other would have 300,315,355,cummins and 315 yanmars.

[Tony Meola]

Scott

After the re-power, I was running at 22 knots at 2200 RPMS and 24 knots at 2400 RPMS. Last year I had to have the props reset and what ever they did, I now have 23 knots at 2200 RPM's and just shy of 25 knots at 2400. Top end at 30 knots. Same top end as before.

Same prop guy. I suspect he tweeked the pitch.

[scot]

I would guess they added a little cup. A lot of prop shops will roll the trailing edge of the fluke (add cup) just a bit to increase the bit. Not enough to decrease your rpms, just enough to give you knot or so. I would ask them if they added some cup, and if so, how much? Good info to have for future re-do. Or, it could be that your props WERE "less than" and they just corrected imperfections.

Adding some cup is typically most noticeable on the bottom end (outta the hole) and cruise. Either way, you done good :-)

[Rawleigh]

Rule of thumb is to keep overhang between at 1 and 1.5 shaft diameters. Too far you have to be concerned about fatiguing shaft and too close you have the issue of reducing water flow thru Cutless Bearing. Then again I have seen people hang a descent size wheel 3-4" past strut on 1" shafts...I made mention, they said its good...I agreed and went back to minding my own business.

Sort of reminds you of an engine lathe doesn't it Carl!!

[Stephan]

Rule of thumb is to keep overhang between at 1 and 1.5 shaft diameters. Too far you have to be concerned about fatiguing shaft and too close you have the issue of reducing water flow thru Cutless Bearing. Then again I have seen people hang a descent size wheel 3-4" past strut on 1" shafts...I made mention, they said its good...I agreed and went back to minding my own business.

Unless you are running Spurs http://www.spursmarine.com/ in which case the spurs will crowd the exit of the cutlass bearing and require an over-extension of the shaft.... oh well.
Best and thanks for this discussion,
S

[bob lico]

however if due to wind,tidal conditions you come in hard and reverse engine 3' from dock and have 0 slip driveline due to Vucan drives with ZF gears and 783 pounds of torque per motor in reverse then slap forward you can rest assure you are not the least bit concerned about water flow thru your structs! you need less then a 1/2 teaspoon at any given moment.

[JimmyG]

I just tested my new props and all is good, heading to Cape May Saturday after 20x23,20 x 24 20x25 finally I reached the correct numbers with a Acme 4 blade 20x26 #8 cupACME 20 X 26 #8 CUP 4/26/2015. Fuel tank of fuel,water tools fishing gear and 2 jumbo size men LOL. I have been thinking of trying 21 diameter props for the spares to see the difference
RPM LOAD THROTTLE FUEL SPEED
600 1-ENGINE 4.6
600 2-ENGINE 5.7

1700 50 45 6.1 19.1
1800 48 53 6.8 21.0
1900 49 53 7.3 22.5
2000 50 57 8.5 24.4
2100 54 61 9.3 25.5
2200 59 65 10.4 27.3
2300 62 69 11.1 28.3
2400 68 72 12.3 29.5
2500 74 77 13.2 30.7
2600 76 81 13.8 32.4
2700 78 85 14.8 32.8
2800 88 90 16.5 33.9
2900 90 94 17.5 35.4
3000 95 98 18.8 36.8
3050 98 100 20.4 37.8

[Carl]

You should be real happy with those numbers.

I love it, after you get exactly what you have been looking for...change it up and start again with new diameter.

[Carl]

however if due to wind,tidal conditions you come in hard and reverse engine 3' from dock and have 0 slip driveline due to Vucan drives with ZF gears and 783 pounds of torque per motor in reverse then slap forward you can rest assure you are not the least bit concerned about water flow thru your structs! you need less then a 1/2 teaspoon at any given moment.

True, when going slow I would think flow is of little issue.

What about at speed?