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Two questions on the prop clearance for 19' barrelback. I'm using a 12" dia. prop and the deluxe speed strut. The strut has a 6 3/4" drop which gives a little more than 3/4" clearance from the prop to the bottom of the boat. The motor installation book recommends minimums of 2" or 10% of the prop diameter for clearance. Should I put a spacer under the strut for more clearance? Also, the plans show 4" of space from the back end of the prop shaft to the rudder in order to remove the prop w/out dropping the rudder. I haven't purchased the prop and shaft yet (they've got this recession on) so can anyone tell me what the distance needs to be from the back of the strut tube?

Dan

wouldn't there be a little more clearance at the prop tips as the angle of the shaft and prop and length of shaft past the strut would bring the prop tips down some?

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From the plans it looks like the most clearance there could be is 1 inch.

Dan

One answer and 1 question:

What is the pitch of your strut?

The technical answer for the clearance between the prop shaft end and rudder is 4.6" which is also equivalent to the amount of shaft you want to protrude past the strut. That is room for the typical prop, nut and clearance between the strut and prop collar. That is the technical answer I recieved when I asked various shops.

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Dave

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Thanks Dave-the strut I'm using is 16 degrees.

Its been my experience that the distance between the beginning of the taper from the backend of the strut towards the threaded end should not be any further than the equivilant of the shaft size.

and the closer the better. You only need enough space behind the prop to be able to get a prop puller in there.

Mike - that would be correct. Nut, a little over 1/2 in, the shaft collar 3 in and clearance 1 in. Using a 1 in shaft. So 4.6" total

Nova - And correct as well, all the work I read is that it is best to move the engine as aft as possible. So the closer to the the rudder the better. I know the specs call for the spacing to be roughly equal to the amount of shaft protruding past the strut but I see a point to be made that a puller does not take up space. It is used only to break the prop free from the lock on the taper on the shaft and then can be removed. The prop will come off by itself after that. Therefore, in theory, one only needs the length of the prop collar . The question would be then to ask what is the overhang from the blades past the collar, if any. Since props could be changed over time with different measurements, I believe the generality of spacing = shaft lenght is probably the safest approach. But who am I to say, this is just what I am told from those that know.

Martin - if you are using a speed strut with a 6 3/4 drop and a 12 in prop your clearance with the hull is only going to be about 1" as UPS states. I believe the specs on your plans call for a 7 1/2 drop using a 13" prop which provides over 2" of clearance. This is the same as my Riviera. Each prop has its performance characteristics as Kens has shown in his discussions. However, all of the them will cavitate if placed too close to the hull. I would suggest that you ask the manufacturer what the clearance specs are for your specific prop. I am wondering why you are choosing the speed strut vs the speced 7 1/2 strut. That is only a 3/4 difference is height from the boat but gives a lot of prop clearance leeway. Just a note also to make. I went to my trusty calculator. That reduction of 3/4in will move your engine aft 2.6 inches.

Ok, now for a thought if you already have the speed strut and staying with it. I have seen some of the modern ski boats (Mastercraft and Correctcraft) actually "hollow or flatten" the keel out above the prop to give the necessary clearance. It had the effect of moving the prop up into the keel for a closer profile. I will let the engineers debate that move.

As Oyster has stated so often, these talks not only help us but tend to gather more information for those builders coming after to help them with their decisions as well.

Good luck and pics please

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Dave

My Riviera build - the Midnight Cry Project

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Few things in the world measure up to the thrill and satisfaction of boating in a boat that you built.

Dave= the plans definately show a strut with 6 3/4 inch drop. however I 'm sure I can exchange it for a 7 1/2 inch strut.

Thanks

Dan

Dan - Before you go to that extent. Check the manufacturer of your prop and see what clearance is needed to prevent cavitation. If that is what your plans spec out then that is the best way to go. Your hull and center balance is taking into account this size strut already. If you go with the larger strut you will move your engine forward of the intended center balance of the boat which is generally not recommended. Just match the prop accordingly

dave

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Dave

My Riviera build - the Midnight Cry Project

Video of Midnight in Action
Blog

Few things in the world measure up to the thrill and satisfaction of boating in a boat that you built.

When I was doing all of this research - I spoke with a prop shop guy at a show and he set me straight.

I was having difficult figuring out where everything went in relationship to each other..
First - I recommend the Hankinson/Glen-L inboard installation book and draw your boat/motor/shaft/strut/rudder cross section full size as suggested.

Step 1 - The rudder must be *under* the boat.. the trailing edge should not stick out or you can loose control due to the air bubbles mixing in just past the transom.

Step 2 - There is a recommended distance between rudder and prop bases on the size of the prop - of course I can't find the charts that I used..

Step 3 - There is a recommended distance between the top of the prop and the hull. If it is too close - there will be cavitation because not enough water can get up in that area... The hull can also wear due to water movement (prop shop guy told me this). This distance is also a ratio based on prop diameter and is on a chart someplace..

Now that you know where the rudder and center of the prop should be on your full sized drawing - you draw a straight line to the output shaft on the transmission. Your strut needs to fit on the line.

I made 1/8" plywood cutouts of all of this stuff to put on the picture.

For me it worked perfectly - I drilled the shaft hole perfectly straight - on line and very tight to the propshaft.
Hope this helps

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Bob Perkins

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I realize that I am late to the discussion here...

Martin... have you done a full-size drawing of your strut/shaft/prop layout?

I found that very helpful to get a sense of how all the different angles play together. Because the shaft and prop are both on an angle, a 12" prop will be more than 6 -3/4" away

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On a lot of flat bottom Vdrive boats they install a "Blast Plate" to protect the hull from this sort of wear. They say it happens fairly often if you dont have the plate. I'm planning on installing one on my Tornado.

You can see the Blast Plate in the picture below. It is just to the rear of the strut. Ignore the red circle around the skid fin

There are some great examples and explanations for sure here. While each setup is so different with a diverse group of engines and transmission and folks measuring tapes, the key is to get the running gear angles and minimum expected clearances correct. Then you can always have the shaft guy cut the shaft to meet the minimum and maximum distances behind the strut end on the stern portion of it.

When I drilled - I was fortunate enough to borrow a shaft that had been modified for drilling.

When it came time to install the *actual* shaft - I had already flipped the hull and installed the motor. I had already picked up a shaft that had not been cut/keyed. I used it to get an exact measurement and marked it for cutting/keying for the prop shop.

Another known quantity in the layout process - the shaft should be exposed from the strut to the beginning of the taper by the same amount as the diameter of the shaft.

so - I used a 1" shaft.. There is 1" of shaft exposed from the strut before the taper begins. The prop shop had all of these *industry standard* formulas available when I was figuring all this out.

The most important part was the full size (and very accurate) drawing I made of the hull cross section so that I got the layout correct the first time.

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Regards,
Bob Perkins

My Current Projects
http://h12restoration.shutterfly.com/
http://nutshellpramconstruction.shutterfly.com/

My Completed project
http://biscayne22.shutterfly.com