Strut and prop placement?

I've been given a lot of mixed info on where to position the strut and prop. I'd really like to do it right so please educate me as to what is correct. I'm using a Hall-Craft nine degree, one inch shaft strut with the second support strut. The shaft center line is at 6.5 inch off horizontal. I need to know first how far away from transom I should place prop. How much shaft should be exposed between prop and strut. My rudder is 6 inches behind transom. I wanted to add a photo but don't know how to.

 

What is the hull type? What is the prop type (i.e. cavitation type, surface, etc.)? Where you position the prop has more to do with the hulls aft lines than anything to do with the strut. The strut should be as close to the prop as possible to prevent damaging whirling.

 

Thank you for your reply! The hull shape and design is a John Hacker 1928 225 hydro Zipper stretched from 18'6" to 25 '. The hull has a single step at 10 foot. The prop needs to be purchased. I currently have a 14 by 19 three blade typical old wood boat prop. Cavitation type I think. This would require a 1.25 inch spacer under the strut and I'm not liking that. The aft lines are close to being just a flat bottom. There might be an inch of rise from keel to chine. I sure wish I new how to post a photo. The shaft is a 1 inch by 10 foot SS high torque alloy.

 

The prop tip should be just inboard the transom. The tip clearance between the prop and the hull should be 0.2 r of the prop. Rudder should be less than 1 r of the prop aft of the prop. Prop strut should be as close to the prop as possible, so with a tapered shaft, less than 1 shaft d between the aft end of the strut and the back of the prop hub.

 

Jehardiman, On behalf of myself and many other forum members, accept my respect and appreciation for all the advice and information that you so generously provide.

We have a few professionals here on the forum who contribute to the general good. To all of you, (you know who you are) Thanks.

 

Regarding photo attachment:

Look for the Camera icon in the tool bar.

Clicking on it will give you the option to “Upload a File”.

There may however be restrictions in place until you have made more posts, or until a certain amount of time passes which has become common on a lot of forums when participants are new.

Your boats sounds like a fun one!

 

Wow! THANK YOU! That is very close to what I had guessed. As soon as I can post a photo, You'll see what I did plan to do.

 

Thanks! I did try to click on the camera and nothing happened. Your most correct about restrictions. I get the words "Gallery im

I managed to post a photo in "Projects in Process" Thanks!

 

I'm going to try to post two photos. Second photo is what I had originally planed using a 14 inch prop. The first photo is showing the changes I'm in the middle of making to use the 12 inch prop. Note things are just cobbled together to get an approximate idea of what they would look like. I just used a dowel to support the props in these photos.

Spent some time today looking at this mess I'm in and found myself unable to even remember how to do basic Trig. The mind just goes blank. If I use the 12 inch prop, I'll need to have 7.2 inches of height on the strut or if I use the 14 inch prop I'll need 8.4 inches. Where I'm running into a problem is with the 9 degree angle of the strut. With the 10 foot long shaft and a need to have an equal height dimension off the internal keel for the transmission flange of close to 7.5 inches or more is OK. I am looking at making a white Oak spacer to be placed under the strut. With the spacer I can correct the strut angle to the lesser angle needed at the same time get the correct heights needed. Does any of this make sense? Another part of the consideration is the hole thru the hull is placed approximately 60 inches from the strut. I just cut a slot hole in the bottom and plan to epoxy in a 1.25" carbon fiber tube spaced around the 1" shaft. Any thoughts or positive critiques are welcome. I just got to get it done and work right of course.
Sunday 7-9-23 Made the white Oak shim for under the strut and found I still needed to add 1/4 " wood spacers to get to 7.2 " height off bottom. Started thinking of using this setup and then adding a Cavitation plate onto the back of the transom. Already have the rudder support bracket sticking out so a SS cavitation plate might look good and provide a more durable surface above the prop.
'

 

Tuesday 7-11-23 After a couple more days of looking and thinking about how to correctly place the strut, I've concluded that I am either going to have to cut and braze a piece into my current Hall-Craft 9 degree strut. With the shaft angle I need and the height off the bottom of the boat, I don't see any other way to build this correctly. Using a 12 inch prop requires 7.2 inches of height or using the 14 inch prop requires 8.4 inches of height. Right now the strut is 6.5. Putting the shaft and strut in place on the hull and then measuring the angles plus calculating them, look like I need an 8 degree strut. I checked out what was for sale and found only one possible strut. It was on eBay and they did not give the critical info I needed. Requested it. Would sure like to hear from someone out there what they have done to solve this issue. Thanks.

 

Originally, these boats had the strut aft of the propeller, which says something about the importance of avoiding disturbance to the inflow to the propeller at high speeds. Any block arrangement for height adjustment will cause problems here.

The correct way would be to use a stut without flange, but with a straight blade coming through the hull bottom and either bolted to a strong transverse carrier, or better; reaching into an inside box where it is fastened by means of an elastic casting compound.

In this way, the shaft is guiding the installation angle and the risk for misalignment is reduced. If it was my boat, I'd have made a welded SS shaft strut to suit. If you have a "soft spot" for polished brass, the current piece could be reshaped by a decent welder.....

 

Thank You for this great reply! I could do exactly what you have outlined. I like it. I've seen this type of strut piece for sale also. I have some 7/16 " wall x 6 x 3 Alum. angle that could be bolted to the keel with the holes I have already made. Two of these pieces on each side of a 1/4 " SS plate welded to the Shaft bearing assy. All this bedded and reinforced internally with some GFlex Epoxy and carbon fiber. I have a bunch of pieces of 12" x 20" carbon fiber to use up. From the hole in the bottom, I have 60 inches of shaft exposed. How long would you make the Shaft bearing assembly? or add a second one. Thanks! Bob

 

Aluminum, bronze, carbon and stainless is a problematic combination. There will be significant electrolytic corrosion between them; with the carbon winning overall.

 

Thank you. I did have a snip-pens of thought about that. Maybe it would be best to purchase two pieces of SS angle. A transverse carrier would that be a fabrication from stringer to stringer with features to bolt the strut too? And would G-Flex be a elastic casting compound?

 

From your pictures this seems to be a boat with a wooden keel and plywood bottom rebated into it and glass covered. What's the internal structure like? Is everything glued on with epoxy? Wood type? Pictures please.

If you use a flangeless strut you need to make a hole in the boat, in the middle of the keel. This hole needs to not weaken the keel, and it needs sealing. The normal way is to double up the keel around the hole using the same wood, plus some structure to bolt the strut onto. There are many options and it depends how the boat is built. You tell us that and we will see from there.

The alternative option is to rebate the flange into the keel on the outside, and use a big wooden doubler on the inside.