Repower: Replacing Prop Strut with Skeg/Flanged Bearing on Ericson 29?

I am repowering my Ericon 29 from an atomic 4 to Yanmar 2GM20F Diesel, and will increase the shaft size to 1" from 3/4". Since I will have to do boat surgery to do this, I am seriously considering building a prop shaft skeg (is that the right term?) and doing away with the strut altogether. I've done structural glass work, and understand how robust this structure would need to be, and how it would integrate into the hull, and am not shy when it comes to projects such as these. My plan would be to carve a block of wood into the skeg shape I want, and build a mold off of that, then make a thin layup up of the part from that. Once it is cut to exact dimensions to fit the hull with the appropriate tip clearances, I'd lay up the thickness to at least 1/2 inch while glassing into the hull (bevelled 7:1 inside and outside), and then finishing it with a couple of mini-bulkheads over the skeg internally.

I am compelled to do it this way because most of the struts I can find for that boat don't offer the 7" drop necessary to accommodate the recommended 13x9 propeller with appropriate tip clearances. Plus, they have become so expensive, and are inherently at risk for bending. Also, I will want to change the shaft angle so the Yanmar can sit lower on the rails, preserving the double aft berth - otherwise the engine cover would need to rise about 4" to accommodate the height of the diesel with the gear reduction - since the A4 is shorter with the direct drive. Many boat designs with the same general profile as the E29 use the skeg and flanged bearing, and I am asking whether this is a common modification for strut alternatives during refit, and how it is best accomplished? Thank you in advance.

 

Build a composite strut, it's the simplest way. Make it out of carbon fiber so it's stiff, then cut a slot into the hull and brace the strut on the inside with knees.
Commercial product for reference: Carbon Fiber P-Bracket https://www.reggianinautica.com/en/?id=products&category=378&product=1699

 

Here is a side profile drawing of the Ericson 29 for reference, copied from https://sailboatdata.com/sailboat/ericson-29/



Can you perhaps post a photo or two showing the existing arrangement with the strut / P bracket?
Re your new prop shaft skeg, have you considered using a length of thick wall fibreglass tube instead?
This tube could extend from the location of the shaft seal at the forward end (the shaft seal would clamp on to the tube) through the hull as far aft as the existing strut.
You could set up everything in place, with the engine on it's beds and connected to the shaft going through the thick wall tube (it will need a cutless bearing or similar in the tube for support) before you glass the tube in to the hull, to ensure that you have the correct alignment.
The shallow vee between the thick wall tube and the underside of the hull could be filled in by a continuous fibreglass strut.

Edit - McMaster Carr have thick wall fibreglass tube - you could maybe use a 2" diameter tube with a 1/4" wall thickness (?) - McMaster-Carr https://www.mcmaster.com/products/pipe/structural-frp-fiberglass-round-tubes/?s=fiberglass+pipe
Here is more detail re the tube :
McMaster-Carr https://www.mcmaster.com/8535K73/

And a 1.5" OD x 1" ID cutless bearing for the new shaft -
https://defender.com/assets/pdf/morse-rubber/morse_bearing.pdf
It is possible to get composite bearings rather than brass, but Defender don't seem to have them - other suppliers will though.

 

@bajansailor I like your suggestion - a lot! Here is a photo of existing arrangement. The thickest G10 tube I can find with a sufficient I.D. (1.25" min, preferably 1.375) is from McMaster and is .125 wall thickness. That can of course be increased with additional layup. Given the forces at the cutlass bearing, the question is how thick the layup really needs to be in order to withstand the forces of the prop, and also raises the following questions: 1) how to attach the cutlass bearing (is it as simple as using a set screw? or bedding in 3M5200?) and 2) should vent openings be cut into the area fore of the cutlass bearing to enable water flow into the tube between the bearing and seal, similar to the bronze flanged bearing (link here: Stern Bearing Housing Oval 00STBO100 https://www.generalpropeller.com/marine-hardware/buck-algonquin-hardware/drivetrain/stern-bearing-housings-oval-flange/stern-bearing-housing-oval-00stbo100)?

 

I think I would go for a 1/4" wall thickness if possible, rather than 1/8" for the fibreglass tube.

Cutless bearings are usually just held with set screws in bronze shaft logs, so it should be possible to do the same with a fibreglass tube.
If you put it in with 5200, you will have difficulty if you ever want to change it.

Re vents, if you use a shaft seal like this one, then you can use a water feed on it (although they say this is only necessary for fast boats), or use the water feed as a vent to the air, high above the waterline.
PSS (Packless Sealing System) Dripless Shaft Seal | Defender https://defender.com/en_us/pss-packless-sealing-system-dripless-shaft-seal

Edit: re the photo that you posted, has the shaft been partially withdrawn? If it has not, then your shaft is much too long for the position of the P bracket. The front of the propeller boss should ideally be no more than 1.5 x the shaft diameter behind the P bracket.

 

I am going for a hybrid approach: I ordered 3"OD x 2"ID structural fiberglass tube from McMaster (5' section, which I'll also use for my Cabo Rico 34 scupper plan which I posted in another thread here), and a 2"OD x 1.375ID bronze pipe which I'll have threaded and attached to the flange I mentioned. Hopefully, the pipe will fit inside the tube and can be bedded with 5200. This was how the Cabo Rico prop housing was set up - and also came apart relatively painlessly since the 5200 was a fine film and not a thick bedding. If the pipe doesn't fit inside the tube, I'll have the machine shop doing the pipe threading also bore the tube so the pipe will fit. I also ordered the Lasdrop shaft seal for the 1" shaft over a 2" tube, and will dry fit it all together for alignment. If I am satisfied with the arrangement, I'll glass the tube to the hull as @bajansailor suggested with >10% prop clearance, and go from there.

I like this setup because 1) the combination of glassing the structural tube to the hull along with the rigid bronze pipe running inside the entire length (and then some for the seal) seems to offer some significant structural benefits, and 2) it was surprisingly inexpensive - $200 flange plus $100 pipe plus $200 tube which I'll use mostly in the Cabo Rico. Machine shop estimates 30 minutes for the threading - Maybe 1Hr total including boring.

Thank you for your feedback - especially @bajansailor! Much appreciated

 

I am a bit confused here (this is easily done, don't worry).
Where is the stern bearing housing supplied by General Propeller going to be fitted?
I get the impression that this would usually be fitted on to the 'deadwood' at the aft end of the keel, but you won't have anything like this with your arrangement (?)
Will it be 'butted' on to the fibreglass tube?
If so, then the ears or flanges (dimension 'D' in the link - Stern Bearing Housing Oval 00STBO100 https://www.generalpropeller.com/marine-hardware/buck-algonquin-hardware/drivetrain/stern-bearing-housings-oval-flange/stern-bearing-housing-oval-00stbo100 ) will be 'standing proud' (?)

And I am wondering if you really need the bronze pipe running inside the fibreglass tube?
Many moons ago I was working in a little boat yard here where we were building 27' and 37' GRP fishing boats with John Deere diesel engines - we used thick wall GRP tube glassed into the keel.
The tube projected slightly from the deadwood so that it could be laminated to the deadwood.
There was a cutless bearing in the aft end of the tube, and a half length cutless bearing at the forward end of the tube, and that was it.
The tube was finally glassed in place once everything had been set up and the shaft coupled to the engine, with everything in alignment.
We fitted PYI shaft seals that clamped on to the tube, with water feed to the shaft from the engine cooling water intake.

 

Hmmmm -- Well, as fate would have it, I also plan to redo the deck drainages on the Cabo Rico, and the 2"OD x 1.5"ID structural tube from the link you provided would be perfect for this, so I'll order it as an alternative to the plan I mentioned above, enabling the plan you just outlined. I'll be keeping both the 3"OD and the 2"OD structural tubes for the CR34, but I'm not getting bronze pipe machined until after dry fit, so I can return the flange and pipe if I decide to go your route.

If I went with the bronze flange, the proud aspect wouldn't be much and could easily be made up with robust glass and maybe a little fairing, with flange secured with coarse thread screws. As I see it, the glass structure using the bronze flange/pipe would be 4.5" shorter than your method due to the length of the flange exterior to deadwood, and only requiring the extra glass/fairing to make up the proud aspect of the flange itself as you pointed out. Going with your method, the 100% FRP skeg (no bronze) is 4.5" longer.

The thing I care most about is structural rigidity in the event of a line wrapping the prop and the torque/sideways forces applied to the structure as a result, so I am comfortable with it being overbuilt. The thick tube, plus the bronze pipe within, would distribute any sideways loads throughout the structure. In your example, the tube went through the keel and had all-around support, but in this case, it will be skegged (is that a word?), and only fixed from above. But, I see your point and would have to agree your method would still be very stable.

 

@bajansailor Looking into this a little further, the tube you suggested is a polyester resin - have you used this same tube before in this application? I know it has good structural properties, but I am concerned about osmosis.

 

I have never heard any reports of osmosis in these fibreglass tubes, and I think that the chances of this happening are pretty much non-existent.
We were using thick wall polyester fibreglass tubes for the shaft logs on the fishing boats we were building 25 years ago, and they are still going strong now.

 

Well I am committed (you can interpret that statement as you wish). I cut out the old shaft log, leaving a hole in the boat that would have sunk her in less than a minute had she been floated. I ground the appropriate bevels on both sides of centerline for the new skeg and cut out old strut. Got engine where I wanted and attached 1" prop shaft, and put bushings around prop shaft fore and aft to center it within 1.5" ID shaft log tube, and glassed all of that into place in alignment. Easier said than done - the last two days were brutal with the cutting, grinding, cleaning, and glassing. The recess that needed to filled for the removed bronze strut took a lot of glass and epoxy, and wanted to I wanted to continue laying up the new prop skeg to achieve the chemical bond - so a long couple of days to say the least. The initial attachment of the tube to hull was accomplished by first laying up a 6" wide x 20" long strip of a few layers of fiberglass separately yesterday morning, and letting that cure until hard enough (afternoon) to cut to a 'skeg shape' attaching that to the top of shaft log and to bottom of hull, and letting that cure last night, creating the foundation for the skeg in its basic form. Today was the job of laying up layers of glass both port and starboard and around tube, after washing the amine blush from last nights cure, then sanding, and wipe down with acetone before laying up bulk today. I used 90-deg/45-deg biaxial with the 90-degree axis going vertically from hull, around tube, and back to hull on other side. I alternated the 45'degree axis so it would criss-cross with each subsequent layer, and got about 4 layers on today, but still with a 2" tube at the bottom hung by a thin skeg of 4 layers per side with a few additional layers of aforementioned pre-made skeg. This was not small feat with the amount of glass and epoxy required, and fairing of hard corners and recesses with new fillets in between layers of glass. After today, the shape is still a tube hung by a thin (but substantial) layup. Next time I'm down, I'll bring some Coosa with me and lay up foam on each side of the skeg 'fin' to build up thickness (making it close to the width of the tube), and then glassing that with several more layers to finish it off in a sandwich construction, and also adding thickness to the fiberglass tube which will house the cutlass bearing.

I had several people observing throughout the weekend, some of whom were veteran boat-builders - and all generally liked the approach and some had suggestions - including the addition of foam board/Coosa to the layup. I didn't take any phot0s, as I was in the thick of it (literally, with epoxy), but will when I'm back down in a week or so and share here.

Lastly, I am going to follow Dave Gerr's advice from his Propeller Handbook about appropriate clearances from skeg to middle of prop blade (from center to tip) and also rudder clearances. This will result in the skeg being cut away about 4" (for 13" prop diameter) at the blade mid-point with skeg being faired thin on the trailing edge.

 

I also posted a question on connecting a new 33c cable to yacht specialties control configured for 63c here: Connecting new 33c cable to Yacht Specialties 63c Transmission Control https://www.boatdesign.net/threads/connecting-new-33c-cable-to-yacht-specialties-63c-transmission-control.68078/

Since it is a different subject altogether, I felt a separate post best

 

Here is the progress so far. I wish I took a photo of the hole in the boat after I cut out the old stern tube and strut, so I'll offer a little detail on that step before the addressing photos posted below: I cut the bare minimum out, allowing the new tube to be inserted with the engine in the lowest possible position, keeping in mind the following factors:

1. adequate space for 'usual' dripless shaft seals (PSS, Lasdrop), combined with SS face, combined with flexible coupling and standard couplings, with room to spare to be able to change seals, access couplings, etc
2. At least 1" clearance between the closest point on the engine to the inside hull. This happened to be the forward corners of the KM2P transmission housing
3. Room to change belts, adjust water pump belt, alternator belt, etc without cutting into more of the liner.

Regarding the photos below, please keep in mind the stern tube is current cut to the midpoint of the taper in the shaft, which is where the propeller tips will align to. I chose to keep the extra length as a frame of reference, and once final shaft, couplings, and prop are here, I can cut that back (about 2" I estimate) to provide the usual 1x-2x shaft diameter spacing between cutlass bearing and prop.

The first two photos show the starting layup of the framework, with several layers of 90/45 biaxial. The third photo shows 1/2" Coosa applied to either side, which was applied today, and tomorrow I'll layup more off the biaxial, then add another layer of Coosa tapered from the hull down to the tube so it creates a 'V' shape. Final steps will be to cut the tube in the appropriate place, cut the skeg back from the tube at that point in a somewhat crescent shape, so the trailing edge of the skeg has some distance between the prop, then fairing the trailing edge of the skeg to a narrower profile, and finally laying it all up in 10oz cloth, fairing, and painting (barrier and bottom paint). Using the 3rd photo (with Coosa board) as reference, I'm guessing the final trailing edge of the skeg will sit around where the wooden handle to upper aft clamp sits, and the end of the stern tube will be 1" - 2" forward of the middle permanent maker mark which is where the propeller face should sit (The angle of the photo makes those marks look farther apart than they really are). Oh yeah, then I have to install the Glide cutlass bearing. Gonna try to complete all that this weekend if prop shaft arrives, and it doesn't rain too much!

 

Latest update photos

 

Your project is certainly looking very impressive, and coming along nicely - this should hopefully be a better / stronger arrangement than simply having a 'P' bracket supporting the shaft.

Have you made sure that the front face of the propeller boss will be a maximum of 1.5 " from the aft end of the new composite shaft log?