FRP shaft strut

I will be installing the engines in my cat soon and I can't see any reason not to make my own shaft strut out of fiberglass and vinylester. This would eliminate 4 below-the-waterline fasteners and a zinc, as well as save some money and make alignment easier. Am I missing something?

Mike

 

Bronze--------------------------if It Is In Saltwater

 

I understand this is the way it is done, but why? My hull, rudders, daggerboards and shaft log are glass. Why does my strut have to be bronze? Is it so that I have a place to hang my zinc? Is it a strength issue? FRP is pretty strong stuff. If I can make my chainplates out of it why not my strut? I still don't get it. Is the emporer nekkid?

 

Composite Strut

Mike,

Check this out... http://www.tigerpropellers.com/yachtdrive.htm

I'm with you on this, composite struts make a lot of sense. Gerrs has some formulas for calculating strut dimensions with standard metals. It's my understanding that in the early days of carbon composites engineers used to calculate for typically alloy strengths and then fabricated the composite parts to similar dimensions. "Black Aluminimum" I think they called it. They didn't achieve any real weight savings of course, but got other benefits that still made it worthwhile. While I trust my abilities to "backyard engineer" steel and timber, composites give me pause. Nonetheless, there are a growing number of products such as these

http://www.bedfordplastics.com/pdf/Fiberglass_Pultruded_Shapes_Brochure1.pdf

that have readily identifiable properties which should be translatable into boat construction. Anyway, I'm curious as to what you eventually used.

Regards,

Doug

 

Mike

No, you're not missing anything. Except ascertaining the design loads to ensure your composite shaft strut is adequate for the job.

 

Ad Hoc,

You write,

"No, you're not missing anything. Except ascertaining the design loads to ensure your composite shaft strut is adequate for the job."

Yes, and then of course determining a laminate schedule to meet those loads...and therein lies the rub.

Gerrs (Boat Mechanical Systems Handbook) gives some pretty detailed formulas to calculate the required section modulus for various standard metals. (They factor in the design loads) Replacing the metal strength values with your laminate values certainly points you in the right direction, but since laminates are not isotropic, thought needs to be given to the alignment of fibers. I suppose if you are designing space shuttles and formula 1 cars, it would be worth it to take the finite element route, but that's where the calculations get beyond my willingness/ability to compute.

I was looking at a carbon rudder stock laminate schedule, (from a reputable NA), and it intutively made sense. Maybe 65% of the fibers aligned along the major load axis with the balance taking up the torsional load, but not how "effecient" the laminate schedule was. I suppose if you really studied it, engineering laminates would really not be that different than anything else, but I never had any of it in school and haven't had the need to invest the time since then to really come to grips with the subject.

It would be interesting to hear from knowledgeable people if there are some "rules of thumb" for laminate design where bleeding edge calculations may not be necessary...such as the strut in question.

Regards to all,

Doug

 

SeaJay

Data from 'technical' sheets only help so far. There is a degree of intuitiveness about the layup too. Such as simply aligning the fibres in the direction that is required; this assumes the basic designs calcs have been done of course. If using a standard GRP a best case is to design to the worse possible strength one could obtain by poor QA. If going down the 'exotic route' one can apply the same methodology. All this is of course better than nothing, if nowt is 'to hand' for reference.

However if time/resorces permit, it is always better to make some samples and test them. Then use that info for design. So whatever layup one selects, make some samples, then the design 'limits' used, are known. This is the approach I've always taken when designing in composites. And it comes as no great surprise that the real tested sample differs from the technical data spreadsheet flyer!

If you wish to go indeep a little more, then trying to ascertain the first fibre/resin debonding helps significantly. This of course being dictated by the resins shear strength. This could be an extension of the actual testing, if the right equipment is to hand...and on it goes!

However, I wouldn't advocate going down the FEA route just to see "...how "efficient" the laminate schedule was...", since it just a matter of establishing the expected loads and then the modulus of said section. (I wouldn't using a standard isotropic material too). For a simple strut, not really worth the bother of FEA. If I'm not pushed for time i may do an FE, but i would not model each individual fibre etc, pointless, too much validation required and impossible to achieve without vast sums of money anyway..

 

Not having any idea how to calculate loads I ended up just laying up something that looks strong. After making a tube using the cutlass bearings as molds I shaped a piece of 3/4" Honduras mahogany and glassed it with 10oz cloth to seal it. I then glued it to the tube and faired it in with epoxy mixed with milled fibers. Then glassed over this with 2 layers of 19oz uni on each side. I will place set screws when I can get some. They seem pretty beefy and of course the tabbing will add some strength as well, we'll see.

Doug,
That Yachtdrive looks pretty slick. I thought of making a combined strut/shaft log or even a skeg that would encompass both, but thought that would make it harder to change the cutlass bearings down the road, so ended up with separate pieces. Here's a photo of the struts in progress:

 

mike

These look great. And if they break, so what, you make the same again, but with a "bit more" laminate. Perfect.

 

Mike,

I'm wrestling with the same question...should I incorporate the strut and sterntube into a skeg or just use a strut and an open shaft. I feel sort of uncomfortable with the shaft and prop just hanging there without protection. But it is done all the time so it can't be that much of a problem. On the other hand, a full skeg is just that much more underwater surface area, and it adds another element to cutless replacement. I'm still undecided but would be interested in hearing pros and cons from members.

Regards,

Doug

 

SeaJay

You need to be a bit carfull here with adding skegs. If you add too much skeg aft, she could become too directionally stable, ie stiff and hard to turn etc.
If you're boat handles well, as the saying goes "if it ain't broke, why fix it?"

 

I think what you missed is the lack of rigidity, what you made will seem rigid but not in comparison to bronze.

Glass also won't take the friction between the moving parts, and if you bushed it the metal bushing to glass joint will fail.

 

I agree, a metal strut is more rigid. You may end up with vibration and shaft failure

 

jonesg,

Thanks for the reply. To be honest I really don't see lack of rigidity being an issue in this application (29hp, 30mm shaft, 100cm long). Besides, a composite strut can be made as rigid as you want simply by adding more layers of reinforcement as Ad Hoc mentions. I would think that for a given thicknes that carbon fiber or even S glass may actually be more rigid than bronze. Add a core and rigidity increases, but I will defer to the engineers on this point. Another consideration is that a composite strut can be glassed directly to the hull eliminating fasteners and potential leak points. Also, bronze struts often require a shim to achieve the right shaft angle.

There is no contact between the glass and shaft as a cutlass bearing is used. A couple of set screws should be all that is needed to hold the bearing in place at the bushing to glass joint. I would think this would be no different to using a bronze strut with a polymer cutlass bearing.

Mike

 

There are several production boats with composite struts ( shamrock is one of them) and those that I have seen have had the hulls laid up in two-part molds with the bulk of laminate in the hull and all the laminate in the strut and aft portion of the keel. After trimming the two halves are bonded together and a laminate is applied down the centerline but the strut is left with only the bonding putty to unitized it.
Although your design method isn't very scientific, I think you'll be fine. What concerns me the most is the wood incorporated in the design. When ( not if) the wood gets wet it will exert about 10,000 pounds pressure as it expands and it is that that will probably be the demise of your strut.
Gerald