Recessing the struts

[jayjays1]

Gentlemen, has anyone had success in recessing the bases of the shafts struts into the hull so that they are flush with it in order to minimise drag? Obviously one would have to fortify the strut backing pad even more than usual!
JJ

[bob lico]

yes i have done that BUT it goes in on a angle to accommodate the 10 degree shaft angle and built up epoxy and laminated two pieces of 1/2" CDX saturated in epoxy . that part of the hull along with rudder tower is two inches thick require new buck Algonquin 10" bronze bolts ,double nut over 3/8" thick bronze plate

[jayjays1]

I thought that you might have Bob! I

will be getting custom struts from Danny with 10-11 degree angle and want to recess the whole strut base into the hull so it disappears. I am thinking of building up the inside pad with several layers of 1/2' structural FG with 1708 in between.

Do you think SFG is a good idea for the pads? Also what is the absolute minimum shaft angle that one can get away with and still swing a 21" prop?

JJ

[Carl]

Surface piercing props will give best angle

[Ironworker]

Before you get too far down the road of buying new struts, I'd suggest you consider building a mock up of what you're attempting to do before you start epoxying stuff. I used my regular bertram struts and was able to end up with a 11 degree shaft angle. I could have forced some stuff to shave off another degree for a 10 degree shaft angle but decided the juice was not worth the squeeze. My strut base is flush with running surface. I used lot of glass and 3/4" and 1/2" coosa laminated together. It was partially recessesed and partially I built up glass on the hull to get them flush

Also very important! You need to maintain a minimum clearance between the hull an the props. Give yourself some working room to play around with the prop size as well.

Lastly remember the weak point of our hulls is the area around the strut pads. I built my pads then went back and tore the out and rebuilt them again because I wasn't satisfied with the strength. We have lots of coral heads/rocks where I boat and Sea Tow is not coming to get me.

[jayjays1]

Will do. Gonna be done right don't you worry! Overkill is is a necessity in the marine world!

[Tony Meola]

When we rebuilt the Strut pads, we used structural fiberglass. Capt. Pat use to say that it was too rigid and to use marine ply glassed in. He felt that more damage would be done if it was rigid and had no give. Without testing it who knows.

My feeling is that all the new boats are using glass for the strut pads, and I have not heard of any issues with them during impact. But then again, around here most of it is soft sand and mud. If you hit a rock that means you just took out a jetty.

But then again there is always that submerged log.

[jayjays1]

Thanks Tony. That was my thinking also. My worry with the wood is that any wood we get today is new growth and doesn't hold up to rot like the old stuff.
Thanks

JJ

[bob lico]

captain Patrick recommended 1/2" CDX plywood saturated with epoxy and i wanted to use B-26 coosa board because i had so many left overs so our brother Dug whom own a bearing factory has a hydraulic material strength tester and the results were capt Patrick was correct (as usual) the two 1/2" cdx plywood saturated and laminated together was 1/3 stronger than Coosa board so that's how i did my strut pad combination rudder support in my boat 15 years ago and still perfect

[jayjays1]

Yup Coosa can't be very strong. There ain't much glass in it by the looks of it. I am hoping that 1/2" structural fiberglass will be stronger than even the plywood.
JJ

[S Ritzert]

In my world (ski and wake boats) the only backing we use are the washers. The fiberglass is built up very thick in the areas of strut and rudders. I have seen the "cheaper" boats that use less laminate on layup completely pull the strut out of the glass. I have seen our brand of boat punch a stump hard enough to embed the strut in a stump, and have to be pulled off the stump with air bags, and have superficial damage.

On my 28, the wood backing was rotten. I removed the wood, ground down to good glass, then laid 3 layers of biaxial glass. While the glass was wet, I used 3/4" coosa. I then glassed the coosa in with 3 more layers of biaxial. Each layer of glass I laid, was 1" bigger than the last.

My opinion of coosa is the rigidity is fine as far as trying to break it, but the squish factor sucks as far bolting to it.

I have had no issues on my 28, I am a firm believer is straight glass, and the backing is more for the tearing aspect of things, and do distribute the load across a broader area.


I agree with ironworker, mock up your own struts, even if its cardboard, and a tube. I also built a laser alignment tool to center struts to the tailstock of the transmission, or use a piece of plastic pipe, or something to that effect. Engine alignment is important all the way through to the strut. You want your shaft to have no load in the strut, and the shaft to transmission coupler to align too. The tolerance on alignment in my world is .003 I managed to get my 28 to .003 at an 1.25" shaft diameter

[Ironworker]

Coosa B26 vs Plywood. I thought long and hard about it but in the end, I wanted to eliminate the wood. I removed plenty of rotted wood from my boat during the demolition including the rudder backing and some or the strut backing materials.

I laminated coosa sheets together with additional glass to and added glass below and above. I'm pretty confident that I won't be poking any holes in the bottom of the boat but you never know. There is a lot of crap floating around in the ocean these days.

[S Ritzert]

Ironworker,

I understand that my knowledge on larger boats is extremely limited. I have learned a ton of stuff Captain Patrick's tips. I wish someone had taught me to use talcum powder to help keep the itch away.

I only know what I have done. We used to cut the caps off of wood stringers, dig out the rot, and backfill them with a mixture of resin and glass. Kind of like a milled fiber slurry if you will. Of course we are talking 2x4 stringers on a mid 80's ski nautique with a whopping mid 200hp 302 or 305.

I am a firm believer in heavy lamination around struts, rudders, and any other thru hull fittings. I am also a firm believer in no wood. It will eventually rot.

I'm with you, and I don't think you'll ever see a problem even if you hit something, I don't think you'll have catastrophic failure.

[Tony Meola]

We also removed all of the old wood backing plate on the Thru hulls and used structural glass to replace them. I also drilled that structural glass and tapped it to give it threads matching a bronze bolt. Rather than go thru the hull with the bolt I just screwed into the backing plate we made. It is more than strong enough to keep the Thru hull from going anywhere.

[Ironworker]

Yup Coosa can't be very strong. There ain't much glass in it by the looks of it. I am hoping that 1/2" structural fiberglass will be stronger than even the plywood.
JJ

B26 coosa is very strong. Frankly I'm surprised just how strong it is. Is it as strong a marine plywood at the same thickness, no. However it is much stronger than rotted wood and as an added bonus about 50% the weight of plywood. Add a couple of layers of glass and laminate the sheets of coosa together you have a very strong assembly.

It is marginal at holder screws unless there is glass but holds bolts well. One small boats like ours weight is a big bonus.