Strut Calculations

[Timm]

Hi all. I am designing a strut for a one off project that will probably have to be welded up out of SS (I prefer cast bronze). I am familiar with the ABYC rules for struts, but does anyone have any other good sources for calculating the various stresses on a strut for a planing powerboat? For instance, ABYC has no guidance for the thickness of the strut palm or sizing the fasteners. How about sizing the welds between the various parts?

Any input would be appreciated. The production version of the boat will use a cast bronze strut, so this is only for the prototype.

[mmd]

I believe that Dave Gerr's book Propeller Handbook describes a method to calculate V and P strut dimensions.

[cyclops]

A phone call to strut companies seems in order. Use their recomendations.

[Timm]

Thanks for the input. I was looking through the Marine Associates catalog and found a stock strut that will work. By dumb luck, they happen to have a few in stock as well. I have avoided the fabricated strut this time, but would still appreciate any input into calculating strut strength. Thanks again for your help!

[RANCHI OTTO]

You will find the calculation method in the Lloyd's register.

[cyclops]

There is a set of standards for horsepower to----- shaft size to------strut size to -----propellor bores. It is all established in the industry.

[Timm]

I know there are standards, but I have yet to see anything to help calculate the actual loads on a welded SS strut. For instance, I haven't found anything that gives guidance on the strut palm thickness. If it was a welded piece, how would I specify the size of the weld between the leg and the palm or between the leg and the barrel? That was my biggest concern. I'll keep looking and if I find anything, I'll pass it along here. Thanks!

[cyclops]

You are trying to design a item that is normally decades of trial and errors with many failures. Do not risk the law suit over the damage a torn loose strut will cause. A marine investagator would get a hell of a bonus on this one. No MFG. marks.

[Timm]

Cyclops, I don't quite understand where you are coming from. Custom boat builders and designers routinely design struts. In fact, I have designed them at a number of boat companies I have worked for, then we paid to have the pattern made and the parts cast. This is a normal part of the business. Although there are a number of "stock" struts available, these were all designed as custom parts for a specific application. The one I found for this project is actually an old Carver strut from 1990. Southeastern Foundries in NC does quite a bit of business with the custom builders in their area casting one off custom struts. As for the manufacturers marks, the struts will usually have the foundry's name or part number on it somewhere. In fact, when I went to the Landing School, our first drafting project was to design a strut and a stern bearing. This was to give us drafting practice as not all the students had drafting classes before coming to the school (I had a couple years in high school). I am getting the feeling that most of the struts available have quite a bit of seat of the pants engineering in them.

[CGN]

Timm, I think in the rules there is some specification of the magnitude of the loads to use for direct calcualtions of the struts, maybe you can have a look at ABS or RINA, they have their Rules online. you may not find for high speed boats but maybe can be a good reference. hope it helps

[cyclops]

Thanks CGN.

[WilliamS]

Also try with LLOYD Special Service Craft rules (SSC), is used for High Speed Craft, Mono-Hull and Multi-Hull design.

[Timm]

Thanks for all the input! I'll have to get hold of some of the rules. Most of my design work has been smaller boats lately, but I have the possibility of doing some bigger things now so I should probably add some more rules to my library.