Catamaran Skeg Strut

My planned hull will have fully protected props and rudders by an aluminum skeg. This boat will rest on the ground frequently and minimum draft is needed to run the boat up the river to its home. I can only get up it at high tide.

The boat a 50' aluminum power catamaran. Displacement will be in the 35,000 lbs range.

The skegs will have a fairly wide 'shoe' (~6" wide) on the bottom of them in order to minimize how far they sink into the grave/mud when grounded in order to keep the prop and rudder clear and protected. They also need to be robust due to a lot of floating logs, whales, etc. in my cruising grounds.

Since the boat will rest on its two hulls, I've conservatively assumed that it will rest on four corners, each bearing roughly 10,000 lbs. I've also conservatively assumed that the skegs will have a point load of 10,000 lbs right in the middle of the span. I've brought the skeg up to about as close to the prop as I want (<15% of prop diameter) before significantly impacting prop efficiency.

I treated the skegs as simple beams and did load calcs to check deformation and ultimate strength. I played with various profiles in both aluminum and steel. If I want to make the skeg a cantilever, I need to increase its depth, building perhaps an I-beam. But draft gets impacted then. Inches matter.

I modeled them with aft struts, creating a simply supported beam from both ends. This solves the deformation problem with an acceptable amount of draft using aluminum in order to control weight.

I'm looking at a single vertical strut behind each rudder approximately 2 1/2 ft high between skeg and hull. Roughly 1/2 inch thick by 6" wide aluminum plate. It would be faired in to a foil shape as much as possible without getting into expensive CNC shaping. It would have a horizontal tube in it aligned with the shaft for shaft removal. I've seen this approach used on some commercial fishing boats in the area though I don't think those boats were designed with efficiency in mind.

So my questions: Have adversely might this affect my overall efficiency (anticipated cruise speed of 11-12 kts)? Any thoughts on how this might affect rudder performance if the strut is fairly close aft of it (say 2-3 inches)? Aside from rudder performance impacts, any thoughts on how this might affect low speed maneuvering when docking?

That's a lot to absorb, but appreciate any thoughts. Thanks.

 

Any pics or line drawings ? If you had a canoe stern all this would be much simpler.

 

Strut jpg

Here you go. Stern beam at this point is 4'-8" per hull. Flat section about at WL, 3' long.

How would a canoe stern make this easier?

 

I followed your arguments carefully and do not understand why you design a sturdy structure, much more than I had ever seen, and then you want to build it with the softer metal of all possible, aluminum.
There are many boats that need to rest on the seabed and none needs a stern so complicated. I would advise you, before inventing anything, copy what exists in similar boats.

 

Your first aspect is to ensure the strut works structurally. So ensure you have covered all the bases with static strength buckling, deflection and fatigue.

If this solves the problems you want then the effect on performance should be secondary. However, you should make the strut as small as possible width wise.

 

Good questions. A few responses:

1. Why aluminum instead of steel? Weight. I could achieve a similar result using steel plate, but at a much greater weight penalty. I'd then also have to deal with the dissimilar metal stuff whereas if I stay with aluminum that problem goes away.

2. Most boats that rest on the seabed do so with their keels supporting most of the weight, not their skegs. This hull is different.

3. I have seen a similar strut arrangement on numerous commercial fishing boats here. I'm not inventing something new. That's where I got my idea from.

 

You have done your beam load calculations. You need to also consider soil bearing versus support area. Six inches wide is pretty skinny to hold up 10 kips. I am skeptical that a half inch thick flat bar will be sufficiently robust.

 

I will answer your questions following the same score as you've marked:
1. Weight gain that produces steel, will compensate the increased robustness of the device. Not only because of its higher elastic LIMIT but above all by its resistance to abrasion.
2. I guess this hull is different because we can not know. If you want to get better answers you should show why your kind of hull forces you to all it rests on only two points. This also can lead to difficult situations, if the seabed is not perfectly flat.
3. I have never seen anything like this and I've done a few commercial fishing vessels. What surprises me also is that the area of attachment to the keel, which is what's going to suffer greater efforts, is the weakest one.

Another question: Have you taken into account the efforts of the rudder shaft when the lower stern knock on the seabed?. But well, I'm sure you have answers for all that.

 

Hello Messabout,

Yes, soil bearing will be an issue in some cases. One location has soft silt and I expect that it'll sink down until the rest of the hull rests on the bottom. But I'm assuming that the prop and rudder will be adequately protected in that case. The other regular place this will ground is gravel and should have adequate bearing.
The skeg shoe is about 6 ft long.

The skeg is a built up channel (3x6x1/2). The vertical strut is plate. I'd feel better if it was beefier, but am compromising between thickness and drag. As Ad Hoc suggested, I ran buckling calcs and the plate looks to be adequate.

Any thoughts on rudder/strut interactions?

 

Hello TANSL,

Yes, I had the same reaction as you regarding steel vs aluminum, but when I ran the beam calcs, the aluminum provided a better strength to weight advantage. By adding a couple inches of draft with a channel beam instead of a flat plate, the aluminum deflection equals the steel plate, but at a fraction of the weight. And I have excess aluminum plate for this project so it saves me from having to buy steel and waste expensive aluminum.

I don't anticipate abrasion to be a major factor since the boat will be resting on the shoe, not skidding along it. But if it becomes an issue I can always weld more material on the bottom.

I don't know that the hull is entirety unique, but the skeg is the lowest part of it. Since it's a catamaran, it will be sitting on 4 points. But your point about uneven seabed is legitimate. The places where the boat will normally sit have level beds, but that won't be the case everywhere. I don't plan to routinely park this on the bottom except in its home berth

You may not be familiar with the strut system on commercial boats you work with, but I assure you that they are not uncommon here in Alaska. I would attach photos but I'm on my mobile.

I'm confused by you comment about the skeg attachment being too weak. Where would you suggest I attach it? You first commented that my design was overly robust, but now you're saying that it isn't.

Yes, I have considered the effects on the rudder post. That's the whole reason for this structure in the first place. To avoid jamming my rudder.

Thank you for your assessment of my skeg design. Now regarding my original question, do you have any thoughts about the potential hydrodynamic effects of the strut on rudder performance?

 

You have studied well all aspects of the problem and therefore is likely to be right about some things. I will not question your design any more.
I do not contradict my self : the structure looks very strong but is poorly attached to their ends. The areas marked in the figure, in my opinion, are weak. Laterally not have the slightest consistency.
Concerning hydrodynamics, I do not think it very appropriate but does not have a significant impact on the rudder. What I would do is place further aft propeller, to increase clearances, and also better hold the horn. But it is an opinion only.
Forgive my poor English.