Radar arch design : aerodynamic, practicality, things to consider etc...

[Joka]

Hi all!

After long consideration, I'm looking into getting a custom arch built for my Farr38, here in Fiji.

I've found a good worker for the job, but he's not a marine specialist at all!


My main concern would be to avoid affecting performance too much.
I've been searching online but can't seem to find much about the ideal design...


How high should an arch be?
Is it better to have it way higher above the bimini, or really close?
My yacht is quite beamy, as long as the metal is thick/big enough to support the weights is this gonna be an issue?
Anything else to consider?

Any advice welcome, I like to overthink but do things once and well!


Fyi ;
- I'd like to have davits on the arch for the dinghy
- I will try to go for aluminium for weight and performance reasons, but may be forced to go SS if not available here.
- I trust this guy for making a strong and reliable structure, and we will reinforce it with a smaller inner arch. But any word on weakness points of the stucture may help!


Thanks in advance for your precious advice!

[psk125]

If you are concerned about impact on performance, don’t put dinghy davits on it. What else are you planning to use the arch for? Radar coud simply go on its own post aft or on the front of the mainmast. Another multitasker poster made his bimini into a rigid solar panel holder.

[SVHarmonie]

Originally Posted by Joka

Hi all!

After long consideration, I'm looking into getting a custom arch built for my Farr38, here in Fiji.

I've found a good worker for the job, but he's not a marine specialist at all!


My main concern would be to avoid affecting performance too much.
I've been searching online but can't seem to find much about the ideal design...


How high should an arch be?
Is it better to have it way higher above the bimini, or really close?
My yacht is quite beamy, as long as the metal is thick/big enough to support the weights is this gonna be an issue?
Anything else to consider?

Any advice welcome, I like to overthink but do things once and well!


Fyi ;
- I'd like to have davits on the arch for the dinghy
- I will try to go for aluminium for weight and performance reasons, but may be forced to go SS if not available here.
- I trust this guy for making a strong and reliable structure, and we will reinforce it with a smaller inner arch. But any word on weakness points of the stucture may help!


Thanks in advance for your precious advice!

Aerodynamics come a very distant second to weight on the transom. All weight above the waterline is bad, and the higher it is, and the closer to the ends of the boat, the worse.

If the arch's primary job is to support a radar, then an arch is over kill. Just use a pole, or mast mount.

If you're really adding dinghy davits and calling it a "radar" arch, then performance isn't a real consideration, do what ever you need to do to get the dinghy high enough to be safe in the worst weather you will ever carry a davited dinghy. When you are thinking about that, remember this

We have an arch on our boat (only to carry solar panels) and it is made of 2" schedule 40 aluminum pipe. It's almost 12 feet wide, and weighs less than 100 lbs. Lasted though one hurricane and 20,000 sea miles. I wouldn't say it is "rigid" but it is strong. We have no davits--and never will.

If you do need an arch, it should be high enough not to hit your head on. No benefit to going higher except for the radar's long range performance, and then a foot isn't making much difference. If you hit your head every time you go out the transom, you'll hate it forever.

The weight difference between SS and Al is large, that's likely a MUCH larger performance hit than windage.

The weakest point on most arches is the fixture to the hull/deck. Be sure the hull/deck is strong and stiff enough, and the attachment is well engineered.

[Joka]

Thanks a lot for these useful answers! I definitely will insist on going aluminium then. I believe it is crucial to go for a good marine grade?

"Radar" was a bit confusing... I thought that was the generic term. The main reason I'd like to have it is for the solar panels, to keep them out of the shade of the sails, secure and off the deck.

The dinghy davits were more of a bonus of convenience to use every now and then...
I guess it wouldn't hurt to have them installed? And then only use them when performance isn't a big deal? Or am I missing something?

[coopec43]

For 200 pound weight of your solar panel/arch on a 38ft yacht probably equates to 800 lb loss of ballast. (of course it would vary with the design of the yacht)

[a64pilot]

Radar arch is a power boat thing, it’s that stylish arch you see on so many and it’s often not at the stern, it’s really a styling feature.
Same thing years ago was I believe called a targa bar.
In my opinion your crazy if you don’t also make it into davits. Davits are so nice when moving short distances from one anchorage to another, merely hook up the dinghy leaving the motor on and use the sheet winches to haul it up, it’s far better than towing, and stowing on deck is fine for serious sailing, but a pain to go from anchorage to anchorage, so you tow.
So use your davits when weather and distance make that the best, and stow on deck for serious passages

[Joka]

Thanks Coopec, good to know, I'll definitely keep that in mind.

A64pilot, I completely agree.
I guess the extra structure for the davits, pulleys and lines will make very few difference, even more so if aluminium...
And it's true that's really convenient to just haul it up!
But I'll definitely have the dinghy on deck anytime I'm looking for better performance.

Thanks!

[Midnight Son]

The most important is as SVHarmonie stated, proper deck attachment. Depending on how the deck is constructed, you might have to do some very serious work, fiberglassing in the backing plates, to properly distribute the dynamic loads, keeping the point loads on the deck to reasonable limits. The loads at 25 kts of wind are far lighter than a +40 kt storm gust.
Cheers all

[hpeer]

There was a thread here a couple of years back about an arch attachment that failed. The arch was attached to the sides of the boat putting the bolts in sheer. They were 1/4” 20’s. There was lots of good discussion and description of strength analysis.

Briefly, bolts develop rated strength on the shoulder, the threaded part is much weaker.

I have a steel boat and added a steel arch. 1 1/4” schedule 40 galvanized pipe. At first it moved a lot and I ended up adding some small braces to stiffen it. I’ve seen an Atlantic “Tower in a Box” installation, it moved a LOT more than ours.

I find that once I had the arch it collects a LOT of stuff. I’m getting ready to make a passage and most will come off for the passage.

The arch supports:
2 310 watt solar panels
Radar
8hp outboard
Fortress FX-37 anchor
All manner of dock lines
Wind generator
Outboard motor hoist

Something I found useful was to put eyes on the aft uprights. I run some light SS chain across the stern, instead of rail. That way I can easily drop the chain and have free access across the transom. The chain has snap shackles.

The arch has been a great addition, but I would do it in aluminum next time.