Keeping Cool Under the Bimini

[The Way]

Originally Posted by Ex-Calif

Exactly! I am also planning to make it out of my old genoa. I've sat under the heavy blue or green canvas and it's oppressively dark. I am hoping it will allow light in yet provide the shade.

Ex-Calif, I'm considering making a dark blue awning in order to cut down on the glare. My current awning is made from white sailcloth, and it hurts my eyes to sit under it.

May I ask, did you find it hotter under a dark awning? Some people say it is, others say it isn't.


Thanks in advance for any advice!
Jack

[brazenarticle]

Seems to me that shade is a product of opacity, not color. A layer of tinfoil, heavy black or blue cloth, plywood or fiberglass will all provide equal shade ( no direct light coming through the material). How they handle heat radiation is a different deal.
A light colored underside will reflect ambient light from the boat, surrounding sea or land surfaces, gleaming brightwork etc., much of that light has spent some of the heat radiation on the reflecting surface so it is not as hot.
A bimini with a silvered or white top and a dark under layer would eliminate opacity and minimize reflected light.

[poiu]

Originally Posted by brazenarticle

When my old (blue) bimini gave up the ghost I replaced it with white 10mm sheet PVC, product name Komocel. I was pleasantly surprised how much cooler it made the cockpit. I also changed the dodger and occasionally use Suncloth between the two on especially hot days.
https://www.flickr.com/photos/brazen...e/11104062065/
https://www.flickr.com/photos/brazen...e/11104062065/

Nice. I've never seen that before. It would be hard to make one big one joining up the two sections without re-doing the frame I imagine. Do you hang side panels off a bolt rope slot on the underside?

[Neptune's Gear]

We added flexible solar panels directly on the bimini (Sunbrella Navy blue). It made a surprising difference - way cooler with the panels on top! :-)

[brazenarticle]

oooops - didn't realize I sent two of the same photo ( and can't figure how to put the photo in a post).
https://www.flickr.com/photos/brazen...e/11104144174/
https://www.flickr.com/photos/brazen...e/11104239493/
My local metal fabricator (David Miller, North East, MD) uses this for tops on the BIG boats. I had a 1 inch stainless frame and figured the Komocel was not much more weight than a soaking wet bimini. Sheets come in 4x8 or 4x10, 5mm or 10mm thick. I used 10mm and did a butt joint with a cover piece in the center. Attached it to the frame with ss straps.
I have a track on the underside for sun cloth side curtains, have not gotten around to finishing a windscreen on the dodger but the plan is to use bolt rope.
This is my third season with the tops, been through some heavy winds (60+ derecho) on the mooring with no problems.
Handy spot for mounting the solar panels too.

[capngeo]

Originally Posted by a64pilot

Your re-inventing the WBGT

Wet Bulb Globe Temperature - Wikipedia, the free encyclopedia

No Sir, I had that specifically in mind when I made the post!

Originally Posted by poiu

The evaporation question is vital to staying cool, but not in answering these questions.

Your observations of inside no breeze made me say that, otherwise breeze has no relevance.

[Ex-Calif]

Originally Posted by The Way

Ex-Calif, I'm considering making a dark blue awning in order to cut down on the glare. My current awning is made from white sailcloth, and it hurts my eyes to sit under it.

May I ask, did you find it hotter under a dark awning? Some people say it is, others say it isn't.


Thanks in advance for any advice!
Jack

Well there are lots of variables - Our environment is predominantly 28-32degC and +80% humidity. I grew up in SoCal and know that "dry heat" allows the bodies A/C system to work properly as long as it is fueled with fluids.

Here we just melt and sweat. We also get thunderstorms - So most people have dark shades here. Sometimes they roll them down to "lifeline" level. I find these cockpits dark, claustrophobic and stifling.

I do know that when sailing and becalmed everyone is looking to get in the shade of the genoa or mainsail - it is a palpable difference. Hence my idea to repurpose my old Genoa. I like it bright - in fact I don't sail with sunglasses on, weird.

I am planning to convert my cloth (canvass) bimini to plexiglass beef up the arch and supports and mount my solar above the plexiglass. Think skylight.

I am not a thermal shade expert but we all experience how hot a "black" car is compared to a white car when we get in it. I think the dark absorbs and radiates heat. White tends to reflect - but what do I know?

I'll let you know in a month or so when I get to that special project - LOL

[The Way]

Thanks, Ex-Calif,

I had suspected a dark awning would be hotter, but also less glare, which I find a problem. The multilayer awnings some have described earlier in this thread would probably be ideal, but much harder to DIY.

A word of warning to you -- according to Don Casey, dacron doesn't filter UV the way canvas or acrylic does, so be careful under your new awning!

Thanks again,
Jack

[Ex-Calif]

Originally Posted by The Way

Thanks, Ex-Calif,

I had suspected a dark awning would be hotter, but also less glare, which I find a problem. The multilayer awnings some have described earlier in this thread would probably be ideal, but much harder to DIY.

A word of warning to you -- according to Don Casey, dacron doesn't filter UV the way canvas or acrylic does, so be careful under your new awning!

Thanks again,
Jack

Point taken about U/V - Banana boat 50 is the norm on our boat.

I am also pretty sure the Dacron will allow water to seep through although with "double layer" I am hoping it mostly drains off. Our rains don't last very long. And really the awning is about end of the day, overnight at dock sundowning...

As described above the bimini is going to end up being a "poor man's hard top."

[Broken-Sailor]

Originally Posted by The Way

Thanks, Ex-Calif,

I had suspected a dark awning would be hotter, but also less glare, which I find a problem. The multilayer awnings some have described earlier in this thread would probably be ideal, but much harder to DIY.

A word of warning to you -- according to Don Casey, dacron doesn't filter UV the way canvas or acrylic does, so be careful under your new awning!

Thanks again,
Jack

I was fixing to ask it the cloth on Bimini was of a anti UV type. I just bought a shirt that anti UV not long ago. There should be some marine grade Breathable Water Proof Anti UV fabric out there somewhere.

[svtadpole]

My solution is a $10 polytarp boom-tent ..... buy the right size to fit the length of your boom .... bungies keep it under tension ..... although polytarps come in various colours, usually one side is silver giving good sun reflection .... in extremis it's easy to rig 2 polytarps in a double-skin with one over the boom & the second rigged on a 'washing line' tied under the boom ...

[Stumble]

Originally Posted by Broken-Sailor

I was fixing to ask it the cloth on Bimini was of a anti UV type. I just bought a shirt that anti UV not long ago. There should be some marine grade Breathable Water Proof Anti UV fabric out there somewhere.

There is, it's called sunbrella and is pretty much the standard for bimminis (mind had to look this up too). The research I did was pretty clear, the darker the fabric the hotter it will be, but with reduced glare. I couldn't find any research on multilayer awnings, but my guess is that the coolest possible would be white on top with a darker thinner layer for glare reduction.

[Cotemar]

We had a huge difference in how cool our cockpit was after changing from blue Sumbrella canvas to an White Azek hard bimini.

[Jim Woodall]

A cruising couple with more than a decade of experience in the tropics swears by the shade-over-shade method. At anchor one might mistake their 45' monohull for a small circus tent, but it is almost always tolerable, even on the days with no wind, and very comfortable onboard their boat if there is even a wisp of wind. They use a fabric shade that covers virtually the entire deck, cabin top, and bimini. The shade is a couple of feet above boom level and leaves plenty of room to move around on the side decks. The times I've been onboard their boat at anchor, I've been amazed at how cool it feels throughout the boat and cockpit. It's a custom-made shade (maybe ShadeTree?), but they love it and say it was worth the expense.


Sent from my iPad using Cruisers Sailing Forum

[socaldmax]

Originally Posted by poiu

I found myself sitting more and more inside in the raised saloon area. It seemed cooler even though there was less breeze than outside in the cockpit under the bimini, so I set about investigating why. The saloon has a lot of windows, all open and they are shaded externally.

This is what I measured by pointing the IR thermometer at the same cushion after allowing the temperature to stabilise. I picked a cream coloured cushion as close to skin tone I could find to try to simulate how the body might heat up.

In the saloon with a light breeze: 30 deg C
Under the cockpit table with a light breeze: 30 deg C
Under the cockpit bimini with a good breeze: 34 deg C
Under the cockpit bimini, behind the windshield with no breeze: 36 deg C
Under full sun, good breeze: 49 deg C (Who doesn't need a bimini...)

So clearly a lot of radiation is getting through the bimini. The breeze also convects a good bit of heat away. The key results from these measurements is that the breeze in the cockpit compared to no breeze reduces the temperature by 2 deg C and full shade i.e. under the table or inside compared to being under the bimini - reduces temperature by a further 4 deg C or probably 5 deg C as a guess. - I'm guessing the difference in temperature between inside and outside would be a degree lower outside than inside as the breeze is better there, but I couldn't find a way to measure that directly.

How much heat is getting through is surprising to me as the bimini is light in colour - a cream Sunbrella, so should reflect a lot of energy. I would expect a dark one would block more IR, but would also absorb more and radiate the heat out again, some of which would be upwards. I wonder if it would be overall better or worse?

I would like to win that 4 deg C of temperature reduction outside. It should improve comfort quite a bit. One option is a fixed white fiberglass bimini. Or I could insulate the current one and use an alu foil.

Anyone have any experience or ideas?

If you have a good water maker, I'd suggest a mist system installed in the cockpit area. They can drop the temp. something like 20* F or more. I've used them under fabric awnings when the air temp. was 110* and it felt more like 80*, very refreshing. With a diaphragm water pump driving it, it used about 4 or 5 gph.