Wind Drag With Hard Top

[jiju1943]

I plan to put a hard top on our toon one day this year, my question is how noticeable is the wind drag when towing with hard top up, vs down. I am trying to decide which way to go, stationary top or a top I can let down.

I still haven't decided how to build it, also if I should I go with a fabric top of a hard top. Any suggestions, which way I should go with it?

 

[Grub54891]

If you build with wood or fiberglass, it will get rather heavy I'd think. How big is the toon? I personally would go with aluminum, and sunbrella fabric. As far as wind drag, I suppose there is some, normally you run with it up, as it's for shade. You cant have it down if you have passengers, unless you are going with a bimi type that folds back out of the way.

 

[HotTommy]

I replaced my bimini style top with an aluminum hard top last spring. The bimini was not perfectly flat (a little higher at the middle support and edges that rolled down a few inches) so on the water it actually caught more air than the flat aluminum top. The structure for my top was made with 1-1/2" wide aluminum angle. The corrugated aluminum sits on top so the currugations run the length of the boat. That means the total cross section of the top that faces the wind when underway is about 2" x 8'. The vertical supports are only 1" wide as they face the wind so they offer much resistance. I estimate the total weight of my 8' x 16' top to be about 60 pounds.

I designed my top so it could be lowered for towing. The vertical supports (three on each side) are bolted to the boat railing near the deck and near the top of the railing. To lower the top I remove the bolts near the deck and push the top toward the rear. It scissors down to about half its normal height. As the vertical supports don't offer much support in the lowered position, I fabricated shorter temporary verticals to carry the weight and keep the top from bouncing. .... As this is a homemade affair, it's not the easiest or quickest thing to put up or down. But I can do it by myself in about 20 minutes. As I typically tow it only 2-4 times per years, it works ok for me.

 

[jiju1943]

Our pontoon is 24 foot with a 85 hp Johnson, I am really not too concerned with weight because I plan to build it light. We really want the top to cover most of the deck area, except for the front fishing area, as we plan to do some over nighting on it. We would like to make it so we can let flaps down to keep rain and such out while camping on the water.

I am concerned with towing drag when pulling the toon. I didn't know if a hard top would have more drag when left up or if it would have less drag by letting the top down while pulling down the road.

 

[jiju1943]

Tommy, we must have posted about the same time. You did exactly what I want to do, same size and all. Do you have any pictures other than your avatar, that you might post?

 

[HotTommy]

The search function is a wonderful thing. .... This recent thread gives all the info: link.

P.S. I have towed the boat about ten miles with the top fully up, max speed 40 MPH. When I did that I ran a strap from the bow of the boat to the front of the top. It was to help keep the top from shifting backward under pressure from the wind.

 

[Reel Kahuna]

Tommy,

Did you noticed any drop in your mpg with the top...

Compared to the Top Down?

@ jiju, is your concern with the mph or just the wind drag?

Just thinking out loud...I would think the mpg would be very minimal change being that roof drag would still be there.

 

[HotTommy]

My only long trip was with the top lowered. I doubt the top made much difference compared with the drag of the rest of the boat and trailer.

P.S. If I had placed the roof supports or the verticals a little differently, the top could have been lowered somewhat farther. I didn't realize there was an obstruction until I was well into the project, so I left it as you see it. I also considered having the top scissor forward. I think it would have worked as well.

 

[jiju1943]

I appreciate the pictures, that is nice. I am not worried about the mpg, I just don't want a lot more drag on my Toyota 4X4 6 banger. It pulls it fairly well now and we live in the mountains. There are some pretty good hills to pull between here and the place we put in so I don't need a whole lot of extra drag.

How tall from the deck is your top, it looks fairly tall.

 

[HotTommy]

The vertical supports are 7' tall (I have a friend who boats with me who is 6'3"). The highest point is about 7'2" above the deck.

 

[ahicks]

From a drag perspective, I would do it like Tom has, but maybe a hair lower. If your boat is like mine, the helm will be the limiting factor regarding just how low you can go. I'd set it up to clear that and call it a day.

 

[HotTommy]

BTW, for anyone who tries to build a top that scissors down like mine, pay close attention to the geometry of the vertical supports. If the spacing between the midpoint bolts (pivot points) is different than the spacing where the verticals meet the top, it can bind as it scissors.

 

[Reel Kahuna]

To make it interesting, here's another option... rather than using corregated panels, how's about using canvas to cover the frame which would be snapped all around the side of your frame? This way the canvas could be removed while towing which would eliminate the drag. And also won't have to fabricate a scissor type frame.

 

[HotTommy]

There's nothing wrong with a fabric top other than having to replace it after several years. But removing it won't do anything to reduce drag. It's facing the wind edge on.

Oops, I forgot about the problem of rain water. Canvas tops tend to sag in the rain and put stress on the fabric and the supports.

 

[mpilot]

Friend has a houseboat that he built an aluminum framed top for the upper deck...it's around 24x12 and his holding up very well. If you go the canvas route don't skimp and go with real sumbrella and their thread. This thing lives on the water and hasn't had anything done to it in 3 seasons. It's starting to let a little water through when it rains but it's threaded on like a T-top on a center console so we haven't taken it off to retreat...no one is upstairs when it rains anyway so not a big deal. The canvas route with some type of snap or toggle would be an easy way to go...the toggles like below work really well and hold up well. The other advantage of canvas is that you could easily have a canvas guy add zippers on it to put a winter enclosure on it.

http://www.duckworksbbs.com/sailmaking/turnbutton/

Edit to Add: Having the top removable will do a ton to reduce drag unlike what the previous poster said. I've seen pontoon boats that run in the 50's with the bimini top tucked in its boot and lose around 10-15% top speed on the same day same conditions with it open, and this is only an 8 foot bimini, not a 20ft roof.. Not having that sail up there when you trailer will make a difference and will put less strain on the frame as well.

 

[HotTommy]

Mpilot,
I think you and I are talking apples and oranges here. A standard bimini top typically has a curve (to help water run off) and puts a big cross section into the wind. It does indeed increase drag as you said. ... The top we were discussing before was a flat structure with very little cross section facing into the wind. As far as drag while towing, it shouldn't matter whether it is covered with canvas or open.

 

[mpilot]

Mpilot,
I think you and I are talking apples and oranges here. A standard bimini top typically has a curve (to help water run off) and puts a big cross section into the wind. It does indeed increase drag as you said. ... The top we were discussing before was a flat structure with very little cross section facing into the wind. As far as drag while towing, it shouldn't matter whether it is covered with canvas or open.

I see the top, but I can't help but think it would drag down the mpg of the tow vehicle as some wind will flow up once it hits the front of the boat off of the tow vehicle and is deflected into the boat. If you've ever watched a vehicle in the wind tunnel all the wind doesn't flow straight in line...patricularly on an SUV there is a dead space behind the vehicle where the air swirls. While it's not acting like a parachute necessarily like a bimini top I still feel it would add to the wind resistance some. Granted towing a pontoon is already sort of like towing a brick but still the removable canvas with no resistance other than the frame would provide less air resistance than the top like in the pictures on the previous page. I am not an engineer so I can't give you exact numbers of how much but I can almost guarantee there will be some difference, granted it may be negligible.

As for the structure of the hard top...you would want to most likely make the top out of aluminium round tubing and not square for that application as you would want a small dome in the top of the structure. My friend made trusses out of a piece of bent aluminum on top, a straight on bottom, and angled braces in the middle. Then he connected these to an outside perimeter frame and connected each truss with flat bar aluminum to help keep the canvas from sagging. This top is significantly over engineered but it's held up in some pretty tough storms out on the lake and hasn't had one issue to date. For the shorter span of a Pontoon width I've seen them forgo the truss and just run a bent piece of aluminum tubing connected to the outer frame and then the flat stock running length wise.

 

[jiju1943]

There's nothing wrong with a fabric top other than having to replace it after several years. But removing it won't do anything to reduce drag. It's facing the wind edge on.

Oops, I forgot about the problem of rain water. Canvas tops tend to sag in the rain and put stress on the fabric and the supports.

I don't really plan a fabric for the covering, but I haven't really decided exactly what just yet. For some reason your roof looks taller, I plan to make ours about 6 foot 2 from deck to bottom of the top, we aren't that tall.

I really appreciate y'alls input, it helps a lot.

 

[HotTommy]

I am not an engineer so I can't give you exact numbers of how much but I can almost guarantee there will be some difference, granted it may be negligible.

I am an aerospace engineer and I spent my first career flying airplanes, so I know a bit more about air flow and mechanical structures than the typical person. I stand by my assessment.

 

[jbcurt00]

If a lower mpg of the tow vehicle is relevant at all to the chi4ice to add a hardtop or not to a pontoon, I suspect putting gas in the tow rig and the boat might be challenging and the boat will likely stay parked quite a bit.

There are plenty of challenges, IMO, to fabbing a nice looking, functional and viable long term hardtop,that will withstand the additional wind load while towing and underway should be a concern, but not because mpg dropping.

Boating is not an inexpensive pusuit.

ReelKahuna hijacked Jiju's topic asking about mpg. Jiju isnt overly concerned about it, as he posted above.

Stay on topic.