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Old 05-09-2020, 10:32   #1
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Adding a Hardtop: how to calc windage/lift forces?

Title kinda says it - the details:

I'm thinking of making a 'free floating' style cockpit hardtop to my 50'Cutter.
Imagine one of those Hanse hardtops, only for dirtbag cruisers C:

Since it'll have a curve to it that matches the cabin trunk roof, I'm wondering
if there's a way to calculate the lift that 10x10 surface will generate.

My early napkin drawing is for it to be held up at 4 corners of the centre cockpit,
with 3" pipe with 4x6 welded bases and supports. The main body will be the usual
honeycomb core with a few GRP layers and brackets for solar panels etc.

I'm pretty sure the 'yachta-palapa' will be fine on most days, but when the wind
kicks up i'm wondering if it'll be enough to make it fly and leave big holes in the deck.

I'm handy enough but no type of engineer. Has anyone tackled this project before?

Many thanks companeros.
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Old 05-09-2020, 11:04   #2
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Re: Adding a Hardtop: how to calc windage/lift forces?

I don't know but with that huge pipe, I think you are good to go, you cant really put on 5" pipe for the ultimate hurricane anyway! Tall tuna towers on Florida motor boats seem to be smaller than 3" ............or no more than that anyway.
Put a diagonal in a corner or two somehow for strength.
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Old 05-09-2020, 12:08   #3
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Re: Adding a Hardtop: how to calc windage/lift forces?

It's not the pipe i'm thinking of - it's if the lifting force of a hardtop/wing/horizontal sail
of 100 sq feet with a curvature of 8"/10' can exceed bolt strength.

[caveat i suck at math] so it think the equation has to include the length, chord, and incidence of the plane (surface)
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Old 05-09-2020, 12:48   #4
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Re: Adding a Hardtop: how to calc windage/lift forces?

You can work out some sort of estimate but the result will be very dependent on what assumptions you make. The primary assumption that will have a drastic impact on the result is what angle you assume the hardtop may take on w.r.t. the wind. The worst case for a monohull would likely be when the boat is perpendicular to the wind and rolling - in this case the angle of incidence of the hardtop could be high.

For a back of the napkin approach I would assume critical incidence angle (e.g. angle producing max lift) and a cambered flat plate airfoil model. You can pick whatever wind speed you think is appropriate.

You can find the equations you will need here:
https://www.grc.nasa.gov/www/k-12/airplane/lifteq.html

Here are lift coefficient curves for a cambered flat plate airfoil:
https://www.researchgate.net/profile...r-Also-the.png

Also, I would design this so that if you did actually have a problem the attachment of the hardtop to the pipe would fail before the attachment of the pipe to the hull.
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Old 05-09-2020, 13:08   #5
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Re: Adding a Hardtop: how to calc windage/lift forces?

Quote:
Originally Posted by LobeliaBlue View Post
It's not the pipe i'm thinking of - it's if the lifting force of a hardtop/wing/horizontal sail
of 100 sq feet with a curvature of 8"/10' can exceed bolt strength.

[caveat i suck at math] so it think the equation has to include the length, chord, and incidence of the plane (surface)
Yeah. I'm too lazy and busy right now to calculate the airfoil lift from that. But my gut says a severe wind burst catching the lip of one corner or side will be worse than lift. Say a 70 mph microburst catching it when the boat rolls. At 70mph 100 sq ft is 1254# of force if the bimini is 90 degrees to the wind. (which it will never be really) So it's a non event really. Then the total load is distributed to from one corner post to both posts on one side.

"Calculate wind pressure. Wind pressure is given by the equation P = 0.00256 x V2, where V is the speed of the wind in miles per hour (mph). The unit for wind pressure is pounds per square foot (psf).
For example, if the wind speed is 70 mph, the wind pressure is 0.00256 x 702 = 12.5 psf."

Or thinking in comparative terms:
Your main turnbuckles on that boat are probably.. 5/8"? to withstand wind on the entire sail.
One 5/8" gr 5 bolt = 19000+ lbs of force.
If your bimini is held with 4 each of 5/16" bolts at each corner, then each 5/16" bolt is worth roughly 4600# of force. So 4 of them are about = to a 5/8" shroud turnbuckle, which is over 10 times what you need.

Just thinking out loud here. Your results may vary!
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Old 05-09-2020, 13:48   #6
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Re: Adding a Hardtop: how to calc windage/lift forces?

We have a center cockpit 60 aluminum. The Bimini is 11 x12 fore &aft.
Aluminum pipe cambered cross ship 6 tubes. 1.5 flat bar supports 4 , 285 watt panels. Two 1 foot alu deck planks down the middle so I can walk on top to cover the main. The Bimini supports are similar to a tuna tower but all cant forward for a nice look. I copied almost everything from a yacht I saw in Newport.
Everything is welded except the supports, the solar panels and the walkway.
If a real bad storm is predicted, I use 4 cargo straps like truckers use only a bit smaller. I talked to two fabricators of tuna towers who just laughed because Ive got the strength of a tall tower but only one layer on the cake. I think the solar panels will blow out before the pipe frame. In very high winds, I had a hum which I cured with a little line on the center line of the Bimini .
It feels solid walking on top. Im not as big as my manatee friends but Im not light. Go talk to a tower builder ...youll be fine.
Happy trails to you.
Mark and his TIG welding manatees.
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Old 05-09-2020, 16:35   #7
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Re: Adding a Hardtop: how to calc windage/lift forces?

A Blue Water Sailor (maybe on this forum) stated he preferred a soft dodger on his (substantial) yacht as he was able to lower it and also place heavy anchors in the bilge in extreme weather. (I am sure his was a steel yacht)




Cooper Maple Leaf 48 Sloop


Arches, Biminis, Dodgers and Stability

https://www.cruisersforum.com/forums...ty-210830.html
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Old 05-09-2020, 17:09   #8
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Re: Adding a Hardtop: how to calc windage/lift forces?

The Administrator won't allow me to edit the above comment but what I wanted to add was this.

I think NoFacey's comment is highly relevant .

Wingssail “But I will say that when I meet the most experienced offshore sailors they almost always have the same preferences: Clear decks, no on-deck stowage (no jerry cans or kayaks), no arch, no davits, solar panels down low, and an easily removable bimini.”

Lol - birds of a feather must flock together. Your boat buddies with clean decks.....rounding Cape of Good Hope last December, amongst a shifting group of about 10 cruisers we’d known mostly since Australia, all had dodgers, most had biminis we sailed with, most had davits, and I can’t think of one that didn’t have a couple jerries on deck.
Agree the davits were all empty on passages - everyone aside from the odd cat carried their dinghy rolled up inside, or on the foredeck


https://www.cruisersforum.com/forums...-210830-3.html
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Old 06-09-2020, 01:36   #9
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Re: Adding a Hardtop: how to calc windage/lift forces?

Here is a calculator to calculate the force on a flat surface at various wind speeds.

My logic tells me that the wind force on a vertical board would be halved if it was rotated 45 but the other half would be converted to lift. If it was rotated further to 90 the force would be zero as the wind would be blowing on the edge: therefore no lift or sideways force.


Wind load on surface - Wind load calculator


https://www.engineeringtoolbox.com/w...ad-d_1775.html


Here's another one


https://www.wikihow.com/Calculate-Wi...oad-Calculator



Let's hear it from an expert.......
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Old 06-09-2020, 03:46   #10
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Re: Adding a Hardtop: how to calc windage/lift forces?

Here is a table


Of course the higher the Bimini is that will increase the leverage. The force on the hull at deck level will be the same as the force on top of the bimini but will have far less leverage

Wind speed converter M/s = MPH

https://www.weather.gov/epz/wxcalc_windconvert
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Old 06-09-2020, 05:09   #11
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Re: Adding a Hardtop: how to calc windage/lift forces?

https://www.dropbox.com/t/8d4uGtXbHlajNqPZ
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Old 06-09-2020, 06:07   #12
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Re: Adding a Hardtop: how to calc windage/lift forces?

Quote:
Originally Posted by LobeliaBlue View Post
Title kinda says it - the details:

I'm thinking of making a 'free floating' style cockpit hardtop to my 50'Cutter.
Imagine one of those Hanse hardtops, only for dirtbag cruisers C:

Since it'll have a curve to it that matches the cabin trunk roof, I'm wondering
if there's a way to calculate the lift that 10x10 surface will generate.

My early napkin drawing is for it to be held up at 4 corners of the centre cockpit,
with 3" pipe with 4x6 welded bases and supports. The main body will be the usual
honeycomb core with a few GRP layers and brackets for solar panels etc.

I'm pretty sure the 'yachta-palapa' will be fine on most days, but when the wind
kicks up i'm wondering if it'll be enough to make it fly and leave big holes in the deck.

I'm handy enough but no type of engineer. Has anyone tackled this project before?

Many thanks companeros.

What kinda winds are you thinking about?

We have a large hard bimini/dodger up and it easily has withstood sustained 60-70kt from Sandy a few years back. We only used 1.5" shed. 40 alum. pipe for the 4 supports for the bimini area and the dodger/ windshield is also thru bolted (6 main thru bolt sets for the whole structure). There is a full alum. grid under the top that adds a lot of rigidity to the structure. The top now is Coosa and has the added benefit of not needing the extra layers of glass, while strong enough to walk on.

Similar to Manateeman, we have heavy cargo tie downs if we ever get into a situation where we think we may need them.

Best plan of action would be to not leave the boat in a potential hurricane zone.
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Old 06-09-2020, 06:26   #13
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Re: Adding a Hardtop: how to calc windage/lift forces?

Quote:
Originally Posted by coopec43 View Post
Sorry!
I was experimenting and forgot to delete it.
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Old 06-09-2020, 17:09   #14
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Re: Adding a Hardtop: how to calc windage/lift forces?

If you include a boom crutch on the top, you could lower the boom and secure the main sheet.
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