So realistically go slow enjoy the view and I can go for hours
Thanks my gut was pretty much telling me this I just don't know the math. My hope was that there might be some intermediate point at around say 4 mph where a foil would provide just enough lift to slow the exponentially drag increase you get with displacement hulls.
Thanks for the help
Nick
Sent from my Bell Samsung device over Canada's largest network.
-------- Original message --------
From: "Rick Willoughby rickwill@bigpond.net.au [harryproa]" <harryproa@yahoogroups.com.au>
Date: 2016-04-30 6:27 PM (GMT-05:00)
To: harryproa@yahoogroups.com.au
Subject: Re: [harryproa] Foiling Hobie Tandem Island
Nick
Without actually doing any calculations the 50lb thruster should get it to 6mph but that will require more than 80W. 80W should get the 4mph. The boat will have steeply rising drag curve above 6mph. If you were aiming to go faster than that using batteries then foiling could be a more efficient option than displacement. Taking L/D at 20 the drag would be 145N (32lbf). If you were to target 6kts (say 3m/s or 7mph) then the required hull power is 435W. Battery drain would be around 750W taking overall efficiency of 60% - that might require a better prop than supplied with the Minnkota. The Minnkota 50lb is rated at 600W (12V x 50A) drain on 12V battery so it would not be capable of the power. If you used a 4 cell (16V) lithium battery you could pull that sort of power:
The rapid development of drones and multirotor camera mounts has pushed the size of LiPoly batteries and the prices have come down. That linked battery is good for 200A. It will hold voltage around 15V at a drain of 50A. The 20Ah LiPoly battery would give last longer than an 80A lead/acid battery at 50A drain. An 80Ah lead/acid batteries are usually rated for 20 hour discharge so an 80Ah battery is good for about 4A continuous. If it is pulled at 50A the voltage will drop quite quickly.
If you do buy a LiPoly battery and have not used one before then ask me what you need to know about them before you do anything with it.
To fly 640lb at 3m/s would require a large foil; something like 3m wide and 0.3m chord. It would have some buoyancy so the actual weight to fly would be less than 640lb. To get the target L/D of 20 would require the foil being set about 1m below the boat. In my view this would not be very practical. A single 20Ah Lipoly would last about 15 minutes giving range of 1.5nm as a hydrofoil.
After doing these rough numbers for foiling I expect that 750W would get the boat above 6kts in displacement mode so the break even would be even higher power, which is beyond the ability of the Minnkota.
Conclusion is that you would need something substantially bigger than a 50lb Minnkota to consider foiling over displacement on this boat.
Note that the LiPoly battery is worth considering. That 20Ah battery weighs 1.6kg. It will run the motor for 4 hours at 80W giving roughly 4mph or range of 16 miles.
Rick
On 30/04/2016, at 8:15 PM, "63urban 63urban@gmail.com [harryproa]" <harryproa@yahoogroups.com.au> wrote:
Ok I don't profess to have understood all of your post but I think I have the answer. The boat is 19.5 feet long and fully loaded for camping including bodies might be 640 lbs. I have a 50 lb thrust minn kota and two 40 watt solar panels. I have yet to put them in the water as we are just getting into nicer weather here so I don't have any speed numbers.The jist is that there is no advantage at speeds of say about 4mph. Which is what I think will be my most efficient speed currently.Nick
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Posted by: 63urban <63urban@gmail.com>
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