I want to put a hydrofoil on my cat. i have a design made up and i have done some calculations for it and it does seem very possible to do. i posted a pic on my account of the general idea. i would make a new centerboard that has hydrofoils that fold out into a V. the rudder will have a T shape foil with adjustable trim for tuning. The centerboard will carry about 365lbs and the rudder will carry about 130lbs. but for starters i might make strap on ladder foils. the original owner of my cat told me he did it on my boat but his broke on a motor boat wake.
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FYC, Nacra 5.2 "Chris's Flyer" & Nacra Playcat & Farrier Tramp
Previously owned: Trac 14, H14, H16, H18, N5.0, G-cat 5.0
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H-18 hydrofoil
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Posted: Jul 16, 2009 - 11:46 AM
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Posted: Jul 16, 2009 - 12:07 PM
You have some large goals for your h18 (pull waterski'ers, add hydrofoils, etc)...
please be sure to keep us posted -
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Posted: Jul 16, 2009 - 01:39 PM
I am thinking of using hydrofoils for my senior design project. i will post everything to break the bubble of people who wont publicly share thier plans
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FYC, Nacra 5.2 "Chris's Flyer" & Nacra Playcat & Farrier Tramp
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Posted: Jul 16, 2009 - 03:39 PM
I would be interested to see how you calculated the loads the boards and rudder will be carrying. I think when the boat is under power there is a lot more force pushing on the boards then 400lbs. Also if you get the boat out of the water all the lateral force currently taken by the length of the hull will be focused on the rudder and centerboard. Just a thought but good luck with the project, sounds interesting. Also what will keep the bow from diving? Will the rudders have a downward thrust to keep the boat level?
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Scott,
‘92 H18 w/SX wings
‘95 Hobie Funseeker 12 (Holder 12)
‘96/‘01/‘14 Hobie Waves
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Posted: Jul 16, 2009 - 04:14 PM
well. a pontoon wieghs roughly 200lbs and i am 165 my gf is 130ish. the torque from the sails will equal that unless we are flipping. the faster the boat goes the less torque there will be. after figuring the wieghts you have to find there independent center of gravities and then do a wieghed average to find the absolute CG. after that you do the reverse to find what the centerboard and rudder must hold. i think that i will make a foil for the front of the pontoons to lift them up. the foil will have the slipping load on it. i do not think that the 1980 centerboard well and my delaminating stern will stand up to the abuse. also the force on the boards is just in the vertical axis. the torque on the wells would be close to normal. since the rudders have wieght on them i can trim to boat to stay relativly level(depends on crew postion). nose diving could be fixed with the standard skii on the bow but i want to see if it does it first.
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FYC, Nacra 5.2 "Chris's Flyer" & Nacra Playcat & Farrier Tramp
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Posted: Jul 16, 2009 - 04:32 PM
Rob,
It has been a few years since my last mechanics class but I think you may need to sketch the free body diagram for this situation again. Consider the condition where the boat is up on the foils and rudders of the lee hull alone and I think you will find that your foils will have to support a lot more than you think. Do not forget to consider the load caused by the wind.
Disclaimer: I may very well be wrong. As a civil eng. I design airports and roadways not boat parts.
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Posted: Jul 16, 2009 - 04:45 PM
i think what smfinley was hinting at is that your not taking account the lateral forces on the boat/centerboards and rudder. the outer sides of your hulls and centerboards keep the boat from slipping sideways because of the force of the wind on the boat. with the hulls out of the water there is going to be all of that force on your centerboards and rudders. your probably going to have to build yourself some stronger centerboards and rudders beacause i dotn think the stock hobie ones will hold up to that. my buddy broke his two old 16 rudders just under regular sailing (not at the same time). just somethin to think about -
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Posted: Jul 16, 2009 - 08:52 PM
I also have not had a mechanics class in over 10 years, but interesting how many engineers there are on this site : ) 1473flyers, yes that was one issue I was hinting at.
Here is another,
In simple terms the torque from the crew and windward hull (against the leeward hull) will equal the 400+ lbs times the 8' beam of the boat right (3200 ft*lbs)? The torque equation for the sails gets more complicated when the mast is connected in the middle of the crossbar and anchored to the windward hull. I have had most of the leeward hull on a SC20 under water, and thinking the only way it got there is the force pushing down displacing the air volume of the hull.
Assume H18 hull was 18' long and averaged 1' wide and 2' tall (just my SWAG). Half the volume of the hull would be 18 cubic feet. Water is 62.5lbs/ft^3 so half full of water you would have 1123 lbs of water in the hull. So if the hull was dry but half under water you have 1123 lbs of force pushing it down. 3/4 under water would be 1685 lbs.
I am sure the force comes from many different dynamics, but I think it will be much greater then your initial estimate. Also think about the additional forces pushing the centerboards backwards when trying to lift the boat up while moving forward, there is a lot of drag forces in water.
This does sound like a great senior design project. Always nice to have a project that is both educational and interesting at the same time. Can't wait to hear how it goes, don't let us stop you, just wanting to make you think about every possibility.
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Scott,
‘92 H18 w/SX wings
‘95 Hobie Funseeker 12 (Holder 12)
‘96/‘01/‘14 Hobie Waves
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Posted: Jul 16, 2009 - 10:43 PM
No intention to discourage you--just the opposite. It has already been done:
http://www.foils.org/gallery/carlson.htm
More info on sailing foils:
http://www.foils.org/gallery/sail.htm
Highly appreciated to keep us posted or even make plans available. In the back of my head is the plan to equip my P18-2 with hydrofoil wings one day...
Can we see a better pic of your GF ? :) -
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Posted: Jul 17, 2009 - 09:20 AM
i have done some research on the topic. the hobie 18's dagger boards are designed for the side load. without them the boat would drift rapidly.
I think that if we consider a static situation on a cat sitting flat, on hydrofoils, with a crew on trap, the forces on each hull are the same. since the boat is going faster the sail becomes more efficent and the boat heels less. with hydrofoils you wont be on trap much.
the drag will be considerable. that is why you need 8-10mph wind for it to work. all of my testing/crashing and building process will be on youtube. infact i already have a bunch of interesting videos. search rob hangen.
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FYC, Nacra 5.2 "Chris's Flyer" & Nacra Playcat & Farrier Tramp
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Posted: Jul 17, 2009 - 09:21 AM
I keep better pics for my own viewing
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Posted: Jul 17, 2009 - 11:35 AM
thanks for the link catdan. it was very helpful. i will definitly be striving to be the first to tow a skier with a foil cat
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Posted: Jul 19, 2009 - 11:45 PM
I don't know anything about the physics of this project nor much about foils. I love the idea of hydro foils and the flying over the water. I think the modifications for the boat sound cool and you should totally do it AND post lots of pictures! -
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Posted: Jan 20, 2011 - 05:21 PM
Just found this picture on the web. I just had to post it for everyone to see.
This is world-renowned moth sailor Rohan Veal on a C-Class cat with foils.
Hopefully that project of yours looks just as good.
http://www.foils.org/gallery/cclass2.jpg
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AJ Sawyer
Nacra 20
San Diego, CA
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Posted: Jan 21, 2011 - 02:52 PM
The person to talk to about foiling is Dave Carlson. His website is the one CatDan mentioned above. I spoke to him at length just last weekend about his foiling efforts and he said that the Hobie 18 was way too heavy. He has a wooden A cat (Catnip) that weighs 200 pounds and he can foil that thing all day. Of course, he is about 165 pounds and he doesn't tow skiers.
Not to be negative about the Hobie 18, but it looks like you'll need 40+kt winds (on a beam reach) and completely flat water to get the 18 to foil. Plus, your girlfriend will have to watch the carnage from shore.
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Posted: Jan 21, 2011 - 03:06 PM
I need to add to my last reply--he said he got the 18 to foil, but it would quickly stall, then crash. It was something about the speed and weight and the technology of the foils he was using. He is being towed in one of the pictures, which I think was his last attempt at that boat before he gave up and went with the A cat.
What started as a friendly conversation turned into a PhD level dissertation on hydrodynamics, angles, etc, which made my brain hurt. That and the beer.
Seriously, if you are working on a thesis, do what you can to contact him. The email link on the site doesn't work but he posts on CatSailor regularly. He is a very nice guy and he really likes to talk foiling. -
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Posted: Jan 21, 2011 - 11:36 PM
