Skirt Wipe

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chris
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Skirt Wipe

Post by chris » June 7th, 2018, 6:23 pm

I own a 7 x 14' Vanguard with 18 HP kicker. I can cruise at around 18 to 20 MPH with it at 2550 to 2700 RPM. If I open it up, I can get 2950 RPM. Sometimes that extra throttle adds a lot of extra speed, and other times it does not. I've noticed that if I can get it perfectly balanced so that there is no quarter skirt wipe, it will climb up to 25 to 30 MPH. However, the slightest wind change, or ripple, or even thrust change causes the quarter skirt to make contact with the water, which causes the craft to want to rotate. So often adding throttle doesn't get me any more speed. I don't seem to lose speed so I'm not talking about a plow in - just drag. The back end feels very lose throughout all of this, so it can be a bit of effort to keep the craft pointed straight as it is like driving on ice with the front brakes grabbing :) Does that sound normal? Does this become more prevalent with longer crafts? I imagine more lift air would likely minimize this.

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bphillip2
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Re: Skirt Wipe

Post by bphillip2 » June 13th, 2018, 6:33 am

Generally the craft sounds a bit slow. My 18hp 14x7 Vanguard tipped the scales at 345lbs empty, seat and no fuel. What does yours weigh? At my weight of about 185lbs I could repeatedly hit 32.4 mph in perfect conditions. The bow would rise up in this condition. Otherwise 25 mph was very easy. I have experienced the skirt drag you describe. Some craft have had poor skirt tailoring, incorrect trim, heavy drivers too far forward, or lots of rough patching on the skirt.

chris
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Joined: August 9th, 2017, 6:19 pm

Re: Skirt Wipe

Post by chris » June 17th, 2018, 2:11 pm

I agree, my performance is slightly poorer than some of the stuff I've read by Barry on alt.rec.hovercraft.

Craft is 385 lbs and I'm 250 lbs. My best case is 105 lbs more than you, but I also had a full 5 gallons of fuel, one chair, small socket set, GPS, lunch, fishing rod and fishing tackle box with me.

The skirt might not be optimally set up.

Have you noticed much for elevation impacts? I'm at 2850 feet. One website suggests I lose 1.4 HP relative to friends at sea level. That isn't much, but it might be 10% of what I have available at these RPM's with this engine.

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bphillip2
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Re: Skirt Wipe

Post by bphillip2 » June 17th, 2018, 8:16 pm

The elevation is going to be noticable, I've run at 6100ft and it takes about 1/3 off.

chris
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Joined: August 9th, 2017, 6:19 pm

Re: Skirt Wipe

Post by chris » June 21st, 2018, 5:51 pm

I can't do much about the elevation. I could do something about skirt trim. Can you detect skirt trim issues by looking at a video? If so, I wouldn't mind your opinion on what you see in these videos:

https://www.youtube.com/channel/UC1Vkfd ... Hu9UqBtHIg

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bphillip2
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Re: Skirt Wipe

Post by bphillip2 » June 22nd, 2018, 7:38 am

The tailoring of the skirt looks good. The stern looks slightly low. Did you setup the craft on a level surface and trim the partition? It should be above the water perhaps an 1".

What does the craft weigh?

chris
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Joined: August 9th, 2017, 6:19 pm

Re: Skirt Wipe

Post by chris » June 22nd, 2018, 8:43 pm

Thank you for checking it out!

I have put a 2 foot long level on the floor when running, and I need to lift the back end of the level about 1/2 to 3/4" of an inch to bring the floor into level. I hope this makes sense? I'm wondering if anyone else has ever done the same and knows how "flat" their craft flies?

I bought this craft second hand. Having said that, I did have it up on blocks to check skirt trim last fall. I don't believe the partition skirt had that much air gap. I think it was closer to 1/2". Now I'm wishing I had taken some pictures, because I don't honestly remember. Should the bow skirt be trimmed perfectly level with the floor when propped up to the right height?

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bphillip2
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Re: Skirt Wipe

Post by bphillip2 » July 7th, 2018, 4:18 pm

With the front propped up at flying height you need at least a 3/4" gap on the bow skirt. I don't block the stern up, just run the engine up enough that it is fully lifted aft, maybe 2100 rpm. The partition needs to be trimmed high enough to allow air to come forward, but not so much that the bow collapses.

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