Glen-L.com

Over the last few months my dad and I have been able to get some real-world performance numbers with our Zip, powered by a 1956 Merc Mark 55 (40 HP). Our hull is on the light side, so we guesstimate our on-the-water weight with two guys on board to be 750--800 lbs. Our motor runs well, but we suspect that its compression numbers may be on the low side, and we're running the cr*ppy pump gas sold in California. The numbers, with a variety of two-blade props, are as follows:

1. 30 MPH @ 5,000 RPM with 11"-pitch brass prop

2. 28 MPH @ 4,800 RPM with 13"-pitch brass prop

3. 30 MPH @ 6,000 RPM with 10"-pitch aluminum prop

The "hole-shot" capability of the last prop tested was impressive. The boat just launched itself onto plane. For the record, our outboard weighs roughly 115 lbs. and is trimmed at the lowest angle. We did make one run with me in the aft cockpit, and we were able to induce some porpoising. The Zip is a small boat and is CG-sensitive.

Since the fuel issue has really grown legs on one of the old-Merc forums, our plan is to repeat this test using 100LL avgas. If we don't see an increase in performance, we'll rebuild the Mark 58 sitting in the garage and see how things pan out with another five ponies.

Mark Shipley

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A boat is just a wooden box with no right angles.

Thats an interesting set of numbers. There are some that still uses those engines on some of the home built boats too.

Mark,

30 mph sounds good but having never ridden in one dont know if that would feel fast or not.

Thanks for the data. I too plan to use a 40hp motor in my Zip. I was hoping for enough power to pull a skier.

I would think 100LL would be good for a couple miles an hour. We run it in some of our vintage motorcycles with good results. The real advantage is the shelf life of 100LL is almost infinite. I hear the shelf life of pump fuel now is less than 6 months. In these small engines that means carbs can gum up much more easily. If you have access to it I would run 100LL just for that reason.

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My Zip build log...
http://www.vupilot.blogspot.com
http://picasaweb.google.com/vupilot/Chr ... O0x7SvsQE#

"Nothing screams poor craftsmanship like wrinkles in your duct tape."

you should only get a boost in performance IF the motor needs the octane(due to compression ratio, timing etc). If not you can actually loss power (although its a very small amount) due to the slower burning of higher octane fuel. Normally speaking as octane goes up the burn time of the fuel goes up as well. Or at least that is my understanding, maybe I'm out to lunch.

We feel that we'll see a slight performance increase with the 100LL due primarily to its increased energy density when compared to the alcohol-laced "politician's blend" sold in California. Old Mercs were designed to run on 80-octane white gas; it's the alcohol that's the problem. It also does bad things to crank seals. If airports still carries 80/87-octane avgas, it would work as well.

Vupilot is correct in that the FAA requires aviation fuels to remain stable for two years, while automotive unleaded goes stale in a matter of months. The big problem is finding an airport around here that will sell it "over the counter."

30 MPH in the Zip is fun. It's a small boat and you're close to the water, so the sensation of speed is definitely there. We just want 33% more of that sensation...

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A boat is just a wooden box with no right angles.

The problem with buying the 100LL at the airport is the road tax issue. Just give them an N number and tell them its for a sea plane or an air boat.

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John Amundsen
Monte Carlo
Winter Haven, FL

Maybe it will be done by G6 and maybe not.

In 1959 we had 14' Tollycraft Mahogany runabout(about the size and weight of a Zip) powered by a 35hp Johnson. The "stock" prop was a 10-3/8"Dia. x 13-1/2"pitch Aluminum. We went to a 10-3/8" X 12" Brass prop and gained a lot of pulling power. We pulled 2-3 skiers all the time and once did 4...The stck prop was slightly faster, but didn't pull as good out of the hole. We later put a 22 gal. tank under the rear seat, so we could ski all day without having to change tanks every 2 hrs or so...AND lost very little performance. The 35 hp always out pulled the 40 hp Merc, because they were never propped right. I did know one person who put the right prop(3 blade)pitched right and it would pull 3 skiiers. The Mercs all came with 2 blade props, which were fast but would not pull...Just a few thoughts from and old fart...Dwain, The Ski King..........

Dwain,We were all much smaller people back then!!!

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Boat building can best be defined as an endless series of
tragedies obscured by the occasional miracle, followed by a good bottle of beer.

Don't Dream Your Life, Live Your Dream

Exactly the way it is supposed to work. (and why ethanol degrades performance)
You want the minimum octane the engine will run on without preignition.

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Some plan to stroll through Saint Peter's Gates, I plan to go through them at 150mph... backwards... in a screaming ball of flame, with a glimmer belt wrapped around my head, and a NOx button in my hand.

uh.... but I'm thinking that if you increase compression, the ethanol would improve performance. I'm mean, if you could distill your own ethanol, you could make up to 105 octane and that would be sweet but only in an engine with some high compression. Am I right?

Ok, yes I know that you would have to apply to the gov for permission to distill it and then you would have to "denature" it so that you couldn't drink it and the best way to do that would be to mix in a little gasoline and then your compression would go down. But I'm being hypothetical here.

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Isn't it amazing!! The person that never has the fortitude to pursue his own dreams, will be the first to try and discourage you from pursuing yours.

I'm going to talk about e85 but most of what I'm saying would apply to home brewing your own fuel too. E85 is 85% ethanol and yes its about 105 octane and would allow you to run 12:1 compression with not issues. (possibly even higher) The added compression along with the other goodies is what makes the extra HP. What you need the high octane for is to stop the motor from pounding itself to death with pre-ignition and detonation. There is more involved then just slapping in a set of domed pistons in a motor and calling it a day. You need to build a fuel system that can resist the rubber degrading effects of that much ethanol (ie most rubber parts in the system would soon fail unless upgraded to alcohol resistant products. PLUS running that much compression means you cant EVER go back to regular pump gas without a total rebuild of the motor to decrease compression. ALSO when your running alky your carb has to be setup to flow a lot more fuel (I believe its in the neighbourhood of 25%) so even if you found high octane gas to run your 12:1 beast on you couldn't just fill your tank with it and run it as you would be SEVERELY over jetted and running way too rich. So there are some issues with going this route. PLUS the availability of e85 or home brewed allky fuel is a little suspect and could make it difficult to wander far from home with your boat.

BUT with all that said if e85 were more commonly available I would run every toy I own on it. Its the cheapest fuel you can get that has that much octane. BAR brewing your own that is. Which has a much higher start up cost but then your price per gallon is pretty low.

Just remember compression is in effect free HP. This is a general rule of thumb but for every compression point you go up you gain roughly 25HP. this is a very general rule of thumb and not written in stone. There are other factors at work here besides increasing compression such as cam and ignition timing that will all effect how much power you gain.(which could be more or less then the 25hp I mentioned)

boy am I ever chatty tonight...lol

I installed a flex-conversion set (just a chip, injectors and sensor) in the DA's wheels. The painfully slow 5-popper got even worse on E-85. It was so bad I worried about getting "the bird" from passing nuns for impeding traffic.

I meant in an existing, mostly-stock gasoline burning motor. In a motor designed for pump-gasoline alcohol is nothing but a fuel-dilution. But yes, in your hypothetical context, if you want to mill for O-rings, hunt up some HC pistons, and rebuild the fuel system for increased volume of corrosive high-alky fuels (not to mention it eating the alloy case on a 2-stroke), increase lubrication pressure to support the added bearing load, be sure that pressure doesn't wash a bearing out (love that clunking noise!) Tolerate frequent oil changes then yes, there is more HP to be had, but at an astounding increase in fuel consumption. Alky burns allot cooler, great for dragsters, not so much for boats.

Ever enjoy starting a cold, blown, alky on a 45 degree spring day? That is a load of fun. Even the mallory magneto has trouble getting a spark to light it off. (insert the notion to use a shot of ether... and the realization that sitting in front of a scoop attached to a roots full of vaporized dynamite is likely to make me look like Kojack... with no eyebrows)

My strange snowmobile drag racing buddy (no accounting for taste, eh?) calls his 1-liter turbocharged 2smoker on alky his "grenade with a 5 second fuse". Normally I like my boating expeditions to last longer than that. (a few haven't )

Any fuel beats a boat powered by a bedsheet tied to a post.

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Some plan to stroll through Saint Peter's Gates, I plan to go through them at 150mph... backwards... in a screaming ball of flame, with a glimmer belt wrapped around my head, and a NOx button in my hand.

I've had a few hand grenade motors in the past. There fun for a while...lol