Flying Saucer 4-stroke horsepower?

Outboard designs up to 14'

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WillieNC
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Flying Saucer 4-stroke horsepower?

Postby WillieNC » Fri Aug 25, 2017 4:51 pm

We are building a two-cockpit Flying Saucer as a project and raffle prize for the North Carolina Maritime Museum.
Our watercraft shop is mostly devoted to sail and oars--I'd like some comments on horsepower for this boat:
Assuming a four-stroke short shaft electric start motor, we are thinking of a 25 hp engine.
Does anyone on this forum have experience with such a motor?
Too big? Too small?
Anyone have experience mounting such an engine into this hull?

Any advice much appreciated

Best,
WillieNC

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BarnacleMike
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Re: Flying Saucer 4-stroke horsepower?

Postby BarnacleMike » Fri Aug 25, 2017 7:44 pm

If you are building it to its standard 12' 3" length, then a 25 hp motor will be about right. That is actually the maximum horsepower it is rated for, though some people have used 30 hp and larger motors on them. (For that matter, some have even used 15 and 20 hp).

25hp should be about right.
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galamb
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Re: Flying Saucer 4-stroke horsepower?

Postby galamb » Fri Aug 25, 2017 9:56 pm

If you are building roughly to spec, so will have an (unrigged) 190'ish pound hull weight it will "fly" with a 25.

My 14 footer, 250 pound unrigged hull weight is powered by a 15 horse and it makes 21 mph with myself (205 lbs) and the wife (just a couple pounds over half my weight and I'll leave it at that :) ) plus another 100 pounds of (junk) gas, anchor, small battery to power the gps/lights etc.
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Re: Flying Saucer 4-stroke horsepower?

Postby PeterG » Sat Aug 26, 2017 5:45 am

Something to consider is the overall weight of the boat, motor and gear. Try not to exceed the design displacement of the boat. If it's not given on the plans it will take some pen scratching. Using Simpson's Rule and the plans you can calculate it (displacement is not the stated hull weight). But to give yourself the best weight margin for people and gear without playing naval architect, try to go with the lightest outboard for the horsepower you want. Typically the 2-cycle outboards are lighter than the 4-cycle so shop around and see what you can find.
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WillieNC
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Re: Flying Saucer 4-stroke horsepower?

Postby WillieNC » Sat Aug 26, 2017 7:44 pm

Thanks for your comments.

I reviewed options for Mercury 4-stroke outboards and we have made a tentative decision.

Their 20 hp motor is a two cylinder with carburetor-- it weighs 115 pounds.
Their 25 hp on the other hand is a 3-cylinder EFI unit weighing 175 pounds. On the light Flying Saucer hull, we've decided that the extra 60 pounds isnt worth it for only 5 additional horsepower at the top end. So, the plan is for a 20 hp engine. I would rather have EFI for better fuel economy, but this engine ought to be okay for the limited excursions likely to be its fate. We are planning to use a smaller gas tank than the customary 6 gallon one, again for weight savings.

On another point: for the electric start, does anyone have experience using a small garden tractor-size 12 volt battery in place of a Size 24 automotive battery? Again--weight saving issues.

Comments on these points, anybody?

WillieNC

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galamb
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Re: Flying Saucer 4-stroke horsepower?

Postby galamb » Sun Aug 27, 2017 6:34 am

So first I will address your battery question.

A garden tractor battery will work fine. I have used one personally (and know a couple other guys as well) that have used them to start our vintage motors that don't have charge capability. If the motor starts up easily (so you aren't cranking it over and over before it fires) you can get almost a long weekend out of the battery, of typical starts.

Given that every electric start model (at least the current ones), have charge capability, if all you are using the battery for is starting a 20 horse, it will work fine, recharge fine and probably last you as well as a cheapie marine group 24.

Now I'll throw my 2 cents at the motor.

I'm a Merc guy, have been for years but given that the 30 horse and below Merc's are made by Tohatsu and are mechanically identical to them - Tohatsu just painted them black and smacked Merc stickers on them; in this case "brand loyalty" would extend no further than the decals.

I will say the 20 horse platform that they use is unique. They designed it purposely to make a "light" 20 horse for smaller/lighter utility boats. But as you note, it kinda sucks that they are carb'd.

For essentially the same money you do have options. Suzuki makes a "fuel injected" 20 horse (DF20A) which comes in at 106 or 108 lbs for the electric start model (short/long shaft).

If you still would prefer the Merc then I will give you one more thing to consider. IF you have a Tohatsu dealer even relatively close you may want to buy the (model) from them - no it won't say Merc on the side but: When you buy the Tohatsu they "include" the prop, the control box and installation of the motor - you need to provide the steering but they provide the throttle and shift cables along with the controls and that's all included in the price of the motor itself - not "add on's" and you get a 5 year warranty.

When you buy "exactly the same motor from Merc" you get the "motor" and a prop with a 3 year warranty. And the price is competitive.

(disclaimer - while I work on outboards I am not affiliated with Tohatsu or Merc or any brand so my "plug" for any brand is purely my own opinion)
Graham

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WillieNC
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Re: Flying Saucer 4-stroke horsepower?

Postby WillieNC » Sun Aug 27, 2017 6:50 am

Graham:

Many thanks! That reply was very informative and certainly gives us more food for thought.

We'll keep reporting on the status of both the boat build and the motor discussion.

Yesterday we cut and installed the chine logs. Now I'm headed down this morning for us to install the shears.

Best,

Willie

craigvansickle
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Re: Flying Saucer 4-stroke horsepower?

Postby craigvansickle » Fri Oct 13, 2017 11:36 am

I'm happy with my 25hp 2 stroke but will be going to a light 30 four stroke due to local regs. Many of the 25s are detuned 30s. The FS design tapers quite a bit to the transom and doesn't like too much weight aft.

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Bill Edmundson
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Re: Flying Saucer 4-stroke horsepower?

Postby Bill Edmundson » Fri Oct 13, 2017 1:09 pm

Here is something to remember. When most of these boats were designed outboard engines were rated at the flywheel. They are now rated at the prop-shaft. That means that an old 25 HP might be rated 20 HP now.

Bill
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Re: Flying Saucer 4-stroke horsepower?

Postby Hercdrvr » Sun Oct 15, 2017 9:09 am

If it were my F saucer build, I’d go for the 30hp and move the tank and battery forward. My Squirt with a 4 stroke 20hp is about right so more would be my choice for the bigger F-saucer. When looking at basic hp/ speed numbers, I’d say a 20 hp would run out at 23mph with a couple guys in it and 28mph with a 30hp.
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