Twin motors - Why?

About inboard or outboard motors.

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Andy Garrett
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Twin motors - Why?

Postby Andy Garrett » Tue Aug 09, 2016 7:20 am

I've been looking at the Hell Cat design.
It might just fit in the garage and give me a lot of room (and a great new project).

I've read about it and power catamarans in general, but I have a question on powering this and other boats in general.

In short:
What is the advantage of twin engines?
The catalog has a link to "A Word on Power Catamarans" which says, "For ultimate performance, twin motors are recommended."
This design suggests twin motors up to 100HP total. Why not use a single 100HP motor? It would be lighter, easier maintenance, far more cost effective...
Where does the performance advantage of twins come from (on this and other boats)?
Andy Garrett

Perhaps the slowest Zip build in Glen-L history...

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hoodman
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Re: Twin motors - Why?

Postby hoodman » Tue Aug 09, 2016 7:52 am

I love those catamaran designs. I think there is a way to mount the engine in the middle with a sort of mini hull like pontoon boats have. It may be detailed on the Glen-l website somewhere. I think back when catamarans were more popular it might have been cheaper to get two smaller engines than one large one. Or the larger engines not available? I'm not sure. Nowadays one engine is definitely more efficient than two smaller ones. Not to mention way cheaper. The complexity of twin outboards just doesn't seem worth it on a small boat. On the Bear-Cat or Wild-Cat I think twins would pretty much be mandatory.

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Andy Garrett
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Re: Twin motors - Why?

Postby Andy Garrett » Tue Aug 09, 2016 9:06 am

I've been studying this since posting.
Lot's of opinions of course...

The most scientific test was conducted by Yamaha on a Youtube video where identical boats--one with a 300HP and one with twin 150s were tested side by side.

With this set up, the results were interesting.
The single had a better hole-shot beating the twin to 30MPH.
They reached 40MPH at the same time,
and the twin got to 50MPH by about the same margin that the single beat it to 30.

All and all, speed and acceleration were a wash with the twin getting about 1MPH higher top speed.

The real story was fuel consumption with the twin burning about 4.5-5 gallons more per hour at 40-50MPH.

The 300HP is about $26.5K at 560lbs.
The 150HPs are about $29.8K for the pair at a total of 960lbs.

If this comparison is representative across the whole range of engine sizes, then it seems the single is the logical option.
Andy Garrett

Perhaps the slowest Zip build in Glen-L history...

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Bill Edmundson
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Re: Twin motors - Why?

Postby Bill Edmundson » Tue Aug 09, 2016 9:16 am

On a cat I can see twin engines. By the time you add that extra appendage and loose the efficiency, I'm not sure it's worth it.

On a mono-hull, I fully agree. The old thought of twins leave you an engine if one goes down just doesn't hold water. When you look at max HP for a boat, min. to plane, the min. to plane is way more than half of max. Modern engines are way more reliable than they once were. Your much better off buying the one big engine and a small "kicker" to limp home or troll.

Bill
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kens
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Re: Twin motors - Why?

Postby kens » Tue Aug 09, 2016 10:44 am

Twin engines does leave you with a redundant in case one engine fails, IF, you have redundant fuel tanks and all to go with it.
Twin engines drawing from a single tank makes no sense when you get a load of bad fuel, you loose both engines.
Twin engines makes no sense if connected to a single common electrical system, if electricals go bad, you loose both engines.

A single 300hp will have less underwater drag than 2x 150hp.
If you apply 300hp to a given boat all performance is exactly the same.
HP is what makes the boat go. period.
Now, how you apply that said HP enters a variance.
Single 300hp outboard may turn a 1:64/1 gear ratio to a 15" dia prop.
Twin 150hp outboard may turn a 1.88/1 gear ratio to 2x 15" dia prop(s)
you just DOUBLED your prop blade area. and turn a different gear ratio.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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Gayle Brantuk
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Re: Twin motors - Why?

Postby Gayle Brantuk » Fri Aug 12, 2016 8:50 am

Here is information on a deflector so you can use one motor instead of twins on our power cats:
http://www.glen-l.com/designs/outboard/aquacat-notes.html

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Andy Garrett
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Re: Twin motors - Why?

Postby Andy Garrett » Fri Aug 12, 2016 9:24 am

Thanks Gayle!

I hadn't even considered that concern.
Andy Garrett

Perhaps the slowest Zip build in Glen-L history...

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Bill Edmundson
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Re: Twin motors - Why?

Postby Bill Edmundson » Fri Aug 12, 2016 9:39 am

Thanks Gayle. I had not seen that detail.

Bill
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Ranny
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Re: Twin motors - Why?

Postby Ranny » Fri Aug 12, 2016 12:23 pm

I agree with Bill about twin engines not being worth it. I had a Pursuit with twin 225s (25 GPH each). When I blew a piston, I got to return 15 miles home in South Florida sunshine and humidity at 4-5 MPH.
Ranny

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Bill Edmundson
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Re: Twin motors - Why?

Postby Bill Edmundson » Fri Aug 12, 2016 1:18 pm

Ranny

You could have come back in on 10-15hp or less.

Bill
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