twin 70hp

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jenny
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Joined: Sun May 24, 2015 9:06 pm

twin 70hp

Post by jenny » Sat May 30, 2015 7:24 am

Hi I recently bought a x coast guard boat which had twin 70hp outboards I always under the impression that this would give you 140hp. Some people are saying to me you don't get 140hp total hp. I'm thinking about mounting just one outboard , what size hp will do the job? thanks
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Roberta
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Re: twin 70hp

Post by Roberta » Sat May 30, 2015 7:40 am

Might be helpful to know what you plan on mounting the motor to.

Roberta :?:
Roberta "Queen of the Boat Builders"
Built Zip "Oliver IV", Super Spartan "Jimmy 70", and Torpedo "The Glen L".

gdcarpenter
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Re: twin 70hp

Post by gdcarpenter » Sat May 30, 2015 8:35 am

Twin outboards versus single is 'moot', that is to say debatable.

One 140 HP will weigh less than 2 70 HP engines, and is considerably simpler to install and operate. Twins provide a 'back up' in the event one engine dies.
This is my first, last and only boat build.

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Bill Edmundson
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Re: twin 70hp

Post by Bill Edmundson » Sat May 30, 2015 2:10 pm

The advantage of twins really doesn't apply anymore. One 70 hp will probable not get you on plane. Much cheaper to get 1 large engine and a small kicker to get you home. You can troll with the small engine for almost nothing. Modern outboards a very reliable. Spend the extra money for a good VHF to call for help.

My opinion :wink:

Bill
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

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Andy Garrett
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Re: twin 70hp

Post by Andy Garrett » Mon Jun 01, 2015 6:42 am

What Bill said.
Andy Garrett

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

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galamb
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Location: Inverary, Ontario - Cuddy Sport (modified)
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Re: twin 70hp

Post by galamb » Mon Jun 01, 2015 4:42 pm

Many years ago I had a fishing rig that came with twin 25 horse motors (marketing ploy or cool factor - not sure which, but that was how it was rigged).

Anyhow, the "twins" did not produce the same top end speed or hole shot that I got when they were ultimately swapped out for a single 50. The weight was a little less with the 50 but not enough to account for the difference.

What I did notice was an increase in fuel usage. The twins didn't use double the gas a single 25 would - running both wide open used about 10-15% less fuel than the 50 (but as stated, with less top end speed - approx 5 mph, so enough to notice, at least on the speedo).

The motors were (originally) a pair of Merc 25XD's which were replaced with a 50 Merc (all 2 stroke carb engines).

It was nice not to have to run both all the time. The way I used the boat I would fire up both to "get out there" then go from fishing hole to fishing hole with one engine only (alternating between the two to try and keep the hours balanced).

I changed them out because you are running two sets of cables, have two motors to maintain, needed "add-on's" for the fuel feed, steering etc etc - ultimately the "marginal safety factor" of "not being stranded" did NOT outweigh the added upkeep work and costs.

Not having a way to test I could not then (or now) have told you whether the twin 25's put out as many horses as the 50 or not. The 50 could turn a "bigger" prop than the two independent 25's and the greater displacement of the 50 gave me a different torque curve - all together or individually could account for the better top end speed.

So for all my babling -

I would calculate the maximum horsepower for your boat (using the Coast Guard formula), unless a max hp tag is provided on the hull and rig it up with a single (main) outboard that is at least 80% of the max allowable.

If you want "insurance", figure out the hull speed and then tack on whatever (portable) will make that speed for you - a 15 horse sail shaft or big foot might be all you need and could also be used for trolling - getting in/out of the docks/no wake zones etc.

That's my (formula) for my Cuddy Sport which has a 140 horse "main" and an 9.9 horse Sailshaft to "kick" with :)
Graham

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kens
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Re: twin 70hp

Post by kens » Sat Nov 14, 2015 2:35 pm

Put a single 175 or 200hp on it.
Then when you cruise you can be at approx 50% power and be up on plane cruising
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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