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Old 19-02-2019, 15:30   #1
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twin 15-20hp outboards on tender

to get up to speed, I am building an all aluminum console tender similar to what Oceancraft in Australia builds for my personal use only.

I am not looking for what size I should run, can decide that on my own.

I have been considering either a 30 or 40hp OB but have been toying with the idea of using twin 20 or 15hp OB the weight is either the same or less than a single large OB



benefits:

-short haul you can run only one motor to save fuel or fire up the 2nd when you have a heavy load, need the speed or just dragging the kids on watersport stuff
-if you are several hours away fishing or up a river etc. and have engine trouble you will have a backup to return
-weight, if you need to remove the motor for long passage or maintenance ~100lbs is doable

negatives:

-slightly more complicated steering linkage (remember this is a console boat so no tillers)
-extra maintenance
-easier to steal

please keep it on topic and discuss
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Old 19-02-2019, 16:23   #2
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Re: twin 15-20hp outboards on tender

I don't have an easy answer for you, but I do have some comments on your rational...

With modern outboards the reliability argument in favor of twins is much less compelling than it would have been 30 years ago. In fact, I think you are actually reversing the equation.

Twice as many engines means (almost) twice the number of failures. "Almost" because a very significant fraction of failures would affect both engines (fuel, steering, electrical, bottom impact...).

Twin engines are never going to be as efficient as a single, although with really good set up and propping they might come close.

No way anybody is lifting a 20 HP engine off the back of your dinghy to steal it. If they steal it, they are going to drive it away, just like the 40, so that should not be in your criteria.

If you have the lifting gear to hoist a tender of this size surely you can hoist a 40 HP outboard off of the boat?

Lastly, you might double check your statement that the two smaller engines are less than the weight of the larger one, and be sure you are comparing apples to apples. I see a 40HP Yamaha speced as 215 lbs, and a 20HP Yamaha as 132 lbs. Just one data point, but the only one I looked at. Be sure you are comparing the same shaft length, and electric start on both (assuming that's the way you would go!)
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Old 19-02-2019, 17:30   #3
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twin 15-20hp outboards on tender

Two engines will be heavier on average, yes you can spoof that number for example the 9.9 Suzuki and the 20 both weigh 99 lbs, because they are the same motor.
You wonít save much if any fuel running one small motor at a higher RPM than a larger one at low RPM.

I donít know but would suspect the cost of twin 20ís is more than a single 40.

Way back many years ago we had a fishing boat with twin motors for safety.
First thing I noticed as a kid that even though it was fast with both motors, it would not plane on one, even with the other trimmed out of the water, reason is it was propped for efficient twin operation and one motor was way over propped so much so that on one engine it would just bog.
We had different electrical systems for each motor of course, only problem I ever had was a whole lot of water in the fuel one time, one gas tank of course so having twin motors as a back up was never used.
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Old 19-02-2019, 18:03   #4
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Re: twin 15-20hp outboards on tender

ok, good comments fellas thank you.

I never did think of the propping, makes sense that is enough to make me throw this idea out as tuning them to work well together could be a pita

that said to answer a couple questions

doing twin Yamaha 20's 130lbx2 vs 220lb there would be weight gain.

twin Tohatsus come in 17lbs or so less than the bigger 40

those are the only two I am considering

yes I will be putting davits on the vessel to match the tender

thanks
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Old 19-02-2019, 18:17   #5
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Re: twin 15-20hp outboards on tender

Quote:
Originally Posted by ironvalleyfab View Post
to get up to speed, I am building an all aluminum console tender similar to what Oceancraft in Australia builds for my personal use only.

I am not looking for what size I should run, can decide that on my own.

I have been considering either a 30 or 40hp OB but have been toying with the idea of using twin 20 or 15hp OB the weight is either the same or less than a single large OB



I think you and I are in a different snack bracket but have at er..
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Old 19-02-2019, 21:18   #6
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Re: twin 15-20hp outboards on tender

I believe that twin motors also need to be running in opposite rotational directions
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Old 19-02-2019, 21:34   #7
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Re: twin 15-20hp outboards on tender

Smaller boats donít need counter rotating twins. You will have some prop steer which can be fixed with proper trim while on plane, but itís an added risk that should probably be avoided. (Hard pull to the right while planing)

I have a Yamaha 15 2 stroke on my dinghy and twin Nissan (tohatsu) 70s on my old aquasport bay boat. Go with a single. Safer, more efficient, and less trouble.
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Old 19-02-2019, 23:48   #8
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Re: twin 15-20hp outboards on tender

Here's my take on it, which is the opposite of most others here I read:

(oh, and congrats on the design; I bought an oceancraft 3.3 meter and run a 20hp on it, that thing is a beast and i'm in love)

I'm making two assumptions:
1) If you weren't gonna use your tender for big things, you would have probably made/bought a smaller one. Ie diving far from the boat, running miles to explore, going far enough off the beaten path that the risk/loss equation becomes more interesting.
2) You have the room, the winches, and the money to do as you please more or less.

With these two things in mind, I would not hesitate to do dual engines with as much independence as I could manage, including two gas tanks. If my sailboat engine dies (as it did a while back), well, I got sails and a watermaker and an SSB and all sorts of stuff to occupy me till I fix it or call for help or sail somewhere better. When I'm out in my fancy-pants dinghy a few miles away, even though everything is new and I have a repair kit made up etc, it is always in the back of my mind that I don't have much if my one engine dies. Sure I got the VHF and big (for a dinghy) mantus anchor and a bottle of water, but you get my drift, so to speak. It sure sounds better to just limp home under one engine and fix it with a beer on the sundeck than bobbing around being carried out to sea
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Old 20-02-2019, 02:15   #9
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Re: twin 15-20hp outboards on tender

How about a single 40 HP and a 2.5 HP?

As a bonus, the small motor could be used on other small boats. Boats light enough to carry up beach.

Steve
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Old 20-02-2019, 02:51   #10
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Re: twin 15-20hp outboards on tender

Interesting dinghy, never heard of them until now.


Ocean Craft All aluminium inflatable style, cylinder craft, OceanCraft boat builder, high speed military vessels used as coastal patrol boats intercept vessels and certified military vessels Caloundra, tubular rigid hull inflatable boat manufacturer,
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Old 20-02-2019, 04:30   #11
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twin 15-20hp outboards on tender

Quote:
Originally Posted by AKA-None View Post
I believe that twin motors also need to be running in opposite rotational directions


If itís possible you really do want that, torque steer can be a bear.
Many small motors like my 20 HP Suzuki donít have the trim tab you can adjust to counter torque steer, which annoys me no end, I know itís not bad, but for very, very little money you could have given me a trim tab that can be both an anode and allow me to zero the torque steer, like itís been done for decades on bigger motors.
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Old 20-02-2019, 04:44   #12
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twin 15-20hp outboards on tender

There are only two good reasons for twins: you need more power than the most powerful available outboard, or you need the security of two because youíre heading somewhere where assistance is difficult to secure.

Youíre not going to be taking one off to save weight and/or fuel. False economy, particularly with the cost of two units over one.

Iím with Steve. Unless youíre headed far offshore or into the wilds, put s kicker on it.

And yes, people still put twins on for security despite the improved reliability of todayís outboards. I know quite a few fishermen who wonít go 30 miles offshore with a single screw.
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Old 20-02-2019, 05:37   #13
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Re: twin 15-20hp outboards on tender

Quote:
Originally Posted by Panope View Post
How about a single 40 HP and a 2.5 HP?

As a bonus, the small motor could be used on other small boats. Boats light enough to carry up beach.

Steve

That's exactly what I would do. Or instead of the 2.5hp (or 3.5hp), maybe even an electric like a Torqeedo.



It depends of course on how you use your dink, but most people use their dinks most of the time (some, like me, even 90% of the time) for very short non-planing harbor or anchorage trips, so more than 2.5hp or 3.5hp is rarely needed.



So if I were to have two motors (and in fact I do ), I would size one of them for the long distance high speed duty, and the other for puttering around, and then you have the best of both worlds. A small outboard when you don't need more is SO nice, lightens the dinghy, so much easier to handle, so much easier in the davits, pull it up on the beach no problem, etc etc .
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Old 20-02-2019, 07:19   #14
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Re: twin 15-20hp outboards on tender

I have a 3.5hp outboard already so i will go that route with a single 40 and small ob for in anchorages and will be great for trolling
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Old 20-02-2019, 08:55   #15
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Re: twin 15-20hp outboards on tender

I went for the twin route.
Put an oar on each side, so I didn't go round in circles.
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