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Old 19-01-2014, 02:57   #1
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Reducing Dinghy Weight

I have an Avon 360 rib with a 25 horsepower remote-steered Mariner two stroke.

I like the remote steering, but the horsepower is overkill, as 95% of my dinghy use is puttering around harbors and rivers with speed limits. The occasional blast in open water, crossing the Solent, etc., is usually solo or two up, and so I don't need 25 hp even for that.

I cracked a davit last year and in general would like to reduce the loads and masses involved. A remote steered motor has to stay on the dink in the davits, so weight is even more important.

So I think about selling the excellent but elderly Mariner and buying a 15 hp Yamaha two stroke, which weighs 36 kilos versus 50 for the Mariner.

What do you guys think? I would consider even going to 10 hp, but they seem to weigh the same as 15 hp - apparently same block.

I realize that going to regular tiller steering would reduce weight even mire, and allow me to take the motor off, but I tank I am rather too attached to the wheel steering, which really makes dinghy trips a pleasure.

Another question is electric starting - I think I could live without this, which would save the mass of the batt; OTOH I could buy one of those very light LifePO batts sold for motorcycles and get almost the same weight.

Grateful for any tips.
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Old 19-01-2014, 04:40   #2
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Re: Reducing Dinghy Weight

The Yamaha is 39kgs, i think. Just looked at one yesterday.
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Old 19-01-2014, 06:22   #3
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Re: Reducing Dinghy Weight

Get the 15hp 2 stroke with tiller, no electric start. They start first pull every time.
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Old 19-01-2014, 06:34   #4
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Re: Reducing Dinghy Weight

I feel like you are caught in the middle, a no-mans land without good answers. There are reasons what you ask is unusual. There are 2 logical answers:

1. Fix the davits, even if that means a complete redesign for the load.

2. Go to a much lighter dingy with tiller steering. Use a tiller extension and you can sit where you like.

You're trying to build something between a car and a motorcycle. I think you would be happier picking sides. Personally, as I get older I am choosing smaller and lighter in many things. It is your call, but I see a lot of older folks that go big because it is more comfortable, only to find that it is heavier. Big surprise.
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Old 19-01-2014, 06:42   #5
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Re: Reducing Dinghy Weight

I agree with Thinwater - you are trying to make a Tardis. It is unlikely that a 10 kilo difference in engine weight is the straw breaking your davits back. You will save 50 kilos getting rid of your console station.

The 10 and 15hp weigh the same.

Electric start will add 3 kilos for the starter motor and solenoid. You can use those light LiFePO motorcycle batteries for that.

If you go with electric start, you are now going through a lot of hassle and cost to save ~6 kilo of package weight.

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Old 19-01-2014, 06:51   #6
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Re: Reducing Dinghy Weight

If you get rid of electric start... you'll be very sorry.

I'm in good physical condition for my age 57 with cycling and racing and all, and we had a pull start outboard (6.5hp) up until 2 years ago on our prior boat. All it takes at my age is one wrong pull.... shoulder surgery.


You don't know how many times I cursed at that brand new damn Tohatsu outboard that was supposed to start with one damn pull.... it never did. I ended up rowing in so many times after trying to start it for 10 minutes and probably 40-50 pulls., Mercury, Tohatsu, etc. they're all the same motors. With my current old crappy Mercury 15hp 2 stroke electric start, when it has difficulty starting... at least I don't have to keep on yanking on the thing. Just turn it over... then look for the problem; then turn it over again.


My right shoulder is already screwed up enough from years of repetitive work related stuff.

Don't switch back to a stone age motor.

Atoll on the post below also makes a very good point obout towing your boat. You need the extra power in an emergency with a 25 ton yacht.

Ken
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Old 19-01-2014, 06:51   #7
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Re: Reducing Dinghy Weight

fix your davit,and get a 2nd light weight dingy,with a 2hp,or even better something you can row,great for late night crew getting back to the boat,or dodgy areas where dingy theft is a problem.

i have a 25hp,and a second rowing dingy,the 25hp is back up if i lose the yachts engine,a 15 hp wont push a big boat like yours easily,
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Old 19-01-2014, 07:28   #8
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Re: Reducing Dinghy Weight

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Originally Posted by atoll View Post
fix your davit,and get a 2nd light weight dingy,with a 2hp,or even better something you can row,great for late night crew getting back to the boat,or dodgy areas where dingy theft is a problem.

i have a 25hp,and a second rowing dingy,the 25hp is back up if i lose the yachts engine,a 15 hp wont push a big boat like yours easily,
I agree regarding HP. Your dinghy has to be able to get you boat out of trouble.

For what its worth, we have a Nissan 4-stroke on my Jon boat. Its too heavy for your needs, however, it has a neat trick that you might look for on the lighter 2-strokes. It has an automatic compression relief for hand starting. If you can find this, it would possibly eliminate the starter and battery.

Otherwise, the reductions you consider won't eliminate the need for beefing up the davit. You must repair it anyway. As long as you want all of the other goodies and you plan to leave the motor on the dink, you are compelled to do this.

Why is it not a consideration to put the motor on a motor bracket? Steering disconnect is only a cotter key & pin.
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Old 19-01-2014, 07:36   #9
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Re: Reducing Dinghy Weight

I have a pair of nearly identical Evinrude 15s from the 1980s. Two, for redundancy, so they can take turns being worked on. But when they are running, they are roaring beasts for their weight (about 75#) which is the most weight I care to wrestle from boat mount to dinghy transom.
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Old 19-01-2014, 07:57   #10
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Thanks for all the great feedback. I hadn't thought about emergency towing. A very strong argument for leaving things as they are.

I will look into the steering disconnect, but I have removed the motor once and it was a complex operation.

Maybe a lighter battery. I don't mind hand starting it; it starts very well by hand - a great old motor which has never given me any trouble. I have a 120 horsepower snowmobile which I also start by hand without the slightest trouble - never wished for electric start even a single time.
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Old 19-01-2014, 10:10   #11
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Re: Reducing Dinghy Weight

I'm hoping to get an aluminum hulled RIB (125#) and a Lehr 15 hp propane outboard motor (110#) this year. The combination is ideal for my boat, sitting on the port quarterdeck, and quick to deploy and retrieve, single-handed, using the halyard. I just clip the halyard into the harness, pull the bow up to a stern roller, and crank the halyard winch. It lifts and slides right onto the deck. My old Zodiac and Honda 9.9 were a great addition to the boat, but lacked the power needed to carry two divers and four tanks on a plane. The RIB is a valuable addition to the boat's cruising tools, being able to shuttle long distances in a minimum of time, for towing and pulling, carrying crew and cargo, and for rescue work. Here's a pic of how it fits:
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Old 19-01-2014, 16:07   #12
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Re: Reducing Dinghy Weight

Stone age motor???

Sheesh, mate and WTF? The now 8 year old Yamaha 15 two stroke on our dinghy may be stone age, but it starts with one or sometimes two pulls, and they are the kinda pulls that this 76 year old yottie can muster up without breaking a bone or into a sweat. It will plane the 3.5 metre alloy hull RIB with four normal size pax and has hip-towed various largish yachts on demand.

I've never desired more HP on this size dinghy. I also agree that the biggest factor in Dockheads quandary is the console steering. If unwilling to abandon that feature, he's gonna have a heavy dink no matter what engine he uses.

Cheers,

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Old 19-01-2014, 16:20   #13
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Re: Reducing Dinghy Weight

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Originally Posted by Jim Cate View Post
Stone age motor???

Sheesh, mate and WTF? The now 8 year old Yamaha 15 two stroke on our dinghy may be stone age, but it starts with one or sometimes two pulls, and they are the kinda pulls that this 76 year old yottie can muster up without breaking a bone or into a sweat. It will plane the 3.5 metre alloy hull RIB with four normal size pax and has hip-towed various largish yachts on demand.

I've never desired more HP on this size dinghy. I also agree that the biggest factor in Dockheads quandary is the console steering. If unwilling to abandon that feature, he's gonna have a heavy dink no matter what engine he uses.

Cheers,

Jim
Roger that, mate, two thumbs up. You can't beat a 15hp 2 stroke for the old weight-to-thrust ratio, in a package that this 56-year-old can still horse from transom to stern rail. With one or two in an 11' alloy RIB, it will really fly, yet it has enough low-end ass to tow the mother ship in calmish conditions. I don't wish for more engine horsepower, a center console, more engine weight, or more "all up" weight. Not unless I was putting a 13' RIB up on a 60 footer. But I ain't, so I don't.
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