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Old 19-08-2009, 02:02   #1
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Yanmar Single or Twin 12hp?

Would a single 12 hp have less grunt pushing a 4.5 ton yacht against a 3-4 knt tide than a twin 12hp? Thank you in advance ! I need advice!

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Old 19-08-2009, 03:47   #2
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against tide - probably marginal as you are looking there at boat speed through water so the tide is irrelevant.

against wind/waves - It may well make a significant difference, because a 2 cylider engine is likely to deliver more torque than the single cylinder, and thus be able to deliver the full revs.
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Old 19-08-2009, 04:42   #3
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Thanks Talbot, its times like this I wish I had trained in motor mechanics ! Thankyou for your time!
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Old 19-08-2009, 06:34   #4
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Just curious Jim Mc, what is the model of the Yanmar two cylinder 12 hp?
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Old 19-08-2009, 07:51   #5
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This has a bit to do with "gearing" as well.

The weight of the boat will factor in when trying to accelerate the hull. The waterline length will set the theoretical hull speed.

When beating against wind, tide and swell the boat will be set back by each element.

You don't state hull length but if it's less than 28 your 12 hp will push it along at hull speed.

A higher revving engine, and lower pitch prop will help the boat accelerate into each swell or reaquire boat speed at the top of the swell.

I don't quite get your choice here. If you are looking at a multi and are thinking one 12hp vs twin 12hp, I would discuss more.

I am not a cat guy but I understand that many will motor or motorsail on one engine only in good conditions. The second engine also provides redundancy and safety factor.

I personally do not plan to have a cat with anything less than twin inboard powerplants. There are lots of reasons that pods, outboards and other options are viable but for all my personal reasons I choose the former.
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Old 19-08-2009, 08:01   #6
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I'm curious if Jim has the choice in a twin why not go with the 18hp model? Would be a no brainer for me.
I didn't know Yanmar made a two cylinder 12hp.
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Old 20-08-2009, 03:30   #7
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I'm curious if Jim has the choice in a twin why not go with the 18hp model? Would be a no brainer for me.
I didn't know Yanmar made a two cylinder 12hp.
Thanks, ex calif and Tellie , I am looking at 30 footers [one hull] .I have seen one that is ok but it has a SVE12 yanmar [single]. I am concerned it wont give me the power needed to put me in position in the 3-4 knott cross tide in and out of the marina. as well as other situations like you metioned . Some other cadidates have small twins 12-15 hp .
[ not a particular model Tellie, sorry i was generalising]

It may be a case of find a boat that ticks most other boxes and put a suitable engine in it later. Very hard to find perfect boat

I suppose my question should have been how much less power would the SVE12 have in critical situations compared to a small twin say 12-15 hp?

Thanks again folks,

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Old 20-08-2009, 05:36   #8
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Thanks, ex calif and Tellie , I am looking at 30 footers [one hull] .I have seen one that is ok but it has a SVE12 yanmar [single]. I am concerned it wont give me the power needed to put me in position in the 3-4 knott cross tide in and out of the marina. as well as other situations like you metioned . Some other cadidates have small twins 12-15 hp .
[ not a particular model Tellie, sorry i was generalising]

It may be a case of find a boat that ticks most other boxes and put a suitable engine in it later. Very hard to find perfect boat

I suppose my question should have been how much less power would the SVE12 have in critical situations compared to a small twin say 12-15 hp?

Thanks again folks,

Regards Jim

Hi Jim, If you're considering a new two cylinder Yanmar you'd be looking at a 2GM20F. They are 18hp. If you wanted to replace the Yanmar with a new one cylinder it would be the 1GM10 which is only a 9hp motor. When you are talking engines this small I don't think worring about overpowering a boat is an issue. If it were me and I was repowering in this range I'd look at the most HP I could get. Yanmars 18hp over their old SVE12hp or the new 9hp again is a no brainer. As I see it, your options with Yanmars is either you rebuild the SVE when needed, because Yanmar doesn't make this model any more, I believe they were only produced between 1974 -1976. Or go with a new 2GM20F. For perhaps a few thousand saved up front on a rebuild, I'd go with the new 18hp instead. These things if properly taken care of will last you a long long time.
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Old 20-08-2009, 13:12   #9
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Yanmar no longer sells the 2GM20F due to emission requirements. They offer a smaller 2 cylinder, 14hp 2YM15 and a 3 cylinder, 21hp 3YM20 that is close in size to the 2GM20F. Specs are available on their website.
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Old 20-08-2009, 13:17   #10
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Yanmar no longer sells the 2GM20F due to emission requirements. They offer a smaller 2 cylinder, 14hp 2YM15 and a 3 cylinder, 21hp 3YM20 that is close in size to the 2GM20F. Specs are available on their website.
Thanks for the update.
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Old 20-08-2009, 13:18   #11
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One consideration is the old Yanmar single was a very rough idling and vibration prone engine. Not very pleasant... two cyls would be smoother. I built a fairly heavy 31 footer once with a 13hp Volvo and it would make hull speed in flat water but generally was pretty underpowered....
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Old 20-08-2009, 14:11   #12
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I have a Yanmar 1GM10 on my 27 at 6000lbs

I would sugest a 2 cyl model over a single for the vibration and noise. I believe you should look at sizing of 1 HP for every 500 lb.

Its not the current thats the problem as you won't do more than hullspeed anyway. Its the wind. I have no problem making hullspeed when its flat and calm but with a strong headwind (when I cant sail) it would be nice to have the extra hp. If your looking at 30 footers you need to be looking higher than 12hp or you will feel underpowered.
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Old 20-08-2009, 14:27   #13
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Current is irrelevant. Current is the water speed with respect to the bottom. You're boat only "knows" of its speed through the water. Torque is a useless figure without speed...and that determines power. You can have a ten thousand foot-pounds of torque on a propeller that turns one RPM...and you will still have very little horsepower.

You wont notice any difference between a one or two cylinder, 12 horsepower engine because the power is directly and linearly converted to forward force..and that is what makes it move. If the prop is sized properly your engine will be able to reach the RPM at which 12 horsepower is generated. Power is power.
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Old 20-08-2009, 14:59   #14
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Don't do as I do...

I built a 7 tonne Hartley RORC 32 a few years ago and put a 7hp hand start in it.

The hand start can make things very simple. The small Yanmar looks to be be hand startable.

That engine would push the boat to 5 knots easy, maybe more under good conditions (getting old, memory hazy).

It was fine round Sydney Harbour with a few crew, and for short trips up the coast.

But now, looking at cruising short handed and over long distances, I went for a 85hp John Deere. Nice to have lots of power.
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Old 20-08-2009, 22:19   #15
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I tried to hand start our Volvo MD2010 - Forget it!

In regard to 30 footers.

Just to push the boat around in and out of the slip 12hp would be fine. These days people seem to want more oomph in terms of acceleration - If I had to I'd go with 12 but I'd be happier with 15-18 and a properly sized prop especially for a heavier boat.
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