Yanmar gearboxes

davidbains

davidbains

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I am seriously considering replacing my 2GM20 KM2P 2.6/1 gearbox with a 3.2/1 box
so that I can replace a two blade "sailing prop" with a three blade prop of larger
diameter and pitch. Possibly a Kiwi prop 15"OD with variable pitch.
The boat is a 39ft of approx 5 tons and has a motoring speed of only 4 knots
unless I thrash the engine in a flat calm. I need more power in wind and sea but
do not want the expense of replacing the whole engine with a three cylinder one.
Comments please!!!!!!
 
Hi David. Interesting question (for everyone else's benefit I once owned a sister ship of Aqua Blue). I used to get 7 knots in flat water with a 20hp outboard, the difference may be down to weight as mine wasn't loaded for cruising. As soon as there was a bit of a chop the prop popped out and my speed dropped, sometimes to zero.

One thing you should consider that isn't relevant to most people is that a feathering or self-pitching prop can bump-start the engine at high speeds. Brunton said their prop was unsuitable for my current boat and I know yours has the potential to be even faster, I saw 18 knots in my tri. I had the same story from suppliers of the Maxprop. In your shoes I would stick with a folding prop, I find my maxprops very effective.

I wonder if you are under or over propped. What rpm do you get at full throttle?

As far as the gearing is concerned, you're probably better off sticking to a faster-turning prop with a finer pitch.
 
I talked to the man on the Kiwiprop stand yesterday. I think their prop, set to a low pitch might be OK without changing the gearbox, and suggest you talk to them as they have experience of lots of different installations, some with smaller engines than yours.
 
[ QUOTE ]
I need more power

[/ QUOTE ] don't see that changing the gearbox is really going to help here.

at any given revs the engine has the ability to deliver a certain (maximum) power output and the combination of gearbox and prop aren't going to change that over and above changing the prop alone.

given the choice between thrashing the engine (high revs) and working it hard (trying to get it to work harder at lower revs - ie overloading it) over the same period and the latter will kill it a lot quicker. however it will run all week at 300rpm off maximum - that's not thrashing it!

establish it' s no load maximum then add pitch/diameter to the prop of your choice as you feel best suits your boat until you start to loose some of those revs (say 100/150).......what's there should be capable of delivering the maximum in terms of speed, economy and low wear.
 
If you feel that the 2GM isn't giving enough power then changing the gear box - especially for a increased reduction in shaft RPM would be a poor choice.

If your engine reaches max RPM easily (ie not at max throtle), then your prop is either too small, or too fine in pitch - try either a bigger prop or a coarser pitch (I'd go for a bigger prop first, then if that's not possible a coarser pitch)

If your engine reaches maximum RPM at max throtle, then you seem to be getting every peeny's worth out of the engine. If you are finding that in rough seas you cannot make headway, then you have two options:-

1/ Change for a bigger engine /forums/images/graemlins/laugh.gif

2/ Change for a gear box with a smaller reduction (ie, higher shaft RPM), and put a put a prop with a finer pitch on. This will give better drive when the boat is moving slower through the water (ie when conditions are bad) at the expense of motoring speed on the calm days.

With regards to Auto-Prop (and the like) with higer speed sailing - I'd strongly suggest taking the advice offered.

["Clink" - sound of 2p being thrown in]

Regards, Jeff.

NB, never drink 1/2 a bottle of red wine then log into the forum.
 
Like you Snowleopard when I had a 25hp outboard Aqua Blue would do 7kts in
a calm. Fitted a "20hp" diesel which probably gives 15hp at the prop and although
I'd got rid of the petrol (for the med) and vastly improved the range, the power
was way down. I feel purchasers should be warned that diesels Hp rating is
so optimistic, I found out the hard way
The Yanmar was overpropped with a 13*10 full blade prop and probably now
underpropped at 12*8 thin blade sailing prop, although I have the cheaper
instrument panel without rev counter.
I was attracted to a Brunton Autoprop but they refused to supply one!!!!!!!!!!!
They wanted me to sort out the power/gearbox/prop size first!!
I don't think Aqua Blue is now capable of speeds high enough to self start the engine
and anyway can't a folding/feathering prop be left in neutral rather than reverse!!
Most experts will tell you that a slow reving large prop is much more effective
than a high revving small prop for the same power output. The 12*8 I have is
effectively an egg whisk.
At least Kiwi prop are prepared to address the problem and the light weight and
variable pitch and moderate price are attractive options.
As someone said I could try the Kiwi at fine pitch and then change gearbox
to increase pitch. With a canoe hull 15" is about max diam anyway.
Autoprop's are particularly good at motor sailing as Rod Heikell has found, but they
are heavy and expensive and those bearings could foul up.
I see that the replacement for the 2GM20 is a three cylinder engine giving about
15% more horsepower but of course we're talking £4000 plus, and I've just
retired!
Thanks for your opinions.
I suppose I was hoping to find someone who had actually changed gearbox
ratio to see if he thought it was worth it.
 
[ QUOTE ]
Most experts will tell you that a slow reving large prop is much more effective than a high revving small prop for the same power output

[/ QUOTE ]

1. this depends hugely on the application
2. as a generalisation for smallish craft though it is probably the wrong way round the high reving small prop is generally more effective - if you want to see what the 'professionals think' look on any outboard motor!
 
Duncan, I think outboards are designed for high speed motor boats which is why
they require fine pitch props when used as a sailing auxilliary.
Most auxilliaries are diesels in slow speed yachts, quite different.
I did fit the finest pitch prop when my auxilliary was a 20hp high revving o'board.
A sailing yacht motors at 4 to 6 kts with a diesel and a large slow revving prop is good for
motoring but bad for sailing!!
Even within these limitations there is plenty of scope for wasting money!!!!!!
 
Dave
You have to remember that you are only getting about 17-18Hp from your 2gm20 no matter what prop you fit - it's a trade-off one will give better low end torque but reduce the overall speed the other will give you better speed at expense of "drive".
Remember that you say that you have 5 tons and 39' unfortunately that tells me your under powered.
FWIW my club has an old ships lifeboat (30' x 4+ tons with Lister 18hp) throwing a huge 3 bladed prop through a big reduction gearbox. What happens is, from a standing start you get huge clouds of black smoke as the engine tries to get the load moving, once it gets it going the smoke disappears and there ain't anything that will stop it - ploughs along at 7 knots, however, when you need to stop it in a hurry or for that matter just coming alongside I have found that the best method is to apply in writting (in triplicate) 2 weeks before you intend to stop /forums/images/graemlins/smile.gif
Unfortunately in the past boat builders skimped on HP to save on costs, now they have realised that owners want power and performance not only from the rig but also the engine. just look at modern boats - most new boats of say 30-35' have 30 hp engines, 35-40 have 40hp, 40-50' have 56hp.
I'm affraid that you have to choose, if you leave the engine alone, prop it according to whether you want overall speed in flat calm or drive in choppy conditions or up-grade to a bigger engine.

Peter.
 
Thanks for that Peter,
Still impressed you get 7kts from "18hp" with a big prop!
Obviously it's overpropped with the delay and black smoke
but 7kts is worth waiting for!!!!!
 
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