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Old 02-09-2013, 23:40   #1
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Yanmar Horsepower

Recently cruising in Whitsundays...Australia, and motored through a passage called Solway Passage near Whitehaven beach. I had timed it to run with tide but the current was very strong AGAINST me. My Yanmar 40hp was struggling,
revs dropped as we fought the current.Finally pulled through but a scary time!
Boat is an Irwin38, approx 19 ton, the engine has been perfectly ok prior to this and since, however, I am wondering if this is to be expected of this horsepower/boat combination?
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Old 03-09-2013, 00:15   #2
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Re: Yanmar Horsepower

At about 2 horsepower per ton, I think your boat may be a little underpowered.

3 or 4 horsepower per loaded ton is usually considered desirable, even up to 5, although it's also not good to overpower a boat (prop will be bigger = more drag; engine won't get a proper load in normal operation).

I am puzzled, however, as to why your engine would struggle "fighting" a current. Your engine shouldn't feel any current. Your boat should go through the water as it usually does; just the SOG will be lower because the water its moving through is moving.

What makes a sailboat engine struggle is more usually trying to bash through head seas, especially steep chop from wind over tide. That's when you need power.
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Old 03-09-2013, 00:23   #3
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Re: Yanmar Horsepower

A quick search suggests that Solway passage is noted for it's strong current. One site mentioned 6 knots.

As your boat on full power may only do 7 or so knots with everything clean slow progress against the current may not be totally unexpected.
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Old 03-09-2013, 00:56   #4
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Re: Yanmar Horsepower

Could need a fuel filter change if it's dropping revs when working harder than usual.
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Old 03-09-2013, 01:17   #5
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Re: Yanmar Horsepower

thanks for the responses....
The engine revs dropped significantly but kept going, SOG was down to 1-2knots
I thought perhaps something was wrapped around prop shaft, but later inspction proved clear.
Fuel filters all recently cleaned/replaced.
I note your comment that engine should 'keep going' but SOG will be reduced.
I thought the current was acting as a break on the prop?
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Old 03-09-2013, 01:53   #6
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Re: Yanmar Horsepower

Is it harder to run on a treadmill than on the ground? Let's say you're running at 5 mph. Your speed over ground on a treadmill is 0, so the current is the same speed that your are running. By your logic it should be significantly harder to run at 5 mph on the treadmill than if you were running 5 mph on the ground.

Another example just hit me. Try running east then west near the equator. Since the earth is rotating at about 1000 mph, one direction should be much harder to run in.

This is a frame of reference problem. If you were in the middle of the ocean with no frame of reference all you or the boat would see is your movement through the water. You have no idea if that body of water is moving.
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Old 03-09-2013, 02:07   #7
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Re: Yanmar Horsepower

Quote:
Originally Posted by our doug View Post
thanks for the responses....
The engine revs dropped significantly but kept going, SOG was down to 1-2knots
I thought perhaps something was wrapped around prop shaft, but later inspction proved clear.
Fuel filters all recently cleaned/replaced.
I note your comment that engine should 'keep going' but SOG will be reduced.
I thought the current was acting as a break on the prop?
A current won't affect your prop in any way, as the previous poster explains.

It simply creates a gap between SOG and speed through the water. Your boat sails (and motors) in water, not over ground, so to judge how your propulsion is working, look at speed through the water, not SOG.

The only way a current will be felt on board is if you get wind over tide or wind against the current, which changes the shape of the waves. That can indeed cause you to labor through the water.
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Old 03-09-2013, 02:12   #8
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Re: Yanmar Horsepower

Solway pass is not a nice place to have an engine failure.

You do need to investigate the cause for the drop in revs. As others have said the current was not to blame.

Fuel supply problems are the most likely problem. With a contaminated fuel system the filters can clog with just a few mins of engine running in the worst case.
You indicated the filters had been recently replaced, but did you inspect them after the loss of power?
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Old 03-09-2013, 02:15   #9
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Re: Yanmar Horsepower

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Originally Posted by noelex 77 View Post
Solway pass is not a nice place to have an engine failure.

You do need to investigate the cause for the drop in revs. As others have said the current was not to blame.

Fuel supply problems are the most likely problem. With a contaminated fuel system the filters can clog with just a few mins of engine running in the worst case.
You indicated the filters had been recently replaced, but did you inspect them after the loss of power?
Indeed.

By the way, I hope you are not like my father, who got tired of cleaning the log impeller on his yacht and just removed it, a few years ago, so that the only measure of speed he has is SOG from his GPS. If you're sailing in tidal waters with currents, you really need to know your speed through the water.
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Old 03-09-2013, 03:20   #10
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Re: Yanmar Horsepower

it does sounds like the prolonged high rev period was causing issues and as others have said most likely fuel starvation or perhaps a heat issue
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Old 03-09-2013, 03:28   #11
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Re: Yanmar Horsepower

OK it seems the majority is aiming at a fuel problem, thank you all or your input, I will check filters again and see if this is an issue. I would be surprised,but will do it anyway.
Am I correct in assuming from your comments that the force of the current against the boat will result in the engine losing revs(power)because of lack of fuel being supplied,also, when that resistance is removed and the current area cleared, he normal revs will return?
(speed was monitored from log impeller and GPS, SOG I think gives a better idea of what is happening under these circumstances than speed through water as it was more a case of the water passing under the keel than me passing over the water)
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Old 03-09-2013, 03:32   #12
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Re: Yanmar Horsepower

thanks but non of that wold explain a reduction in revs
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Old 03-09-2013, 03:37   #13
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Re: Yanmar Horsepower

No the engine does not know, or care about the current it will not effect the engine in any way.

However with a poor SOG its likely you were using higher than normal revs and the choppy seas around the area (particularly the bottom of Whitsunday island if you were traveling in that direction) are good at stirring up tank.

Motoring in current the STW ( from the log) reading should have been normal and unaffected by the current. The SOG (from the GPS) will show the actual speed over the ground and will be effected by the current.

For example if you do 7 knots at 2500 rpm with no current when motoring with a 6 knot current against your log should show 7 Knots and your GPS 1 knot @ 2500 rpm.

BTW
It would also be worth redoing the tide calculations to see what went wrong.
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Old 03-09-2013, 03:45   #14
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Re: Yanmar Horsepower

Perhaps I am missing something, but your collective advice ( appreciated) indicates that
the current doesn't affect the prop/revs
If this is so, why did the engine revs return to normal as soon as we cleared the area?
Engine was not revving highly for any prolonged period, no heating problems were evident from alarms.
Knowing my relative position because of reference points did not reduce the revs and being on a treadmill would also not reduce the revs.
Coincidence?
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Old 03-09-2013, 03:53   #15
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Re: Yanmar Horsepower

Quote:
Originally Posted by our doug View Post
Coincidence?
Possibly. I think its more likely you increased the engine revs to compensate for the slow SOG and this resulted in a fuel starvation, or overheating as Stevensuf has suggested. There are other possibilities such as a blocked exhaust elbow, or even a plastic bag around the prop.

The main message is the engine problems were not caused by the current. Therefore you should do some investigation to determine what caused the problem.
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