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Old 17-02-2018, 02:20   #1
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Boat Speed Variation between tacks

Good morning all.

I would appreciate any views on the cause of a difference in boat between port tack and starboard tack on my catamaran. The difference is several knots and more apparent the narrower the apparent wind angle. On the port tack the boat performs well, on starboard it does not! Here is an example data set that I took from the instruments:

Port tack TWS 10.4, AWA 44,, boat speed 10.1ns

Starboard tack TWS 10.7, AWA 45, boat speed 6.2

Starboard tack TWS 11.2, AWA 45, Boat Speed 6.4.

Daggerboards were the same, the props are folded OK. I am assuming rigging? We played with the sheets but couldn't correct the differential.

Any views would be appreciated
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Old 17-02-2018, 03:11   #2
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Re: Boat Speed Variation between tacks

Good evening Naughty Cat (well, it's evening here)

With a difference that extreme, the first thing I'd look at is your wind indicator.

I don't know what sort of boat you have (you may like to update your profile with that info so that it shows beside your post), but I find 10.1 knots in 10.4 TWS at 44 AWA hard to believe for a cruising cat.

If your wind indicator is 10 out, you are pinching at an actual 35 AWA on Starboard and sailing at 55 AWA on Port - which could make your figures much more believable.
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Old 17-02-2018, 04:30   #3
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Re: Boat Speed Variation between tacks

Thanks StuM. Interesting thought and thank you for responding. It's a 2017 Catana 47 - daggerboards, new sails etc. so I was pleased with the port tack performance rather than doubting!
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Old 17-02-2018, 04:35   #4
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Re: Boat Speed Variation between tacks

One possible reason for this is the mast not being quite upright ie leaning to one side or the other. However I think for this to be the cause of your large difference it would be very noticeable indeed!
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Old 17-02-2018, 04:43   #5
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Re: Boat Speed Variation between tacks

On my Endeavourcat I used to sail pretty much the same on both tacks when close hauled, but once I got loaded down for cruising I am a knot or two faster on a port tack than a starboard tack. I suspect it's weight distribution, I have not been super careful about making sure the two hulls are perfectly balanced.
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Old 17-02-2018, 05:08   #6
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Re: Boat Speed Variation between tacks

Its common for boats to have small performance diffs on tacks but, thats pretty extreme. I agree w Stu, confirm your instruments are correct first.

Are you getting speed from paddle wheel or GPS?
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Old 17-02-2018, 05:37   #7
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Re: Boat Speed Variation between tacks

I would turn off the instruments, and record a track with a GPS out of sight.
Sail the boat by the telltales over several tacks and compare the port tack track to the starboard tack track.
This will reveal if your problem is boat or gizmo
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Old 17-02-2018, 05:49   #8
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pirate Re: Boat Speed Variation between tacks

This can be seen in mono's as well where it is down to the rig trim..
In a cat it could be rig trim and/or load variation/distribution.. is it an owners suite in one hull model..???
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Old 17-02-2018, 05:56   #9
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Re: Boat Speed Variation between tacks

Its common for boats to have small performance diffs on tacks but, thats pretty extreme. I agree w Stu, confirm your instruments are correct first.

Are you getting speed from paddle wheel or GPS?
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Old 17-02-2018, 08:30   #10
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Re: Boat Speed Variation between tacks

Belizesailor's comment was the first thing I thought of as well.
Our paddlewheel is slightly offset and shows differences between tacks as well.
The GPS doesn't.
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Old 17-02-2018, 09:27   #11
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Re: Boat Speed Variation between tacks

Agree with Stu & others, wind instrument calibration.

Good perf, can you tell us more about your cat ?
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Old 17-02-2018, 11:18   #12
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Re: Boat Speed Variation between tacks

It is an owners' configuration - starboard owners' hull. The boat speed on the instruments is from a paddle wheel in the port hull, but I am getting the same off the GPS. To be honest you can feel the difference. Like most cats, it feels stationary below 4 knots and gives feedback at 7+ knots. With the differentials we are recording you can feel the difference. Port tack is fun, starboard lacklustre. I took a look at the mast - it's easier with a cat - and there was no apparent bias.

Its a relatively standard C47 - just over 10 tonnes unladen, owners configuration, square top main, solent and reacher. Standard rigging, no genset, light tender/outboard (Takacat + Torquedo), small 4kg washing machine in starboard hull and 80m 10mm chain + upsized Spade. we are very pleased. It's hard to find a good passage-maker that's also gives a fun day sail. I used to sail 420's, and we outpaced a fleet of 420's racing in the bay off Xabia last summer, but as a monhuller you cant believe how slow it is to tack and it's a big thing to have in crowded water.
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Old 17-02-2018, 11:30   #13
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Re: Boat Speed Variation between tacks

I would say there are four major causes, as others have pointed out:

1. Wind instrument calibration. One easy way to check if there is an issue is this: sail on one tack as closed hauled as possible (main on the verge of luffing), note down the jib sheet angle and trim. Tack without releasing the main sheet, get as closed hauled as possible on the other tack with similar jib trim. Is there still a huge difference? If yes, then the cause is something else, if the difference is much less, then the wind instrument calibration is playing a trick on you.

2. Tune the rigging. It is very likely that port and starboard sides are tuned very differently. The mast does not necessarily have to be leaning on one side to see a big difference, it can just be that it is flexing on one side much more than the other. Get or borrow a tension gauge and measure it. One easy way to spot a tuning issue on one side is to observe the main as you go close haul, do the telltails break in the same way? Is it easier to fly them on one tack than on the other? Also, look at the forestay (laying on the bow, with your eyes pointing at the top of the mast) on the two tacks, does it sag more on one tack than on the other?

3. Weight distribution. On a monohull, this would make a noticeable difference, on a cat, not sure, but worth checking.

4. Below water line appendices and finish. Has one side more growth than the other? did one rudder get a hit and now is slightly off? It can certainly happen.
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Old 17-02-2018, 11:51   #14
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Re: Boat Speed Variation between tacks

And speaking of paddlewheels... ours works fine for the first few days after a bottom wash and then stops. We sort of think it's marine life clogging it, and if so would love some ideas about how to prevent that. But are there other explanations?

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Old 17-02-2018, 12:05   #15
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pirate Re: Boat Speed Variation between tacks

My rationale is pretty simple.. the owners side is loaded much lighter so on one tack the heavy hull is being lifted by wind and hydro dynamic's, less resistance more speed.. on the other tack the wind is pushing the heavier hull under increasing resistance and slowing the boat down.
But.. I'm probably wrong..
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