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Post by alenka on Nov 22, 2019 9:18:37 GMT
That’s an impressive speed increase (assuming like-for-like comparison). I’m still musing moving to a 3B Flexofold as well but have kind of got used to my crappy folding Volvo which has taken the initial urgency away. The 9.7kts fig sounds like a bit of over-reading given your hull speed is about 8kts. It’s quite common to get slightly over hull speed under power with a modern engine and clean hull & prop... but probably not by quite that much 😊 Impressive figures indeed. The THS for the 39i is around 8.3kts so your original prop giving 8.5kts flat out would suggest that the pitch was pretty well matched. To achieve a 9.7kt speed (17% improvement) on your new prop might suggest that it is over pitched and making your engine work harder with consequential increase in fuel burn. As your goal is to achieve every once of speed out of your boat I am surprised you have not invested in copper coat antifoul or a hard, low friction, type. Enjoy.
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Post by zaphod on Nov 22, 2019 19:42:51 GMT
I am certainly wondering if something happened to knotmeter calibration, as that top speed number really does seem too good to be true. It would be strange considering the knotmeter has been pretty much bang on gps speed every time I have checked it in the year we have had the boat. I'm not sure what could have happened in the 2 weeks the boat was out of the water. Hopefully the weather will cooperate and I will be able to get out sailing this weekend and check it again.
In terms of the theoretical hull speed, I don't think it is as simple as just plugging the waterline length into a formula. If that were the case all 40ft boats would have the same maximum speed, and that certainly is not the case! My boat has hit 9kts under sail, and we certainly were not surfing or planing.
I am not trying to squeeze every last 10th of a knot out of the boat. I have done the hard bottom paints, fairing and wet sanding to make an ultra fast bottom on race boats, but this is not a race boat! I don't have the time or patience for that anymore, besides, all that work will be negated by all the cruising gear and 1000lbs of water we carry! Not to mention the crappy dacron cruising sails we have!
Changing from a fixed prop to a folding prop is probably the single biggest performance improvement one can make on a sailboat regardless if it is a cruiser or a racer.
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Post by alenka on Nov 22, 2019 20:57:29 GMT
I am certainly wondering if something happened to knotmeter calibration, as that top speed number really does seem too good to be true. It would be strange considering the knotmeter has been pretty much bang on gps speed every time I have checked it in the year we have had the boat. I'm not sure what could have happened in the 2 weeks the boat was out of the water. Hopefully the weather will cooperate and I will be able to get out sailing this weekend and check it again. In terms of the theoretical hull speed, I don't think it is as simple as just plugging the waterline length into a formula. If that were the case all 40ft boats would have the same maximum speed, and that certainly is not the case! My boat has hit 9kts under sail, and we certainly were not surfing or planing. I am not trying to squeeze every last 10th of a knot out of the boat. I have done the hard bottom paints, fairing and wet sanding to make an ultra fast bottom on race boats, but this is not a race boat! I don't have the time or patience for that anymore, besides, all that work will be negated by all the cruising gear and 1000lbs of water we carry! Not to mention the crappy dacron cruising sails we have! Changing from a fixed prop to a folding prop is probably the single biggest performance improvement one can make on a sailboat regardless if it is a cruiser or a racer. You are correct many factors enter into the boat speed equation, and as others have already stated you can get any boat beyond THS by applying brute force power from either wind or engine. However, getting beyond the THS is an uphill slog because the hole in the water the boat leaves behind it acts like suction and wants to hold it back; unless you are in an ultra light displacement and/or can get onto the plane. Most switch to a feathering/folding prop do so to reduce drag whilst under sail. There is always some drag associated from a prop under power, hence all the really fancy designs you find on speed boats to maximise the power from the engine is turned into propulsion rather than wasted energy. If you compare the HP output of a direct drive electric outboard at the prop it is pretty much as stated, whereas a 4hp petrol OB will lose around 25% of its power along the drive shaft before it gets to the prop.... lots of things in the equation. It is very conceivable that the factory fitted prop was an inefficient design. You seem to have struck upon a winning combination for your boat if you are getting anywhere close to indicated speeds. It would be interesting to know what fuel burn you are returning to achieve those speeds. I can vouch for the copper bottom paints create a very slippery hull and require minimum maintenance. If only I could bring myself to scrape off the old stuff back to the gel coat !
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Post by zaphod on Nov 25, 2019 6:21:04 GMT
We had the boat out today and I did some more testing, and unfortunately the performance gains were not as significant as I thought. It seems my knotmeter is acting up, and although it agrees with the GPS at lower speeds, the faster I go the further out of calibration it gets! At full speed the knotmeter reads .75kts high. This is a new problem that I now need to solve.
Unfortunately there was no wind so I wasn't able to test sailing performance...
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Post by alenka on Nov 25, 2019 11:18:14 GMT
It could be that when it was reading in harmony with your GPS there could have been a bit of weed or something around the paddle wheel slowing it down and if the weed has now gone following the lift out then it has reverted back to over-reading.
I am sure you know all this but others might be interested...
I start at the paddle wheel and work back. I remove calcium deposits with a mild hydrochloric acid. It dissolves all deposits within seconds. A quick rinse off and its good to be re-installed. Getting it correctly set against the water flow is very important of course.
If there are still a discrepancies you need to put a bias in on the display. On my Raymarine I have the option to adjust the paddle wheel speed automatically to match the GPS, or enter it manually. Now here is a gotcha. If you have previously entered a bias manually and then perform the auto calibration it will still apply the manual bias to the auto-calibration so the reading will be incorrect - Auto-calibrations does not remove any manual calibration.
In the end I found it easiest to enter the correction manually. Although we have some currents in my non-tidal part of the Med it is not a problem to find slack water and just add increments a bit at a time. Once the two speeds were matched a quick 180 turn to make sure they still agreed on the reverse direction and then it's down to a few days of observation.
With my system the error is not linear so no one bias reads accurately across the scale. In the end I opted for a correct reading around 5.8kts which is a good average speed for us under sail. This means we get accurate true/apparent wind speed and direction indications.
In truth on passage I rarely use paddle wheel speed indications as they are of little use for estimating eta's, etc.
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Post by zaphod on Nov 25, 2019 21:23:12 GMT
Yes, I suspect there is a manual correction in there somewhere, but I cant find it.
When I go into the calibration mode my ST70 instruments allow me to sync to gps at 5 different speeds so there are 5 data points. I deleted all data points, and then started again. I sync'd to GPS at 7 its, and the knot meter momentarily read 7kts and then it switched to reading 3.5kts! I put 4 more data points in at different speeds, and it still reads 3.5kts at 7kts. The hz readings from the impeller at the different speeds seem to have a linear relationship, and when I monitor the hz in real time the reading is consistent with the data points. That tells me there is a manual offset in there somewhere, but I cant seem to find it.
I like to have both speed over water and speed over ground, because that tells me how much current I am in. My area has a lot of currents and eddys, and often they are very localized. Knowing how much negative or positive current you are in is useful information.
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