Mph = Knots?

Alistairr

Alistairr

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Could someone please explain to me how you convert mph into knots? I have been running my new boat in, for 10hours at 3/4 throttle, and the next 10 hrs at up too full throttle, the boat now has 12hrs on it and on monday it was a lovely day i deceided to open her up, At 5,500rpm she was doing 55mph, is that good for my type of boat?, and what does that convert into knots? Cruising on moderate waves for economy i think about 3500rpm which gives me about 37mph, is that about average? And why are speedo's on some boats in mph and not knots?

What is your top speed and what do you cruise at? (answers on a postcard please) or a simple reply will do. Thanks.

Cheers....

Alistair....

P.s. And yes BrendanS I did beach on Monday, no probs..( unlike the targa).




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Re: 1kt = 1.15mph

http://www.grc.nasa.gov/WWW/K-12/WindTunnel/Activities/knots_vs_mph.html

<A target="_blank" HREF=http://www.themeter.net/beaufort_e.htm>http://www.themeter.net/beaufort_e.htm</A>


Hope this proves interesting



MarkDJ

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cruise at whatever speed is comfortable in the given sea conditions ! If you are mean, you may want to go to boattest.com and find a similar size boat and engine and have a look at the fuel chart. Roughly the last 10pct of speed doubles your fuel consumption. Can be interesting to see quite where is a good economic level to cruise - maybe one time you start wondering how much fuel you have left !
Your speed seems pretty high, Id have thought for 23-24ft and 5.0litre, but maybe the prop you have.If you have a gps, does it correlate?

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That's fast. (too fast?)
A larger cruiser will do 30 knots or thereabouts flat out, depending on sea state, and loading characteristics. (and engine options)
A smaller sports boat will do what yours does.
Some speedo's are more accurate than others, esp in smaller boats- try using GPS and even a handheld one will tell you in KPH, Knots or MPH to a greater accuracy.
In lakes and lochs it's flatter and easier to go flat out, but you run out of space sooner- however you are not too far from the re-fuelling pontoon!!
Howard

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I don't own a gps, i thought i might borrow one from a mate and
try it again, would be interesting.

Mark, Interesting sites, Thanks.

Cheers..

Alistair.

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Loch Lomond, can be as rough as the west coast at times, I have been on it for over a year now i haven't run 'out of space' yet. I think it is 5miles wide and 22miles long with plenty of island to see and visit,You will never run out of space!!
As for fuel as far as i am aware there are only 2 fuel stations on the Loch, 1 right at the south at Cameron House and the other right at the top at Ardlui, where i berth.

cheers

Alistair..


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surprised!

Wow, 55mph (47kts) is very fast. It is 260bhp in a 23/4 foot boat, right? I would have guessed 38-40 kts top, I am amazed it does 47! If it does, then great. But I'd check it with a GPS. It sounds like your speedo is a pitot tube type (is the scale on the gauge non linear? - that means a pitot tube) and these can be quite inaccurate.

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if the boat is more designed for inland lakes and waterways, then navigational issues aren't too important, and mph is more meaningful for newcomers to boats.

Otherwise, if the boat went to sea, then knots is what you wantto measure cos it relates to the charts nicely with one minute (=1/60th of a degree of latitude) being 1 nautical mile so yercan just measure it and use estimated speed in knots to work out how long it will take to go places, and allow for tide in same way.

Most small powerboats top 40 knots or more, as yours does. Bigger powerboats 40+feet will see 30 knots as more of the limit and crusie at 25knots tops. I have decided to stay longer on the boat for short trips, so we laze around on the foredeck or stand around on the bows at 18knots - which is about as fast as you can go without losing towels and food over the windscreen.

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Strictly speaking, of course, a nautical mile is one minute of arc of latitude at the equator. Also, as the earth is not a perfect sphere but an oblate spheroid, there are other small differences that creep in but hey, what the hell, they're probably not going to make any differences to the number of bottles of wine you drink....assuming swmbo gives you enough latitude.../forums/images/icons/smile.gif

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Re: surprised!

jfm,
I have no idea what kind of speedo it has on it. I don't pretend to think that speedo are accurate, i don't ever 100% believe them on boats cars or anything else, my job is in transport so i have to deal with tacho's they have to be calibrated every 2 years, and its amazing how much they can be out by.

tcm,
Speed was not an issue when we bought this boat, although its good to have, as i am relatively new to this game i am curious to what average speed people cruice at? Navigation is quite important to me as i am learning new things every day. I dont plan on being in the Loch forever, i hope to learn as much as i can before venturing out to sea, and i think the loch is a great place to learn.
Cruising at speed can have its drawbacks, fuel consuption will never be great, and as you said, wind, i don't know how many times i 've had to practise my man over board technigue with my base ball cap.

Thanks all

Alistair...



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lost cap cure

If it's a pitot speedo it will have a tiny pipe facing forwards into the flow of water rushing past the hull as you drive forwards. This will be located on a plastic bracket at bottom or transom, or set into the leading edge of your bravo leg, visible just as a tiny hole in the gearcase. The speedo guage is then just a simple bowden tube pressure gauge, and the speed markings on the face of the gauge will be non-linear.

Regarding your cap problem, try these. they attach to your collar wiv a crocodile clip <A target="_blank" HREF=http://www.musto.co.uk/index.cfm?articleid=511>http://www.musto.co.uk/index.cfm?articleid=511</A>

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Re: lost cap cure

I know the plastic bracket you are talking about, i had 1 on the transom of my old fletcher, however there isn't one on the new boat, its possibly set into the bravo leg as you say, i don't know, you are probbaly right.
Thanks for the cap tip, i'll need to head down to the chandlery and get one, though it won't be as much fun!!

cheers...

Alistair..



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Re: lost cap cure

I was at a water ski race last weekend, friend on mine running a 21ft F1 boat with a big block Chevy with superchargers has a new speedo that is controlled by a built in GPS, I will drop into his workshop and find out the make and cost.

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Re: lost cap cure

And what sort of speed are they doing?

The fuel consuption must be hellish?

Cheers..

Alistair..



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Re: surprised!

Alistair,

I would agree with JFM here, my boat happily reads up to 55mph on the speedo.

Cross check with the GPS and it just reads 40knots so my speedo is well out.

Simmillar setup to yours 5.0 mercruiser with an alpha leg.

Cheers

Wake Up

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Re: lost cap cure

On Sunday with a slight chop on the water the leading F1 boats reached just under 100 mph, F2, 21ft with a 2.5 fuel injected Mercury outboard around 75mph, the skiers are on single racing skis on 220 ft lines.

The outboard's burn around 33 gallons per hour flat out bearing in mind that most are heavily worked on, some F1 boats run twin outboards, in Belgium we have one F1 with 3, 2.5 Mercury's on the back, it has been tried here a number of times but was never that successful, the inboards are horrendous will find out the fuel consumption and let you know.

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Re: lost cap cure

WHY THE HECK WOULD ANYONE WANT TO SKI AT 75-100MPH BEHIND A BOAT???????????????

Are they mad???? i presume thats why the line is so long?
I presume when the skiier falls they are out the race? If not dead!!!

33 gph is not bad i've heard of worse.
Do you race?



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