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Old 27-10-2014, 06:21 PM   #1
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Speed calculation

Hi All

Does anyone know how to calculate rough achievable speed with horsepower,revs,gear ratio,weight and water length?

ie

370hp 1.46 gear ratio 9 tonnes and approx 12m

Rob
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Old 27-10-2014, 07:13 PM   #2
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for a speed calculation you need rpm ; ratio ; slip ; pitch

rpm ratio pitch slip
5000 1,46 15 20%

gives you 62km/h in every speed calculator

Is this a existing boat and motor ? If so, look up the WOT rpm and the proppitch. If not, start with a similar boat.

Hope this helps a little,
Kind regards, T.
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Old 27-10-2014, 11:34 PM   #3
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Old 27-10-2014, 11:44 PM   #4
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Boat speed App on IPhone App Store !
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Old 28-10-2014, 08:22 AM   #5
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I have looked at the iphone App. The thing Im not sure about is that you cant put in engine revs anywhere and they must play a part?
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Old 28-10-2014, 09:42 AM   #6
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There's two basic separate calculations that can be done.

One takes boat weight, horsepower, and a coefficient constant for what I assume is drag, which is always something of a guess based on some guidelines. This will give you a top speed estimate assuming you have it propped correctly and are turning the appropriate RPM

The second is a propeller slip % calculator. This involves speed, revs, gear ratio and propeller pitch.

In both cases there are lots of calculators online that allow you to calculate the missing value if you have the others. For instance if you know your current boat weight, speed and horsepower then you can fairly accurately work out your coefficient, which then means you can see what effects weight saving or increased power will have on your speed.
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Old 29-10-2014, 06:06 PM   #7
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Quote:
Originally Posted by Ubatuba45 View Post
Hi All

Does anyone know how to calculate rough achievable speed with horsepower,revs,gear ratio,weight and water length?

ie

370hp 1.46 gear ratio 9 tonnes and approx 12m

Rob
Sunseeker should be able to help with this?
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Old 29-10-2014, 06:43 PM   #8
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30mph !!
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Old 29-10-2014, 06:52 PM   #9
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sounds like a Volvo D6? if it is and you message me all your specs you have i'll do a prop calc for you
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Old 05-11-2014, 09:46 AM   #10
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A scopo didattico ricordiamo la formula per ricavare la massima velocità TEORICA di un'imbarcazione (in nodi):
Vt = (RPM / R) * P / 1215,2
dove:
Vt = velocità teorica
RPM = giri/minuto motore
R = rapporto di riduzione al piede
P = passo dell'elica in pollici
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Old 25-11-2014, 09:47 PM   #11
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D6-370

Looks like they will now be Volvo Penta D6-370 on DPR drives!
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Old 25-11-2014, 11:30 PM   #12
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Boat name: atlantic11, P9, no name.
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Looks like they will now be Volvo Penta D6-370 on DPR drives!
hi, is this for the apache? twin or triple? managed 48 mph on pdc with twin kad 300 (285hp) with clean hull, would think 55/60 given the extra 30% increase in power, less the increase in drag at the higher speed. how well will the D6s fit on the old engine beds? Hope it all goes well, keep us informed pete L.
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Old 26-11-2014, 08:23 AM   #13
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Originally Posted by peter law View Post
hi, is this for the apache? twin or triple? managed 48 mph on pdc with twin kad 300 (285hp) with clean hull, would think 55/60 given the extra 30% increase in power, less the increase in drag at the higher speed. how well will the D6s fit on the old engine beds? Hope it all goes well, keep us informed pete L.
Hi Peter

Yes this is for my Apache 45. She has twin KAD42 in her at the moment. i have owned her for about 12 years. The D6's are slightly wider than the KAD's so have a little work there to do but doesn't look too bad. I'm not sure what speed she will do, I'm thinking 42-45 knots depending on props and weather we try to squeeze a little more out of them.

Do you have an Apache?

Rob
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Old 28-11-2014, 12:42 PM   #14
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I made my own speedcalculator :-)

http://www.fletcher-sportboote.de/ap...hnerV12eng.exe

BR
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Old 30-11-2014, 11:25 AM   #15
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Country: solent
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Boat name: atlantic11, P9, no name.
Boat make: sunseeker, bonito,arrowbolt. ex. dry martini, scavenger
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Quote:
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Hi Peter

Yes this is for my Apache 45. She has twin KAD42 in her at the moment. i have owned her for about 12 years. The D6's are slightly wider than the KAD's so have a little work there to do but doesn't look too bad. I'm not sure what speed she will do, I'm thinking 42-45 knots depending on props and weather we try to squeeze a little more out of them.

Do you have an Apache?

Rob
hi rob, have tried to reply 3 times but they keep getting lost, will reply when can sort this out, all the best, pete L
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Old 01-12-2014, 06:32 PM   #16
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Country: solent
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Boat name: atlantic11, P9, no name.
Boat make: sunseeker, bonito,arrowbolt. ex. dry martini, scavenger
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Quote:
Originally Posted by peter law View Post
hi rob, have tried to reply 3 times but they keep getting lost, will reply when can sort this out, all the best, pete L
hi rob, hope you get this mail. We replaced the 43s about 7/8 years ago with the 300s. use to get 34knts. through the water with the 43s. Volvo techs got 48 mph on the GPS on sea trials with the 300s. gps is variable with tide,wind etc. it is not possible to work out exactly the predicted speed from this as you need more hp per mph as the speed increases. it would seem likely that 50knts 58mph would be about right with the 370s Not sure i would like to drive the apache at that speed in a rough sea as they are a touring/sports boat, have had ours completely in airborne but dont think it would stand a lot of that treatment. Did you get a good deal on the d6. just a note on props, we were over propped on sea trials with C6s with on 1.5 drives, down 150 rpm which is not good for the motors, hope all goes well, pete L.
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