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Old 13-08-2013, 05:23   #1
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Repowering a 42' sailing catamaran

It looks like the TDME 385 (perhaps a copy of a Massey Fergusson -
AGCO - tractor engine that has been marinized) 3 cylinder 32 horsepower engine Is the most likely prospect at the moment. The sales guy has asked me "required ratio for gearbox, 1.88:1 or 2.74:1?" and I don't know the answer. The engine, as I recall, idles about 800 rpm and max is 3000 rpm. I am interested in what experienced catamaran owners think is the best gearbox and why. There is only the one engine and the boat probably weighs five tons or so.
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Old 13-08-2013, 06:55   #2
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Re: Repowering a 42' sailing catamaran

You say it's 32hp, and you only have one. That seems a bit low. ex: my Catana40S has two 36hp engines, and I never feel like it's too much...

but for gearing, you might want the torque since you only have the one engine?
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Old 13-08-2013, 09:02   #3
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Re: Repowering a 42' sailing catamaran

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Originally Posted by tamicatana View Post
You say it's 32hp, and you only have one. That seems a bit low. ex: my Catana40S has two 36hp engines, and I never feel like it's too much...

but for gearing, you might want the torque since you only have the one engine?
The builder put in one Volvo-Penta model 2002 which I believe is only an 18 hp new - and this engine is not new. It has a V-drive so the prop shaft is at a significant downward angle which also likely detracts from delivered thrust. The builder said that this would drive the boat at six knots with no problem - or it would if I could get water to run through it.

I was originally looking at the TDME-480 rated at 37hp but it is a four cylinder engine that weighs 280kg - and I am not sure I can get it into the engine compartment without tearing a larger hole in the boat, so I was thinking that the smaller three cylinder engine would be a better fit.

Note that I am so stupid that I am not sure which gearbox gives the most torque. Would this be the slowest turning propeller? Does 1.88:1 mean that for every revolution of the crankshaft the propeller shaft turns 1.88 times or the other way? Likewise for the 2.74:1? I am not familiar with the standard convention here.
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Old 13-08-2013, 09:34   #4
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Re: Repowering a 42' sailing catamaran

HowStuffWorks "Torque and Low-End Power"
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Old 13-08-2013, 10:15   #5
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Re: Repowering a 42' sailing catamaran

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Does 1.88:1 mean that for every revolution of the crankshaft the propeller shaft turns 1.88 times or the other way? Likewise for the 2.74:1? I am not familiar with the standard convention here.

Other way around - every 1.88 turns of the crank = 1 turn of the prop.

FWIW, my saildrives are 2.64:1 - the prop turns 1 time for every 2.64 turns of the engine.
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Old 13-08-2013, 10:15   #6
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Re: Repowering a 42' sailing catamaran

Quote:
Originally Posted by Bruce626 View Post
The builder put in one Volvo-Penta model 2002 which I believe is only an 18 hp new - and this engine is not new. It has a V-drive so the prop shaft is at a significant downward angle which also likely detracts from delivered thrust. The builder said that this would drive the boat at six knots with no problem - or it would if I could get water to run through it.

I was originally looking at the TDME-480 rated at 37hp but it is a four cylinder engine that weighs 280kg - and I am not sure I can get it into the engine compartment without tearing a larger hole in the boat, so I was thinking that the smaller three cylinder engine would be a better fit.

Note that I am so stupid that I am not sure which gearbox gives the most torque. Would this be the slowest turning propeller? Does 1.88:1 mean that for every revolution of the crankshaft the propeller shaft turns 1.88 times or the other way? Likewise for the 2.74:1? I am not familiar with the standard convention here.
ratio is for a reduction of engine rpm to shaft rpm,so 3000 engine rpm gets divided by 1.88 or 2.74 to give shaft rpm.

this inturn will govern what size(diam) and pitch of propeller you need for the given horse power of the engine.

try Victoria Propeller, Marine propeller sales, service, and design for prop size calculations
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Old 13-08-2013, 19:17   #7
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Re: Repowering a 42' sailing catamaran

Atoll wrote: "ratio is for a reduction of engine rpm to shaft rpm,so 3000 engine rpm gets divided by 1.88 or 2.74 to give shaft rpm." ==> OK, so assuming idle is 800 rpm and max is 3000 rpm the 1.88 will spin the propeller from 425 rpm to 1596 rpm; and the 2.74 will spin the propeller from 292 rpm to 1095 rpm. I asked the sales guy several times for the maker recommendation on propeller - diameter/pitch - but with no response. I looked at the propeller calculators but they ask me for information I am trying to calculate, the propeller size (actually pitch as they don't have a variable for diameter which seems odd as I would think that would be important in calculating thrust) - seems like a catch 22. One of the variables is the speed in mph (oddly not in knots). I would guess that my catamaran will not likely move at more than hull speed with the engine - whatever that is for a catamaran and not a single displacement hull. So what is an accurate estimate of real hull speed for a catamaran with a waterline length of 40'? Since one does not usually run the engine at max rpm, what would be the best value to use in the calculations - 2/3 max, 2000 rpm? With thousands of catamarans sailing the seas, I was thinking that there should be some good rule of thumb experience that could be used as a starting point. Thanks for the help thus far, I really do appreciate it even if it seems like I am whining about the lack of specifics.
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Old 14-08-2013, 04:50   #8
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Re: Repowering a 42' sailing catamaran

firstly for using the calculator,you need to work out what is the largest diameter propeller that would fit on the propshaft,leaving a few inches clearance from the hull.

second to need to work out the maximum weight of the vessel,loaded.

3rd the waterline lenth.

for a catamaran you would also be looking at the formula for a "semi displacement" vessel,and the beam needs to multiplied by 2,ie if one hull is 6 ft wide,your calculation is for a semi displacement vessel of 40ftis,(aprox) waterline lenth 38ft,by beam 12ft by draft by total weight = hull speed

once you have a theoretical hull speed,then you can calculate how many horsepower you need to achive that speed.

you then plug in to the calculator,you hull speed,weight,hp of engine,gearbox reduction ratio,and max rpm of engine and max prop diameter,the calculator will then give you the pitch your propellor needs,changing the diameter of the prop will change the equation an give a new pitch.

generally the lower the rpm at the shaft the bigger diameter prop,and bigger pitch needed to work efficiently.......but the bigger the drag if the prop is not featering or folding when sailing.

on my prout 37ft catamaran the calculation worked out at a 13'diamx9'pitch propellor
with disp 4500kg,wl35ft,beam9ft,draft 2"9',hp 33 ,rpm 3000, reduction 1:1.6, max diam 13'

here is maybe a better calculator that you can download,with more variables that you can play with

http://www.castlemarine.co.uk/pitch-calculator.php
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Old 14-08-2013, 20:20   #9
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Re: Repowering a 42' sailing catamaran

Atoll wrote: "firstly for using the calculator,you need to work out what is the largest diameter propeller that would fit on the propshaft, leaving a few inches clearance from the hull." ==> Tried again on the recommended calculator - still will not load here. I won't know the prop diameter until the engine is installed along with the propeller shaft - so much for prior planning.

I used the Boat Speed Calculator calculator for a semi-displacement hull and it thinks my hull speed is over 15 knots as opposed to the traditional 8.5 knots. It indicates I need around 85 hp to push the boat this fast - but the good news is that the curve indicates that 32 hp will push the boat at around 9.5 knots (an 85 hp is just not going to happen). No place to put the beam into the equation though.

Once I have all the required variables I can try to do the propeller calculation - although actually finding a propeller in the Philippines is likely to be a nontrivial task in itself. The engine installer has instructed me to get the 1.88:1 gearbox which is why I originally posted. I really appreciate all the replies and I have learned a lot (not to mention learning how much I don't yet know - for example I still don't understand what the Buttock angle is or why it is important in semi-displacement hulls).
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