Does more power mean changing gearbox ratios?

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uncledekka

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Hi all, here's my question to the wise- if I double my engine horsepower, should I change my gearbox ratio?

I've swapped my tired old Lister HW3 for a youthful Perkins 4236. The Lister put out 36hp through a Borg Warner with 2.1:1 reduction, and the sweet spot was around 1800 rpm giving me 5 knots.
The Perkins will also be happiest around the same revs for fuel consumption, but it'll also be producing around 60hp- twice what the Lister was capable of.
Should I stick with the original box at 2.1:1, or should I get a different ratio box to take advantage of all that extra power?
I should mention that this is a heavy-displacement 35ft motorsailer, and all I really want is to achieve hull speed- around 7 knots!
(I appreciate that there will be some fettling to be done to the prop regardless)
Hope that makes sense!
 
As it is won’t the existing g box / prop only take out what ever Hp it needs ie more or less the same on its prop curve as it did with the old engine ?
With a displacement motor sailor it cannot plane anyhow so what iam saying you cannot benefit from the extra Hp anyhow .
Arguably the new engine will be less stressed and maybe last 2 x long , but there are many other variables to longevity which I think will end its life .

They are not like petrol engines whereby ( I think you have this mindset ? ) , that at a certain throttle position the same it gonna throw 60 Hp out , I won’t .
It the reverse so to speak the prop will ask it via the g box to deliver the same Hp to push the boat as it did before @ 5 knots .
Ball park 3/4 of the old 36 Hp so 27 Hp .

Except the Perkins will deliver that demand very easily without arguably ever getting up to the desired, optimism cylinder temps and pressures, cica 550 degrees EGTs .If it does reach optimum cylinder temps on 27/60 , less than 50 % then it last a very long time as it’s very lightly loaded - a good thing btw

The fuel consumption curves rapidly steepen , as does the EGTs and loads in the final , last quartile of the rpms .
They also drop down in the two middle quartiles , This new engine will be in the 2 nd quartile if the boundaries between 2nd and 3rd are @ 50 .Fourth being 75-100 .
All you have done is shifted the place the engine operated in from the 4 th to the 2 nd quartile as it still needs 27 Hp , which it gets .
So if you borrowed one of my 13 L 700 Hp engines and dropped that in it would still go 5 knots in theory.
How ever they do about 100 Hp on tick over 600 rpm so they would chew your g box .
If you then fitted a beefed up g box that could survive the 2400 kg of torque and the 700 Hp all that would happen would be the the boat reach max hull speed and push a pretty big bow wave and your stern start to sink scarily.
Still extract a tiny amount of the Hp .
So you then fit a bigger pitched prop and so on , all that would happen is it would start to slip more and cavitate egg whisk like at the back .Again it only ever needed 27 Hp to push your tub 5 knots .You might see 7 knots and a bigger bow wave + stern dipping .

The reason mine goes faster with more Hp is it planes , it gets up out of the water and looses its drag the faster it goes .
Yours is a different hull shape .It can’t loose its drag so any extra Hp is useless .
 
To get to 7 knots approx the fastest hull speed * for this it’s gonna need approx 36 Hp from the 60 so the second quartile .
This means a bigger pitched prop .
Seeing as the original 36 Hp gearbox survived if it ever was WOT ed , it’s just about gonna cope .

How ever it’s aged now and on the edge boundary wise , so to get 7 knots I would buy a stronger g box to cope with the bigger pitched prop that will demand this sort of Hp out the 60 available.

* it’s got a keel being a motor sailor .The sq root of WL length is a rule of thumb , bit of a noddy calc tbh .
 
How long is a piece of string? At a minimum the gearbox ratio or the prop pitch may need changing, but it's a complex business; as mentioned above will your GB take the extra power? Can a larger diameter prop be accommodated? Speak to Darglow or Bruntons or one of the other prop specialists and they will want all that info and more and in return will suggest a likely prop spec with an error band, because even they cannot calculate exactly what is required.
 
Damn- thanks Scottie and Portofino for pointing out the painfully obvious!! I feel stupid now...
Just to clarify- sister boats to mine have been successfully fitted with upgraded engines and can cruise at hull speed with no adverse effects. And there are plenty of other boats running the same engine/gearbox combination, so that's not an issue either.
I guess I should rephrase my question: how best to achieve hull speed at the optimal engine rpm?
 
There are a number of factors to consider when increasing HP.
Engine revolutions at full revs.
Gearbox ratio, i.e. 2:1, 3:1, etc.
Prop diametre.
Pitch,
and finally blade area.
The bigger the diametre, the slower the prop should turn i.e the lower the gear ratio and diametre is much more potent at absorbing power than pitch. To prevent cavitation the blade should not be overloaded. To calculate minimum blade area use this formula by D.Gerr: (100 x Shaft HP) / (Knots x Sqr root knots). Simply adding HP will cause the prop to be overloaded. A larger prop will require more clearance all around. A modest increase in power might be absorbed by increasing pitch, but not when you are increasing it by nearly 100%. You can have an existing prop pitched up by a maximum of about 2". Except on high speed craft the diametre to pitch ratio should be kept below 1.4 for optimum efficiency.

The easiest approach for you would be to use a propeller calculator such as Vicprop (free), just google it and enter the data for your boat. You can play around with certain parametres such as HP, gear ratio and desired speed to find the optimum set-up for your boat.

To note: to reach hull speed (relative speed of 1.34 x sqr. root DWL) you need 1HP per 500 lbs displacement at the shaft. Engine HP is not equal to shaft HP as there is at least a 4.5% loss for gearbox and bearings etc.

Hope this helps. Went through the same on our MS.
 
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