Which Gearbox?, technical help needed

[Guest]

Hi,

I hope that someone can give me an answer to my problem, hear goes.

I have a Roberts 34 in steel which is going to weigh approx 5.5 tones be approx 8.25 meters LWL.

My engine is a Ford XLD416 which will give me 42HP@4800RPM (which is enough to power two of my boat) and 26HP@3000RPM which is more than enough for one.

Using a tool on the Nanni web site (and I do appreciate that this only a guide) I get a result that my boat will achieve 7knots with 16.1HP@3484RPM.
If I change the Max RPM to say 3000 (or what ever the optimum fuel efficient RPM turns out to be and it probably around 3000) then I get a result that my boat will achieve 7knots with 16.1HP@2555RPM.
I assume the reason that the same speed is achieved is that the prop will be different.

My question is,

Why am I being told by a gearbox manufacturer that I should be using a gearbox rated at the max (42HP@4800RPM)?. I would much rather govern the engine to the optimum fuel economy and save on fuel and avoid a gear box which would cost more.

The response from a large marinisation company is that I can do it, the response from the gearbox manufacturer is that I should have the next size box up.

Any help would be appreciated.

Paul.

 

[yoda]

The simple answer is that if you fit the smaller gearbox you are stuck with limiting the power output of the engine for ever. Fit the larger one and you can still limit power output and hence control fuel consumption. Should you find you then need more power for any reason you would easily be able to achieve it without an expensive gearbox change. My one thought is that if I was fitting an engine I would maximise the possible power output and limit the Revs by some means that could easily be over riden should I need the power in an emergency.

Yoda

 

[cameocrew]

Paul,
You do not mention the'Grunt' (torque) factor It is very easy to xceed this
factor at any RPM so a gearbox size has to be rated at max +++ for the
power curve developed. The +++ is to give the unit a reasonable service life
and a bit in reserve for the marrage of the egg whisk with Lobster pot lines
mooring ropes etc. I would fit the biggest unit +a rope cutter

 

[Guest]

The max torque of the engine is 61Nm the max torque the box can take is 80Nm, however at full revs (4800RPM) the box is only rated to 31Hp and my engine will put our 42Hp. I cant help looking at this and thinking, well, if my boat reaches 7 knots (which is max for its LWL & Weight) at something like 16HP@2500 or 3300 RPM why do I need the bigger box?

 

[cameocrew]

Paul,
Torque developed by the engine and driveline torque are two different things
as are HP .SHP and BHP . but to answer your question- is in my first reply

 

[brian_neale]

The calculated power requirement is presumably for driving the boat at cruising speed in flat water. In waves or with strong head winds, you may need more power to achieve the same hull speed, and it may be that for whatever reason you are willing to forget fuel consumption for a little while! But you would probably feel happier knowing that the gearbox will take the strain.

 

[spanners]

Go with Yoda ,,,, you never know when you will need the little extra. also think about the feelsafe factor.Doyou want to ruin your enjoyment? You will always have a niggling little doubt in your mind if you choose a box which is not really up to the job

Tis meself, so it is.