PRM Stepdrive

[Scubadoo]

In the last few months in the boating mags I've seen an advert for PRM Stepdrive. This is basically a automatic two speed gear box that directly replaces the stand gearbox on the Volvo Duoprop drives.

Has anyone tried it? it would be ideal for me being slightly under powered - will have a look at the boat show next week.

Regards,
RM.

 

[Guest]

No info available, really, apart from their website that says: being launched at LBS... Let me know what you think, as I would be interested (subject to cost, obviously).

I'm going to guess at £2300 + VAT + fitting per leg. Any other takers? My bet is it would be cheaper to re-engine, in the long run...

 

[jimg]

There was an article in either MBM or MBY about their test boat. I am sure the dealer will have a ccopy of it and could give you a re-print

 

[adarcy]

Re: Prob no help anyway

RM
<<it would be ideal for me being slightly under powered>>

Even a simple angled bevel drive only has 98% efficiency so added gearing is not going to help an underpowered boat unless you have an odd setup.

I always thought they were used to allow engines to rev up into the meaty part of the torque curve to get on the plane ( or pootle around a slower speed on tickover) and then change up the gearing to allow a high top speed. Unless you have difficulty getting up on the plane because the engine is lugging (which I doubt) then you just need to be sure your are able to pull maximum power revs.

If problems a prop change is an awful lot cheaper.

 

[tr7v8]

Re: Prob no help anyway

Thats my understanding as well. But given that diesels have a very limited power/torque range it seems a very good idea. The Class 1 powerboats used to use 2 speeders (later 4 speeders, now banned) to do exactly the same thing, low gear to get the engines spinning at peak torque to get it over the hump and a higher gear to accelerate once beyond the hump. Without it they wallow along with clouds of black smoke as the engines have to much load.

Jim
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[adarcy]

Re: Don\'t agree

Sorry Jim but I don't agree with <<diesels have a very limited power/torque range>>
Max torque on most smallish turboD's is around 1900 and max power 4200 (lightweight ones not heavy lumpers like big Cats etc) whereas petrols often don't get max torque until 2500 and also top out 4200-4500. Our 375 Cats rate about 1500 and 2800 so that is a narrower range.
In any case, it matters little except on v fast boats with racing style props as there is no direct gearing to the water. A car or truck will bog down an engine as it is directly geared to the tarmac whereas a prop acts almost like a torque converter. The prop spins and transfers its thrust (or suction power - let's not be pickey) regardless of the boat speed. As the boat picks up speed then the pitch works more effectively. But as long as it does not cavitate I doubt if a gearing step is going to do much good. Again I repeat a prop for a v fast boat will gain but not a "normal" engine/outboard.

 

[duncan]

Re: Don\'t agree either....

Have to take issue with some of your post (if only to drive out a better level of understanding for myself - so please don't take offence)
With TD engines being put into smaller lighter boats, and the use of duoprops, there is an increasing issue with the relationship between the resistance hump and the engines power curve. This is presumably why some smaller engines have a supercharger as well as a turbo.
In my case if I increase props from 22 to 24 it simply will not pull more than 1600rpm whatever because the boat hits the hump before the turbo comes in at 1700rpm. With 22 it gets to about 1750 rpm for the same boat speed, turbo is in, power comes piling in and we are off. At the other end of the scale the engine will pull the same revs - 200 rpm over max power - whether lightly or really heavily loaded /fouled, indicating that the larger (coarser) props would probably increase top end and maybe even fuel consumption in between.
The duo props just don't seem to allow the initial slip you refer to in your post, especially when combined with the diesel throttle/ rev relationship. Single prop on an petrol engine being the exact opposite of course - behaving exactly as you indicate.
One could argue that this is a mismatched engine / gearbox / prop / hull ; equally a pair of large trim tabs would probably enable boat to achieve more hull speed for less power initially and the engine to attain sufficent revs with 24s. But a 2 speed gearbox would seem a perfect solution......

 

[tr7v8]

Re: Don\'t agree

Sorry but your comment:-
"In any case, it matters little except on v fast boats with racing style props as there is no direct gearing to the water"
Slow turning props are not a light load as they actually slip very little and a large cruiser off the hump with props optimised for high speed planning represents a large load.
Hence as Duncan talks about above your margin with a given set of props is actually quite small. eg. increase the load on the boats by kit or more people etc. and you're likely to find the previously optimum props overload the engines and you wallow around and create huge amounts of black smoke! Gearboxes help overcome that disadvantage.
Petrols are worse that I fully agree with but in PLANING boats diesels aren't much better hence the reason why the Class 1 boats used to use aeration via pumps to reduce the prop loads, before they started using gearboxes etc.
Further as people want higher output diesels most of the higher output moves the power curve to be peakier (multi valves, peakier camshafts, turbocharging) and reduces the low down torque, the only things that don't are supercharging or capacity increases. The later increases the weight so is unattractive.

Jim
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[Guest]

Re: Don\'t agree either....

I have to agree, hence my interest in this technology. We have AD31's on duoprops, and if the boat is heavily loaded - full fuel and water and more than 4 people - it struggles to get over the hump. The reason: the turbos aren't spinning hard enough to generate any significant boost much below 3000 rpm, but with no prop slip, 3000rpm is the point where the bow wave is highest, and hence resistance is highest. A two speed box that lets me hit that highest resistance part of the hump at 3200rpm, then shifts up to a 'normal' ratio at maybe 3800, dropping the revs back to 3200 for the same speed would be ideal.

The KAD32 has a supercharger to give boost before the turbo kicks in, for just this reason. My only concern is that I doubt they'll market the thing at a 'reasonable' price - i.e. sufficiently below the cost of re-engining with KAD32s to make new engines unattractive.

 

[pistonbroke]

Re: Don\'t agree either....

The reason for developing them is to enable you to run props best suited for max speed, which would normally result in difficulties getting onto the plane. This occurs as the turbo engines develop little power at low revs due to insufficient boost.

The cost of the conversion is £1,700.00 per drive, and this includes removal of drive, conversion, refitting and sea trials to set up shift point.

Very reasonable and an excellent product. They are also developing a similar setup for the Yanmar/Bravo package.

 

[duncan]

Re: Don\'t agree either....

now that does interest me....here comes 26 pitch props and 42 knots!

 

[adarcy]

Re: Fascinating

Duncan
Please don't apologise - I may be well off beam if you are having such trouble
I will admit to surprise as I thought these new CDIs had much greater torque low down specifically useful in a marine installation specifically to get up on the plane.
I'll have to look at the engine specs again.
I accept duoprops not cavitating therefore transmitting the "thrust". We had no probs with our admittedly twin 205 petrols and never needed to bother with trimming in and out. Dead neutral was the best for fast cruise and top speed so, after a bit of playing, we just left them there.
I don't want to preach but if you are having probs with 24 pitch I assume you have tried trimming in fully until up on the plane and then trimming out until it propoises?
You are obviously underpropped if you can get over max revs regardless of weed/load etc

 

[Guest]

Re: Don\'t agree either....

Likewise, that is an interesting price. I think you could buy a pair of KAD42s for around £10k a pop, and get maybe £6k for the 31's (inc legs), so it's £3,400 vs £8,000. That's worth a look...