We visited the Boat Show in London in January and decided to replace our old engine, a Volvo MD17C, with a Perkins M30. We are currently finalising all the details so that the engine can be delivered to our boat which is in Spain.

The Perkins M30 originally specified a Hurth gearbox HBW50. We visited the Hurth stand also at the boat show and were advised that we should fit an HBW100-3 as the engine will probably have more use than the usual "pleasure" rate of 400 hours per year. I should perhaps explain that my husband retires at the end of this year and we intend to spend about six months of the year cruising the Mediterranean and it is possible that we may occasionally have to run the engine continuously for 24 hours or so.

Furthermore as we would like to keep our existing propeller we would need to run the gearbox in the B position.(Also our permanent berth is very tight and it would be quite difficult to get into with the propeller rotating in the opposite direction). It was therefore agreed that we should fit an HBW100-3 and the technical data (given to us by Hurth) stated reduction ratios of A 2.72 and B 2.15. We then received a chart from our suppliers which showed the technical data for the HBW100- 3 of A 2.72 and B 1.86. This was of particular concern as we feel that a reduction ratio of at least 2 (as per the old engine) will be required.

We contacted our suppliers regarding this who now inform us that the Hurth gearbox they propose to fit is an HBW100-2.5 with reduction ratios of A 2.53 and B 2.15. However they now stipulate that we need to also fit a gearbox oil cooler.

I am totally confused and would be grateful for your advice. Should we go ahead with the HBW100-2.5? Why should we need a gearbox cooler for that and not for the HBW100-3 or is it necessary to fit a gearbox cooler anyway? Should we consider changing the propeller and if so which gearbox should we then go for?

We are currently living and working on a very small island (6 miles x 1 mile) in the Caribbean so it would be helpful if you could reply by email as our post takes two or three weeks to reach us.

I look forward to hearing from you.

Yours sincerely

Nicky Constantine

You are probably aware that the M30 is marketed in the UK by Volvo Penta as the 2030 and is fitted with the Volvo Penta MS2 gearbox. I think that your M30 has come from the USA and is probably being sold by Lancing Marine who have some of these engines in stock.

When run in the B position, there is a reduced torque limitation because the gearbox is being used backwards and this could be important in your case. Lancing recommends the use of a gearbox oil cooler with the HBW100 when used in the B position. An alternative gearbox would be the HBW 125, which has a better torque capability for your application and is only £30 more expensive. The UK main agent for Hurth is Duffields, Tel: +44 (0)1603 426360, who should be contacted to confirm the warranty for the HBW100-2.5 with the alternative ratios. I suspect that this gearbox is a hybrid.

I think that the need for the oil cooler for the HBW 100 when used in the B position is due to the reduced torque capability and the fact that you will be pretty near (or exceeding) this limit. With a large difference in forward and reverse ratios, if the box is run backwards for ahead, the extra torque may always require extra cooling.

Your MD17 engine had a maximum hp of 35 at 2500 rpm and the Perkins M30 29hp at 3600 rpm. The gear ratio for the MSB box on the MD17C is 1.91:1 according to my figures. Taking account the different power developed you would need a reduction ratio of around 2.7 or 2.8:1 for your existing prop to be the correct size. A ration of 2:1 would need a pitch of about 9 inches and 1.8:1 a pitch of 8.5 inches. With a gear ratio of less than about 2.5:1, I think your existing prop will be too big and achievable rpm considerably less than 3600, with a corresponding loss of power. You must be able to achieve at least 70% of rated rpm at maximum throttle to avoid overloading the engine.

The PRM 120 gearbox has suitable ratios (2.5:1 in both directions) and is about half the price. However, as you need a heavier duty box, Lansing may consider this box not to be suitable. On paper it has more power handling capability than the HBW100 and does not need a gearbox oil cooler.

Yours sincerely

Pat Manley