replacing BMC engine

[lilianroyle]

This summer my 40 year old BMC 1.5 finally decided to call it a day. It was very reliable until nearly the end and I am thinking to replace it with another reconditioned BMC. I do have a couple of questions.Lilian Royle is 31 feet OA (maybe 27 feet WL) and weighs around 8 tonnes. The old engine at full belt gave me 6 knots, comfortable cruising speed would be more like 4.5-5.0. I have no idea of revs as theres never been a rev counter. I was wondering what difference a larger similar engine (BMC 1.8/2.2/2.5) would make? Would a more powerful engine with (presumably) a larger prop drive the boat faster at the same revs or the same speed as the old engine with less revs. Given these engines are quite noisy, I like the idea of going the same speed with less revs as much as I do the idea of having some extra power in reserve. How much extra power is going to make a difference to a boat like mine. All advice or illumination of the tech issues welcomed.
Thanks

 

[Tranona]

The hull speed (speed at which power required to go faster rises rapidly) is about 7 knots.

You can achieve this with 35hp and a 2.6 reduction box swinging an 18*14 prop. 40hp will give you a bit more, but not much as you will have reached the maximum displacement speed of your hull.

If you are going to replace the engine, suggest you forget about using an obsolete engine type, but go for a modern engine such as a Beta Nanni Vetus Volvo or Yanmar. Lighter, smaller, more efficient and no concerns about spares in the future.

 

[Jcorstorphine]

I would stick with the BMC 1.5

If you go for a different engine, cost will be twice that of keeping to the same unit. If you buy a reconditioned unit and perhaps a new box to give say 2.5:1 reduction and swing a bigger prop you may get better speed. My main worry over the BMC however is the availability of parts for the injection pump as these CAV rotary DPA pumps are getting long in the tooth.

John (BMC 1.5 fan)

 

[Lakesailor]

I've often fantasised about using a modern car diesel like a VW or a Fiat as a boat engine. Smoother, lighter, powerful and more economical.
An indirect cooling system would make it an easier job, wouldn't it?

Or would it just be too much hassle?

 

[Monique]

You can get a VW marine diesel directly from VW. Looks very tidy too.