Practical Boat Owner's experts answer readers' questions

Did you know that PBO strives to get expert answers to all readers’ questions sent in – even if they do not get published?

Our popular Ask the Experts section calls on the help of 16 professionals, all with different specialisms.

These include: sea safety, insurance, cruising, sails, masts and rigs, surveys and corrosion, electrics, engines, gas fittings, paint and antifouling, yacht design, toilets and plumbing, trailer-sailing, electronics, boatbuilding, and wood.

Now we’re making it available to our online readers for the first time. Each week we’ll be sharing more questions and answers on a diverse range of boating topics.

Here’s a recently published question from the October 2013 issue of PBO.

Find the latest Ask the Experts’ content on pages 30 to 32 in the November 2013 issue, on sale tomorrow, Thursday 10 October.

Do I need a larger pitch propeller?

QUESTION: I am re-engining a 20-year- old 34ft Prout catamaran (4,000kg displacement) with a larger engine than original to allow for all the extra electrics we now find ‘essential’.

The original Prout engine was 20hp, but I now propose to use a 30hp Beta replacement.

I don’t know the original propeller pitch, but am I right in thinking I’ll also need a larger pitch propeller to suit the bigger engine?

I use a Sillette drive leg which offers 13in-16in three-bladed props of various pitches.

Frank Robertson County Down, N. Ireland


A propeller harnesses the engine’s power and translates it into thrust to drive the boat. That much is pretty obvious.

The mystique comes in the combination of prop diameter and pitch.

Within reason, the bigger the diameter the more efficient the prop: that’s why modern jet engines are short and fat, rather than the old-fashioned long and slim.

The pitch of the propeller equates to the angle of attack of a wing, keel or rudder. The greater the angle of attack the greater the lift, until it reaches a critical angle at which the blade stalls and the lift falls away rapidly. The thrust of the propeller is a combination of its diameter and angle of attack. The angle of attack depends on the forward speed of the boat, the rpm of the propeller and the prop’s pitch.

The pitch of a propeller can be likened to a wood screw’s pitch. If a wood screw has a pitch of 5mm, for every turn of the screw it will pull itself into the wood 5mm. The big difference between a wood screw and a water screw is that because water isn’t solid a prop doesn’t pull itself forward by the pitch of the screw. There is slip, and the difference between the pitch of the propeller and how much it actually pulls itself forward is the angle of attack – and that is the function that contributes to the thrust of a propeller.

Fixed-pitch prop

Normally, a fixed-pitch propeller is matched to the maximum power of the engine, maximum rpm and maximum speed of the boat, which means that at all other combinations it’s not going to be efficient. It’s like having a car with only one gear, where the gear ratio is chosen to give the car its maximum speed. The only way of starting from a standstill is to slip the clutch until you’re going fast enough for the car to accelerate to its maximum speed in ‘top gear’ – a very inefficient process.

Variable-pitch props are available for a boat of course, but they are pretty expensive.

Twin complications

A catamaran has two slender hulls that have less drag than a similar displacement monohull, but things are changed by the spacing of the hulls as their inside bow waves interfere with each other.

Normal monohull displacement speed formulae for hull drag don’t apply – it’s an area that requires the knowledge of a specialist.

Sillette have a long association with catamarans, and they will be the best people to advise you on both the suitability of your drive leg to accept a 30hp engine and also to advise you of the correct propeller size. Visit

Engine size

It’s worth remembering that a diesel engine should be run at a minimum of 50% power for prolonged engine life. With a fixed-pitch prop, this equates to about 75% max engine rpm, or in your case, 2,700rpm. With 30hp, this could provide too high a cruising speed, which may be uneconomical.

A Beta 25 with the optional 70A alternator may be better – it would be worth discussing with Beta. The engine power absorbed by an extra 30A of alternator power would be no more than a couple of hp.

It also depends on your actual cruising displacement, of course, which could be far in excess of the nominal 4,000kg.

For more on props, see ‘Understanding your propeller’ PBO October 2010. You can buy a copy online or by calling the Copy Service on tel: 01202 440 832.

Pictures show: The Ask the Experts pages, October 2013 issue; a Prout 34 catamaran like the boat Frank Roberts is re-engining; a prop diagram – pitch is the distance the prop ‘screws’ forward in one complete revolution.