Question about fuel consumption-Colvic 43

[brianchesters]

I have just returned to motor boating but am now in the typical dilemma. Previously, I had the money to use both engines flat out but no time to enjoy boating. Now I have retired, I have the time but not the money.

I read in this months MB&Y about the cost of motor cruising and a displacement 53ft boat that uses very little fuel at about 7 kts. I have just bought a 1990 Colvic 43 with 370 HP Ford Sabres. I believe the consumption is about 1.3 miles per gallon flat out. Is this correct? What if I cruised at 7 knots with the engines just ticking over. What would the cost per mile be then? Finally if I do the same speed with just one engine and remove the unused prop for the journey? Any ideas on fuel consuption then? Brian in Lancashire.

 

[longjohnsilver]

Usual calculation is 20hp = 1 gallon so flat out 740hp would be approx 37 gallons per hour which I guess for your boat would be nearer 0.5 mpg! At 7 kts I'd expect you to get about 2.5 mpg, give or take a bit.

I've never come across the idea of removing one prop for a longer trip, how would you practically achieve this bearing in mind the usual difficulty of getting the darned things off when you want to! As for doing it in the water........

 

[Whitelighter]

From your description I guess you have conventional shaft drives rather than outdrives on that boat.

I would expect the consumption of the boat to be about 1.3mpg on the plane, but not flat out. At the top of their range I think you would probably be lucky to get 1mpg if the throttles are on the stops.

You will find consumption much better at displacement speed, which for your hull is probably just under the 7knt mark. At this speed, with both engines running you will probably find the consumption drops to around 3mpg.

I think running just one engine is a bit of a false economy, but ehile the mpg figure will probably not be much better, it will keep the engine hours down if you alternate which engine you use.

All mpg figures are subjective to see state, tidal flow, hull condition etc etc which is why you will fing most consumptions quoted in lt/hr for given revs

 

[volvopaul]

I would say that you will get about 0.9 mpg on the plane at 18 to 22 knots I would think your boat wil be flat out at 25 knots with a clean bottom and reasonably loaded at half tanks etc.
Flat out you would burn a lot more fuel and see the mpg drop to 0.7.

The colvic hull is a good hull but not inline with todays faster planing hulls that give better mpg per given hp etc.

If its the 43 im thinking of its a nice design there used to be one on the broads at Brundall some years back owned by the marina owner, some were fitted out as home projects others by reputable builders.

 

[Bandit]

My boat 47ft waterline about 44ft 18 tons 2x 370hp.

Cruising speed 19.5 knots, tops 23 knots 6 litres per mile 1.32gallons per mile 0.76 mpg

 

[brianchesters]

Here are a few more details. This Colvic is the 'home built' type in that the hull was bought in by a boat builder. She has conventional shafts. I am pleased to read that at 7 kts or just under, I will get about 3 miles per gallon. She is berthed in Deganwy and is a bit dated. One owner from new in 1990 and no upgrading of electrics since new. The owner died last year. I plan to make the first trip to the Isle of Man TT race next year and then down to Brest in Brittany for 2 years.
My previous craft was a 2000 Cranchi Zaffirro 34 called Bi-Polar which is now berthed in Brixham. I plan to replace all upholstery and carpets and headlining. Possibly a new chart plotter etc. Brian Chesters