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Written by Staff of the Quadrant Marine Institute
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Q. I have a 28' boat with a 7.4-litre MPI 310-horsepower engine in it
with a Bravo 2 leg, and I would like to increase my travelling distance
due to the fact that we do not have many gasoline facilities on the
water where I boat. I was thinking of repowering to diesel or updating
my leg to duoprop. If I go diesel, do I need 300 horsepower or can I go
with less and stay on plane? Would there be any advantage to staying
with gas and switching to a duoprop leg, other than it being the
cheaper route to go and a faster change over? —Bob Bugg
Q. I have a 28' boat with a 7.4-litre MPI 310-horsepower engine in it
with a Bravo 2 leg, and I would like to increase my travelling distance
due to the fact that we do not have many gasoline facilities on the
water where I boat. I was thinking of repowering to diesel or updating
my leg to duoprop. If I go diesel, do I need 300 horsepower or can I go
with less and stay on plane? Would there be any advantage to staying
with gas and switching to a duoprop leg, other than it being the
cheaper route to go and a faster change over? —Bob Bugg
A. There are many areas to consider when deciding whether or not to repower a boat and every situation
is different. Our answer can only deal with general principles, and
we’ll have to make some assumptions about your specific case:
1. The boat is valuable and worth putting serious money into;
2. You plan to keep it for several years, at least; and
3. You use it enough, and burn enough fuel, to realize cost savings over a reasonable time period.
First of all, you indicate that your main problem is range and
availability of gasoline fuel. The obvious and infinitely cheaper
option would be larger fuel tanks, if space and weight constraints
permit. However, this would not increase fuel economy; probably reduce
it.
If you are considering repowering to a diesel engine, you need to take
into account that they are more expensive to purchase than gasoline
engines; diesel engines of comparable horsepower weigh more than gas
engines; diesel engines can provide substantially greater fuel economy;
and diesel fuel is less volatile and not explosive so it is safer on
board your boat.
Since you have a planing hull design, you must be careful about adding
weight to the aft section of the hull, as it will affect trim and
planing ability substantially. Also, the engine beds may or may not be
adequate for the additional loads of a diesel engine.
Changing over to a new diesel engine with a similar performance curve
that weighs close to the same as your original engine would be
suitable, and it would most likely have a lower BHP (break horsepower)
rating—although enough to drive your boat. The new diesel engine with
the duoprop drive leg would certainly help with fuel economy; however,
the cost of purchase and installation may outweigh the fuel savings.
This will depend on how long you intend to keep the boat, if you want
to realize the accumulated fuel savings over years.
The duoprop only leg option will also increase fuel economy by
approximately 10–12 percent, although not to the extent of the diesel
engine option. This transition is also quite expensive, although less
so than the diesel changeover. Bear in mind that any damage from debris
in the water will be a costly repair compared to a conventional
single-propeller drive leg.
Find an experienced mechanic/technician or marine surveyor who will
give you an honest evaluation of the situation and who will stand
behind the advice given before the next step. Repowering with a
different engine or drive train is never simple or cheap.
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