SAIL - April 2015

ASK SAIL

BOAT

WORKS

my 40-year-old engine and transmission. How

would you recommend sizing a new prop for

this configuration?

Nikos Singelis, via [email protected]

NIGEL CALDER REPLIES

First, there should be no problem

in running the engine at higher

rpms, although you will see the fuel con-

sumption go up quite dramatically for small

increases in boat speed as you get closer to

your boat’s nominal hull speed. Second, it

seems to me that 6.3 knots at 1,500 rpm is not

a bad place to be! Sizing for a new propeller

is a moderately complex process that involves

the relationship between engine power, shaft

rpm (which in this case is the same as engine

rpm), propeller aperture size (which relates to

propeller diameter), and propeller pitch. Dave

Gerr’s Propeller Handbook has an excellent

description of all these factors, together with

various nomographs and tables that make it

relatively easy to work through the process to

size the propeller. Most propeller manufactur-

ers will also provide a similar service for free

as part of the process of selling a propeller.

In addition to the Kiwi propeller, you might

look at some of the folding propellers. In

testing I have done, the latest generation of

these propellers (we tested the Flex-O-Fold,

the Volvo-Penta propeller and the Varifold)

all performed extremely well, and were also

impressive in reverse.

FUEL TANK EXPANSION

Q: I have a Beneteau 42s7 and plan to do

extended cruising. The stock fuel tank is

an aluminum 40-gallon unit, just aft of

the engine. Just abeam of it is a 70-gallon

fiberglass water tank. I plan on installing a

watermaker and would like to convert the

70-gallon water tank to a fuel tank. I will

probably remove the fuel tank and install a

smaller water tank to supplement the other

70-gallon water tank located forward. That

would still give me two separate water tanks

for safety. Is there any reason the fiberglass

water tank wouldn’t be a suitable fuel tank?

Any tips for the conversion?

Russ Whitford, Milwaukee, WI

DON CASEY REPLIES

I cannot encourage this conversion.

While it is true that diesel fuel

is widely stored in fiberglass tanks, unless

Beneteau contemplated fuel storage in this

particular tank at the time of its construction,

it may be entirely unsuitable. Diesel fuel may

exploit weaknesses in the tank construction

and penetrate or even dissolve the tank

walls. Biodiesel additives are a particular

concern that could grow larger in the future.

Even today, bio additives may be included

in the diesel you bring aboard without your

knowledge. It is worth noting that many

fiberglass fuel tanks originally engineered

for gasoline had to be replaced (at significant

expense) when ethanol gasoline became

the norm and these tanks began to dissolve.

Unless you can get assurance from Beneteau

that this tank is safe for diesel storage, you will

be wiser to take a different tack. Also be aware

that if you decide to “give it a try,” you cannot

convert back. Once diesel has been introduced

into the tank, it can never be used for storing

potable water again. s

[email protected]

http://www.varipropusa.com

747 Aquidneck Ave.

Middletown, RI 02842

AB Marine, Inc.

401-847-7960

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