Cruising speed & RPM's - 28' Salty Dog

[Josh]

I purchased a 1973 28' Salty Dog with twin Chrysler 318, 235 hp, and I
am trying to decide if I have the right propellers. At 3250 rpm I am
only getting 18 to 19 knots, which seems slow to me. I can get up to
23 knots at 3800 rmp, but feel I am putting a lot of pressure on the
engines not to mention my gas bill. Any thoughts what the cruising
speed of the boat should be and at how many rpm's? Any input is
appreciated.

[fishinfuul]

Josh, I have a 28 foot Mega, I think it is the same hull as yours or
at least similar, powered with twin 305 Crusaders.

I have the original shipping checkout list from Uniflite, it came with
16 X 14 3 blade cupped props.

The manual for the boat suggests the choice of props should allow the
engines to meet their rated RPM at wide open throttle, in my case
4,000 RPM. I have read some information on the net from a noted Marine
Surveyor suggesting the correct props will give you a wide open
throttle approximately 500 RPM lower than the engines rated RPM at
wide open throttle.

Google David Pascoe for more information. (the surveyor I mentioned)

[Josh Rosen]

Thanks Jim. I have 16 x 14 props, but I think I will take them in to S&S to
see if they are still OK. They have been refinished a few times after
hitting logs, so probably worthwhile checking.

Out of curiosity, what speed to you get at 3000 rpms?

- josh

[Fred]

I repowered my 28' Sport Sedan with 5.7 L GM Small Blocks last year.
I heard different horsepower ratings but I would say 275 HP each is
the real number (Pre Vortex Heads). It has straight drives with 1:1
ratios. At 3000 I get 21-22 knots off the GPS. At 3200 with the 4
barrels open more like 24 knots. Right now I believe I am a bit over
proped as I top out at 4000 doing 27 knots on the GPS.

The Salty Dog and Sport Sedan are the same hull but I suspect the
Sport Sedan probably weighs a bit more and has more drag due to the
larger cabin. The above numbers were with a clean bottom and full
fuel and water tanks with the Binimi Top up and 5 people on board.

With 30 hours on the engines at 3000 rpm I figure very roughly about
20 gallons an hour. In reality its probably less.

Fred

On May 31, 5:02 pm, Josh <J...@Stellar-Alliance.com> wrote:

[Josh Rosen]

Hi Fred,
I think I forgot to get back to you, but wanted to thank you for your input.
I am still running a bit slow, and burning a bit more. Maybe next season I
will get new props.

The engines are 1986, so the compression may be a bit low as well.

Thank you again and hope you had a good summer season.
Josh

-----Original Message-----
From: Unifli...@googlegroups.com [mailto:Unifli...@googlegroups.com]
On Behalf Of Fred
Sent: Monday, June 11, 2007 9:18 PM
To: UnifliteWorld
Subject: [UnifliteWorld] Re: Cruising speed & RPM's - 28' Salty Dog

[waterguy]

Josh -

It's not so important what your cruising speed is; to determine
whether you're propped right you need to determine whether you're
turning the max rated RPM for your engines at wide open throttle.
Assuming ignition timing is right, carburetors are in decent shape,
hull is clean, props are in good shape and the engines have the
adequate compression, your 318 Chryslers should (I believe) turn 4,400
to 4,600 RPM at wide open throttle (if someone has the specs on the
small-block Chryslers, please confirm me - I have 440's and the manual
for those specifies 4,000 rpm max, but the small-blocks should turn
higher).

If you can't achieve the max rated RPM at wide open throttle (again
assuming everything else is right), you're over-propped, which is
overloading your enginea at all throttle settings above hull
displacement speed - very, very hard on them. Each 1 inch reduction
in propeller pitch should add about 200 rpm at the high end/