23' Inboard 350 Mag

[CValle]

Looking to install a Mercruiser 350 MAG HPI in a 23 inboard. Has anyone done this with that engine? What is the performance like? Also, any reccomendations on improving backing-up performance. Bigger rudder? Boat does not back-up well at all.

Thanks in advance.

[Fr. Frank]

Forward & reverse performance in a single inboard is a compromise in propeller design. A propeller designed to be very efficient while turning in one direction get that efficiency at the expense of efficiency in the opposite direction. To gain efficiency in reverse will necessarily mean a sacrifice of efficiency going forward. The sacrifice is not always proportional, however. A 10% sacrifice in forward rotational efficiency can mean a gain of 30-40% efficiency in reverse.

That being said, one thing most seamen tend to forget is that a single screw inboard hull, while backing, can only turn in one direction. An inboard propeller is often called a wheel, which helps to understand how it works. A right-hand rotation screw is turning counter-clockwise while shifted into reverse. If the wheel (prop) was touching the ground, which direction would the wheel want to roll? When backing, the hull will want to go in that directions, also. This is called "prop walk". Also bear in mind that in reverse, thee thrust of the propeller is directed AWAY from the rudder. For these reasons, a right-hand rotation single screw inboard will not back to starboard, and usually won't back up straight, either. AT best, they will have only a very slight tendency to back to port, being nearly straight.

A bigger rudder won't stop that, although the offset of a rudder and the shape and can help a little. For this you need to find the proper Reynolds number of the foil, and how changes to the foil affect you particular hull shape. (A rudder is nothing but a vertical hydro-foil)

After all that, know that squared blades on an inboard wheel are generally very inefficient in reverse, as are any wheels with cupped blades, although both are very good forward. If you spend a lot of time offshore at trolling speeds, a four-blade is good, and generally gives you better acceleration as well. Three-blade heavy-cup props are better for running on plane in rough water, have a much lower slip ratio, a lower cavitation factor, and are less likely to ventilate in a chop.

Beyond this, for propellers you get into rake, angle of attack, progression of pitch, and pretty detailed stuff.

Bottom line: learn to use what you have well first, then consider changes, and test each change carefully.

[1bayouboy]

Great explanation Fr Frank....

[riprunner]

CValle,
It's never going to back up well, it's just the nature of the beast. I went with a bigger rudder and it did help a bit, I'll get you the specs.

I have the 2004 350/350HP MPI and my numbers aren't really any better than anyone elses. I basically have a 19-21 knot cruise with a full fuel and a couple people. I also have a big T-Top so I think that might make a difference as well.

I'm just down the harbor from you so feel free to hop on for a ride anytime after the first of May!

[Entourage]

I have a 1973 Tsunami i just repowered with a 350 mpi mercruiser with bravo 1 outdrive. WOT is 42 knots cruises at 28 knots at 3600 rpms, tacks out at 4800rpms. Fuel injection is the best thing since sliced bread for that motor. Oh yeah 19 pitch prop mirage.

[Miles Offshore]

my rudder is slightly oversized as well and will back to the right everytime!!!!! If you have a large area of water i can eventually get her to back straight. To back to the port when docking you basically have to walk the boat in , forward with a little throttle wheel hard right, then reverse and wheel left. If you are doing this against an incoming tide with the pier on your port you will be in for a treat. Just dont try showing off your docking skills with this scenerio! But hey- once you can run a single screw inboard with proficiency you can run any boat out there.

[CValle]

Riprunner,

I know your boat and have always liked it. My dad's boat is the 23' inboard "Questor" and mine is the recently restored 23' outboard "Sara B". We have waved back and forth a few times. Would love to hook up for some fishing this spring.

Thanks for all the replies.

Christian