Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

PART ONE: DRIVETRAIN INSTALLATIONS

Figure 1-8A.
Pocket viewed
from the stern
(rear)

Figure 1-8B.
Pocket viewed
from starboard.
The arrow indi-
cates the forward
end of the tunnel,
which is too short
on this boat

vibration, and the smaller diameter should

reduce the portion of blades in interrupted

water flow.

POCKET-ORTUNNEL-EDGEALIGNMENT TO

THEPROPELLERAnother thing to check in a

pocket or tunnel is the geometry in section.

You want to ensure that two blades don’t

line up with the pocket or tunnel corners at

the same time. For instance, if a prop were

tucked up exactly halfway into a tunnel, you

wouldn’t want to use a 4-bladed wheel,

because two blades would line up with the

tunnel corners at the same time, which can

cause vibration. Instead, use a 3- or 5-bladed

prop (probably 5 for adequate blade area).

Other vertical prop locations and tunnel

configurations might line up with a 3-blader

or a 5-blader. Such alignments should be

avoided.

Some deep pockets with V-shaped cross

sections pose another potential pitfall. The

V shape can have the blades approach closest

to the underbody (minimum tip clearance)

twice instead of once (Figure 1-10). This dou-

bles the potential tip noise. Once again, a

smaller-diameter prop with more blades

(maintaining adequate blade area) is the

answer.

V-STRUTS ANDPOCKETS Still another diffi-

culty with a pocket or tunnel is the strut.

Keep in mind that the pocket or tunnel

already restricts water flow, at least to some

degree. For this reason, I don’t like to use

V-struts in pockets or tunnels; I prefer I-struts

(also called P-struts). This introduces one

less strut leg to further restrict water flow

and cause vibration. Struts are discussed in

Chapter 2.