Prop size questions

[tinderbitzin]

I recently bought & rebuilt an 11' Bobcat Mini Tunnel. The boat has a stock Tohatsu 40 HP 2 stroke with a 3 blade SS cleaver on it. After I rebuilt the boat, I struggled with getting the boat to plane. I've since lowered the engine so that the cavitation/ventilation plate is about even with the top of the tunnel. If I set the engine angle pin to the 3rd hole from the bottom, it planes well, but to much of the hull is in the water. If I move up one hole, the boat struggles to plane, but more hull is out of the water. According to my speedo, I'm only going about 34MPH. The engine max's out at only 3600RPM, so I'm figuring the prop is too big. Unfortunately there's no #'s on the prop, so I don't kknow where I'm even starting from. How do you measure prop size, & what is the concensus on what size prop is "proper"? (pun intended). The engine while in neutural will rev over 4500. I remember with a 20' Baker Tunnel I had, someone once gave me a larger cleaver prop for it & I seemed to have similar problems.

Thanks, Tony

[ZUL8TR]

Assuming the engine is in good shape with spec compression and spec carburation the prop appears to have too much pitch and or diameter since the rpms are to low. I would think a 40 hp Tohsu should rev to 5500 - 6000 if proped right (ask dealer for the max operation revs for this engine). You need to get the prop to deliver the RPM that provides the service you want from the rig like: max speed, circuit racing, water skiing, drag racing, or?

You can easily measure diameter, that is the distance from the center of the hub to the blade tip x 2. As for pitch it is variable all over the blade and is measured on the pushing side of the blade (blade that faces aft called the blade face) and its value depends on the radius from the center of the hub. The pitch of the propellor is usually taken as the average pitch over all the blades. And each blade will not be the same but should be close (if not damaged?) so pitch measurement is not straight forward.

So here goes:

If you are adapt with measureing angles you can get an approximate pitch number for your prop to see where you are. For pitch at any point on the blade face you need to measure the angle on the pushing side of the blade from the face slope to 90 degrees to the prop shaft centerline. This measurement can be made along the circle at about 70% of the max radius to the blade tip. Take measurements at the leading edge and the trailing edge (not including any cup) and a few places inbetween at that radius.

Calculate the pitch at each angle measurement as (sorry for a little math):

Pitch (inches) = 2 x Pi x radius(inches) x Tangent of angle (tables or from calculator)

Pi approx = 3.142

Take the average of all the pitch calculations for the single blade, then do this for the other blades at the same radius and average the results for each blade to get one pitch value (remember this is approximate but representative)

As far as judging your boat speed for a given pitch prop here is the fastest the boat can go with no prop slip (ie no drag to hold the hull back) and assumes the prop advances like a wood screw as it is rotated into wood.

Speed (mph) = Pitch (inches) x RPM divide by (1056 x gear ratio)

For gear ratio you need a decimal number that is usually greater than 1 and is determined by knowing the number of teeth on each of the forward gears in the gear case. Then divide the larger number of teeth by the smaller number of teeth. Or check the owners manual. Or with engine in forward gear rotate by hand the flywheel one revolution and measure the portion of a revolution the prop turns, then divide the flywheel rev (1) by the portion of the prop rev (less than 1 rev). To get more accurate gear ratio you could rotate the flywheel say 5 times and count the prop revs and do the same division of flywheel revs divided by prop revs.

Here is a link that will define prop terms and operation

http://sites.mercurymarine.com/porta..._schema=PORTAL

If you are interested in max speed forget where the cavitation plate is and deal with where the center of the prop shaft (CL) is relative to the bottom of the lowest sponson planing surface at the rear. I would start with the shaft CL at about the bottom of the lowest sponson surface. Then try different angles for lift of the hull.

Success for your application will only be achieved by lots of testing so see if you can borrow props to test to keep the $ in hand.

Hope this helps.

Pete

[tinderbitzin]

Prop size??

Ok. I removed the prop & stamped into the side of the hub is "11x18RQF". So considering I've got a stock 40HP Tohatsu 2 stroke on this 11' tunnel hull, is this too much prop? Additionally. I'm running at 1200' elevation.

[ZUL8TR]

Prop size question

Ok. I removed the prop & stamped into the side of the hub is "11x18RQF". So considering I've got a stock 40HP Tohatsu 2 stroke on this 11' tunnel hull, is this too much prop? Additionally. I'm running at 1200' elevation.

Not sure, but if you can get the gear ratio and the max recommended engine rpm a better estimate can be made.

Being at 1200 feet will reduce power somewhat and the fuel ratio might need to be adjusted to compensate for the reduced air density.