Prop Angle

[casampson]

Hi everybody -- mounted my new Tohatsu 5 hp propane outboard in the well of my CD 25 recently. Fits nicely and is smaller than the gas-powered 6 hp. It actually turns more than 90 degrees left and right on the mount, whereas the the old motor just barely fit and was locked in. I noticed, however, that the propane motor comes out of the well at a very slight angle upward, whereas I think -- although I am not sure -- the old motor was straight up and down. Will this be a problem?

[John Stone]

No problem. All our boats have a slight down angle on the prop shaft. I think it helps counter squatting at higher RPM. My jon boat is the same to help it attain a level'ish plane. The shaft log I installed on the FR has a 5° down angle.

IMO you are good to go.

[Jim Walsh]

Not ideal, the cavitation plate on the outboards leg should be level with the waterline. All outboards have an adjustment on the mounting bracket to compensate for this.

[John Stone]

Not ideal, the cavitation plate on the outboards leg should be level with the waterline. All outboards have an adjustment on the mounting bracket to compensate for this.

I didn't even think to mention there are bracket adjustments for outboard trim. Good catch Jim. So definitely check that if you have not done so. Beecause casampson is not using the outboard on a planing hull you think the angle of the cavitation plate matters? It will cause more drag not level but it won't hurt the engine and I don't think it will noticeably affect performance underway on a displacement hull. Outboards with power trim use that surface along with the vector of the thrust to get the most beneficial trim on the boats hull when planing.

It looks better of the engine is vertical when out of the water and usually if something looks right it is right.

But it would be an interesting experiment to check the influence of the trim under power and under sail, via GPS, on a displacement hull.

[Jim Walsh]

I had a 26 foot sailboat with a 7.5 Merc. outboard for propulsion eons ago. It actually had a cut-out in the stern as opposed to a bracket or a well installation. The best starting point was to keep the cavitation plate parellel to the waterline then use the adjustments to fine tune the angle to determine what was best under most conditions. I had to run down a circuitous tidal river channel for a couple miles before entering Long Island Sound where I could set sail. I had an old Datamarine knot “rough estimater” so it took several trips under different conditions to arrive at the best overall angle. It a process…every boat will differ, but it comes to fruition eventually.

[casampson]

Thanks Jim and John for the helpful responses. I just went out to look at my outboard more carefully, and I'm a little embarrassed to report that the pitch of the shaft is completely adjustable. One of these days I'll pull her out and adjust the shaft so that it hangs vertically in the well.

Thanks again for the help.

Chris

[John Stone]

Thanks Jim and John for the helpful responses. I just went out to look at my outboard more carefully, and I'm a little embarrassed to report that the pitch of the shaft is completely adjustable. One of these days I'll pull her out and adjust the shaft so that it hangs vertically in the well.

Thanks again for the help.

Chris

Well that's good news. And a +5 for Jim W on that one.