Outboard trim angle and prop depth

[kpgraci]

All info I can find online deals with planing hulls.

First trip with the Nissan 6hp 25" shaft on the oday 22 at WOT I got good speed (6.2 knots) but the bow was higher than I thought it should be. We had 2 adults in the back and the motor was trimmed to 90* (based on how it looked while trailered). My motor mount cants back, maybe 12*-15* or so. To get the motor to 90* I had to go up 5 or 6 notches, which seemed like a lot.

With the bow raised it appeared I needed to lower the trim a bit, so I went down one notch with no noticeable difference. Circumstances prevented me from trying other settings.

It occurred to me that with the 25" shaft the prop is about 5" below the bottom of the hull and this may be causing the lifted bow due to the leverage achieved by this distance. I imagine a 10' shaft then pushing on the prop, the bow will raise pretty much regardless of the trim angle. If I trim all the way down the thrust will be pushing down quite a bit which seems wasteful. I could raise the motor 5" which would put the center of the prop right at the bottom of the hull.

Another issue could be the speed, 6.2 is above my hul speed (by .3 knotrs or so) , so perhaps the bow was trying to climib over the bow wave...pluus we were sitting all the way back in the boat, one manned the motor the other the tiller.

 

[kloudie1]

Yup.. Bow wave is most likely what is doing this, Ken.. This is indicating a couple of things.. one: ya have plenty power to push through nasty chop.. and two, with only about 50 more horsepower ya could have her planing! ;~)

 

[GeorgiaBII]

Hehe wouldn't it be a hoot to plane out a standard sailboat?!

I've played with my trim a lot trying for the best solution with no luck. I'm using a 9.9 2 stroke and have the motor straight down to even a bit tilted forward and no improvement.

If I go full throttle the stern sinks to the point where I have water coming in the cockpit drain!

 

[kpgraci]

I suppose the thing to do is get the boat to just under hull speed and then play with the trim, with a more standard crew balance as well.

 

[sailormanbigd]

The info is for planning hulls, the depth of the outboard should be, the bottom cavitation plate should be in line with the bottom of the boat hull. That is the usual position to get a good bite with the prop, good water flow to the cooling water pickup and to minimize drag from the lower unit or foot. the plate should be parallel to the bottom of the hull as well. By adjusting the tilt you can change the trim of the bow/stern, tilt it back a little and you raise the bow slightly, tilt forward and the bow goes down and stern rises a little.
For a non planning hull it is less of a concern. You want the prop deep enough in the water so that it bites when you go forward to the bow to ready the anchor, hoist the jib, or when riding over waves. Having the motor break out of the water when hit by a 3ft boat wake is very hard on the motor as it over revs and temporarily looses cooling water. Remember it is not a planning hull or power boat. You have more power than the boat requires, pushing it too hard causes the stern to squat and bow to lift and consumes a lot more fuel. Put the motor as deep as you can, tilt the motor to 90d or less and throttle back to 50%. Boat will handle good, motor will last longer, etc. Be comfortable in the fact that in a blow, or stiff current the boat motor will be up for the task. If you want to plane get a power boat or one of those Macgreggors with
the 50hp motor.

 

[kpgraci]

The info is for planning hulls, the depth of the outboard should be, the bottom cavitation plate should be in line with the bottom of the boat hull. That is the usual position to get a good bite with the prop, good water flow to the cooling water pickup and to minimize drag from the lower unit or foot. the plate should be parallel to the bottom of the hull as well. By adjusting the tilt you can change the trim of the bow/stern, tilt it back a little and you raise the bow slightly, tilt forward and the bow goes down and stern rises a little.
For a non planning hull it is less of a concern. You want the prop deep enough in the water so that it bites when you go forward to the bow to ready the anchor, hoist the jib, or when riding over waves. Having the motor break out of the water when hit by a 3ft boat wake is very hard on the motor as it over revs and temporarily looses cooling water. Remember it is not a planning hull or power boat. You have more power than the boat requires, pushing it too hard causes the stern to squat and bow to lift and consumes a lot more fuel. Put the motor as deep as you can, tilt the motor to 90d or less and throttle back to 50%. Boat will handle good, motor will last longer, etc. Be comfortable in the fact that in a blow, or stiff current the boat motor will be up for the task. If you want to plane get a power boat or one of those Macgreggors with
the 50hp motor.

Right. That sounds reasonable. I was pushing that day to avoid squalls, normal motoring in and out of the harbor will be at a more leisurely pace. I will play with the trim a bit more at lower speeds to see what works best.

My old motor (15") would come out in chop and when I went forward, I'm really looking forward to that not happening anymore.

Thanks all!

 

[caguy]

Unless you are off setting that transom angle is your motor really 90* to the boat line?

 

[kpgraci]

Unless you are off setting that transom angle is your motor really 90* to the boat line?

Transom rakes forward, motor mount rakes backward. (or is it the other way round?) In the picture this is where I had it trimmed for the sail.

 

[caguy]

That looks about right.