Motor Height...

Ok, so I know this type of question gets asked all the time, but I have some detailed questions that I didn't find answered.

I'm trying to decide whether my motor height is acceptable. First off, does it look reasonable? I don't have any cavitation (clearly, if anything it's mounted too low).

Second, If I do want to move it, do I even have the ability to do it safely? My motor (details in sig) only has screw clamps, and from what I can tell there is no sort of adjustment on the motor to change the height it attaches to the clamp assembly. Can I just screw it on an inch higher on the transom to try it out? This seems like a bad idea to me, because then the top end of the clamp wouldn't be contacting the top (cap) of the transom.

Third, I want to compare with the common opinion of having the anti-ventilation (cav) plate 0-3" below the bottom of the hull, but when figuring this out do I measure from the bottom of the flat part of the hull as labeled #1 in the pic, or from the bottom of the center keel (I think that's the right term...) which is labeled #2 in the picture.

Maybe I'm just overthinking this, because I don't really have any performance issues, but then again, the motor has always been mounted this way so maybe I don't know what I'm missing.

1 by Photo 144, on Flickr

2 by Photo 144, on Flickr

If you don't care about getting more speed than you currently have, don't need to be concerned about running in shallow water or hitting underwater obstructions, and you're happy with the mpg and overall performance of your boat, I wouldn't mess with it. If you'd like to improve any of those, then you might want to play around with the height of your motor. When I want to do some testing with motor height, I use wood shims (cut to ¼" and ½" thickness) to put on top of the transom under the C clamp. Then I tighten the clams down good and go test. I've been playing around with mine lately trying to get the best combination of speed and time to get on plane. Right now I've got 2 x ½" shims under mine and I've picked up 3 mph vs having the motor all the way down. But I'm still playing with the tilt/trim and shifting weight around to get on plane quicker. Once I get done with my adjustments and am sure it's where I want to leave it, I plan to bolt the motor to the transom for more of a permanent attachment. I used #1 in your pics as a ref/starting point for motor height and have adjusting up and down from that for my testing. One other advantage of getting the motor to the best height for performance is that you could probably loose the whale tail and pick up even more top end speed. If you've got the height right and the weight distribution correct, I wouldn't expect you to need the whale tail for a 40 horse to get that boat on plane quickly unless your boat and contents are extremely heavy. Opinions will vary but that's mine.

He's right. That whale tail is a bunch of parasitic drag.

All good advice above.

Looks too low to me.

I'd measure from #1

Whale tail can solve some "hard to plane" issues, at the expense of speed. Tabs are better, but might not work for your hull. Take the tail off and see what happens, before you change anything else. Often "hard to plane" issues have more to do with weight distribution, than anything else, especially in our small hulls.

However,.....I still believe in the old adage....if it ain't broke, don't fix it......

richg99

This may help.

Thanks much, guys! I actually just put the hydrofoil on recently. I like it, and I know it just covers up underlying issues (mine being a stern-heavy condition which is inherent in how I built my rig) but I really like it and I think I'll keep it on.

As for parasitic drag, I know this has been beaten to death all over the internet and there are very mixed opinions, but I agree with the thought that there ought to not be any extra drag (or top speed loss) if your motor is set at the correct height, hence the reason I started this thread. If properly set up, the hydrofoil should be almost, if not completely, out of the water when on plane. Sure there's extra drag while getting to that point, but the lift provided by the hydrofoil shortens the time you spend in the hole.

Sorry, I got a bit off track there. Regardless of the hydrofoil, I want my motor at the proper height. I will use your suggestions, starting with some shims, to test different heights and related performance. It's no racer, so I'm not going to get too caught up about it, but I like to have things as they ought to be.

Thanks again to all who replied, and I'll post my results once I get around to it.

Gotta admit , I've never seen a fin / foil on a lower unit gear case. Almost seems like you would not need a foil but from the build thread I know the boat is "butt heavy"






I had a 20 ft Cajun like that,wallowed like a pig in slop on the hole shot, but once the 27P prop started hooking up , she took off

mgros483 said:

... but I agree with the thought that there ought to not be any extra drag (or top speed loss) if your motor is set at the correct height, hence the reason I started this thread. If properly set up, the hydrofoil should be almost, if not completely, out of the water when on plane.

Click to expand...

Me too! IMHO and in testing a 16' skiff w/ 40hp equipped w/ GPS-enabled fuel
computer, when properly setup, the benefits are many!

-Shorter time to plane
-Improved visibility while planing, keeps bow down
-Ability to run on plane at much slower speed (improves fuel
economy up to 50% vs not being on plane
-Increased stability tracking
-Increased stability in turns and when crossing wakes
-Boat becomes LESS sensitive to typical out-of-trim conditions and trim to a small boat is everything!
-Makes the boat safer, but that is more of my opinion than fact-based, less the known stability improvements