Leg Height Myths.-

[Sea Rider]

This experiment is the result of testing same portable engine with standard delivered prop on a light weight rib but sitting leg at 2 diametrically opposed transom heights.

Being reading about boaters advising to raise transom engine height a bit so prop spins closer to water flow to achieve better top speeds. Not knowing exactly what to expect and counting with ideal close environments with flat, no wind water conditions went for the on-site water test.

Was pretty sure that running a prop with anticav plate riding close to water flow would increase slightly top speed on flat straight courses, but not necessarily better overall performance. A lightly loaded 420 rib, with just driver, a 18 HP 2 strokes engine and 2 gallons of fuel was used on both test trials.

Test 1: 1 mile run at wot, water flow passing slightly under small upper plate : 41.3 Km/h at 5780 rpm (Pic-2)

Test 2 : 1 mile run at wot, water flow passing slightly over anticav plate : 39.5 Km/h at 5580 rpm. (Pic-3)

According to both test trials, the prop that sat at ideal transom height which is the sweet height spot (pic-2) for Tohatsu engines, achieved best prop grip, top speed, 200 + rpm increase, best hole shot and superb close turns than on same test 2.

The myth about achieving better engine performance with standard prop and raised leg proved to be not true at least with Tohatsu horses.

Happy Boating

 

[colbyt]

Re: Leg Height Myths.-

This is of serious interest to me.

Did I read it right? You did better with the cav plate higher than factory specs?

 

[JB]

Re: Leg Height Myths.-

I really think your interesting results are unique to your boat, Sea Rider.

Though we sometimes take generalities as firm rules about AV plate height relative to a boats bottom, I have found that the peak performance changes with changes in boat loading and balance. That is why so many invest in jack plates to make fine adjustments.

 

[Sea Rider]

Re: Leg Height Myths.-

Hi JB,

Assume my interesting results are unique to my engine brand, not boat, brand which has been tested in other light boats too with same results, it's just only a engine height optimization for best water performance, not trying to discover gun powder again.

To be spot on as the boating sac is enormous and not mixable, this post is related to only portable, tiller, mechanical trim, 8 to 40 HP OB's, placed on light boats category, probably the post should have been posted under sib/rib inflatable boats, but also applies under engines. This tested fact applies perfectly to Tohatsu and all rebadged same HP brands engines too.

Colybit, distance from lower hull to anticav plate stated on all Owners Manuals is just a refference from whete to start, as there are many variables such as boat tyype, hull, weight, HP among other parameters to have into consideration. But definitely best boat/engine transom height performance will be only achieved under trial and error and up to you if pleased or not with your actual engine height setting & overall performance.

Happy Boating

 

[Bronlonius]

Re: Leg Height Myths.-

I'm not sure I'm interpreting your picture correctly, Are you saying you got better performance with the motor lower? Every boat is different, but I'm really surprised to hear this.

I started out with the motor in a similar position to what your picture shows, but ended up raising it considerably. The difference was literally night and day. I have a 12' aluminum boat, The transom is 15", the lower unit is 20", so I initially built my jack plate because it was cheaper than converting to a 15" shaft, thinking I'd just set it to 5" of lift and be done. I had no idea how important motor height was until I started playing with this. Keep in mind my jack plate has 5" of setback, so I am probably able to run higher than most people would, but I have it jacked up 8.5"(So 3.5" higher than it would be if the motor matched the transom). The difference between 7" and 8.5" of lift on my boat is literally "to plane or not to plane". At 7" I got pretty much the same result with any trim angle, the bow would go up but never get on plane. I moved my gas tank and battery to the front and that still wasn't enough, but the final adjustment to 8.5"(basically 3.5" higher than it would be clamped to a 20" transom) of lift is what really changed the boat and made it feel like I wasn't dragging my anchor behind me.

I attached a picture what my height looks like, you can see that the jack plate brings the motor back pretty far, so that's one reason I can probably have my motor higher than most boats.

 

[Sea Rider]

Re: Leg Height Myths.-

Bronlonius,

If that's a 20" Tohatu engine hanging on 15" transom will experiment excessive water splash probably over transom if not out trasnom, including excessive water drag at lower tail because water flow at speed is hitting the middle round leg portion and not the lower portion where cuting edge is located to cut water flow nicely.

Nice to read that you have done extensive trial and error research to have your combo set spot on, most boaters assumes that is just a matter to sit engine on transom and that's it. Re check original post have added new pics for better clarification.

Happy Boating

 

[Bronlonius]

Re: Leg Height Myths.-

Bronlonius,

If that's a 20" Tohatu engine hanging at 15" transom will experiment excessive water splash probably over transom if not out trasnom, including excessive water drag at lower tail because water flow at speed is hitting the middle round leg portion and not the lower portion where cuting angle is located to cut water flow nicely.

It was completely unusable for anything other than idling without the jack plate. hanging it directly on the transom put the cav plate 7 inches below the keel. It moved the boat, but it was impossible to control and very slow, it made a crazy wake/vortex behind the boat. I did this test really just to see if the motor ran okay, thinking I'd need to convert to short shaft, but a few people on this forum suggested the jack plate instead, I'm really glad I took their advice.

I started with the jack plate at 5" of lift, which still put the anticav plate 2" below the keel. So I moved it up to 7", and it was better, but I still couldn't get the bow to go down. The final adjustment to 8.5" was as high as I can get it, and the the boat handles like a sports car now.

I should probably try it without the whaletale fins, they were on the motor when I got it but I don't think they are necessary now.

Nice to read that you have done extensive trial and error research to have your combo set spot on, most boaters assumes that is just a matter to sit engine on transom and that's it. Re check original post have added new pics for better clarification.

Happy Boating

Thanks. The new pictures help. My upper splash plate has a nice thin spray of water coming off of it like your picture, and the anticav plate is not poking out of the wake. So the splash you have is very similar to what I'm getting.

I've done a lot of tweaking to get the motor height and weight distribution right, but there are a few other things I did:
1. I made a steel gas line extension with tohatsu quick connect fittings at each end that allows me to remove tank(s) from the front and connect in back if need be to adjust weight distribution.
2. I got rid of the 6 gallon tank in favor of a pair of 3-gallon tanks so that I have more control over weight distribution. Also, for short trips, I don't have to carry 2 tanks.
3. I bought a sliding seat mount for the driver's seat. This lets me slide my own weight to help balance the boat better.

These all seem like little things, but they all add up. The jack plate was the best thing I could have done to the boat.

 

[Sea Rider]

Re: Leg Height Myths.-

Good to read you have finally reached the sweet height leg spot for that engine confirming top improvement over previous run tests at different leg heights. If you are still using a doel fin screwed on anticav plate and boat is not riding paralell to water level whether boating alone or with load, doel fin will create tail drag as it will be runing at an angle and not paralell to water flow so to slide smooth & flat.

Test same configuration without doel fins, and check if useless or not to have one installed. On my boat with engine perfectly height seated & trimmed as in pic 2, no need to ballance boat as it jumps on plane passing 1/2 throttle while boating solo. And that's me sitting next to engine like a gondolier.

Happy Boating