What effect does motor height have on attitude (bow lift, porpoising, etc)?

[rakucana@omegafive.net]

In retrospect, whales tales are for cruising/hole shot, not WOT performance. For that, they are just something else to add to the drag factor. On trim being touchy, bet you look at how much of the tail gets wet at your different WOT trim angles and you'll see the reason. On 6k rpm, I ran a 115 hp 1988 Merc I6 Tower of Power (rated at 5500 max) at 6k every chance I got for 7 years. Sold it to my SIL and last time I checked, many years ago now, he was still running it. It loved rpms.

Once the anti-vent plate ceases to dip down into new water (water flowing out from the transom) you will essentially lose the ability to lift your bow (front of the boat). Trim positions will have minimal affect as compared to having the plate in the fresh water. Any bow lift occurring at-near WOT when set high will depend solely on hull design and thrust angle.

With the AV plate even, you are currently able to dip into that water with the front of the AV plate and have a pronounced effect on it's attitude. With that said, your best bow down configuration is max neg trim....midsection up against the boat transom.......tilt pin removed. Also firewalling the throttle to get past the bad spot as fast as the boat will let you. As you come up on plane and the bow falls, trim out as you go till you get the desired boat "attitude" relative to the speed and water conditions.

 

[Faztbullet]

Since the K style prop is designed to surface a bit ..loose the whale tail and retest

 

[JimInAlabama]

Since the K style prop is designed to surface a bit ..loose the whale tail and retest

Yep, good advice. I spent an hour with it yesterday and there's nothing to not like about it but with the motor raised it might do just as well without the hydrofoil.

 

[Sea Rider]

Yep, good advice. I spent an hour with it yesterday and there's nothing to not like about it but with the motor raised it might do just as well without the hydrofoil.

Just for the record, with motor raised high, would you say that the combo is an excellent water performer on all water cond say : flat calm, windy, wavy, rough and at tight close turns at speed ? If with prop aeration on other than on calm water cond it's only good for running at straight water courses at speed.

This is the recommended boat/motor parameters for larger motors and boats as well for fast cruising, family oriented boating. Over speeding which has their own parameters is not considered...



Happy Boating

 

[Faztbullet]

That information is way out of date and Tohatsu recommendations to not apply to other OEM Engines.
BRP states to run engines at highest trim position without excessive ventilation and to mount engine AV plate even with bottom of boat and raise 1 hole until you loose performance, then lower one hole. You cannot argue with results that work and MFG recomendations

 

[JimInAlabama]

Just for the record, with motor raised high, would you say that the combo is an excellent water performer on all water cond say : flat calm, windy, wavy, rough and at tight close turns at speed ?



Happy Boating

Yes, I ran in calm water, chop, upwind, downwind, crosswind, and made turns at top speed (as tight as I dared) and it performs really well, I think it handles choppy water better than before. And remember also that the boat performed well before, my only goal when I started this was to be able to stay on plane at slower speed.

And since I raised the motor after putting on the foil, I will pull the foil off next time it's out of the water and test it without and post results.

 

[Sea Rider]

That information is way out of date and Tohatsu recommendations to not apply to other OEM Engines.
BRP states to run engines at highest trim position without excessive ventilation and to mount engine AV plate even with bottom of boat and raise 1 hole until you loose performance, then lower one hole. You cannot argue with results that work and MFG recomendations

Sorry Fuz, that fantastic speculation of yours is not posted in any Tohatsu Owner's, Service Manuals or Bulletin, each motor brand has their own particular installations which by the way NONE states that AV plate must sit even with the boat's bottom and that motor must run fully trimmed up while cruising at speed, if running so matching the first drawing won't see a thing up front, a fantastic recommendation for direct disaster boating.

Why are you so darn parameterized with Tohatsu, you think that all my posts are related with this brand, well no, the post corresponds to modern 4 strokes large HP motors installations stated on their respective Owner's Manual which you don't read nor have the slight idea about their existence.

Good to read that the OP's combo handles nicely at different water cond at speed, way to go...

Happy Boating

 

[Texasmark]

Just for the record, with motor raised high, would you say that the combo is an excellent water performer on all water cond say : flat calm, windy, wavy, rough and at tight close turns at speed ? If with prop aeration on other than on calm water cond it's only good for running at straight water courses at speed.

Happy Boating

Obviously NO! You knew that....reason you posted as you did.

 

[Texasmark]

Once the anti-vent plate ceases to dip down into new water (water flowing out from the transom) you will essentially lose the ability to lift your bow (front of the boat). Trim positions will have minimal affect as compared to having the plate in the fresh water. Any bow lift occurring at-near WOT when set high will depend solely on hull design and thrust angle.

With the AV plate even, you are currently able to dip into that water with the front of the AV plate and have a pronounced effect on it's attitude. With that said, your best bow down configuration is max neg trim....midsection up against the boat transom.......tilt pin removed. Also firewalling the throttle to get past the bad spot as fast as the boat will let you. As you come up on plane and the bow falls, trim out as you go till you get the desired boat "attitude" relative to the speed and water conditions.

This looks like verbatim what I said earlier in this post......................before I commented on chunking the tail.

 

[Faztbullet]

Sorry Fuz, that fantastic speculation of yours is not posted in any Tohatsu Owner's, Service Manuals or Bulletin, ​

Well you must not read your manuals... it plainly states in a large HP (250Hp 4 stroke) on page 33 of the Tohatsu manual to mount the engines AV plate 0-25MM or 0-1in ABOVE bottom of boat. So Tohatsu does state along with BRPs and Mercury manuals to mount engine AV plate even with bottom as starting point. I known you haven't read a BRP manual. Maybe one day someone will take you for a ride in a real boat and show you how trim at speed really work, not at cruise. I don't need to read owner manuals as I know how a engine and hull should be set up and operate for best performance. One of my service areas with a variety.....

 

[Sea Rider]

Hi Jim Alabama,

As you know there's an eternal Hanky Panky going on on the forums with the nonsense that AV plate must be set higher or lower under the middle hull. As you have answered positively that now your combo performs much better with raised motor than did before, can you please enlighten the audience as to which lower leg height (1 or 2) is water flow now passing by at plane on flat calm water cond. For that must visually look what's going on at back transom, have someone kneel aft transom, pull head over transom side of motor and check. Report your visual findings...

..

The lower 2 strokes Yam 140 HP motor had excessive prop aeration on tight close turns and on bit rough water cond while flow was passing around the red line, the previous transom height was modified and raised conveniently, now the flow skims right under the yellow line, water performance is awesome, with trim set at 90 º, deck load evenly distributed, with a maximized prop to run full wot rpm range the combo literally jumps on plane at just 3/4 throttle



On a dry motor/combo installation, personally don't care at which lower leg height the AV plate remains with respect to the boat's bottom as won't know once combo is on plane at which lower leg height is water flow passing by at speed which is of extreme importance to know so to improve the combo performance and take the max HP out of any motor. Second pic speaks for itself...

Happy Boating

 

[Faztbullet]

Above information is incorrect as usual as the AV plate must be above water not under when running. Thousand of pictures, videos and MFG instructions that prove that it incorrect. The reason that motor lost bite in turns is the round hull bottom design and incorrect prop. That problem could have be easily fix by swap from that K series prop to a round ear prop with cup.Would have picked up speed and hole shot.

 

[jimmbo]

Agreed, totally incorrect. He is the one shoving the Hanky Panky,

 

[Sea Rider]

Hey guys why don't you mind your own business and Let the OP post what was asked if he's willing to help to determine at which lower leg height 1-2 is water flow passing by. So AV plate must ride Above, oh yeah, tell me the story in colors. What was posted was tested on different water conditions and not under your plain Theoretical BS you like posting and boating with. Besides I'm down here and you up there so you've are proving nothing.

Happy Boating

 

[Faztbullet]

Sea...I had to turn my intelligence level down two notch's to try and get you to understand the BS you post is incorrect. It might work fine on underpowered dingys and rubber boats but not up here where real boating happens. It is very apparent that you do not understand that hull design, insteps, set backs ,jack plates ,different prop design , and trim is used in determining prop type, pitch and engine height, MFG's install recommendations.

 

[jimmbo]

Ok Sea Rider, If your yellow dashed line indicates where the AV Plate should be for higher performance, then I will apologize.

 

[Sea Rider]

Ok Sea Rider, If your yellow dashed line indicates where the AV Plate should be for higher performance, then I will apologize.

Jimmbo,

To understand, the dashed yellow line indicates where is the current hardened water flow pasing by at speed while motor sits at 90 deg, the previous flow used to pass right through the red dashed line close to the AV plate. With that position achieved prop areration on choppy windy, white caps water cond and at close tight turn as the distance from the upper plate to the AV plate is only 2 inches high. The motor needed just a fine tunning transom height optimization if you know what you're doing.

The transom height was modified, now the motor sits lower than before, there's more water bed height and prop bits much better than before. For me it's irrelevant at which AV plate height now sits with respect to the boat's bottom. Is this too complicated understand ?

Will totally disagree with Fuz that by just looking at a posted pic have found out that the round hull shape and the current prop are not doing their homework right and are the direct culprits for motor to aerate. The round rear hull is only located there and the remaining hull lenght is same as any other boat.

In real boating world it doesn't matter at all the shape of the hardened water flow exiting right the middle rear hull, could be round, shallow or deep V or even flat if you plan powering your dining table with an attached motor As Long the flow passes Completely Undistrurbed and engages between any of both plates, the one you prefer as this is a personal choice.

Opinions are welcome...

Happy Boating

 

[jimmbo]

Here are Mercury's recommendations. The only time they recommend putting the AV Plate below the hull, is on Heavy barges. Other Outboard makers have similar Philosophies

 

[Sea Rider]

Hey Jimbo,

What's the comparison for ? What has a 130 HP Yam to do with a 200-250 Merc motor ? Each boat/motor installation is unique and all will need to be fine tuned to a certain degree if wanting to get the max out of any boat/motor combo.

I've seen those Merc parameters before. No other motor brand Owner's nor Service Manuals I've read indicates that their motor's lower leg heights should duplicate the Merc's parameters. Only indicates a variable height distance between rear middle keel/hull and the AV plate depending on HP model and motor brand, which if you haven't figured it out corresponds only to the initial installation process of any motor period.

Those Merc parameters corresponds to oldie motors in which the distance of both plates are at the most 2 inches high, the different motor heights seen on pic only performs well on near flat calm water cond at straight water courses. What works OK with a 2 inches plates distance will not work same with a 4 inches ones, that's the difference between boating with oldies and newer motors.

Happy Boating

 

[jimmbo]

You truly are Delusional about engine heights, except maybe for barges. What is your fixation with some splash deflector some engines have? Mounting the engine so low as to have the the water level between the AV Plate and Splash Deflector when at planing speeds goes against every engine makers recommendation on engine heights. Maybe you need to use better props