boat steering

[2763steve]

i have just purchased a new 25hp f/wd steer/elec start short leg tohatsu for my 13ft f/glass runabout that used run great with with my old evinrude 25,shortleg and rope pulley steering,got a new helm steering with new motor,now boat wont steer but seems to veer onto its sideand this is at 10kph and is pretty scary.i notice the new motor leg is not as long as the old evindrude and the cav plate is not below the hull line?could this be the problem,hope you guys can help,thanks steve

[Roughasguts]

Your probably right, the leg needs to drop down further to be usefull as a rudder and it also helps keep th e boat on an even keel.

Is the motor bolted on tight, but still lower the motor first.

[FNQCairns]

How far above the bottom of the hull is the Cav/vent plate in mm?

cheers fnq

[Lancair]

If my etec40 is trimmed right in or down, it will cause my 4.2 stessco tinny to lean to the right alarmingly at planning speeds or just under . If the breeze is from the right too, look out. So I dont trim it right in unless Im doing like 4 knots or less.
I believe its all to do with torque from the prop, due to the props plane of rotation being non perpendicular to the direction of water flow when trimmed right in. The prop blade angles, relative to the water flow mean the down going blade (on the rhs) has a great angle of attack and pulls more water, pulling the motor right. If I were to let go of the tiller, it goes into a hard right turn. By holding the motor straight, that force has to go somewhere, and it translates into a push sideways, therefore the boat leans. I can demonstrate this easily, by trimming out or up, to the point where the turning force on the tiller is neutral, the boat will ride level. I can further demonstrate that trimming up more, leads to a left turn force on the tiller and a lean to the left when held straight. However we'd probably reach cavitation/ventillation point before the lean got excessive.

The postion of the trim tab on the cav plate has a huge effect on where the "neutral point" will be. So assuming you dont have tilt n trim, your trim tab needs to move more to the right, (facing the direction of travel) or your motor needs to be set on a higher trim pin.

Its similar to the force that causes mono hull power boats to lean into wind whilst running at speed. The wind from the right, is trying to push the bow left, your turning slightly right to hold your course, theres no right turn happening, so the force translates to a lean.
If I could draw 3D force vector diagrams it would/may be easier to understand.

Andrew

[Hornet Rider]

How far above the bottom of the hull is the Cav/vent plate in mm?

cheers fnq

FNQ, while your waiting for steve to get back to you with his measurements, just how far should the cav/vent plate be about the bottom of the hull? I'm about to replace my evinrude 50 with a jono 70. Pic shows current height of existing 50 hp, so I was going to replicate that same height on the 70 when I fit it.

[2763steve]

thanks mate ,boat shop trying to tell me that it doesnt matter about cav plate,they think chines arent sharp enough,cav plat is about 20mm above bottom of hull,thanks steve

[2763steve]

cav plate about 20mm above bottom of hull but boat shop says this doesnt matter and that the chines are no longer sharp enough but the boat wont turn just leans over and seems to be running on its nose?thanks steve

[2763steve]

thanks for your reply,complained to boat shop about fitting of motor and steering and we took it for a water test,when he stood at the back oft the boat it went and steered like it should. trying to tell me chines are worn?but boat went great with old motor/cav plate about 20mm above hull line?boat wants to turn on its left handside even at fast walking pace, wont turn just leans over.boat shop reckons cav plate is ok?thanks steve

[FNQCairns]

FNQ, while your waiting for steve to get back to you with his measurements, just how far should the cav/vent plate be about the bottom of the hull? I'm about to replace my evinrude 50 with a jono 70. Pic shows current height of existing 50 hp, so I was going to replicate that same height on the 70 when I fit it.


HR as high as you can get it without undue negatives, the degree to what constitutes a negative will be different for each person, on average a ski boat will be set lower than a fishing boat due to the need to execute tight turns.

Low V and light can mean 5mm above the bottom is too high, deeper V and heavier hulls can often go higher, regardless testing is the only way to find out what suits your individual setup.

Foils can rob near an entire inch of Pitch from the prop whenever it is being dragged through the water so to make them work without this big negative the engine needs to be as high as possible so they clear the water as early as possible. Often it's not possible to get the engine high enough without too much ventilation.

Yours looks close to the bottom but it's hard to tell from the pic, try up one hole, if it works try another, I notice there is no room to drop it so up might be worth a test.

cheers fnq

[ryank]

will tinnies you want the cave plate just above. it sounds like it is in perfect position. having stingrays can make your boat lay over if you have them on.