optimum height?

[boating brad]

first, im not sure if i should post this here or in the just boat questions area, but anyway here it is. i have a 14ft capri bowrider, 50hp evinrude W/hydrofoil, manual tilt. i have the tilt stop rod in the 2nd hole from the transom. when i adjust to the 3rd hole it will ventilate. my question is, is optimum height based on height of motor at the verticle point? or at say 2nd hole from the transom? basically im wondering if my motor is at optimum height. when i set it up, i mounted the motor cavitation plate level with the bottom of the transom, but maybe i can go higher?

 

[Kenny Bush]

Re: optimum height?

Catfish, your starting point is right where it should be. The cavitation plate should run on the surface plane. On most boats, the cav plate is about 2 to 3 inches above keel. But it still is on the surface plane of the water. If your transom is notched, you will even bring it up higher. The pin settup on the transom is trimming out(raising up) the motor. I am assuming that you do not have tilt and trim on that motor. Anyway, one should trim out(up) the motor as much as possible, as to not cavitate(spinout) the prop. This position lets the weight of the boat ride on the sterns keel, resulting in smoother and faster ride.

 

[boating brad]

Re: optimum height?

"starting point" ok but, can or should i go higher with the motor if it ventilates at the 3rd hole from the transom? or is it setup perfecly as is? id kinda like to know because its over a 1hour drive to the closest outboard frendly lake. just to see if it will ventilate at one more hole up on the transom plate. i know that at the 2nd hole from the transom is perfect! but will it be better to lift the motor and set the tilt stop at the 1st hole from the transom? i know tilt up for speed, down for hole shot. so if i lift the motor plate and put the tilt stop at the first hole, what will be compromised? hope im not over complicating things. maybe i should leave well enough alone, but i want perfection!

 

[boating brad]

Re: optimum height?

^

 

[Walker]

Re: optimum height?

Since you don't have tilt & trim do this. Have someone sit on the bank or in another boat watching and you come blasting down the lake. Your motor should be level with the horizon at full plane.

 

[Dhadley]

Re: optimum height?

Catfish - dont confuse height and trim. Height or X dimension refers to a verticle adjustmet position up or down in a straight verticle line. Usually references where the AV plate (cavitation plate) is in relation to the true bottom of the hull when the plate is horizontaily parallel (doesnt have to be even) to the hulls running surface. On race boats we reference the centerline of the propshaft because the propshaft is usually at or slightly above the true bottom.<br /><br />Trim is measured as positive, neutral or negative and moves in an arc, the axis being the steering tube or clamp bracket bolt. Positive trim means the mid section and lower unit is moved out away from the transom, while negative trim angle means the mid section and lower are closer. Neutral trim means the centerline of the mid section and lower unit (or driveshaft centerline) is 90 degrees to the running surface. <br /><br />Good luck!

 

[boating brad]

Re: optimum height?

Dhadley. thanks for the reply, i learned the difference between tilt/trim, and x dimension but i dont think my question(s) were answered.<br />or were they? <br />i just want to hear your opinion on my question, i promise i wont sue if your wrong, which i doubt you would be. i've read your replies and feel you are very knowledgeable.

 

[Dhadley]

Re: optimum height?

Optimum height (X dimension) and trim angle at any given speed is a relationship between the hull design and prop technology. <br /><br />On boats that have power trim but no hydraulic jackplate, usually (there are exceptions) you start with the motor trimmed in (negative trim) and trim out as you gain speed. <br /><br />The better the prop you have the more trim and height you can use and not ventalate the prop.<br /><br />In theory, optimum height would be where the outboard's gearcase is completely out of the water (hidden behind the transom) so there is no drag from it and the prop still hooks up. 0% slip is what we all try for and is the theory behind the surface drives like the Arneson and Kaama. Imco and Mercruiser's Blackhawk outdives also follow the surface piercing technology.<br /><br />In the case of motors without power trim and a fixed X dimension, the otpimum motor positioning is a comprimise between best holeshot, top speed, best ride and cornering depending on water conditions. Without power trim even load distribution becomes critical.<br /><br />Good luck!

 

[Spidybot]

Re: optimum height?

Thanks, Dhadley. Golden Nuggets as always <br /><br />Just want to add, that a Bayliner 14 with a 50hp and Hydrofoil may not have the power needed to fully explore the fun of these principles. The really serious setups uses powertrim and powered jackplates to trim all these parameters while under way.<br /><br />Alone the cost of the special design props needed when you're taking surfacing to it's limits could make one reconsider.

 

[boating brad]

Re: optimum height?

thanks guys. im thinking now, that its set up pretty good as it is. it is a evirude outboard and does have slotted adjustments for xdimension but i raised it 1 inch above the transom when i drilled the holes for motor mounting. its probably fine. thanks for the info.

 

[Outrage 19]

Re: optimum height?

I’m by no means an expert but let me see if I can explain in simple terms and if I’m wrong someone please correct me. With your lower unit trimmed in, negative trim, it will have the effect of pushing your bow down. This helps your boat get up on plain if your load is more to the back on the boat. With your lower unit trimmed out, positive trim, it will have the effect of pushing your bow up. This keeps your boat from plowing if your load is more to the front of the boat. Hope this helps.

 

[boating brad]

Re: optimum height?

outrage. thanks i do know what effects trimming in & out makes and i do know that the motor mounting plate can be raised "x dimension", what i didnt know is the setup for optimum x dimension. there is nothing i could find on this. im guessing its a trial & error thing! to get the motor to the optimum height without cavitation.