jackplate question

[BUDDY123]

I have a new tiller short shaft 20hp 4cy engine which adds to much weight to my current 14ft alum V. If I find a boat that required a long shaft engine can I use a jackplate to make the necessary adjustment or will motor be to low in the water or is this just s stupid question, be nice. Quess its not stupid until you ask it. THANKS in advance

 

[ssobol]

Usually a jack plate is used to raise the engine, not lower it. As such it may be difficult to mount a jack plate to the boat so that the motor plate is lower than the mounting plate. The biggest problem I see with mounting the engine low is that the tiller arm may be below the top of the transom and that will make steering difficult. Usually the head of the engine also hangs over the transom a bit. Mounting the engine low may have the engine hitting the top of the transom or may limit how far you can turn it to steer. You might be able to mount a hull extension that will allow you to move the motor aft and down, giving you the clearance that you need. However, on a smaller boat (that needs a 20hp engine) this might be difficult. Another option is to cut down the transom to allow clearance for the short shaft motor. Depending on the boat this might also be difficult and result in a transom top that is too close to the water level (water will come aboard over the transom when you stop quickly as the wake catches up to the boat or in following seas). You may be able to convert your short shaft engine to a long shaft by changing the leg between the power head and the lower unit. This requires changing the housing and the shafts and linkages that connect the two pieces together. It can be fairly simple to do this, but depending on the engine this will cost some hundreds of $ for the parts. In the end converting the engine or trading it for the correct length may be the best solution.

 

[Texasmark]

Jack plates cause "setback" which requires the engine to be mounter higher than the normal line of the hull. Depending on who's doing the talking, the AV plate needs to be inline with a straight edge protruding aft from the bottom of the boat +/- about an inch with a normal transom mounted engine and conventional hull (with no molded in setback like you find on Bassboats). Somewhere in the archives is the rule of thumb that people in the business use for initial settings of engine height (AV plate) vs setback.

You are looking at 5" of height difference and with 3 or 4" of jackplate setback from the transom you probably will be close too what you need with the engine mounting position on the jackplate straight out from the 20" shaft length mounting position on the transom. This should put your AV plate 5" above the line of the hull which is probably pretty close to what you would need.

Couple of problems however: 1. With tiller steering, tight turns may be awkward; tiller in your way turning toward the side on which you are sitting (usually Starboard), and long reaches in the opposite direction. 2. Heavier engine, weight of jackplate, and distance, add up to a weight at the end of a cantilever that projects more weight on the transom than you would normally have and your 5 inches of additional height with a different boat with a 20" transom may not result in the transom "freeboard" being any higher with the boat at rest than what you currently have.

My 2c,
Mark