outboard height problems....

[kmercury]

I recently bought a 1961 Starcraft Jet Star with a 1973 Johnson 40hp on it. I took it out for a test run and had back splash problems. Went home and saw I had a long shaft on a 15" transom. So, I built a jackplate and raised the motor 5". Took it back out and still had some back splash but not a much. I looked up how to set the outboard height based on the plate above the prop. Problem is, that plate is still well below the bottom of my boat (5"). I do see two other flange type plates on the shaft.... Please see the diagram attached.... Which plate should it be lined up with.... I assume the bottom plate but that means another 5" of raising... ugg This old Johnson must be set up for a 25" transom maybe?

 

[sutor623]

Re: outboard height problems....

I recently bought a 1961 Starcraft Jet Star with a 1973 Johnson 40hp on it. I took it out for a test run and had back splash problems. Went home and saw I had a long shaft on a 15" transom. So, I built a jackplate and raised the motor 5". Took it back out and still had some back splash but not a much. I looked up how to set the outboard height based on the plate above the prop. Problem is, that plate is still well below the bottom of my boat (5"). I do see two other flange type plates on the shaft.... Please see the diagram attached.... Which plate should it be lined up with.... I assume the bottom plate but that means another 5" of raising... ugg This old Johnson must be set up for a 25" transom maybe?





View attachment 137532

Hey buddy. Yeah measure the top of transom mount to cavitation plate. That lowest plate with the trim tab is definately the one that needs to be "lined up" as you say. I believe you do have a sailboat shaft.

 

[kmercury]

Re: outboard height problems....

Here is a better diagram

 

[kmercury]

Re: outboard height problems....

Darn... anyone have a standard drive shaft and shift lever for this motor so I can take the 5" extension out?

 

[steelespike]

Re: outboard height problems....

Isn't your jack plate 5".A 20"(long shaft) usually measures 21-22 inches.Your 15" transom might actually be a hair less than
15. While a short shaft is the right way to do it Seems your allready on the way with your jack plate.
You might check ebay for parts or parts motors.Can you shorten a 73 motor pictures seem to indicate there may be a 5" extension piece.

 

[kfa4303]

Re: outboard height problems....

You can look online for a short shaft conversion kit, but if the motors running well, I would try to let sleeping dogs lie and try to use a jackplate of some sort. You'd like to have the anti-vent plate level, or even 1" - 1 1/2" above the keel of the boat. Here are a few pics that may help. You can also make adjustable jackplate using pieces of angle aluminum.

 

[kmercury]

Re: outboard height problems....

Thanks for the pics.... I already have it on a 5" jackplate so either I need to go up another 5" or convert to a short shaft.....

 

[kmercury]

Re: outboard height problems....




Here is my situation. Even with a 5" jackplate I have a little over 2" to go.....

 

[oldman570]

Re: outboard height problems....

Have you had the LU off of the motor yet? You might be able to remove the extension and all others if the motor has them on the shift rod and the driveshaft at the top of what is there. It could be that a extension kit was added that would have these items.
oldman570

 

[kfa4303]

Re: outboard height problems....

I see your problem kmercury, you have a long shaft motor which adds, guess what, about 5" to the normal length of the leg. The good news is that you can get a short shaft conversion kit which includes a short drive shaft, water uptake tube(s) and shorter shift rod. I would try to remove the extension then use your current jack plate to get just the right height. I've circled the removable, longs shaft extension in the pic below.

 

[steelespike]

Re: outboard height problems....

It might look a little funny but it seems to me you could modify the jack plate to add 3" or more.
Looks like the transom is a V; maybe if you wanted to get fancy you could add 2 or 3" to the transom and then
increase the jack to the desired height.Less splash problems when backing .

 

[sutor623]

Re: outboard height problems....

Wow that is a short transom. Looks like youd need a jackplate even with a shortshaft.

 

[unleaded]

Re: outboard height problems....

lol i have the same engine (but evinrude) in a short shaft but i need a long.

 

[kfa4303]

Re: outboard height problems....

Here are some pics of the jack plate I just made from 1/4" aluminum for my short shaft '66 20 hp which has sort of the odd 18" leg length. It was a little too long to be a 15" short shaft, but too short to be a 20" long shaft. My anti-vent plate was about 2"-3" below the keel even with the motor clamped as high on the transom as I could get it. Since adding the jackplate, it is now about 1/2" above the bottom of the boat with room for me to clamp the motor even higher if need be. I haven't had a chance to get her on the water yet to see if there's been any improvement in top end speed, but I should at least get a little bit better handling and be able to slip into a some skinnier water than before.

 

[Sea Rider]

Re: outboard height problems....

Float your boat and trim engine to be perpendicular to water level, sit 1 adult passenger up front, go for a wot test and check visually at which tail height is water flow passing by. Ideal engine height is when water flow is slightly passing under yellow small upper plate while flow is being cut by blue edge. Play with engine height untill you can match that position.

Happy Boating