79 35hp short shaft transom height or prop?

[JMG]

I could use some advice on the proper transom height for my '79 35hp short shaft(I think?). the measurement from the top of the mount openning to the AV plate is 17.5" and my transom is cut down to 18". the AV plate is 1/2" above the bottom of my 1648 v jon boat. my problem is at a hard take off and at WOT the prop starts to slip and the rpms go up and the boat speed goes down. the prop is in good shape and it is a 10x15. Can anybody please help on what to do next.

 

[F_R]

Re: 79 35hp short shaft transom height or prop?

The problem is obvious. The motor is too high. So, you have cut a 20" transom down to 18" right? You have two, maybe 3 choices:
1: Cut some more out of the transom to get the AV plate below the surface
2: Build the transom back up to where it was and get a long shaft motor.
3: Live with it.

 

[JMG]

Re: 79 35hp short shaft transom height or prop?

I know it sounds obvious but if you research past posts you get answers that go from the AV plate could be 2'' higher to 2'' lower and all will work well. Just a little confusing to someone new to boating. Is a short shaft transom 15" ? that would place it 3" below the rear or should I go one more inch and try it? it's a long process to cut it and make it like new so I would prefer to do only one more time.Is it possible that some of my problems come from my prop? does 10x15 sound ok?Thank you for the help F R.

 

[JB]

Re: 79 35hp short shaft transom height or prop?

Is that transom 18" on the surface or 18" vertical drop from the top? Many transoms are angled, leading to misleading measurements.

You should not be getting ventilation with the AV plate only 1/2" above the keel unless the engine is tilted too far aft. Set the engine angle so that the AV plate is parallel with the bottom of the boat. When on plane the AV plate should be right at the surface of the water, not under or above.

 

[F_R]

Re: 79 35hp short shaft transom height or prop?

Well, in the first place every boat is different. There is no magic height. The usual way of setting them up is to start off with the motor sitting down on the transom and raise it in steps till you get it as high as you can without ventilation. You are doing it backwards.

In my personal opinion and obversations, it is a rare boat indeed that will run with the A/V plate 2" above the bottom. We are talking about race boats here. And they all ventilate like crazy till you get it up and planing and are worthless at less than flying speeds. Normal pleasure boats run with the plate even or below the bottom.

Just my 2c.

 

[JMG]

Re: 79 35hp short shaft transom height or prop?

The measurement for the transom was taken along the surface of it. It only changes by a 1/4" or less when you do it at the motor. Right now the AV plate is at the surface of the water when it's up on plane and in line to the bottom of the boat, the motor is in the 2nd trim hole and I tried to raise it more to get the bow out of the water some and it would ventilate even easier, so I am assuming that tells me it's still to high in the water?. AM i correct in saying the a short transom is 15" even in 1979 or only on the newer boats because if I make my transom 15" that puts my AV plate 3" below the bottom of the boat and from the old posts that should also cause problems is this correct?

Thanks for the replies JB and F R