Ideal transom height for 1962 28hp short shaft

[kevinwd1]

I have brought a 13ft aluminum boat which has had the transom cut down to accommodate a short shaft motor .I haven't tried the motor on the boat yet, as the motor is stored at work, but rough measurement seem to indicate that the transom height of 16.5" will be a little bit too high to completely cover the water intake plate for the impellor. Is there an ideal height for the transom to suit this older type motor? I?m thinking that you should have the cav plate around 1' to 2" below the bottom of the boat to cover the water intake plate sufficiently. Most of the information says that the cav plate should be level with the bottom of the boat but this would put the water intake just above the water line when the boat is planning. Also seems that most new motor have the intake below the cav plate .Any feedback would be appreciated.
Regards Kevin

 

[orbanp]

Re: Ideal transom height for 1962 28hp short shaft

Hi Kevin,

Actually, water intakes for outboards are usually below the cav plate (have a look), for the exact reason that they would pick up water even when the boat is planing.
And you are correct, you want your cav plate 1" - 2" below the bottom of the boat.
If the cav plate were higher, your real problem would be cavitation (the prop would suck in air) as soon as you would go faster.
This is definitely not a surface piercing drive set up ;-)

Good luck, Peter

 

[kevinwd1]

Re: Ideal transom height for 1962 28hp short shaft

The water intake is just above cav plate.I have tried to load an image that shows a similar lower unit to mine,

 

[kevinwd1]

Re: Ideal transom height for 1962 28hp short shaft

After doing some more searching I think I may have been mistaken in thinking the water intake was the three small holes above the cav plate? I now believe the intake is on the fin behind the prop? Can anyone confirm this?

 

[bbstacker1]

Re: Ideal transom height for 1962 28hp short shaft

Kevin, you are correct, that "fin" sticking down is the primary water intake when you are going forward. It is important to make sure it is clean and open (if it has been painted over make sure the holes are clear).

 

[kfa4303]

Re: Ideal transom height for 1962 28hp short shaft

+1. The primary water intake is in the fin just in front of the prop, but you are correct that there is a smaller water intake found just above the anti-vent plate which functions at idle/test tank conditions. Ideally, the anti-vent plate should be level or slightly higher than the keel of the boat. Most OMC (Johnson/Evinrude) motors of this era have sort of an odd length of around 18" whereas most transoms are generally 15" or 20". As a result, they tend to sit a little low on some transoms and a little higher on others. I have a 15" transom myself, so I had to build a small transom riser to raise the motor about 1"-2". It's not a must have modification, but it helps a little bit. As long as the motor is pumping sufficient amounts of water at all times, you should be fine. There is no "pee stream" like on modern motors either. Instead the cooling water and exhaust gasses are mixed and expelled out of the back of the motor and beneath the waterline.

 

[F_R]

Re: Ideal transom height for 1962 28hp short shaft

OK, first off, the main water intake is on the forward edge of the exhaust outlet snout, aft of the propeller. The holes in the rectangular plate above the cavitation plate are to supply additional water when running in reverse. With that cleared up, remember that motor was originally designed for wooden boats with keels and run deeper than modern motors. The hull's bottom line should fall somewhere between the cavitation plate and water pump bulge. Get it too high and you will be plagued with cavitation (actually ventilation for the purists). Get it too low and you will get excess spray and loss of speed. Really, the conclusion is that it would be rare to find a boat with unsuitable transom height. But feel free to experiment if that is your inclination.

 

[phillnjack2]

Re: Ideal transom height for 1962 28hp short shaft

cav plate level with bottom of boat is the very lowest you want to go..

the statement about cave plate being lower than bottom is wrong, very wrong, like said above it causes spray drag and steering problems.

15 inch shaft engines were designed for 15 inch transoms, 20 inch for 20 inch transoms, back in the days of non-though prop exhaust.
when props went through prop exhaust it changed as water pick ups went lower.

personaly I would have that engine with cav plate 1 inch above transom as see if I still get water pressure,if so then try a touch higher.

 

[F_R]

Re: Ideal transom height for 1962 28hp short shaft

cav plate level with bottom of boat is the very lowest you want to go..

the statement about cave plate being lower than bottom is wrong, very wrong, like said above it causes spray drag and steering problems.

15 inch shaft engines were designed for 15 inch transoms, 20 inch for 20 inch transoms, back in the days of non-though prop exhaust.
when props went through prop exhaust it changed as water pick ups went lower.

personaly I would have that engine with cav plate 1 inch above transom as see if I still get water pressure,if so then try a touch higher.

Confusing apples with oranges here. The question concerns a 1962, 28hp, a NON-Prop Exhaust motor designed for 15 or 20 inch transoms.....as you said. If you hang the motor on such a transom, you will find the cavitation plate is considerably below the hull bottom. As I said, the motor was designed for wooden boats with a keel, and the service manual and installation instructions will confirm that. The O.P. said his transom measures 16.5 inches, which is about average for modern fiberglass boats. That puts the cavitation plate up closer to the bottom, but probably still a bit below. I stand by my statement that if you raise it up much you are going to get excessive ventilation, in MOST cases. That will be a pain in the rear for the recreational boater. Now if we are talking about race boats, the whole story changes. There you are going to put up with ventilation and often will barely be able to get it up out of the hole. But once you get it planing, it will go maximum speed. BTW, these lower units are not the most sleek design, adding to the ventilation and cavitation problem.

 

[kevinwd1]

Re: Ideal transom height for 1962 28hp short shaft

Thanks for all your replies. I picked the boat up today and will put the motor on in a few days. It seems from rough measurements that the cav plate will sit just below the bottom of the boat which should be ideal .The boat is only going to be used to pull the kids around on a tube and going up and down rivers etc .So as long as the boat gets up and planes and pumps water I’ll be happy
Regards Kevin

 

[phillnjack2]

Re: Ideal transom height for 1962 28hp short shaft

modern transoms 16.5 inch ?????????????

.from 1972 in short shaft configuration are standard 15 inch, not 16.5 inch.
I don't know of any boat with a 16.5 inch transom, definitely no UK or European boats.
we have had lots of USA boats here as well with short shaft transom and none have been 16.5 inch to my knowledge.

and 1.5 inches too low is a hell of a lot and will kill boat speed by a long way, especially for an older engine of just 28hp.

this engine will only be a clamp on bracket so experiment with it low and high and see what you get.
just make sure water pressure is goo.

phill