Transom Height

Hey Pappy,
I am at a quandary here. Getting ready to cut the transom wood for my Crestliner
and double checked the motor - I "assumed" I had a short shaft (15") RDE-19C
and it is actually 18" as pictured. or do they even come short and long ??
anyways - this is the original transom for the '58 30hp Sea Horse.
what do you suggest for a proper transom height for the RD-19 ?

I may well be wrong on this but wouldn't you have to add height to the transom? If it's 16.5" and the engine has 18" I'd think you'd be adding 1.5".

I'd drop the engine on the back as it sits and measure where the anti-cavitation plate hangs, measure from that point and go high, you can always trim down if needed.

Crazyboat said:

I may well be wrong on this but wouldn't you have to add height to the transom? If it's 16.5" and the engine has 18" I'd think you'd be adding 1.5".

I'd drop the engine on the back as it sits and measure where the anti-cavitation plate hangs, measure from that point and go high, you can always trim down if needed.

Click to expand...

I'd have to agree. Start out tall and trim to the proper height.

I am usually up to date on transoms - but this one caught me
completely off guard....... I know the anti-ventilation plate plate has
to be (close to) even with the hull bottom for modern motors.
My quandary is the dimensions of the motor itself.
when the Crestliner was purchased new in 1959, it had a '58 30hp on it and it ran great
for 20 years. If this is a long shaft, I will not raise the transom up or put a jack plate on it
to the point of being ugly - I will sell the motor and buy another short shaft RD-19. (if this is the case).

Thanks Pap !!
that clears it up for me. I am almost positive it had a RD-19 on it when purchased new in 1959.
and I am trying to remember back to which one of my knucklehead brothers had it
after me and cut the transom down.... that is when it changed over to a 1975 Johnson 25hp short shaft
and it has had short shaft Johnson 25s on it ever since.
I measured the RDS-20 today and it is also 18" - same as the RDE-19.
- So I will build the transom flush with the top of the boat @ 16.5" and call it a day.
and yes - it has been another learning moment for me..... for the rule "one size does not fit all".
especially in the vintage marina.
and yes - those measurements were taken with a builders square perpendicular to the hull bottom.
and the boat is set up for remote cable & pully system.

Thank you.


Edit: This is what I mean by Ugly it Up.


I will do a bit of creative woodworking to make it look pretty again !!




.

Learning something here. ( I hope).

Do I understand that you will wind up with the cav (anti-cav) plate one or two inches under a line from the bottom of the hull? If that is so, why?

Were older motors built so differently than those of today?

Won't you have a much tougher time getting onto plane with the 16 1/2 distance?


after looking at the pictures, I agree with the ugly factor...

richg99

Rich, at one time, I had a '79 Johnson 25hp that was almost 4" too deep -
and it roostertailed something terrible.
I chose to install an aluminum plate to act as a spray shield instead of a new transom
or jack plate and it worked fine until I could find a better motor.

my outboard history is: when it needs fixin - put it in the shop.
I attempted VERY few repairs myself - I am just not mechanically inclined.
Pappy is the "go to" guy for vintage motors like this one - so maybe he can
clarify the issue........... I will be running the RD-19 about 2-3" below the line.






.

richg99 said:

Learning something here. ( I hope).

Do I understand that you will wind up with the cav (anti-cav) plate one or two inches under a line from the bottom of the hull? If that is so, why?

Were older motors built so differently than those of today?

Won't you have a much tougher time getting onto plane with the 16 1/2 distance?


after looking at the pictures, I agree with the ugly factor...

richg99

Click to expand...

If you have read much of what I have written through my "tenure" here you will note that I have written about the different measurements you will get when measuring engines for transom height.
Lets take short shaft engines as an example. We commonly call these engines "short shaft" or 15" engines.
Example here would be a 15hp from Johnson/Evinrude. Measuring from the top inside of the transom bracket to the cav plate will yield one measurement.
Grab a Mercury/Mariner of the same vintage and you will find a different measurement.
Yamaha/Suzuki/Tohatsu, etc.....all different.
But they are all listed as short shaft engines yet not a single one of them will measure out at 15".
Vintage engines are another story. Cavitation plates were designed more for aesthetics back then than something that would actually keep air from being sucked under it and into the propeller. The exhaust, behind the prop, was just waiting for a good excuse to curl around and get into the propeller blades as well.
Propeller design was fairly crude compared to the more modern "thru-hub" propeller designs. All this added up to engines that were routinely destined to run with the plate well below the bottom of the boat in order to run properly for most applications of the day.
To throw another wrench in the works, a typical outboard transom angle is around 15 degrees. Not all manufacturers built to that spec. Not all manufacturers built transoms at the same height either. Transom heights varied greatly and they still do to this day!
This is why it is sometimes difficult to get an answer across to someone wanting "instant satisfaction" about setting up their boat properly. I find that I typically lose almost all of them when this information comes out and trying to get them to make one change at a time and run new numbers with the new change. Then we have someone contribute that he has a similar boat but different brand with a brand X engine on it and he runs this that or the other and to just do this and everything will be perfect, lol